Geosynthetic Clay Liners 9781003077848, 9054105194, 8022763837, 9789054105190

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PROCEEDINGS OF AN INTERNATIONAL SYMPOSIDM NURNBERG/GERMANY I 14-15 APRll..1994

GEOSYNTHETIC CLAY LINERS Edited by

R. M. KOERNER Geosynthetic Research Institute, Drexel University, Philadelphia, USA

E. GARTUNG & H. ZANZINGER LGA-Grundbauinstitut, Niimberg, Germany

Authorization to photocopy items for internal o.r personal use, or the internal or personal use of specific clients, is granted by A.A. Balkema, Rotterdam. provided that the base fee of US$1.50 per copy, plus US$0.10 per page is paid directly to Copyright Clearance Center, 222 Rosewood Drive, Danvers, MA 01923, USA. For those organizations that have been granted a photocopy license by CCC, a separate system of payment has been arranged. The fee code for users of the Transactional Reporting Service is: 90 5410 519 4/95 US$1.50 + US$0.10 . Published by A.A. Balkema, P.O. Box 1675,3000 BR Rotterdam, Netherlands (Fax: +31.10.4135947) A.A. Balkema Publishers, Old Post Road, Brookfield, VT 05036, USA (Fax: 802.276.3837) ISBN 905410 519 4 © 1995 A.A.Balkema, Rotterdam

Contents

Preface R. M. Koerner, E. Gartung & H. Zanzinger Foreword

Vll

IX

R. Floss

I Regulatory perspectives On the equivalency of landfill liner systems - The state of discussions in Germany K. Stief US EPA experiences with geosynthetic clay liners D. A. Carson

3 17

2 Fundamentals Characteristics and sealing effect of bentonites

31

F. Madsen & R. Niiesch

Properties and test methods to assess bentonite used in geosynthetic clay liners T. Egloffstein

51

A suggested methodology for assessing the technical equivalency of GCLs to CCLs R. M. Koerner & D. E. Daniel

73

3 Testing and design Basic examination on the efficiency of GCLs D. Heyer

v

101

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Geosynthetic clay Liners

Guidelines on the use of liners in highway construction H. Rathmayer

113

Landfill cap designs using geosynthetic clay liners 1. M. Fuller

129

On the long-term shear behaviour of geosynthetic clay liners (GCLs) in capping sealing systems G. Heerten, F. Saathoff, C. Scheu & K. P. von Maubeuge On the slope stability of landfill capping seals using GCLs D. Alexiew, H. Berkhout & R. Kirschner

141 151

4 Applications Groundwater protection using a GCL at the Franz-Josef-Strauss airport Munich, Germany G. Heerten

I6I

Design and installation of a state-of-the-art landfill liner system R. Trauger & K. Tewes

175

Test field for the capping system of the rnichelshohe landfill V. Kreit

183

Capping landfill surfaces and contaminated areas with geomembrane supported clay liners 1. T. Pape & R. B. Erickson

189

GCL installation in a water protection area for the A96 motorway near Leutkirch, Germany R. Schmidt

199

Measurement and control system for the upper basin of the Reisach-Rabenleite pumped storage power station M. Rau & 1. Dressler

207

5 Closure Quality assurance in the manufacture and installation of GCLs H. Zanzinger

219

GCL summary and conclusions

229

E. Gartung

Authors' addresses

235

Advertisements

239

Preface R. M. Koerner. E. Gartung & H. Zanzinger

A new type of geosynthetic material which is an excellent blend of natural soil and geosynthetics in the form of a composite barrier material is currently available as wide width, factory manufactured products. Called Geosynthetic Clay Liners (GCL) or Geokunststoff-Ton-Dichtungen (GTD), the current variations of these products are as follows: • an adhesive bonded layer of bentonite between two geotextiles, • a powdered or granulated layer of bentonite between two geotextiles and then stitch bonded together, • a powdered or granulated layer of bentonite between two geotextiles and then needle punched together, • an adhesive bonded layer of bentonite directly on a geomembrane. Since the book is in English the products will be called geosynthetic clay liners, usually by reference to the acronym of 'GCLs'. The various products are typically 7 to 10 mm thick, 3 to 5 m wide with a unit weight of approximately 5.0 kg/m2 • The purpose of the bentonite is to hydrate and swell as moisture is encountered forming the barrier component. The geosynthetics (either geotextiles or a geomembrane) initially act as carrier materials. Subsequently, the geosynthetics also act independently to reinforce, further decrease permeability, provide structural integrity, etc., depending on the particular product. There are currently six manufacturers of GCLs, each with unique variations of their basic style. However, it should be recognized that manufacturers regulary modify their materials and new products are regularly appearing. In the environmental area, GCLs are used most frequently to construct waste containment liners or covers, e.g., GCLs are used, • by themselves as a barrier layer • as the lower component of a composite geomembrane/geosynthetic clay liner • sandwiched beneath a geomembrane and above a compacted clay liner as a triple component composite liner • placed beneath a geomembrane/compacted clay liner as a triple component composite liner • placed above a geomembrane for puncture protection. GCLs have also been used within vertical cutoff walls, as secondary containment for underground storage tanks and beneath a wide variety of reservoirs and surface impoundments. Numerous application papers in this book will describe some of VII

vm

Geosynthetic clay liners

the above applications. Clearly the use of GCLs in environmental applications is the major use at this point in time. Transportation applications of GeLs, however, are also important and rapidly growing. The various products have been used beneath airfield pavements for the containment of deicing liquids, to protect against accidentally spilled chemicals from highway accidents (also roadway deicing salts) and for groundwater protection at fueling stations and related depots. Several papers in this book relate to, and describe, describe, these applications. While geotechnical applications of GeLs have not been widespread, their use should be considered by engineering designers and facility owners. Cutoffs beneath hydraulic structures and impervious barriers within them are clearly potential areas for utilization. Surface dressing above walls and slope coverings to avoid hydrostatic pressure are candidate uses as well. Thus it is seen that the entire spectrum of environmental, transportation and geotechnical engineering applications can utilize GCLs as hydraulic barriers. Indeed, this book will show that such activities are ongoing. Necessary before describing these applications, however, is the presentation of the technical background of the materials used (i.e., the bentonite and the various geotextiles and geomembranes) and their identification via test procedures and standards. Thus the structure of the book is in discrete parts dealing with preliminary details, followed by regulatory perspectives, then testing and design, followed by applications and finally issues of quality controUquality assurance and the closing summary.

Foreword R. Floss

ChaimiGTI of the 'Synthetics in Geotec/mics' section of the German Geotechnical Society and President of the German Chapter of the International Geotextile Society.

The 'Synthetics in Geotechnics' section of the German Geotechnical Society and the German Chapter of the International Geotextile Society welcome the initiative shown in arranging a symposium on geosynthetic clay liners (bentonite liners). The symposium has been jointly organized by the Geotechnical Institute of the LGA and the Geosynthetic Research Institute (GRI) of Drexel University, Philadelphia, USA. The purpose of the symposium is to pool the experience gained in Germany and America. The subjects discussed cover current know-how on fundamentals and experimental investigations, as well as the state of the art in designing and installing bentonite liners. This book is a direct outgrowth of the symposium which was held in Nuremberg, Germany on April 14-15, 1994. These pre-fabricated products are composite liners, with the bentonite, either in powder or granular form, contained between two textile layers, generally geotextiles, which are bonded by needle-punching or other methods. The composite can take a certain amount of tension and stretching. In recent years in Germany, comprehensive scientific and practice-oriented investigations have shown these products' basic suitability for various applications in earthwork engineering and landscaping, in landfill construction, in traffic systems and hydraulic engineering. Accordingly, these liners have grown significantly in importance, in particular for the soil and ground water protection of constructions in water protection zones, for protection from erosion and in covering systems near the surface. The first basic investigations were published on the occasion of the first 'Synthetics in Geotechnics' congress in Hamburg, in 1988. Since then, a number of institutions have been involved in the development and testing of GCLs, within their range of research activities. These scientific investigations have mainly been concerned with saturation and permeability behaviour, and have included a number of influences, such as freeze-thaw and wet-dry cycles, the effect of overlying coverings and hydraulic gradients, as well as the effect of saline defrosting solutions and hydrocarbons. One further important area of investigation was permeability behaviour when subjected to bi-axial stretching. Since 1988, the Testing Institute for Foundation Engineering, Soil and Rock Mechanics at the Technical University of Munich has been engaged on practical trials to put scientific findings on these liners to the test. These practical investigations looked at the effectiveness of the products under working conditions, with particular attention paid to installation and seams, as well

IX

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Geosynthetic clay liners

as the effect of the supporting and covering material under static and dynamic stress. It was possible to study these fmdings further within the framework of special tenders and large-scale tests carried out at a number of major construction projects. Some such examples were: • Ground water protection at the Franz-Josef Strauss airport, Munich-Erding. • Double lining system for the Lech canal, Lechwerke AG, Augsburg. • Ground water protection for the A 96 motorway between Munich and Lindau, in the area of the 'Leutkircher Heide' water protection area. • Reconstruction work on the lining system for the Raben Ieite elevated storage basin, operated by the East Bavarian Energy Authority. By now, we have a number of years' experience in the construction and behaviour of these lining systems, which in principle are to be assessed positively. However, they have also led to the conclusion that GCLs must be differentiated in accordance with certain general quality requirements. Working group 14A of the 'Synthetics in Geotechnics' section is currently engaged in working out reconunendations on use, examination and quality control. The technical contractual terms will be included in the new draft for the 'Additional Technical Contractual Terms for Earth Works in Road Constructions' (ZTVE-StB). The products currently on the market are widely different in their manufacture and joining methods. Accordingly, those responsible for planning and quality control are obliged to pay great attention to the suitability of the candidate products for each individual case, and, if necessary, to decide on their use on the basis of special evaluation. Moreover, one must assume that the development of GCLs and their related construction measures is not yet at an end, but will lead to even further improvements.

Regulatory perspectives

Taylor & Francis

Taylor & Francis Group http://taylorandfrancis.com

On the equivalency of landfill liner systems ­ The state of discussions in Germany K. Stief

German Federal Environmental Agency, Berlin, Germany

ABSTRACT: According to Federal regulations in Germany, landfills must be constructed with composite Liners of well specified characteristics. Alternative lining systems are accepted provided that they are equivalent to the standard systems. The criteria for the assessment of equivalency have to take many different aspects into account. The fundamentals of the assessment criteria are discussed in the present paper. I INTRODUCTION A range of landfill liner systems are currently available, some that are constructed significantly different from others. It may be assumed that liner systems which are constructed differently will also vary in efficiency. But even if the construction of landfill liner systems is not different, their effectiveness may vary considerably if different liner materials are used. Information is needed on the effectiveness of any landfill liner system to be used, in particular if these differ from the 'usual', the 'proven', or those liner systems specified in administrative regulations. T A Abfall (fechnical Instructions on the Storage, Chemical, Physical and Biological Treatment, Incineration and LandfLJling of Waste) and TA Siedlungsabfall (Technical Instructions on the Recycling, Treatment and other Management of Municipal Waste) outline and describe landfiiJ liner systems for class I and I1 landfills. Class I landfills are those, that contain material from construction demolition and other well defined predominantly mineral solid waste which contains the lowest potential for contamination, while, in the future, class II landfills will contain the residues of thermally treated domestic waste. Landfills, operated at the time theTA Siedhmgsabfall has come into force, have the status of 'old landfills (Aitdeponien)', which have also to fuJfill particular requirements. Landfills of class m are regulated in TA Abfall, since they are hazardous waste landfill facilities. T A Abfall states the following in item 9.4.1: • The performance effectiveness of landfill liner systems may not be affected by any deformation of the liner layer due to overlying weight. Accordingly, settlement and deformation are to be calculated. • Any pipe penetrations of the liner system in the slope area are to be carried out in such a way that they may be monitored and repaired.

3

4

Geosynthetic clay liners

• The requirements made of these landfill liner systems in accordance with paragraphs I to 3 may be waived, if it can be proved that an alternative system is equivalent. The requirements given in paragraph 4 shall also apply accordingly. T A Siedlungsabfall states the following items: • Landfill liner systems are to be planned and constructed in accordance with items 13.4.1.3 and 13.4.1.4, or with equivalent systems. • A landfill liner system is to be constructed on the Landfill subgrade, in accordance with item 13.3.2, and on the slope surfaces. The type of landfill liner system for the slope surface shall depend on the gradient. • Vertical penetrations of the liner system are not permissible. • The landfill bottom liner system is to consist of the system components shown in Figure I, or an equivalent liner system. • The landfill bottom liner system is to be installed in accordance with Figure I, or to consist of an equivalent system, whose material and test requirements are given in Appendix E ofTA Abfall. • After a landfill section has been filled with waste, a cap lining is to be installed on top of the landfill body. • If required by the intended, permissible subsequent utjlization, the cover soil layer may be replaced by a covering, suitable for the appropriate use, which gives the cap system equivalent protection. • The following requirements shall apply for the individual cap elements: For class I landfills, the lining shall be a mjneralliner or with an equivalent liner system. For class II landfills, the linings shall be composite liner as in with Figure 2, or with an equivalent liner system. These extracts from TA Abfall and T A Siedlungsabfall clearly show that great emphasis is placed on the determination of equivalency of landfill liner systems. An aim, to be achieved with the Technical Instructions in accordance with the Waste Management Act, is federally standardized requirements for landfills. Accordingly, there must also be standardized federal procedures and criteria to determine the equivalency of landfill liner systems. Although T A Abfall has been in force since I April, 1991, the administrative practice of the federal states has not achjeved a standardized procedure, or, at least, such a procedure is not known. This may be due to the fact that, since T A Abfall came into force, no landfills for waste requiring special supervision ('Hazardous waste landfills', 'Class ill landfills') have been planned or approved, for which 'alternative' landfill liner systems were to be used. As T A Siedlungsabfall bas only been in force since I June, 1993, there has, as yet, been no reason to officially assess as equivalent any landfill liner system for municipal solid waste (MSW) landfills other than composite liner systems. Inrurectly, however, there has been such an assessment, indeed such a comparison, as, in practice, the composite liner is considered the best sealing system using state-of-the-art technology. In November 199I, a working group for the 'Assessment of Landfill Liner Systems' (AK BEDAS) was set up at the German Federal Environmental Agency (UBA), with the aim of working out some basic criteria for the assessment of equivalency. This initiative has not been expressly supported by the Working Group on Waste of the Federal States (LAGA). The LAGA's technical commjttee on waste merely decided, in January 1992, 'to observe' the further work of AK BEDAS. The work of AK BEDAS has not resulted in any conclusions ready for publication. In the opinion of the Federal Ministry of Building and Construction, and of the

On the equivalency oflandfillliner systems

5

Ministries of Construction for the Federal States, the German Institute for Building Technology (Deutsches Institut fur Bautechnik, DIDt) is responsible for the approval of building materials and components which also comprises landfill liner systems. This claim to responsibility is based on the Constructional Guidelines (Musterbau­ ordnung, MBO) and the Building Products Act (Bauproduktengesetz, BPG). The Federal Minister for Environment, Nature Protection and Reactor Safety shares this opinion. Therefore, the AK BEDAS working group at the Federal Environmental Agency has concluded its activities. lhis work is now being continued by the German Institute for Building Technology and its advisory committee GDSA (Grundsatze des Deponiebaus und der Sicherung von Altlasten (Basics of landfill construction and remediation of abandoned hazardous waste problem sites)). This paper attempts to portray and discuss some opinions on the equivalency of landfill liner systems, in order to re-state the discussion to a wider circle of interested parties. Here, reference is made particularly to a publication by Engelmann (1993). The composite liner, in accordance with T A Siedlungsabfall, is the reference point for any considerations concerning equivalency. 2 AN OVERVIEW OF LINER SYSTEMS In practice, a range of landfill liner systems are used, or are considered to be used, which differ from the composite liner. • Composite liner in accordance with T A Abfall • Composite liner in accordance with T A Siedlungsabfall (Figures I and 2) • Double composite liner (without leak detection layer), (August 1986) • Double composite liner (with leak detection layer) • Composite liner, with a bentonite geosynthetic clay liner (GCL) as mineral layer (Daniels & Koerner 1992) • Inverse composite liner (with the synthetic geomembrane below the mineral lining layer), (Collins 1991) • Composite liner, with asphalt concrete liner and mineral liner • Asphalt concrete liner (Ryser 1993 and Schonian 1991) • Multi-layer mineral liner with leak control layer • Compacted clay mineral liner (Dtillmann 1992) • Mixed grain liner (Hom 1989) • Special mineral liner (DYWIDAG liner), (Jessberger et al. 1993) • Composite liner, with geomembranes and capillary block (Melchior 1993) • Capillary block (Melchior 1993 and Mock 1991) • Composite liner, of asphalt concrete liner and capillary block These liners are being offered, or used, for both bottom liners and caps. What is not yet clear is how the various liner systems are to be assessed for effectiveness, when comparing them with composite liners in accordance with T A Abfal l and T A Siedlungsabfall. Decisions are currently being made on an irregular basis. In Switzerland, it is not necessary to carry out assessment procedures for the various landfill liners. In Appendix 2 of the Swiss Technical Regulations for Waste (TV A), 'Requirements made of the site, construction and completion of landfills' three different liners are permissible under item '22, Lining' (TV A 1990): The liner must give long-term assurance that leachate cannot seep away; subsoil conditions, the slope of the landfill bottom and sides, and the condition of the drainage

6

Geosynthetic clay liners

layer are to be taken into consideration. One of the following liners will generally be sufficient: • Mineral liner. This must have a minimum thickness of 80 em and a coefficient of hydraulic conductivity k of 1 x 10·9 m/s or less, and must be installed in at least three layers, with each layer individually sealed and protected from drying out. • Asphalt coating seal. This must have a minimum thickness of7 em, be instalJed above a suitable foundation and binder layer, and be sealed in such a way that the porosity determined from a sample is not greater than 3 per cent. • Synthetic foil seal, i.e., a geomembrane. This must have a minimum thickness of 2.5 mm and be installed above a mineral liner with a minimum thickness of 50 em. • Other liners. Laboratory and field tests are to prove that these are at least equivalent to the liners described in the above three items. The effectiveness of the liners must be tested and documented during installation and before covering. 3 WHAT DOES 'EQUIVALENCY' MEAN? Strictly speaking, it is clear that equivalent means equally good when subjected to the same stresses and demands. In the context of assessing landfill liner systems different from the composite liner specified in T A Siedlungsabfall, the question arises (from planning and licensing practice) if another alternative liner system may not also be accepted under certain circumstances if it is sufficiently good. At issue is if equivalent in the sense ofTA Siedlungsabfall also (only) means sufficiently good. The need to have sufficiently good landfill liner systems accepted as equivalent becomes particularly great if the installation of composite liners is made difficult or impossible, or if the durability of the sealing layers is affected, due to the following: • Construction features (for example steep slopes), or problems in meeting a deadline, combined with unfavourable weather conditions. The inclusion of weather conditions in any standardized Federal assessment procedure is likely to present problems. • Demands from the commissioning body, often a high temperature difference between top and bottom edge of the mineral liner layers, which may be caused by high temperatures in the leachate. These difficulties lead to the search for landfill liner systems which withstand the requirements specified in each case, while also being capable of fast, simple and economical instaJlation. It is acknowledged that, under the pressure of conditions, some liner systems have also been taken into consideration which almost certainly were known to be not equally good, but under the existing conditions could be rated as sufficiently good. 'Cost effectiveness' is said to be even better, because the sufficiently good liners could be produced less expensively, were less liable to malfunction, etc. T A SiedlungsabfaJI, in comparison toTA AbfaJI, has opened a few possibilities for the use of other liner systems. The greatest of these is to be found in Item 13.4.1.3: • A landfill liner system is to be installed on the subgrade, in accordance with item 13.3.2 and on the slope surfaces. The type of landfill liner system to be used on the slope surface is to be determined depending on the gradient. This reference to the special feature of liners on slope surfaces may be understood

On the equivalency oflandfill liner systems 7 as a reference to the fact that not all liner systems can be used on all slope gradients, as well as to the fact that the use of other landfill liners is to be exa.rrllned, if the installation of a composite liner is considered too difficult or simply too expensive. It is worthy of note that equivalent liner systems are not expressly stipulated here. Is this supposed to mean that only sufficiently good liners are considered necessary? It would have been stricter to require that steep slopes, on which composite liners could not be installed, were to be avoided. Assessments of the equivalency of landfill seals are constantly made in the course of licencing procedure. If, for example, a double mineral liner with two mineral liner layers, each 75 em thick (with a leak control layer in the middle), is authorized instead of a composite liner consisting of a 2 .5 mm thick synthetic geomembrane and a 75 em thick mineral liner layer, this liner system will be indirectly assessed as equivalent or of higher quality, but at least as sufficiently good. An assessment of the clay mineral liner or a mixed-grain sealing layer is also carried out if a single mineral liner is authorized instead of a composite liner. However, in such special cases, it can hardly be assumed that equivalency means equally good. It is much more likely that the responsible authorities assessed the effectiveness of the mineral liner as sufficiently good under the site specific water management conditions. The assessment criteria for sufficiently good are at least as important as for the assessment of equally good, indeed they may be more important. Generally speaking, the demands concerning the water management requirements made on the landfill liner system were merely that the best liner system technically available had to be selected. That developed to the point where, with reference to Section 34, Paragraph 2 of the Water Conservation Act (Wasserhaushaltsgesetz), the so-called 'High-safety landfills' or 'Containment structures' were stipulated (IWS 1987). Rating criteria are lacking, such as permissible levels for pollutants which may penetrate the liner under certain landfill operating conditions, while taking the existing geological barriers into account. For this reason, and only for this reason , certain liner systems are specified in T A Abfall and T A Siedlungsabfall. It is to be hoped that the option that TA Siedlungsabfall permits by stating'... or equivalent liner systems' will really lead to a further development of the technological state-of-the-art. However, it is to be feared that what will really happen is that better, but more expensive systems, will be avoided. In the attempt to have landfill liner systems recognized as equivalent, the quality of the geological barrier is often drawn into the discussion. When that is the case, it becomes particularly clear that what is concerned is the choice of a sufficiently good and not an equally good liner system. It is argued, for example, that the bottom liner system needs no special adsorption capacity if the geological barrier is very homogeneous, and has an adsorption capacity many times that of the clay mineral liner. On the other hand, the requirement for a composite liner is more readily accepted if the properties of the geological barrier (homogeneity, pollutant retention capacity) can be rated as evidently not particularly good in the near vicinity of the landfill. Moreover, it is then often the case that one, two or three additional clay mineral layers are offered as 'replacement for the missing geological barrier' . As long as there is no express withdrawal from the multi-barrier concept for landfills, the geological barrier may of course not be replaced by just any type of liner. But administrative regulations such as T A Siedlungsabfall are not intended to stand in the way of reasonable interpretation. It is indeed worthy of consideration whether an excellent geological barrier (great homogeneity, great thickness, great pollution retention potential) really needs to be covered with another 1.50 m thick

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Geosynthetic clay liners

clay mineral sealing layer with high pollution retention capacity. A sealing layer might also be used which has no great pollution retention capacity, but which possesses very good properties relating to sealing effectiveness, plasticity and sensitivity to shrinkage fissuring and cracking. 4 PROPERTIES OF LANDFILL LINER SYSTEMS TO BE TAKEN INTO CONSIDERATION IN ASSESSING EQUIVALENCY

The characteristics and properties of landfill liner systems may be classified under four main groups (Engelmann 1993): • theoretical effectiveness • durability • ease of construction at the particular site • system characteristics The theoretical effectiveness is characterized by the transport of pollutants through the liner system considering both quantity and time. The transport of pollutants is determined by convection, diffusion and adsorption behaviour. For this purpose, the results of laboratory tests are to be taken into consideration, as are the charac­ teristics under real load conditions. However, the establishment of maximum loads under real landfill conditions is based on assumptions for pressure gradients and for chemical exposure (types and concentrations of pollutants). Theoretical effectiveness may be achieved under ideal laboratory conditions, but not, however, in practice. As for durability, the variability of the sealing characteristics under long-term external conditions (mechanical, biological, chemical, physical) are to be considered. External stress may result from freeze/thaw, wet/dry, erosion, suffusion and colrnation, dissolution etc. Under constructability, such criteria are considered as the sensitivity of materials and construction to weather, the feasibility and reliability of connections and penetrations, the permissible tolerances of certain parameters, quality management, the need for and possibility of carrying out rapid checks on sealing characteristics at the construction site. In particular, the possibilities and limits of constructability on steep slopes must be taken into consideration, as must the question of stability if the landfill base is sloped. System characteristics which may influence the result of a comparative assessment are, for example, the ability to check individual liner components (for example by leak detection systems), the installation of multi-layer liner where any errors made in the individual layers may be compensated for, the reliability of the parameters which must be used for proof of stability, as well as the redundancy of the system, i.e., the stability of one component under the same condition of stress, where another component fails. In a broader context, system characteristics may also include financial and ecological aspects. These may, for example, include: energy consumption during construction, during extraction and transport of the liner material, during installation of the materials, as well as the dependency of installation on the weather, with its effect on the time needed for installation, or the use of recycled liner materials.

