Earthworks: Theory to Practice - Design and Construction 1032104708, 9781032104706

Citation preview

Earthworks

Case studies are used to show how theory is applied in practice. In the design and construction process, various models are used – geotechnical, laboratory, analytical, delivery, and economic models as the project is developed from planning to construction. This book explores the use and limitations of these earthwork models to be understood and appropriately applied. This book evolved from an earthworks course to practicing engineers over a ­10-year period. Theory alone is not enough. Experience alone without relating back to theory can sometimes be misleading if transferred without understanding the fundamentals. The book benefited from the experiences of those many practicing engineers and the author’s experience in multi-disciplinary consulting companies as well as specialist geotechnical companies and government departments. The basics of soil, rock and compaction mechanics as applied to field conditions are covered. Material typically not covered in other textbooks, include the applications and limitations of associated “standard” laboratory and field testing. Specific chapters are dedicated to excavation, subgrade and expansive clay assessment and treatment. Useful design practices as well as the development and application of specifications is covered. A specification, test or design in one climatic condition or geology may not apply in another.

Earthworks Theory to Practice – Design and Construction

Burt G. Look, OAM

Cover image: Burt G. Look First published 2023 by CRC Press/Balkema Schipholweg 107C, 2316 XC Leiden, The Netherlands e-mail: [email protected] w w w.routledge.com – w w w.taylorandfrancis.com CRC Press/Balkema is an imprint of the Taylor & Francis Group, an informa business © 2023 Burt G. Look The right of Burt G. Look to be identified as author of this work has been asserted in accordance with sections 77 and 78 of the Copyright, Designs and Patents Act 1988. All rights reserved. No part of this book may be reprinted or reproduced or utilised in any form or by any electronic, mechanical, or other means, now known or hereafter invented, including photocopying and recording, or in any information storage or retrieval system, without permission in writing from the publishers. Although all care is taken to ensure integrity and the quality of this publication and the information herein, no responsibility is assumed by the publishers nor the author for any damage to the property or persons as a result of operation or use of this publication and/or the information contained herein. Library of Congress Cataloging-in- Publication Data Names: Look, Burt, author. Title: Earthworks : theory to practice — design and construction / Burt G. Look, OAM. Description: First edition. | Boca Raton : CRC Press, Taylor & Francis Group, [2022] | Includes bibliographical references and index. Identifiers: LCCN 2022004454 (print) | LCCN 2022004455 (ebook) | ISBN 9781032104706 (hbk) | ISBN 9781032104713 (pbk) | ISBN 9781003215486 (ebk) Subjects: LCSH: Earthwork. Classification: LCC TA715 L66 2022 (print) | LCC TA715 (ebook) | DDC 624.1/5 — dc23/eng/20220527 LC record available at https://lccn.loc.gov/2022004454 LC ebook record available at https://lccn.loc.gov/2022004455 ISBN: 978 -1- 032-10470 - 6 (hbk) ISBN: 978 -1- 032-10471-3 (pbk) ISBN: 978 -1- 003-21548 - 6 (ebk) DOI: 10.1201/9781003215486 Typeset in Times New Roman by codeMantra

Contents

Preface

xiii

1 Introduction 2 1.1  1.2  1.3  1.4 

Introduction 2 Why an earthworks book 4 A short history of earthworks 8 Ground models 12 1.4.1  Geological model 14 1.4.2  Geotechnical model 16 1.4.3  Earthworks model 20 1.5  Earthworks cost 24 1.6  The business of geotechnical engineering 26 1.7  Case study – Geological model for a deep basement excavation 28 1.8  Summary 30

2 Site investigation 36 2.1  Influence of the ground 36 2.2  Planning and staging of a SI 38 2.2.1  Depth of SI 42 2.2.2  Extent of investigation 44 2.2.3  Sampling 44 2.3  Field work of SI 46 2.3.1  Deep investigation 50 2.3.2  Shallow investigation and subgrade assessment 52 2.4  Testing variation 54 2.4.1  Shallow foundations 54 2.4.2  Deep foundations 58 2.4.3  Counting blows 58 2.4.4  Energy transfer 62 2.4.5  N-value strength varies with geology 64 2.4.6  High and low SPT values 66 2.5  Case study 1 – No geotechnical investigation 66 2.6  Case study 2 – Auger and cored drilling 68 2.7  Summary 70

vi Contents

3 Site safety 74 3.1  Site-safety awareness 74 3.2  Failure of trenches 76 3.2.1  Temporary supports and slopes 80 3.3  General safety considerations 82 3.4  Operating plant 84 3.5  Safe work method statement 86 3.6  Case study 1 – Sink hole failure from pile installation 88 3.7  Case study 2 – Incorrect as-constructed services drawings 90 3.8  Case study 3 – Slope failures 92 3.9  Summary 92

4 Phase relationships and soil classification 94 4.1  Soil elements and classification 94 4.2  Phase definitions 96 4.3  Soil types 100 4.3.1  Water retention 100 4.4  Soil classification 100 4.4.1  Gradings 102 4.4.2  Atterberg limits 108 4.5  Engineering use chart 112 4.6  Case study – Gradings pre and post compaction 114 4.7  Summary 114

5 Theory of compaction 116 5.1  Introduction 116 5.2  Mechanics of densification 118 5.2.1  Theory of compaction 118 5.2.2  Compactive effort 120 5.2.3  Compaction curves for different materials 128 5.3  Strength from compaction 130 5.4  Sample preparation 130 5.5  Field versus laboratory compaction 134 5.5.1  Oversize correction 138 5.6  CBR test 140 5.7  Compactor performance in the field 140 5.8  Case study 1 – Importance of curing times 142 5.9  Case study 2 – Representative sampling 142 5.10 Summary 144

6 Soil and rock strength 6.1  Introduction to soil and rock types  148 6.2  Rock types  150 6.3  Soil types  150

148

Contents vii

6.4  Types of soil strength  152 6.4.1 Critical strength 156 6.4.2 Residual strength 156 6.4.3  Compaction induced strength  156 6.5  Classification of clay strength  158 6.6  Classification of strength of granular soils  160 6.6.1  Standard penetration test  164 6.6.2  Dynamic cone penetration test  164 6.6.3  Cone penetration test  166 6.7  California bearing ratio  168 6.7.1  Interaction with underlying layer  170 6.7.2  Laboratory versus field conditions  172 6.7.3 CBR soaking 172 6.7.4  CBR from DCP test  174 6.8  Various methods of subgrade investigation  176 6.8.1  Plate load test  176 6.8.2  DCP to estimate modulus  178 6.8.3  LFWD to estimate modulus  180 6.9  Rock properties  182 6.9.1 Rock weathering 182 6.9.2 Rock strength 184 6.9.3 Rock modulus 184 6.10 Degradable materials 188 6.11  Case study 1 – CBR subgrade assessment  192 6.12  Case study 2 – SPT field values  194 6.13 Summary 196

7 The compaction process 198 7.1  Prequel to compaction 198 7.2  Principles of compaction equipment 200 7.2.1 Number of passes and lift thickness 204 7.2.2 Travel speed 208 7.3  Targeted moisture content 210 7.3.1  Water required for compaction 214 7.4  Productivity of compaction plant 216 7.5  Influence depth 218 7.6  Compaction equipment 220 7.6.1 Small-sized equipment 220 7.6.2 Large-sized equipment 222 7.6.3 Impact compaction 224 7.7  Deep compaction 228 7.8  Case study 1 – Targeted field moisture ratios 228 7.9  Case study 2 – Laboratory testing variation 230 7.10 Case study 3 – Effect of roller type: dynamic force monitoring 232 7.11 Summary 236

viii Contents

8 Excavations and bulking 240 8.1  8.2  8.3  8.4 

Introduction 240 Definition of rock in contract documents 242 Excavation equipment 244 Open excavation assessment 248 8.4.1  Excavation assessment based on rock mass rating 250 8.4.2  Excavation assessment based on seismic wave velocities 250 8.4.3  Excavation assessment based on various ratings 250 8.4.4  Excavation assessment based on production rates 250 8.5  Equipment balance 252 8.5.1  Plant output 258 8.6  Confined space excavation assessment 258 8.6.1  Diggability index 258 8.6.2  Trench, shaft, and tunnel excavations in rock 262 8.7  Bulking factors 262 8.8  Case study 1 – Unit weight of excavated material placed as fill 266 8.9  Case study 2 – Variation of material through a cutting 268 8.10 Summary 270

9 Slope stability in cuttings and embankments 9.1  Introduction 272 9.2  Causes of slope failure  272 9.3  Quantitative risk analysis  274 9.3.1  Landslides as compared with other hazard events  276 9.3.2  The perception of risk  278 9.3.3  Case study of landslides with varying consequences  280 9.4  Factors of safety  284 9.4.1  Factors of safety for new slopes  284 9.4.2  Factors of safety for existing slopes  286 9.4.3  Factors of safety based on consequences class  288 9.4.4  Factors of safety for dam walls  288 9.5  Typical slopes for cuttings and embankments  290 9.5.1 Rock slopes 294 9.5.2  Rock cut stabilisation measures  296 9.6  Soil erodibility  298 9.6.1 Erodibility hierarchy 300 9.6.2 Erosion control 300 9.6.3  Benching of slopes  302 9.7  Case study 1 – Mechanisms of landslide failures  302 9.8  Case study 2 – Riverbank failure  304 9.9  Case study 3 – Landslide zonation by GIS analysis  304 9.10  Summary 308

272

Contents ix

10 Expansive soils 310 10.1  Introduction 310 10.1.1  Pavement design and distress 312 10.2  Cost of damage 316 10.3  Mechanical damage from tree roots 318 10.4  Volume change behaviour 320 10.4.1  Index tests 322 10.4.2  Embankments and cuttings 324 10.5  Calculation of movement using the shrink – swell index 326 10.6  Weighted plasticity index (WPI) for residual soils 328 10.7  Soil suction and saturation 332 10.8  Relationship of WPI with CBR test 338 10.9  Compaction 344 10.10  Design CBR 348 10.11  Equilibrium moisture content compaction 350 10.11.1  Index parameters which indicate the seasonal changes 352 10.12  Swell pressure tests for assessment of stable zone 354 10.13  Zonal use of expansive clay 356 10.14  Effect of trees on ground movement 358 10.15  Case study 1 – Long-term monitoring of existing embankments 360 10.15.1  Trial embankment 362 10.15.2  Construction monitoring 364 10.15.3  Key considerations 366 10.16  Case study 2 – Effect of desiccation cracks on modulus 366 10.17 Summary 368

11 Subgrades 11.1  Introduction 370 11.2  Sampling survey  372 11.3  Subgrade considerations  374 11.3.1  Site investigation versus construction requirements  376 11.4  Analytical proof of subgrade depth  378 11.4.1 Boussinesq analysis 380 11.4.2  Finite element analysis  380 11.4.3  Hertz contact mechanics  382 11.5  Proof rolling for subgrade assessment  386 11.5.1  Tyred equipment for proof rolling tests  388 11.5.2  Rollers for proof rolling tests  388 11.6  Rail track permissible pressure on the formation  388 11.7  Case study – Subgrades for heavy loads  390 11.8  Summary 392

370

x Contents

12 Improved subgrades

396

12.1 Introduction  396 12.2 Remove and replace  400 12.2.1 Design basis for R&R   402 12.3 In-­situ stabilisation  404 12.3.1 Lime stabilisation 406 12.3.2 Cement stabilisation 408 12.3.3 Soil stabilisation with bitumen  410 12.4 Geosynthetics  410 12.4.1 Geotextiles for separation and reinforcement  412 12.4.2 Establishing geotextile strength class  414 12.4.3 Geotextile strength class for horizontal and vertical placement  414 12.4.4 Establishing geotextile strength class adjacent to walls and slopes  416 12.4.5  Geotextile overlap 418 12.4.6 Geogrids for subgrade improvement  418 12.4.7 Bearing capacity factors using geotextiles  418 12.4.8 Modulus improvements with geosynthetic inclusions  420 12.4.9 Geotextiles as a soil filter  420 12.5 Working platforms  422 12.5.1 Subgrade testing 424 12.5.2 BR470 design considerations  424 12.5.3 Adjacent to a slope  426 12.5.4  Platform maintenance 426 12.5.5 Track bearing pressure  428 12.5.6 Platform material 428 12.5.7 Design alternative using geotextiles  430 12.6 Case study 1 – Adjacent to a creek  430 12.7 Case study 2 – Dredged sand subgrade over very soft clays  432 12.7.1  Approach 436 12.7.2 Track pressure loads  438 12.7.3 Geotechnical parameters 438 12.7.4 Risk based analysis  440 12.7.5 Acceptable displacement criterion  442 12.7.6 Allowable stress criterion  442 12.7.7 Analysis summary 444 12.7.8 Proof rolling deflections  444 12.8 Case study 3 – Lime stabilisation and a reinforced soil slope  446 12.9 Summary  448

13 Design considerations 450 13.1  Introduction 450 13.2  Embankment considerations 456 13.3  Factors of safety for slopes 460 13.3.1  Factors of safety for new and existing slopes 462 13.4  Probability of failure 462

Contents xi

13.5  Stable slope batters 464 13.6  Embankment foundations 464 13.7  Foundation movements 468 13.7.1  Immediate to total settlements 468 13.7.2  Free surface movements for light buildings 468 13.7.3  Free surface movements for road pavements 468 13.7.4  Tolerable deflection for proof rolling 470 13.7.5  Rail track deformations 470 13.7.6  Road surface movements on compressible soils 470 13.7.7  Differential settlement of reinforced soil structures 472 13.8  Design value – risk based 472 13.9  Typical CBR values 476 13.10  Applying CBR values 480 13.11  Design interface with hydraulics 482 13.12  Case study 1 – Back-analysis of a failed slope 484 13.13  Case study 2 – Design detailing and analysis input 490 13.14 Summary 494

14 Construction considerations 500 14.1  Introduction 500 14.2  Quality control 502 14.3  Specifications 504   14.3.1  Characteristic values 508   14.3.2  Frequency of testing 512   14.3.3  Specification development 512   14.3.4  Effect of climate and geology 518   14.3.5  Effect of traffic 518 14.4  Blending 520 14.5  Rock specifications for roadway embankment fills 522 14.6  Rock durability 526 14.7  Ballast grading 530 14.8  Backfill for buried pipes 532 14.9  Observation and instrumentation 534 14.10  The zero air voids line 538 14.11  Compaction specifications 540 14.12  Non-density quality control 546 14.13  Case study 1 – Uneven rock surface 548 14.14  Case study 2 – Earthworks tender considerations 550 14.15  Case study 3 – Spatial variation and blending 552 14.16 Summary 554

Permissions Abbreviations References Index

558 563 567 583

Preface

This book was derived from my course notes compiled over 10 years of presenting a 2-day course specific to “Earthworks” for Education Engineers Australia. The earthworks course was designed for practicing civil engineers, site inspectors, and project managers with some experience in earthworks and was not a geotechnical engineering course. Those with little to no experience did not benefit as much as those with some field and project experience. The course evolved from on-the-job issues and was refined through discussions with attendees. A case study approach was adopted in the course to examine everyday design and construction practices where the application of theory in its purest form was not self-evident. This book utilises a similar approach – diagrams and tables alternate with case studies to provide the context. The emphasis is on the application of theory to practice in both design and construction. This text documents my experiences over 40 years as a practicing engineer and incorporates issues raised by my colleagues over the years. An inspector or project manager requires a different level of detail to that of a design engineer – both aspects are considered in this text. There are many excellent earthworks books that provide more comprehensive details on theoretical earthworks aspects. Preface Tables 0.1 and 0.2 provide a list and overview of key references for texts and standards, respectively. Apologies to those who acquired this book (hopefully, legally) and, one page into the text are swiftly being referred to read other publications for details. As stated previously, this text aims to link the theory with practice. To that end, some of the “basics” are widely and competently addressed and available elsewhere, and my brief treatment of them does not diminish their importance. Knowledge and understanding come from many sources, and I believe those texts are useful supplements on the broad topic of earthworks. The profession has developed excellent working models, rules of thumb and specifications drawn from experiences accumulated over the years. These heuristic techniques assist in making time-sensitive decisions, but also lead us into complacency – a “that’s the way we do it” approach, even void of rationalisation. In general, many practicing construction engineers discover theory may not match practice. This is a troubling situation – researchers are continuously enhancing our knowledge but there is an ever-widening gap in its application. This means earthworks practice significantly lags the state of knowledge, and is decades from state of the art. Fortunately, this does not deter leaders in the field who continue to present theoretical and experimental developments. Unfortunately, these state-of-the-art “science” developments do not trickle down into practice. The motivation of this text is to help fill the lacuna between theory and practice.

xiv Preface

The simplistic approach of using the compaction curve of maximum dry density (MDD) and optimum moisture content (OMC) as a goal is challenged. This is the current teaching position on compaction – a necessary starting point and reference but it ignores consideration of many specifics that enable such fundamentals to be appropriately applied in practice. This is not new: Holtz (1990) stated, “… MDD is only a maximum for a specific, compactive effort and method of compaction. This does not necessarily reflect the MDD that can be obtained in the field”. Many are wedded to this compaction theory and are critical of any suggestion this is not how compaction works in the field. Some may mistakenly believe the validity of these fundamentals is being questioned. I clarify, I do advocate and do not question such theories being taught as the fundamentals in all basic courses. I have much admiration and respect for the pioneering work of Proctor et al. However, I do advocate that, alongside the theory, examples should be provided to illustrate how the theory is applied in practice. We will then avoid the binary discussion of those for or against fundamental compaction theory, or the practice vs academic positions. Examples of how “must teach” fundamentals do not match practice. In the first year of primary school, we were taught one can only subtract a smaller number from a larger number. Essential for learning subtraction, but everyone with a credit card or mortgage, acknowledges a larger number can be subtracted from a smaller number. We were also taught 1 + 1 = 2. In secondary school we were introduced to binary addition (1 + 1 = 10), and vector addition (1 + 1 cos 45° = 1.707). Site inspectors and construction workers seldom write technical papers or attend conferences. Without that feedback, construction practice may seem disconnected from theory. Thus, this book aims to connect earthworks theory with practice by providing examples of inconsistencies that occur on a regular basis. While I do hope to develop the understanding of practicing earthworks engineers, hope must be tempered with realism. The subject matter is so broad that I must omit some theoretical details (well covered in several texts) on many related aspects of soils and rock mechanics, site investigation and geotechnical analysis to keep this text earthworks focused. Simplification is intentional, yet each project may present its own complex details. In particular, and while not specifically identified in its title, this text places more emphasis on residual soils and Australian experiences. There are many fundamental concepts that apply directly to transported soils but should not be applied directly to residual soils. Similarly, a universal earthworks specification is not possible though we often strive to apply specifications universally. In Australia, specifications differ between States to reflect local conditions. It is unwise to automatically apply an Australian specification to a European site just as it is ill-advised to unthinkingly apply a European specification directly to an Australian location. This situation is amplified in Australia where there is a large immigrant engineering population employing their overseas experience which may not always be directly applicable to the Australian environment. The Eurocodes recognise these location variabilities where member countries adopt variations to suit local practices. Earthworks practice then relies on an understanding of local specifications and standards. To state the obvious, a construction procedure used in a semi-arid environment should not be applied to temperate or tropical climates. Yet this concept is not widely applied.

Preface xv

I have taken a conversation-like approach in parts of this text to keep the reading interest. We can often take life too seriously and presenting technical data in its raw form can be boring (no pun intended) to fundamental geotechnical engineers. I would like to acknowledge the many volunteers who proof-read the many technical papers that contribute to the knowledge base of all engineers. I learn much from such feedback. Those insights have been incorporated in my 60+ peer-reviewed technical papers on earthworks (mainly from a practitioner viewpoint) and I have liberally included some extracts from my papers in this text. I have used photos and extracts of reports taken from work colleagues. I acknowledge using material from joint projects and discussions with Adam Kemp, Dr David Lacey, and Jared Priddle. There are many others who have contributed to the content of this book through the questions they raised, and solutions sought that arose from many diverse projects over the years; industry practice constantly challenges, and there is much to be learned from each project, in every week. Feedback and comments from many practicing engineers (1,000+) who attended the Earthworks courses provided valuable input. I acknowledge all the contributors for sharing of their thoughts, dilemmas, and experiences in this fascinating area of engineering. I have replicated diagrams and tables and referenced them accordingly. In some cases, modification of the original referenced material was carried out to simplify or expand that concept. Dennis Tiong assisted in the illustrative diagrams. Dr Mogana Sundaram and Natalie Campbell also provided feedback to some chapters of this text and their efforts are appreciated. So many are time poor with life and their day job when asked to provide feedback. Most importantly, I would also like to acknowledge my wife Gina Look, an ­accountant by profession, who took many months and leave from her work to edit, format, and review this text with her eye for detail, and to question my meaning. I am grateful for her patience, diligence, and support when motivation was lagging, as I o ­ ften questioned whether this project was worth the effort. That assistance was so ­vital, as even when the manuscript was completed, there was the onerous requirement of the 100s of hours to summarise and then gain permission for use of every table and figure, even when it was from my own previously published work. The many ­permissions are tabled at the end of the book and gratefully acknowledged. Burt G Look November 2021

xvi Preface Table 0.1  Useful earthworks texts R. Holtz – Editor (1990). Guide To Earthwork Construction State of the Art Report No. 8, Transportation Research Board

L . Forssblad, (1981). Vibratory soil and rock f ill compaction. Dynapac Maskin AB. N. A . Trenter (2001) and updated by P. Nowak and P. Gilbert (2015). Earthworks: A Guide. 2nd Edition. Telford Publishing.

