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theory and practice of foundation design

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Theory and practice_of_foundation_design_readable

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  3. 3. Theory and Practice of Foundation Design T h.1e On e
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  5. 5. Theory and Practice of Foundation Design N.N. SOM Prof.ssor of Civil Engin<ering Jadavpur Univ~rsity, Kolkala S.C. DAS Professor of Civil Eng inuring JiUUJvpur UniverJ;ry, Kolkalo Prentice, Hall of India ~ [lli[U)~ New Delhi - 110 001 2006 Copyrighted material
  6. 6. Rs. 275.00 THEORY AND PRACTICE OF FOUNDATION DESIGN by N.N. Som and S.C. Das 0 2003 by Prentic&>Hall o1 India Private limited., New Delhi. All tigh!S resetved. No part ol tNs bOOk may be reproduced il 8frJ fonn, by mimeograph or any other meens, vNiout permission h writing from the J)l.C)Iistwtr. ISB,..._I1-203·2191)..1 The export l"ighls ot this book are vested solely with the publiShet. Thh'd Printing June, 2006 Published by Asoke K. Gh0$h. Prentice-Hall ot India PMte Umited. M-97, COnnaught Circus, New Oelhl-1 10001 and Printed by Jay Print Pack Private Limited, New Oelhi-110015. Copyrighted material
  7. 7. To Professor NOEL E. SIMONS and Indian Geotechnical Society
  8. 8. Copyrighted material
  9. 9. Contents Prtau I SOIL AS AN ENGINEERING MATERIAL 1 I I Introduction I 2 N amrc of Soil 1.3 Th~ phase 1.3. 1 1.3.2 2 System J Defiojcjons 4 Weight/Volume Relationships 1.4 S 1.6 Soil C lass jfication 4 Index Propenies of Soil 4 1.4.1 Plasticity Chan 6 1 7 Relative Density of Granular Soil 8 I. 7 Some Special Soil Types 9 1.8 Groundwater /0 1.8.1 Types of Water-bearing Formations 10 I 82 Ojy jsjon of Subsurface Water I I 1.9 Engineering Properties of Soil /2 1.9.1 Permeability /2 1.9.2 The Principle of Effective Stress JJ 1.9.3 Pore-pressure in Soil due to Applied Lood 1.9.4 Shear Strength of Soils /7 1.9.5 COM(')Iidation 21 1.9.6 Properties of Soil 26 1.10 Soil Deposits of India 29 Rel~renceJ 30 2 I-3I SITE INVESTIGATION /6 32-53 12 2.1 Jotmductjon 2.2 2.3 Information Extracted from Site Investigation 32 Stages of Site Investigation 3.1 2.3.1 Reconnaissance Study 33 Boring (Detailed Soil Investigation) 34 2.4 J4 2 4 1 Trial Pirs 2.4.2 Wash Boring J4 Ytl Copyrighted material
  10. 10. YIU • Ctmteuu 2.6 2.4.3 Auger Boring 35 2.4.4 Rotary Drilling 36 2.4.5 Percussion Drilling 36 2 4 6 Srahiljz;)tjon of Boreholes l6 Sampling 37 2.5. 1 Sampling from Trial Pils 37 2.5.2 Sampling· from Boreholes 37 2.5.3 Preservation of Samples 39 Testing of Soil 40 27 Field Iesrs 2.5 41 2.7.1 2. 7.2 2.7.3 2 7 4 Vane Shear Test 2.7.5 2.7.6 Direct Shear Test (In-situ) 47 Plate Bearing Test 48 2 7 7 2.8 Standard Penetration Test <SPD 41 Dynamic Cone Penetration Test (DCP1J 43 Static Cone Penetration Test (SC!'D 44 J?u:ssurc:merer Tesr 46 49 Lahoratory -Tests 51 51 2.10 Planning of Exploration Programme 51 2.10.1 Layout and Number of Boreholes 51 2.10.2 Depth of Boreholes 52 2 9 Ground Water Table Rtkrtncu 53 3 SOT!. DATA AND DFSJGN PARAMETERS 3. I Introducrion 54 Soil Investigation 55 3.2.1 Responsibility of Designer 55 3.2.2 Information Required from Soil lnveotigation 55 3.2.3 Soil Test Report 55 Reference 64 3.2 4 FOUNDATIONS: TYPFS AND DESIGN CRITERIA 4.1 4.2 Jnu!lduction 65 Iypes of Foundation 65 4.2. I 4. 3 65- 75 Shallow foundadons 66 Deep Foundations 68 4.2.2 4.2.3 Choice of Foundation Type 70 Design Criteria 70 4.3.1 Bearing Capacity 70 4.3.2 The SenJemeoJ Criteria 71 References 75 Copyrighted material