On the equivalency of landfill liner systems

9

5 THE COMPOSITE LINER A frame of reference is needed if the equivalency of landfill sealing systems is to be assessed. The frame of reference which should be referred to is given by the composite liner systems specified in TA Abfall and TA Siedlungsabfall (Figures 1 and 2). The characteristics of composite liner systems were specially examined in a research program (BAM 1992). There are no reliable test results available from practice for composite liners or for clay mineral or mixed grain liners.

A verbal description of the characteristics of composite liners might be as follows

(Engelmann 1993): • The HDPE geomembrane is resistant to chemical and biological attack. • The HDPE geomembrane is an absolute convection block for liquid materials. No flow takes place. • As a result of their good affinity for organic pollutants, these are concentrated in the geomembrane and between the geomembrane and the mineral liner, because they are diffused through the geomembrane, so that levels of concentration and therefore also diffusion are reduced. • If there is no intimate contact, a lateral flow may result beneath possible punctures in the geomembrane. • It is quite possible for pollutants to be transported through the mineral layer by convection flow. Nevertheless, the mineral layer may be regarded as a convection

Waste Drainage layer

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Leachate pipe Protective layer

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Landfill subgrade Geological barrier

Figure l. Landfill bonom liner system for class II landfills, in accordance with TA Siedlungsabfall (1993).

I0

Geosynthetic clay liners

............ .. ................... .. .. -.. .............. .. .. .. .. .. .. - .. ..

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Figure 2. Landfill cap liner system for class II landfills, in accordance with TA Siedlungsabfall (1993).

block if the pressure gradients are very small and if the effective pore volume is very small. • If there are no leaks in the geomembrane, the mineral layer will not be made use of as a convection block. The mineral layer, however, functions as an element of the composite liner. It acts as a diffusion block against the organic materials which are diffused through the geomembrane, if the mineral layers have an appropriate adsorption capacity. • Diffusion times, the pollutant break-through times through the mineral liner layers, are intended to be very long. The diffusion times are related to the square of the thickness of the mineral liner layers. One argument against the composite liner is the aging of the geomembrane. Available examination figures on the course of aging over several decades are interpreted as proof of the decomposition of the geomembrane in a few decades. If these assumptions for the loss of effectiveness of geomembranes in composite liners were correct, similar assumptions on the reduced effectiveness of other geosynthetics would have to be made, for example for geogrids of the type used to improve the load-bearing capacity of the subsoil in the extension to the Hausham landfill in Bavaria. The possibility cannot be ruled out that in the long term, liner materials will suffer a loss of effectiveness; this will be very difficult to take into account when assessing the equivalency of liner systems.

On the equivalency oflandfill Liner systems

11

6 COMPOSITE LINER OF GEOMEMBRANES AND GEOSYNTHETIC CLAY LINERS In the USA, the Environmental Protection Agency (US EPA) requires double liners for landfLIIs with hazardous waste. Double liners consist of the following components (from top to bottom): • upper drainage layer • geomembrane • lower drainage layer (leak detection layer) • composite geomembrane/compacted clay liner Only small quantities of water are permitted to leak from the lower drainage layer. If these levels are exceeded as defined by the site specific action leakage rate, ALR, investigatians must be carried out to determine the reason for the increased leakage. If the leakage increases further to a so-called rapid and large leakage rate, RRL, the landfill must be closed until the damage has been stopped (US EPA 1989). Leakages well above the ALR and, indeed, greater than the RRL are said to have occurred often at many landfills. It was possible to stop these problems by installing geosynthetic clay liners (GCLs) below the upper geomembrane. The function of these installations is to ensure that leaks in the lower drainage layer (leak detection layer) are kept close to zero. Table 1 shows the advantages and disadvantages of compacted clay liners (CCLs) and geosynthetic clay liners (GCLs). These results are increasingly being used to claim that the composite liner, consisting of geomembrane and CCL, is equivalent to the composite liner consisting of geomembrane and GCL (Daniel & Koerner 1992). There are considerable reservations about GCLs, because of their small thickness (it is suspected that it is easily penetrated in spite of all tests) and because of misgivings about the long-term durability of the geosynthetic fibres (the discussion of the long-term durability of polymer fibres is developing along similar lines to that concerning the long-term durability of synthetic geomembranes). 7 PROBLEMS OF COMPARATIVE ASSESSMENT Every landfill sealing system has advantages and disadvantages. Comparative assessment becomes particularly difficult if the advantages and disadvantages arise from different characteristics and properties of the sealing systems, for example a system could be devised as follows: Criterion A

B

c

System I ++ 0

System II

0

++

If points are given instead of the +/0/- system, the assessment does not become any easier. For example, can a liner system whose theoretical effectiveness is poor, but whose construction is very simple and whose costs are very low, be awarded more points than a liner system with high theoretical effectiveness, but whose construction may only be carried out by trained specialists and under favourable weather conditions, if necessary under a roof? Is the former liner system of higher

12

Geosynthetic clay liners

Table I. Advantages and disadvantages of CCLs and GCLs Compacted clay liners (CCLs)

Geosynthetic clay liners (GCLs)

Large volume required Large weight Heavy installation equipment Lengthy installation Correct installation difficult Penetration impossible Large constructional thickness necessary Field trial areas may be required Specia.l data on sealing material required May develop dry fissures Difficult to repair Sensitive to frost High, variable costs Sensitive to senling

Smail volume required Light weight Light installation equipment Rapid installation Correct installation easy Penetration possible Can be fabricated in small thicknesses No field trial areas required Data for GCLs available Cannot develop fissures if moisture available Easy to repair' Less sensitive to frost Low, better predictable, costs Less sensitive to settling

• Applies only to landfill caps

quality because of its greater nwnber of points? Or is the system of assessment wrong? Is it at all possible for a simple (one layer) liner system to be equivalent to a multi-layer liner system (for example a composite liner), if one component of the multi-layer liner system is identical with the simple liner system? Must not a two-layer composite liner system in which one layer functions as a convection block (for example a HOPE geomembrane) and the other as a diffusion block (for example a mineral liner layer) be assessed as of higher quality than a composite liner system which contains no convection block? . Of what significance to the assessment is the proven working life of liner layers in landfLilliner layers, for example of geomembranes, asphalt concrete sealing layers, or geotextiles? How can new liner materials be put to use if proof of long-term durability is always going to be required? What working life of landfill liners is to be required? In the case of landfill bottom liner systems the answer is 'forever', as they are practically impossible to repair. (With the so-called 'repairable liner systems', where it is possible to inject sealing medium into the leak detection layer, the mentioned problem arises after the leak has been closed by injection. What this produces is quite simply a composite liner system which is no longer controllable and repairable, with the disadvantage that the injected liner layer cannot have the same quality as the Liner layers constructed in open installation.) But how can it be demonstrated that their characteristics will remain unchanged 'forever'? Can we count on a medium-term effectiveness of, say, 100 or 200 years when assessing the equivalency of a landfill bottom liner in a composite liner, for example a synthetic geomembrane or an asphalt sealing layer, without taking into account the significance of the increased safety in the operating period and the effectiveness of the landfill cap seal (in principle, always easily repairable)? 8 POSSIBLE ASSESSMENT STRATEGIES It will take many years to work out strictly scientific and generally acceptable

On the equivalency oflandfill liner systems

13

assessment procedures. By that time, the widest range of landfill liner systems will have gained acceptance in practice, because of decisions based on individual cases. An assessment that is only based on the feelings of a specialist, or, more likely, a department manager, cannot be the right strategy either. Results are most likely to be achieved if a group of recognized experts discuss the pros and cons of the liner systems available for assessment and present the arguments in print, so that they can be understood in public, or at least acknowledged. Engelmann (1993) suggests 'listing the characteristics and criteria to be examined only as points of reference, and following up this list to the best of one's knowledge and conscience, while evaluating laboratory results, field trials, the results of calculatory models and practical experience. The interim results. if available, should be listed quantitatively, together with a verbal appraisal; if they are not available, then only a verbal appraisal should be given. A group of experts in the field of landfill liners should then make the decision on the permissibility of a certain liner for the specific conditions at one specific site. The group of experts should be chaired by the authority responsible for the construction of the landfill.' A further possibility for evaluation, which he considers the simplest to apply and the most practicable, given the present level of knowledge on how to describe the matter or the feasibility of manufacture is quoted by Engelmann (1993): 'the standard liner system of TA Abfall or TA Siedlungsabfall is thus taken as the given and approvable system, and any liner system to be assessed differently is questioned on the nature of its difference in a scientific 'quiz' . In other words, the only problems considered will be those which lead to divergencies from the standard liner system, both negative and positive. Any doubts as to the equivalency (this must mean the sufficient effectiveness) in individual aspects are to be removed or confirmed by quantitative and qualitative arguments, and to be placed in perspective alongside the confirmed advantages. The overall view of individual aspects concerning their short-term and long-term effects on the pollution retention of the whole system eventually leads to confirmation of equivalency or to its rejection.'

9 FINAL REMARKS There is an urgent need for statements as to what is to be understood by landfill liner systems, which are equivalent to the composite liners in accordance with TA Abfall (1991) or TA Siedlungsabfall (1993). Strictly speaking, equivalent means equally good. In the broader sense, equivalent means sufficiently good . The assessment of sufficiently good landfill liner systems makes it necessary for hydrology assessment criteria (permissible pollutant transport) to be specified, in which, of course, the characteristics of the geological barrier of a landfill may also be taken into account. If it is possible to take into account local or regional hydrology conditions, including the characteristics of the geological barrier of a landfi!J, in deciding on sufficiently good landfill liner systems, then it will not be possible to establish, on a Federally standardized basis, which liner systems are sufficiently good, as these conditions vary considerably throughout the country. However, it is quite possible, and indeed urgently necessary, to establish Federally standardized criteria for the assessment of equally good liner systems.

14

Geosynthetic clay liners

The call for the specification of hydrological criteria to establish whether landfill liner systems are sufficiently good, is based not only on the requirements made in the Water Conservation Act (Wasserhaushaltsgesetz), but also, in particular, on the requirements of EC (European Community) directives. Here, above all, the EC directive on the protection of groundwater (EWG 80/68 1979) should be mentioned, which, together with the first general administrative regulation (on the Waste Management Act) concerning requirements for the protection of groundwater when storing and depositing waste (VwV AnSchGw 1990), has been adopted in Gennan law. The technical interpretation of the EC Directive on Ground Water Protection and the VwVAnSchGw (1990) frequently leads to very high demands being made on landfill liners in Gennany. REFERENCES August., H. 1986. Untersuchungen zur Wirksamkeit von Kombinat.ionsdichtungen. Fehlau, Stief (eds) Abfa/lwirtschaft in Forschung und Praxis Bd. 16 'Fortschrille der Deponietechnik 1986'. Stuttgart: Erich Schmidt Verlag. August, H. 1990. Neuere Forschungsergebnisse zur Sperrwirlrung von Kombinationsdichtungen filr Deponien. Fehlau. Stief (eds) Abfa/lwirtsclwft in Forschung und Praxis. Bd. 16 'Fortschrine der Deponietechnik 1990'. Stuttgart: Erich Schmidt Verlag. Collins. 1988. 1st eine Folie auf einer mineralischen Deponieabdichtung vertretbar? Mii/1 und Abfa/1. 20. Jg.• Heft 8, p. 362. Daniel, D. E. & Koerner, R. M. 1992. Landfill Liners from Top to Bottom. Civil Engineering. Dec. 1991, pp. 46-49. DGEG. 1990. Empfehlungen des Arbeitskreises 'Geotechnik der Deponien und Altlasten' · GDA. Deutsche Gesellschaft filr Erd- und Grundbau e. V. (ed). Berlin: Ernst & Sohn. Diillmann, H. 1993. Qualitiitssicherung bei Planung und Bau von Kombinationsdichtungen fiir Deponien. Abfa/lwirtsclwft in Forschung und Praxis. Bd. 54 'Fortschritte der Deponietechnik 1992'. Fehlau. Stief (eds), Stuttgart: Erich Schmidt Verlag. Engelmann, B. 1993. Zur Bewertung der Gleichwertigkeit alt.emativer Deponieabdichtungssyst.eme. Ablagerung von Siedlungsabflillen - Geologische Barriere Deponieabdichrungen Deponiebetrieb, Tagungsunterlagen z:ur UTECH BERLIN '93, Seminar 08. FGU Berlin 30. EWG 80/68. 1979. Richtlinie des Rates vom 17. Dezember 1979 tiber den Schutz des Grundwassers gegen Verschmutzung durch bestirnmt.e gefahrliche Stoffe (80/68/EWG). Miil/handbuch. Kennzi.ffer 0274,65. Lfg. V/82. Finste!Walder, K. & Mann, U. 1990. Stofftransport durch mineralische Abdichtungen. Jessberger (ed) Berichte vom 2. Deponie-Seminar, Bochum, 10. -JJ. Okt. /990: 209-221. Rotterdam: Balkema. Hom, A. 1989. Mineralische Deponie-Aachendichtungen aus gemischt-komigen BOden. Bautechnik 66. Heft 9 ( 1989). Berlin: Ernst & Sohn. IWS. 1987. Symposium 'Die Deponie - ein Bauwerk'. lnstitut filr wassergefahrdende Stoffe an der Technischen Universit.iit Berlin. IWS-Schriftenreihe 111987. Jessberger, H. L., Omnich, K .. Finste!Walder, K. & Mann, U. 1993. Versuche und Berechnungen zum Schadstofftransport durch rnineralische Abdichtungen und daraus resultierende Mat.erialentwicklungen. BMFT-Verbundvorhaben 'Weiterentwicklung von Deponieabdichtungssystemen, BAM, Berlin, 2. Arbeiwagung 17.03-/9.03.1993. Melchior, S., Berger, K., Vielhaber, B. & Miehlich, G. 1993. Ergebnisse der Langzeitlibe!Wachung von Oberfliichenabd.ichtungssyst.emen auf der Deponie Georgswerder (Hamburg). Altlastensanierung 93, Beitriige zum Vierten intemationalen TNO!Kfl< Kongress, Berlin, 3. -7. Mai 1993. London: Kluwer Academic Publishers and BMFT· Verbundvorhaben 'Weiterentwicklung von Deponieabdichtungssystemen ·. BAM. Berlin, 2. Arbeitstagung 17.-19. Miirz 1993. Mock. J., von der Hude, N. & Jelinek, D. 1991. KapiUardichtungen filr Deponieoberflachenabdichtungssyst.eme. Fehlau, Stief (eds), Abfallwirtsclwft in Forschung und Praxis. Bd. 47 'Fortschritte der Deponietechnik 1991'. Stuttgart: Erich Schmidt Verlag. Ryser 1993. Erfahrungen mit der Planung und Herstellung von bitumi.isen Deponiebasisabdichtungen (Asphaltbeton) in der Schweiz. Ablagerung von Siedlungsabfiil/en · Geologische Barriere. Deponieab­

On the equivalency oflandfill liner systems

15

dichlungen, Deponiebetrieb. Tagungsunterlngen zur UTECH BERUN 93, Seminar 08. FGU Berlin 30 Schon ian, E. 1991. Aspha.ltbeton-Dichtungen im Deponiebau. Miill und Abfall 23. Heft I ( 1991 ): p. 12 and Heft 3: p. 173. TA Abfall. 1991. Gesamtfassung der zweiten aUgemeinen Verwaltungsvorschrift zum Abfallgesetz (TA Abfall), Teil I: Technische Anlo:itung zur Lagerung, chemisch/pysikalischen, biologischen Behandluog, Verbrennung und Ablagerung von besonders iibewachungsbediirftigen Abfallen vom 12. Man 1991, GMBI, 42 Jg. (1991), Heft 8: p. 139. Ktiln : Carl Heymanns. TA Siedlungsabfall. 1993. Dritte allgemeine Verwaltungsvorschrift zum Abfallgesetz, (T A Siedlungsabfall), Technische Anleitung zur Verrneidung, Verwertung, Behandlung und sonstigen Entsorgung von Siedlungsabfallen; Kabinettsbeschlull vom 27.08.1992, Buodesratsdrucksache 594/92 und Anderung des Bundesrates vom 12.02.1993, Bundesratsdrucksache 594/1/92. TV A. 1990. Technische Verordnung iiber Abfalle (TV A) vom I 0. Dez. 1990. US EPA. 1989. Requirements for Hazardous Waste Landfill- Design, Construction. and Closure. Seminar Publication . EPN625/4-89/022, August 1989, chapter 10, pp. 121-125. VwV AnSchGw 1990. Erste Allgemeine Verwaltungsvorschrift liber Anforderungen zm Schutz des Grundwassers bei der Lagerung und Ablagerung von Abfallen vom 31. Januar 1990. GMBJ: p. 74.

Taylor & Francis

Taylor & Francis Group http://taylorandfrancis.com

US EPA experiences with geosynthetic clay liners D. A. Carson United States Environmental Protection Agency, Cincinnati, USA

ABSTRACT: The United States Environmental Protection Agency (US EPA) is interested in the appropriate application of geosynthetic clay liners (GCLs). With academic and industrial participation, US EPA is investigating the engineering properties and performance characteristics of these materials to gather information necessary for the proper selection and use of GCLs. US EPA hosted two workshops assessing current knowledge, identifying and addressing issues surrounding these relatively new materials. The proceedings of the workshops is summarized in this paper. 1 INTRODUCTION The United States Environmental Protection Agency's (US EPA's) Risk Reduction Engineering Laboratory studies many aspects of waste containment technology in support of national regulations, and for better understanding of performance. Research has led to the use of composite containment and cover systems comprised of both natural (mineral) and synthetic materials. While both have desirable and specific properties, it has been shown that the composite application of these materials offers superior theoretical performance in a waste containment scenario than either material used independently. GCLs offer a new opportunity in waste containment. These manufactured clay liners offer many potential advantages. In order to evaluate the use of a GCL at a particular site, the functionality of that layer must be analyzed. 2 US EPA LANDFilL LINER AND COVER REGULATIONS US EPA regulations have been promulgated for two categories of waste disposal: • Hazardous Waste The Resource Conservation and Recovery Act (RCRA) Subtitle C (as amended by Hazardous and Solid Waste Amendments of 1984), 40 CFR 264. • Municipal Solid Waste The Resource Conservation and Recovery Act (RCRA) SubtitleD, 40 CFR 257 & 258. Those implementing the regulations are required to meet or exceed the minimum design or performance guidelines established in regulations. The goals of the 17

18

Geosynthetic clay liners

recommended designs are to protect human health and the environment through the isolation of waste from man. This is accomplished through a top cover designed to minimize contact between infiltrating water and waste, thus minimizing the for­ mation of leachate, and the bottom liner is designed to contain any leachate, detect any leakage, and provide for the prompt removal of any liquids. The design engineer must use judgement to fit established regulations and practice to the specific site that the landfill will be constructed. US EPA regulates landfill construction through a system of permitting that involves dialogue between regulators and potential private landfill owners. Further, the waste containment facilities need to perform their intended design function throughout the active life of the facility. The facility should minimize the entrance and exit of any liquids or gases. The facility should not foul groundwater or the atmosphere. US EPA regulations state that landfill owners are responsible for the facility for 30 years after closure. The notion of applying GCLs as a substitute in a lining or cover system requires the landfill owner and engineers to demonstrate that the GCL will provide equivalent or superior performance to that of a compacted clay liner (CCL) in liner or cover systems. As can be seen in the following figures, GCLs are being considered as an 'auxiliary application' to enhance the performance of the engineered cross-section, and also as 'substitute' for compacted clay liners. 2.1 Bottom Lining Systems Bottom lining systems for 'hazardous' waste landfills, consist of a double liner system with at least one liner using a composite geomembrane/compacted natural or amended soil layer. Bottom lining systems for 'municipal' waste landfills are considered somewhat differently. There are two fundamentally different approaches to satisfy the regulation, one being to meet or exceed a minimum design standard, or second, to meet a performance standard (§258.40). These two options are shown conceptually in Figure 1. Designers may choose to meet the design regulation as a minimum, or provide an alternative design intended to minimize pollutant migration. Minimum design requirements are provided, but the performance standard was designed to allow the introduction of new engineering materials into the design as developed in the years after the regulation was promulgated. Landfill owners and engineers have the burden of proving equivalency/superiority of the design over recommended minimums to the US EPA regulators. Potential bottom lining designs incorporating GCLs are shown in Figures 2 and 3. 2.2 Cover Systems Recommended designs have common features between hazardous and municipal waste landfill covers, however, the regulations are much more flexible for municipal waste landfill covers. A cover system for a hazardous waste landfill contains, from the surface down, a vegetation/soil layer (or a natural stone armored top for arid regions) that is a minimum of 60 em of soil graded to a final slope of between 3 and 5%, this is underlain by a 30 em thick drainage layer with a hydraulic conductivity of at least I x I 0 4 rnls, which is underlain by 60 em of compacted or amended natural soil with a hydraulic conductivity of at least 1 x 10-9 rn/s in intimate contact with a 0.5 mm geomembrane liner. A figure showing the recommended cover design is shown in Figure 4, and a possible cover design with a GCL is shown in Figure

5.

US EPA experiences with geosynthetic clay liners

19

COMPOSITE LINER AND LEACHATE

COLLECTION SYSTEM DESIGN

Leachate

Collection

System

"

0

0 0

Flexible Membrane Liner

Compacted Soil

(permeability

!t 1 x 10 •7 cmtsec}

DESIGN THAT MEETS PERFORMANCE STANDARD AND

APPROVED BY AN APPROVED STATE

Approved Design Relevant Point of Compliance

-

I

1

Less than Allowable Constituent Concentration ~

1

--Uppermost Aquifer

..L.

