Construction, Maintenance, Restoration and Rehabilitation of Highway Bridges 9781642875508, 1642875503

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CONSTRUCTION, MAINTENANCE, RESTORATION AND REHABILITATION OF

HIIGHWAY BRIDGES

..

"This page is Intentionally Left Blank"

CONSTRUCTION, MAINTENANCE RESTORATION AND REHABILITATION OF HIIGHWAY BRIDGES

K S Rakshit

BSc, BE (Cal), FIE (Ind)

Recired Chief Engineer, Public Works Deparcmenc Governmenc of Wesc Bengal

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CONSTRUCTION, MAINTENANCE, RESTORATION AND REHABILITATION OF HllGHWAY BRIDGES

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n

he author Shri K. S. Rakshit, B.Sc., B.E.(Cal), F.l.E. (India). a retired Chief Engineer of P\VD, West Bengal, is a knowledgeable bridge designer and construction engineer. His previous book "Design and Construction of Highway Bridges"-published in January 1992-is now very much in demand. Activities relating to bridge construction and maintenance in India have. by this time, gained a fair amount of momentum. Failures and distresses noticed in a few bridges in India and abroad, and constructive suggestions made in a review of his previous book, have prompted Shri Rakshit to go in for bringing out this useful book "Construction, Maintenance, Restoration and Rehabilitation of Highway Bridges".

Although there may be some cases of "force majeure''. in many cases restoration and rehabilitation might have been necessary on account of shortcomings in investigation, design, construction and maintenance. Vagaries of nature cannot be ignored at any stage. Necessary appropriate precautionary steps are required to be taken in construction of bridges in saline area and snow regions. Deleterious effects of some gases and factory wastes are also required to be guarded against. Regular inspection and timely maintenance of bridges can, under no circumstances. be ruled out. All these aspects have been duly considered by Shri Rakshit and some specific cases have been elaborated in this book with a number of case studies supported by necessary sketches and photographs. Shri Rakshit was a member of various IRC Bridge Codes and was actively associated with the investigations of causes of various failures and distresses encountered during and after construction of a number of bridges. It is this background and his vast experience in design and construction of bridges which have helped him in writing such an important and excellent book. I believe this book will be of immense help to students, bridge designers as well as construction and maintenance engineers. Enormous efforts made by Shri Rakshit in publishing such a useful book is commendable. I had the opportunity to write a "Foreword" for his previous book. Now it is my proud privilege to write the "Foreword" for this book too.

R. B. Sen Retd. Engineer-in-Chief & Secretary, PWD and Housing Commissioner, Govt. of West Bengal

T

he Book "Construction, Maintenance, Restoration and Rehabllltatlon of Highway Bridges" has been written for the benefit of bridge engineers associated with the construction. maintenance. restoration and rehabilitatio~ of bridges. The engineering students having a special subject on bridge engineering or construction technology will also find the book very useful. The book contains six chapters on bridge construction including durability considerations. two chapters on bridge inspection and maintenance and seven chapters on failure and restoration of bridges as well as rehabilitation of distressed bridges. The appendices contain proforma for original bridge report. proforma for routine inspection and proforma for principal/detailed inspection. To make the subject easily understandable, sketches have been profusely presented. Moreover. photographs of actual construction, failure. restoration and rehabilitation of various Indian bridges have been included

in the book. Case studies have been reported for 25 actual bridges which are put to service after either restoration or rehabilitation or reconstruction when they became unserviceable fully or partly due to failure of or distress in bridge components. The problems encountered and the solutions adopted for these bridges will immensely help the departmental engineers. consultants and contractors who might be faced with similar problems.

-rHE '.AUTHOR

n

he author is a retired Chief Engineer of the Public Works Department of the Government of West Bengal

and was associated with the design and construction of various types of bridges including r6lstoration and rehabilitation works throughtout his service career in various capacities starting from Assistant Engineer up to the rank of

C~ief

Engineer. In addition. he devoted considerable time in studying practical problems and

published as many as 32 technical papers dealing with theoretical and practical aspects of. highways and highway bridges based on his long experience and field studies in the science of bridge engineering. He

has also published a book on bridge engineering under the title "Design and Construction of Highway Bridges" which has been highly appreciated in the Book Revie1111s of various technical journals. The author is a Life Member of the Indian Roads Congress and a Life Fellow of the Institution of Engineers (India). The author acted as1 a member of the Bridge Specification and Standards Committee as well as various other Committees of the Indian Roads Congress in connection with the preparation of the Standard Specifications and Code of Practice for Road Bridges. The author was also a member of few code- making Committees of the Bureau of Indian Standards (ISi). Due to his expert knowledge in bridge and structural engineering. the author was appointed both by the Ministry of 5urface Transport (Roads Wing), Government of India. as well as by the Government of West Beng'."I

a~

a member of several Technical Committees to investigate into the causes of failure of some bridges

and other structures.

PREFACE lllHlliS!llllll:ll!t

lllJilll:lllllll.lllllSl:lllll!tlll!tlll!tilllllill!tlil!:Ul!tl.1111.l!tl:llllllllilsi!lmll.l:IUll!llilillUallllll!!ll.11.111.11.11.i.li!.l!ill.llrnSUlllllllU:m.11111111:.11.:l!tl.IUll!tl8

y first book on bridge engineering under the title #Design and Construction of Highway Bridges# has been published in January t 992. This book contains 25 chapters covering all aspects of design and construction of highway bridges. Although, in my above book. construction aspects of various bridge components have been dealt with, the main emphasis in that book was on the design side for which design of bridge components has been illustrated with the help of structural theory. codal provisions. sound engineering practices and sketches backed by worked-out examples. On the other hand. construction aspects in great details cound not be covered for various reasons. Maintenance of bridges was simply touched upon covering only two pages. In one of the Book ·Review of my aforesaid book. although the comprehensiveness of the book embracing nearly all aspects of bridge engineering and the usefulness of the book as a whole to the students of engineering, practising engineers and consultants have been admitted. the Review has put forward some constructive suggestion regarding the construction and maintenance of bridges. viz. '"Considering the importance that durability aspects have assumed in recent years. one would have wished that the author could have treated the topics of construction and maintenance of bridges in much more details. Probably this could be done in the next edition of the book'". Further, the particular issue of the technical journal in which the abovementioned Book Review appeared. entirely covered a very importanr and timely theme "Repair & Rehabilitation" of which all civil engineers including the bridge engineers are now seriously concerned. The suggestion in the Book Review and the theme of the technical journal referred to above inspired and encouraged me to write the present volume of the book under the title "Construction, Maintenance, Restoration and Rehabilitation of Highway Bridges:· The book is divided into 5 parts. viz. Part I : Construction of Bridges. Part II : Maintenance of Bridges. Part Ill : Failure and Restoration of Bridges. Part N : Rehabilitation of Distressed Bridges, and Part V : Lessons from Failures and Distresses. In this book. the construction and maintenance of bridges including the durability aspect as well as the restoration and rehabilitation of bridges have been dealt with in great details as may be evident from the contents of the book. Since steel bridges are not much used in our country as highway bridges except for long span bridges. less emphasis is given in the book to steel bridges. Proformas for Original Bridge Report as well as for Routine Bridge Inspection and Principal/Detailed Inspection have also been included in the Appendices.

ix

To make the book practical and realistic. attempts have been made to include actual cases of construction. restoration or rehabilitation of bridges as far as possible in addition to theoretical discussions which are considered necessary to make the subject more clear and understandable. For this purpose. photographs of actual construction as well as those of restoration and rehabilitation works have been presented in the book. Sketches have been profusely used to explain the subject matters clearly. Case studies of 25 bridges which had been affected "either by failure or distress and which were either restored or rehabilitated or reconstructed have been presented in the last few chapters of the book.