  11. 11. Contellls + ix 5 STR FSS QISTR IB I rtiON IN sou s 76:116 76 5 I Introduction 52 5.4 76 Stresses due to Foundation Loading 78 5.3. 1 Boussinesg Analysis: Point Load 78 Venical Stresses below Unifonn Rectangular Load 8/ SS Vcnical Srrcsses below IJnj form Circular l nad 5.6 Other Common Loading Types 85 5 6 I I fnjform I joe I oad 85 5.6.2 Unifonn Strip Load 85 5.6.3 Triangular Load 86 5.6.4 Embankment Type Loading 87 Stress at any Point below Rectangular Load 89 5.3 5. 7 S8 S9 In-situ Srress 8! Newmark's O,an 89 Pressum Bulb 9/ 5.10 Rigidity of Footings: Contact Pressure 92 5.1 I Non-homoge.neous Soils 94 5.11.1 Two-layer System 94 5.1 1.2 Three-layer Systems 97 5.1 1.3 Multilayer Systems 99 5.11.4 Non-homogeneous Medium /00 5 12 Nonljnear $ojl /Ot 5. I 3 Approximate Method of Detennining Venical Stress Re(erenc~s 6 /06 11) BEARING CAPACITY OF SHA!.I.OW FOUNDATIONS 6 I 62 l ntrodnrljno 117 Fajlure M ecbanjsril 117-137 118 6.2.1 6.2.2 6.2.3 6.2.4 Prandtl's Analysis 120 Terzaghi's Analysis 122 Skempton Method 125 Meyerhors Method 125 62 S Ham;en·s M ethod 6.2.6 Vesjc's Method /26 126 6 3 1 «a! Shear Eajlure 6.4 6.5 Square and Circular Footings 6.6 Limitations of Theoretical Analysis 6.7 Factors Affecting Bearing Capaciry /29 6.7. I Effect of Ground Water Table on Bearing Capacity 130 Gross and Ne.t Soil Pressure: Safe Bea.ring Capacity 131 Bearing Capacity from Field TestS I 3 I 6.8 6.9 126 127 Bearing Capacity of Non·homogeneous Soil 127 /29 R•fere11ces /36 Copyrighted material
  12. 12. x + Comenu 7 SETI'LEMENT ANALYSIS 7 I 7.2 lntrOOnctinn 1 JR 7 I I Definjrjons 138- 163 118 7. 1.2 Methods of Settlement Analysis I 19 Stress-path 141 7.2. 1 Stresses During loading and Consolidation in the Field 7 .2.2 Influence of Stress-path on the Drained Deformarion of Clav 148 7.2.3 Settlement Analysis by Stress-poth Method 149 7,3 Rote of SettlemenJ 7 ,4 Foundatjon on Sand 7 .4.1 7 .4.2 146 151 152 Elastic Theory I 52 Semi-empirical Method (Buisman. 1948) 7,4.3 Plate Load Test 74 4 Stj)tjc Cone Tt-SJ I 52 / 51 151 Re(trenets 161 8 FOOTINGS AND RAFI' DESIGN 164-!98 164 8 I lntmductjon 8.2 Design 8.2. 1 8.2.2 8.2.3 of Footings 165 Depth of Footing /65 Allowable Bearing Capacity 166 Effecl of Ground Wa1er Table 168 82 4 Senlement 8.2.5 8.2.6 8.2. 7 8.2.8 De.<ign 8.3.1 8.3.2 Dimens ioning Footing Foundations 170 l nterfcrenre Effect /72 Design for Equal Settlement 172 Structural Design of Footings 174 of Raft Foundation 175 Types of Rllft Foundation 175 Bearing Capacity 177 8.3 169 177 8 .3.3 Scttlcmenl 8.3.4 Aoating Foundation 8.3.5 Basement Raft 178 /81 8.3.6 Structural Design of Raft Foundation Referenc-. I 98 9 182 PIJ.E FOUNDATIONS 9 I 9.2 9.3 lntroductjon 199 C!qs..,jfie.ation 200 9.2. L Classi fication Based on Composition 200 9.2.2 C!assjficatjoo Based on Merbod of lnsta!larjoo Pile Behaviour Under Axial J.ood 201 199- 265 201 Copyrighted material
  13. 13. Coments • xi 9.4 9.5 Pile Capacity to Resist Axial Forces 203 9.4.1 Structural Capacity of Piles 203 9.4.2 Pile Capacity from Static Analysis 203 EricljonaJ Resjs(inee 206 9,5. I 9.5.2 Fric1jonal Rcsjstance jn Cohesjve Sojl 207 Frictional Resistance in Cohesionless Soil 210 9.7 9.8 End Bearing 2I 3 9.6.1 End Bearing in Cohesive Soil 2/3 9.6.2 End Bearing in Cohcsionless Soil 213 Critical Depth 215 Pile Capacicy from l n-siw Soil Tests 217 2 .9 Under-reamed Pj!es 9.6 21 7 9.10 Allowable Load on Piles from Static Analysis 218 9.11 Dynamics of Pile Driving (Dynamic Analysis) 2/9 9. II.! ENR Formula 220 9.11.2 Hiley Formula 22/ 9.11.3 Simplex Formula 22/ 9.1 1.4 Janbu's Formula (Janbu, 1953) 222 9.1 1.5 Wave Equation 222 9.1 1.6 Umitations of Dynamic Analysis 223 9.12 Pile Groups 223 9.12.1 Capacity of Pile Group 224 9. 