""~

Figure I. Conceptual Design Alternatives for Municipal Solid Waste Landfills per 40 CFR 25k7 & 258 (Federal Register 1991 ). New MSWLF units and Lateral Expansions must have one of the designs shown above.

20

Geosynthetic clay liners

0.6 m Select Fill

Figure 2. Potential Landfill Double Composite Liner Incorporating GCLs (US EPA 1993).

Figure 3. Potential Single Composite Liner Design (US EPA 1993).

US EPA experiences with geosynthetic clay liners

vegeLation/!cil top layer

21

{

---------------------------------­ -----------------­ --------------------------------------------------

filter layer

drainage layer

0.5 mm (20 mil) geomembrane low hydraulic conductivity

geomembrane/soil layec

waste

Figure 4. US EPA Recommended Landfill Cover Design (US EPA 199lb).

vegeLation/SJi I top layer

drainage layer

GCL (S IQ-9 em/sec) _____., fine-lextured soil. compacted { to 95·100% Standard Proctor.

Wet of optimum moisture

T 60cm

~~~ tl_~=iill

mer layer

30cm

-:J=

0.5 mm (20 mil) geomembrane

t30cm ~~~~~~~_L_

conlml

waste

Figure 5. US EPA Recommended LandfiiJ Cover Design (US EPA 199lb).



22

Geosynthetic clay liners

Landfill covers for municipal waste landfills have the requirement of having a hydraulic conductivity less than that of the bottom liner, but not greater that I x 10-7 m/s. Covers present a challenge to construction personnel that bottom liners do not, and that is the process of compaction over land filled municipal waste becomes more complicated. The reason is that the waste often absorbs compactive energy, and doesn't supply the necessary reactive force to achieve compaction leading to satisfactory hydraulic conductivity values. In these situations, GCLs have another advantage over compacted or amended clay. A possible application of GCLs to a cover system is shown in Figures 4 and 5. Also, cover systems at municipallandfilJs should contain a system for the removal of decompositional gases.

2.3 Application of Covers as Remedial Measures In addition to being used on municipal and hazardous waste landfills, covers are conunonly used as a remedial measure on old landfiUs constructed prior to regulation. When used in these situations, there are other regulations that come into play. Guidance is issued under the Superfund Amendments and Reauthorization Act of 1986 (SARA) which adopts and builds upon a provision in the National Contingency Plan (NCP) of 1985 that invokes the concepts of applicable or relevant and appropriate requirements (ARARs). In essence, the landfill site falls jurisdiction to any laws that are more rigorous than those imposed by federal law that are relevant to the site, such as those originating in individual states. The influence of how a GCL material might be used from the technical perspective is no different than when used any other application. The material will need to prove viability when used to cover a specific kind of waste at a site. 3 US EPA's GCL WORKSHOPS Geosynthetic clay liner (GCL) products were first investigated by US EPA in the late 1980's. The products were being considered for use in liner and cover systems for landfills, waste impoundments, site remediation projects, secondary containment structures around tanks, and elsewhere. The materials had certain obvious potential advantage areas, and generated significant interest. Because of a desire to expand the knowledge of the material and address uncertainties about the engineering properties of the material, a workshop was held in Cincinnati, Ohio in 1990. 3.1 Summary of EPA Geosynthetic Clay Liner Workshop 1, June 7-8, 1990 The focus of the workshop was to discuss the content and composition of the materials themselves, and for users of the materials to share experiences about their utility. The workshop first described the guidance associated with the proper design and construction of compacted soils. The discussion included a sununary of materials selection, geotechnical variables such as hydraulic conductivity and density/moisture relationships, construction techniques and compaction, construction protection techniques, and quality controL GCL manufacturers described the bentonite and geotextiles employed in the individual products, reconunended installation technique, engineering properties such as shear strength, hydraulic conductivity of both water and chemical permeants,

US EPA experiences with geosynthetic clay liners 23 Table I. Differences Between GCLs and CCLs (afler US EPA 1993) Property

Geosynlhetic Clay Liner

Compacted Clay Liner

Material

Bemonite Clay Adhesives, Geotextiles and Geomembranes

Native Soi.ls or Soil/Bentonite Blend

Construction

Manufactured in Factory and Installation in Field

Constructed Entirely in Field

Thickness

"'lOmm

0.5 - 1.0 m

Hydraulic Conductivity of Clay

LQ- Il

Speed an Ease of Construction

Rapid. Simple InstaJlation

Slow, Complicated Construction

Water Content at Time of Construction

Essentially Dry; Cannot Desiccate During Construction and Produces No Consol.idation Water

Nearly Saturated; Can Dessiccate and Can Produce Consolidation Water

Cost

$51m2 to $11Im2

Highly Variable (Estimated Range $8/m1 to $32 /m2)

Experience Level

Low

High

to

LQ·IO

mfs (typical)

10" 10 to

w·• mls (typical)

and provided typical candidate sites where GCLs might be used. The ability of the GCL materials to recover from damage via the tremendous swelling potential of granulated bentonitic clays was described in laboratory studies. Composite action between GCLs and geomembranes (GMs) was discussed. Finally, the recommended technique for joining panels of GCLs was described by each manufacturer. There were several points of discussions that took place at the first workshop. The primary focus of the discussion revolved around the notion of technical equiv­ alency. A technical analysis of the functionality of the clay layer in a composite liner ensued, and after much discussion, the various properties and characteristics were pointed out. (This comparison carried over to the next workshop where it was further developed and is shown in this document as Table 1). A list of concerns about the materials and unknowns was generated at the workshop, and included such items as sparse independent data on engineering properties and field performance, chemical resistance of GCLs, effects of settlement, effects of wetting/drying cycles, field seam performance, shear strength and slope stability, and equivalency with CCLs. At the conclusion of the workshop, attendees expressed the following prioritized list of data needs pertaining to GCLs: • shear strength • hydraulic properties • seams • long-term performance The workshop resulted in clear paths for which further study was warranted, and the proceedings were summarize in a compilation document (US EPA 1991 a).

24

Geosynthetic clay liners

3.2 Summary of EPA Geosynthetic Clay Liner Workshop 2, June 9-10, 1992 In 1992, with product interest remaining very high, research results becoming available, and field experiences growing more numerous, another workshop was hosted, to determine if questions raised in the 1990 meeting could be answered, and to further gather user experiences. Like the previous workshop, manufacturers of GCLs described innovations to their product since the last meeting. The manufacturers further investigated a comparison between GCLs and CCLs, and discussed research and application information. The next topic discussed was that of test standard development, and while test standards remained sparse, some advances had been realized. The lack of test methods remains as an issue for continued persistence by all involved. Comparisons between GCLs and CCLs led to discussions of composite action. Since many GCL materials have textiles on both top and bottom, the ability of the GCL and geomembrane to perform as a composite was challenged. It was suggested that composite action was prohibited by the geotextile that will lie between the geomembrane and the bentonite clay layer of the GCL. While more research is necessary, opinion at the time of writing is that the geotextile can be manufactured thinly so that the intimate contact between geomembrane and GCL is preserved. Landfill owner/operators discussed their experiences with GCLs. Applications varied greatly, with the most comfortable situation as a component of the cover system. Owner/operators had many reservations that were shared by other attendees, such as longevity issues, and the general lack of engineering data. Owner/ operators expressed difficulty in gathering regulatory approval of the intention to use GCLs. Ongoing research on GCLs was summarized. The projects focussed on the areas of hydraulic conductivity of the GCL seam overlap, the effect of differential settlement on hydraulic conductivity, the stability of slope covers constructed with GCLs, and the direct shear behavior of GCLs when immersed in liquids other than water. The comparisons between the two materials continued. The functionality of the layer was further analyzed and remains an important topic. The discussion and presentations resulted in an equivalency assessment that is displayed in this document as Table 2. Fundamental performance issues about both CCLs and GCLs remain to be investigated. Long-term performance is the most difficult to measure for any new engineering material, and composite landfill linings are no exception. 3.2.1 Testing Standards Laboratory testing is a critical part of product development, and later for conformance testing. The tools necessary to evaluate a material are generally found in consensus standards-making organizations like American Society for Testing and Materials (ASTM) in the USA. Generally, there is a lag time between the time when a material is developed and when standards are developed to measure properties or performance of the material. While the development of testing standards continues, design engineers are left to determine engineering properties of these new materials by independently modifying previous test standards that may have been intended for soils or textiles. As a result, it is difficult to compare test results generated by the variety of laboratories examining GCLs. lbis is an issue than can benefit from industry and user participation. This second US EPA workshop resulted in a summary report (US EPA 1993).

US EPA experiences with geosynthetic clay liners 25 Table 2. Summary of Equivalency Assessment (adapted from US EPA 1993) Category

Productor SiteSpecific

Relevant to: Liners

Covers

Water Steady Flux

,/

,/

,/

Solute Steady Flux

,/

,/

Criterion for Evaluation

Probably Equivalent

Probably Not Equivalent

Hydraulic Issues

Adsorption Capacity

,/

Water Breakthrough Time

,/

Solute Breakthrough Time Consolidation Water

,/

,/

,/

,/

,/

,/

,/

,/

Freezeffhaw Perfonnance

,/

,/

,/

Wet/Dry Perfonnance

,/

,/

,/

Total Settlement

,/

,/

,/

Differential Settlement

,/

,/

,/

Physical/Mechanical Issues

Slope Stability

,/

,/

,/

Erosion

,/

./

,/

Bearing Capacity

,/

,/

ConstnJction Issues Puncture Resistance

,/

Subgrade Condition

,/

,/

,/

,/

,/

Ease of Placement

,/

,/

,/

Speed of Construction

,/

,/

,/

Availability of Materials

,/

,/

,/

Weather Constrains

,/

,/

,/

Quality Assurance

,/

,/

,/

26

Geosynthetic clay liners

4 RECENT EVENTS The popularity of GCLs continues to grow. Manufacturers continue to refine GCL products with varying geosynthetic materials, GCL design, and attaching techniques. The attempts to characterize the material in a comparative manner with CCLs will continue. Many engineers are eager to apply GCLs in a setting where the GCL would serve as a substitute to all or part of the CCL. Without performance data, these substitutions have proven an uncomfortable situation for regulatory bodies within the USA. In the critical application area of landfills, selection of an inappropriate material is unacceptable, and pieces of the engineering puzzle remain to be fully understood. It is clear that GCLs are a new material that cannot and will not possess all of the engineering properties of traditional CCLs.Perhaps it is best to resist direct comparison, an opt for a better definition for the role of GCL materials, fully taking stock of the strengths and weaknesses of this exciting new product, and carefully applying it in a way that enhances the performance of linings and covers. 5 CONCLUSION GCL materials offer substantial potential benefits, yet as with any new engineering material, some technical data gaps exist. US EPA is interested in the appropriate application of these materials, and is assisting in the study of these materials for use in landfills in the USA and around the world. When testing procedures mature, accompanied with performance and technical data, the materials stand to gain more acceptance in the engineering and regulatory community through better landfill performance. 6 ACKNOWLEDGEMENTS The research efforts of the manufacturers of GCLs, landfill owner/operators, Drexel University, the Geosynthetic Research Institute, The University of Texas at Austin, LandesgewerbeanstaJt Bayem and Umweltbundesamt are sincerely appreciated. REFERENCES Federal Register, Part D, Volume 56, Number 196, October 9, 1991, (codified as 40 CFR Parts 257 & 258) United States EnvironmentaJ Protection Agency. 199Ia. Compilation of Information on Alternative Barriers for Liner and Cover Systems. EPA 60012-911002. (NTIS P89/-14/846). Risk Reduction Engineering Laboratory, Cincinnati, Ohio. United States EnvironmentaJ Protection Agency. 199lb. Seminar Publication- Design and Construction ofRCRA/CERCLA Final Covers, EPA/530/SW-9/1025. Risk Reduction Engineering Laboratory, Office of Research and Development, Washington, DC. United States EnvironmentaJ Protection Agency. 1993. Repon of Workshop on Geosynlhetic Clay Liners. EPA/600/R-931171. Risk Reduction Engineering Laboratory, Cincinnati, Ohio.

RELATED PUBLICATIONS United States EnvironmentaJ Protection Agency. 1985. Draft Minimum Technology Guidance on Double-Liner

US EPA experiences with geosyntheric clay liners 27 Systems for Landfills and Surface lmpoundments-Design, Consauction, and Operation. EPA/530-SW-85-014. Office of Solid Waste, Washington, DC. United States Environmental Protection Agency. 1989. Technical Gujdance Document: Final Covers on Hazardous Waste Land !iUs and Surface Impoundments, EPA/530-SW-86-031. Office ofSolid Waste and Emergency Response, Washington, DC.

Taylor & Francis

Taylor & Francis Group http://taylorandfrancis.com

Fundamentals

Taylor & Francis

Taylor & Francis Group http://taylorandfrancis.com

Characteristics and sealing effect of bentonites F. Madsen & R. Ni.iesch ETH Zurich, Zurich, Switzerland

ABSTRACT: Bentonites are naturally occurring rocks or soils with a high clay fraction content. Generally, this clay fraction consists mainly of montmorillonite, a clay mineral capable of swelling. Montmorillonite has a large specific surface area, a high cation exchange capacity, excellent plasticity, a high capacity for swelling and water adsorption, as well as low hydraulic conductivity and low apparent diffusion coefficients for cations. Because of these properties, bentonites are used for numerous purposes, for example as foundry clay, in the chemical and pharmaceutical industry, in food processing, for pets (e.g. cat litter), in drilling and underground workings, and last as sealing materials. Often, bentonites result from the transformation of volcanic ashes or rocks. They are found world-wide. There are large deposits of natural sodiumbentonite in Wyoming and Montana, USA. However, compared with calciumbentonites, natural sodiumbentonites are relatively rare. Deposits of calciumbentonites are to be found, for example, in the Slovakian Republic, on the island of Milos in Greece, in Turkey, Iraq, Syria, Italy, France and the south of Germany. Almost half of the world's bentonite production takes place in the USA (Jasmund & Lagaly 1993). As sodiumbentonite is better suited for many applications, calciumbentonites are converted into sodiumbentonites by adding soda. These are then distributed as so-called soda-activated sodiumbentonites.

1 CLAY MINERALS Clay minerals are among the smallest of minerals, with a particle size generally smaller than 2 J.lm. In chemical-physical classification, they are included among the colloids. Mineralogically, clay minerals are classified as silicates. Because of their layered structure, clay minerals are designated phyllosilicates. Ideally, the clay crystals have a flake structure. As a mineralogical rarity, clay minerals are negatively charged. !fle negati~e charge results from the isomorphic substitution of hi§he_r­ grade catiOns, e.g. S1 4+ or Al 3+ for those of a lower valency, e.g. Al 3+ or Mg +, 10 the ideal crystal grid. The negative charge is distributed over the surfaces of the clay crystals. The charge may be regularly distributed in the crystal, i.e. with all elementary sheets in the crystal having the same charge, or it may be irregular. The clays possess different characteristics, depending on the strength of the charge and the way it is distributed. While the surfaces always have a negative charge, the edges of the particles have 31

32

Geosynthetic clay liners

a positive charge in the acid and neutral area, and a negative charge in the basic area. The clay crystals' charge is compensated by ions stored in the clay crystal surfaces. Their habitus, normally flake-shaped, combined with the small particle size, results in clay minerals having very large specific surfaces. In extreme cases, they may achieve sizes of up to 750 - 800 m 2 /g. Depending on structure, charge and composition, clay minerals are divided into groups, sub-groups and species. Table I gives the international names of clay minerals. Table I. Structure and Type of Clay Minerals (from Brindley & Brown 1984) Layer type Group (x = change per formula unitY

Sub-group

Species'

1:1

Serpentines

Chrysotile, antigorite. lizardite, amesite Kaolinite, dickite, nacrite

Serpentine-kaolin (x = 0)

Kaolins 2: 1

Talc-pyrophyllite (lt = 0) Smectite (lt = 0.2-0.6)

Tales Pyrohyllites Saponites Montmorillonites

Vermicu Iite (x = 0.6-0.9) Mica (lt = 1.0) Brittle mica (lt = 2.0) Chlorite (lt variable)

Trioctahedral vermiculites Dioctahedral vermiculites Trioctahedral micas Dioctahedral micas Trioctahedral brittle micas Dioctahedral brittle micas Trioctahedral chlorites Dioctahedral chlorites Di,triotahedral chlorites

Talc, willemseite Pyrophyllite Saponite, hectorite, sauconite Montmorillonite, beidellite, nontronite Trioctahedral vermicul.ite Dioctahedral vermiculite Phlogopite, biotite, lepidolite Muscovite, paragonite Clintonite, anandite Margarite Clinochlore, chamosite, nimite Donbossite Cookeite, sudoite Sepiolite. loughlinite Palygorskite

SepioliteSepiolites 2:1 invened palygorskite Palygorskites (lt variable) ribbons • Only a few exemples are given

· x refers to an 0 10(0H), formula unit for smectite, vermiculite, mica and brittle mica.

2 MONTMORILLONITE We now look at the general structure of the clay mineral, montmoril1onite. The description is intended to help explain terms used later. As with all clay minerals, the components are: cations: Si4 + and Ae+ anions: 0 2. and OH· In addition, cations such as Mg2+, Fe2+, Mn 2+ are included in the structure by isomorphic replacement. As the following illustration from Mitchell (1993) shows, the ions are distributed in planes and ordered in tetrahedrons of Si4+ and 0 2-, as well as octahedrons of Ae+, 0 2. and oH·. The tetrahedrons are connected together in planes by common 0 2'ions, with the comers of the tetrahedrons pointing in one direction. The octahedrons are also built together into planes. The Si tetrahedrons and AI octahedrons in Figure I form overlapping tetrahedron

Characteristics and sealing effect ofbentonites

33

planes (T-planes) and octahedron planes (0-planes), consisting of rings of six. In the case of montmorillonite, two T -planes and one 0-plane are superimposed to fonn a sheet, also known as an elementary sheet (Figure 2). The oxygen ions of the tetrahedron plane simultaneously belong to the octahedron plane. A number of sheets combine to fonn a layer or one clay crystal .

Figure l. View from diagonaUy above, showing the surface of a T-0-T clay mineral structure.

Figure 2. The edge of a sheet, comprising two T-planes and one 0-plane.

34

Geosynthetic clay liners

The icon for the tetrahedron layer is:

'\:..___...J7

The icon for the octahedron layer is: Montmorillonite has one sheet of two T-planes and one 0-plane. The diagram below shows the charge of the unit cell. The unit cell is the smallest unit, from which one montmorillonite crystal is composed.

Charge

6

o2 ·

12

4 Si 4 + 4

o2 ·. 2

16+ OH.

10 I I I/] +

Instead of 4 AI] +

2 4 0 ".20 11

10

4 4 S1 +

1 6~

o o2 ·

12

Total charge

2/3 ­

===================== The chemical formula for montmorillonite is generally given for the half unit cell and is as follows (including exchangeable cations):

(I) Montmorillonite's negative charge is less than that of illite, for example, a clay mineral incapable of swelling, similar to mica. Moreover, the isomorphic substitution mainly takes place in the octahedron plane and not, as in the case of illite, in the tetrahedron plane. Montmorillonite's negative charge is slightly shielded by the tetrahedron planes and therefore has less outward effect. The negative charge is compensated by cations such as Na+, Ca2 + and Mg2+, which are present both in the crystal's interlayer spaces (Figure 3), and in its external surfaces. Because the negative charge is relatively weak, these cations are not strongly bonded to the clay crystal surfaces. On the contrary, these cations can easily be exchanged, and the cation exchange capacity of the clay mineral corresponds largely

Characteristics and sealing effect ofbentonites

35

Figure 3. Montmorillonite partjcles made up of two T-0-T sheets with e;o;changeable cations between them.

to the negative charge of the clay crystal surfaces. A small exchange also takes place at the edges of the clay minerals. Because of the relatively weak charge, the attractive force of the sheets is also weak. The cations between the sheets are (depending on humidity) differently hydrated, i.e., surrounded by several water molecules. This means that the distance between the sheets varies according to water vapour tension. The clay minerals do not lose their adsorbed water until dried in an oven at 105°C. The montmorillonite crystal consists of several T-0-T sheets (generally between 5 and 20). The distance between the sheets varies (Figure 4), and in the case of sodiummontmorillonite may become so large in suspension that the individual sheets separate completely. Extensive use is made of this property, e.g., in thixotropic drilling muds. However, in the case of calciummontmorillonite, the distance between the sheets is never greater than about four water molecule layers, i.e., approximately 1 nm. Accordingly, the swelling capacity of sodiummontmorillonite is much greater than that of calcium­ montmorillonite. As montmorillonite can store water between its sheets, the clay material is said to be capable of swellinr. Montmorillonite has a very large specific surface accessible to water (750- 800 m /g). The surface between the sheets is called the internal surface, in contrast to the external surface of the crystals.

36

Geosynthetic clay liners

•••

•••

Na-Montmorillonit

Ca-Montmorillonit

§

• • •• • § • •• ....-•• •

§

~

~

Austauschbare Kationen und WasS~er

____.

§

••

~

••• •••

§ •• §

§

•• •

Figure 4. Swollen Na- and Ca-montmorillonite with adsorbed, ell:changeable cations.

3 MINERALOGICAL COMPOSITION OF TYPICAL BENTONITES Montmorillonite has a relatively large cation exchange capacity, normally in the range of 100 to ISO meq/1 OOg. This means that I kg of montmorillonite can adsorb 1 to 1.5 equivalent ions, e.g., between 23 and 35 g Na+ ions for other ions, e.g., heavy metal ions. 1 equivalent Na+ ions are = 6.02 x 1023 ions, see Table 2.

Table 2. Base Exchange Capacities of Bentonites (Data from Madsen & Kahr 1993a) Na-bentonite MX-80" Montmorillonite content [% by weight] Specific surface [m 2/g] Ell:change capacity [meq/IOOg] Wyoming, USA •. Bavaria, FRG

75

560 76

Ca-bentonite Montigel""

66 490 62

Characteristics and sealing effect ofbentonites 37 4 WATER ADSORPTION

Oven-dried clay minerals (at 105°C) have no adsorbed water on their surfaces. The clay minerals immediately take water from the atmosphere in the fonn of water vapour. This water vapour is initially adsorbed around the exchangeable cations, i.e. the ions hydrate. Depending on the relative humidity (water vapour pressure), more or fewer water molecules are adsorbed around the cations. The water adsorption capacity in the vapour phase depends on the specific surface, the charge per unit cell and the type of cations. Montmorillonite has the greatest specific surface of all natural clay minerals and adsorbs, here in the fonn of Ca-bentonite Montigel and Ca-bentonite STx-1 (Texas/USA), as shown in Figure 5, more water at the same water vapour pressure than clay minerals with smaller specific surfaces (here Kaolinite and Illite). In general, not more than four water molecule layers are adsorbed from the vapour phase. The distance between the elementary sheets accordingly represents approximately 1 nm. At a relative humidity of 50 - 60%, the Ca-montmorillonite in Ca-bentonite has incorporated two water molecule layers. In Figure 5, this corresponds to a water content of approximately 15% by weight. A Na-bentonite incorporates one water molecule layer at 50 to 60% humidity. This fact is used, among other purposes, for the identification of Ca- or Na-montmorillonites, or Na-bentonites, by X-ray

gr-------------------------------~

0

Isotherms

•

0

0

11> 0 "' C: N

p/p, Keeling

"'

o

(ij

o

0

~

:J

"'

0

I

0

•O

Oo

STx-1

o

1

0

0

p/p, Orchiston

'

0.1

0.2

0

.

•

I

' '

0.3

...

0.