I. Vanicek and M. Vanicek (2008). Earth Structures: in Transport, Water and Environmental Engineering. Springer Publishers. M.R. Hausmann (1990). Engineering Principles of Ground Modif ication. McGraw Hill Publishers. J. Burland, T. Chapman, H. Skinner, M. Brown (2012). ICE Manual of Geotechnical Engineering – 2 vol set.

L . I. Gonzalez de Vallejo and M. Ferrer (2011). Geological Engineering. CRC Press.

• A basic text of 107 pages. A useful overview of earthwork construction. This is the minimum level of awareness for anyone associated with earthworks. 1990 “State of the Art” is slightly outdated with only minor updates, but still extremely useful. • Visit TRB site for free download http://www.trb. org/main/blurbs/153713.aspx • Practical handbook approach, in its 175 pages, for anyone involved in construction of earthworks. • Simply explained and illustrated. • A comprehensive text (2001) of 257 pages → updated to 360 pages (2015). Useful theory, design and practice guide on earthworks for design engineers. • Outlines the essential earthworks processes and theoretical background to the compaction process and available f ills. • A comprehensive text of 637 pages for design engineers. Useful design and practice guide on earth structures. • Wide application and examples as per the title e.g., earth structures in transport engineering. • 631 of geotechnical gold. But not earthworks focused. Well organised and readable text with insights not found elsewhere. • An excellent and essential text for earthworks design engineers. • A comprehensive two volume resource of 1,537 pages, focused on geotechnical engineering. There are a few chapters specif ic to earthworks. • Provides the core geotechnical engineering principles that engineers should keep in mind when dealing with real-world engineering challenges. • 678 pages of essential geology in the design and construction of infrastructure. Well illustrated. • Integrates geological conditions into engineering design and construction. Practical Geology applications to engineering.

Preface xvii Table 0.2  Useful earthworks standards and texts on residual/unsaturated soils Australian Standard 3798 (2007) Earthworks.

Austroads, Guide to Pavement Technology (2009), Part 4I: Earthworks material. AS1289 (Various years), Methods of testing soils for engineering purposes.

Quality Control of Soil Compaction – Using ASTM Standards (2011).

BS6031:2009 Code of practice for earthworks.

J.L Briaud (2013). Geotechnical Engineering: Unsaturated and Saturated Soils. John Wiley & Sons, Inc.

G.E. Blight and C. Leong (Editors, 2012). Mechanics of Residual Soils. CRC Press.

• Specific to commercial and residential developments. Useful basic introduction to “Standards”. Qualifies that specialist advice is required when the depth of filling exceeds 5 m. • Not intended to be used for pavements, major earthworks, or water –retaining structures. • 43-page document for roads. Overview level for construction engineers. • Emphasis on material properties. • Various standards for testing. Essential for an earthworks engineer to gain an understanding of applicable tests. However, testing details are not directly useful to most. • While generally similar, a few “Standards” have minor differences to the British or American equivalent. • 95-page document based on USA standards (non-metric). Covers both lab and f ield testing as well as overall applicability of various construction equipment. • Useful overview for construction engineers and inspectors. • 128 pages; updates the 1981 version which has served the industry well, and is in line with Eurocodes. • Planning, design, construction, management and decommissioning of unreinforced earthworks. • Comprehensive 1022 pages to cover both saturated and unsaturated soil mechanics. • Covers both fundamentals and useful analytical and design applications of soil mechanics to a wide variety of topics, including key details of advanced topics such as erosion, earthquake and geosynthetics in a very practical manner. • 392 pages specif ic to residual soil characteristics and engineering properties which can differ signif icantly from those of the more familiar transported soils. • Practical professional guide for engineers working with residual soils. Sections on unsaturated soil mechanics and instructive case histories.

To those who keep me grounded: Gina, Matthew and Alexander

Chapter 1

Introduction

1.1  INTRODUCTION Earthworks is the process where the earth (soil, rock, and artificial materials) is excavated, transported to, and placed at another location. This may be for use as an earth dam, a road or rail embankment, or as platforms for buildings. Most developments, such as large commercial or industrial building sites, typically require site levelling. To minimise earthwork costs, the design would use a balanced cut-and-fill approach. For roadwork projects, the earthworks component represents a significant cost factor. It typically comprises 20% of direct costs and indirectly affects other costs such as temporary works, retention systems, foundations, and pavements. This is a highly variable cost that varies depending on the type, size, and location of project, and costs can increase if ground improvements or difficult rock excavations are required. The fill can be obtained from a longitudinal cut to embankment fill (Figure 1.1-1) or from a side long cut to fill (Figure 1.1-2). Suitable side slopes need to be assessed; stable cut slopes for embankments differ from those required for slope cuttings, despite being of similar material. Benches may be required for stable slopes when the height increases above 6 m. This requirement also applies to embankment fills, both externally and internally, where the fill joins with the natural ground. Figure 1.1-2 illustrates the internal benching in a side long fill. The vertical alignment is set by: • • • • • • •

The cut-to-fill balance longitudinally Land ownership of the corridor Flood levels to fix over water bridge or culvert levels and lead up embankment heights Minimum heights below bridge crossings at overpass (typically 5.5 m clearance) Connections to the existing infrastructure Drainage requirements Vehicle type and volume usage

DOI: 10.1201/9781003215486 -1

Introduction 3

Figure 1.1-1  Terms and definitions: longitudinal cut to fill.

Figure 1.1-2  Terms and definitions: side long cut to fill.

4 Earthworks

In the transverse sections, stable slopes are required. These slopes do not directly contribute to vehicle or rail movement, but the associated corridor space is a necessary expense and needs to be minimised. Where the required slopes are not possible, retaining walls are used: • • •

To minimise land resumptions Where road grades taper towards each other and the slopes become excessive To reduce the interaction effects on adjacent services

The materials used may be soil and rock, as well as processed materials, such as pavements or industrial waste (e.g., fly ash or recycled concrete). When the in-situ materials are unsuitable, the material would need to be improved (stabilised) or imported materials would be required. Earthworks include excavations for deep basements (cut only), mines, and canals (cut and fill). Vanicek and Vanicek (2008) suggest that “Earthworks” is limited in its meaning, while “Earth Structures” include the earthworks process and a wider interpretation of its application. In its common usage, the word “earthworks” covers both the process and the structures. Earthworks and earth structures are reported in this book. An understanding of geomechanics is essential for earthworks design and construction (Figure 1.2-1) as well as equipment use, and the testing required. Some diagrams and tables in this and other chapters have been taken from Look (2014). 1.2  WHY AN EARTHWORKS BOOK Soil and rock are the most abundant (inexpensive) construction materials available. There is, therefore, a benefit to understand the engineering properties of these extensively available materials. These earthwork notes are developed for practitioners, so the emphasis is on applications in various areas (Figure 1.2-2). These include earth structures in the areas of: • • •

Transportation infrastructure – engineered fills and cuttings for roads and rail Buildings – building pads and industrial sites with cut-and-fill platforms Water and environmental – pipelines, canals, dams, and landfills

Regardless of the type of application (buildings, transport, etc.), they each have many common geotechnical considerations, and the general phases of a geotechnical project equally apply – geotechnical investigation, site assessment, ground improvement, excavatability, slope stability, retaining structures, and quality control, as shown in Figure 1.2-2. There are not many formal courses on earthworks, and this topic is not usually included in an undergraduate engineering or geology degree. University syllabuses for engineering and geology degrees may include a few lectures on compaction. This is not enough. Higher-level university courses may include aspects in dams or ground improvement or pavement engineering, but few provide specific lectures on earthworks. This book fills this gap but is not intended to be a specialist geotechnical engineering

Introduction 5

Figure 1.2-1  Earthwork elements.

Geotechnical investigation

Site assessment

Ground improvement

Excavatability

Slope stability

Retaining structures

Quality control testing

Buildings

Transport

Water

Environmental

Platforms

Roadways

Canals

Land fills and waste disposal

Land development

Railways

Levees

Airports Deep basements

Reclamations

Ports and harbours Pipelines and tunnels

Tailing dams Earth and rock fill dams Marine

Mine waste

Figure 1.2-2  E arthwork applications showing common aspects across the industry specific area.

6 Earthworks

text, rather a working reference for non-specialists in the earthworks field – civil designers and constructors as well as project engineers and site supervisors. Due to the breadth of earthworks-related skills learned on the job, the practitioner may forget and/or discard the fundamentals learned in formal education as too basic or academic and fall into the trap of relying only on experience from job to job. This experiential approach is a useful starting point, but if used only, it may fail to identify when an uneconomical (conservative) process is occurring, or worse, when a nonconservative (risky) process is occurring. This may result in financial loss or risk to life. The more subtle condition of serviceability failure, which is a long-term failure of performance for its intended purpose, can result in additional maintenance costs or risks to the users. One must be careful not to convert experience to general rules. Typical examples are: •

•

A contractor (often a qualified engineer wearing their contractor’s hat) suggests a modification of a design slope based on “their experience” at a previous project cutting where a steeper slope was used. This modification is suggested on the basis that it will save time (and money) in reducing the excess cut material. The previous slope experience may have been for a different geological material and/or a different geological structure, or a different height slope and/or a different risk exposure; previous experience is inadequate. At a particular location, slopes can vary from one side of a cutting to the next, even for the same material. This is determined by the type of material, height, joints, and dip of the different materials. Figure 1.2-3 illustrates where different slopes apply on each side of a roadway, even for a given material. “Experience” suggests that compaction to a high density is desirable as it improves strength of the subgrade, so constructing to high densities across a site must be a good practice. When the same compacted density for a road is applied to the landscape area, the ground is over-compacted and unsuitable for planting; plants may not survive after a few months because the ground becomes too impermeable (over-­ compacted) for root growth to occur or for water to penetrate. A lower compaction density is required in such areas. Figure 1.2-4 shows the compaction levels applicable to landscape areas, base of embankment subgrades and pavements.

These examples illustrate how experience used without consulting theory can be misleading. The opposite also follows, theoretical applications without consulting the experience of others, and without having regard to local practices and standards, may lead to an inefficient design. Many related topics impact earthworks design and construction such as: • • • • • • • • •

Engineering geology, rock mechanics, and site investigation Geotechnical assessment and site characterisation Ground improvements Slope stability and retaining structures Detailed testing standards The range of equipment used in construction Pavements – rigid and flexible Dams, dredging and water structures Mining and tunnelling

Introduction 7

Figure 1.2-3  Stable slopes in different materials.

Figure 1.2- 4  Compaction varies with intent of earth placed.

8 Earthworks

Only the fundamentals of these topics are covered in this book, with little attention given to the last two, as the focus of this text is on road infrastructure. One must therefore consult detailed references, such as those recommended in the preface or cited in the references, for a more complete understanding in those areas. For example, refer Thompson et al. (2019) for mine haul roads where special purpose off-road trucks (exceeding legal road limits) are used. Potholes and rutting serviceability criteria are different for legal road vehicles. Embankment dams are covered comprehensively in Fell et al. (2005). Earthworks modelling involves using the existing contoured surfaces to balance cut and fill and minimise haul distances and material to be removed off site (Figure 1.3-1). This assumes that suitable material is available on site. If the earthworks is not in balance, the designer should adjust grade lines to be in balance, allowing for wastage. If suitable material from the adjacent cuts is insufficient, imported material is required. Conversely where there is an excess of excavated material, disposal off site is required. In such cases, it may be more economical to reduce hauling costs by integrating the excess material on site by widening shoulders or placing the material in disposal areas. Setting grades and layouts are the first and most important phase of earthworks design and construction. However, such earthworks alignment details are not covered herein. 1.3  A SHORT HISTORY OF EARTHWORKS This section presents a short history of earthwork developments. Water supply canals and tunnels in Persia (Mesopotamia) date back to 2,800 BC, and irrigation canals and dams date back to 3,200 BC. They represent some of the earlier civil engineering projects. Canals were later employed to shorten ship routes between the Nile and the Red Sea. While several earthen tracks existed in BC times, it is the Roman road network that is best known – they introduced drainage systems and hard-wearing surfaces to resist rain and flooding (Figure 1.3-2). The road widths allowed the movement of horse-drawn vehicles (chariots, carts, and carriages), and the stone surfacing was able to withstand the high-impact wearing of the iron-shod wheels. However, the gradients of 10%–20% in ordinary and mountainous terrain were limited by the earthmoving tools of the day. The Incas had an even longer road network, but as wheeled traffic was not yet in existence, the “standard” of the pavement was less rigorous as it was only required for foot traffic and pack animals. Agricultural tools such as the shovel, hand pick, hoe, plough, and basket (for moving soil or stones) were the construction earthmoving equipment of the day. Animal power, mainly horses and mules, provided additional capacity to dig, move, and even compact the ground. Construction improvements occurred with the introduction of the wheelbarrow; substantial gains were achieved from using baskets to move material by around a factor of ten; previously, less than 2 m3 of material was moved approximately 100 m in a 10-hour working day. The modern-day Bobcat excavator has further transformed earthmoving capabilities, possibly up to a factor of 100.

Introduction 9

Figure 1.3-1  Cut to fill (longitudinal section).

Figure 1.3-2  Elements of Roman roads compared with current road layers.

In one of our family banters, my sons who work in the health profession, remarked at the number of lives they helped that day. I retorted, “my” foundations enabled 30,000 cars to safely cross the bridge and more than 100,000 persons benefited just today from the water pipeline project in which I was involved; clean, potable water and engineered sewerage systems provide arguably greater health benefits than direct medical interventions. A safe road can be considered “preventative” medicine and is another of the many benefits engineers provide to society (I am also proud of the lives that benefit from my sons’ direct work).

10 Earthworks

Developments constructed on the ground have been mainly forged by trial and error, with a rather broad understanding of “good” and “bad” ground. The implications contained in the statement “Upon this rock I will build my Church” were well understood at the time, even by the layperson – the concept of good (strong) ground and bad (weak) ground was being conveyed. Nowadays, that representation of rock strength does not always hold true, as failure on rock can be high when additional strength potential is extracted by stressing the rock to higher loads. Figure 1.3-3 summarises some key compaction and soil mechanics historical events. A few noteworthy projects are mentioned below. The industrial age brought a greater need to transport and connect; in the UK, extensive canal networks were developed for transporting goods and materials between 1760 and 1830. In the early 1800s, Telford and MacAdam provided a scientific approach to roads. Coulomb and Rankine proposed concepts of earth pressures for retaining wall designs in the eighteenth and nineteenth centuries, respectively. At the same time, geology as a science was evolving. By 1982, Cauchy had introduced the theory of elasticity, and the concept of stress (force per unit area) and strain led to other developments, with Boussinesq providing the fundamental theories on stresses from vertical load by the late nineteenth century. Graphical methods of representing stress relationships were introduced by Culmann (1866) and Mohr (1882). Note that Cauchy was an engineer who is best known for his works in mathematics, and Mohr’s contribution was in both the strength of materials and the theory of structures. None of their work was specific to geotechnical engineering, yet these pioneering contributions to mathematical models are the basis of modern-day numerical geotechnical analysis. The construction of railroads in the 1830s required flatter grades and alignments, bringing into play an associated risk level that was new to the time, and consequently was not considered, evaluated, or incorporated into the design. After several landslide disasters resulting in fatalities, the Swedish Railways established a Royal Commission in 1923 to investigate the cause of failure. The key technology for the analysis and understanding of landslides started with the Swedish Circular Arc Method, and issues of embankment and cuts stability developed (Figure 1.3-3). The Panama Canal was started by the French in 1880 and by 1889 the project was only quarter completed; it was abandoned due to costs, time overruns and deaths (25,000 mainly from tropical diseases of typhoid, cholera, smallpox, malaria, and yellow fever – 70% of those infected died). Medical technology had to develop further before construction could progress (Figure 1.3-3). The USA resumed work on the Panama Canal in 1904, and the canal was opened in 1914. Close collaboration with doctors reduced deaths from the above diseases; this major civil engineering project prompted medical developments with an understanding of tropical diseases and mosquito-borne infections that emerged from this civil engineering earthworks project. Excavatability, landslide, and shale degradation understanding developed as The American Society of Civil Engineers formed committees to investigate the problems of massive slope failures. Below-ground understandings were occurring with the construction of the London Underground (1860s–1940s). Engineering geology had also developed as a science at this time.

Introduction 11

Compacting placed soils with animals ... BC • Light horse drawn rollers ... BC  AD • Wheel barrow was used in China from the 3rd century for transporting military supplies, although there was earlier historical evidence of the wheel barrow being used to move people. Introduced in Europe circa 17th century. Steam powered rollers • Late 1850s: Construction of railroads – flatter grades and alignments

Slope stability – Swedish Railways: 40 lives lost after several landslides • Royal commission 1923: Swedish circular arc method of analysis and development of concepts related to embankment and cut stability • Panama Canal - started by the French 1880–1889 with only quarter completed. Abandoned due to costs and time overruns, and 25,000 dead mainly from tropical diseases : USA resumed work 1904 – 1913.

Theory of consolidation (do not confuse with compaction) • Terzaghi (1925)

Theory of compaction – OMC concept ... Proctor (1933) • California Bearing Ratio ... Porter (1938)

Figure 1.3-3  A brief history of compaction and soil mechanics.

2800 BC

1930s

Soil Mechanics

Civil Engineering

1970s

AD 1700s

Rock Mechanics

Figure 1.3- 4  The geotechnical evolution.

Geology

Early 1900s

Since 1990s

Engineering Geology

Specialisations in •Environmental Geotechnics •Hydrogeology / Dams •Tunnelling •Pavements •Analytical Geomechanics •Deep Foundations •Ground Improvement •Mining

12 Earthworks

Major highway systems developed from 1950 onwards, allowing greater vehicle speed, which required flatter grades and, consequently, higher embankments and deeper cuttings. Rail lines require even flatter grades, hence greater cut-to-fill volumes for such alignments (Figure 1.3-1). Cut-to-fill balance minimises the importing of material and its associated costs. The use of somewhat less-than-desirable material represents a significant earthworks consideration. Compaction concepts have been understood since BC times. Steam-powered rollers permitted greater quantities of fill to be compacted. Compaction mechanics principles were introduced by the California Division of Highways (1920s–1930s), following the mechanisation of earthmoving equipment. These were: • •

Proctor’s (1933) findings – MDD and OMC concepts. Independently, Kelso ­performed similar experiments during the construction of the Silvan Dam in ­Victoria, Australia. Porter’s (1938) California Bearing Ratio – an index of bearing strength.

Key concepts on consolidation and effective stress (fundamental principles of soil mechanics) were being put forward at that time by Karl Terzaghi. Casagrande and Atterberg’s procedures on soil classification provided a framework to associate properties with certain types of soils. Figure 1.3-4 depicts the geotechnical evolution as the science moved into an engineering discipline. Soil mechanics only became a science in the 1930s and rock mechanics in the 1970s. The profession continues to evolve with speciality areas such as analytical geomechanics and environmental geotechnics. The latter has grown in emphasis in recent times and does not diminish the principle that civil engineers should always seek to provide sustainable and environmentally-friendly solutions in developments. One must also be conscious of factors outside core engineering considerations and their possible impacts on the project. 1.4  GROUND MODELS Models (whether numerical, financial, or laboratory used in testing), are simplifications of the real world and contain several in-built assumptions. The constructed models representing simplified conditions allow us to solve complex physical-world problems. Models contain many assumptions and limitations, and we must be alert to the degree of divergence between the model and real-life conditions. The model requires verification to ensure the model was constructed accurately. The model is also validated against real-world conditions. Figure 1.4-1 shows how a variable ground is translated across to a simplified geological model with “uniform” strata derived from the site investigation. Similarly, laboratory and load models are simplified to “one” number. This is then used in the analytical model. Sampling disturbance needs to be minimised during the field investigation. The sample tested in the laboratory has its confining stress re-applied to represent the confinement on site, with the expectation this also reduces part of the sampling effects. The applied load over the load expected on the sample now represents a laboratory model.