12.2 Pile Spacing 225 9.12.3 Pile Group Subjected to Vertical Load and Moment 227 9.13 Senlement of Pile Groups 228 9.13.1 Pile Groups in Cohesive Soil 228 9.13.2 Pile Groups in Cohesionless Soil 230 9.14 Uplift Resistance of Piles 231 9. 15 Piles under H orjzoocal Forces 232 2 .15.1 Faj!ure Mechanisms 232 9.15.2 Stiffness Factors and Subgrade Modulus 233 9. 15.3 Uhjmate Laleral Resistance 234 9.15.4 Deflection, Moment, and Shear under Working Load 237 9. 15.5 I.S. Code MeJhod 240 9.16 Negative Skin Friction 242 9.17 Testing of Piles 245 9.17.1 Purpose of Pile Testing 246 9.17.2 Causes of Defect in PHes 246 9.17.3 Integrity Testing 246 2 . 18 Load Test on Piles 247 9.18.1 Test Procedure 248 9.18.2 Maintained Load Test 248 9. 18.3 Constant Rate of Penetration (CRP) Test 249 9. 18.4 Pile Driving Analyzer 251 R•for.nus 262 Copyrighted material
  14. 14. IXii • Cnntem.r 10 266-293 WELL FOUNDATIONS 10 I 10 2 lntrodncrjon 266 C lassificarjon 266 I 0.3 Physical CharocteriSLics-shape and Size 10.4 Componen<s of Well Foundation 268 10.4.1 Steining 269 10 4 2 Well Curb 10.5 I 0.6 I 0. 7 10.8 10.9 267 269 10.4.3 Culling Edge 269 10.4.4 Bouom Plug 269 10.4.5 Dredge Hole 269 10.4.6 ln<ermediatefl'op Plug 270 10.4.7 Well Cap 270 Sinking of Wells 270 Physical Characteristics-Depth 270 10.6.1 Scour Depth 270 Allowable Bearing Pressure 27I Forces Acting on Well Foundatlon 272 Lateral Stability 273 10.9.1 Analysis Based on Bu.lkhead Concept 273 I0 9.2 IRC Method 275 10.9.3 Mode of FaUure 281 10.9.4 Recommeodarjons of IRC. 78-. 1283 281 10.10 Well Sinking 283 References 293 II 294=303 FOUNDATIONS ON EXPANSIVE SOII.S I I I lntroductjon 294 11.2 Nature of Ex)?!lnsive Soil 294 1L .2.1 fret·swell Test 294 11.2.2 Differential Free Swell 295 I I 2 3 t Jnrestmjned Swell Tcsr 295 11.2.4 Swelling Pressure 296 11.2.5 Cla.•sification of Swelling Potential 297 11.3 Effect of Swelling on Building Foundations 297 I 1.4 Foundation Design in Expansive Soil 298 11.4.1 Isolating the Foundation from the Swelling Zone: Under-reamed piles 298 11.4.2 Controlling Swelling 300 I J .4.3 Me.a~ures to Wj!hstand Seulernent 102 Re[uences 302 12 GROUND IMPROVEMENT TECHNIQUES 12 I lntaylucrjon 304=345 10:1 I 2.2 Principles of Ground Improvement 304 12.3 Ground T renJment i n Cohesive Snj! 106 12.3.1 Preloading with Venical Drains 306 I2 3 2 Sronr O J!umns V4 Copyrighted material
  15. 15. Conttllf.f + xiii 12.4 Ground Improvement in Granular Soil 326 12.4.1 Heavy Tampir.g or Drop Hammer 326 12.4 .2 ·Dynamic Consolidation 328 12.4.3 Vibroeompaction 330 12.4.4 Compaction Piles 332 12.4.5 ConlrOI of Field Work 332 Rtkr~nus 13 344 EART HQUAKE RESPO NSE OF SOILS AND FOUNDATIONS 13.1 lnlrOduction 346 13.2 Earthquake Characteristics 346 13.2.1 Magnitude 346 13.2.2 Energy Release 347 13.2.3 Intensity 347 13.2.4 Ground Acceleration 348 13.2.5 Response Spectrum 348 13.3 Effects of Earthquake 350 13.4 Ground Settlement 350 13.5 Liquefaction 352 13.5.1 Liquefaction Potential 354 13.6 Effect of Earthquake Loading on Behaviour of Fine-grained Soils 13.7 Building Damage due to Liquefaction 360 13.8 Measures to Prevent Liquefaction 362 13.9 Effect on Superstructure 362 13.10 Dynamic Properties of Soil 363 346-370 358 ReJere11ces 369 14 CONSTRUCTION PROBLEMS 14.1 lntroduction 371 14.2 Common Construction Problems 371 14.3 Stability of Excavation 372 14.3.1 Design of Bmced Cuts 373 14.4 Dewatering 378 14.4.1 Rate of Seepage 378 14.~.2 Methods of Dewatering 380 14.4.3 Field Con1r0l 384 14.5 Land filling 385 14.5. 1 Cohesive Fill 385 14.5.2 Granular fill 387 14.6 Effect on Adjoining Structures 389 14.6. 1 Effect of Vibration 391 R~f tmmces 392 371-392 Appendix 393-394 lntfu .195 .!99 Copyrighted material