The reference to IRC or IS codes is based on existing codes. Revised codes shall be followed whenever these codes are revised. Further. to get complete information, the readers are requested to consult the original codes. I acknowledge my indebtedness to Shri R. B. Sen. Retired Engineer-in-Chief & Secretary. PVVD. and Ex-Housing Commissioner. West Bengal. for kindly writing the ""Foreword"" of my book. I am very much grateful to Dr. lndrajit Sen. Associate Planner. CMDA. for kindly going through the manuscript of my book and making useful and constructive suggestions. The assistance received from my son Shri Chandan Rakshit, a postgraduate engineer. during the preparation of the manuscript is also appreciated. I express my deep gratitude and sincere thanks to Mis. New Central Book Agency (Pl ltd., for bringing out the book with such good and beautiful get-up. type-setting. printing and binding. They have made an all-out and whole-'.hearted effort to publish the book in t~. best possible manner to keep their reputation as a publisher of quality books. .I hope. the book-which has been written for the benefit of engineers of the highway departments and construction firms associated with the construction. maintenance, restoration and rehabilitation of bridges-will be very useful in their profession. This book will also be useful to the graduate and post-graduate students of engineering who have special subject on bridge engineering or construction technology. I have no doubt that the users of the book will be immensely benefited. In conclusion. I request the readers of my book to kindly send their suggestions for the improvement of the book. They are also requested to bring to my notice such mistakes that might have crept in in spite of all care and best efforts; for this act of kindness I shall ever remain grateful to them.

P 509 CIT Scheme 47 Calcutta 700 029 April 1995.

K.

x

s.

Rakshit

AcKNOWLEDGEMENTS

A

number of books, journals, literatures and codes of various authors and publishers have been consulted in writing the book. I gratefully acknowledge the contributions of these authors and publishers as mentioned under ffReferencesff at the end of the chapters where necessary assistance from their publications has been received for the book. A number of f!gures and few tables have been taken from some of the aforesaid publications. This has been separately acknowledged mentioning the source within parenthesis where such materials are utilised.

Although adequate care has been taken to acknowledge the contributions of all the authors and publishers, whose assistance has been taken in preparing the manuscript there might be few omissions which are unintentional and may kindly be excused.

·Calcutta. April, 1995

K. S. Rakshit

"This page is Intentionally Left Blank"

CoNTENTS PART I': CONSTRUCTION OF BRIDGES CHAPTER 1 : 1. 1 1.2 1.3 1.4

1.5

1.. 6

'

PROJECT MANAGEMENT, SPECIFICATIONS OF MATERIALS AND WORKMANSHIP

Introduction Components of bridges + Main items of works for bridges Preliminaries to Start Work - Mobilisation of Men, Materials and Equipments Material Specifications and Storage of Materials Quality Control and Quality Assurance for Serviceability and Durability Quality control + Quality assurance + Statistical quality control Workmanship Brick, stone and block masonry + Falsework + Bending and placing of reinforcement + Laying of post-tensioned prestressing cables + Fabrication and erection of structural steel + Concreting +Tensioning of prestressing tendons + Grouting of prestressing tendons +Workmanship from durability consideration + Protective coatings from durability consideration + Cathodic protection for corrosion prevention References

CHAPTER 2 :

3 - 36 3

4 6 8 1O

35

CONSTRUCTION OF FOUNDATIONS

37 - 77

2.1

Types of Bridge Foundations

37

2.2

Fixation of Centre Line of Foundations

37

2.3

Construction of Open Foundations Setting-out of foundation footings + Excavation of foundation trench + Dewatering of the foundation site + Formwork, bending & placing reinforcement and concreting + Foundations on rock + Filling up of foundation trenches '

39

2.4

Construction of Pile Foundations Types of piles + timber piles + Precast concrete piles + Cast-in-situ concrete piles + Tubular steel piles + Screw piles + Pile driving + Pile grouping + Evaluation of load bearing capacity by dynamic formulae . + Load test on piles

43

2.5

Construction of Well Foundations Cutting edge and well curb + Steining + Bottom Plug + Sand filling + Well cap + Shapes of wells + Depth of wells + Sinking of wells + Method of sinking + Tilts and shifts + Bedding of wells + Bottom plugging + Filling the well + Top plug + Well cap References

54

2.6

xi

76

CHAPTER 3:

CONSTRUCTION OF SUBSTRUCTURES

78 - 85

3.1

General

78

3.2

Construction of Piers

78

3.3

Construction of Abutments Weep-holes + Back-fill materials

80

3.4

Construction of Wing Walls and Return Walls Weep-tioles + Back-fill materials

82

3.5

Joints between Abutments and Retaining Walls

83

3.6

References

85

CHAPTER 4:

CONSTRUCTION OF SUPERSTRUCTURES

86 - 128

4.1

General

86

4.2

Temporary Bridges Timber bridges • Prefabricated steel bridges

87

4.3

Semi-permanent Bridges

94

4.4

Submersible Bridges

95

4.5

Low Cost Bridges

96

4.6

Simply Supported Steel Bridges Plate girder bridges + Trussed girder bridges • Cqnstruction methods of s.imply supported steel bridges

J6

4.7

Continuous and Cantilever Steel Bridges Truss bridges + Plate girder bridges

101

4.8

Steel Arch Bridges Types of arch bridges

102

4.9

Small and Medium Span R C Bridges other than Arch Bridges Types of bridges + Casting of superstructure over falsework +Casting of hollow-box superstructure using formwork suspended from steel trusses

105

4.1 O

Reinforced Concrete Arch Bridges Special centering for R C arches + Cantilever construction of arch rib

109

4.11

Simply Supported Prestressed Concrete Bridges Erection of PSC beams by gantry cranes + Erection of PSC beams by tilted derricks + Erection of PSC girders by use of launching truss + Launching of PSC gii'ders by travelling towers

111

4. 12

Continuous, Balanced Cantilever and Cantilever Prestressed Concrete Bridges Casting of superstructure over centering erected from ground or river bed + In-situ cantilever construction + Cantilever construction of superstructure using precast segments

xii

1 18

4.13

Cable-stayed Bridges Second Hooghly Bridge at Calcutta + Chaco-Corrienties Bridge, Argentina