Introduction 13

Geological Model

Laboratory Model

Load Model

Analytical Model

Figure 1.4 -1  Analytical models and their sub-models (assumptions) (Look, 2014

*P1).

Table 1.1  Ground models (Look, 2014 *P1) Model

Real life

Depth of thickness of strata

Interpolated/extrapolated between/from test locations and accuracy is only at such locations which could be 20 m or over 50 m apart. On-site sample selection may unintentionally ignore large sizes as they are too heavy, or they cannot be used in the laboratory classification tests. Additional sample material is removed when performing tests for the same reason. The quantity removed may have significant implications for the applicability of the test results, especially in residual soils. Even in a homogeneous layer, no two samples are likely to produce the same test result – a design value is adopted which allows for such variations. Assumes no sample disturbance. Highly variable and depends on time of year. Can also vary with change of land use either at or in surrounding sites. Lower bound value or moderately conservative values may be used. Different factors of safety apply. Requires a minimum volume (say five data points, but ideally above 20) of testing to arrive at such values. An analysis without such site-specific testing data and using universal correlations, may not be reliable.

Laboratory classification tests

Strength parameters from field or laboratory tests Ground water during site investigations Design value

14 Earthworks

The load was derived from a combination of effects such as the wind, intended use, likely material and dimensions used for construction, etc. Thus, the analytical model used for settlement or bearing capacity calculations has many of these model assumptions. Table 1.1 presents some of the underlying assumptions contained in ground models. The use of an appropriate safety factor (or geotechnical strength reduction factor) should therefore be used to account for the various model uncertainties. A safety factor applied varies with the project risk and its consequences. A safety factor is a decision-making tool and not an end to itself. The Factor of Safety (FS) in the traditional “working stress” design accounts for uncertainties in: • • • • • • • •

Loading and type of structure Ground conditions — type and variability Extent of site investigation (SI) Consequences of failure Temporary versus permanent exposure Probability of failure Construction uncertainty Input assumptions e.g., lowest credible value versus moderately conservative or typical value

Groundwater models are also part of the ground model. Understanding the site geology is integral to any ground model.

A “safety factor” is a politically correct word as clients can accept that terminology rather than the same explanation associated with an equivalent term “ignorance factor” which is what is being applied.

1.4.1  Geological model The development of ground models precedes any analytical models. Work can be done by an engineering geologist or geotechnical engineer in some but not all cases. To develop the geological model, one must obtain site-specific, historical, and regional data. Using this model, the key ground-related issues can be classified and identified. The geologist would work with the geotechnical engineer to develop a site-specific solution. Terrain unit classification and assessment form part of this process for longitudinal projects. The case study at the end of this chapter will examine the effects and consequences of an inappropriate geological model. Stratigraphy models are developed as part of the ground model. Plans and sections are 2D models; a 3D model should be developed for major projects. The soil origin is obtained from geology maps as well as from site and material observations. This information provides valuable insights that feed into design. Soils are broadly classified as either transported or residual soils. Figure 1.4-2 illustrates the typical ground stratigraphy for transported soils and underlying residual soils. It would be incorrect to classify any soil origin as uniform. An alluvial soil may have an upper, middle, or lower alluvium, which may be firm clay,

Introduction 15

Figure 1.4 -2  Illustration of various soil origins.

4,600 - 2 million years 10,000 years Geological 500 - 1,000 years • Time of Holocene 50 - 100 years planet earth • Last glacial Geological to man 10 - 25 years period appearing in stability Life of • 60 m sea Quaternary level change • Earthquakes permanent Life of period • Floods structures maintained • Buildings structures • Bridges

• Pavements • Mine slopes

Figure 1.4 -3  Time scales.

Anthropogenic (man-made) fill, land, and climate changes suggest another classification age. However, while the Anthropocene is a proposed geological epoch based on human impact, this has not been formalised.

16 Earthworks

loose sand, or stiff clay, respectively. A geotechnical model attempts to classify each uniform material as a distinct stratigraphic unit for analysis purposes. However, each unit may have a different geological origin. Transported soils have more distinct layers than residual soils which grade into the parent rock material without a distinctive change. The presence of significant amounts of coarse-grained particles (stones) in residual soils means the classification based on plasticity index (PI) may be misleading. This is discussed further in later chapters. Even a bedrock has different properties with depth, although the rock type is not clearly layered. The rock may grade from a residual soil at the surface to an extremely weathered (XW) rock (soil-like in its description) to fresh rock (no weathering at depth). Each grade of weathering then has an associated strength with its structure governing rather than its intact strength. This weathering classification provides a method to develop the geotechnical model. Both the recent site history and geological history should be understood ­(Figure 1.4-3). The time scales demand special mention as geological stability cannot be assured, and the life of any earth structure earth must be considered part of the design. Maintenance is also required during its design life. Soils formed in the Holocene epoch have a different structure than for any other geological time. Our time horizon is often too limited to see changes as they occur. For example, a mountain may be considered stable, but in a longer time scale of several million years, that stable entity was the result of dynamic processes of both uplift and erosion. The time of exposure is also important for design and construction. A permanent slope (of say 100-years design life) is not assessed in the same way as a temporary slope required only for a few weeks. An extremely weathered rock such as phyllite or claystone may be “hard” when tested in situ; however, when exposed during an excavation it can quickly degrade to a weak material. The exposure time can significantly change the material’s strength properties. Recent anthropogenic fill and changes require their own soil classification.

1.4.2  Geotechnical model Characterisation of the ground is developed for the various stratigraphy units and may involve statistical models if sufficient data is available. This is usually performed by a geotechnical engineer (Figure 1.4-4) but may be conducted by an engineering geologist in some, but not all cases. Geotechnical engineering is a specialisation within the civil engineering discipline and is focused on the understanding and effect of ground conditions on the proposed development. The building blocks of a geotechnical engineer are shown in Figure 1.4-5. A trained civil engineer must learn and develop in the areas of engineering geology and rock mechanics while acquiring engineering skills in site investigation, analysis, and design. It would be usual for a geotechnical engineer to be more aligned to one side of the triangle, i.e., emphasis on analysis rather than site investigation or vice versa. A postgraduate qualification in geotechnical engineering assists in advancing this knowledge base; there are also many targeted industry short courses which are useful. Irrespective, the key to knowledge and development is to regularly attend local events, short courses, webinars, and conferences. Time-based-only learning has an

Introduction 17

Figure 1.4 - 4  Geological and geotechnical models (Look, 2014

*P1).

Figure 1.4 -5  Building blocks for a geotechnical engineer (Look, 2014

*P1).

18 Earthworks

inherent “learning from experiences and mistakes” philosophy; formal training has the advantage of learning from the experiences and mistakes of others. Geotechnical engineering is the branch of civil engineering concerned with the engineering behaviour of earth materials. Geotechnical engineering is not simply the sum of soil mechanics + rock mechanics as closely allied terminology and overlapping fields of expertise include geo-environmental engineering, geological engineering, and engineering geology. In the late twentieth century, geotechnical engineering was considered a specialisation. Yet further specialisation is occurring within the “broad” field of geotechnical engineering. One must be wary of over specialisation though geology and civil engineering are the building blocks of a geotechnical engineer. That said, a worrying trend observed in civil engineering university degree courses is the broadening of the curriculum to include minor elective subjects (from the Arts say) at the expense of removing directly relevant subjects (such as Engineering Geology) which results in skills gaps. That debate is left to educators. An earthworks engineer is a subset of a geotechnical engineer and can be its own speciality – no different from those who specialise in dams, pavements, foundations, or any of the many specialist areas. Figure 1.4-6 illustrates the fundamental skills and training required in the development of a geotechnical engineer. Figure 1.2-1 showed the three key elements of “Earthworks”: 1. Geomechanics 2. Earth structures 3. Earth process Regardless of experience, testing to an acceptable level is required before designing the geotechnical models (parameters for design). These models recognise and provide site constraints e.g., slopes or allowable bearing capacity, and suggest ground solutions. Geotechnical models therefore develop from: • • • • • •

Terrain classification and assessment Ground characterisation Statistical models Analytical and design models Numerical models (if required) Documentation with drawings and specifications

Each of these requires some model development. There is an unfortunate trend to substitute numerical models for actual site data (refer to text box on verification and validation). For example, a budget that only caters for an analytical model using assumed parameters rather than the budget allowing (i.e., allocating expenditure) for these parameters to be obtained through a site investigation sacrifices the ground characterisation model. “What if” analysis and sensitivity models alone are not a substitute for absent site investigation or test data. Geotechnical reporting can involve:

Introduction 19

Figure 1.4 - 6  Interactions of a geotechnical engineer across disciplines.

The terms “verification” and “validation” are often used ­interchangeably – this is incorrect. Validation is the process to ensure that the model is representing the real world as much as possible. The validation process is performed after verification. The key question for verification is whether the model was built correctly. The key question for validation is whether the correct model was built. After validation, one should be confident that the model represents the real-world process and can be used to predict the behaviours of a process. Validation relies on the data collected and checks whether the model represents the real-world conditions to a sufficient degree of accuracy to be useable.

20 Earthworks

• • • • •

A desktop study report with existing data and the geological setting. A factual site investigation report. This includes a description of the work carried out, the borehole, test pit and other in-situ test data and associated laboratory tests. An assessment report which can be integrated with the factual report. There is an interpretation on recommended slopes and types of foundations, and design parameters for the design of pavements, foundations and retaining walls. A geotechnical baseline report (GBR) which forms part of a contract document. Factual data is included, and any assessment downgraded to a technical note. The GBR applies only after the design is advanced to at least a 50% level. A design report which provides the basis of the design. This will include the ground characterisation, analytical and design models used.

Be wary of reports that assume values rather than provide test values. Even when tests are carried out, a copy and paste assessment from a previous report may bypass such actual test results. Correlations from index tests are an extremely useful first approximation for preliminary assessment and validation of other test values but are not a substitute for a more thorough testing of critical layers.

1.4.3  Earthworks model Compaction theory represents only one aspect of a multi-faceted consideration in earthworks engineering. The understanding of the principles of soil and compaction mechanics and phase relations is essential base knowledge. Some engineers associate earthworks with the compaction process only; although a core activity, this is a very narrow perspective. Compaction is often the word associated with earthworks, but the transporting and quality of material are also significant components of the compaction process. Key elements of the earthworks process are presented in Table 1.2 and Figure 1.4-7. This is not an all-inclusive list as temporary works such as trenching, slopes, and working platforms are also important. Where excess borrow material occurs on a project, a steep slope is desirable to minimise cutting. Conversely, when limited fill is available then a flatter cut slope may be beneficial – provided land acquisition cost do not govern. These elements, shown in Table 1.2, cover: 1. Site investigation 2. Earthworks design 3. Earthworks process Aspects of retaining walls and slope stability are related elements. Later chapters use the basics explored here to discuss the interaction of these elements with the earthworks design and process. An earthworks engineer requires additional geotechnical subject matter due to interactions with civil, pavements, and dam engineers (Figure 1.4-6). Each is its own discipline requiring specialised knowledge of their topic though all fields are anchored in geotechnical engineering. One should acknowledge the design constraints and limitations of other disciplines and work towards the common goal of a cost-effective delivery.

Introduction 21 Table 1.2  Earthworks elements Activities

Earthwork elements

Site investigation

Safety and approvals Geological setting Terrain classification and terrain assessment Appropriate field and laboratory testing Stratigraphy Site characterisation Cut and fill balance Interactions with adjacent property or infrastructure Material specifications Slopes and retention systems Ground improvements; long-term changes after construction Buried structures (buried pipes, geosynthetics) Compaction and quality control Statistical representation Testing standards Excavation, transport, and placement; Equipment Temporary works (working platforms, trenching) Remove and replace (“soft” spots)

Earthworks design

Earthworks process

Excavaon

Grubbing inially

Transport

Loose state

Bulking

Placement

Loose soils  Low density

Low strength

Compact

Increases density

Quality Control

Test

Some materials may be difficult to remove

Strength ▲ Compressibility ▼

Destroys natural structure

Compressible

Moisture Density relaonship

Evaluate results

Figure 1.4 -7  T he earthworks process. Compaction is a key consideration, but other factors are part of earthworks assessment.

22 Earthworks

Consider the three competing design perspectives. The civil design (roads/alignment) engineer may prefer a balanced cut and fill which may result in high embankments on an alluvial flood plain. The hydraulics engineer’s preference is for the embankment to be as high as possible to attain a suitable height above the one in 100-year storm event. The geotechnical engineer’s requirement is for a low embankment due to soft ground issues. In this example, if the final design leans towards the perspectives of the civil design and hydraulics engineers, the geotechnical engineer may need to consider additional ground improvement options, which comes, of course, with additional “geotechnical” costs. If the hydraulics does not require a high embankment, then it is incumbent upon the road engineer to reduce the vertical alignment at this location. An optimal cost for balanced cut and fill without consideration of a high ground improvement cost is not an optimal project cost. A best-for-project solution dictates a meeting of the disciplines – the civil design, hydraulics, and geotechnical engineers balancing their specific requirements with overall costs. The geotechnical design of excavations and fills requires consideration of: • • • •

Foundations and settlement Stability of cuts and fills (slopes and volume stability) Excavatability Retention systems and temporary stability

Furthermore, these considerations must be factored for other design elements such as balanced quantities (cut and fill as well as bulk up on excavation). For example, a slope design is based on slope stability analysis (a model), but this represents only one of many components (models) that impact the final design of this rock slope. An appropriate rock fall, kinematic or global analysis is a fundamental requirement in slope design, but land resumptions, the material excavatability, its properties, and the quantity of fill required are other important considerations in ­determining an acceptable slope requirement. Figure 1.4-8 illustrates the case where slope analysis, though important, is only one element in the slope design. The limitations of testing must be recognised. For example, basic laboratory tests require some form of standardisation for the tests to be repeatable. Yet no two persons are likely to produce the same results, even with testing standards. This can be due to natural soil variations and/or operator variability. Standards reduce that variability. To appropriately use test results, earthwork engineers require an understanding of laboratory “models”, their assumptions, and limitations. A few common testing misinterpretations, and discussed in later chapters, are: •

A high plasticity index (PI) indicates a soil is highly sensitive to moisture content and may be classified as a highly expansive clay. Based on the geological origin of the soil and the testing procedure, one would not necessarily classify a residual clay with a high PI as expansive. Sixty percent of such soils typically contain particles larger than the 425-μm sieve. Atterberg tests only use the material passing the 425-μm sieve. With such a test discounting more than 50% of the material suggests

Introduction 23

Figure 1.4 - 8  R ock slope design procedure. Slope stability is an important factor, but other factors may govern. (Madhava et al., 2010 *P2 ).

Table 1.3  Relative cost of earthworks – projects based in Queensland, Australia Type of project

Services

New roadway 10 km Minor with four bridges

Temporary works

Construction element

Relative cost

Minor

Pavements (150,000 t) Earthworks (330,000 m 3 ) Unsuitable Bridges (4 No.) Seal and A .C. Bridges Earthworks Drainage Seal and A .C. Noise walls/landscaping Pavements Earthworks Ground improvement

20% 20% 14% 13% 6% 23% 18% 16% 9% 6% + 4% 3% 9% 4%

Duplication of 6 km ring road with an interchange near residential areas

Some 2% Traffic management 7%

Road widening of a major highway

Some

Transportation of material in chainage • 1.0

Significant

Transportation of imported material to site

Transportation of unwanted material off site

• ~ 1.1

• ~ 1.3

Work done by machinery • ~ 2.8

Figure 1.4 -9  Earthworks carbon emission relative effect. (After Pantelidou et al., 2012.)

24 Earthworks

• •

that high PI value is not representative of the bulk of the residual clay material. This will be discussed in detail in Chapter 10. Similarly, during a compaction test, oversize particles are discarded as part of the standard test. If the volume discarded exceeds 20% then the compaction test may not be valid. Density and moisture content corrections are required. A clay sample not prepared properly (i.e., over several days) prior to a 4-day soaked California Bearing Ratio (CBR) test will give different CBR values compared to a sample that has been appropriately prepared.

1.5  EARTHWORKS COST The earthworks cost component of any project varies considerably. This cost is usually significant for any new roadworks project, but less so for rehabilitation or urban projects where the cost of structures may dominate. Temporary works cost is significant when construction is for a widening or a highly trafficked corridor. The costs of services protection and relocation are also significant. Table 1.3 provides an indication of relative earthworks cost for Queensland, Australia projects and illustrates the cost variability associated with each type of roadway project. Geotechnical engineering aspects associated with bridge foundations, pavements, and ground improvements are additional costs. Pantelidou et al. (2012) provide a case study of a major 24 km highway construction and compared the CO2 contribution of various elements. Earthwork emissions were calculated to be less than those from the structures and pavement construction. The embodied carbon attributable to each of the components of this construction ­example was: • • •

Structures and Foundations – 52% Pavements – 42% Earthworks – 6%

Most of the emissions/energy consumption are locked in the construction materials used (production of concrete, steel, asphalt, etc. from raw materials) with small contributions from the transportation to and from site, and the associated construction work on site. Figure 1.4-9 illustrates the relative earthwork carbon emissions. These values are expected to vary from project to project. Sustainability considerations advocate the use of in-situ earthwork materials and the reduction of material removal off site, thereby reducing carbon emissions during construction. However, efficient use of machinery during the process governs. Figure 1.4-10 shows the relative construction costs with ground conditions which are based on the soil and rock strengths. Low soil strength has associated ground improvement costs.

Introduction 25

Figure 1.4 -10  Effect of ground strength on relative cost of construction.

Figure 1.5-1  Geotechnical input into various phase of a project with key elements.

26 Earthworks

1.6  THE BUSINESS OF GEOTECHNICAL ENGINEERING The geotechnical engineering business can be divided into three predominant ­components – professional (consultancy), field testing, and laboratory testing. These components may be combined/offered by companies providing geotechnical services i.e., consultancy with laboratory testing, or field and laboratory testing. The type and level of geotechnical input are aligned with the requirements of each phase of the project delivery – what is mandatory in a later phase may not be essential now. Throughout the development and preliminary engineering phases, the geological model for the site is critical; detailed testing and analysis are less crucial (Figure 1.5-1). Conversely, the use of analytical software is more important in the detailed design phase and less so during the development model phase. Ideally, a two-phase geotechnical site investigation (SI) is required as analytical and design models progressively develop (refer Chapter 2). Geotechnical design integrates the models into drawings and specifications. A geotechnical engineer may be involved in a project as an asset owner’s representative, a civil design consultant with geotechnical capability, a geotechnical consultant, or in a testing company. In each case, the emphasis and objectives are different. ­Figure 1.5-2 depicts the business of geotechnical engineering where: • • • •

A testing company may be more concerned with 1,600 PI or 1,000 compaction quality tests during construction to earn income. This is a testing-business focus. A geotechnical or civil consultant will focus on attaining 1,600 chargeable hours per year. This is an assessment-business focus. A drilling company will focus on 1,600 m drilled per year – an equipment-business focus. A design engineer is interested in geo-integration by producing 160 drawings per year. This is a drafting-business focus.

An excellent geotechnical consultant focused on producing SI reports may not be able to produce adequate geotechnical designs and vice versa. The consultant focused on design may use software as the main tool, while the SI consultant uses testing equipment. Bonaparte (2012) provides further discussion on the business of geotechnical engineering, including future directions and career trajectories in this field. Lab testing companies employ mainly technicians while consulting companies focus on engineers and engineering geologists. An array of business models exists; some companies specialise in one aspect, say pile testing, while others offer a more extensive range of services, for example design as well as site investigation. It is not unusual for talented analytical design engineers to be out of their comfort zone when producing design details for construction drawings. Likewise, there are many competent site investigation engineers who are unable to carry out numerical analysis. It is not uncommon in recent years to find a modelling engineer unable to classify samples on site. Similarly, an excellent practicing engineer does not automatically make one an excellent lecturer in engineering studies, and vice versa.

Introduction 27

Figure 1.5-2  The business of geotechnical engineering.

Figure 1.6 -1  Geological model can affect design and construction analysis.