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  17. 17. Preface Over long years of association with the profession of teaching and research in civil and geotechnknl engineering. our experiences sugges1 that foundations arc ofte- designed without n lllking all the ess<:nlial parameters into consideration particularly those with regard to geology of the site. soi.l dara! ground conditions. type of structure. and )and use pattern in the vicinity of the construction area. Though the theoretical aspects of foundation design are well understood. the fie ld situations are usually not given the due imponance. As a c·o nsequence, the implementation of the proposed design runs into problems necessilllting changes in constrUction methodology or in some cases. even the design. The book covers the essentiaJ features of foundation design through fourteen chapters. The foundation design requires underslJinding of the soil type, its strength and deformation characteristics. ground wate- tab1e, and other details of the site and the structures. We have r striven to incorporate all these criteria so as to acquaint the reader with all necessary aspects or foundation design. This treatment begins with Chapter I that details the engineering properties of soil as required for the design of sound foundations. Chapter 2 discusses site investigation as lhe next step towards foundation design. This chapter elaborates on the various methods of soil exploration and testing. The design parameters and the importance of proper interpretation of the data collected from soil i11vestigation arc described in Chapter 3 while Chapter 4 introduces different types of foundations and lheir charac.tetistic- . s Chapters S through 7 offer de<ailed study of the mechanical prope11ies of soil including the stress distribution, the bearing capacity of foundations, and settlement. The effect of non~ homogeneity and nonlinearity of the stress-strain relationships on the stress distribution in soils is elucidated and the concept of streSS-path method of settlement analysis is introduced in these chapters. The imponant considerations for obtaining the design parameters from a Jarge amounl of soil test data are highlighted. The design procedures for shallow and deep foundations. a..;; also for well foundations are presented in Chapters 8 through 10. The special requirements of expansive soils are covered and the earthquake response or soils and foundations are empha..;;ized in Chapters II and 13 respectively. The ground improvement techniques commonly used in practice and the constJUCtion problems generally encountered at site are adequately dealt with in Chapter 12. The reader should also find in the book a comprehensive treatment or the design procedures vis~ a ~vis the construction problems and practices (as discussed in the final chapter). The chapters on analylical aspects are followed by worlced-out examples taken from real-life problems which make the reading bolh topical and interesting. Besides, numerous references have been made to actual cases of foundations for better clarity and understanding of the topics covered. XV Copyrighted material