123

4.14

Suspension Bridges

125

4.15

Steel-concrete Composite Bridges

126

4.16

References

127

CHAPTER 5 : MISCELLANEOUS CONSTRUCTIONS AND PROVISIONS 129 - 144 5.1

General

129

5.2

Wearing Course

129

5.3

Expansion Joints

131

5.4

Approach Road

132

5.5

Approach Slab

135

5.6

Protective Works for Shallow Foundation for Minor Bridges Protective works for open rafts + Protective works for multiple boxes

135

5.7

Protective Works for Major Bridges For bridges with waterway from high bank to high bank + For bridges with waterway much less then the width between high banks

136

5.8

Instrumentation for Performance Monitoring Need for instrumentation + Technique of instrumentation

141

5. 9

References

144

CHAPTER 6 : SPECIAL PRECAUTIONS DURING CONSTRUCTION

145 -149

6.1

General

145

6.2

~ome

145

6.3

Precautions to be taken during Construction of Foundations Foundations resting on rock + Pile Foundations + Well Foundations

146

6.4

Precautions to be taken during Construction of Substructures

147

6.5

Precautions to be taken during Construction of Superstructures In-situ constructions + Erection/launching of precast PSC girders + Release of centering of balanced cantilever superstructure

147

6.6

Precautions in respect of Miscellaneous Items Bearings + Expansion joints + Drainage spouts + Joints in concrete wearing course + Guide bunds

149

6.7

Night Construction

149

6.8

References

149

Common Points to be Looked into

xiii

PART II : MAINTENANCE OF BRIDGES CHAPTER 7 :

l

INSPECTION AND DOCUMENTATION FOR THE MAINTENANCE OF BRIDGES 153 - 164

7. 1

Introduction

153

7.2

Alms of Bridge Maintenance

154

7 .3

Inspection of Bridges Bridge Register• Necessity for bridge inspection • Objectives of bridge inspection • Categories of bridge inspection • Access for inspection • Bridge Inspection Cell • Bridge Inspection Personnel + Frequency of bridge inspection + Bridge Inspection Reports • Documentations

155

7 .4

References

163

CHAPTER 8 :

IMPLEMENTATION OF THE MAINTENANCE REPAIRS

165 - 177

8. 1

Categories of Bridge Maintenance

1 f,5

8.2

Steps Involved in Maintenance

lb6

8.3

Methodology for Repair Works Materials for repair works + Techniques of repairs

166

S.4

Formulation and Implementation of Repair Plans Repairs. to !oundations • Repairs to substructures t ~airs_to superstructures • Repairs to miscellaneous works

172

8.5

Documentation of Repair Works

17l

8.6

Evaluation of Performance

176

8. 7

Reference~

177

PART III: FAILURES AND RESTORAION OF BRIDGES CHAPTER 9 :

i

FAILURE AND RESTORATION OF FOUNDATIONS AND SUBSTRUCTURES 181 -202

9. 1

Failure and Restoration vs. Distress and Rehabilitation Failure and restoration • Distress and rehabilitation

181

9.2

Causes and Nature of Failures of Foundations Open raft foundations + Pile Foundations + Well Foundations

182

9.3

Some Foundation Failures and Their Restoration, Strengthening or Reconstruction Rupnarayan Bridge+ Hasdeo Bridge+ Chambal Bridge+ Haldi Bridge

xiv

182

.9.4

Causes and Nature of Failures of Substructures Piers + Abutments, wing walls and return walJs

9.5

Some Substructure Failures and Their Restoration, Strengthening .or Reconstruction Bhagirathi Bridge + Wainganga Bridge + Vansadhara Bridge + Gautami Bridge + Narmada Bridge

9 .6

193

193

References

202

CHAPTER 1 0 : FAILURE AND RESTORATION OF BRIQGE SUJ>ERSTRUCTURES

203-226

1O.1

Causes and Nature of Failures of Bridge Superstructures Failure during construction + Failure during service

203

10.2

Nature of Collapse of Bridge Superstructures

203

10.3

Failure of Some Notable Bridges of the World

204

10.4

Some Superstructure Failures and Their Restoration, Strengthening or Reconstruction First Tacoma Narrows Bridge + Vansadhara Bridge + Yamuna Bridge + Narmada Bridge

10.5

10.6

206

Some Superstructure Failures during Construction-Their Causes and Their Preventive/Remedial Measures Rupnarayan Bridge + Slab and girder bridge + Solid slab bridge + PSC box girder bridge+ Bhagirathi Bridge

214

References

226

PART IV: REHABILITATION OF DISTRESSED BRIDGES CHAPTER 11 : DISTRESS IN BRIDGES

I

229-240

1 1. 1

Types of Distress Distress in foundations + Distress in substructures + Distress in superstructures

11.2

Causes of Various Forms of Distress Corrosion of Steel + Cracks in concrete + Shrinkage cracks + Plastic settlement cracks + Cracks due to surface crazing + Cracks due to alkaliaggregate reaction + Cracks due to sulphate attack + Cracks due to porosity + Spalling of concrete + Cracks due to improper detailing + Cracks due to overloading + Cracks in wearing course + Disintegration +Abrasion+ Scaling + Delaminatlon + Deformation+ Efflorescence + Carbonation + Deterioration + Malfunctioning of bearings + Settlement and tilting of piers & abutments + Misalignment of superstructures

1 l.3

References

229..

. 231

239

xv

CHAPTER 12 : ·NEED AND PROCESS FOR REHABILITATION OF BRIDGES 241 - 253 12.1

What Is Rehabilitation?

241

12.2

Why Rehabilitation is Necessary?

241

12.3

Process of Rehabilitation General + Condition survey by visual inspection + Investigations through study of documents + Investigations by testing + Formulation of repair and rehabilitation plans + Implementation of repair and rehabilitation plans

242

12.4

Monitoring and Instrumentation Need for monitoring + Methods of monitoring + Instrumentation

252

12.5

References

252

CHAPTER 13 : REHABILITATION OF DISTRESSED FOUNDATIONS AND SUBSTRUCTURES 254 - 263 13.1

Causes and Nature of Distress in .Foundations Open raft foundations + Pile foundations + Well foundations

254

13.2

Distress in Some Foundations and Their Rehabilitation Rasulpur Bridge+ Chambal Bridge+ Sunbhola Bridge+ Godavari Bridge

254

13.3

Distress in Some Substructures and Their Rehabilitation Pile ~restle bridge + Approach Viaduct + Ram and Bhogawati Bridges

259

13.4

References

263

CHAPT~ll 14 : .REHABILITATION OF DISTRESSED SUPERSTRUCTURES 264 - 278 14.1

Causes and Nature ·of Distress in Bridge Superstructures

264

14.2

Distress In Some Bridge Superstructures and Their Rehabilitation Thane Creek Bridge + Little Rangit Bridge + Akharaghat Bridge + Gulsakri Bridge + Ram and Bhogawati Bridges + Godavari Bridge + Ghaggar Bridge