28 Earthworks

1.7  CASE STUDY – GEOLOGICAL MODEL FOR A DEEP BASEMENT EXCAVATION This case study highlights how models develop and the importance of having a geological model as subsequent geotechnical, analytical, design and construction models are affected. Figure 1.6-1 shows the stratigraphy used for a seepage analysis for a deep basement excavation model using in-situ tests in a simplified one-layer ground model. Later ground investigation data developed the model further into a two-layer model with an upper alluvium of mainly clays and a lower alluvium of interbedded sand and clays. The sand varied with each borehole – very loose in the middle of the site and dense at all the corners. In-situ permeability tests were carried out, and these tests formed the basis of the design for likely water inflow into the basement during construction. Particle size analysis was also used to estimate the permeability. Boreholes and cone penetration tests (CPTs) were external to the building site due to access limitations for an existing structure. This resulted in a two-dimensional stratigraphy for the geotechnical model. A sensitivity analysis was carried out to assess the likely upper 95% confidence level flow rates. However, this still failed to correctly predict the flows from the temporary excavation wells during the deep basement excavation. Changes in basement and wall levels, and the types of walls also affected the predicted flow rates. The effect of an underlying gravel layer was also incorporated as a three-layer model (Figure 1.6-2). These were all contributing factors in their own right – but were not THE governing factors. In a relative sense, the analytical model did not use the correct ground model (Figure 1.7-1). In this deep basement case study, the geological model had not been properly developed to identify the paleochannel at the building location where the excessive flows occurred. In this case study, the models used included: • • • • •

Geological model Geotechnical model Analytical model Design model Construction model

Each model should incorporate key aspects of the earlier developed model. However, design is a dynamic process with changes occurring throughout the project. The geological and geotechnical models are often lumped together as the ground model. The analytical model may include the design model. Analysis reporting is not the same as design when drawings and specifications are required – there are many design details that are not part of analysis. When the design drawings are issued for construction, the design model is complete. The contractor may develop their own model which has some adjustments subject to approval from the design engineer. Project management, cost, and delivery then govern. Since the construction model is two steps removed from the analytical model, some intent/assumptions from the analytical model may not be reflected in the construction drawings.

Introduction 29

Figure 1.6 -2  Geological model incorporates historical land changes perpendicular to river.

Figure 1.7-1  Analytical model and its sub-models (assumptions).

30 Earthworks

The construction model in this case study includes the use of pumps for the temporary lowering of the water level using extraction wells. Employing unsuitable pumps that are unable to meet predicted flow rates (from the geotechnical analysis model) affects the time for construction. This error is not easily corrected because often a “suitable pump” is not an off-the-shelf item and is a significant cost, but more importantly, it leads to time delays during construction if incorrectly sized. In this case study, forensic data-gathering revealed a historical creek that, 150 years prior, ran through the middle of the development site. In recent times, this area is subject to flooding. The creek was infilled in the late nineteenth century, and the area was left undeveloped for another 50 years. That was the paleochannel surface “signature”. The extraction wells for the basements had high flows in the middle of the site but low (and manageable) pumping flows at the corners. The in-situ test data measured the external middle alluvium properties, which were external to the building footprint. An appreciation of the 3D geological model with a paleochannel would have significantly changed the approach formulated mainly from the 2D analytical model. The initial two boreholes in the preliminary phase showed only interbedded medium dense sands for the middle alluvium. Several months later, during the detailed design phase, the additional borehole testing and CPTs suggested local loose sand for the middle alluvium. Nevertheless, the preliminary design parameters were carried forward to the second detailed design phase with only “minor” adjustments and sensitivity checks. More importantly, the geological site model was not fully developed. Figure 1.7-2 shows the various models developed for this building site in the forensic stage. The geological model was missing in the geotechnical and analytical models. Design considerations involve considering the type of wall, the toe of the wall, the basement level, and the surrounding ground conditions. However, analytical models prior to and after the excessive flows failed to predict the flows unless excessively high permeability values were used. These values did not match the soil types or the data from the (corner) pump tests measured before construction. The geology model envelopes the other models. 1.8  SUMMARY This is an overview chapter; specifics become especially important in later sections. Earthworks engineering must combine theory with practice. This is because many of the current ways in which we carry out analysis, design, and construction stem from the historical base of “that’s the way it’s done”, rather than a true appreciation of the uniqueness of each project. There are benefits to using experience, but there are also pitfalls if we fail to update ourselves in current and more appropriate technologies. Key earthwork elements are provided in Figure 1.4-7. They indicate an extension beyond the compaction and earthwork processes. The understanding of the mechanical behaviour of soil or rock is necessary to quantify improvements in the three key properties: strength, deformation, and permeability. The moisture content-density relationship is a core principle in the compaction process. This compaction model is simple, yet significant issues occur if applied

Introduction 31

Geology model Geotechnical model Analycal model

• Geomorphology and topography • Site history (me) • Sedimentaon environment and sequence • Stragraphy (soil layers and thickness) • Ground water levels • Soil properes (permeability) • Wall and basement depths • Type of wall • Predicted flow rates

Figure 1.7-2  Various models developed to assess flow into a basement.

Figure 1.8 -1  Geo-evolution experience. Reduction of hands on technical to residual value.

32 Earthworks

directly rather than being used as a framework. Understanding testing standards is mandatory to know when a test result is applicable. Models are simplification of real-life situations/conditions. During the life of a project, we are continually developing models, whether it be a load path model or a ground and numerical model. This includes our field and laboratory tests, which are classification, or strength and deformation models. Model simplification is required to solve and derive a solution, but one must evaluate and check the developed end-product model against real-world conditions, otherwise our geo-evolution may not be moving forward. Our technical geo-experience may “soften” with time although our ability to look beyond a simplified model and see the big picture may increase (Figure 1.8-1). Other non-technical experiences also increased in later years. Importantly, there is a large volume of technical literature produced yearly that indicates geotechnical areas are still expanding, and with many uncertainties. Yet so little of this research literature seems to advance to “application” (Figure 1.8-2). The  percentages are indicative and observational only from a practicing engineer’s view, but they highlight the issue with technology transfer. Peer-reviewed papers presented at an international conference are not an indicator of industry acceptance. This is disturbing. The papers may be viewed as being either too “academic” or too material/project-specific, but there is also a reluctance on the part of industry to change current practice. However, experience (precedence) is part of the requirements for a successful application. The latter chapters of this book seek to include “new” technology (which is over 20 years old but still not widely adopted in industry practice). This text is intended to bridge that application gap. Theory to practice does not follow a linear path but is an iterative process. At university, one is trained to solve problems and questions set by others. In practice, defining the question is not a simple matter and needs the greatest amount of time. “Text book” answers seldom occur. There are many specialities in the geotechnical subject matter. Investigation, assessment, and analysis are not the same as design. Procedures and specifications help standardise the output. One should aim to minimise the gap between theory and practice, expectations and delivery (Figure 1.8-3). Classification is but another simple model to “solve” common issues. Common usage and practical usefulness of classification are not the same as truth (correctness) because over-simplification deviates from the classification mode. The rush to “solve” and “classify” leads to convergent thinking, which works well for simple projects, but less so for complex projects where divergent thinking is required to explore the key design issues. Soil and rock mechanics principles form the basics to understanding earthworks theory. Residual soils, extremely weathered (XW) and highly weathered (HW) rock are overlap materials. Water affects the performance of all materials (Figure 1.8-4) and is often a governing factor. All of this should be understood against the framework of Engineering Geology. Material properties change during processing (Figure 1.8-5). A case study was used to demonstrate the importance of a geology model and incorporate additional data from a phased investigation approach. While stratigraphy plans are usually developed as a 2D model, a full site assessment may require a 3D geological model.

Introduction 33

Figure 1.8 -2  R esearch evolution: Does this apply to the geo-evolution? Most useful fundamental research seldom reaches industry practice.

Figure 1.8 -3  Earthworks theory to practice framework.

34 Earthworks

Figure 1.8 - 4  S oil and rock mechanics framed by Engineering Geology and water effects.

Figure 1.8 -5  P rocessed fill

Chapter 2

Site investigation

2.1  INFLUENCE OF THE GROUND During the data-gathering phase of any earthworks project, one must obtain survey, property ownership, environmental, hydraulic, and geotechnical information and assess their effects on the project. You pay for a site investigation (SI) whether you have one or not. The SI in the Construction Series (1993) gave this clear message, and it is still relevant 25 years on. It appears this advice has not been heard or heeded. The SI is an upfront cost when it is included as a planned component of the project. Avoiding this cost often leads to even greater costs being incurred when complications arise due to the use of insufficient design data – a situation that could have been avoided with an appropriate SI. The topic of SI is a specialist area, and this chapter aims to illustrate its importance As a guide, I have found the and to highlight some lesser-known aspects additional project costs to be of testing. Understanding applicable tests and greater than 20 times the cost their interpretation is central to a cost-effecof the SI if one had been contive project that has given due consideration ducted or between a poor and a to the relevant geotechnical risks. Clayton good SI. et  al. (1995) and Mayne et al. (2002) are recommended reading resources. This chapter presents a few anomalies that may occur in practice during the SI but are not typically covered in SI texts. Figure 2.1-1 outlines some of the ground-related issues that may occur at a development site: 1. What is a stable cutting slope angle? Is land acquisition required? Are there any services or other structures at the top of the slope? Will the services or structures be relocated, acquired, or retained? Will the design be a retaining wall or soil nail/ anchor system? 2. Can the material be excavated easily? The strength and defects of the rock must be assessed. If the material cannot be removed by standard excavation equipment, significant costs may be added to the project. 3. What types of foundations are required for the building? Shallow foundations are the least cost, while deep foundations (piles) add significant cost to the project. Weak layers may require piled foundations, rafts, or large spread footings. DOI: 10.1201/9781003215486 -2

Site investigation  37

Figure 2.1-1  Geotechnical site considerations (Look, 2014

Impact Assessment Study (IAS)

Planning

Preliminary Design

*P1).

Detailed Design

Construction

Desk study - geology / business model

Site reconnaissance

Terrain classification and assessment Formulate investigation required

Preliminary site investigation - geotechnical model / stratigraphy Depth Thickness Composition of soils and strata

Detailed site investigation - design model Quantitative testing

Figure 2.1-2  Staging of site investigation (Look, 2014

Assess critical or founding strata

*P1).

Site characterisation

38 Earthworks

4. What is the design subgrade strength? The subgrade must be assessed to determine the type of pavement required. The pavement is a significant cost item. Expansive clays are sensitive to changes in moisture content. Stabilisation may be required or removed and replaced within the active zone of movement. 5. Are there any compressible layers? Soft clay, if intersected, has stability and compressibility design issues. The ground settlement must be evaluated if fill is placed. Ground improvement may be required. The effect of settlements on services (pipes), the building, drainage, and roads requires examination. These considerations affect the timing of project delivery and the performance of the infrastructure or structure. There are two approaches for acquiring geotechnical data: 1. Accept the ground conditions as a design element, i.e., based on the structure/development design location and configuration, and then obtain the relevant ground conditions to design for/ against. This is the traditional approach in the civil engineering investigation which contrasts with the mining engineering approach, which is driven by the need to identify the depth and location of key deposits to extract (refer Text box). 2. Geotechnical input throughout the project by planning the structure/development with the ground as a considered input, i.e., the design, layout, and configuration are influenced by the ground conditions. This is the recommended approach for the minimisation of overall project costs. Figure 2.1-2 presents the typical procedure. At each stage, the ground issues may require: 1. Identifying the most suitable sites or parts of a given site that provide an economic or the least environmental impact development. This influence does not occur with a one-stage SI approach. 2. Data to identify ground issues, quantify their effects, and produce solutions. 3. Evaluating the changes that may occur in the ground or surroundings due to the proposed development. 2.2  PLANNING AND STAGING OF A SI SIs are required to assess ground conditions and their impact on the proposed development. The SI should be conducted in phases depending on the stage of the project. This staged SI approach need not translate into additional SI expenditure, but rather the SI budget is applied progressively as the project gathers momentum and circumstances demand investigation. From a financial perspective, spending the entire SI budget, as a lump sum, in the early stages of the project is risky, as a project can be mothballed as a business proposition at any stage of its life (refer Figure 1.5.1). A phased SI approach allows targeted investigation to be undertaken to meet project needs as and when the need arises. Accurate and reliable SI data collected at each phase of the investigation plays no small part in assisting the project to proceed to construction. A three-phase model is illustrated in Figure 2.1-2. The essential considerations are:

Site investigation  39 Table 2.1  Geotechnical involvement at various stages (Look, 2014 Project phase

Feasibility/IAS Planning

Geotechnical study for types of projects Small

Medium

Large

Desktop study/SI

Desktop study

Desktop study Definition of needs

SI

Preliminary SI Detailed SI

Monitoring/inspection

Monitoring/inspection

Preliminary design Detailed design Construction Maintenance

* P1)

Inspection

Inspection

Mining exploration – investigate then decide on location. Planning and design then follow. Civil industry investigation – decide location, then investigate the ground conditions. Early investigation should be used to investigate alternative alignments and locations to influence planning and design.

Figure 2.2-1  Ground-related problems during construction. (Data from Clayton 1991.)

40 Earthworks

1. At the impact assessment (IAS) or planning stage, a desk study and site reconnaissance may be sufficient. Planning may occur before or after IAS, depending on the type of project. Terrain classification and assessment are carried out with reference to geology maps, topographic conditions, and any other existing information. During this stage, the extent and type of ground truthing are formulated based on the geological model. 2. At the preliminary design (or planning) stage, a preliminary SI is carried out to broadly evaluate the types of underlying materials and their likely thicknesses. A geological model is used to develop the site stratigraphy. This is the beginning of the geotechnical model. 3. The detailed SI is undertaken during the detailed design. Not all planned locations are feasible as site access may be dependent upon permission to enter the land, environmental approvals, road closure permits, ability to construct working platforms for drilling rigs for variable topography, or removal of obstructions to access location. Without a staged SI approach, there is an implicit assumption of accepting the ground conditions as a design element. With a staged approach, due consideration is given to the relevant ground conditions at significant phases of the project. The number of stages varies with the size of the project (Table 2.1). In small projects, some stages are combined. The method of delivery may also influence the combining of stages (e.g., in a design and construct). During the first phase, the focus of SI is on the stratigraphy across the site, utilising mainly index laboratory testing. Correlations are used to establish material properties for this preliminary design. Soil characterisation and upper and lower bounds of values govern the second phase. The benefits of a two-phase SI are evident in the following examples: •

•

Moisture content or index tests may be used to assess the likely settlement in preliminary design using correlations, but this approach is unacceptable in detailed design. The expense of more accurate and reliable consolidation tests may not be warranted during the preliminary SI as the precise horizontal or vertical road alignments are not yet fixed, or the position of columns or loads of the building are still at the preliminary engineering stage. A common error is to transfer the parameters obtained from index testing in the preliminary phase to the detailed design phase. This is not an optimal design. A case example of non-optimisation occurred in the design and construct for a major bridge river crossing. Tenderers were advised to use a rock strength of 10 MPa in the design of the rock sockets for tender assessment only and represented a conservative value. The successful tenderer was subsequently required, as part of the contract, to carry out additional boreholes and testing at each bored pile location (24 No. at each bridge pier location). This stipulated over-water SI had a significant cost. The final design value ignored the data collected (a few hundred additional point-load index (PLI) and unconfined compressive strength (UCS) testing) from the mandated SI and instead used the tender-provided estimate of 10 MPa rock strength. Ignoring the detailed phase SI data indicates a reluctance of

Site investigation  41

Figure 2.2-2  Shallow foundation site investigation (Look, 2014

*P1).

Figure 2.2-3  Canal investigation site investigation (Look, 2014

*P1).

42 Earthworks

•

the design engineers to provide their own design value, anchoring the final design to the lower preliminary tender value. Extensive consolidation tests during a one-phase SI in the preliminary design phase may ultimately not be useful if deep foundations are required and does not represent value-for-money testing for any client.

Figure 2.2-1 adds commentary to the data in Clayton (2001), who highlights the ground-related problems during construction with issues segmented as follows: 1. Soil boundaries (22%), and soil properties (20%) 2. Ground water/contamination (24%), obstructions (10%), and SI adequacy (9%) 3. Services (6%), detailed design (5%), and other (4%) The predominant ground-related concerns, while important, are not necessarily the main cost impact on the project. Services-associated issues tend to govern costs. The impact on a project of services and obstructions, while not occurring frequently, may require special attention, particularly in an urban or constrained environment. Services owners may not place relocation as a priority. This usually results in delays or relocation becoming a project cost. When left in place, these services may require bridging or alignment adjustments. Whether the decision is made to relocate or not, managing services is typically associated with large project costs, often due to stringent requirements from the asset owner whose main concern is the integrity of their infrastructure, and with no constraints on cost and time.

2.2.1  Depth of SI In addition to ensuring the number and type of testing, and the quality of sampling, the geotechnical client advisor should check that investigations are carried out to the appropriate depths, related to the type and intent of the project. Typically, investigations should be taken to a depth where the stress influence is less than 10%. This theoretical approach is not directly applied initially when loads or position are unknown but is considered at the final SI stage. The load and foundation type or even the location may not yet be confirmed at the preliminary stage; hence, a 10% influence is a guiding parameter only. This is an iterative process (which does not always happen in practice), i.e., based on the investigation, confirm the foundation width AND the depth of current investigation are adequate. Locating competent material in a preliminary SI is then an objective as the width of the foundation is unknown. The SI targets “competent” strata before terminating the borehole or test pit. The adequacy of the preliminary depth should be confirmed during the final design stage. If competent bedrock is intersected before those depths, there may be a basis for early termination of the borehole. The 3 m depth into “rock” is a check that a boulder or thin layer was not intersected.

Site investigation  43 Table 2.2  Guideline to extent of investigation (earthworks) (Look, 2014

* P1)

Development

Test spacing

Approximate depth of investigation

Embankments

25 –50 m (critical areas) 100 –500 m as in roads

Cut slopes

25 –50 m for H > 5 m 50 –100 m for H  40 m width

At compressible alluvium, near structure zones, alluvium to a depth beyond the base of compressible alluvium at critical loaded/ suspect areas: otherwise as in roads. Five metre below toe of slope or 3 m into bedrock below toe, whichever is shallower. A minimum of 2 m below subgrade level for pavement assessment. Below the slide zone. Frequent test spacing suggests DMTs or CPTs. As a guide, (because the slide zone may be unknown) use 2 × height of slope or width of zone of movement. Five metre below toe of slope or 3 m into bedrock below toe, whichever is shallower. Two metre below formation level for subgrade assessment (refer Chapter 10); deeper if compressible material intersected. Three metre below formation level for subgrade assessment. One metre below invert level for subgrade assessment. Three metre below invert level or one tunnel diameter, whichever is deeper; greater depths when contiguous piles are required for retentions. Target 0.5 –1.5 linear metres drilling per route metre of alignment; lower value over water or difficult to access urban areas. Two × height of dam, 5 m below toe of slope or 3 m into bedrock below toe, whichever is greater; extend to zone of low permeability. Three metre minimum below invert level or to a zone of low permeability. 2B – 4B but below base of compressible layer

One Borehole One at each end One at each end and one in the middle with maximum spacing of 20 m between boreholes Two BHs for  400 parks

Car parks

Notes: In soft /compressible layers and fills, all boreholes are extended to the full depth of that layer. Samples/testing performed every 1.5 m spacing or when changes in strata occur. More frequent testing required in landslide investigations. Obtain undisturbed samples in clays and carry out standard penetration tests (SPTs) in granular material. Boreholes (BHs) can be replaced with cone penetration tests (CPTs) or other investigation type testing as required. In large linear projects geophysics should be used. Geological mapping of existing nearby cuttings (if in similar materials) provides reliable design information for cuttings.

44 Earthworks

Figures 2.2-2 and 2.2-3 depict the minimum depth considerations for a building and canal development, respectively. The former is based on the depth of influence of the foundation pressure while the latter is based on seepage considerations. Other relevant depths of SI are shown in Table 2.2. This varies based on the material intersected. Figures 2.2-2 and 2.2-3 illustrate some of the geotechnical considerations for a typical building site development. The considerations include depth of influence of the (new) imposed load (typically two to four times the foundation width), the susceptibility to movement (settlement and seasonal shrink/swell), and construction considerations on excavatability. Similar considerations exist for a canal where the canal geometry is evaluated to determine the investigation depth (Figure 2.2-3). Site data is required to assess slope stability, seepage, excavatability, and reuse of material. Permeability, in addition to strength parameters, is also assessed. However, permeability has a wider zone of influence than pressure bulbs.

2.2.2  Extent of investigation The extent of the SI is based on the relationship between the competent strata and the type of loading/sensitivity of structure and the known geology. Usually, this information is limited at the start of the project. Hence, the argument for a two-phase SI approach for all but small projects. The extent of available information and use of the structure determine the type and extent of SI, and depth of the investigation. Table 2.2 provides guidance on the extent and depth of investigation associated with different type of earthworks projects. Table 2.2 should be used with the following considerations: • • •

Ensure boulders or layers of cemented soils are not mistaken for bedrock by penetrating approximately 3 m into bedrock (Figure 2.2-4). Where water-bearing sand strata is present, ensure exploratory boreholes, especially in dams, tunnels, and environmental studies are sealed. Repair any destructive tests on operational surfaces (travelled lane of roadways).