  18. 18. xvi • Preface We have drawn upon our experiences in teaching and consultancy in geotechnical engineering to compile this volume. Almost forty years of tcachjng and research at Jadavpur University have given us the opportunity to face many field situations which hod to be treated in unconventional ways. The inter.& ction with our students has been very helpful during all these years. This book is designed to serve undergraduate and postgraduate students of civil engjneering with interest in foundations-their design, development, and maintenance. lt will be equally useful for practising civil and structural engineers who have to design fou ndations of structures in difficult subsoil conditions. We acknowledge the immense help derived out of our association with colleagues in che civil engineering department at Jadavpur University, Kolkata. We are thankful to Prof. R.O. Purkayastha, Prof. P. Bhattacharya, Prof. S.C. Chakraborty, Prof. S.P. Mukheljee. Or. S. Ghosh, and Or. R.B. Sahu who participated in many fruitful discussions on the subject and gave their invaluable suggestions. We are also grateful to our staff at the soil mechanics division. particularly Mr. Robin Pal, Mr. Apurba Banerjee, Mr. Sisir Monda ), Mr. Ranm Jana, and Mr. Bhupesh Ghosh for their skilled help in many ways. The painstaking task of compiling the manu.~eript was undertaken by Mr. Bivas Roy, Mr. Subhasish Ghooh, and Mr. Hrishikesh Nayak with great interest and patience. We express our sincere thanks to them all. Last but not the least, we express our sincere gratitude to our spouses Smt. Rita Som and Smt. Sikha Das for bearing with the demand of time that was needed to complete the task often under trying circumstances. N.N. SOM S.C. DAS Copyrighted material
  19. 19. Soil as an Engineering Material 1.1 INTRODUCTION From an engineering viewpoint. soils and sof1 rocks comprise all the loose ond fra-gmented materials that are fou nd in the em1h's crust They arc distinguished from solid rock. i.e., the hnrd ond compnct mass i n the earth's body, which cannot normally be excavated by manual means. Soils 3re fonned by disintegr:uion or decomposition of a parent rock by weathering or they moy be depOsited by cr:mspOnation from some other soun.·e. Soils which are formed by the disintegration of 3 parent rock and remain 01 their place of formation are known as re.sidual soils. The disintegration of the. parent rock is caused by physical agents such as tempcroture changes. freezing. thawing etc. or by agents like, hydration etc. When the soil is transpOned from its original bed rock by forces of gravity. wind, water or ice and re-deposited at another location it is known as tra11sporttd soU. Tmnsported soils arc gcnerully sorted out according to their gmin s ize as the velocity of the transporting medium gets reduced away from the source. A fter deposition at a new place, these soils may be subjected to further weathering with the passage of time. Tnmsported soils tare c lassified into differe nt types according ro their mode of 1 ransponmion. r.>eposits of soil thar nre formed by willd 11re called Aeolian deposi1s. Sand dunes and loess are examples or these de.positS. Loose sand is geJle.rally swept by wind and transported close to the surface. If the motion is stopped. it is depOs ited in the form of sand dunes. The common lrllllSported soils are, however, those which have been carried by water or ancient glaciers. Mariue soils which have been can·ied by sea w:atcr and Alluvial soils which have been carried by rivers 3nd streams constilute probably the largest group of transpor1ed soils on earth. These deposits may also be called sedimemary deposits ns they hnve be.en formed by deposition from e ither standing or moving water. The deposilion is primarily caused by the graduaJ decrease in 'elocity or river carrying the sedi ment~. A larger part of the great Indian plains is made up of alluvial deposits. Gltrcinl deposit~ are remnants of the icc age thai were carried along by the moving ice. They are generally found as big boulders at places away from their parent rock and nrc heterogeneous in nature with linle or no stratification. Other sedi mentary deposits :;ti'C the Lacustrine .soils which are depos ited on u lake bed and the Esluaritre soils which arc deposited at the mouth of ::tn estuary. 1 Copyrighted material