264

14.3 · References

277

J

PART V : L&§ONS FROM FAILURES AND DISTRESSES CHAPTER 15 : LESSONS FROM CASE STUDIES TO MINIMISE FAILURES OF AND DISTRESSES IN BRIDGES 15. 1

281 - 315

Lessons from Foundation and Substructure Failures Rupnarayan Bridge + Hasdeo Bridge + Chambal Bridge + Betwa Bridge + Bhagirathi Bridge + Wainganga Bridge + Vansadhara Bridge + Gautami Bridge + Narmada Bridge

xvi

281

15.2

Lessons from Superstructure Failures First Tacoma Narrows Bridge + Yamuna Bridge + Rupnarayan Bridge + Slab and Girder Bridge + Solid Slab Bridge + PSC box-girder bridge + Bhagirathi Bridge

285

15.3

Lessons from Distress in Foundations and Substructures Rasulpur Bridge + Chambal Bridge + Sunbhola Bridge + Godavari Bridge + Pile trestle bridge + Approach Viaduct + Ram and Bhogawati Bridges

287

15.4

Lessons from Distress In Superstructures Thane Creek Bridge + Rangit Bridge + Akharaghat Bridge + Gulsakri Bridge+ Ram and Bhogawati Bridges+ Godavari Bridge+ Ghaggar Bridge

289

15. 5

Summary of Lessons from Failures and Distresses Actually Occurred in Various Bridges

15.6

Focus on Some New Grey Areas Prone to Failure or Distress and Their Remedies Segmental roller bearing + Defective detailing + Settlement of approach slab + Precast footway slab + Contrete wearing coarse

300 303

15. 7

Some ·Dos· and ':'Don'ts· to Avoid Failures and Distresses in Bridges Site selection + Hydrological investigations + Geotechnical investigations + Design discharge + HFL + Waterway + Scour depth + Submersible bridge + Design + Detailing + Construction +Maintenance

309

15.8

References

313

xvii

APPENDIX A

Proforma for Original Bridge Report

317

APPENDIX B

Proforma for Routine Inspection

319

APPENDIX C

Proforma for Principal/Detailed Inspection

321

.\UTHOR 'NDEX

327

SUBJECT INDEX

329

xviii

LIST OF TABLES

TABLE NO.

PAGE

1. 1

Diameter of HoJes. for Rivets

17

1.2

Proponions for Nominal Mix Concrete

18

1.3

Target Mean Strength ·for Various Grades of Design Mix Concrete

19

1.4

Size of Coars~ A$Jgregates in Various Concrete Works

19

1.5

Slumps for Various Types of Concrete Works

20

1.6

Average Output of Concrete per Hour for Various Mixers

21

1.7

Assumed Standard Deviations

28

1.8

Minimum Cement Content and Maximum Water u.. I- u.. ::> a:: wZ a:: ::> -'U. a:: Vl

-·

:.,.

..

(\ ·,;.

{.)

I~

J ••

Ut3'::

0

U:t:

...

o::i:

o:~

0

luUJ

0.

:z:u z .,.,z ;/) WO a::u oth friction and end bearing. 2.4.7.4 Vibro piles are quite similar to the Simplex type and the casing pipe is driven into the ground by hammering it at top and by providing a cast iron shoe at the bottom. The principal difference in: this pile is that instead offilling the pipe with concrete in stages, it is completely filed with concrete of fairly fluid consistency. During lifting of the casing pipe, a special type of hammer which hits an attachment of the pipe upwards is used. The vibration created by the hammer in the pipe and the static head of the fluid concrete helps to withdraw the pipe as w~U as to make a continuously vibrated shaft of the pile. The surface of this sort of piles is smooth and no corrugation is formed. 2.4.8. Bored Cast-in-situ Concrete Piles 2.4.8.1. Bored cast-in-situ concrete piles are found useful in places where the vibrations caused by the driving of the casing tube may be harmful to the neighbouring structures. These piles are cast in the hollow space made by removal of the earth by means of boring. Precautions should be taken to prevent the incoming of the earth into the borehole which should also be protected from necking .:aused by soft soil or piles should be protected during casting from loss of cement due to movement of subsoil water. 2.4.8.2 In recent years, large diameter bored piles (sometimes known as cylinders) are being used in large numbers. When the diameter of these piles exceeds 75 cm, these piles are known as large diameter piles. It has been reported that bored piles of 'loo cm dia may be installed without casing the borehole up to the bottom. Only a casing of about 5 metres in length is driven in the ground initially. The soil inside the casing is loosened by use of a cutting toolwhich moves up and down as well as sideways. The loosened earth is removed by heavy pumping and thus a borehole is made at top. In order to protect the borehole from collapsing, bentonite slurry is poured into the borehole. This bentoni te slurry is a mixture of bentonite and water making a density of 1.08 and bentonite content of 6 per cent by weight of water. The bentonite has a special property known as thixotropy which prevents the w,all from collapse. The loosening of the soil at the bottom of borehole, removing the same by sucking JS.y; a heavy pump and lilling the borehole by fresh bentonite slurry is continued till the final levelof the borehole is reached. After the completion of the boring to the required level, the reinforcement cage is lowered into the borehole and the same is concreted by use of tremie pipe of 200 to 250 mm dia fitted with a hopper at top. The tremie pipe covers the full depth of the borehole. High slump (about 150 mm) concreting is used. Concreting is done always at a constant head and the tremie pipe is withdrawn gradually to prevent ingress of water into the green concrete.

48

CONSTN., MAINTCE., RESTORATION AND REHAB. OF HIGHWAY BRIDGES

2.4.8.3 Removal of soil from the borehole may be done by using an alternate method which consists of a hea:vy cylinder 2.0 to 2.5 m long fitted with flap valves at bottom. These flap valves open only upwards. This cylinder which is known as a bailer is dropped from a height of 3 to 4.5 m into the borehole which is filled with bentonite slurry as described before. The bailer is operated from the ground by means of a winch and tripod and when the bailer is filled with soil after a few fall, the same is lifted up and emptied. This operation is repeated till the borehole reaches its final level. Lowering of the reinforcement cage and concreting are done as usual taking precautio1i.s as before. 2.4.8.4 Special type of piles comparatively of large diameter were used in Thane Creek Bridge, Bombay (Maharashtra). Piers P 7 to P 38 were founded on six Hachstrasser RC C bored piles ~ach anchored to rock by 57 mm deformed bars 16 nos. per pile. External dia of each pile was 1.531 m. The piles were hollow with a wall thickness of 230 mm (Fig. 2.12). '

~--~~c-:;_,. ___'_. _ I -

.llOS.O·OllTmfA~ aoo•.

t

FIG. 2.12 HACHSTRASSER RCC PILES USED FOR THANE CREEK BRIDGE, BOMBAY (IABSE) 4

CONSTRUCTION OF FOUNDATIONS

49

2.4.8.5 These piles were braced with partly precast and partly in-situ bracings at pile cap level. Precast bracings were also provided at 90 cm above LT L whenever necessary. Hachstrasser piling equipment (imported) along with 100 tonnes and 30 tonnes floating cranes were used for pihng operation (Fig. 2.13).