2.2.3  Sampling Sampling is carried out during the SI for laboratory testing (Figures 2.2-5 and 2.2-6). Different drill bits are used to advance the borehole between sampling (Figure 2.2-6). Undisturbed or disturbed standard penetration test (SPT) sampling would occur typically every 1.5 m, mainly due to the length of the drill rod extension. However, testing should occur at every change in soil type/layer or strength as noted from the drill returns. The size of the undisturbed samples can affect the reliability of the laboratory results (Table 2.3).

Site investigation  45

Figure 2.2- 4  Confirm boulders are not misinterpreted as basement rock.

Figure 2.2-5  Sampling during site investigation (Look, 2014

(a) TC and V Bits

Figure 2.2- 6  Drilling bits used.

(b) RR bits

*P1).

(c) Drag Bits

46 Earthworks

The sample size should reflect the intent of the test and the sample structure. ­ ecause the soil structure may be unknown (local experience guides these decisions), B it is prudent to phase the SI as suggested in Figure 2.1-2. An economic investigation (smallest size sample) versus the quality of testing (large size sample) is the regular trade-off, when neither the soil type nor fabric is known prior to the investigation. The SI using a large diameter hole for obtaining 100 mm sample size can cost 50% more than an SI obtaining 50 mm samples. Undisturbed size samples and other samples are typically annotated: • • •

U50 (Undisturbed 50 mm tube sampler). The sample obtained is 38 mm for a thinwalled sampler. Disturbed sampling is annotated “D” and can be from a test pit or split spoon sampler. SPT – standard penetration test sample.

The split spoon sampler evolved into the ubiquitous SPT for counting the number of blows during sampling. Originally intended for use in obtaining disturbed samples for classification purposes, the SPT has become the most common in-situ test and now provides both disturbed samples and a numerical index (SPT N-value) on relative strength. The SPT N-value is measured by counting the number of blows in 300 mm increments (Figure 2.2-7). This test is for soils mainly but is often used in weak rock. N* is the inferred value when the SPT value (end of counting) is extrapolated in hard or dense material. A standard weight hammer and drop height are used, and the number of blows at increments of 150 mm are measured. The first 150 mm increment provides the seating blows, and the following 150 mm increments are the test drive, aggregated to give the 300 mm increment for the N-value. This field N-value is corrected for overburden and energy to obtain a design value. While the test data collected from this test is generally useful for many design situations, the “missing” 1.5 m test data at the surface is inadequate for the design of pavements or temporary works such as working platforms. In these situations, shallow boreholes or test pits are required. Dynamic cone penetrometers (DCPs), PANDA DCPs, or light falling weight deflectometers (LFWDs) may be used for the assessment of such shallow depth profiles. 2.3  FIELD WORK OF SI Safety plans, environmental approvals, and buried services checks are required for all SI sites, but particularly so for brownfield sites. Site access may need to be created. SI conducted near major roadways may require traffic management and night works. Often these overhead costs account for more than a third of the SI cost.

Site investigation  47 Table 2.3  Specimen size 10 −10

Mass permeability k (m/second) Soil type Macro-fabric Fissures

10 −9

10 −8

Homogeneous clays below zone of weathering

↓

m v, c v

Non-fissured (sensitivity  5)

C u , c′, φ′, m v, C v

50 –250 mm

Pre-existing slip

c r ′, φ r ′

150 mm or remoulded

^ A pedal is a general soil science term indicating that soil structure is present, or a soil is made up of Peds. Peds are aggregates of soil particles. Such lumping creates natural planes of weakness. Source: Modified from Rowe (1972).

Figure 2.2-7  Standard penetration test (Look, 2014

*P1).

48 Earthworks

Borehole logs should record the following information: •

• • • •

Identification of the test log with the following data: • Client/project description/project location/project number • Sheet No. --- of ---/Date started and completed • Reference: Easting, Northing, Elevation, Inclination/Reference map or drawing • Geomechanical details only; environmental details separately covered • Comment on access, services, and weather Drilling information Soil or rock description Field testing Strata information

A typical record of information for soil and rock is illustrated in Tables 2.4 and 2.5, respectively. “Borehole log” refers to all logs – test pit and borehole log. Subgrade and pavement testing borehole logs may have slight variations to the soil log. The water level should be recorded when first observed during and after drilling. The PLI or Is (50), an index of rock strength on a 50 mm sample is a common rock strength index used to derive the UCS. This is the intact rock strength and may not necessarily represent the field rock strength. This topic is developed in later chapters. The PLI test is predominantly performed on diametral or axial rock cores, but the test can also be carried out on an irregular sample. The type of bit is an important index data on relative strength, especially in residual soils and should be recorded on the borehole log. To avoid wear and tear on testing equipment, drilling bits are selected and changed based on the driller’s subjective assessment of drilling resistance of the material, inferred from the thrust of the equipment. During soil drilling, different drill bits are often used, including the V bit, the Tungsten Carbide (TC) bit, and the rock roller (RR) bit (Figure 2.2-6). During rock coring, the selected core cutting bit may be one of the following: • • •

Saw-tooth – for soil or very soft rock Carbide – for soft to medium strength rock Diamond – for soft to extremely hard rock

Drag bits (also called blade bits) are faced with a TC tip. Regular type drag/blade bits (Step) are recommended for drilling in slightly softer formations e.g., clays, weathered sandstones, light shales, gravels, etc. Chevron-type drag/blade bits are recommended for drilling in harder and more consolidated formations e.g., sandstone, hard shales, limestones, etc.

Depth

Elevation

Graphic log

Rock mass defects

Depth

Elevation

Graphic log

Origin

Dynamic cone penetrometer

Pocket penetrometer*

Field testing

Origin

Defect description (Depth, type, angle, roughness, infill, thickness)

Intact strength

Defect spacing

Shear vane test

Standard penetration test

Soil description`

Unconfined Compressive Strength

Point Load Index (Diametral)

Moisture

Consistency

Structure

Plasticity/particle description

Table 2.4  Borehole log for soil (Look, 2014

Point Load Index (A xial)

Rock description

Estimated strength

Moisture

Rock Quality Designation (RQD)

Colour

USC symbol/ soil type

Sample type

Water level

Drilling method

Depth

Drilling information

Structure

Drilling information

Colour

Weathering grade

Core Recovery

Water level

Drilling method

*

Depth

Site investigation  49

* P1)

Strata information

My personal view is that this is an unreliable test, and hand squeezing is more reliable. But tactile assessment has no number value, and as engineers, we love our numbers.

Table 2.5  Borehole log for rock

Strata information

50 Earthworks

The V and TC bits are typically expected to display “refusal” within extremely weathered and moderately weathered rocks with significant defects, respectively, ­depending on rock type and assuming the employment of a heavy drilling rig ­(Figure 2.3-1). Table 2.6 compares the hardness of different drill bits with various soil and rock materials. Thus, there is a relationship between the depth of drill bit refusal and rock hardness, and this data should be recorded. However, hardness is not strength, and observation of drill bit refusal alone is not enough to provide a sound basis for rock strength or weathering assessment. When combined with other key rock indices, drill bit data provides an improved basis for the assessment of subtle strength changes that may occur across the soil/rock interface. Rock coring is generally preferred, though it is more costly. This chapter focuses on the interface of drilling in residual soils grading into weathered rock. Once bit refusal occurs, rock coring will generally be required. Some investigations use rock coring as early as possible, without taking the auger to refusal. This has the advantage of recovering continuous samples but comes at a cost – typically 4–5 times higher when compared to non-core techniques (Figure 2.3-2). There will also generally be significant core loss and low rock quality designation (RQD) associated with low strength and highly weathered rock. Basic statistical values of the N* and point load rock strength index [Is (50)] observed at the depth of refusal for various types of drill bit are tabulated in Table 2.7 from various data points (n) as described in Look et al. (2014). These values vary with rock type as only SPT (uncorrected) values were used in the comparative analysis shown in the table. For a residual profile, V bit refusal is interpreted to be generally associated with a soil/extremely weathered rock interface. Examining the median and mean values presented in Table 2.7, the key findings include: • • • •

SPT N* value – the RR bit refusal value records the highest SPT value of all considered bits, and the RR bit can penetrate materials at least 1.5 times stronger than the V bit. Rock strength – the TC and RR bits appear to be able to penetrate rock materials up to medium to high strength; the blade bit stops in comparably weaker (low strength) rock. RQD – limited relationship. RR bit penetration may be limited to materials with lower RQDs than either the blade or TC bits. Rock defect spacing – no difference between bit types. Limited to ~200 mm average spacing. For larger spacing, rock coring is usually performed. Note: rock coring can also be carried out on close spacing.

2.3.1  Deep investigation Using the data in Table 2.7, as a first indicator, driven piles are likely to penetrate beyond V bit and TC refusal. For driven piles, the borehole should extend below the tip of an inferred pile refusal + an additional 2 × diameter depth or 3 m, whichever is the larger. The depth will vary with ground conditions and pile type. However, the type of foundation is unlikely to be known at the time of the SI. Extending the boreholes to “competent” rock is typical.

Site investigation  51

Drill type

V bit

TC bit

• Affects drilling depth • A 12 t drill rig will have a different refusal depth to a 20 t drill rig • Table 2.7 refusal indicators are for 12 t drill rig and heavier

• Steel bit • Can drill to XW/HW rock • Mohs hardness 7 • Low strength rock

• Tungsten carbide • Mohs hardness 9 • Can drill to SW rock • High strength rock

Figure 2.3-1  Drill bit attributes. Table 2.6  Comparison of hardness of drill bits with material types Material

Mohs hardness

V bit – Steel

7

TC bit – tungsten carbide

9

Basalt

6 5.5 7 2.5

Sandstone Clays

Comments

Low-cost bit

Medium cost bit With 50% Feldspar With 50% Mafic With 80% Quartz Smectite, Kaolinite, Illite

Table 2.7  Summary of material properties at drill bit refusal depth (Look et al., 2014 Test (n)

Bit refusal

Test value at percentile 25%

SPT N* value (n = 74) Is(50) (MPa) (n = 15) RQD (%) (n = 25) Defect spacing (mm) (n = 21)

V TC/drag RR TC Drag RR TC/Drag RR TC Drag/RR

* P3 )

40 60/70 70 0.4 0.1 0.1 25 10 45 50

50% 80 100 120 1.0 0.2 0.3 50/35 30 100 100

Mean 125 140/125 210 1.5 0.6 1.2 45/50 30 230 200

75% 150 170/150 205 2.1 0.5 0.8 75 55 235 210

52 Earthworks

An SPT N-value with N > 50 may be due to cemented materials, very dense sands, or rock. A common (incorrect) rule of thumb application is to infer that this indicates likely refusal for driven piles. Look (1997, 2004) showed the SPT Nvalue is not a reliable indicator of pile refusal. A pile will not reach refusal at N = 51 but at N = 500 refusal is likely to have occurred. Both values are N > 50. An SPT is used to obtain samples for classification purposes and is considered a multi-purpose test. The following tests are more reliable and useful in-situ tests when values are to be used in design: • • • •

Vane shear test Cone penetration test (CPT) Dilatometer test Pressuremeter test

The first three tests apply to mainly soils; pressuremeter tests can be used in both soil and rocks. However, each model would have its testing range. In soft to firm clays, the vane shear and CPT are superior tests. The CPT measures: • • •

Cone resistance (MPa) Sleeve friction (MPa) → Friction ratio (%) Pore pressure – when this is measured, this is a CPTu or piezocone test

These various measurements add redundancy in being able to interpret the CPTu data in various ways. A more confident strength and material assessment can then be made. A seismic CPT (SCPTu) measures the shear wave velocity through the soil and provides additional data on the small strain shear modulus of the soil.

2.3.2  Shallow investigation and subgrade assessment Shallow foundations are typically used for light buildings such as houses, and where high-strength materials exist within 3 m. A load capacity of 200 kPa or less (typically 50–100 kPa) is required and can be obtained from very stiff clays, dense granular materials, and various rock types. The DCP is a commonly used hand tool (Figure 2.3-3) for in-situ testing in shallow foundations or subgrades. It is performed by counting the number of blows per 100 mm or the penetration (mm) per blow. Various correlations are applied to convert to strength or in-situ California Bearing Ratio (CBR). Figure 2.3-3 illustrates the main components of the DCP equipment. The DCP test has the advantages of being a lowcost test, commonly available, transportable, and easy to use. The DCP has a large variability at low test values. In such cases, the PANDA DCP is considered more reliable. However, on stiffer or stronger soils, the standard DCP is preferred. The SI for subgrade assessment should be at least 0.5 m deeper than the defined subgrade level. This does not account for possible underlying compressible material which is relevant for settlement assessment when high dead loads apply. The type of testing for subgrade assessment is discussed in Chapter 11.

Site investigation  53

Rock Coring

Solid Augering

4x$

4 x times faster

Poor core recovery for low RQD / strength

Can penetrate rock with low RQD / strength

Rock logging of defects and strength

Returns of crushed rock  soil SPT used

Figure 2.3-2  Comparison between rock coring and augering.

Figure 2.3-3  Dynamic cone penetrometer (Look, 2014

*P1).

54 Earthworks

2.4  TESTING VARIATION In theory, an SI should envelope the adverse conditions to which the site is subjected, i.e., when rain and flooding are occurring. Due to safety considerations, an SI is usually conducted in dry conditions i.e., when showers are not occurring. Where the most adverse conditions are a normal occurrence and have not been tested in situ, for example adverse and intense rainfall in tropical areas that usually occurs for 3 months of the year, the test results must be interpreted with caution. For deep foundations, excessive rainfall may affect only the water level. For shallow foundations and subgrades, the ambient weather conditions can have a significant effect on near-surface strength tests. Even with standard testing, variability can occur due to: 1. Spatial and material variability 2. Testing variability 3. Time of testing Characterisation of material testing variability is provided by Phoon and Kulhawy (1999).

2.4.1  Shallow foundations Shallow foundations, pavements, and subgrades are sensitive to seasonal and daily changes in strength and stiffness. Yet an SI is usually conducted once and not at a specific time of year. Though useful data, it is uncommon to record the time of year and rainfall in the preceding weeks of an SI. The following example illustrates the relevance of the time of year to test results. Figure 2.4-1 shows the coefficient of variation (COV) using the DCP in a uniform site of residual clays, and the associated variation due to: 1. Measurement – repeated tests at the same location 2. Spatial – three tests in a triangle 12–15 m apart in a uniform site 3. Temporal – five tests repeated over a 12-month period in approximately the same three locations The testing demonstrated that measurement errors and inherent variations had a testing variance of up to 30%. When combined with spatial and temporal considerations the variance increases to 60%. Geotechnical investigation data collected on a given day may not envelope an adverse condition. Depth variation should be part of the analysis for each stratigraphic layer. At the near surface and for the same material, one can expect an active zone where moisture variation occurs (discussed further in Chapter 10). This can be further divided into cracked and uncracked zones. The stable zone at depth would have less variation. Figure 2.4-2 illustrates these cracked and uncracked active zones and the DCP variation at the same site as Figure 2.4-1

Site investigation  55

Figure 2.4 -1  Total variation of DCP results (Mellish et al., 2014

0

0

5

10

15

20

25

*P4 ).

Cracked depth 0 - 0.8 m

Depth (m)

-0.5

Active depth 0 - 2.2 m

-1

Uncracked depth 0.8 - 2.2 m

-1.5

-2

Stable depth > 2.2 m -2.5

DCP - Blows / 100mm Min

Max

Figure 2.4 -2  Active and stable zones from DCP profiling (Mellish et al., 2014

*P4 ).

56 Earthworks

The active zone is subject to seasonal variation with resulting moisture content and strength changes. Below the active zone, seasonal changes are unlikely, although groundwater variations may occur. In-situ CBR samples were obtained using 150 mm sample tubes driven into the ground (Figures 2.2-3 and 2.2-4). These were extruded from the tube, wrapped and taken to the laboratory for testing. Intact CBR samples were tested initially, and those samples were progressively dried back and moisture contents of the top 20 mm were taken with the corresponding CBR value. This is shown in Figure 2.4-4. The corresponding wet and dry period moisture contents are also shown. The CBR would vary by a factor of 2 over these periods. Note the wettest and driest periods do not correspond to the peak or least rainfall as a time lag occurs (Figure 2.2-5), as is shown in the case study in Section 10.15, with long term moisture content monitoring at other sites.

Figures 2.4-1–2.4-5 represent data from tests performed on a residual soil in my back yard – hence unlimited access to carry out repeated tests over 1 year. Brisbane, Queensland has a sub-tropical climate and high summer rainfall between December and November. The following design values apply by factoring time of year: • • •

Testing from December to March – after rainfall starts and for a few weeks thereafter, is a “weak” period for subgrade strength; Typical design characteristic value is between the median and 30th percentile values. Testing from April to May – a shoulder period of rain events; Typical design characteristic value is at the 30th percentile value. Testing from June to November – a generally dry and “strong” period for subgrade strength; this should not be considered representative. Typical design value for tests in this period is at the lower quartile value.

Such considerations would vary for each climatic condition.

Site investigation  57

Figure 2.4 -3  S chematic of the sliding hammer apparatus used to drive CBR sample tubes into the ground.

Figure 2.4 - 4  M C @ 0.5 m depth and corresponding CBR. Wettest and driest period lags rainfall.

58 Earthworks

2.4.2  Deep foundations The SPT is one of the common in-situ tests used in geotechnical engineering to determine the properties of subsurface soils at depth. The N-value is used to estimate the approximate shear strength properties of the soil (Clayton, 1995). Various corrections are made to the in-situ value to allow for type of hammer, energy corrections, etc., (Skempton, 1986). As testing “standards” vary among countries, N* (refusal) can have different values. Figure 2.4-6 compares the SPT standards across three countries when the test is discontinued at “refusal”. The Australian standard is explicitly titled for “soils”, and states there are limitations when applied to rock. Yet in Australia, in the absence of a standard specifically applicable to rock, this standard is, in practice, applied to weathered rock materials. Look (2004) showed that an inferred SPT extrapolation less than 100 had little relevance to rock strength for engineering design and only SPT values above 120 indicate some correlation with strength. The UK and USA standards better account for these high blow counts. Both the UK and USA standards allow for the SPT to be used in rock while the Australian standard, with an early refusal, has N* of 90–150 – this is too low to be meaningful in design. The larger N* values in the USA standard suggests that the USA standard is more appropriate to residual soils and weathered rock.

2.4.3  Counting blows The SPT requires an accurate count of the SPT blows (N-values), and an energy conversion to be appropriately applied to design. At the time of SPT standardisation (which was before the digital age), measurements using chalk marks were appropriate, but corresponding digital readings indicate the manual counting of blows between increments consistently has an error.

Site investigation  59

Figure 2.4 -5  Median MC with depth versus month and comparable rainfall.

Australia

UK

USA

Total of 30 blows cause less than 100 mm penetration at any stage. No measurable penetration or the hammer is bouncing for 5 consecutive blows. Use HB (hammer bouncing).

For soils: maximum blows of 25 and 50 for seating and test drive, respectively. For rock: maximum blows of 25 and 100 for seating and test drive, respectively.

Total of 50 blows during any one of the 150 mm increments. Total of 100 blows applied. No observed advancing for 10 successive blows

N* ~ 90 - 150 • 30 / 100 * 300 to • 5 / 10 mm * 300 (HB)

N* ~ 125 - 200 • soils 25 + 50 • rocks 25 +100 •100 / 150 * 300

N* ~ 100 - 300 • 50 / 150 * 300 to •10 / 10 mm *300 (HB)

Figure 2.4 - 6  Comparison of test procedures when test is discontinued (Look, 1997

*P5 ).

60 Earthworks

Look (2016) showed that 150 mm is a counting target. It is not practically measurable (Figures 2.4-7 and 2.4-8 and Table 2.8) as any blow does not stop at 150 mm. An eye measurement is 150 ± 41 mm. The reported Nvalue counted at 150 mm increments therefore represents the closest integer value to that increment. The counting errors alone account for 15% COV. Figure 2.4-8 displays an example where the experienced drilling supervisor measured blow counts of 4/6/9 for each 150 mm increment (N = 15). The following should be noted for this ubiquitous test when digital measurements are compared: • • • • •

Increments were at 158/296/442 mm rather than at 150/300/450 mm. Energy ratio also varied with each blow count. The energy at the hammer is shown in this example, but the energy at the rod below the anvil would be even lower. The first four blow counts at the seating drive varied from 76% (Blow 1) to 84% (Blow 4). At the test drive the two “150 mm” increments varied from 81%–84% to 79%–89%. N “average” energy correction applies for the N-value. Seating stopped at the third blow which is at 131 mm. The seating drive count stopped at 158 mm for four blows. This is also evident in the energy ratio with each set for each blow shown, with a steady increase in hammer energy ratio and for sets less than 30 mm.