  20. 20. 2 • Theory and Practice of FoutJdatiou Desig11 1.2 NATURE OF SOIL Properties of soil are complex and variable, being primarily infl uenced by the geological environment under which they have been deposited. An understanding of soil composition is important in appreciating the behaviour of the soil. Natural soil consists essentially of discrete solid particles which are he.ld together by water and/or ga.< filling the pore space. These particles are. however. not bonded as strongly as the crystals of a metal are and can. therefore. move freely with respect to one another. The si?.e :md shape of grains and the minemlogical composition of soil panicles varies widely in nature. However. in c.oarsegrained soils the most important propenies do not depend on the constituent minerals although locally, the minerals may control the frictional c- aracteristics of the individual h grains. In these soils, the panicles are so large that the forces between the grains other than those due to externally applied forces and gravity are smaJI. The minerals suc.h as mica. feldspar, and quanz which constitute sand and silt do not render any plasticity and cohesion to the soil. Thus. the influence of the constituent minerals becomes appreciable with the decrease in size. Clay minerals are hydrated aluminium silicates in crystalline form. These are generally of three different types (Scott 1965): A 14 Si0 4 0 1o(OH)a Kaolinite Montmorillonite : (AI~.67 MgNao.33)Si,0 10(0H)1.H20 Ulile K,(AI4 Fe4 Mgu)(Si8_7 I y)0 20(0 H), .A Kaolinite has a very stable struciUrc. h generally resists the ingress of water and consequently, undergoes little volume chan,ge whe,n in contact with water. On the other hand, Montmorillonite auracts water and undergoes large swelling and expansion when saturated with water. Most of the black cotton soils of India contain clay minerals of this variety. Illite is less expandable than montmorillonite. A s ingJe panicle of clay consists of many s heets of clay mine,rals piled one on another, as shown in Fig. 1. 1. As each sheet has a definite thickness but is large at right angles to itS thickness. clay particles are believed to be plate shape:d. The Oat surfaces carry residua) negative charges, but the edges may carry either positive or negative charges depending upon the environment 4 ":w=o::::oooc:o=; Potasslum ' L,r----~ Strong r - - - o - l>OO<I Weak bOOd (a) Kaolinite "-'r-----{/ Slrong r---o-bond ( molecules ":w:=-:=c:o::::Jc:~ Fa;rty strong '< bond ) =weak <:Jooc::c:c:~ Potassium I '(b) Montmorillonite Fig. 1.1 ~ molecules (c) tulle Structure of day particles. Copyrighted material
  21. 21. Soil as 011 E.11ginetri11g Maten'al • 3 Particle orientation has an imponant bearing on the engineering properties of a soil. Spacing 1nd orientation of panicles influence the development of interparticle bonds. For cohes.ionless soils. individual grains may be approximated as spheres with loose. dense, or honeycombed structure, as shown in Fig. 1.2. A dense structure is more stable than the loose or honeycombed structure. Figure l.3 shows some simplified structures found in clay. The development of structure is influenced by the origin and nature of deposition of the soil. Thus. the flocculated structure is typical of clay deposits in salt water. This structure may change due to leac.hing or by external influences. auch as loading, drying. freezing. electroosmositic processes. and so on. (a) (c) (b) Fig. 1.2 S bwture of c:oheslonle$$ soits. (a) (b) Fig. 1.3 Structure of eohesl~ SOilS. 