,

FIG. 2.13 INSTALLATION OF HACHSTRASSER PILES FOR THANE CREEK BRIDGE, BOMBAY (IABSE)' 2.4.9 Tubular Steel Piles 2.4.9.1 Tubular piles may be driven open-ended or with cast iron shoes as in casing pipe of cast-in-situ concrete piles. The piles when driven open-ended are filled with soil automatically during driving. The piles with closed end may be kept empty or may be filled with concrete. 2.4.10 Screw Piles 2.4.10.1 A screw pile consists of a circular steel shaft of various diameter ranging from 75 to 250 mm and ending in a large diameter screw blade at the bottom. The screw is a complete turn, the diameter of the blade being 150 mm to 450 mm. The base of the screw piles is installed by screwing them down by means of capst~ with long bars fitted at the tope of piles with the help of manpower. Electric motors are nowadays employed for this purpose but the use of screw piles is becoming rarer day by day. 2.4.11 Pile Driving 2.4.11.1 The piles may be driven by any type of hammers. However, it may be ensured that the · piles penetrate to the design depth or attain the required resistance without being damaged. The weight of the hammer has some relation with the weight of the pile to be driven successfully. This weight generally is about half the weight of the pile but it is always desirable to use the heaviest practicable hammer and to limit the drop or stroke of the hammer so as not to damage the pile. Whenever the piles cease to penetrate, long continued driving shall be avoided as it damages the piles. Over-driving of piles shall be avoided. Any sudden change in the rate of penetration which is not due to the nature of the subsoil strata shall be noted. Driving of piles in such situation shall be discontinued till the cause of the sudden change in the rate of penetration can be ascertained and reconciled. 2.4.11.2 Piles are driven by means of either drop hammer or steam hammer. The hammer is supported by a special frame known as pile-driver which consists of a pair of guides. The hammer moves within the guides and falls from the top of the guide on the top of the piles to be driven. The hammer which I~

50

CONSTN., MAINTCE., RESTORATION AND REHAB. OF HIGHWAY BRIDGES

is lifted by manual labour or by mechanical power and is then released to fall freely by gravity is known as drop-hammer. Nowadays steam hammers are used for pile driving. The steam hammer which is ·lifted by the steam-pressure and is then allowed to fall freely is a single acting steam hammer but the one which is also acted on by the steam-pressure during dQwnward movement and adds to the driving energy is known as double acting steam hammer. 2.4.11.3 Driving of piles in loose sand compacts the sand and as such, increases the skin friction. Therefore, driving friction piles in a group shall proceed outwards from the centre. If the piles are driven otherwise, i.e., from the outside towards the centre, it will be difficult to drive the inner piles to the same depth as the others. Similarly, in case of stiff clay, the driving a group of piles shall proceed from the centre towards the outer side. However, in the exceptional case of very soft soil, the driving of piles shall proceed frnm the outside towards centre in order to restrain the soil from flowing out during driving operation. 2.4.11.4 Piles shall be driven as per design, viz., for vertical piles, they shall be as vertical as possible and for batter piles, they shall be driven true to the prescribed batter. 2.4.12 Pile Grouping 2.4.12.1 Spacing of Piles In case of piles founded ori very hard stratum and deriving their load bearing capacity mainly from end bearing, minimum spacing of such piles shall be 2.5 times diameter of piles. Friction piles derives their load bearing capacity mainly from friction and as such shall be spaced sufficiently apart since the cones of distribution or the pressure bulbs of adjacent piles overlap. Generally, the spacing of friction piles shall be minimum 3 times the diameter of piles. 2.4.12.2 Arrangement of Piles in a Group Typical arrangemer.t of piles in a group is shown in Fig. 2.14. The spacing, S, indicated in Fig. 2.14 shall be as recommended in Art 2.4.12.l.

(a) ·7:PILE

GROUP

(b) 9-PILE

GROUP

s

Id)

15-PILE

GROUP

0

14-PILE

(c)

(e)

s

20-PILE

s

GROUP

s

s

GROUP

FIG. 2.14 TYPICAL ARRANGEMENT OF PILE GROUPING (Teng) 22

CONSTRUCTION OF FOUNDATIONS

51

2.4.13 Evaluation of Safe or Ultimate Load Bearing Capacity of Piles from Driving ResistanceDynamic Formula 2.4.13.1 This method takes into account the work done by the piles in overcoming the resistance of the ground during driving and as such equates the energy of the hammer blow. In some realistic methods, allowances for losses of energy due to the elastic compression of the piles and the soils are also made. 2.4.13.2 Formula for Determining Safe Load R on Piles (Engineering News Formula) (aJ For piles driven with freely falling drop hammer 16.7WH R=~-­ S + 2.54

(2.1)

(b) For piles driven with sigle-acting steam hammer R= 16.7WH s + 0.254 (c) For piles driven with double acting steam hammer R =16.7 H (W + Ap) s + 0.254 where, R

(2.2)

(2.3)

1Safe load on pile in kg,

W = Weight of hammer iri kg, H

Height of fall in metres,

S

Average penetration of the pile in cm per blow measured as the average of the last 5 to 10 blows under a drop hammer and 20 blows under a steam hammer,

A

Effective piston area in sq. cm,

p

Mean effective steam pressure in kg/cm2•

2.4.13.3 Formula for Detennining Ultimate Load Ru on Piles (Modified Hilley Formula) Ru= WHN

s +J:. 2

where,

(2.4)

Ru= Ultimate resistance in tonnes. The safe load is obtained by applying a factor of safety of 2.5, W = Weight of hammer in tonnes, H

l-feight of fall in metres (full value of H for trigger operated drop hammers, 0.9 H for single acting steam hammer and 0.8 H for winch operated drop hammers),

N

Efficiency of the blow, i.e., the ratio of energy after impact to the striking energy of • ram and is given by Equation 2.5 or 2.6,

S

Penetration per blow in cm,

C

Sum of the temporary elastic compression in cm of the pile, dolly, packings and ground as given in Equation 2.7.

52

CONSTN., MAINTCE., RESTORATION AND REHAB. OF HIGHWAY BRIDGES

where Wis greater than P. E. and the pile is driven into penetrable ground, W+PE2 N

=

W +p

(2.5)

where W is less than P. E. and the pile is driven into penetrable ground,

N= where,

w + PE2 W+P

IW - PE]

2

l}v+Pj

(2.6).