Industry asserts an SPT N-value is “factual”, yet it is only possible to count whole blows. A decision must be made by the drilling supervisor to specify which 150 mm increment to assign to the blow. The true blows are NOT at 150 mm intervals but at the nearest integer value either before or after the 150 mm chalk mark, which can be 5–10 mm thick. Figures 2.4-9 compares the supervisor’s blow count with digital measurements. In this case, driving to refusal occurred in 42 seconds with an automatic trip hammer. The experienced supervisor measured 30 blows at 40 mm for refusal while the digital measurements showed 30 blows occurred at 48 mm. This is a 20% counting error.

Some common fallacies with measuring and using the SPT N-values are: • • • • •

N-value is factual NField  = NDesign Similar correlations for all geology N ≤ 1 implies a clay is very soft N > 50 implies a pile will refuse

Site investigation  61

Figure 2.4 -7  SPT measurements (n = 54) by eye and digitally compared (Look, 2016

*P6 ).

Figure 2.4 - 8  V ariation of penetration and penetration hammer energy to obtain N-value (Look et al, 2015 *P7 ).

62 Earthworks

2.4.4  Energy transfer The in-situ SPT N-value provides an indicator of relative change. However, correction factors are required for design. Typically, “textbook” correction values are used but these vary widely between references. As a minimum, the field SPT N-value (NSPT) must be converted to the (N60) energy value where the latter is the energy at 60% and is the percentage of the theoretical free-fall hammer energy. N60 E60 = NSPT ESPT Australian standards do not currently specify energy requirements in the SPT procedure; consequently, energy is rarely measured. Any correction (if applied) is based on international literature. There is a wide range of energy correction factors described in the technical literature. Aggour and Radding (2001) summarised the SPT correction factors applied by different researchers and concluded the most significant factor affecting the measured N-value is the amount of energy delivered to the drill rods (Table 2.8). The SPT energy correction for a trip hammer varies between 80% and 172% depending on one’s chosen  reference. Thus, only actual energy measurements provide confidence in an appropriate correction factor. The correction range is larger for rope and pully or donut hammers. Energy is transferred from the hammer to the anvil, then to the drilling rods, which drives the split spoon sampler into the soil. The energy value is not constant and depends on the length of the rod and the soil hardness (Seidel, 2014). The most significant factor affecting the measured N-value is the amount of energy delivered to the drill rods (Aggour and Radding, 2001). Energy transfer is affected by the type of drill rig and hammers, operator skills, as well as ground conditions. Most design correlations that use the SPT N-value are based on the N60 (energy corrected value). The in-situ N-value (NSPT) should not be used as advocated by several researchers for many decades. The energy below the anvil is not the same as the energy at the hammer. Skempton (1986) uses an anvil correction factor of 0.6–0.8, a significant number yet this is not included in the correction factors by others. The anvil correction factor warrants further research. As a result, site-specific energy corrections should be used wherever possible. The full list of correction factors includes: • •

Overburden correction factor – CN Energy correction factor (C ER) to account for: • Hammer – CH • Rod length (depth) – CR • Sampler – Cs, Borehole diameter – CB, Anvil – CA

Site investigation  63

Figure 2.4 -9  D igital measurements compared with counting between chalk marks (Look, 2016 *P6 ).

Table 2.8  Energy correction factors for trip and automatic hammers Reference

Energy ratio

Anvil

Bowles (1996)

1.14 –1.72 (N 70 used)

None listed

Skempton (1986)

None listed

Small 0.6 – 0.7; Large 0.7– 0.8

Robertson and Wride (1998)

0.8 –1.5

None listed

Seed et al. (1984)

1.67

None listed

64 Earthworks

2.4.5  N-value strength varies with geology Figure 2.4-10 illustrates that results from residual soils derived from Greywacke are near constant once the overburden correction factor is applied (No). Without the correction, one may misinterpret an increase in N-value for an increase in strength. Such soils would then appear to have an undrained strength (Su) = 110 kPa (lab measurements on U50 samples) at No = 13 (qu = 220 kPa). This can be compared with widely varying published relationships. For this residual soil Su = 8 N60 which fits between the two relationships.

( ) 4.5 N60 ( N m 2 ) K Stroud (1974 )

Su = 10.5 N60 N m 2 K Décourt (1989 ) Su =



A conversion of N-value to strength should be based on local geology and correlations rather than general “universal” relationships in the literature. While correction factors are emphasised for liquefaction studies, many design correlations for strength and settlements also require an energy correction. The value (No)60 is used in design and is the corrected field (N) value for overburden and energy. At σ′vo = 100 kPa, a similar correction applies for normally and over consolidated sands, with the former requiring a higher correction.

( No )60 = CN CER N N = C

CN =

100 ( NC sand ) Liao and Whitman (1986) σ '   vo 170 (OC sand: OC = 3) Skempton (1986) 70 + σ vo ′

Clayton (1995) provides the following relationship between the UCS and the corrected SPT value (N60) for weak rocks: UCS ≤ 10 N60 ( kPa ) However, Look (2004) found the UCS/N ratio greater than 20 for extremely low strength rocks in Queensland. The UCS/N ratio of 10 is more applicable to residual soils and probably denotes the factored (reduced) value. The ratio represents the minimum value likely for preliminary assessment and not a design value. The UCS/N-value was affected by the geology of the material and its strength classification (Table 2.9). The variability in the UCS/N ratio was derived by comparing the SPTs to immediately adjacent PLI values, which also required a strength conversion. The data was at the coring interface where the driller extended the drilling to SPT refusal (N > 50) and auger bit refusal. Rock coring was used to extend the borehole.

Site investigation  65

Figure 2.4 -10  ( a) Uncorrected and (b) corrected SPT N-values versus depth for residual soils (Priddle et al., 2013 *P8 ).

Table 2.9  UCS/SPT N-value ratio for various rock types (Look, 2014

* P9 )

UCS/N (Field) Strength

Extremely low to low Medium High Very to extremely high

Phyllite

50 50 –75 75

Sandstones/ siltstones

Greywacke/ argillite

30 30 –50 50 –100 100

40 –50 50

40

A table has four legs. If one observes four legs, then one does not conclude the object is a table. This could be a dog. Similarly, a very soft clay (Cu ≤ 12 kPa) has N ≤ 1. A measured N ≤ 1 is not necessarily a very soft clay. Values of Cu = 60 kPa have been measured with adjacent CPT, vane shear and laboratory tests on undisturbed samples.

66 Earthworks

Note the uncorrected N-value was used as the energy correction was unknown for the data used. As in any “intact” UCS, a suitable field conversion value applies. Residual soils are derived from weathered rock, and often there is little distinction between extremely weathered rock and residual soils. Priddle et al. (2013) found there was no relationship between strength (Su) and the SPT N-value in soils derived from Phyllite or Tuff. A UCS/N ratio of 14 was applied to residual soils derived from sandstone and mudstones; Greywacke-derived residual soils had a UCS/No ratio of 8 compared to a “constant” No = 13 (Figure 2.4-10).

2.4.6  High and low SPT values The incorrect use of N-values occurs at both high and low SPT N-values. Many borehole logs record a very soft clay with an SPT N-value ≤ 1. The flawed logic being employed here is as follows (refer to Text Box): • •

A very soft clay has an N-value ≤ 1 is correct Therefore, any clay with an N-value ≤ 1 means the clay is very soft

This is incorrect as there are many cases when alternating SPT and undisturbed tube sampling show Cu = 40–60 kPa values have N-values ≤ 1. A simple bearing capacity check using the weight of hammer, rods, and anvil will indicate the SPT split barrel will “fail” into firm clays (Cu  50 as a basis for evaluating pile driving refusal. Adams et al. (2010) present data for driven prestressed concrete piles in Queensland and the refusal criteria based on the SPT N-value factored for geology (Figure 2.4-11) Note: the capacity of the pile may have varied and is unknown. This pile refusal (a near constant set mm/blow) varied from: • •

N (Field) ≥ 160 for shales at a set of 4 mm/blow N (Field) ≥ 90 for sandstones at a set of 2 mm/blow

Thus, the use of a singular high SPT value as “refusal” is flawed, yet it is regularly seen used in practice. The rock or soil type, and the type of pile, influence the depth of driving. 2.5  CASE STUDY 1 – NO GEOTECHNICAL INVESTIGATION Figure 2.5-1 illustrates a 1988 case study where the stratigraphy of the material located at two nearby sites, separated by a road, varied remarkedly from high-level rock to deep compressible alluvium (a buried channel). The project was eventually abandoned due to additional construction costs because the developer had incorrectly assumed the ground conditions at the second site to be the same as those in the first site across the road, less than 20 m away. The developer, therefore, did not allow for the cost of deep piled foundations.

Site investigation  67

Figure 2.4 -11  SPT N-value required for pile refusal (Adams et al., 2010

Figure 2.5-1  Case study 1 of changing ground conditions.

*P10 ).

68 Earthworks

Figure 2.5-2 depicts the time sequence of events in this project. There was significant “off the plan” interest in the initial project, with demand exceeding supply. So, when additional land became available during the initial construction phase of the first site a decision was made to duplicate the project on the second site – the second site was located just across the road from the first site and the land area was of similar size to the first. The developer decided against an SI as this would reduce expenditure. They deemed an SI was unnecessary expenditure because they had “experience” from building on a site across the road. The assumption that ground conditions are similar between adjacent sites is a common error. In this case, the ground conditions at the new site were vastly different with a buried alluvium channel. The shallow foundations were no longer appropriate, and more costly piled foundations were required. The increased costs ultimately led to the cancellation of the project and the return of deposits to all prospective purchasers who had bought “off the plan”. 2.6  CASE STUDY 2 – AUGER AND CORED DRILLING In this case study, the on-site project manager during construction requested additional drilling and then mistakenly believed a different material was intersected. The change in the appearance of the material was due to a TC bit auger drilling technique using being employed. Different drilling techniques to the design drilling data lead to a misclassification. One must be careful not to mistakenly identify a sample as “soil” for what is crushed, highly weathered rock due to the drilling technique used to extract the sample (Look et al., 2015). Figure 2.6-1 compares “soil” samples taken from probe holes (just prior to construction), using a TC bit to rock core to the same depth as an immediately adjacent borehole. The amount retained on each of the three sieve sizes is shown. The “soil” samples comprised low to medium strength basalt rock fragments which, as the rock core exhibited a natural fracture spacing of approximately 100 mm, had resulted in the TC bit crushing the in-situ rock mass. Thus, based on the material conditions present, the rock material was able to be easily penetrated by the large (heavy) rig fitted with a TC bit. Figure 2.6-2 shows the probe hole immediately adjacent to the cored hole in the one stratigraphic profile. This “new” “soil” stratigraphy versus the design rock stratigraphy led to an incorrect interpretation of the ground conditions. Additionally, highly weathered rock can be classified as a “soil” description. The drilling supervisor can provide a more accurate description from the rock core rather than from the SPT sample or non-core drilling just above, especially if wash boring techniques are used to advance the boreholes. Therefore, the material just above the start of coring is often conservatively assumed to be a weaker grade of material (Look, 2004). This is the implicit assumption contained in many borehole logs produced with residual soils and weathered rock profiles. Rock core samples are then used to carry out the visual strength classification, PLI, and other types of testing.

Site investigation  69

Figure 2.5-2  C ase study 1: Site investigation was not carried out – time frame to cancellation.

Figure 2.6 -1  M oderately weathered basalt obtained via TC bit and core drilling techniques (Look et al., 2014 *P3 ).

70 Earthworks

2.7  SUMMARY SI is the data gathering phase of projects and is used to inform the design model. These early costs are often compromised. A staged SI approach can provide more relevant data for the designers and target key issues as the project plan develops. Geotechnical involvement should occur throughout the life of the project. The input varies depending on the phase of project. The phasing of the SI improves the quality and relevance of geotechnical data to the project. The extent and depth of the SI must be relevant to the project. The type of drill bit used in the test is an indirect index test, yet that data is often missing from the site record of information. The choice of bit is dependent on the hardness and type of material to advance the auger. Details associated with the many types of field and laboratory testing have not been covered in this chapter; associated references should be consulted. Field and laboratory testing have many assumptions and variabilities. These include testing, spatial, and temporary variability; they should be taken into consideration when deriving the design characteristic value. Some of these variabilities were outlined in the chapter. Figure 2.7-1 provides a summary of considerations for the SI process leading to a geotechnical report, which may be factual or include an engineering assessment. This input forms the basis of the design model. The site assessment of safety and access is a prerequisite (discussed in C ­ hapter 3) to any mobilisation of equipment or ground truthing at the site. Similarly, the geological model should be a priori to the geotechnical model and engineering assessment. Even simple and common tests to standards have assumptions. The interval of 150 mm in SPT blow counting is an estimate and is not attained in practice, and the nearest number to that increment is what is used. Overburden and energy corrections are required. In summary, the field “factual” SPT N-value is useful for comparisons but should not be directly applied to design. Since the early 1980s, this practice has been proven by several researchers to contain inaccuracies. Yet the field SPT N-value is still being applied in daily practice. This seems to be the most common oversight in practice. Improved quantification of site for design is obtained with other tests such as CPTs. Soil and rock strength are discussed further in Chapter 6.

Site investigation  71

Figure 2.6 -2  B H A13 with cored data and probe prior to construction, obtained via TC bit (Look et al., 2014 *P3 ).

An associated error by contract administrators (CA) is to mistake extremely and highly weathered rock for soil as an excavator can typically remove such materials. The CA makes a judgement on site for “Top of rock” for a founding layer and then replaces with compacted imported soil at significant costs, but that represents 20% of the strength of removed XW/HW rock. The CA is on-site full time while the Geotechnical Engineer is often on a call-out basis.

Figure 2.7-1  S ummary of site investigation procedures required for engineering assessment.

72 Earthworks

Chapter 3

Site safety

3.1  SITE-SAFETY AWARENESS Prior to, during the site investigation, later at the construction phase, and throughout all phases, safety must be considered, and procedures implemented to mitigate risks. Securing the work area and safe work method statements (SWMS) are required that appropriately consider risks to workers and the environment. The boundary for the work activity is established, but as the work activity moves, its boundary moves accordingly. A risk matrix is usually required to identify each sub-activity in terms of: • •

Likelihood – almost certain, likely, possible, unlikely, and rare Consequences – insignificant, minor, moderate, major, and severe

A risk control method is used to mitigate the risk. In SI work, this could involve keeping a safe distance from traffic or implementing traffic control measures if the site is within that “unsafe” distance. Consequently, targeted locations may be varied to satisfy safe work practices. Excavation failures are especially dangerous as they occur quickly. Excavations are required during an SI, or for the laying of underground services, access chambers, and underground tanks. Temporary or vertical slopes are allowed for short-term conditions. Short-term conditions can vary from a few minutes to several days or even weeks. There are many instances of a short-term, temporary excavation, anticipated to be open for several days (for temporary works design (TWD) purposes), being left open for several weeks. This has been the cause of many failures. Often conditions can change over short distances. The operation and movement of construction equipment, its working platforms, and its effects on all adjacent activities or structures are a considered part of any SWMS. Other site hazards include adjacent public users and traffic. These are important considerations, especially when the site is unattended. Nearby underground and overhead utilities must be identified. Working platforms are discussed in a later chapter. The design of such and other temporary works can add significant costs to a project. Due to site complexities and relatively high levels of risk and failure, only a competent engineer should design ­temporary works. Final temporary design documents should consider: DOI: 10.1201/9781003215486 -3

Site safety  75

Figure 3.1-1  Test pits in similar materials with groundwater at the base in the left photo.

Figure 3.1-2  O bserve trench ground condition during excavation – which may suggest safety issues/collapse pending.

76 Earthworks

• • • •

The design of the working platform, shoring, or other elements Effects of the excavation on nearby structures, services, or travelling public Safe work methods Controls such as hold points and certification before proceeding to construction

A TWD engineer working with the contractor may have different considerations than the design engineer for the permanent works. Safety and construction issues govern the TWD engineer. Applying a Factor of Safety (FS) of 1.2–1.3 for temporary works and an FS > 1.5 for permanent works ignores the site-specific range in probability of failure. This is a binary consequence of failure versus no failure, irrespective of the range in probability of failure or FS. Instrumentation monitoring and daily observations are mandatory for any FS ≤ 1.2. 3.2  FAILURE OF TRENCHES During SI, test pitting is often used to assess the near-surface ground conditions. The two test pits in Figure 3.1-1 are 80 m apart and excavated in similar materials (silty sand), to similar depths (approx. 2.5 m) and with groundwater at the base affecting stability in one case. Both photos were taken during the subgrade SI, directly after the excavations were completed. In the photo on the left, the sidewall of the test pit has collapsed, whereas the photo on the right shows the walls are vertical and appear stable. Such a collapse would be enough to critically injure or cause death if a person were inside the excavation at the time of collapse. The stability of excavation is largely material and water-dependent. Silty sands, such as those shown, sometimes give the illusion of stability, but this stability is only for a short period of time and is due to soil suction. Once soil suction is lost (due to an increase in saturation), the collapse, as shown in the photo on the left, may occur. In this instance, the groundwater undermined the excavation and led to the collapse. Loose or soft materials at the ground surface are an obvious indicator of likely trench instability. A less obvious indicator is when weak or loose layers occur at depth. A stiff crust often overlies such materials and hides the potential trench instability. Figure 3.1-2 illustrates other considerations based on ground conditions. Continuous observation and corrective action are especially required. A sudden trench collapse may occur due to: • • •

Water flow seepage Weak/loose underlying layer intersected Adverse dip of soil or rock layer

Safe work methods require excavation pits greater than 1.5 m in depth, and sometimes to depths as short as 1.0 m, to be supported if the pit is being accessed by testers and other workers. The practical maximum depth of any excavation pit is chest height (nominally shoulder height less 100 mm). Therefore, safe trench entry varies with the height of the person. Greater excavation depths require bracing. The stability of adjacent buildings, services, and retaining walls may be affected by construction activities (Figures 3.2-1 and 3.2-2). The latter shows the loss of passive pressure may result in movement if within the passive zone.

Site safety  77

Figure 3.2-1  R estrictions on trench depths adjacent to existing utilities and buildings.

Figure 3.2-2  R estrictions on trench depths adjacent to retaining walls.

78 Earthworks

A value of 1 (V)ertical–1 (H)orizontal is typically used for temporary excavation, but flatter slopes apply where there are soft or loose materials. For retaining walls, the passive zone in front of the wall uses 1V:2H. A retention system is required for increasing excavation depths, even temporarily. Lower factors of safety ( 1.0 m deep typically requires shoring before it is considered safe to enter the excavation. When B > 5H, i.e., a wide-open cutting, this excavation is considered an open cutting rather than a trench. The distances indicated in Table 3.1 are dependent on groundwater conditions, soil or rock types, and their stratigraphy. Other factors of influence are: • • • •

Distance of stockpile and equipment to excavation (to minimise the additional load and risk to the trench) unless trench bracing is designed to accommodate these loads (Figure 3.2-3) Structures and services within the zones of influence require consideration Movements are likely when placed at 75%

Common/frequent occurrence

More than one event per month

Likely

Commonly occur 25% –75%

Is known to occur or “It has happened regularly”

More than one event per year

Possible

May occur occasionally 10% –25%

Could occur or “I’ve heard of it happening”

One event per 1–10 years

Unlikely

Could infrequently occur 1% –10%

Not likely to occur very often

One event per 10 –100 years

Rare

The threat may occur 200 mm). Some road authorities extend the compaction standard to 35% oversized material to bridge this grey area. Predominantly coarse-grained granular material and rock fill use a mechanical interlock approach for compaction. This approach relies on rolling the material for about six passes, and until no further permanent deformation is observable, or less than 10 mm in the last two passes for deep lift rock fill. 5.6  CBR TEST Associated with the compaction test is the California Bearing Ratio (CBR) test. This is a strength index test and is a quasi-static bearing capacity test. In this test, a 50 mm diameter plunger is pushed into the compacted sample, which may be soaked or unsoaked. A CBR mould size applies, which is larger than the 1-L mould, hence allowing larger sizes to be tested. Many of the concerns (e.g., sample curing) associated with the compaction of ­samples also apply to the CBR test discussed further in Chapter 6. 5.7  COMPACTOR PERFORMANCE IN THE FIELD Selig (1971) developed a numerical method for analysing compactor performance, due to non-existent data on production, and power requirements of equipment at the time (Table 5.6). This table serves as a useful indicator of the equivalent compactive effort for various thicknesses and passes. The comparative energy values for the standard and modified tests are shown below the table. The modified energy is not achieved in the field directly. This illustrates some of the many field variables that influence compaction energy in the field. Table 5.6 indicates the volume of soil influence is at a depth of 0.3 m for the compaction equipment. Current compaction equipment is twice the size of what was available at the time these findings were obtained. Additionally, this early approach does not recognise the change in the depth of influence with each pass of the equipment as the surface modulus changes. Chapter 7 covers other aspects of field compaction when compared to laboratory considerations.