0.9), Na- and Ca-bentonite adsorb approximately the same quantity of water in the vapour phase, some four water layers. If water in liauid form is presented to an oven-dried clay mineral, the first thing that happens is that the cations hydrate. In the case of calcium montmorillonite, the incorporation of water ceases after approximately four water layers. The double-charged cations move into the middle, between the elementary sheets, and, with their greater charge. hold these together. In the case of Na-montmorillonite, the Na ions distribute themselves throughout the space available, and are not as strongly connected to the clay surfaces as Ca ions. These facts explain the greater swelling capacity and greater water adsorption of Na-bentonites compared with Ca-bentonites. To illustrate bentonite's different behaviour, there is the moisture adsorption capacity, as given by Enslin-Neffs (Demberg 1991) suction test, for an Na- and Ca-bentonite. Figure 6 shows the moisture adsorption capacity of a soda-activated Na-bentonite and a Ca-bentonite.

Table 3. Various Moisture Adsorption Capacities Material

Ca-bentonite [% by weight]

Na-bentonite [% by weight]

Moisture adsorption capacity as given by Enslin-Neff

100-300

400-700

450

-'i. u..

400

~

350

:

300

(,)

c

.2

250

~ 200

----------

0

CIO

.,

~

-

150

~ 100

---Na·Benl.

CIO

0

:I

Ca-Benl

50 0

0

~

~

~

100

1~

1~

1~

Suction test duration (minutes] Figure 6. Moisture adsorption capacity of a soda-activated Na-bentonite and a Ca-bentonite. as in the method used by Enslin-Neff.

Characteristics and sealing effect ofbentonites 39 5 SWELLING BEHAVIOUR The clays' swelling behaviour is closely connected to the water adsorption capacity of the clay minerals. Even if the swelling behaviour in underground construction principally gives rise to problems, it has a positive significance in the form of the so-called 'self-healing effect' of these materials which can swell. An existing swelling potential can, for example, close dry cracks. The better the swelling behaviour, the greater the self-healing effect. In connection with its lower water adsorption capacity, theCa-montmorillonite swells less than the Na-montmori!Jonite, where the osmotic swelling has a greater effect, due to the weaker bonding strength of the cations at the clay mineral surfaces. The double-layer theory (Madsen & Muller-Von moos 1985 and 1989, Madsen & Niiesch 1990) shows that the swelling pressure (P) is principally dependent on the distance of the elementary sheets (2d) from each other and the valency of the exchangeable cations (v): p

=

constant

(2)

d2v2

With the same distance between the elementary sheets, Na-montmorillonite develops four times the swelling pressure of a Ca-montmorillonite in osmotic swelling. The swelling behaviour may be examined using the guide-lines of the International Society for Rock Mechanics (1989). The swelling pressure of low-compacted bentonites was examined by Schuster (1986) (Figure 7). Swelling pressures for Na-bentonite MX-80 of approximately 0.25 MPa were measured at dry densities of up to 1.2 Mg/m 3• In the case of highly densified bentonites (Bucher & Spiegel 1984), swelljng pressures of up to 40 MPa were measured at dry densities of approximately 1.95 Mg/m3 • The swelling pressures are therefore, as equation 2 suggests, extremely dependent on the dry density of the material.

250

~

II

200

a.

~

.:f...

: 150 Cl

•

100

/v

"i

0

50

0

0. 1

0.2

-

~

0.3

~

0.4

/

/

/

v

v

...............

0.5

0.6

0 .7

0.8

0.9

1.0

1. 1

1.2

Dry density (Mgtm3)

Figure 7. Swelling pressure of Na-bentonite MX-80. depending on dry density (from Schuster 1986).

40

Geosynthetic clay liners

6 ADSORPTION BEHAVIOUR Because of the negative charge and the relatively weakly bonded exchangeable cations which compensate for this charge e.g., sodium, calcium or magnesium ions, clay minerals are particularly suited as adsorbers for heavy metals. In general, clay minerals prefer cations of a higher valency e.g., lead and zinc ions, to cations of lower valency, e.g., sodium ions. As Kruse ( 1993) and Pliiss ( 1993) have shown, preference is also given to the adsorption of heavy metals rather than calcium ions. The cations on the clay mineral may first be complexed with a mercaptane bond, in order to improve the adsorruion behaviour towards heavy metal ions. One product of this kind is Sillitonit . According to Kruse (1993), this is Ca-bentonite Montigel, where the Ca ions have been exchanged for S-Trimercaptotriacine, in the form of a sodium salt. Sillitonit is an excellent adsorber of heavy metals. According to Stockmeyer (1993 ), better adsorption behaviour towards organic bonds is achieved if the cations on the clay minerai (in this case montmorillonite) have first been exchanged with organic cations. In most cases, dimethyi-dioctadecyl­ ammonium chloride is exchanged. If, for example, Ca-bentonite montigel is used, a so-called organophile bentonite is thus produced. If all charges are occupied by organic cations, this may no longer be moistened with water. The bentonite has become hydrophobic (water rejecting). Products of this kind, such as Tixosorb ® (half organophile) or Tixogel ®(fully organophile), are particularly suited as adsorbers for organic bonds. Such products could be of great value in landfill barriers where the leachate contains a high proportion of heavy metal ions or organic bonds. 7 PLASTICITY AND SHEAR STRENGTH The plasticity characteristics (DIN 18 I 22) of bentonites depend on the quantity of montmorillonite (representing approximately the proportion J

I

-- ~

1.o f-H++H 111--t--t+H ttlll- ~'"-i-J·t Hlt1---J-t-H;nt

- -- •'>11-Q;

.!! a. u~

ffi~

'

.....· /

Steady state concentration

' - - - --

Diffuse layer

-

­

Stern layer

Clay mineral surface

Concentration of electrolytes

II

Figure 4. Expansion of electric double layer, dependent on concentration of electrolytes (Reuter 1988, Ustrich 1991).

are considerably lower than in the case of inner-crystalline swelling. The 2nd water layer is 100 MN/m 2 , 3rd and 4th water layers are 27 MN/m 2 (Madsen & Mitchell 1989). The swelling volume (free swelling) resulting from the inner-crystalline swelling (Ca-bentonite) is approximately two to four times as much; the swelling volume ofNa-bentonite (inner-crystalline+ osmotic swelling) is approximately 8 to 15 times as much. In the case of swelling under load, the extent of swelling (dhfho) is consid­ erably less, and varies with applied loads in a range of20 to 100 kN/m2 • from approx. 5 to 1% (Ca-bentonite), to approx. 40 to 20% for Na-bentonite (AGK 1988/89). 3 METHODS OF EXAMfNATION 3.1 Mineralogical, chemical and soil physical characterisation of bentonites - methods of examination The relevant properties of bentonite for use in GCLs are described is this section. 3.1.1 Watercontent For the purpose of quality assurance, it is important to determine the water content of bentonites, as, because of their hygroscopic characteristics in a 'dry' state (bentonite powder, bentonite granulate), bentonites generally show water contents of approx. 5 to 15% by weight (DIN 18121: 105°C). When drying bentonites for further laboratory examinations (e.g., water adsorption), they should not be heated above 80°C, as the loss of any hydration water bound in the interlayer may cause irreversible changes in their characteristics (Demberg 1991). With bentonites, the release of captive water generally ceases at temperatures of up to approx. 80°C, as the example of a differential thermo-gravimetry curve of active bentonite shows. The subsequent loss of weight

56

Geosynthetic clay liners Temperature ("C)

so

100

DTG (Differential Thermo-Gravimetry) Weight of sample + !- 30 mg Heat-up rate 1"/min

-o.oooso

---"'E

00

Figure 5. DTG curve for an active bentonjte (Golde 1994).

-

an

lav ,._,

t1adiu•

fillt

(uru

_I

tur.n

HtC..

----~-~r-==~~ - --­ ....

-c----­

-­ -

---1-----­ -

- .::::;)··""'1::::-­ -­ - ­ - -1- ­ -H-1+1---+_...JI-1--1­ -------­ ~- -- - - - - ­ · ---· ' - - - - - ­

--~- 1- --

f--

v

---

- - - - -- --- -- - - - -

I/

0.01

I ,~.

Q_Ql

CJ..OIS

~-

:1-

i - f-HH+H+--I----l---l--1l

o.e

0.1

iO 80 70

00 50

-I-I;>'---··-- -- - ---- · '--1--t-1--+-I+H+--+---Jf-t-

V-/ -1-/--,.. 8 is negative, like the clay mineral surfaces. The now general reciprocal repulsion of the clay mineral crystallites results in a disperse structure with high swelling volume and low permeability. Concentrated mineral bases such as caustic soda dissolve the silicon from the tetrahedron blocks of the clay mineral structure while producing amorphous silicic acid (Madsen & Mitchell 1989, Kohler & Mortiani 1984). This leads to an almost complete loss of argillaceous properties. As a result of the aggregation of the clay minerals, bentonites behave like tine, sandy silts after treatment with concentrated caustic soda (Golde 1994 ). See the above for the likelihood of the occurrence of contact with concen-trated bases. No negative effects on the permeability or long-term stability of bentonites are to be expected with dilute bases however, a reduction of permeability might be expected. 4.1 .2 Organic compounds The large group of organic molecules may generally be divided into electrically neutral (polar or nonpolar) and organic cations and anions. Their interactions with clay minerals may be roughly estimated from their electric charge (anions, cations), the polarity of neutral molecules (dielectric constant), their size and shape. Specific interactions cause exceptions, which often can only be registered empirically.

Neutral polar organic compounds The polar character of these non-ionized molecules allow them to be mixed with water. They include such compounds as acetone, methanol, glycol, etc. At high concentrations of over approx. 70% by weight (Fernandez & Quigley 1985), these polar organic compounds may lead to a concentration of the diffuse layer, because of the lowering of the dielectric constants in the diffuse layer, thereby shrinking the clays, resulting in increased permeability. Nonpolar or slightly polar organic compounds This group of non-polar or slightly polar compounds includes a wide range of solvents and detergents which are only slightly soluble in water, if at all. These substances are characterized by a low dielectric constant (e.g., n-heptane = 1, water= 81) or by a high KOW value (division of a material between nonpolar octanol and polar water). This group also contains the aromatics (benzole, toluole, xylole, ethyl benzole) and the highly volatile hydrocarbons (per, tri, cis, trans, etc). Because of their low solubility in water, their low polarity and the absence of electrostatic interaction, the non-polar, organic molecules hardly interact with the clays (with the exception of organophilic bentonites) see Figure 14. No influence of these compounds on clays in a hydrous solution is anticipated. Examples of the spectacular effects of concentrated solutions of non-polar compounds on clays (shrinking and cracking) have been reported (Madsen & MitcheU

Properties and test methods to assess bentonite used in geosynthetic clay liners

67

Figure 14. Effect of liquids with low dielectric constant on pure clays.

1989). If a non-polar compound comes into contact with a clay that has been saturated with water, it cannot at ftrst enter the saturated pore space of the clays, as the water is the better moistening phase to the clay and cannot simply be displaced. However, a diffusion of these non-polar materials does take place in the pore water and in the hydrated envelope of the clay minerals, which lowers the dielectric constant of the pore solution. This results in a compression of the diffuse layer and a reduction of electrostatic repulsion forces between the clay particles, causing the clay to shrink. However, these effects are only to be feared with pure (concentrated) organic compounds, which are hardly ever found in practice.

Organic ligands of complexes Organic complexing agents (e.g., EDTA in rinsers) can convert the positively charged heavy metal cations into a negative charge, by the creation of stable anionic meta.! complexes, during which they cannot interact with the negatively charged clay mineral surfaces. Because of the largely absent adsorption process, the barrier effect is limited to the unchanged low permeability of the clay. Organic acids Organic acids (e.g., aliphatic mono carboxilic acids such as acetic acid and propionic acid) have a polar character and are readily soluble in water, with the creation of H30+ ions. As well as the effect of the electrolytic content on the diffuse layer, there may be an exchange of cations in the metal interlayer cations for protons from the acidic pore solution. The effects are a reduction in water adsorption capacity and swelling capacity. In contrast to the much stronger mineral acids, however, damage to the mineral grid need not be feared. Reuter ( 1988), in flow-through tests with 5% acetic acid/propionic acid (50 : 50), registered penneabilities greater by the power of ten than that measured with water.

68

Geosynthetic clay liners

Organic cations Possible disadvantageous changes to a bentonite (Ca-montmorillonite) by the intercalation of organic cations may be illustrated by the example of di-ammonium dodecan ((NH3) 1C 11H 24), a by-product of the soap industry (cationic surfactants). The organic cation, with two positively charged ammonium groups at the ends of the chains was included in the intermediate layers of the montmorillonite after a diffusion time of 16 days, with the base interval shrinking from 2 nm to 1.3 nm. After intercalation of the chain-formed molecule, the cohesion of the chain is so great that no water intrudes and the interlayer interval expands no further (Hasenpatt 1988). In the case of technically modified bentonites, use is made of the properties of long-chain, organic molecules for their adsorptive capacity, in contrast to that of neutral polar and non-polar organic molecules. 'Normal' bentonites, i.e. those with Na+ or Ca2+ as counter ions, have only a very limited capacity to adsorb, in contrast to these organic compounds, because of their lack of electro-static bonding processes. The bentonites are made organophilic by ion exchange for organic cations with an asynunetric building principle (e.g., alkyl ammonium, k-tenside) with a positively charged, hydrophile head and a hydrophobe, organophile hydrocarbon chain. Such bentonites display hydrophobic behaviour and, through the adsorption of organic compounds, swell at the organic cations forming in the interlayer space (Stockmeyer 1990), see Figure 15. Because of their hydrophobic behaviour, such modified bentonites are not suited as sealing elements. Depending on the degree of organophilation and boundary conditions, the water permeability is higher than that of normal bentonites by a factor of 10 to 1,000. The group of substances of the organic bases, e.g. cti-methylarnine, generally tends to behave like cations. When dissolved in water, the molecule with dipole character adsorbs H+ ions from the water, with the solution taking on a basic character from the raising of the OR ion concentration. The resulting cation may be adsorbed at the negative surface of the clay minerals; this may result in a change of the interlayer interval (Ustrich 1991). In the case of di-methylarnine, the molecule's 'good fit' (specific adsorption) in the hexagonal indentations of the tetrahedron layers results in a reduction of the layer thickness of the resulting di-methylamine smectite. This reduces water adsorption and swelling (Golde 1994).

Negatively charged clay flake

Counter ion

Hydrate envelope

®=· Alkyl ammonium cation 'NH 3CnH2n+t

-

adsorbed cluster of organic material

Figure 15. Swelling of montmorillonite with water and through the adsorption of organic compounds.

Properties and test methods to assess bentonite used in geosynthetic clay liners 69 5 USE OF GEOSYNTHETIC CLAY LINERS IN CONTACT WITH CONTAMINATED SOIL Often, there are no data available on the materials to be expected in leachate, or their concentrations, when GCLs are used for sealing purposes with leachate, e.g. in the storage of contaminated soil, or in surface or underground construction in areas of contamination. Often, conclusions as to the concentrations to be expected in the soil permeant or in the leachate must be drawn from figures of total content of pollutants. As solution equilibrium is dependent on material and environment, and because of the interaction with other materials contained (ion competition, dissolving intermediary, complexation processes), it is extremely difficult to make any statements on the concentrations to be expected in leachate. By examining samples of extracted water with a high ratio of solid matter to solution ( l : 5 to l : 2), a general inference may be drawn on the dissolved compounds to be expected and their concentrations. However, the concentrations and concentration behaviour in the soil solution are not necessarily identical with the concentrations in leachate in the soil. The concentration of soluble ions and compounds in leachate depends on: • The solubility of the material in water. • The concentration of the material in the soil. • The chemical environment of the soil (pH, Eh index). • Pressure and temperature conditions. • The retentive capacity of the soil. • The mineral content, organic material, carbonate content of the soil. • The permeability of the soil. • The pH index of the liquid. The variety of possible pollutants, and also natural ground salts may be roughly divided into the following groups for their solubility behaviour: • Salts with alkaline and earth alkaline cations (e.g., Ca, K, NH4 ). • Heavy metals (e.g., Pb, Zn, Cd, etc). • Anions (e.g., chloride, sulphate, nitrate, phosphate). • Organic compounds. • Neutral nonpolar (e.g., BTXE aromatics, highly volatile hydrocarbons). • Neutral polar (e.g., glycerine, glycol). • Organic cations/anions (e.g., tensides). The alkaline and earth alkaline cations and the appropriate anions in a wide pH spectrum are easily washed out. However, heavy metals with index of pH between 6 and 10 have little mobility and are therefore not to be expected in high concentrations. The soil index of pH and carbonate content are important. Neutral non-polar organic compounds are only slightly soluble in water and therefore also not to be expected in higher concentrations (the dissolved concentrations have, however, a great potential for mobility. The content of organic material is important. Neutral polar compounds are easily soluble in water. Organic cations have a high potential for accumulation and therefore a low potential for mobility. Similarly to the inorganic anions, anionic organic compounds (e.g., anionic tensides) ought to be very mobile. Organic complex builders (e.g., EDTA) and other dissolving intermediaries may cause considerable damage to this simple plan (e.g., by building anionic heavy metal complexes). Even for generally valid concentration areas for the natural content of alkalines and earth alkalines in the soil solution or in leachate, only a wide spectrum is possible.

70

Geosynthetic clay liners

The Ca2+ and K+ concentrations in the soil relevant for interaction with bentonites (ion exchange) may be between I to 600 mg/1 for calcium and 1 to 60 mg/1 for potassium. Generally we may say that only ions or compounds dissolved in water may interact with bentonite.

6 SUMMARY There will continue to be controversy on the question of the long-term stability of clay minerals and clays when clays are used in contact with chemical compounds of natural or man-made origin. When discussing stability in the context of clays, the writer believes it necessary to make a basic distinction between stability against processes attacking the structure (e.g., attacks on the clay mineral grid by highly-

(lis)

·1·10" 6

·1·10" 10 -1·10"

9

-1·10-B -1·10"

7

----

~

-------

/

/

I 05

~-

·;:

ttme (hi 10 1000 1 100 1---------+-------+------=-::->""'-~/-f---------1

..--· /

1---------:-==~~"""""""'"'=---------+-------+--------1

-1·10" 6

Figure 3. Development of permeability wiLh time after drying out of test specimens.

Comparing Figures 2 and 3, however, the dried-out test specimens demonstrated a longer phase of water adsorption and therefore saturation, which may be traced to the less efficient surface of the bentonite crumbs, compared to the dust-like bentonite. The test specimens examined were dried three times in all. There were no differences in the subsequent determination of the saturation and permeability behaviour, even with the number of drying-out processes. The permittivities determined after stationary flow conditions had been achieved were, as in the tests described in the previous section, 2.0 to 2.5 X 10"9 s·'. Some even had a tendency to decrease. As only one side of the GCL installed in the field test is acted upon by rain, in contrast to the laboratory tests, where water adsorption is possible on both sides, the process of water adsorption and swelling takes longer, which is accompanied by a reduction in water flux. There were flows of short duration, immediately after the start of the test, and before the phase of water saturation, with some dried-out test specimens and with brand-new GCL test specimens. This phenomenon cannot be attributed to boundary flow, as tests with coloured liquids showed. Further examination is required to show if this is due to a technical problem during testing, or is a phenomenon specific to the product. This phenomenon is, however, of minor significance for the practical use of GCLs in construction work, as bentonite always finds natural moisture in the soil for swelling, and therefore cannot dry out completely, as simulated in the tests. Moreover, such a pressure head as that of the laboratory tests is unlikely to develop suddenly, but would probably develop in the course of a period of time.

106

Geosynthetic clay liners

S EFFECT OF FREEZEffHAW CYCLES Prior to the freeze/thaw cycle tests, two test specimens from the GCL, which had previously swollen for three days under water, were frozen to -S°C and then thawed out, to enable an initial visual examination of the structure of the test specimens in a frozen and thawed-out state. This showed that the bentonite released water under the effect of freezing; this was associated with the formation of ice crystals. The bentonite layer received a lamina to fan-shaped structure, which is not completely reversible in the thaw period, suggesting that the bentonite does not completely re-adsorb the water released in the thawing process. For further tests, a two-chamber freezing cabinet was used, in which a total of eight test specimens with a diameter of 16 em could be installed. The test installation for the examination of the test specimen is shown in Figure 4. The test specimens were subjected to a total of IS freeze/thaw cycles, with a loading of S.S kN!m\ and with half of the test specimens receiving moisture from below. The temperature in the lower chamber was a constant +S 0 C. In the upper chamber, a daily cycle of -S°C for the frost period and +20°C for the thaw period was set. The settlements and heavings were read daily. Immediately on completion of the freeze/thaw cycle, test specimens of I 0 em diameter were punched out of the test samples, in a frozen state, for the permeability tests. The bentonite layer of the test specimens, which had been allowed to swell for three days before the freeze/thaw cycles, now showed a crumbly structure, with dry crumbs. The bentonite crumbs of the test specimens with moisture supply showed slightly more plastic qualities. It was not possible to determine any basic difference in the structure of the test specimens without moisture supply. The settlement measurements basically confirmed the visual examination of the structure of the bentonite layer and the results of the associated preliminary tests. The crystalline water produced in the freezing period was not completely re-adsorbed by the bentonite particles in the thaw period, which led to settlement of the test

Nonwoven boundary loading devise 1.s'and 2nd faa mIa ye

Compacted crushed gravel------' Figure 4. Test installation for freeze/thaw cycling.

Measuring devise Geosynthetic clay I iner ( GCL) Fi Iter stones

plate

Basic examination on the efficiency ofGCLs

I07

specimens of approx. 3 to 5 mm, related to an installed height of I 0 mm. In the permeability tests, it was established that water flowed through the test specimens immediately. There was no initial phase of water adsorption, as with the tests on the dried-out test specimens. These initial permeabilities immediately following the installation of the test Specimens were 5 X lO-S tO 5 X 10-9 m/s. After further flow through the test specimens, it was possible to determine a reduction of permeability in all cases. The maximum coefficient of permeability after a minimum of seven days' flow was k =5.6 x 10" 10 m/s. No significant difference in the settlement and permeability behaviour of the test specimens could be observed in these tests, whether with or without moisture feed. In the meantime, further examination of the influence of freeze/thaw cycles on the saturation and permeability behaviour of GCLs has been carried out. The test specimens were installed as used in practice, being saturated and permeated, as in the case of the permeability tests carried out, until steady-state flow conditions had been achieved. Subsequently, the test specimens were frozen and thawed. In contrast to the tests described above, the water level in the plastic ring was set at the level of the bottom filter stone (Figure 4), so that the water had a capillary connection to the test specimens. After the frost and thaw phase had been carried out, the saturation and permeability behaviour of the test specimens was again determined. The whole procedure was repeated five times in all. In the permeability tests after the freeze/thaw phases, the test specimens first consolidate by releasing water which was adsorbed in the freeze/thaw phase. The coefficients of permeability determined values in this case showed no difference from the testS with non-frozen test specimens ranging from 1.0 tO 2.5 X 10-ll m/s. The same results were achieved in further test series, to study the effect of stopping the capillary connection of the test specimens with water, the prior treatment of the samples (saturation in the permeability cell or pre-swelling under slight loading) and the degree of frost (-5° to -15°C). A comparison of the coefficient of permeability of thawed-out and frozen test specimens was also made. These results, clearly different from the first series of tests, may only be attributed to specific product developments, e.g., an improvement of the composite material. The only test parameter still needing clarification is the effect of the number of freeze/thaw cycles. Initial examinations show, however, that the freeze/thaw cycle places demands on the new development of such products, which will have to be closely examined. 6 INFLUENCE OF LOADING Use must be made of the compression permeability device or oedometer (viz DIN 18130 KD-ES-ST-SB) to examine any dependence of the permeability on the load. As was known from previous comparative examinations (Heyer 1991) smaJier coefficients of permeability were determined with the compression permeability device, compared with the triaxial cell, given the same test conditions, as long as there was no boundary flow. When using the compression permeability device to determine the permeability of fine-grained soils, there must be a minimum load of around 50 kN/m 2 in order to avoid boundary flow. The test specimens were installed in the compression permeability device in a dry state and the load of 5, 10 or 50 kN/m 2 was immediately applied. Subsequently, deaired tap water flowed through the test specimens, with a decreasing pressure