1.3 THREE-PHASE SYSTEM Figure L4 shows a typical soil skeleton consisting of three distinct phases-solid(mineral grains). liquid(usually water). and gas(usually air). These phases have been separated to facilitate the quantitative study of the proportional distribution of different constituents. Fig, 1.4 Thre&1)t'lase system,. Copyrighted material
  22. 22. 4 + Tlteory and PmCiice of Foundation De.sigu 1.3.1 Definitions r.. = Water (a) Unit weigh~ W..,IV., Solid particles r,= WsfV1 Specific gravity : Gs = = W,IV"y_. rirw (b) Water conte.nt. IV = (W.IWJ x 100% (c) Void ratio, e = Vl'IV_, = (V., + V8 )/V" n = V,JV = V/(V, + V,) S = V.JV, = V.I(V. + V ) 1 (d) Porosity, (c) Degree of sawnuion. 1.3.2 (a) Weight/Volume Relationships 11 ( b) Se = el( I + e) or =n/( I - n) = wG, (c) Bulk de nsity. r= (d) Dry density. 1fl = (e) e r= (I W + v = Se1 + G, )( r. e ~ = G, 1 + t0 r. X + w) Yd ( f) Submerged densi<y, r' = r - r. c . (G - I) - (I - S)e I I For saturated soil, S = 1.0. ... X r• G - I . r'= t.f+ e x y_. The range over which the typical values of the above parameters vary are as follows: (i) G,= 2.60-2.75 r (ii) = 1.60-2.25 glee = 1.30- 2.00 glee (ii i) (iv) " = 0.25-0.45 (for sand) 0 (for dry soil}-100% (for fully saturated) (v) S r, = 1.4 INDEX PROPERTIES OF SOIL As already menrioned. the clay minerals in fine-grained soils have sufficient surface forces to attract water molecules to the clay particles. The interaction between the clay minerals, water. and various chemicaJs dissolved in the water is primarily res ponsible for developing the consistency of these panicles. Pure water mainly consisL.:; of molec- les of H1:0 but a few of them get dissocialed iniO u hydrogen ions, H+ and Hydroxyl ions. 0 11. If impurities such as acids and bases are present. Copyrighted material
  23. 23. SoU as au Eugiuuring Maltrial + 5 they also dissociate into cations and anions. Sail. for example. breaks up into Na• and CJ- . Since plane surfaces of the clay mineials carry negative charges, cotions including H+ from water are attracted towards the surface of these particles. The water molecules closest to the clay particles. called the adsorbed water. are tightly held to the clay and exhibit propenies which are somewhat different from those of ordinary wa:er. This adsorbed water is believed to give cohesive and plastic propertie~ to clayey soils. It is obvious, therefore. that the amount of water present in a clay will determine its plasticity characteristics and. in tum. ilS engineering properties. The Atterberg limits are designed to serve as an index of the plasticity for clayey soils. refer Fig. 1.5. Plastic 5em1Uquld state stale ! - - - - + - - -;.....----;.....--+Increase of moisture Solid state, SemiSOid state content Legend Sl-Shri'lkage limit PL~astic limit U -4.iquid limit Moisture content Fig . 