CBR is an index test of soil strength and is not a ­fundamental soil property.

Theory of compaction  141

Above six passes, crushing of the rock fill surface could occur without adding compaction to material below

Figure 5.5- 8  C ompaction test boundary does not match “rock” boundary. “Soil” tests may not be applicable but there is a gap when a “rock” standard does not apply for compaction purposes.

Table 5.6  Compactor performance Roller

Weight

Width Speed

kg

m

km/hour m

Steel wheel

4,536

1.0

4.8

0.10

Steel wheel

9,072

1.3

3.2

2.3

4.8

Pneumatic – 22,680 2.2 7 tyres Sheepsfoot

15,196

Vibratory

9,979

Pneumatic – 14,515 17 tyres

Thickness Passes Compactive Power Production effort kJ/m 3

kW

m 3 /hour

5

517.5

14.9

103

0.15

5

565.4

19.4

125

0.15

8

488.8

20.8

214

4.8

0.20

6

460.0

46.2

360

3.4

4.8

0.20

10

436.1

40.3

329

2.0

2.4

0.30

6

436.1

26.8

245

Standard compaction 95%/90% at OMC

596 563/534

Modified compaction

2,703

Source: Adapted from Selig (1971).

142 Earthworks

5.8  CASE STUDY 1 – IMPORTANCE OF CURING TIMES An example of the effect of curing time is shown for a Bundamba clay with a plasticity index of 47% and grading with 97% passing the 425-μm sieve and 69%–82% fines. Samples from the site were prepared and cured for 0, 1, 4 and 7 days, compacted, and the CBR (soaked and unsoaked tests) determined at various times. The following observations were made from compaction and CBR tests on these samples: • •

•

Curing for 0–1 and 4–7 days produced higher MDD values. This leads to more compaction effort in the field being required, or the in-situ material is more likely to fail the DR test (Figure 5.8-1). Lower curing days produced higher CBR swell values which incorrectly require more remove and replace of material than is necessary (typically 250 mm for the swell values produced in the tests). This of course leads to increased c­ onstruction costs. There is a significant difference in swell values depending on whether the initial sample was wet or dry i.e., the field moisture content at the time of sampling influences the soaked CBR swell test results when there is inadequate curing. Lower curing days produced higher CBR (unsoaked) values which incorrectly result in a reduced pavement thickness and increased pavement failures in the long term. A curing time of 0 days produced a lower CBR (soaked) value which increased as curing time increased. This results in over-design of the pavement. There was little effect on FMC for samples initially wet or dry (Figure 5.9-1).

5.9  CASE STUDY 2 – REPRESENTATIVE SAMPLING The grading and percentage breakdown change when source material is compacted. When the XW – SW sandstone siltstone source material from the stockpile (n = 4) was rolled with the smooth drum and padfoot rollers the % passing the 37.5 mm sieve ­decreased from 100% pre compaction to 82.8% and 80.3% for the smooth drum and padfoot rollers, respectively (Table 5.7). Only a minor increase in the reported proportion of fines occurred between preand post-compaction samples (pre compaction = 10% and post compaction ~ 11%). The pre-compaction result is clearly incorrect as these samples were biased towards the finer component and excluded material above 37.5 mm; no material was noted to be retained on the 37.5 mm sieve (i.e., 100% material passing in the test). On average, 75% of material was reported to pass the 19 mm sieve. However, rocks measuring up to 300 mm were visually observed in the stockpile (Figure 5.9-2).

Theory of compaction  143

Figure 5.8 -1  Curing – effect on MDD.

Figure 5.9-1  C uring – effect of soaked CBR. Table 5.7  A verage grading curves for XW – SW sandstone-siltstone pre and post compaction AS sieve (mm)

Average percentage passing (%) Pre compaction (n = 4)

Post compaction – smooth drum roller (n = 12)

Post compaction – padfoot roller (n = 12)

75 37.5 19 2.36 0.425 0.075

100.0 100.0 75.5 33.0 27.0 10.0

100.0 82.8 65.4 25.0 21.6 11.1

100.0 80.3 61.8 23.9 20.4 10.8

144 Earthworks

The post-compaction particle size distribution (PSD) testing (n = 24) indicated a maximum particle size of less than 75 mm for both rollers. Visual observations confirmed the maximum particle size at the surface of the embankment after the compaction effort was 300 mm (Figure 5.9-3). Visual monitoring of the embankment surface during compaction also confirmed effective material breakdown of large XW – SW interbedded rock particles due to direct heavy roller vibratory loading. This example highlights the inherent inaccuracy in the testing and industry’s misplaced confidence in accepting the test results as evidence that material specifications have been met. 5.10 SUMMARY The quality control process involves assessing the laboratory MDD value with the field density. The aim is to reduce air voids, measured by the DR achieved after compaction. The DR is dependent on the thickness of the layer and its moisture content. These two criteria are considered supporting considerations to achieve the required air voids reduction (Figure 5.10-1). Climate and equipment-dependent aspects of compactions are discussed in later chapters. Using an air void only approach has the shortcoming of low air voids can be achieved by simply increasing the moisture content (Mokwa and Fridleifsson, 2007). For example, at low energy compaction at wet of OMC, low air voids are obtained, and with a low DDR. This is unacceptable. Importantly, OMC and MDD are derived from a laboratory compaction model that may not be characteristic of the field condition, due to unrepresentative soil samples, placement, or surrounding conditions. Efficient use of high-energy compaction equipment is achieved by compacting dry of OMC. However wet of OMC could also be a target for long-term equilibrium conditions. The compaction process in the field is different to that undertaken in the laboratory. The particle sizing found in the field (particularly for residual soils and weathered rock) may not match the sizing used in the laboratory test. The rigidity below and at the sides of the site are not the same as that of the test sample due to the boundary set by the mould. The combination of these factors may produce entirely different results between the laboratory and in the field. More compaction energy in the field is introduced by employing heavier rollers, conducting more passes, or constructing thinner lifts. This is the “energy” input for compaction. The layer thickness achievable (for a given targeted DR) is equipment-dependent. Sample preparation and curing are mandatory as part of any test to avoid meaningless moisture-density test results. The compaction and CBR tests require a maximum size be applied with correction factors required to adjust the MDD and OMC

Theory of compaction  145

Figure 5.9-2  E xamples of oversize (>300 mm) XW – SW interbedded materials included in embankment but not reflected in pre-compaction grading results.

Figure 5.9-3  L arge particle breakdown at embankment surface under smooth drum vibrating roller effort (a) 600 mm particle prior to rolling (b) after two roller passes and (c) after eight roller passes.

Figure 5.10 -1  Quality control process.

146 Earthworks

for these oversize particles. Figure 5.10-2 provides some of these considerations for curing times and over sizes in compaction standards applicable to earth fill. Above 35% (or 20% in some standards), a rock fill specification should apply. This interface may be unclear as the material may then not match a rock fill specification. Visual monitoring of in-situ material is essential – do not blindly accept the ­“factual” grading curve, and cross-check with the MDD/OMC and CBR values. Oversized materials can invalidate these results. Figure 5.10-3 illustrates other types of testing associated with quality controls discussed in Chapter 14.

Theory of compaction  147

Figure 5.10 -2  Considerations for applying compaction standards to earth fills.

Figure 5.10 -3  Past - Future testing (Look, 2021)

Chapter 6

Soil and rock strength

6.1  INTRODUCTION TO SOIL AND ROCK TYPES Three types of rocks are formed from different processes (Figure 6.1-1): •

• •

Sedimentary rocks are the most common rock types found at the earth’s surface and sea floor. They are formed from soil sediments or organic remains of plants and animals, which have been lithified under significant heat and pressure of the overburden, or by chemical reactions. This rock type tends to be bedded. Igneous rocks are formed when hot, molten rock solidifies. They are also known as primary rocks and are classified mainly on their mineral content and texture. Metamorphic rocks are formed from other rock types when they undergo pressure and/or temperature changes. Metamorphic rocks are classified as either foliated or non-foliated.

The earth’s radius is 6,371 km and an earthwork’s process can be as little as 0.5 m, but Magma is the molten rock that typically less than 50  m, in depth. Earthis under the earth’s crust. Lava works, therefore, represent a depth of less is the magma that comes out than 8 × 10−4% of the ground being affected to the earth’s surface due to (Renton, 2006). Igneous and sedimentary volcanic activity. rocks dominate the earth’s crust, but near the earth’s surface, sedimentary rocks are more noticeable. Additionally, rocks are often covered by soils, whether transported or residual (in place). Residual soils are formed from the weathering of rock. Transported soils are ­deposited from erosion. Both transported and residual soils may experience additional pressure changes from successive sediments being deposited or reduced pressure changes from erosion effects. Over many millions of years, these soils may again become sedimentary rocks. Each soil type and origin are assessed differently. Coarse-grained soils are assessed differently from fine-grained soils, with friction only as the shear strength in the former. The soil’s origin affects the angularity and even the grading which also affects the soil strength. DOI: 10.1201/9781003215486 - 6

Soil and rock strength  149

Igneous

Cooling of molten material •~ 80% of volume of earth's crust but generally covered by sedimentary and metamorphic rocks. ~15% of earth's surface. Hard and crystalline texture. Examples: •Basalt - dark grey / black / fine-grained •Granite - light colured (white / greyish) / medium to coarse-grained •Rhyolite - light coloured (light grey, pale red) / fine-grained Settling and deposition of material from transported sediments; subsequent sedimentation •~ 8% of earth's crust. > 70% of earth's surface. Variable in colour. Examples: •Sandstone - shades of grey / brown / red •Conglomerate - rounded fragments •Limestone - lighter shades Sedimentary

Metamorphic

Squeezing and heating; fault zones formed as consequence •~ 15 of earth's crust. ~ 12% of earth's surface. Variable in colour. Examples: •Slate (black / brown) •Quartzite (white to grey with other colour impurities) •Phyllite (grey / black / greenish but weathers to a tan or brown)

Figure 6.1-1  Rock formation and types. (Data from Renton, 2006.)

Figure 6.1-2  Typical changes in rock properties with depth (Look, 2014 *P1). Weathering (W) Grades: X, Extremely; D, Distinctly. This can be sub-divided into HW/MW – Highly/Moderately, S, Slightly and Fr, Fresh.

150 Earthworks

Similarly, rock types may have their own strength and structure, with the combination of both elements providing the design value. The degree of weathering and jointing affects the rock strength. Compaction of the soil destroys the natural structure and creates a different soil strength envelope. In all cases, water affects the strength properties and/or stability. 6.2  ROCK TYPES The engineering characteristics of rocks are examined from three general conditions: • • •

Competent rock – fresh, unweathered, and free of discontinuities; reacts to an applied stress as a solid mass Decomposed rock – weathering of the rock affects its properties: increased permeability, compressibility, and decrease in strength Non-intact rock – defects in the rock mass govern its properties; joint spacing and opening, width, and surface roughness are some features to be considered

Weathering typically decreases with depth while strength increases. Typical changes in these types of rock are shown in Figure 6.1-2. However, there are cases where strength inversions occur. For example, a sedimentary rock over an igneous rock may have an abrupt change at the rock interface from (say) a moderately weathered sandstone to a highly weathered tuff below. The discontinuity at the rock interface may be a weak zone. Soils form from the continuous and slow process of the breakdown of rocks through weathering. Even transported soils were derived from the degradation of rock and then transported and deposited elsewhere. 6.3  SOIL TYPES Soils are predominantly derived from the weathering of rock and the decay of organic matter. Residual soils develop and remain at their place of origin whereas transported soils are moved from the location where they were originally formed. Transport can be achieved by ice (glacial), water (alluvial), or gravity (colluvial). Generally, transport is through a combination of these modes and to a lesser extent, other processes such as by wind or man. Various types of transported soils (alluvial, colluvial, etc.) are presented in ­Table 6.1. The transporting mechanism determines its origin classification: There may be an upper young alluvium (say, loose sand) • Alluvial – deposited by water ­overlying a middle alluvium (say, • Glacial – deposited by ice soft clay) overlying an older • Aeolian – deposited by wind lower alluvium (say, gravel or • Colluvial – deposited by gravity dense sands). • Fill – deposited by man

Soil and rock strength  151 Table 6.1  Soil classification according to origin (Look, 2014

* P1)

Classif ication

Process of formation and nature of deposit

Residual

This soil overlies bedrock and is the result of chemical weathering of parent rock; becomes more stony and less weathered with increasing depth. Materials transported and deposited by water; usually pronounced stratification; gravels are rounded; a relatively high water table is often associated with this deposit. A relatively homogenous deposit for a given stratigraphy as compared to residual soils or colluvium. Above the water table, a stiffer crust may occur due to over-consolidation from wetting and drying. Material transported by gravity; heterogeneous, with a large range of particle sizes. Material transported by glacial ice; broad gradings; gravels are typically angular. Material transported by wind; highly uniform gradings; typically silts or fine sands. Formed in place by growth and decay of plants; peats are dark coloured. Ash and pumice deposited in volcanic eruptions; highly angular; weathering produces highly plastic, sometimes expansive clay. Materials precipitated or evaporated from solutions of high salt contents; evaporites form as a hard crust just below the surface in arid regions.

Alluvial

Colluvial Glacial Aeolian Organic Volcanic Evaporites

Figure 6.3-1  Predominance of soil type (Look, 2014

*P1).

152 Earthworks

Table 6.1 provides a classification of the main soil types according to origin. The process of formation and nature of deposit for each classification is described. In many cut-to-fill earthworks, residual soils tend to dominate. However, colluvial deposits at the base of slopes should be identified as potential unstable elements. A gradation test and site observation can often identify the different soil types present at a site. Figure 6.3-1 illustrates the predominance of soil types with alluvium material found in the flat flood plains and colluvium material at the base of hill slopes. Understanding the geomorphology and geology of the site is essential to the development of an appropriate ground model. Coastal areas (within 25 km of the coastline) are likely to have deep alluvium material made up of older and younger layers. The predominant materials tend to be finegrained silt, clays, and sands. The residual profile tends to be deep – (say) 20 m or even more if nearer the coastline or rivers. Away from the coastline, shallow alluvium units and coarser materials are more likely to be present. Shallow residual materials are also present. Note that the definition of coastal areas varies by country – a much greater distance is found for a country with large alluvium flood plains and extensive inland rivers. 6.4  TYPES OF SOIL STRENGTH The shear strength of a soil is its maximum resistance under a given load. This is usually at a given water content and confinement. As water content may vary over time, so too its strength can vary, even with a constant load. The soil may have: • •

A drained strength with some minor cohesion (c′) but with the friction angle (Ø′) dominant for the long-term condition An undrained strength (cu) which is a short-term condition

Strength is expressed in terms of: • •

Shear strength for cohesive material Relative density for granular material

Cohesive soils are sensitive to moisture changes while granular materials are sensitive to confinement. For sands and gravels with little to no fines, the long-term and shortterm strengths are the same, and with c′ = 0. For materials with fines, the long-term and short-term strengths are different. The Mohr-Coulomb (M-C) model is widely used in geotechnical engineering. The M-C shear strength (τ) is a combination of cohesion (c) and internal friction (Ø). Mohr’s circle of stress expands to a failure envelope, depending on the principal stress and the confining stress (Figure 6.4-1). Cohesion is considered independent of normal stress (σ), but may be stress-­ dependent at low stresses i.e., is not necessarily a constant value. Above that envelope of shear strength, a failure region applies. The M-C model has evolved as shown in ­Figure 6.4-2 with (a) the traditional Mohr-Coulomb model and (b) the updated model with peak (φp) and constant volume (φcv) values of friction. These graphs have the ­abscissa and ordinate as σ′ and τ, respectively for a saturated soil.

Soil and rock strength  153

Figure 6.4 -1  Mohr’s circle of stress expands to a failure envelope.

Figure 6.4 -2  Mohr-Coulomb model and its current variant.

Table 6.2  Critical degree of saturation Soil type

Sat crit

Gravels Silts Clays

≥20% 40% –50% ≥85%

154 Earthworks

τ = c ′ + σ ′ tan ∅ ′ For short-term conditions in clays, the undrained strength applies and c = cu, and Ø = 0.

τ = cu For undrained conditions

σ 1 − σ 3   = 2  cu The effective stress σ′ = normal stress (σ) – pore water pressure (u). This is the first part of Terzaghi’s principle of the effective stress relationship and accounts for shear strength changes (Atkinson and Bransby, 1978). The second part of the principle states, “all measurable effects of a change in stress such as compression, distortion and a change of shearing resistance are exclusively due to changes in ­effective stress”. The effective stress principle applies for saturated soils. Refer to the large text box in this chapter for further discussion on shear strength for unsaturated soils. Additionally, when deformation is required, a yield surface that combines failure with stress strain behaviour is required. Volume change behaviour at the critical state is discussed in Atkinson and Bransby (1978). Chapter 4 presented the volume-mass equation: S   e = w  Gs For saturated soils (S = 100%), changes in moisture content are directly related to volume changes as represented by the void ratio (e). For unsaturated soils, there is water storage that presents an added complexity for volume change behaviour. Moisture changes affect soil suction that affects volume change behaviour. Jennings and Burland (1962) showed a critical degree of saturation (Satcrit) where there is no unique relationship between void ratio and effective stress (Table 6.2). Thus, soils above Satcrit still exhibit saturated soil behaviour. Well-graded gravels or clayey gravels with significant fines are likely to show “clay” behaviour. Fredlund and Rahardjo (1993) discussed, in detail, (refer to text box) the extended Mohr-Coulomb failure envelope for an unsaturated soil with a third abscissa axis of soil suction, (ua − uw).

Unsaturated soil mechanics often govern in compaction mechanics. However, in current construction controls such measurements are not state of practice.

Soil and rock strength  155

The effective stress (σ′) was first defined by Terzaghi in 1936. This is the stress state variable for a saturated soil. Bishop (1959) suggested an adaptation for unsaturated soil behaviour with independent stress variables (σ − ua) and (ua − uw) to describe the mechanical behaviour of unsaturated soils. The latter term is also called the matrix suction.

σ ′ = ( σ − ua ) +   χ  ( ua − uw ) where ua = pore air pressure uw = pore water pressure χ = a parameter related to the degree of saturation of the soil When the soil is saturated χ = 1, ua = 0 and uw = u →σ′ = σ – u When the soil is dry χ = 0, In the field ua = atmospheric pressure and is used as the datum (zero) →σ′ = σ – ua Fredlund et al. (1978) extended the Mohr-Coulomb shear strength equation to

τ = c ′ + (σ − ua ) tan ∅ ′ + ( ua − uw ) tan ∅ b where c′ and Ø′ are the effective cohesion and friction angles (ua − uw) = matrix suction Øb = a soil property that reflects the influence of the suction on shear strength Further details on unsaturated soil behaviour can be found in Fredlund and Rahardjo (1993). Vanapalli et al. (1996) developed the model further for the prediction of shear strength with respect to soil suction by including more easily measurable parameters. The volumetric moisture content is related to the degree of saturation, gravimetric moisture content and density.   θ − θr   τ = c ′ + (σ − ua ) tan ∅ ′ + ( ua − uw ) tan ∅ ′    θ s − θ r    where τ = unsaturated shear strength c′ = effective cohesion φ′ = effective friction angle σ = total confining stress uw = pore water pressure θ = volumetric moisture content θs = volumetric water content at saturation θr = residual volumetric water content The latter terms introduce the essential soil water characteristic curve, which relates the volumetric moisture content to the soil suction.

156 Earthworks

6.4.1  Critical strength At low confining stress, the failure envelope may also be affected by confining stress as well as whether the soil is saturated or unsaturated. Typically for disturbed soils (such as recompacted material), cohesion does not apply or is very low. The updated M-C model (Figure 6.4-2b) with a constant volume strength (φcv) applies to many cutting slopes. This allows for a reduced, or no, cohesion at high strains which applies in the long term. This is the critical strength value. The “apparent” cohesion is considered an interlocking that reduces at increased normal stress and as water is sucked into the soil at that stress level. Although the effective stress relationship includes a value of cohesion, Skempton (1970) showed the decay of strength with time to a “fully softened value” for first time slides in overconsolidated clays. This “softened” value is also called the critical strength. This is a low value (1–5 kPa) and not necessarily the constant volume zero value. Discussions in earthworks embankment generally concern a peak strength – a maximum shear strength. Strain at the peak strength is relatively small (say, 1%). At larger strains, a critical state strength (constant volume) may then apply – i.e., the strength where distortion continues at constant volume. At very high strains of about 10%, a residual strength may apply. This is lower than the critical strength. Strain affects the strength (and modulus) values obtained in the laboratory, and this should be considered when these parameters are applied in design. Embankments on soft clays experience a large strain as the underlying soft clay deforms near its peak strength (Figure 6.4-3). The embankment material and underlying soil act at the same time at failure (strain compatibility). Analysis requires a peak strength of the underlying soft clay and a post-peak value of the overlying embankment material (which is a remoulded peak strength during compaction). In this slope analysis case, a peak embankment strength does not apply. If the underlying clay is a very stiff layer, then the peak embankment strength governs the stability (Figure 6.4-4).