108

Geosynthetic clay liners

head from bottom to top. The pressure head difference fell from approx. 30 em to approx. 10 em. The test results are given in Table l. Even with low loads of 5 and 10 kN/m2 , there were coefficients of permeability comparable in size with those for a load of 50 kN/m2 , so that these tests showed no grounds for deriving dependence of permeability from load. It was evident from the settlement and heaving measurements that with a load of 50 kN/m2, the swelling process after the addition of water somewhat compensated for settlement of the dry GCL, so that the installed height of the dry mat is approximately equal to the height of the water-saturated mat on removal after the tests. However, with lesser loads, representing approximately a gravel or sand fill of25 to 50 em, the swell heaving, which is in the range of 15 to 20%, predominates. The final thickness of the saturated seal with even surface pressure of this kind should therefore never be greater than 9 mm.

Table I. Results of permeability tests with different loads Test specimen No. 25047n 25047/5 2504711

Initial thickness (mm] 7.0 7. 1 6.6

Load

Initial settlement

Swelling

'V

[kN/m2 )

(nun)

(nun]

(s"']

5 10 50

0. 1 0.9

1.4 1.3 0.6

1.4 2.4 1.5

k [m/s] X

X X

9

10· 10·• 10·•

1.2 2.0 9.5

X X X

10" 11

10·"

10·"

7 INFLUENCE OF HYDRAULIC GRADIENT The triaxial permeability apparatus was again used to clarify any possible dependence of the coefficient of permeability on hydraulic gradients. The test specimens were installed in a dry condition and, as described previously, the saturation and permeability behaviour was observed at a pressure head difference of 25 em. The pressure head differences were then increased in stages to 50, I 00 and 200 em. On achievement of stationary flow at these stages, the permittivity was determined. The results are shown in Table 2. The test results clearly show that the coefficient of permeability is not dependent on pressure head, and therefore on the hydraulic gradient. All the figures for \jl are 2 x 10"9 s·', and therefore in the same range as in the other tests. The multiplication of flow speed resulting from the increase of the pressure head may, presumably,lead to particles of bentonite being washed through the geotextile supporting layer. As very fine-grain synthetic filter stones had generally been used in the above-mentioned tests, which would tend to work against such a process, or stabilize it, the synthetic filter stones at the exit side of the test specimen were replaced with coarse sand in two test installations (test specimens no. 25047/1 and 25047/7). The results, however, showed no difference from the other tests. Nor was it possible to see any bentonite scouring or any embedded bentonite in the supporting geotextile at the top of the test specimen. If, in other tests, the GCL is able to swell up without being subject to load, or is practically dry and then, in these cases, is hydrated suddenly with a large pressure

Basic examination _on the efficiency ofGCLs

109

head, this may well lead to different results. However, the procedure chosen for the tests, of saturating the dry test specimen at a low pressure head, with subsequent increase of pressure, should be reasonably near to the conditions prevailing in practice, when used for hydraulic engineering. Tests carried out with pressure heads of up to lO m have, in the meantime, shown no changes in permeability behaviour.

Table 2. Permittivities in s·' determined for different pressure heads Pressure head difference 611 (em]

25 50 100 200

Test specimen No.

25047/2 2.3 2 .3 2.3 2.4

w· w·• x w·• x w·• x x

9

25047/13

25047/15

25047/1

w·• w-• w-

2 .3 X 10-9 2.3 x w·•

3.3 X J0-9 9 3.2 x 3. 1 x 3.0 x w-•

2.2 2.3 2.4 2.4

,..

x x

9

"' 10-9

2.3 x 2.4 x

ww-• 9

w· w-•

25047n

w-• w-• 1.1 x w­ 1.6 x

u

x

9

2.2 x

w·•

8 TESTS WITH HYDROCARBONS AND SALINE DEFROSTING SOLUTIONS The permeability of GCLs, when acted upon by saline deicing solutions and hydro­ carbons, is of particular importance when GCLs are used to protect groundwater in areas of traffic. As confirmed by examinations of another product (Daniel et al. 1993), the permeability of GCLs to hydrocarbons is dependent, to a decisive degree, on the water content or degree of swelling of the bentonite. If the liner is not dried out, showing water contents of approx. 100 to 150%, the permittivities determined in our own examinations, of I to 4 x 10·9 s·', are in the same range as in the other permeability examinations. In contrast, permeability values (with water content of less than 50%) increase by four orders of magnitude, according to Daniel et al. ( 1993). Our examinations tend to show the same results, using Na- and Ca chloride solutions with concentrations of 5 and 50 mg/1. If the GCLs were subject to direct contact with the saline solution, without being able to adsorb water beforehand, perntinivities in the range of 1 X 10'7 to 3 X 10'6 s· l resulted. In the long-term tests carried out with NaCI, where saturation with deaired tap water had been carried out before exposure to the saline solution took place, there was at first only slight divergencies from the previously established 'I' of 2 to 3 x 10·9 s· 1, up to a period of I ,000 hours. However, in further tests of up to 5,000 hours, values fluctuated considerably, reaching up to 6 x 10·8 s·' . The perntittivities plotted over this period had a charac­ teristic 'fever-chart' appearance. This is not observed with standard permeability tests carried out with back pressure, suggesting that ion exchange occurred in the test specimens. 9 SELECTION OF FILL MATERIAL The following information, based on experience of the maximum grain permissible for fill material, depending on non-uniformity, may be taken from the manufacturers'

instructions for installation.

Geosynthetic clay liners

110

For soil with Cu > 15 : dmax = 63 nun For soil with Cu < 5 : dmax = 16 mm From both the economic and technical point of view, and because of dynamic strain caused by traffic, these criteria are no longer applicable in many cases, so that individual examinations are necessary. For example, this is the case if larger grains are to be sieved out of the soil, or if an adequate water permeability must be ensured for the fill material in hydraulic engineering, which can only be achieved with a comparatively narrowly-graded, coarse-grained material. In the course of construction of the A96 motorway (Schmidt & Heyer 1993), a test site was laid out which was subjected to earthwork operations for a period of seven weeks. Three different soil types were used as fill material; their grain size curves are shown in Figure 5. A protective geotextile with a surface mass of 600 g/m2 was additionally laid on the GCL, to half of the extent in each of the three areas. The depth of fill in all areas was evenly decreased, from approx. 85 em to approx. 40 em. After the test site had been exposed, large-scale samples were taken. The permittivity of these was established, with respect to permeability properties. Any loss of mass of the bentonite and any percentage of the surface found defective was identified. 'Defective' was defined as meaning any area where the geotextile supporting layer had been pressed, with the result that there was no more bentonite present between the two layers. This was easy to determine because of the white coloration in the area. This assessment was chosen, after it had been seen from previous permeability tests that there was a significant rise in the permeability of the GCLs when the two geotextile layers came into direct contact with each other. The loss of bentonite presumably resulted from the cyclic and dynamic load resulting from the construction site traffic. Currently, large-scale laboratory tests are being conducted to simulate the in situ load, so that more exact parametric variations may be determined. Although sufficient

Grain - size -curve Sedim•nhtion Cl•y 100

Silt

-d;w,

fin•

co.r~•

,.

Sie v•

,..

S•nd

,.,.d....'"

t~n•fysis Gr~vcl

lin•

c.o.rJ•

'"'"

.

~n•div~r~

co~ r ••

,.

I I

••

I

70

I

TO

J lj~

L

'I ]

.s

JO

~ 10 ,!! •

10

0

100

-

qoot

--- -

60

lO

•o I

~ P'~

JO

/

:::-.:..... '­

10 I

~

qooz

D,OOCI

O,Ol

qoa

O,Z

O. fJ

1,0

10

6,J -

Figure 5. Grain size curves of lbe lbree fill materials examined.

df mm)

6J

100

0

Basic examination on the effteiency ofGCL.s

111

results are not yet available, some important conclusions for practical use may be drawn from the examinations and observations at the test site. GCLs are not only to be installed in a dry state; it should also be ensured that the fill material is compressed, even if the liner has not yet hydrated. At the present state of knowledge, with further, purely static load, the above-mentioned grain criteria for the fill material may be used. With the installation of a protective geotextile, an additional buffer effect may be achieved. Should there also be dynamic load from construction site traffic, in particular in the case of intensive earthwork engineering, an extra protective geotextile will be disadvantageous, as its open-pore structure considerably reinforces the bentonite's loss of mass. The minimum covering depth of the GCL, of 70 to 80 em under such load conditions, is to be observed.

10 SUMMARY Examinations carried out in recent years confirm the basic suitability of GCLs for various applications in earthwork. road construction, hydraulic engineering and landfiiJ construction. This does not, however, exempt authorizing bodies from their responsibility to test the suitability of this new sealing element for individual cases, and, if necessary, to have additional examinations carried out. REFERENCES Bartels. K ..Scheu, C.& JohannBen, K. 1988. Ein neues Dichtungssystem in Sandwichbauwe ise aus Bentorut und mechanisch verfestiglen Vliesstoffen. / . Kongreft Kunststoffe in der Geotechnik. Hamburg /988. S. 193-202, Eigenverlag Deutsche Gese/lschaft for Erd- und Grundbau e. V., Essen. Daniel. D. E., Sban, H.- Y. & Anderson, J. D. 1993. Effects of Partial Wetting on lhe Performance of the Bentonite Component of a Geosynlhetic Clay Liner. Geosynthetics '93. Van couver, Canada /993. pp. 1483-1496. Floss, R. & Heyer, D. 1989-1994. Priijberichte des Priifamts for Grurulbau, Bode~chanik und Felsmechanik

der TU Miinchen und Gutachten mit der Projekt-Nr. 9926 im Auftrag der NAUE-Fasen echnik GmbH & Co. KG, mit der Proj.-Nr. 7200 im Auftrag der Flughafen Miinchen GmbH, mit der Proj.-Nr. /0373 imAuftrag der Bauleitung Wangen des Aurobahnbetriebsamtes Heidenheim und mit der Proj.-Nr. 10429 im Auftrag der OBAG. (unpublished). Heyer, D. 1991. Die Prtifung der Durchlassigk.eit mineralischer Dichrungsstoffe. 7. Nurnberger Deponieseminar, Geotechnische Probltme beim Bau von Abfalldeponien. Niimberg 1991. Heft 59, pp. 147-164, Eigenverlag Landesgewerbeanslall Bayem. Heyer, D. & Ross. R. 1993. Suitability Tests for a Composite Sealing Mat made of Geotextil.e s and Bentonite. Proceedings Sardinia 93. Founh International Lnndfill Symposium, S. Margherita di Pula. Cagliari. Italy. 11.-15. Oct. /993. pp. 389-396 Schmidt, R. & Heyer, D. 1993. Grundwasserschutz mil Bentonitdichtungsmanen bei SrraBen in Wassergewinnungsgebieten am Beispiel der A % in Baden-Wi.irttemberg. 3. Jnfonnarions- und Vomagsveranstaltung iiber 'Kunststoffe in der Gtotechnik', Miinchen. 15.-16.03.93. Geo!echnik. Sonderheft 1993, pp. 35-39

Taylor & Francis

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Guidelines on the use of liners in highway construction H. Rathmayer VJT Communities and Infrastructure. Espoo, Finland

ABSTRACT: The Finnish National Road Administration, in cooperation with the Technical Research Centre- VTT and the National Board of Water and Environment, has drawn up new guidelines for groundwater protection in the area of highway projects. The slope seal is intended to function as a construction measure in the protective area of water supply facilities or groundwater reservoirs, primarily in the case of accidents, e.g., with oil or chemical transporters. The construction measures stipulated for slope areas are intended to keep pollutants from penetrating the groundwater area for at least 12 hours, a period which must suffice for emergency services to take the requisite corrective measures. The use of deicing salt in winter also means a significant potential danger for groundwater contamination. Sodium and calcium chloride, because of their greater density in a hydrous solution than water, tend to concentrate in depressions where groundwater is present. In the long term, salt has a contaminating effect and its penetration in the groundwater area is therefore to take priority over accident risk as a planning factor. Because of the depth of frost experienced in Finland, and the flat nature of the country, the cross section of the road is generally carried out as an embankment. The slope inclination varies, depending on the importance of the road, and in the case of new construction, is generally made flat (e.g., 1 :5), for safety reasons and to simplify maintenance work. In planning protective measures, it is necessary to contend with a number of unknown factors. Because of the variety of environmentally-dangerous loads carried on the road, the contaminating medium is unknown in the case of an accident. The season and the weather constantly affect the condition of the slope and the area that the road influences. There are innumerable variations of current conditions, from a solid blanket of snow to saturated, baseless soil layers, or desiccated, cracked earth crust and other possible situations. There are comprehensive investigations on the territorial extension of the area to be protected beside the roadway. One third of the vehicles coming off the roadway continue their path over the area of the facing slope, that is to say, beyond the drainage ditch. The main routes for chemical transporters representing an environmental risk have been statistically registered. However, as there is no rigorous obligation to report, deviations from customary routes are always to be reckoned with, and are also to be expected, particularly in the case of traffic jams. 113

114

Geosynthetic clay liners

1 SHOULDER CONSTRUCTION An examination of existing shoulder seals revealed serious shortcomings. The degree of sealing of the earth material used was generally much too low. Erosion damage was often observed. The earth protection layers showed deep erosion in places and in the subsoil, earth material was often washed into the body of the road from the protective layer, generally crushed rock. In the road sections examined, no geotextiles with a separating function had been used. The slope seals had principally been made of moraine soils or dry crust clay soils. Because of their low permeability, these materials, if appropriately compacted, are well suited for sealing purposes. However, their susceptibility to erosion, which may be compromised by planting vegetation, is a disadvantage, as is their sensitivity to frost and their tendency to shrinkage cracks when dried. In two of the cases examined, a thin plastic geomembrane (PETP), 0.35-0.5 mm thick, was installed in the protective layer at a depth of approximately 0.3 m. Experience with this resulted in two findings. The soil material below the geomem­ brane still had acceptable moisture behaviour and therefore also an appropriate low coefficient of permeability. Above the geomembrane there had been several local slips, especially during periods of thaw. 2 COMPACTED CLAY LINERS: REQUIREMENTS AND EXPERIENCE The permeability of clay liners is influenced by a range of factors. Chemical and weather influences (freeze/thaw cycles) are predominant in the applications discussed here. Each parameter that comes in question may change the coefficient of permeability by at least three orders of magnitude. Added to this is the influence of the deicing salt on the qualities of the protective layer and therefore on the possible degree of pollution of the groundwater. In both tield and laboratory tests, compacted clay liners were analyzed at five selected road slopes. As well as material parameters, the influence of various chemicals (petrol, diesel, deicing salt solution, xylenes and methanol) and of2 to 4 freeze/thaw cycles on the coefficients of permeability were established. In short-term trials with different hydrocarbons, no significant change in the coefficient of permeability values was established. Salt, however, caused a seven-fold rise and freeze/thaw cycles from 2 to 2.5 times the original value. The guidelines established give recommendations for the fine grain content and the necessary degree of compaction for clayey soils and silt moraines. If the fine grain content (d $ 0.074 mm) is greater than 70%, the modified Proctor density has to be greater than 85%. If the fine grain content (d $ 0.074 mm) is less than 70% but greater than 60%, then a modified Proctor density of greater than 90% is necessary. ln laboratory tests to be performed for the selection of suitable materials, a water permeability value of k < 5 x I o-s m/s at a modified Proctor density of 90% and optimal water content is required. In field tests, a water permeability of k < 5 x 10-6 mls is required to fulfill the 12-hour retention requirement. These require-ments take into account any shrinkage cracks and variation factors listed above.

Guidelines on the use ofliners in highway construction

115

3 NEW GUIDELINES FOR GROUNDWATER PROTECTION 3.1 General Groundwater protection is generally to be provided for roads in a groundwater sensitive area, to avoid damage in road tanker accidents with the possible release of toxic substances, and to reduce the risk of contamination caused by the use of large quantities of deicing salt. The guidelines apply to all new road construction and to the reconstruction of existing roads. In the case of existing roads, the special conditions given are to be observed.

The guidelines give the principles of construction for shoulder seal necessary to protect the groundwater. Other protective measures than those given here may also be used, after prior consultation with the responsible authorities (National Board of Water and Environment, Water Works, Road-Building authorities, etc). Points not covered by the guidelines are: • the orientation of roads in the groundwater area • compaction methods for shoulder layers, including related quality control measures • determination of requirements for any subsequent compaction of existing shoulder seals • detennination of requirements for quality control of groundwater

3.2 Requirements Classified groundwater protection areas must be protected in accordance with their area classification from I to Ill (1: groundwater important for drinking water supply. II: groundwater suitable for drinking water supply. ill: other groundwater.) In practice, all gravel and sand areas count as possible groundwater protection areas, even if the strata are overlain with more impermeable layers. Shoulder protection is always to be carried out in category I groundwater protection areas, i.e. groundwater important for drinking water supply. In this case, shoulder protection includes the whole groundwater area. In category II (groundwater suitable for drinking water supply) and category ill (other groundwater) areas, shoulder protection is sometimes used, depending on each individual case. In such areas, slope protection becomes necessary if no category I groundwater is in the vicinity, or if there is often dangerous transport in the area in question. The responsible authority for water and environment decides whether this is necessary. The extent of shoulder protection is in accordance with the protected zones for drinking-water supply. If a protected zone has not been designated, or has not been clearly planned, the extent of shoulder protection depends on each individual case. The basic principles for the extent of protected zones for water supply facilities are given in Appendix 2 of the regulations. Public roads shall not be built in the central area of a water works (0.5 hectares). Shoulder protection is not necessary if the subsoil bearing groundwater (e.g., at the edge of a groundwater area) is overlain with impervious strata, which naturally meet the requirements for shoulder protection. A minimum figure of 35% is sufficient for fine-grain content (d < 0.074 mm), e.g., silt sand moraine, if the thickness of the stratum is at least 2m. Dry crust clay with cracks is alone, however, insufficient. In the case of roads with low traffic volumes (less than 1,500 vehicles per day),

116

Geosynthetic clay liners

the shoulder protection may be checked to see if it is sufficient. If it is known that dangerous materials are often transported on such a road, then the road shoulders are to be protected. Occasional transport, e.g., of heating oil, does not generally necessitate any shoulder protection measures. The need for shoulder protection may not be abolished because deicing salt is no longer used, if dangerous materials are transported to industrial facilities on the road. This may increase the risk of black ice causing accidents to road tankers. In the catchment areas of wells that supply Jess than 10 households, a decision is to be taken in each individual case on whether shoulder protective measures are necessary. Roads with traffic volumes of more than 3,000 vehicles per day require the following shoulder protection: • inner protective zone of a groundwater works highest requirements

• outer protective zone strict requirements

• other groundwater area normal protection required

• groundwater areas not normal protection, if

designated as drinking shoulder protection

water supply area is installed

These categories are raised by one level if the soil is very permeable, i.e. when the fine grain content d < 0.074 mm is Jess than 8%. The categories may be reduced by one level if the volume of traffic is less than 3,000 vehicles per day, if practically no dangerous goods are transported, or if less than 250 m 3/day of drinking water are withdrawn. In less important cases, checks are carried out, depending on use, to see whether normal protection is necessary or not. The protective zones of old water works are to be brought into line with the new regulations. In cases of groundwater where there is no water works, the need for shoulder protective measures is to be decided on in each individual case. 3.3 Construction Natural clay sealing materials are generally used for shoulder seals. Geomembranes and other liners are used as additional protective measures. The advantage of geosynthetic liners is their complete sealing effect. Their proneness to damage, e.g., by tearing or perforations, as well as their high price and the difficulty of carrying out repairs are disadvantages. Shoulder seals are sub-divided as follows, depending on their sealing effect (the alternative given on the right is envisaged for cases where space is short or where there is only limited availability of suitable sealing material):

Slope protection to highest requirements: 1 ME: 0.6 M + K: 0.3 m protective layer 0.2 m protective layer 0.7 m special soil seal 0.4 m soil seal and geosynthetic liner The upper part of the shoulder protection is given a GCL or similar seal.

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Slope protection to high requirements: IM: 0.8 ME: 0.3 m protective layer 0.3 m protective layer 0.7 m soil seal 0.5 m special soil seal In this case, the upper slope seal is also sealed with a bituminized geotextile or similar material. 0.6 M + K construction may be used instead of 0.8 ME construction. Normal slope protection: 0.8 M: 0.3 m protective layer 0.5 m soil seal

0.5 M + K: 0.2 m protective layer 0.3 m soil seal with bituminized geotextile or fine construction membrane (0.3 mm) with overlapped joints In this case, the upper seal is also covered with bituminized geotextile or similar material. The bituminized geotextile or thin plastic foil reduces the drying out of the soil seal. The term K means that a geomembrane is used on top of an additional mineral grain seal. The term 0.6 M + K for the slope protection construction means that a geomembrane is used below a mineral grain protective layer on top of a sealing layer. The thickness of the construction is 1.0 m.

Light slope protection: 0.4 M: 0.1 m protective layer 0.3 m soil seal (not clayey) The upper part of the protective layer is covered with a layer of gravel. This type of construction is not used with new projects. The difference in cost, compared to the next better shoulder protection, is minimal, but the efficiency of this construction is much lower. Mode of operation of shoulder protection to high requirements: • In the case of a truck accident, the tyres of the vehicle may penetrate 0.3 m into the upper soil layer, thus not reducing the mode of operation of the underlying soil seal. The sealing layer itself must prevent the penetration of dangerous toxic materials into the subsoil for at least 12 hours, so that the ftre brigade (emergency service) can seal off the dangerous material in the ditch or dispose of the portion of contaminated soil in the protective layer. • With minor damage (hydrocarbon spills or similar pollutants), the shoulder seal layer can absorb these materials and store them until they have evaporated or decomposed. A geomembrane or thick sealing layer prevents dangerous materials from seeping through the seal. • A large proportion of the deicing salt flows away with the spring thaw. Asmall proportion of the deicing salt penetrates the soil and may, in the course of years, seep through the sealing layer in insignificantly small quantities. Normal shoulder protection is, in comparison, less effective. 3.4 Lateral extension of the shoulder seal In general, the width of slope protection may be taken from Figure I. If, as in the