P

Weight of pile, anvil, helmet, etc., in tonnes,

E

Coefficient of restitution of the materials under impact and is equal to 0.5 for steel ram .of double acting hammer striking on steel anvil and driving RC pile, 0.4 for cast iron ram of single acting or drop hammer striking on the head of RC pile, 0.25 for single acting or drop hammer striking a cap and helmet with hard wood dolly and driving RC Piles. The value of C in Equation 2.4 is given by the following formula : (2.7)/

where, C1 = Temporary compression of dolly and packing, C2 = Temporary compression of piles, and CJ = Temporary compression of grouhd. Again, the values of C1, C2 and.CJ may be obtained from the following formula: i) ii)

Where the d1fving is without dolly and the cushion is about 2.5 cm thick I Ru . c1 = t.77A Where the driving is with short dolly up to 60 cm long, helmet and cushion up to 7.5 cm thick Ru

c =9.os-;;;:1

Ru. L

~

~=M~A

iv)

Ru CJ= 3.55-p:-

where, Ru= Ultimate resistance of piles as per Equation 2.4,

2.4.14

L

Length of pile in metres,

A

Area of the pile in cm2 •

Load Test on Piles

2.4.14.1 The pile formulae for capacity of piles given in the previous articles as also the static formulae for pile capacity derived from soil parameters predict approximately the safe load the . piles will carry but it is always desirable to verify the load carrying capacity of the piles by actual load tests.

CONSTRUCTION OF FOUNDATIONS

53

2.4.14.2 Initial Tests and Routine Tests There shall be two c;ategories of test for piles, viz., initial tests and routine tests. Initial tests are carried out on test piles at the beginning prior to driving of working piles to determine the length of piles to sustain the design load. Initial test shall be carried out on minimum two piles. Routine tests are carried out on working piles to verify the capacity of piles as obtained by initial tests. While initial tests may be conducted on single pile, the routine tests may be carried out on single pile or a group of piles, two to three in number, the latter being preferable in clayey soils. Routine tests shall be carried out on 2 per cent of the piles used in the foundation. 2.4.14.3 Procedure for Vertical Load Tests The test load may be applied in stages directly over a loading platform as shown in Fig. 2.15 or by means of hydraulic jack with pressure gauge and remote control pump, reacting against a loading platform similar to Fig. 2.15. The difference between the former and the latter method is that while all the test load placed on the platform is transferred on the test piles in the former method, the reaction of the jack is only transferred as load on the piles in the latter method though the load on the platform normally exceeds the required reaction. Pile testing by reaction method may also be done by taking advantage of the adjacent piles which give the required jack reaction by negative friction. For testing of piles by direct loading method, RC pile caps are usually provided on the top of piles for using it as loading platform as well as for transferring the load on the piles uniformly. TEST

CAP "'1

I

I

II

1

I

I

L.l

T E'ST

PILE'S

I

Ll

FIG. 2.15 LOAD TEST ON PILES (Rakshit) 16

•

2.4.14.4 .Procedure for Lateral Load Tests on Piles Lateral load tests may be conducted by jack reaction method with the hydraulic jack and gauge in between two piles or two groups of piles. The reaction of the jack as indicated by the gauge is the lateral resistance of the pile or the pile group. 2.4.14.5 Application of Test Loads, Measurement ofDisplacements and Assessment of Safe Loads for Vertical Lpad Tests (A) For Initial Load Test- The test loads shall be applied in increments of about 10 per cent of the estim'lted total test loads and measurements of displacements shall be done by three dial gauges for single pile and four dial gauges for a group of piles. Each stage of loading shall be maintained till the rate of seWement is not more than 0.1 mm per hour in sandy soils and 0.02 mm per hour in clayey soils or arrurximumof2hours, whicheveris greater. The loading shall be continued up to the test load which. is twice the safe load (safe load as estimated by using static or dynamic formula) or the load at which the total displacement of the pile top equals the following specified value :

54

CONSTN., MAINTCE., RESTORATION AND REHAB. OF HIGHWAY BRIDGES

(a) The safe load on single pile shall be the least of the following : (i) Two-thirds of the final load at which the total settlement attains a value of 12 mm. (ii) Fifty per cent of the final load at which the total settlement equals 10 per cent of

the pile diameter. (b) The safe load on groups shall be the least of the following : (i) Final load at which the total settlement attains a value of 25 mm. (ii) Two-thirds of the final load at which the total settlement attains a value 40 mm.

(B) For Routine Load Tests - Loading shall be carried out up to one and half times the safe load or up to the load at which the total settlement attains a value of 12 mm for single pile and 40 mm for group of piles, whichever is earlier. The safe load shall be given by the following: (a) Two-thirds of the final load at which the total settlement attains a value of 12 mm for single pile. (b) Two-thirds.of the final load at which the total settlement attains a value of 40 mm for a group of piles. 2.4.14.6 For Lateral Load Testl"

The loading shall be applied in increments of about 20 per cent of the estimated safe load after the rate of displacement is 0.05 mm per hour in sandy soils and 0.02 mm per hour in clayey soils or 2 hours, whichever is earlier. The safe lateral loads shall be taken as the least't>f th~ following :

..

(a) 50 per cent of the total load at which the total displacement is 12rrul14t ~E'. cut-off level. (b) Total load at which the total displacement is 5 mm at the cut-off level. 2.4.14.7 Pull-out Test on Piles

For this test, clause 4.4 of "IS : 2911 (Part IV) - 1979 : Code of Practice for Design and Construction of Pile Foundation-Load Tests on Piles" shall be referred. ' 2.4.14.8 Cyclic Load Tests & Constant Rate of Penetration Tests

Datails of these tests are given in Appendix A and Appendix B of "IS : 2911 (Part IV) - 1979", respectively which may be referred in this regard. 2.5

CONSTRUCTION OF WELL FOUNDATIONS

Where pile foundations are unsuitable due to site conditions, the nature of the soil strata, or for the reason of comparatively deep scour, well foundations are adopted.

2.5.1

2.5.2

Cutting Edge and Well Curb

2.5.2.1 At bottom, wells are provided with a steel cutting edge made of mild steel plates and angles

rivetted or welded together and anchored into the well curb by means of anchor bars. Concrete well curbi are triangular in section in order to assist in removing the earth by grabbing and to help easy sinking of th~ wells. The inclination of the well curb should not exceed 35 degrees with the vertical. These curbs are properly reinforced so as to make it strong enough to resist the stresses during sinking. Usually, reinforcement both in the form of stirrups and longitudinal bars are provided not less than 72 kg per cu.m excluding bond rods of concrete. Link bars are used to keep the longitudinal bars and stirrups in position. The concrete to be used in well curb shall be generally of grade r..J 20 but in situations where-very stiff clay with kankar is anticipated, the grade of concrete shall be made of M 25

CONSTRUCTION OF FOUNDATIONS

55

in order to resist the iqipact force of the chisels to be used in loosening the hard strata (vide Art. 2.5.10.1 ). Typical cutting edge and reinforcement for well curb actually used are shown in Fig. 2.16. The cutting edge shown is for a very big circular well of diameter 12.0 m for Brahmaputra Bridge. For normal circular wells, the angle used is 150 X 150 mm backed by a plate of 250 to 300 mm.

fp0..,&1~c__:.r_=-_....-.. .!:!L~lf

••

(b)Well curb

rcinforcemeni

.