6.4.2  Residual strength Chandler and Skempton (1975) related the height of the embankment to long-term slope stability for cuttings in fissured clays. Residual strength applies to very large strains and pre-existing failures. Applications of residual strengths include slope stability with pre-existing failure zones, where large strains would have occurred.

6.4.3  Compaction induced strength In compaction mechanics, material compacted dry of OMC will likely have an unsaturated soil strength. Yet this is not typically accounted for in the design, despite the large influence of soil suction on soil behaviour. Very loose soils (prior to compaction) have a metastable structure with large voids. There is also low suction in this state. During compaction, collapse occurs with the initial roller pass, and suction and other shear strengths increase with each pass.

Soil and rock strength  157

Embankment Soft Layer

Stress

Embankment strength

Strain at failure is post peak strength of embankment material

Foundation strength

Strain at failure is peak strength of soft clay

Failure strain

Strain

compatibility for embankment design on a soft clay

Figure 6.4 -3  Strain compatibility for embankment design on a soft clay.

Residual strength should not be confused with residual soils.

Embankment Very stiff Layer Figure 6.4 - 4  Peak strength for embankment design on a very stiff clay.

“Loose” refers to compaction state strength and not the density of granular soils.

158 Earthworks

In dumped soil, there is little confining stress initially (σ 3 ~ 0). This is a “prequel” to the compacted failure envelope. The soil “fails” in collapse with the compaction stress (σ1) induced by the roller, i.e., it is above the failure envelope. After a few passes of the roller, the voids reduce, and the material acquires a confining stress. The M-C failure envelope then applies at n passes (Figure 6.4-5). After n passes to achieve the material’s “minimum” strength, the roller has a lower contact area (B) with the ground at the n + 1 pass, which results in an increased compaction stress (σ1) for the same roller (Figure 6.4-6). With a locked-in confinement stress (σ 3), the failure envelope expands as compaction stress (σ1) increases due to reduction in B. This contact stress is discussed further in Chapter 11. A minor change in σ 3 may also occur. Chapter 4 discussed the fundamentals which control the soil properties. These are: 1. The state of the soil – its air-water-solid phase mix, and 2. The type of soil – its classification, grain size, and plasticity Compaction removes the air component, but the type of soil affects its shear strength. Natural soils are a combination of fine and coarse-grained materials although ­classified as either one group or the other. Coarse-grained soils are affected more by the normal stress applied (σn). For example, Vallejo (2001) carried out direct shear testing by using binary mixtures (5 and 0.4 mm beads) subjected to static compaction ­(Figure 6.4-7). When the coarse-grained concentration is greater than 70%, the mixture is totally supported by the coarse grains (depending on the normal stress). When the coarse grains are less than 40%, the mixture structure is completely supported by the fine grains. The porosity of the mixtures decreased as the level of compaction increased. There was a minimum porosity of the mixtures which represented the boundary between a coarse grain controlled structure and a partially fine-grain controlled structure (Figure 6.4-7a). 6.5  CLASSIFICATION OF CLAY STRENGTH Strength of clays are commonly measured by different means: •

• •

Field measurement • Cone penetration test (CPT), dilatometer, vane shear, and pressuremeter • Standard penetration test (SPT), dynamic cone penetrometer (DCP); these tests are more reliable in granular materials and less reliable in clays Laboratory • Triaxial, shear box • Unconfined test (qu); this test is less reliable for fissured or soft clays At subgrade level • Visual observations are also part of testing. A vehicle becoming bogged or the creation of surface depressions from simply walking across the site (heel tests) are also “tests” on subgrade bearing capacity – albeit non-quantitative values. • Plate load tests (PLTs) and light falling weight deflectometer (LFWD) are more reliable.

Soil and rock strength  159

Figure 6.4 -5  Initial passes induce soil “failure” until soil acquires confinement (σ 3 ).

Figure 6.4 - 6  A fter confinement, compaction stress σ 1 increases, and failure envelope expands.

160 Earthworks

Typically, less reliable tests are less expensive. In a limited budget situation, the tradeoff is between the minimum number of tests (i.e., costs) and quality of data. Table 6.3 provides clay strength classifications and field identification. The classifications are broad, and the field and laboratory tests mentioned above are used to refine these values. For example, for a 5 m embankment load, a firm clay at 28 kPa is likely compressible with foundation or stability issues, whereas a firm clay at 48 kPa may be adequate for that load – depending on layer thickness and depth to layer. Adjacent to waterways, high water content is likely and loose sand or soft clays may be present. Water has no shear strength that presents both a construction and long-term design risk (Figure 6.5-1). As the water content and liquidity index of the material increase, its strength decreases. On-site assessment can provide an early indication of the material’s strength at the surface. Figure 6.5-2 illustrates the allowable capacity (qall) for no depressions to occur for common surface-trafficking loads. A simplified expression for qall is: qall ~ 2Cu ( factor of safety ~ 2.5 ) Tracked plant typically imposes less ground pressure than a wheeled vehicle or person. However, the zone of influence is usually much deeper. Deep underlying material may govern the site’s stability and settlement, which is not evidenced by the surface depressions. An overconsolidated crust often occurs at the surface. On land, the crust is often stronger due to repeated drying and wetting, or changes in the water table (Figure 3.4-3). 6.6  CLASSIFICATION OF STRENGTH OF GRANULAR SOILS Granular soils are considered non-cohesive. However, most soils are mixtures with a clay matrix, and while predominantly granular, the fines content provides some (temporary) cohesion. Granular behaviour changes between 15% and 35% fines, with the soil no longer permeable and classified as “clay” or silt” with 35% fines. This strength discussion applies to predominantly granular soils (clean sands and gravels). Strength assessment is considered in terms of relative density (Dr). Dr =  

emax − e emax −   emin

where emax = void ratio of soil at loosest state emin = void ratio of soil in densest state e = in-situ void ratio (refer Chapter 4 for various phase relationships) A well-graded gravel may have a value of 0.46–0.26 for emax–emin, while a clayey gravel would vary from 0.28 to 0.18. Typically, the in-situ e value ranges from 0.2 to 0.4. Void ratios of sands greater than 0.6 and with the PI of fines 200

Exudes between fingers when squeezed Can be moulded by light finger pressure Can be moulded by strong finger pressure Cannot be moulded by fingers; indented by thumb pressure Can be indented by thumb nail Difficult to be indented by thumb nail

Do not confuse the terms “relative compaction” (used in compaction testing) and “relative density” (used for strength assessment of granular materials).

Relative density is not typically used for compaction control.

162 Earthworks

Other values are shown in Figure 6.6-1. Note emax shown is theoretical only. Das and Sivakugan (2011) provided some useful relationships for granular materials and ranges of emax − emin (Table 6.4): emax ~ 1.62 emin emax = 0.6042 D50−0.304 where D50 is in mm emin = 0.3346 D50−0.491 estd = 0.4484 D50−0.356 where estd is the void ratio from the Standard Proctor test The relative density, Dr, is also expressed in terms of unit weight. Dr =

{γ {γ

d

}

− γ d ( min )  γ d ( max )

d ( max )

}

− γ d ( min )  γ d



where γd(max) = dry unit weight in the densest state γd (min) = dry unit weight in the loosest state γd = in-situ dry unit weight Typical values are provided in Figure 6.6-1. However, none of the above parameters are easily measured at depth. Engineering professionals prefer an easier (although less accurate) measure. Table 6.5 provides the strength classifications for granular materials, based on the field standard penetration test (SPT). These relationships are commonly found in books and standards. Note that Terzaghi’s original classification was simply three parts: • • •

Loose 65%

The current application extends the strength classification range to include: • •

Very loose 85%

Table 6.4  R ange of e max  − e min for sandy soils with  200 kPa

Sands

Very loose Loose Med dense Dense Very dense

0 –1 1–3 3–8 8 –15 >15

ϕ  45°

Gravels, cobbles, boulders*

>10 >20

ϕ = 35° ϕ > 40°

Rock

>10 >20

c′ = 25 kPa, ϕ > 30° c′ > 50 kPa, ϕ > 30°

Notes: • Lowest value applies; erratic and high values are common in this material. • The table should be interpreted left to right for clays. For example, firm clay has an n-value of 1–2. A value of 1–2 is not necessarily a firm clay e.g., this could also be a stiff to very stiff clay in fills and residual profiles. • The top 0.5 –1.0 m of most clay profiles can have a lower DCP value and is indicative of the depth of the desiccation cracks.

Table 6.7  Cone penetration tests (Look, 2014

* P1)

Symbol

Test

qc qt fs FR u0 ud Δu T t 50

Measured cone resistance (MPa) Corrected cone tip resistance (MPa) Sleeve frictional resistance (kPa) Friction ratio = F s /q c In-situ pore pressure (kPa) Measured pore pressure (kPa) Δu = u d  − u 0 (kPa) Time for excess pore pressure dissipation (seconds) Time for 50% dissipation (minutes)

Table 6.8  Soil type based on friction ratios (Look, 2014

* P1)

Friction ratio (%)

Soil type

5%

Coarse to medium sand or gravels Fine sand, silty to clayey sands Sandy clays, silty clays, clays, organic clays Peat (also at FR > 2% and q c   5%. Austroads (2012) uses The Japan Road Association (1989) equation that considers the thickness of materials. The Japan Road Association equation provides a model which determines an equivalent subgrade strength (CBRE) for a multi-layered subgrade system.   CBR E =  

∑ h CBR ∑h i

i 

i

i

i

0.333

3

   

where CBRi = CBR of ith layer hi = thickness of ith layer Σhi = layers to 1.0 m depth The maximum CBR from the use of this formula is 15%.

Soil and rock strength  171

In the field

CBR test

Relatively “soft” support Rigid steel base on top of concrete floor in laboratory

Figure 6.7-1  Rigid (laboratory) and “soft” field supports for CBR assessment.

Pavement base: Lab CBR = 50%

Pavement base: Lab CBR = 50%

Ballast: Lab CBR = 40%

Subgrade: Lab CBR = 5%

Subbase: Lab CBR = 20%

Capping: Lab CBR = 15%

Χ

Subgrade: Lab CBR = 5%



Subgrade: Lab CBR = 5%



Figure 6.7-2  E xample of a laboratory CBR being achievable only with an intermediate layer.

Road pavement example: Subgrades of CBR 5% can only support a maximum CBR 26% above them (5.23 x CBR 5%). The lab CBR of 50% for a granular pavement base cannot be achieved in the field. A pavement design should therefore use a CBR = 20% with underlying subbase below CBR = 50% base material. The base and subbase ratios are also then compatible.

Rail modulus example: Ballast has a typical modulus of 150–250 MPa (CBR ~ 30—50%). If the ballast were to be placed directly onto the subgrade, the subgrade should have a CBR > 8%. Therefore, for any subgrade with a CBR less than 8%, a capping layer (AKA sub-ballast) is required. In the example of a subgrade CBR = 5%, for the ballast to achieve its “lab” value, a capping layer of CBR = 15% is required, irrespective of ballast thickness requirements.

172 Earthworks

6.7.2  Laboratory versus field conditions Figure 6.7-3 presents some of the key differences between the laboratory and field CBR tests. These differences are the same as those between the laboratory and field compaction models. These include: 1. 2. 3. 4. 5.

Underlying support and interaction Side support Compaction energy Limit on material test size allowed Rate of loading

The CBR test is not related to fundamental engineering properties but enables the formulation of empirical correlations. A few aspects of the CBR testing are: • • •

•

• • •

CBRs  100% test result can be obtained, but do not use. The test has a large variability; Rallings (2014) showed less than 60% of results are within ±30% of the median value when multiple laboratories are compared with the same samples in assessing reproducibility of tests on similar samples at different laboratories. The test can use a soaked or unsoaked condition; the soaked condition is representative of a 4-day flood which can produce a reduced strength; however, in Australia, a 4-day flood is not representative of extreme flood events and 7 or 10-day soaked tests are more appropriate for soaked conditions for low permeability materials (Figure 6.7-4). A soaked CBR test is over-conservative in arid environments. A swell value is measured at the end of soaking and is also an important assessment parameter. For low CBR values, the swell value is arguably more important than the CBR test value. A 4.5 kg surcharge is used during the soaked test; this mass should be varied to be representative of the overlying material; both the CBR and swell values are affected.

6.7.3  CBR soaking A high plasticity clay CBR sample soaked for 4 days seldom achieves 100% saturation, with 90%–95% saturation typically achieved, depending on the level of compaction and the clay permeability. When the moisture content in the top 30 mm after soaking is not the same as within the sample, then this is an indicator that saturation throughout the sample has not occurred. Granular materials generally achieve close to 100% saturation in that time.

Soil and rock strength  173

Figure 6.7-3  C BR test – laboratory versus field (Look, 2014

*P1).

Unsoaked material

Unsoaked conditions

•Soaked

•WPI < 200 •Granular materials •Uniformly graded

•< 15% fines •Excellent drainage •Low rainfall (< 500 mm)

•Rainfall > 500 mm •Poor Drainage •High water table

4 -day soaked

4 -day soaked

• WPI < 2200 • Granular material • > 35% coarse material • May apply as design value

• WPI > 2200 • Do not use as design values • Apply corrrection factor if subject to wet conditions

7 -day soaked

10 -day soaked

• WPI = 2200 - 3200 + wet conditions • Mixed with a high fines content • Design value / lower bound

• WPI > 3200 + wet conditions • CH clay materals • Design value / lower bound • Swell likely governs design

Figure 6.7- 4  A pplying unsoaked and soaked CBR tests (Note industry is not generally tolerant of large waiting times to obtain results).

Common industry practice does not follow much of the above. A typical procedure is a 4-day soaked CBR laboratory test targeting 98% MDD and OMC. This model is not representative of site conditions without consideration of climate and environmental conditions. This design model is also a mismatch with the compaction standard for the subgrade which would typically have 95% MDD specified.

174 Earthworks

Saturation (DOS = 100%) represents the likely minimum strength at a given density. To achieve an appropriate design CBR value, it is prudent to carry out one of the following (Figure 6.7-4) when dealing with clays of high PI (which are sensitive to moisture changes) placed in a “wet” condition: • •

Apply a correction factor to the 4-day soaked CBR value in areas of high rainfall or high water tables, or where poor drainage conditions occur Use a 7 or 10-day soaked test, instead of a 4-day soaked CBR test

“Wet” conditions are not limited to flood-prone or waterway areas. “Wet” climates also influence the clay subgrade, and a soaked value may still apply, as is discussed in Chapter 10. For material with low % fines, low rainfall environments, or when the material is not subject to inundation, an unsoaked CBR test generally applies.

6.7.4  CBR from DCP test The DCP is often used for the determination of the in-situ CBR. Various correlations exist, depending on the soil type. A site-specific correlation should be carried out where possible. The correlation is not as strong for values ≥10 blows/100 mm (10 mm/ blow), i.e., CBR > 20%. CBR values from DCP data, specific to soil type, are provided in Table 6.12 ­(Webster et al., 1992). The values in the table are based on: •

Log10 (CBR) = 2.465 − 1.12 (Log10 (DCP)) Where DCP = penetration (mm)/blow

Webster et al. (1994) suggest the following CBR relationships for various soil types: • • •

Gravel, sand, and silt – CBR = 292/(DCP)1.12. This is similar to Austroads. High plasticity clays – CBR = 1/0.002871 DCP. CBR values above 25% are not shown in the table. Low plasticity clays – CBR = 1/(0.017 DCP)2

For subgrades, upper values of CBR of 20% and CBR of 10% apply for low and high plasticity clays, respectively. Hence upper limits have been applied herein to Webster et al. (1992) equations in Table 6.11. Webster’s work is based on the 8 kg cone as per Figure 2.5-4 in Chapter 2, while Austroads uses the 9 kg cone. The energy is similar, but the cone tips are different. The DCP is highly variable in the active zone as was discussed in Chapter 2.

Soil and rock strength  175 Table 6.11  DCP-CBR relationships for varying soil types (Look, 2014 Blows/100 mm

20

* P1)

In-situ CBR (%) Austroads (%)

General (%)

Gravel, sand, and silt (%)

Low plasticity clay (%)

High plasticity clay (%)

50

40

50

 3 MPa) metamorphic or igneous rocks. Conversely, rock with IS (50) 10

Note: A  xial and diametral tests should be carried out in anisotropic rocks. An anisotropic rock has different properties in different directions.

Figure 6.9-3  C hange of drilling techniques shown as change in rock weathering – common error.

Figure 6.9- 4  Rock quality designation (RQD) measurement (Look, 2014

*P1).

186 Earthworks

Small strain rock shear modulus (G max) is increasingly being used in routine design by applying a reduction factor applicable to foundations under normal serviceability conditions. The shear modulus (G) can be measured by shear wave velocity. While the velocity measurement provides a reliable means of determining the small strain modulus, this represents only an upper bound, and an appropriate modulus degradation factor is required. In practice, as the strain increases, G will decrease, as shown in Figure 6.9-5. The secant modulus may be 20%–40% of G max for a practical range of a factor of safety (Poulos, 2015). The small strain Young’s modulus (E 0) is related to G by using the Poisson ratio (ν): E0 = 2G (1 + ν ) Look et al. (2016) used the PUNDIT (portable ultrasonic non-destructive digital ­indicating tester) to compare with UCS and modulus values measured traditionally. A database of various rock types in Southeast Queensland, tested with the intact rock modulus derived from both a non-destructive pulse velocity and traditional UCS testing to failure were compared. Ultrasonic pulse velocity (UPV) testing was used to establish if this would be a reliable test method on Brisbane rock cores and provide the modulus at low strain. The UCS-modulus ratios at low strain levels and at failure were also compared. The results show that PUNDIT is a reliable tool for measuring rock core properties. Figure 6.9-6 compares the traditional strength modulus conversion method versus the UPV method which measures modulus directly. The latter seems to be a more reliable approach to obtain the rock properties with less uncertainty for obtaining UCS and the modulus. However, this is offset by introducing a strain reduction factor. The UCS ~ E 0/1,200 but varied with rock type, strength, and weathering. Overall, this shows that one should avoid second-order correlations which are derived from PLI to establish a modulus. This unreliability is especially apparent in “soft” rocks. Table 6.15  Use of correlations to determine rock strength and modulus Test

2020 (AU $)

Correlations

Possible error

PLI

50

UCS ~ 24 PLI For soft rocks UCS ~ 11 PLI Varies from 4 to 40 PLI

UCS ± 50% with sitespecific UCS/PLI UCS ± 300% without UCS/PLI correlation

UCS only

175

Reference test

UCS ± 25% if measured UCS ± 50% with sitespecific UCS/PLI

UCS + modulus (from velocity) + Poisson ratio

350

E = 200 –500 UCS

E = ±25% if measured E ± 75% (25% + 50%) with E/UCS correlation

UCS + modulus measured + Poisson ratio

500

E ± 125% (75% + 50%) with E/UCS/PLI correlation

Soil and rock strength  187

Figure 6.9-5  Stiffness variation and strain ranges (Clayton, 2011

*P21).

188 Earthworks

Figure 6.9- 6  Traditional method versus UPV method to indirectly determine UCS and modulus (Look et al., 2016 *P22 ).

While modulus is often derived from correlating to UCS, this data also showed that deriving the UCS from modulus measured by the UPV may be just as reliable for high strength and fresh rocks. On a cost basis, this UPV approach is comparable to using the PLI and its correlations for obtaining UCS or modulus values. Figure 6.10-1 summarises the main consideration and multitude of correlations that exist for: • • •

SPT N-value/Point load index Weathering/RQD Rock strength UCS/Modulus

Note that none of these represent the design value as these measurements are focused on the intact strength or modulus and would apply for RQD > 90%, with the field value reducing to 15% of the intact value at RQD > 50%. 6.10  DEGRADABLE MATERIALS Some rock strengths degrade when first exposed during the construction of slopes or excavations. This occurs mainly with sedimentary rocks, which are relatively close to the surface.

Figure 6.10 -1  R ock testing of strength and modulus.

Soil and rock strength  189

190 Earthworks Table 6.16  Degradation assessment (Look, 2014

* P1)

Test

Rock

Intermediate

Soil

Slake durability test (I d ) – two cycles Jar slake test (I j)

>90 6

60 –90 3 –5

 80% • plasticity if slake durability  3  1 90 85 8   7

1.0 – 0.1 83 80 5 4

Properties Lift thickness (mm) Major problems Minor problems Compacted field density, t/m 3 Varies with I s , I d and PI

Potential voids 750 600

Easy to break down 750 550 500 250

≤1