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Geosynthetic clay liners

case of the application shown in Figure l, a vegetative clearing important to the landscape would have to be cut down, narrower shoulder protection may be carried out, as in Figure 2. Surface water and thaw water containing salt are then collected on the sealed inner and outer shoulder. If a road tanker accident results in toxic materials escaping beyond the outer slope, the natural humus layer delays their seeping into the subsoil. The width required for normal and light shoulder protection, as in Figure 2, may also be chosen for other applications. If required, guard rails or safety rails, at least 0.8 m in height, may be installed to prevent road tankers from leaving the road. The upper part of the inner shoulder and the lower part of the outer shoulder may be made flatter, to prevent road tankers from turning over. 3.5 Shoulder protection for existing roads Shoulder protection is to be carried out for existing roads in the following cases: • when the body of the road is to be thoroughly reconstructed • at endangered areas near groundwater works, if the soil layer above the groundwater level is thin and permeable, and if there is regular transport of hazardous goods • as part of protective measures for more comprehensive groundwater protection • if suitable material for shoulder protection is available from nearby construction sites Subsequent installation of thick, wide shoulder seals in existing roads is often difficult. Either there is insufficient space, or the shoulders are too steep and there is obvious risk of the sealing layers slipping, or the excavation and installation of earth would be too expensive with traffic still running, and would in any case be

Figure I. Determination of width of shoulder seal in excavation, ground compensation and embankment sections. The slope protection is extended to the point where the line drawn at 10m distance, with an inclination of I : I, meets the original ground. Surface water must be prevented from flowing onto the unprotected shoulder, even in the case of embankment fill. In the excavation, the shoulder seal must be raised 2 m above the height of the road ditch.

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.. .. ... ·... ­ natural humus layer ~6m

Figure 2. Shoulder protection made narrower. to protect vegetation or for olher reasons. The shoulder protection extends slightly beyond !he outer extent of !he road ditch. or in !he case of an embankment, slightly beyond !he inner ex lent of !he road embankment, so that no water from the shoulder may flow into !he surrounding area. In the culling. !he shoulder prorection is lo be raised 2m above the heigh! of !he road ditch. The natural humus layer of the intact ground of the vegetation acts as a replacement for the shoulder protection.

dangerous to traffic. It is preferable to install shoulder protection in connection with repaving or other reconstruction measures. Where possible, the same sealing system should be used on existing roads as on new construction. The following exceptions may be considered: • The liner of the shoulder protection construction is replaced at the top of the inner shoulder by a bitumen emulsion spraying, if traffic signs, prevent the use of a liner. • If it is very difficult to install a thick shoulder protective layer, the 0.5 M + B or 0.4 M construction may be used instead of the 0.8 ME construction. • The light protective construction (0.4 M) may be used instead of the normal shoulder protection if the available topsoil shows a fine grain content (d < 0.074 mrn) of at least 8%. • The width of the shoulder protection may generally be constructed as in Figure 2. • A thickness of 0.1 to 0.2 m is sufficient for the protective layer in all applications, if there is insufficient space for greater thickness. Because of its low thickness and width, the light shoulder protection in no way guarantees sufficient groundwater protection for all cases involving road tanker accidents. Nor does the light shoulder protection stand up to the strain of road maintenance vehicles, especially if the shoulder is steep. However, the greater part of the dissolved deicing salt can be drawn off with the surface water. 3.6 Drain The surface water drawn off from the shoulder protective layer (soil mineral or geosynthetic liner) is fed to a safe drain via road ditches or drainage pipes. In the

120 Geosynthetic clay liners rule, these are moraine areas, clay areas or rivers, lakes and moors outside the groundwater area to be protected. The water led off can also be allowed to seep away in areas unsuited for the withdrawal of water. Surface water from busy roads is first diverted to an oil separating basin, before being Jed to drainage areas or especially sensitive waters. Sometimes it is also necessary to pump the surface water out of depressions in the groundwater protection area.

Length of the oil separating basin

=

II

Freezing volume volume for ice layer Oil retention volume Traversed volume Sump for chemicals and sludge

Figure 3. Oil settling basin, example of dimensions.

The thickness of the 'freezing volume' is 0.6 - 1.0 m. The oil retention volume (also envisaged for other light liquids) is generally 20m3, sufficient for the quantity involved in a road tanker accident. The sump for sludge (and other heavy media) may be between 10 and 30 m3• At right angles to the direction of flow, the cross-section of the traversed volume (A= width x height) is to be chosen so that the average flow speed (v) is a maximum of 54 metres per hour (A > Q I v). The quantity of flow (Q) is measured in accordance with the 20-year spring discharge. The length of the basin (L) is so designed that the duration (t) in the basin is at least 9 minutes (0.15 hours) (L > t x v). With the Board of Waters' approval, other methods of calculation may be used, e.g., a flow speed of 75 metres per hour and a duration of at least 0.25 hours. The base of the oil settling basin is to be constructed as a shoulder seal to the highest requirements. 4 CHARACTERISTICS OF THE PROTECTIVE LAYER AND SOIL SEAL The task of the protective layer is to protect the soi I seal from outward strain, erosion and drying out. It also serves as a fertile soil for plants. The materials best suited are gravels, sands and silt moraines, all without stone content. The material used for the protective layer may be the same as that for the soil seal, if it meets the required characteristics for both layers. If the surface water has high flow speeds, the base of the road ditch may be lined with stones. The surface of the protective layer must be finished with humus. To prevent drying out, a robust grass topsoil is to be provided, ideally with clover added. In the case

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of steep shoulders, it is advisable to use ready-made lawn or erosion protection matting. In areas with bushes and trees, a 0.3 m layer of humus packing may be installed between the protective layer and the soil seal. The roots of the trees and bushes may not penetrate too deeply into the soil seal, in order not to endanger its effectiveness. For special soil seals, silts, silt moraines or clayey soils are suited, whose proportion of fine grain material (d < 0.074 mm) is greater than 70% and whose density is ;;::. 85% modified Proctor density, or whose proportion of fine grain is greater than 60%, at a density of::::- 90% of the modified Proctor density. This assumes that equal material properties can be ensured, without the inclusion of sand layers or any organic materials which may decay. Other mineral bases, whose water permeability measured in the laboratory have a maximum of 5 x 10·8 m/s at a degree of compaction of 90% of modified Proctor density, are also suited for use as special soil seals. It may be assumed from this that the soil examined on the construction site, given careful compaction, will achieve a water permeability of k < 5 x 10-6 m/s, which meets the requirements. The differences in the coefficient of water permeability are to be explained by the fact that both the sealing effect and the uniformity of grain size distribution in nature differ from those in small-scale laboratory sampling. On a slope, there are additional cracks and disturbances as a result of erosion, frost and drying out, as well as from variations in grain size. Apart from the above-mentioned mineral materials, other silt moraine soils (with fine grain content d > 50%) are also suitable for normal shoulder seals. The necessary compaction figures may also be smaller. 4.1 Sample cross-sections - slope protection with mineral seal, GCLs and geotextiles

Construction 1 ME, 0.8 ME, I M and 0.8 M: • It is recommended that GCLs (t ::::- 8 mm) or similar materials are used for shoulder protection to the highest requirements (nigh traffic volumes, important groundwater protection area). For other categories of shoulder protection, it is recommended that a needled geotextile, of geotextile category 2, with rough surface is used, and that this is sprayed with 1.3 kg/m 2 bitumen emulsion (BE-0) or sprayed hot. • In the cases of construction 1 ME and 0.8 ME, it is recommended to install a special soil seal with fine-grain content::::- 70% and a degree of compaction of ;;::. 85% modified Proctor density, or alternatively, a soil with fine-grain content;;::. 60% and degree of compaction::::- 90%. The corresponding nominal thicknesses h2 are 0.5 m or 0.7 m. The lateral covering layer at the upper part of the inner shoulder may be less than this nominal thickness, but not in an area nearer than 0.5 m, measured from the base of the road ditch. Other silt moraine soils may be used for construction I M and 0.8 M . • Protective layer of silt, sand or gravel moraine soil, resistant to load and erosion by surface water and giving nourish.ment to plants. The nominal thickness is in general 0.3 m, but may be less if the thickness of the layer of the soil seal is correspondingly larger. The upper part of the layer is generally humus or earth, if the whole protective layer does not consist of it. • Broken stone material meeting the requirements of the wearing layer of unsurfaced (gravel) roads.

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Geosynthetic clay liners

• Grass upper soil with good moisture retention capacity (category 2 grassy soil and clover). • The construction is generally to be thicker on frost-proof subsoil, and if required, to be supplied with drainage. • Geotextiles are to be installed as a separating layer below the seal base, if the subsoil is of coarse rock or such gravel or broken material whose grain content$ 2 mm is< 50%. • The surface of the rubble layer is secured and levelled with finer crushed stone material. • The uprights for the guard-rails are driven into the ground without any excavation. Before doing so, the upper surface of the GCL or the bitumenised geotextile is exposed. The hole made by driving in the upright is sealed with bentonite powder or bitumen. • The width of the shoulder protection is determined as in Figures I and 2. The term I M of the slope protection layer refers to the total thickness of the protective layer and the soil seal. 5 CHARACTERISTICS OF GCLS AND GEOTEXTILES IN ROAD SHOULDER PROTECTION For shoulder protection to high requirements, the roadway slope or roadway demarca­ tion is made with-GCLs (Figure 4). Approximately 8- 10 mm of activated bentonite clay material is installed between two geotextiles. The bentonite swells to three times its original volume when it makes contact with water. Bentonite liners must be installed when dry. Overlaps 300 mm wide are made, with bentonite powder spread in the overlap (1 kg/m). Bentonite liners can resist minor expansion without the seal tearing. Any damage caused to the liner is repaired by the 'self-healing effect' of the bentonite powder. Other expandable synthetic materials which withstand contact with coarser stone material, resist chemicals and are easy to repair, may be used for the other slope protection categories, instead of GCLs. There are, for example, pre-fabricated bituminized geotextile liners or geotextile liners which are bituminized on site. A quantity of 1.3 kg/m2 of bitumen emulsion BE-0 or warm (140- 150°C) sprayed bitumen injected into the raw, needled surface structure of the category 2 geotextile. A width of 2.5 mat the inner slope is recommended for the slope seal, or 1.9 m for an inner slope gradient of 1 : 3 or 1 : 5. The seal is installed in such a way that it extends 0.5 m below the road surface. Coarse-grain, crushed stone may be installed between liner and road surface in a thickness of 0 - 0.25 m. The underside of the liner is to be leve!Ied, so that no stones can be pressed into the liner. GCLs are to be watered several times before the roadway surface is installed. Geomembranes may be used if the soil seal is at least 0.5 m thick. In general, it is recommended that the soil sealing layer be made thicker, so that road fixtures are not installed through the geomembrane. The recommended thickness of soil seal is 1 m. Guard rails are installed to a depth of I m, lighting to 2m and drainage shafts to approx. 2-3 m. The minimum thickness of the geomembranes is 1.0 mm. A greater thickness may be necessary to ensure that a tension-proof overlapping seam can be made. The most suitable material for the geomembrane is high density polyethylene (HDPE).

Guidelines on the use ofliners in highway construction v

Excavation

- . ..

lm

~

- - - .. - - -- - - - - - - -- - - /" -~- -

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Rock

Rock excavation

Embankment

Rock or soil fill

Figure 4. Shoulder proto::.:tion with mineral liner, GCLs and geotextiles.

123

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Geosynthetic clay liners

The seams of the geomembranes must be waterproof. A simple overlap is possible above a perched water table if the angle of slope of the geomembrane is 1 : 1.5 or steeper. To prevent earth layers slipping on the geomembrane, its angle may not be between I : 3 and 3 : 1 and the incline of the soil surface may not be steeper than 1 :4. The danger of slipping may be reduced by using a textured geomembrane. A steep slope may be secured by using a reinforcing geogrid that is anchored at the top. The edges of the geomembrane are to be pulled up sufficiently above the level of the drainage ditch, or above the level of a perched water table. To increase friction and produce a protective effect, geotextiles are normally installed on the geomembrane. If there are outcrops of sharp-edged stones below the geomembrane, a protection layer of sand is installed. Geomembranes used for nonnaJ slope protection are of high density or low density polyethylene, of at least 0.3 mm thickness. Their principal function is to prevent the sealing layer of mineral earth from drying out. The seams of these this geomembranes do not need to be watertight. 5.1 Use of geomembranes - sample cross sections (Figure 5) In the guidelines, geomembrane liners are only recommended for particularly endangered places and additionally for areas where suitable soil material is not available within an economical distance.

Constructions 1 K and 0.6 M + K:

• Location of geomembrane, if guard rails definitely not required. The safe distance from the surface of the shoulder to the geomembrane is generally at least 1 m for new roads. A simple overlap is sufficient, as the shoulder inclination is made steeper than I : 1.5, and the subsoil is drained. The lower geomembrane is anchored to the slope with nails. • Location of the geomembrane in cross section with guard rail. • The geomembrane is installed at least 0.2 m below discharge pipes, lighting mast foundations or other fixtures. Alternatively, mast foundations or manholes may be led through the geomembrane. • A protective layer without stones is necessary if there are sharp stones in the subsoil. • Frost-resistant filling material. • Root space for bushes needs a layer thickness of 0.6 m; trees need 1.0 m. A thicker topsoil layer may also be used for the root space of trees. • Partially waterproof topsoil layer. • Reserve for soil (mud) deposits. • In the case of manholes, the geomembrane is fitted to the manhole pipe with a circular clamp. The geomembrane is to be raised, waterproof, at least 0.5 m above the highest perched water table. • The upper part of the road (inner) slope may either be sealed with a GCL or with a geomembrane as with the central strips. The geomembrane is then to be laid at a depth of at least 0.5 m. • The geomembrane may also be installed in a terraced manner if there is a I : 2 steep slope, to reduce the danger of slipping. The tenn 1 K means that a geomembrane is used at a depth of 1.0 m without any additional mineral grain seal. The term 0.6 M + K for the slope protection

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Guidelines on the use ofliners in highway construction

construction means that a geomembrane is used at a depth of 0.6 m below a mineral grain seal consisting of protective layer and sealing layer.

5.2 Slope protection for existing roads - sample cross section (Figure 6) Figure 6 illustrates the type of slope protection required for existing roads. Some important details follow.

Narrow Central Strip

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Figure 5. Slope protection with geomembranes. ·

f-:-.:-~-::-/:

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7

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126

Geosynthetic clay liners

Cutting Existing slope

Extension of cutting

Cutting in rock

Embankment

Figure 6. Shoulder seal for existing roads.

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127

• The slope incline is flattened by raising the road ditch. • In the case of steeper slopes, the naturally available soil is replaced by seaJing materiaJ (silt moraine) that is naturaJly more stable. A protective layer is placed on top. The grass topsoil is fixed using geotextiles or a geogrid. • Where possible, a liner seal is at top of inner slope, and geomembrane is below the GCL. Terraces are used, where required, to prevent the shoulder seaJ slipping on the thin geomemebrane. • By extending the excavation, it is possible to flatten the upper area of the inner shoulder and the lower area of the outer (opposite) shoulder. • The naturaJly a vailable soil is replaced by soil liner material. A protective layer is placed on top. • Where possible, a liner seaJ is at the top of inner slope, and a geomembrane is below the mineraJ grain sealing layer. • A 0.3 m excavation is removed from the slope. A bitumenised geotextile is installed on the stone-free subgrade, with 0.25 m of broken stones and a 0 .0 5 m thick bitumen or bitumen emulsion covering layer above it. • Cracks in the rock are filled with shotcrete or similar. • Installation of a liner on an existing slope. • Drainage ditch for surface water, with reserve for sludge settling. • Drainage to collect surface water. The ditch is sealed at sides and base with geomembrane. The drainage pipe is bedded in drainage gravel. A coarse sand layer is placed on top. 6 SUMMARY This paper illustrates the extreme care taken in Finland against accidentaJ spills of chemicaJs and deicing salts entering the groundwater adjacent to roadways. Various schemes of protection are required. Clearly, GCLs are used in many of these schemes and they are actuaJly required by regulations. In this regard Finland is a leader in the use of innovative methods for groundwater protection in transportation applications. REFERENCES Ehrola, F. 1981 . Running off the road. A study ofcar encroachment accidents and road conditions in Finland in 1971-1975, University ofOulu, Series C: 19. 144 pp. (Finnish+ english Abstract and Figure texts). Oulu. Ralhmayer H. & Juvankoski, M. 1993. Validity of geosynlhetic clay liners and geomembranes for groundwater at roads. Investigations and recommendations. Helsinki. Finnish National Road Administration.. Research report 26/1993. 83 pp. (Finnish + english Abstract). Pohjaveden soujaus lien kohdalla (Protection of groundwater at roads . 1991. Helsinki. Finnish National Road Administration.. Guidelines. 32 pp. (in Finnish). Rathmayer H. & Juvankoski, M. 1993. Geomembranes- function and selection criteria. Helsinki. Publications of the Water and Environment Administration - series A 153, 82 pp. (Finnish +english Abstract).

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Landfill cap designs using geosynthetic clay liners J. M. Fuller James Clem Corporation, Fainnount, USA

ABSTRACf: Increased environmental awareness and recent changes in US environ­ mental regulations have forced many existing landfill sites to close. The owners of these sites want a simple and affordable capping system that will also allow them to meet the new stricter regulations. Geosynthetic clay liners (GCLs) provide an attractive alternative to traditional compacted clay liners (CCLs) as the barrier layer in a final cover system. GCLs provide many performance advantages including better freeze-thaw and wet-dry behavior and greater tolerance of differential settlement. GCLs are also much easier to install than traditional CCLs. This paper focuses on GCL design considerations of particular importance to caps and highlights two projects in the United States where GCLs were used as the sole barrier layer in the final cover system. 1 INTRODUCTION

Recent changes in US environmental laws require all operating landfills to have sophisticated bottom lining and final cover systems. A number of landfill operators have found these new requirements impossible to meet and have decided to halt their operations. The final closure systems constructed for these older landfills (and for all new landfills) must meet the new stricter requirements. The owners of these sites want a simple and affordable final cover system that will meet the new stricter mandates. Compacted clay liners (CCLs) are required for the barrier layer portion of the new fmal cover systems. Because of the high costs and long time needed to construct a CCL, geosynthetic clay liners (GCLs) often represent a cost-effective alternative. GCLs also provide several performance advantages. This paper concentrates on the benefits of using GCLs in landfill final cover systems and discusses some of the important design issues engineers should consider when evaluating a GCL for their landfill cap. Brief descriptions of 2 landfill projects in the US where a GCL was used as the sole barrier layer are also included.

129

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Geosynthetic clay liners

2 PURPOSE OF THE FINAL COVER SYSTEM While a final cover system serves many purposes, its main function is to separate the waste from the surrounding environment. By separating the waste, the landfill cap prevents disease vectors, such as birds and rodents, from causing public health problems. The final cover system serves another purpose. It protects the local groundwater regime from contamination by limiting the amount of rainfall infiltration. Reducing the amount of infiltration correspondingly reduces the amount of leachate generated. As a result, less leachate is available to contaminate the groundwater. 2.1 Components of the Final Cover System Modern land capping systems comprise 5 basic layers: • Surface Layer • Protective Layer • Drainage Layer • Barrier Layer • Gas Collection Layer Although each component plays an important role in a properly functioning final cover system, this paper will concentrate on the barrier layer. An effective barrier layer is critical to the long-term success of the final cover system. Traditionally, the barrier has been a 0.3 to 1.0 m thick CCL with a hydraulic conductivity coefficient of I x I o-9 m/s. However, CCLs have several inherent drawbacks. 2.2 CCLs as the Barrier Layer A CCL is difficult to construct properly. Frequently, the contractor is required to construct a test pad to demonstrate that he knows the proper combination of compaction equipment, number of passes and soil moisture content needed to achieve the required soil permeability. These efforts are often wasted because changes in the borrow soil, equipment operators or the weather make it difficult to attain the proper level of compaction. Attaining the proper soil compaction in the final cover is also difficult because of the relatively soft subbase, the waste. The waste's low density and high compressibility prevent it from providing an adequate reactive force during compaction. These difficulties often result in the contractor repeatedly compacting a section of the CCL before an adequate soil permeability has been achieved. A CCL also has several performance drawbacks. The barrier layer in any final cover system will be exposed to environmental factors such as wet/dry and freeze/ thaw cycles. Researchers have shown that freeze/thaw or wet/dry cycles can severely and permanently crack a CCL (Koerner & Daniel 1993). These cracks can extend up to 1.0 m (Koerner & Daniel 1993), leaving the barrier far less effective than the designer intended. The limited swelling potential of most natural clays means that these cracks will never fully heal when water is reintroduced. Other factors may also damage the barrier layer. All Municipal Solid Waste Landfills (MSWLFs) experience settlement during their postclosure period. While some total settlement does occur, it does relatively