FIG. 2.16 TYPICAL CUTTING EDGE AND REINFORCEMENT FOR WELi. CURB 2.5.2.2 Where pneumatic sinking is to be adopted, the internal angle of the well curbs shall be steep enough for easy access of the pneumatic tools. In case blasting is to be resorted to sink the wells, the full height of the internal face and half height of the external face of the curb shall be protected with mild steel plate of 6 mm thicknes.i; properly anchored to the curb by anchor bars. Rl'.ltlF"ORCEMV'T n~o... eTE.1MIMG U•41'0

wE.U.· cu"" I

WltLL·CUR&

ACHOR aAR "DA

f"ntlNO 'THC. CUTTINCS EOOE WITH THE. WICLL·CUIU!>

CUTTINO l:OGL.

c.;un11o0 ror.r ,,,,,,.

~

ANCl..L ,lNUPLATL RIV~1'£0 OR "'LLDCD TOGE1HE.A

( Q) DETAIL.S -OF WELL.

( b)

1somm

QET-'JL. ~ OF WE.LL. CURI!>

ANO CUTTINO

E.OGE..

FIG. 2.17 COMPONENTS OF WELL FOliNDATIONS (Rakshit) 16

56

2.5.3

CONSTN., MAINTCE., RESTORATION AND REHAB. OF HIGHWAY BRIDGES

Steining

2.5.3.1 The steining is made of brick or stone masonry or of mass concrete. Nominal reinforcement for mass concrete steining shall not be less than 0.12 per cent of gross section area of steining to resistthe tensile stress that may be developed in the well steining in case top portion of the steining is stuck to a layer of stiff clay and the remaining portion is suspended from top. Two layers of vertical steining bars with binder are preferred to one central layer only. Typical concrete steining with two rows of bond bars is shown in Fig. 2.18.

FIG.2.18 TYPICAL CONCRETE STEINING FOR CIRCULAR WELLS 2.5.3.2 In case of brick or stone ma sonry steining, vertical bond rods shall be provided at the middle of the steining at a rate not less than 0.1 per cent of the gross steining area . These bars shall be encased withconcreteofM20gradewithina column of 150 x 150 size. These columns shall be tied with RC bands

CONSTRUCTION OF FOUNDATIONS

57

of suitable width not less 30P mm and of 150 mm depth. The spacing of such bands shall be 3 m or 4 times the thickness of the steining, whichever is less (Fig 2.19). RC BANC (1:2:.t.) {

300 WIOri

150 x150 CONC. COLUMN B~IC~ MASONRY

(a)

BONO

p LAN

Al

c-o-£-F

BAR----~

(b}0£TAtl-Q,

(c)S£CTION-AB

FIG. 2.19 DETAILS OF BRICK MASONRY WELLS (IRC)1 2.5.4

Bottom Plug

2.5.4.1 The function of the bottom plug is to transmit the load from the steining to the base of well and ultimately on to the foundation soil. The bottom plug is generally in cement concrete of 1:2:4 mix. The depthofbotlomplugshallnotbelessthanhalfthediameterofthedredgeholemeasuredfromthe1'evel point of the well curb up to the bottom of sump. In practice, the bottom plug depth is kept more which is about 30 cm above the top of well curb for small diameter wells and 60 cm above the top of well curb for large diameter wells. 2.5.5

Sand Filling

2.5.5.1 The well pockets are usually filled with sand or sandy clay bt1t sometimes the pockets are kept empty to reduce the dead load of well on the foundation. It is desirable that at least the portion of the pockets below maximum scour level should be filled with sand for stability of the wells. In each case, a top plug is provided over the sand filling. 2.5.6

Well-cap

2.5.6.1 Load from the piers and abutments are transferred to the well-steining through the well-caps which should, therefore, be reinforced adequately to withstand the resulting stresses caused by the superimposed loads and moments. 2.5.7

Shapes of Wells

2.5.7.1 Wells of various shapes are used, depending on the type of soil through which they are to be sunk, the type of pier to be supported, and the magnitude of the loads and moments for which they are to be designed.The shapes shown in Fig. 2.20 and described l\ereafter are very common :

CONSTN., MAINTCE., RESTORATION AND REHAB. OF HIGHWAY BRIDGES

58

(Oxb) ·"

c 0 ) ooue.LE..- o.

..

0

.·:

• ... ·,·'•"",·

( b) OCTAGONAL.

00 ..

( C) OUMf>- &LL.

.

.

.

. ..

( d) SINGLE. CIRC\JLAR ,

(I) TWl"l CIRCULAR. ( f) MULTI-ORE.DC£. HOLE. MOl.IOLITH.

FIG. 2.20 VARIOUS SHAPES OF WELLS (Rakshit)1 6 !'°

2.5.7.2 Double-D, octagonal or dumb-bell shaped wells have generally twin pockets or dredge holes

due to which greater control over the shifts and tilts of wells is possible. In addition, dumb-bell shaped wells offer greater resist.:.nce to tilting in t!te longitudinal direction but while brick or concrete can be used in the construction of well steining in both the double-0 or octagonal wells, labour cost is more if brick steining is used in dumb-bell wells. Nowadays, circular wells with concrete steining are used in most cases. ~ost economical where the moments in both the longitudinal and trans verse directions are more or less equal. Moreover, for the same base area, these wells have lesser frictional surface on account of which lesser sinking effort is required to sink the wells. Twin-circular wells are more or less similar to single circular wells but these are suitable where the length of pier is more but twin-circular wells are not favoured where possibility of differential settlement between the two wells is not overruled.

2.5.7.3 Single circl,llar wells are

2.5.7.4 Multi-dredge hole wells or monoliths are adopted in supporting piers or towers of long span bridges. This sort of monoliths was used in supporting the main towers of Howrah Bridge at Calcutta. The size of the monolith was 55.35 m x 24.85 m with 21 dredging shafts each 6.25 m square. The multi-dredge hole circular well of 24 m diameter has been used in Second Hooghly Bridge at Calcutta. The well has 9 nos'. compartments formed by 4 nos. diaphragms crossing each other at right angles. 2.5.8

Depth of Wells

2.5.8.1 In deciding the founding levels of wells, the following points should be duly considered:

i)

The m~nimum depth of well is determined from the considerations of maximum scour so as to get the minimum grip length below the maximum scour level for the stability of the well.

ii) The foundation may have to be taken deeper if the soil at the founding level as determined from the preceding para is not suitahle to bear the design load.