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little damage to the liner because the entire system settles as a single unit. Differential settlement, which is more damaging to the liner, predominates because of the heterogenous nature of the waste. Unfortunately, CCLs, the traditional choice for banier layer in landfill caps, can only withstand 0.85% strain before cracking (Koerner & Daniel 1993). Consequently, the CCLs used in most MSWLFs are likely to crack and thereby be less effective than the engineer intended. The cost of constructing a CCL can be exorbitant if there is no natural clay on-site that can meet the permeability requirement Hauling distances of only a few kilometers can raise the construction costs of a large landfill cap dramatically. Construction of a large CCL can take months and the cost is often further increased by weather delays. A significant or prolonged rain storm can increase the water content of the borrow soil to the point where the contractor must take exceptional measures (and incur great expense) to dry-out the soil so it is useable. A CCL's poor long-term hydraulic performance and high cost make the search for an alternative barrier layer material almost imperative. GCLs offer an effective and economical alternative to the traditional CCL.

2.3 GCLs as a Barrier Layer GeLs have several performance advantages over the traditional CCLs. GCLs have an extremely low permeability compared to CCLs even under the relatively low confining pressures likely to be encountered in final cover systems. By using the Hydrologic Evaluation of Landftll Performance (HELP) Computer Model (Schroeder 1988), it has shown that GCLs significantly reduce the amount of infiltration. Using the following simple cross-section and other assumptions, the HELP Model predicts up to 18 times less flow through the GCL barrier layer than through the traditional CCL (Figure 1): 0.46 m thick Topsoil Layer, 0.3 m thick Drainage Layer,

Barrier Layer (GCL or 0.3 m thick CCL),

Waste.

CCL

*40.5 Hectare Site Figure I. Help Model Comparison, Average Flow Lhrough Cap.

GCL

132 Geosynthetic clay liners The site was assumed to be 40.5 hectares in area and to have only moderate vegetative growth. Also, rainfall data collected from a 5 year period in Sacramento, California was used to determine the potential flow through the two barrier layers. GCLs are not permanently damaged by either freeze/thaw or wet dry cycles (Boardman 1993 and GeoServices 1988). When researchers dried a GCL that was placed under 0.6 m of cover soil, they found that the bentonite did exhibit "alligator cracks". These are small, thin desiccation cracks in the bentonite. However, shortly after reintroducing water to the system, the GCL regained its original low penneability (Boardman 1993). GCLs also are able to withstand greater settlements than CCLs. By using large indoor tanks with false bottoms and a liquid filled bladder to control and simulate settlement, researchers have demonstrated that GCLs can withstand strains due to settlement of 10% to 15% (LaGana 1992). This is true despite the fact that bentonite itself can only withstand strains of 3.4% (Koerner & Daniel 1993). The textiles used to carry the bentonite seem to take up most of the strain. The construction benefits of GCLs also make them an attractive alternative to CCLs. Because the GCLs are manufactured under highly controlled conditions, comparatively little construction quality assurance (CQA) testing is required. A GCL can be installed at a rate of 4,000 to 8,000 m 2 per day. This installation rate often results in an 80% reduction in the barrier layer construction time. Finally, while the cost of GCL will vary with the size of the project and the location relative to the manufacturing site (there are only 4 plants in North America and 6 worldwide) the GCL cost is often competitive with a CCL even if adequate clay is available on-site (if construction and CQA costs are considered). 3 DESIGN CONSIDERATIONS Engineers design landfill final cover systems with steep slopes to minimize the amount of construction materials needed and to maximize the available airspace within the landfill. A major drawback to this approach is that it is difficult to construct a CCL on slopes approaching 2H : 1V .GCLs, however, are ideally suited for steep slope installations because they can be simply unrolled into place. While GCLs facilitate the construction of steeper side slopes, there are serious design issues in this type of application that the engineer must address. Because of bentonite's inherently low shear strength, the engineer must carefully analyze the stability of the liner system when it includes a GCL, even a reinforced GCL. There are two components to the stability question: • the internal stability of the GCL, • the interface friction of the GCL and adjacent liner materials. Each of these issues is discussed in the following sections. 3.1 Internal Shear Strength There are two types of reinforced GCLs: those that are stitch bonded and those that are needlepunched. Differences in the two types of reinforcing mechanisms result in different shear behavior and vastly different failure mechanisms. These differences may influence the engineer's selection of reinforced GCL depending on the site conditions.

Landfill cap designs using geosynthetic clay liners

133

The following are the results of direct shear tests on stitch bonded and needle­ punched GCLs (Fuller 1994). These tests were run for the purpose of directly comparing the shear behavior and failure mechanisms of these two reinforced GCLs. All the GCL samples were hydrated for 5 days under the same confining load as was applied during shear (9.6, 69, and 345 kPa). The samples were sheared at a rate of 0.1 mrnlmin in a 300 x 300 mm direct shear box. The samples were sheared until 100 mm of displacement was reached. The reinforcing mechanism and the bentonite contributed to the shear strength of both types of GCLs. The bentonite contributed an equal amount of strength to both types of GCLs. However, the stitch bonc:ling mechanism provided more internal shear strength than did the needlepunching mechanism. This can be seen in the shear strength vs. displacement and shear stress vs. normal pressure plots. 3.2 Shear Stress vs. Displacement Figure 2 is a plot of internal shear stress v. displacement for the stitch bonded GCL. The shear stress of the first sample (9.6 kPa of confinement) increased steadily throughout the test to a peak of 31 kPa. The second sample (69 kPa of confmement) reached a peak shear stress of 53 kPa and settled to a residual shear stress of 50 kPa. The third sample reached a first peak of 132 kPa and a second peak of 108 kPa before reaching a residual shear stress of 72 kPa.

3,000 . -- ------ --------------------------. (144 ~

cf.

2,500 (120

~

::-2,000

g_

(96}

Normal Putssures

g 1,500 ~

Cii

-

;u 1 .ooo (])

(48}

...c

500

(/)

200 p$f (9.6 kPa)

+ 1440 p$f {59 kPa)

(72}

·• 7200 p$1 (345 kPa)

·· · ···· · ~------ - -. - - - -" - - -. - ­.

(24} . - - - - - ­

O L-----------------------------------~

0

0 .5

(12)

1

1 .5

(2~}

(38}

2

l51.}

2 .5 {63)

3

(76)

D1splacemen , 1nches (mm)

3 .5 4

(89)

{102}

Figure 2. Stitch Bonded GCL - Internal Shear Strength -Shear Stress v. Displacement.

Figure 3 is a plot of the shear stress v. displacement curves for the needlepunched GCL. The first sample (9.6 kPa of confinement) reached a peak shear stress of 13 kPa and a residual shear stress of 10 kPa The second sample (69 kPa of confmement) reached a peak shear stress of 49 kPa before setting to a residual shear stress of 15 kPa. The shear stress of the third sample (345 kPa of confinement) reached a peak of 84 kPa before decreasing to a residual shear strength of 46 kPa.

134

Geosynthetic clay liners

3,000 r - - - - - - - - - - - - -- -- - -- -, (144

lil2,500 ~

(120

:;; 2,000 a.

(96)

Normal Pressures

::: 1,500

_g

en ~

~ .c.

-

200 psi (9.6 kPa)

+ 1440 psi (69 kPa)

(72)

1,000

7200 psi (345 kPa)

(48)

en

500 · 0./ 0

0.5

(12)

1

1.5

(2!?)

(38)

2

(51)

2.5

(63)

3

3 .5

(76)

4

(89)

Dtsplacement, tnches (mm)

(102)

Figure 3. NeedJe Punched GCL - Internal Shear Strength -Shear Stress v. Displacement.

3,000.---- ----------------------------------, (144)

i

2,500 (120)

~2 000 ····· Stitch-Bonded GCL ~ ­

iii ' a.

(96)

~

(72)

........

g 1,500

en

--

iii 1,000 ... (])

.c.

en

500 .-:-. (24)

+

--

--.....

'-

'!"'

­

Needle-Punched GCL

OL---------------------------------------_J 0

1

(48)

2

(96)

3

(144)

4

(192)

5

(240)

Normal Stress,ksf (kPa)

6

(288)

7

(336)

Figure 4. Peak Shear Strength Envelopes - Comparison of Stitch Bonded and Needle Punched GCLs.

3.3 Peak Shear Strength Figure 4 compares the peak shear strength failure envelopes of the stitch bonded and needlepunched GCLs. The stitch bonded GCL has a internal friction angle of 14.5° and an apparent cohesion of 33 kPa The needlepWlched GCL has a peak internal friction angle of 10.5° and an apparent cohesion of 23 k.Pa.

Landfill cap designs using geosynthetic clay liners

135

3,000.---------------------------------------~

(144)

lil

a..

2,500 (120)

~2.000

Cii

a.

(96)

~

(72)

Stitch-Bonded GCL -._

g 1,500 iii

;o 1 ,ooo

~

C/)

(48)

500 (24)

+ - - -

it> 13 k.Pa (more than 4-times safer).

The relations shown in Figure 4 are modified in Figure 5 for a soil covering h with a unit weight ofy= 20 kN/m 3. The required peel strength is shown as a function

Peel strength fv,r•q. (N/1 Ocm) 80

1.5 : 1

70

2: 1

110

2.5: 1

50

n

3 :1

40

4: 1

30

Slope Inclination n: 1

20

10 0 0

2

3

4

Soli covering (m)

5

with

6

'Y =

20 kN/m'

Figure 5. The required peel strength of a needle-punched GCL in undrained condition (free hydration) as a function of soil covering and slope inclination.

On the long-term shear behaviour ofgeosynthetic clay liners (GCLs)

149

of the soil covering and slope inclination. A peel strength of Fv.""' ~ 16.7 N/10 em is for example required for a soil covering of h = 1.25 m (a= h x y = 1.25 m x 20 kN/m 3 = 25 kPa), which is common for capping sealing systems, and a slope inclination of 2 : l. As mentioned above the minimum peel strength laid down for a needle-punched GCL in the quality assurance programme is Fv 2 60 N/10 em (approx. 3.5-fold safety factor). To estimate the long-term performance of this common case a long-term tilt-table test with slope inclination 2 : 1, cr = 25 kPa and Fv = 60 N/10 em should be carried out. ln order to accelerate the end of the test, the peel strength was reduced to Fv = 30 N/10 em by maintaining the other test conditions, although a higher value is guaranteed due to the production technique. This test results in a safety factor of 1.8. Nearly no sliding had occurred in the tilt-table test at the time this paper was printed (7500 hours = 0.85 years had elapsed). The above mentioned evaluations are thus confirmed. At low normal stresses, as for instance in case of a landfill capping seal, a safe transmission of the shear stress on a long-term basis can be proven for needle-punched GCLs without difficulties, even in undrained condition of the bentonite layer. The undrained condition should be taken into consideration since the mentioned loadings can quickly occur during the earthworks. In case of high loads, as for instance for a landfill base seal, the loads which have to be considered for project-related reasons (loads resulting from the waste) will increase, however, step-by-step over a longer period; the shear coefficients in drained and/or undrained condition have specifically to be determined and considered for the stability of the structure. ln the drained condition, considerable shear stresses can also be transmitted by the bentonite layer together with the fibre reinforcement.

5 SUMMARY GCLs used in landfill capping applications are often placed on steep slopes. Under such conditions the long-term shear strength of the mid-plane is an important design consideration. Under worst case assumptions, the GCL will be in a hydrated state under low normal stress. This paper has investigated the condition described above from an analytic perspective using the concept of partial factors of safety. Stitch bonded GCLs were compared to needle punched GCLs. Thus a design methodology has been presented for a long-term assessment. REFERENCES Heenen, G. 1994. Geotextile Dichtungselemente als mineralische Kompom:nte in OberfUichenahdichtungen. 10. Fachragung 'Die sichere Deponie ·, SKZ Wiir:;burg: pp. 141-170 (in Gennan). Heenen, G., Saathoff, F., Scheu, C. & von Maubeuge, K. 1994. Ober das Langzeitscherverhalten von geosynthetischen Tondichtungsbahnen in OberfHichendjchtungssystemen. Veroffenrlichungtm des LGA­ Grundbauinsrirurs. Heft 71: pp. 177-183 (in Gennan). Saathoff, F. 1991. Geolrunststoffe in Dichtungssystemen. Miueilungen des FRANZIUS-Jnsritutsfiir Wasserba.u und Kiisreningenieurwesen der Universittit Hannover. Heft 72: pp. 1-3 16 (in Gennan). Saathoff, F. & Ehrenberg, H. 1992. Dichtung von der Rolle. Baumaschinendienst. Heft 9: pp. 848-856 (in Gennan). Saathoff, F. & Heenen, G. 1994. Geosynthetic Clay Liners in Capping Sealing Systems. Geosynthe tic World.

April 1994: pp. 18-19 & 23.

150

Geosynthetic clay liners

Scheu, C., JohannSen, K. & Saathoff. F. 1990. Non-Woven Bentonite Fabrics- A new Fibre Reinforced Mineral Liner System. Proc. of the 4"' International Conference on Geotextiles, Geomembranes and Related Products, The Hague. Vol. 2: pp. 467-472.

On the slope stability of landfill capping seals using GCLs D. Alexiew & R. Kirschner Huesker Synthetic. Gescher, Germnny

H. Berkhout Akzo Nobel Geosynthetics. Amhem, Netherlands

ABSTRACT: The slope stability of three different landfill capping systems using GCLs is analyzed. Results of tilting plate and shear box tests are given and the factor of safety for every possible shear plane is calculated. The critical shear plane is directly determined or calculated. System I with a single GCL sealing layer is stable for slope inclinations up to 1 (v) : 2.5 (h). Systems ll and Ill, consisting of a GCL and different embossed geomembranes, can be inclined up to 1 (v) : 3 (h). 1 INTRODUCTION

The German regulations and technical instructions for municipal waste disposal (TA Siedlungsabfall 1993) envisage a mineral seaJ, i.e., a compacted clay liner, as cap seal for landfills in category I, and a combination seal, i.e., a geomembrane over a compacted clay liner, for landfills in category ll (Figure 1). Alternatively, other equivalent cap seals may be used. However, the precise meaning of 'equivaJent' is still a matter for discussion in Germany. For many reasons, alternative systems may have advantages, which, however, will not be examined at this point. Our aim is rather to consider three alternative systems with GCLs from the point of view of slope stability. 2 SLOPE STABILITY In many cases of cap seals on steep slopes, the critical failure mode is a slipping of the sealing system on a shear plane within the layer construction. Proof of slope safety may be carried out in accordance with GLR reconunendation E 2-7 (1993) (Figure 2). Several potentiaJ slip surfaces must be exarnined. The determining shear parameters q>' and c' (within one materiaJ) oro' and a' (in a shear plane between two materials) are to be aJJocated to the slip surface.

151

Geosynthetic clay liners

152

Category I landfill

c

v

= =

Recultlvation layer (cover soli)

"'

.s

layer

=

.....,

~:-

c

u

Mineral sealing layer

::;

Compensating layer or gas drainage layer

"'

Waste

Figure I. Landfill cap seals in accordance with the German regulations and technical instructions for municipal waste disposal (TA Siedlungsabfall 1993).

Category II landfill

E

u

-

l

-

Recultivatlon layer - (cover soil)

===~~~~~~~~==~

or gas drainage layer Compensating layer Waste

Figure 2. Proof of slope stability in accordance with GLR recommendation E 2-7 (1993).

3 SYSTEMS EXAMINED System I, whose sealing element consists of a stitch-bonded GCL, represents the scheme without geomembrane envisaged for landfill category I. Furthermore, the mineral drainage layers have been replaced by geocomposites (Figure 3).

On the slope stability oflandfill capping seals using GCL.s

153

Vegetation layer

Drainage Geocomposite NaBento (GCL)

Figure 3. System 1: GCL without geomembrane.

Systems ll and ill are planned to meet the needs for combination seals for landfill category 11. The major difference between systems II and lii is in the sequence of GCL and geomembrane used, see Figures 4 and 5. The further difference is in the use of different types of geocomposites, and the embossing or texturing of the geomembrane. The different sequence of geomembrane and stitch-bonded GCL was chosen because both alternatives are currently being discussed and used. These alternatives arise from various approaches to the question of safety and different safety philosophy.

4 ESTABLISHING SHEAR PARAMETERS The shear parameters are generally determined in shear boxes measuring 100 x 100 mm or 300 x 300 mm. So-called tilting plates measuring at least 500 x 500 mm may also be used as test equipment. In carrying out the tests, the following points are, among others, to be taken into account: • moisture conditions (wet test) • realistic surcharge loads, as low as possible, e.g. l0, 20 and 40 kPa • low shear rates, if possible less than 1 mmlh • possible boundary condition effects. Because of the inaccuracies in the tests, particularly with the shear boxes, reduced shear parameters, in accordance with EAU (E 96) ( 1990) have to be used for design. It is to be noted that the result of each shear test is related to the material and product involved. If it is necessary to relate test results to similar materials or products, great caution should be observed; this should only be done by an experienced expert.

154 Geosynthetic clay liners

Geomembrane (rough) NaBento (GCU

Drainage Geocomposite

Figure 4. System ll: GCL below geomembrane.

Vegetation layer

Drainage Geocomposite NaBento (GCL)

Geomembrane (stuctured) Drainage Geocomposite

Figure 5. System ill: GCL on geomembrane.

Note, that the angle of internal friction of the GCL used in the Systems I, II and ill here amounts to 40° (wet test) due to stitching. Colliequently, the internal sliding stability of this GCL is not a problem in common capping systems.

On the slope stability oflandfill capping seals using GCLs

155

Lead plates Aluminium box

~----Dra i nage

geocomposite

Critical shear plane NaBento (GCL)

Drainage geocomposite Non-slip underlay

Figure 6. Direct examination of sliding stability of System I on the tilting plate.

30 E

E

.!:

18.5°

25°

~20 (( g­

iS

10

Angle of inclination



I +--~-----------

I

o Bottom (drainage geocompositel + Middle (stitch-bonded GCU ~ Top (drainage geocompositel

40

20

60

Time, days Figure 7. Displacements of the components in System I, failure in the interface Drainage Geocorn-posite/GCL.

4.1 System I (in wet condition) System I was examined as a complete system, using a tilting plate for a period of 2Y2 months (Figure 6). The displacements were measured at all layer levels. In all cases, the angle of inclination was not increased until slip displacements had ended. The critjcal slip surface is registered at an angle of inclination of 29.5° in the interface between the top of the GCL and the bottom of the geocomposite (Figure 7).

156

Geosynthetic clay liners

Table I. Shear parameters of System II and factors of safety with a slope inclination of I (v) : 3 (h). Position of shear plane

Shear parameters

Factors of safety

(0')

c' (a') (kPa>

Vegetation layer/drainage geocomposite (side with nonwoven)

27.1°

2.6

1.54

1.95

Drainage Geocomposite (uncovered core) HDPE geomembrane, rough surface

23.8°

0

1.32

1.32

HDPE geomembrane, rough surface/stitch-bonded GCL

14.4°

7.0

0.77

1.88

Stitch-bonded GCUDrainage Geocomposite (uncovered core)

19.2°

5.8

1.04

1.97

Drainage Geocomposite (side with non-woven)lsupport.ing layer

33.4°

0

1.98

1.98

Supporting layer (sand)

34.1 °

2.03

2.03

qJ'

without cohesion (adhesion)

with cohesion (adhesion)

Table 2. Shear parameters of System Ill and factors of safety with a slope inclination of I (v): 3 (h). Position of shear plane

Shear parameters

Factors of safety

(0')

c' (a') (kPa)

without cohesion (adhesion)

Vegetation layer

31.30

22.3

1.82

5.36

Vegetation layer/Drainage Geocomposite from nonwovens

32.6°

1.5

1.92

2. 16

Drainage Geocomposite/Stitch-

17.9°

6.5

0.97

2.00

Stitch-bonded GCLIHDPEgeomembrane, structured surface

23.7°

0

1.32

1.32

HOPE-geomembrane, structured surface/Drainage Geocomposite

27. 1°

1.5

1.54

1.77

Drainage Geocomposite/Supporting layer

23.3°

3.8

1.29

1.90

Supporting layer

33.4°

13.8

1.98

4. 17

qJ'

with cohesion (adhesion)

bonded GCL

4.2 Systems II and Ill (in wet condition) The shear parameters of the layers and the shear planes were established separately for each of the components, in shear boxes sized 300 x 300 mm. The lower bound shear parameters may be seen in Tables 1 and 2.

On the slope stability ofLandfill capping seals using GCLs

157

5 INTERPRETATION OF THE TEST RESULTS 5.I System l It can be seen from Figure I that displacements clearly increase at all layer levels when the angle of inclination is increased from 28.1 o to 29 .SO. The critical shear plane occurs between the top of the GCL and the bottom of the geocomposite. The relative displacement between these two layers is approx. I0 mm at incipient failure. This failure plane could be seen to emerge in the earlier stages of the test. Time-dependend displacements in the layer construction proved negligible. Taking into account the reduced shear angle in accordance with EAU (1990) and the required safety in accordance with DIN 4084 (1981), a maximum slope angle of 21.6° is permissible for System I. This corresponds to an inclination of slope of 1 (v) : 2.5 (h). With this test installation, it was not possible to investigate the influence of seepage forces. The same System I was tested in a field test site at the Michelshoehe landfill (Kreit I995, Weiss & Siegmund 1993 and Weiss et al. I995). Here, seepage forces were created by intensive and frequent inigation. With a slope inclination of I (v): 3 (h) the system proved stable. 5.2 System II In tests using the tilting plate, the critical shear plane can be obtained directly as a test result. If the shear parameters are established in the shear box, the critical shear plane must be found by calculation. The factor of safety for Systems II and III was determined under the following assumptions: • thickness of vegetation layer 1 m • unit weight of vegetation soil y = 20 kN/m 3 • no seepage flow • slope inclination l (v) : 3 (h) The calculated factors of safety are given in columns 4 and 5 of Tables I and 2. Neglecting the cohesion/adhesion, the critical shear surface for System ll is between the bottom of the embossed HDPE geomembrane and the top of the stitch-bonded GCL (factor of safety T] = 0 .77). This would lead to slope failure. However, it is not realistic to neglect adhesion, because the shear angle used was established in a wet condition and a considerable adhesion (a' = 7 kPa) is realized. With adhesion, this shear plane has a safety factor ofT]= I.88, which is in excess of the customary value of 1.4. The determining shear plane is then between the geocomposite and the top of the geomembrane (T] = 1.32 < I.88); however, this 'new' critical shear plane is already sufficiently stable (TJ = 1.32 > 1.30). Recent requirements in Germany are that the geomembrane must remain free of tension, i.e., the shear resistance at the bottom of geomembrane (corresponding: T]b) should be noticeably greater than at the top (corresponding: TJJ This condition is achieved with System II, as can be seen: TJ/fl, 1.88/1.32 = 1.4 (1)

=

158 Geosynthetic clay liners 5.3 System lll

m

The situation for System may be interpreted in a sirrular way to that of System IT. Neglecting adhesion, the critical shear plane would be between the bottom of the geocomposite and the top of the stitch-bonded GCL. The safety factor of 0.97 would not be sufficient. However, as with System II, the safety factor with adhesion (TJ = 2.00) is sufficient. Here again, this results in the critical shear plane being between the GCL and the geomembrane (TJ = 1.32 < 2.00). The safety in this 'new' critical shear plane is, however, sufficient (TJ 1.32 > 1.30). A tensile stress of the geomembrane is not to be expected (TJt/TJ, = 1.77/1.32 1.3).

=

=

6 MEASURES FOR STEEPER SLOPES If System [ has to be installed on slopes steeper than 1 (v) : 2.5 (h), stability may be achieved by the use of a geogrid reinforcement of the layers above the critical shear plane. This also applies to Systems II and ill in the case of slopes steeper than I (v) : 3 (h). Considerations and design approaches for such reinforcements are to be found in Kirschner (1993) and Alexiew (1994). 7 SUMMARY This paper has been focused on the mid-plane shear strength of hydrated GCLs. Even further a complete system of different geosynthetic interfaces has been evaluated. It is felt by the writers that investigations such as presented herein are necessary for most landfill capping systems and are espcially important as the slope angle increases. Insight into this situation was also presented. REFERENCES Alexiew, D. I994. Bemessung geotextiler Bewehrungselemente fiir Dichtungssysteme auf geneigtcn Flachen. 10. Fachtagang Die sichere Deponie. Wiirzburg: SKZ. DIN 4084. 19!11. Geliinde· and Boschungsbruchberechnangen. Berlin: Beulh Verlag. EAU. 1990. Empfehlungen des Arbeitsausschusses 'Ufereinfassangen, Hafen and Wasserstraflen'.Berlin: Ernst & Sohn. GLR. 1993. Geotechnics ofLandfill Design and Remedial Works· Technical Recommendations- Gil?. Berlin: Ernst & Sohn. IGrschner, R. 1993. Zur Bemessung von Geoginern fur die Bewehrung von Deponieabdicntungssystemen auf Biisctmngen. Lehrgang Nr. 17469 Geokanststoffe im Deponiebaa, Esslingen: TA. Kreit, V. 1995. Test Field for Lhe Capping System of the Michelshiihe Landfill. In Koerner, R. M., Gartung, E. & Zanzinger, H. (eds), Geosynthetic Clay Liners. Rotterdam: Balkema. TA Siedlungsabfall. 1993. Drilte Allgemeine Vem•altangsvorschrift vmr Abfallgesetz. Cologne: Bundesanzeiger. Weiss, W. & Siegmund, M. 1993. Einsatz von NaBcnto-Dichtungsmanen zur Oberfliichenabdichtung von Altdeponien. Versachsbericht Nr. 7193. Hochschale for Archirekrur and Baawesen Weimar and MFPA Thiiringen, Weimar. (unpublished). Weiss, W., Siegmund, M. & Alexiew, D. 1995. Field Performance of a GCL Landfill Capping System under Simulated Waste Subsidence. 5th Nonh American Regional Conference on Geosynthetics - Geosynrhetics '95. Nashville. Tennessee (to be published in February '95).

Applications

Taylor & Francis

Taylor & Francis Group http://taylorandfrancis.com

Groundwater protection using a GCL at the Franz-Josef-Strauss airport Munich, Germany G. Heerten Naue Fasertechnik, Liibbecke, Germany