CONSTRUCTION OF FOUNDATIONS

2.5.9

59

Sinking of Wells

2.5.9.1 The principal features in the sinking of wells are : a) To prepare the ground for laying the cutting edge. b) To cast the well-curb after laying the cutting edge. ::) To build the steining over the well-curb. d) To remove the earth from the well pocket or dredge hole by manual labour or by dredgLTlg and thus to create a sump below the.cutting edge level. The well will go down slowly. e) To continve the process of building up the steining and the dredging in alternate stages. 'fhus, the well sinks till the final founding level is reached. f) If necessam, .kentledge load may be placed on the well steining to increase the sinking effort for easy sinking of the wells. 2.5.9.2 In preparing the ground for the cutting edge, it is not a problem when the location of the well is on a land or on a dry river bed but when the well is to be sited on the river bed with some depth of watt::r, ~ome special arrangements are..tp be made for preparing the ground. These are : a) Open islanding. b)

Island~ with

bullah cofferdam.

c) Islanding with sheet pile cofferdam. d) Floating caisson. a) Open Islanding (Fig. 2.21-a) When the depth of water is small, say 1.0 m to 1.2 m, earth. is dumped and an island is made such that its finished level remains at about 0.6 m to 1.0 m higher than the W Land sufficient working space (say 1.5 m to 3.0 m) round the cutting edge is available. b) Bullah Cofferdam (Fig. 2.21-b) When the depth of water exceeds 1.2 m but remains within 2.0 m. to 2.5 m., cofferdam is made by driving close salbullah piles and after placing one or two layers of durma mat, the inside is filled with sand or sandy earth. Sometimes, two rows of bull ah piles at a distaf\C:e.Of about 0.6 m between the rows are used and the annular space is filled with puddle clay. The inside and the ou~ide rows, being tied together, gives more rigidity. This sort of islanding is adopted in comparatively deep water. c) Sheet Pile Cofferdam (Fig. 2.21-c) Islanding with sheet pile cofferdam is resorted to when wells are sited inside river where the depth of water is considerable and bullah pile cofferdams are unsuitable for resisting the pressure of the filled up earth inside the cofferdam. The sheet pile cofferdams are stiffened with circular ring stiffeners. Fig. 2.22 shows the sheet pile cofferdam actually used in the construction of Rupnarayan Bridge 14 having a minimum depth of water of 6.0 m and a daily tidal fluctuation of 4.0 to 4.5 metres. d) Floating Caissons (Fig. 2.21-d) In very deep waters, the sheet pile cofferdam is not a solution because the hoop tension developed due to the earth pressure of the filling material is tremendous. In such cases, floating caissons are usually employed. The well curb and the steining is made up to certain height with steel sheets braced inside with proper bracings. The space between the inside and outside surface is kept void. The caisson is floated and brought to the actual location. The launching of the caisson is done by

60

CONSTN., MAINTCE., RESTORATION AND REHAB. OF HIGHWAY BRIDGES

filling the annular void space with conc~ete in stages. Before concrete filling, the caisson is carefully centered at its correct position. Due to theweightof the filled up concrete, the caisson goes down slowly and ultimately it touches the bed and it is grounded. The sinking is done as usual by building steining over the caisson and dredging. The grotlnding of the caisson in correct position may not sometimes be possible, specially in rivers with high velocity or in tidal rivers. In such cases, the caissons are refloated by pumping tl1e water kept either in some cells of the multi-cell caissons or in water tanks over the caissons and then regrounded in correct position. Caisson launching of foundation No. 3 of the Second Hooghly Bridge at Calcutta is sho~n'in Fig. 2.23. This foundation had twin circular wells of24.0 m dia having 9 compartments formed by 4 nos. internal diaphragm crossing each other at right angles. The depth of water at the foundation site was -w.:1-1.

( o.)

oPDJ

( b)

1~1..ANO

&UL.LAM

..

conr::~o 11

•IU'L •11~1!.T'"" SHUT PILI!. S

wA111' LLVll! •

CONCRl!.T~

INSIDI!.

THI& AH .... UL.AR $P#rrr.C.E.

( C)

( d)

STU:L SHIEET Pru: COl'"FERD>.M.

F"L.OATIHG

CAISSON.

FIG.2.21 VARIOUS METHODS QF STARTING WELL FOUNDATIONS (Rakshit) 16 I

about 15 m during high tide with tidal vari Ha If- o I an FIG. 2.22 SHEET PILE COFFERDAM FOR LA YING WELL CURB (Ghosh et al) 14

62

CONSTN., MAINTCE., RESTORATION AND REHAB. OF HIGHWAY BRIDGES

1111. CAI'! MO!IPRESING ANO OEWATERI~ TliE WEU.

3 TONS OIESll.

Ol'ERATEO WN:H

FIG. 2.31 EQUIPMENTS AND ACCESSORIES FOR PNEUMATIC SINKING (Bhati)ll The minimum thickness of the carpel should be equal to the thickness 9f the steining having adequate provision for anchoring the same with the steining. The diameter of the corbel opening is '' generally 2.5 mwHh 20 mm dia anchor bolts at 15 cm centres along the periphery to receive the well · adopter which again is fixed in position on the corbel with bolts and nuts with good rubber packings and washers in between the corbel and the adopter. The adopter, which is made of steel plates, is conical in shape with a{\ opening of 1.10 m diameter to receiye the vertical shaft. The lock adopter connects the · air loc.k with ~he vertical shaft. The workmen enter the man-lock through the door and then it is closed. The lock opei:ator opens the air valve and builds up the air pressuxe gradually. When the pressure of the air-lock rmd man-lock gets equal, the air-lock's door is opened and the workmen enter the air-~ck and then into the working chamber through the vertical shaft with the help of ladder. After waiting for few minutes, the workmen start working with pick-axes, crow-bars and other pneumatic tools to excavate th~ hard soil/rock. For excavation in rock, pavement breakers are usually used but where blasting is necessary, the well is decompressed and under-water blasting is done using gelatine sticks.

The advantages of pneumatic sinking over the open sinking are : a) The process facilitates the sinking of wells under almost all odd conditions, such as large size boulders, large tree trunks, presence of hard strata, piping action in the river bed, or where the foundation is laid on a shelving bed ofrock which requires cutting at a considerable depth below the surface. b) Visual examination of sub-strata at various levels during the process of sinking. c) The bottom plug is laid in a dry condition.

The disadvantages of pneumatic sinking are : a) Experienced labours and supervisors as well as special plant and machinery are required. b) The sinking rate is very slow. c) Wxking under pneumatic condition is hazardous to health as people working under pressure experience pains in body joints. ' d) Very senior and experienced engineers' advice is not available since they are not permitted to go to the working chamber under pneumatic condition because of their old age.

70

CONSTN., MAINTCE., RESTORATION AND REHAB. OF HIGHWAY BRIDGES

2.5.10.3 Sinking of Wells under Artesian Conditions Artesian conditions are not commonly encountered during well sinking but where such situations arise, the wells require careful handling. The wells to be sunk under artesian conditions may be liable to shifting or tilting due to continuous flow of water unless the artesian head is balanced which may be done by raising the well steining higher than the spring head. the well thereafter may be sunk under still water condition. When a well penetrates a sandy strata having very fine sand in an artesian condition, the sand m