ABSTRACT: In order to prevent groundwater contamination due to runway deicers, automatically working, maintenance-free purification networks have been created along the runways of the new Munich airport. The 'IDS - In Situ Decomposition System' (ASG - Abbausystem im GeHinde) ensures the natural bacteriological decomposition of deicers into water and carbon dioxide during seepage through this system which is sealed towards the groundwater. The sealing of the IDS/ASG is carried out using an impermeable needle-punched geosynthetic clay liner (GCL). This paper reports on the installation of approx. 700,000 m2 of GCL, including the technique carried out with regard to overlaps and connections. Comparative tests performed in the lab and in the field have shown the effectiveness of the sealing bentonite layer. The detailed work- carefully canied out by the 'Institut fur Grundbau, Bodenmechanik und Felsmechanik der Technischen Universitiit Mtinchen'- which was absolutely necessary for meaningful field measurements, will be described with examples. The documentation of water level measurements in the operating decomposition system confmned the effectiveness of the solution. 1 THE 'IN SITU DECOMPOSITION SYSTEM - IDS' The deicing of the aeroplanes with a mixture of hot water and glycol as well as service works on the runways and taxiways are daily routine to ensure a safe air traffic in winter. According to the decision of the authorities the deicing agents had to be prevented from contaminating the groundwater at the new Munich Airport. The contaminated surface waters from the runways and taxiways are conducted to the purification plant of the local administration union. Automatically working, maintenance-free purification networks have been created along the runways for the waters contaminated with deicing agents. The 'In situ Decomposition System - IDS' is based on the experience that glycol based deicing agents are transformed by soil bacteria to the harmless components water and carbon dioxide during a 20-days seepage through the soil region above the groundwater. The total cost of the system was approximately$ 15M. The effectiveness of the system which was developed by the Consulting Engineers Dr. Blasy and Mader had been proven beforehand by several years of large-scale testing. Over a length of approximately 28 km along the runways and parking areas, the decomposition system with an average width of 22 m was installed. 161

162 Geosynthetic clay liners conra.miruued water

emting qullt!etiWy gnvels (soil group B +C) ~----------- ZO bls ZSm

Ci) Q)

sealing mat (BentofUsite system consisting of geotextiles and a bentonite layer. A composite consisting of a mechanically bonded nonwoven filter fabric and a woven fabric serves as carrier layer. A mechanically bonded nonwoven fabric is the cover layer. Sandwiched between these two geotextile layers is the sealing element, a bentonite powder layer with a thickness of approx. 3 mm which swells in contact with water. All layers are connected by needle-punching over the entire mat. The needle-punching considerably improved the application related properties of GCLs. Since this type of GCL, which was developed by Naue Faser-technik in 1988, made it possible for the first time to transmit for long-term periods shear forces between the geotextile components encapsulating the bentonite layer, these products have increasingly been used for several years as the sealing component of sealing systems in groundwater protection and landfill construction. In 1993, a total quantity of approx. 3 million m2 of this type of GCL was used in Europe and North America. During the needle-punching process felting needles with barbs pull fibres from the top nonwoven fabric layer through the bentonite layer and anchor them in the carrier geotextile. The so created fibre bridges provide a tension and shear resistant connection (fibre reinforcement) between the cover nonwoven fabric layer and the carrier geotextile. Figure 3 schematically shows a needle-punchedgeosynthetic clay liner. The needle-punching technique makes it possible to grant the transmission of shear forces and to maintain the good sealing properties of the bentonite or even

I64 Geosynthetic clay liners

+ - - fibre-reinforced bentonite

Figure 3. Schematic sketch of a needle-punched geosynthetic clay liner.

to improve them by the normal tension of the needle-punched connection (swelling counterpressure). 2.2 Properties of the needle-punched geosynthetic clay liner 2.2.1 Water impermeability, laboratory and field tests The most important parameter for the evaluation of a mineral sealing is the water permeability coefficient k in [m/s]. Several testing institutes carried out water permeability tests with different boundary conditions for the proposed GCL. Tests carried out by the 'Institut fiir Grundbau, Bodenmechanik und Felsmechanik der Technischen UniversiUit Miinchen' conf1m1 that - after a one-day saturation phase during which the bentonite absorbs water and after a non-stationary percolation of one or two days, k-values are determined according to Darcy's formula ranging between approx. 10" 12 m/s and 10" 11 m/s. The final k-value of 2.5 X w l-l m/s was determined after steady state flow conditions had been achieved. Further tests show that the water permeability coefficient is not dependent on the pressure head, or on the hydraulic gradient. Tests with fressure heads of 25 em tO 200 em result in k-values ranging between 1.3 X 10" 1 m/s and 3.3 X JO-lt m/s. It was a result of construction circumstances during the installation of the IDS that the GCLs could not always be immediately backfilled after they had been installed. Therefore they were exposed to changing weather (precipitation and sunlight) for longer periods. Therefore laboratory tests had to clarify the time-dependent development of the sealing effect of the GCL caused by drying out and moistening processes. As a reference value for the quantitative description of the sealing effect of the GCL the pennittivity \jf = k IT was chosen, since the thickness T of the swollen sealing layer cannot be detennined. This allows independent comparisons of test results from laboratory and field tests. Several phases in the test behaviour of the virgin and the dried samples can be detected in the test where a sudden hydraulic gradient of i 25 was a test condition. In individual cases, a short-term flow-through of the GCL took place at the beginning of the test. In the subsequent phase the samples absorbed water from the top and bottom side which the bentonite requires for swelling. During this period the material is water impermeable since the imposed hydraulic gradient is overlapped by the suction potential of the bentonite. During the last test phase a stationary condition appears in which the in-flowing

=

Groundwater protection using a CCL at the Franz-Josef-Strauss airpon

1·10"6

I

virgin mat

~

~ ~~~ -1·10" 1

2 cycles

~ \o . J ..

·.

"'

0 20 (0 60 80 100

I

in-flowing water quantity

l

2 cycles

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water quantity corresponds to the out-flowing water quantity. When a thickness of 1 em of the sealing membrane is assumed the permeability coefficient lies beneath 5 X 10"11 m/s. The tests show further that the fissures caused by the drying process heal on their own in the course of time. In the laboratory test even swollen pieces of the GCL with failures caused by gravel indentations or foot prints showed a similar self-healing effect. Figure 4 shows the results of the testing of dry/wet cycles. After the mats had been installed special tests were carried out to check the impermeability of the mats and the overlapping areas. These test set-ups were executed during further construction works. The 'Institut fur Grundbau, Bodenmechanik und Felsmechanik der Technischen UniversiHit Mtinchen' developed a pressure permeability test apparatus for short­ term tests which consists of a hollow steel cylinder with removable lid. Apart from a scale for the time-dependent measuring of the pressure head and the evaporation, two vertical pipes are attached on the lid (Figure 5). To carry out the test, the apparatus is put on the test area and filled with dyed water to detect boundary flows in the contact area. In order to avoid boundary flows in the contact area of the cylinder it is sealed with a circular roll of a bitumen modified polymer dispersion ('WEBAC 5611 ') which hardens very slowly when it is mixed with Portland cement PZ 35 F. The expansion of the sealing roll within the test area is limited towards the inner side of the cylinder by a formwork ring. In order to counteract the upward pressure and to achieve a contact pressure on the test area the test apparatus is weighted with a load of up to 160 kg. The mat is squeezed together in the region of the contact area by the contact pressure. This avoids water losses resulting from a horizontal flow in outward direction in the upper nonwoven fabric layer.

166

Geosynthetic clay liners

measuring scale

C covering

rain gauge

vertical pipe

inner diameter 10 mm

gravel filling •

•

benlOnile pule 0

•

0

•

sealing mal (BenlOflx® Typ B)

cylinder

JRS~we

penneabilily test apparalUS

concrete manhole ring r.est

Figure 5. Test set-ups for lhe impermeability testing of lhe GCL (Gruber, Aoss & Schmidt 1992).

In order to examine butt joints even after the GCLs had been covered, several 1 m high concrete manhole rings with an interior coating of bitumen were arranged in the IDS on still uncovered mats and open joints. To improve the conditions of support the cover nonwoven fabric in the contact area of the rings was initially cut out and the edges between ring and mat were sealed with a bentonite paste. Later on the cover nonwoven fabric was not cut out so that the original state in the overlap area of the mats was maintained. The gravel load in the test set-ups protects the test area. prevents the overlaps from opening and nearly corresponds to the conditions in the finished IDS (Figure 5). During the test with the pressure permeability apparatus the sealing of the contact area was difficult since irregular bearing pressures resulting from roughness in the gravel surface and therefore different compressions of the bentonite mat under the edge of the test cylinder are possible. Therefore the horizontal flow in the cover layer of the GCL may differ depending on the contact of the cylinder edge along its periphery. The results achieved with the cylinder permeability apparatus are therefore influenced by the marginal conditions given in the particular test area. Increasing experience, however, made it possible to control the marginal influences to a degree that comparable results could be achieved. In case of stationary conditions the permittivities ranged between 8 x w-s and 3 x 10·7 s· 1• They show at least qualitatively that the sealing properties of the mat are also existing in the installed state. 2.2.2 Deformation and settlement behaviour Needle-punched geosynthetic clay liners have a comparably high defonnability. The fibre labyrinth of the fibre bridges penetrating the bentonite layer between geotextile carrier and cover layer provides, with the bentonite, a homogeneous deformability of the whole structure and thus supplies good sealing properties even in the two-dirnensionalJy deformed condition. This was tested at the 'Franzius-lnstitut fur Wasserbau und Ktisteningenieurwesen der Universitat Hannover' and at the 'Institut fUr Grundbau und Bodenmechanik der UniversiUi.t Hannover'. The needle-punched geosynthetic clay liner was biaxially stretched, samples were taken from the elongated

Groundwater protection using a GCL at the Franz-Josef-Strauss airport

167

mat and the water permeability coefficients of the elongated and deformed mat were determined. The results show that in the case of an area elongation of up to 20% a k-value in the range of 10-11 rnls is consistently maintained. 2.2.3 Shear behaviour and absorption of shear forces As already mentioned, the covering of the GCL was partly delayed due to constructional circumstances, so that the mat remained uncovered for long periods and sometimes had to be covered in a wet and swollen condition which was a result of the weather conditions. In order to withstand this situation and to minimize displacements or damage of the mat during covering the needle-punching had to provide over the whole mat a constant shear resistant connection of the geotextile layers. During the needle-punching process a multitude of individual fibres are pierced from one geotextile component (cover nonwoven fabric) through the bentonite layer into the opposite geotextile component (carrier geotextile). These fibre bridges (2 to 3 million fibres/m 2) form a regular shear connection independent on the direction. Local shear stress concentrations are avoided. Dependent on the efficient normal stress, the consolidation condition of the bentonite layer and the peel strength of the GCL the decisive shear plane will become apparent. • outside the GCL between carrier geotextile or cover nonwoven fabric and the adjacent soil or geosynthetic (e.g. geomembrane) or • within the GCL in the bentonite layer. When, for instance, the shear coefficients of a sealing system designed with GeLs are determined in shear tests, the materials to be used and the actually efficient stress and state conditions have to be considered (Heerten 1994). In case of low loads and due to the installation stresses resulting from the covering process the shear plane will always appear outside the GCL, since the efficient shear tensions load the fibre reinforcement, which has in the peel test a peel strength of at least 60 N/10 cm(quality assurance value), only to a low degree. 3 INSTALLATION AND INSTALLATION DETATI.,S The construction schedule was very tight and approx. 700,000 m2 GCL had to be installed in a rapid manner. Up to 14,000 m2 per day were installed, the installation being nearly independent on the weather conditions. The delivered rolls were rolled out using a spreader bar attached to a wheeled front loader (Figure 6 ). Figure 7 shows how the GCL was rolled out on a transverse separating dam integrated into the sealing area which divides the individual IDS sections and thus helps to control them separately. The overlap areas were sealed with bentonite paste. To ensure the daily installation rate also for the sealing of the overlap areas, a big mobile machine for mixing the paste and a special pump with a feed pipe and a specially designed outlet nozzle for the coating of the overlap areas was developed and used. Figure 8 shows this machine in service. Figure 9 shows the use of the special nozzle which catches and seals the overlap areas in a single working processs. Figure I 0 shows a turned up seam with a uniform paste sealing of the overlap area. The connection of the sealing system to the concrete structure of the runways required special care with respect to design and realization. The GCL was butt-jointed to the macadam (aggegrate, hydraulically bonded) of the runway on the slope of

168

Geosynthetic clay liners

Figure 6. Installation of the GCL with a spreader bar.

Figure 7. The GCL is rolled out on a transverse sand dam.

Groundwater protection using a GCL at the Franz-Josef-Strauss airport

Figure 8. Machine for mixing, pumping and installing the bentonite paste into the overlap area.

Figure 9. Sealing the overlap areas with bentonite paste.

Figure I0. Controlling an overlap area.

l69

170

Geosynthetic clay liners 0.50 m

0.35 m

overlap :? 0.40 m

/

Bentofix L=1.20 m

bituminous grouting ace. to bit. Fug. on undercoat, 1 em thick

n

grouting of the seam between Bentofix GCL and concrete of the pavement with bituminous grouting Figure II . Connection of the sealing system to the concrete construction of the runways.

Figure 12. Connecting the GCL to the concrete construction of the runways.

the earth works. A GCL overlapping strip was rolled into a joint grouting bed according to TL-bit.-Fug. in the macadam edges and additionally connected with a grouting of the joint to the runway concrete. Figure ll shows details. Figure 12 gives an impression of the practical execution of the connection works to the concrete construction of the runways.

Groundwater protection using a CCL at the Franz-Josef-Strauss airport

4

•

mm

t

•

•

•

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Figure I. Construction of Geomembrane Supported GCL

Table I. Forms supplied and typical properties Standard form Geomembrane

HDPE

0.5

mm

Covering

Na-bentonite

4.9

kg!m1

Width of roll

5.3

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Length of roll

61.0

m

Weight per roU

1.790

kg

Typical properties Effective hydraulic conductivity

no measurable conductivity

Coefficient of permeability, geomembrane (ASTM E96)

2.7

Coefficient of permeability, bentonite (ASTM 05084)


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This will be reported on separately, however, Figure 5 illustrates the concern ove coarse gravel backfill materials. It proved possible to install the GCL with great accuracy, because of the optimized boundary formation, the geometrically even installation surface and the short sheet lengths. The overlapping was therefore reduced from 40 em to 30 em in standard cross-section in water protection zone lll. Even in the very rainy autumn of 1992, installing the GCL proved exceptionally trouble-free. The insta.lled sections were covered daily with gravel, so moistening never reached the point where the uncovered GCL had hydrated so much that the section had to be replaced. Examinations carried out by the test office for Geotechnics, Soil and Rock Mechanics of the Technical University, Munich showed, as did experience during construction work, that GCL installation is also possible during conditions of frost, down to approximately -10°C. This is, however, dependent on having a dry frost. 5 CONCLUSION In the roadway cross-section designed for the use of specific materials to meet the requirements of the water authority, both the novel drainage system and the use of GCL have shown themselves to be effective, economical and, in practical construction terms, an exceptionally advantageous system for ground water protection. It is to be expected that this 'Leutkirch System' will be used at other locations. REFERENCES Abwassertechnische Yereinigung (ATV). Arbeitsblan A 142. 1992. Abwasserkanale und -leitungen in Wassergewinnungsgebieten. St. Augustin, Oktober 1992.

206

Geosynthetic clay liners

Floss, R. & Heyer, D. 1992-1994. Prlijberichte des Prlifamtsfiir Grundbau, Bodenmechanik wui Felsmechanik der TV Miinchen u.nd GuJachten mit der Pro). Nr. 10373 im Auftrag der Bau/eitung Wangen des Landesamts fiir StrajJenwesen Baden-Wiintemberg. (unpublished). Forschungsgesellschaft fiir Stra.Ben- und Verkehrswesen. 1982. Richtlinienfiir bautechnische MajJnah.m en an StrajJen in Wassergewinnungsgebieten (RiStWag). Ausgabe 1982. Heyer. D. 1992. Eignungsuntersuchungen fiir eine Verbunddichtungsmatte aus Geotextilien und Bentonit. 2. KongrejJ Kunststoffe in der Geotechnik K-G£0 92, Luzem. 20-22 Mai 1992, Schwei;;erischer Verband der Geotexti/fach/eute.

Measurement and control system for the upper basin of the Reisach-Rabenleite pumped storage power station M. Rau & J. Dressler EDR, Munich, Gennany

ABSTRACT: The Energieversorgung Ostbayern AG (OBAG) operates the Jansen pumped storage facility in the southeastern part of Germany. The upper storage reservoir, called the Rabenleite, is currently being rehabilitated. The main difficulty with the existing seal was the lack of an effective control system. Therefore, a measurement and control system was one of the important points in the development of rehabilitation work. The measurement and control system contains the following features: • subdivision of the reservoir into control sections • installation of geosynthetic clay liners for proofing of the subsoil • collection of drainage water within each control section • measurement of drainage water in an inspection gallery • transmission of data to the control center in the power plant. This paper presents the design and construction details of the measurement and control system.

1 GENERAL The Energieversorgung Ostbayern AG (Energy Authority for Eastern Bavaria, or OBAG) operates the Jansen pumped storage scheme in the southeastern part of Germany. This scheme was put into operation in the period 1951 to 1955. The heart of the system is the Reisach-Rabenleite pumped storage power station. Its upper basin, the Rabenleite elevated storage basin, is undergoing a general rehabilitation after an operating period of nearly 40 years. The Rabenleite elevated storage basin was built on the top of the Rabenleite mountain, approximately 180 m above the Pfreimd river, where the lower basin and the Reisach power station are located (Figure 1). The summit of the mountain was removed for the construction of the upper storage basin, the soil material being used as an enclosing dam. The solid bedrock consists mainly of gneiss, intersected by granite seams, some of them of large size. Near the base of the elevated storage basin, the gneiss and granite are interspersed by several kaolin fissures up to 2 m wide, which in the past resulted in recurring damage to the seal of the reservoir bottom.

207

208

Geosynthetic clay liners Rabenleite elevated storage basin Tanzmuhle

Eulengrund tunnel Approx. 2. 7 km

I'

-tApprox. 180m Reisach power stationTrausnitz barrage

lApprox. Weinberg tunnel lReisach tunnel l 1.6 km Approx. 1.2 km

,

1

1

Figure I. General plan and longitudinal cross-section of the Jansen pumped storage scheme.

2 PREVIOUS CONSTRUCTION OF THE UPPER STORAGE BASIN SEAL AND MEANS OF CHECKING The installed slope seal consisted of unreinforced concrete slabs, sized 7 x 7 m and 20 em thick, placed on concrete beams, with an intermediate bituminous sealing system. A so-called ·mammoth skin', consisting largely of several layers of bituminously adhered roofing pasteboard, had been developed to seal the base. Because of the considerable alternation of stress due to pumped storage operation, with water level differences of over 15m, the sealing system had incurred major damage after almost 40 years of service. The concrete slabs at the slopes were mostly cracked, and the mammoth skin also showed considerable damage and signs of aging. Checks were carried out on the elevated storage basin by observing the ground water levels and by using seepage water collection troughs at the downstream face of the dam. The quantity of seepage water was indirectly deterrruned by container measurement, with the elevated storage basin being filled and the level observed over a period of 48 hours of operating stoppage. By monitoring the amount the decreased water level, and allowing for precipitation and evaporation, the amount of seepage water could be calculated. In recent years, the results of these container measurements showed a large increase

Measurement and control system for the upper basin ofthe power station

209

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Natural

Sodium Bentonites Available

• Contaminant Resistant Polypropylene Geotextiles • Activated with Water and Most Leachates • Custom Engineered Designs Available • Simple Seaming Technologies Used • Durable and Economical VOLCLA Y UMITED SCOTISQUAYS BIRKENHEAD MERSEYSIDE L41 1 FB Tel: 0151-638 0967 Fax: 0151 - 638 7000

COLLOID ENVIRONMENTAL TECHNOLOGIES COMPANY 1500 Wesl Shure Dnve • Arlington Hei;

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BLANKETS· · ENTONitE · B . .._: . _, . . .

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The~Vcintag"~s ·; ·.·.' .Of Bentonite '· · With POlyethylene · :: ·· Membrane· . · · : ..., · Bac;king. . bl~nket has

a perme2biliry rated at 4 x 10 -•z

bentonite , · . Gundseal

·. I

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c:m/sec., at least 100 times less

than olhcr bentonilc blankrts. This

pcnltcability is equivalent to a

thickness of much more than 3 ft.

• '

of compaCted clay. • No geotextile interferes with com­

PQiiie liner action. The bentonite

. layer provided by Gundseal

de liven truly "intimate COIIIJICt"

under a synthetic liiJU. . ··

• The prcscoce of a membrane · backing separates the bentonite ··

from moisture in underlying

soils, kccpiitg tbe bentonite dry

aJ?'f SJSblc .

. ii.The 17.5-fr.-widc, membrane­

. bllckrd iolls of 6 und,c;U speed

. . instaUation·a,nil proteCt the benton­ ite from installation damage. . • Gundscal can be applied as a sub- ,

grade replacioj trucked in clay or installed as a clay lay.:r in CQID·

posite dcsigns or by itself.

It becomes clear-specifying Gundsc-.ol lets you "roll out the clay," and im·

prove your _contaioment n~s .

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Gunclle Ljnng Systems Inc

Gu(ti)dl@

"lF IT NEEDS LINING, IT NEEDS GUN OLE: '

19103 Gundk Road Houston, 1aas 77073 U. S.A.

Phoce: (7 13) 4-43-8564 lOll Free: (800) 435-2008

"Wcx: 166657 Gundlo:Hou

Fu: (713) 875-6010

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Geosynthetic clay liners

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11 Welham Road · Gt. Bowden · Market Harborough

· Leics · LE16 7HS · Tef.IF~: 0658-463648

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P.O. Box 1262 • D-48705 Gesc:Mr/Germany

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P.O. Box 1262 • D-48705 Gesc:Mr/Germany TeL 49125421701·0 ·Fax 49125421701 :r7 P.O. Box 1262 • D-48705 Gesc:Mr/Germany TeL 49125421701·0 ·Fax 49125421701 :r7 P.O. Box 1262 • D-48705 Gesc:Mr/Germany

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P.O. Box 1262 • D-48705 Gesc:Mr/Germany TeL 49125421701·0 ·Fax 49125421701 :r7 P.O. Box 1262 • D-48705 Gesc:Mr/Germany TeL 49125421701·0 ·Fax 49125421701 :r7 P.O. Box 1262 • D-48705 Gesc:Mr/Germany TeL 49125421701·0 ·Fax 49125421701 :r7 P.O. Box 1262 • D-48705 Gesc:Mr/Germany TeL 49125421701·0 ·Fax 49125421701 :r7 P.O. Box 1262 • D-48705 Gesc:Mr/Germany TeL 49125421701·0 ·Fax 49125421701 :r7 P.O. Box 1262 • D-48705 Gesc:Mr/Germany TeL 49125421701·0 ·Fax 49125421701 :r7 P.O. Box 1262 • D-48705 Gesc:Mr/Germany TeL 49125421701·0 ·Fax 49125421701 :r7 P.O. Box 1262 • D-48705 Gesc:Mr/Germany TeL 49125421701·0 ·Fax 49125421701 :r7 P.O. Box 1262 • D-48705 Gesc:Mr/Germany TeL 49125421701·0 ·Fax 49125421701 :r7

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