1.5 limits of consistency. Slarting with a low water content a clayey soil first appears to be a solid and moves to the plastic state with increasing water contenl. The word plastic here refers to the ability of a soil to be moulded into different shapes without breaking up. At even a higher water content. the soil begins to flow as a viscous fluid. The Atterberg limits. that is, the liquid limit (LL), the plastic limit (PL), and the shrinkage limit (SL) indicate the limits of water content at which the consistency of clayey soil changes from one state to another. The Atterberg limits along with the natural water content give useful indication of the nature of the clayey soil. A natural water content close to the liquid limit indicates a soft compressible soil while a natural water content close to the plastic limit is characteristic of a stiff and Jess compressible c lay. Plaoticlty Index, PI The range of water content over whic- a soil remains plastic is called the plastic limit. h i.e. PI = U - PL(%) ( 1.1 ) Liquidity Index, LI It is the ratio of natural water content, w of a soil in excess of its plastic limit to its plasticity index and is indicative of the state of the water c.ontent in relation 10 the liquid limit and the plastic limit of the soil. U= IV - PL LL- PL X 100% ( 1.2) Copyrighted material
  24. 24. 6 • 11r~ory 1.4.1 and Practice of Fourul.arion De.sigll Pluticity Chart It has been observed that propenies of clay and s ih can be correlated at least qualitatively with the Auerberg limits by means of the Plruricil)l Chart, as shown in Fig.l.6. The liquid limit and h the plastic limit or a soil are ploned on the plasticity c- art and the soil is classified according 10 the region in which it falls, the A-line being an arbitrary boondary between inorganic clays and inorganic silt/organic c lays. Table 1.1 gives the liquid limit of some cohesive soils. 70 // 60 ~ U-lino 50 PI • 0.9 (LL - 8} Ci: '> ~ 40 ~ i l 30 / / 20 10 0 0 . /! 10 / ' v 20 CL or OL /, Cl-ML / / / / or OH /' / / A-tine PI • 0.73 (LL - 20) v,; L MH or or OH OL 30 / CH / 40 50 60 Liquid limit, LL (%) 70 60 90 100 Fig. 1.8 Plasticity chart Table 1.1 Soil Consistency limits of some soils Liquid Limit (~) Plastk Limit (e.fl) Alluvial Deposits Boston Blue Clay Chicogo Clay Normal CaJcuua Soil 41 20 ,S8 21 55 28 MariMIE.'Itwtrint London Clay 75 Norwqian Quick Clay Bombay M:uine Cay Cochln Marine Clay Shellbaven Clay 40 90 Illite Kaolinite MontrooriJJonite 29 17 90 40 45 97 )2 Clay Minerals 100 45 so 25 500 50 Copyrighted material
  25. 25. 8 • Tlu~ory and Practice of Fouudatio11 Desig11 Sand Gravel 100 .........__ ' [' .: eo 1 ]; 60 ~ ~ & ~ ~ 6 10 0.6 2 -.... k I j'"'j 0.2 0.06 0.1 1 0.02 .........__ t' Oispeo>ed kaolinite Ctaye~ I'- 0 sandy silt I Flocculated Gra¥elty sand 20 ..... '""--J~montmo1111onlte) 00•um ~ton""' ' Silty ne s*nd Ctay nne r--- [' "() !'I c .. Silt fine coarse meclk.m ooarse "' ....... 0.006 0.002 0.01 .... ' 0.0006 0.001 0.0001 Particle diameter {mm) Fig. 1.8 Grai~size distribution w rve. Dro is the diameter of panjcles correspOnding to 60% fi ner and D 10 is the diameter or the panicle corresponding to 10% finer. The gradation of soil is detennined by the following criteria: c11 = l Uniform soil Poorly-graded soil I < c. < 4 c. > 4 Well-graded soil It must be considered, howeve,r. that the pilrtic1e size alone is not an adequate criterion for the classification of a soil. as the shape or grains and clay fraction may vory widely depending upon the constituent minerals. More elaborate soil classification systems. making use of the Auerberg limits. in addition to che particle size distribution. have since been evolved. The roost comprehensive of lhese systems are the Unified Soil classification system and the Indian Standard Classification System. The Unified Soil Class(ficotio" System divides the soil.into coarse-grai ned soil (having more than 50% retained on number 200 sieve) and 6 ne grained soil (more than 50%. passing through number 200 sieve). Further subdivisions are made according to gradation for coarse-grained soils and plasticity for fine-grained soils and each soil type is given a group symbol (Table 1.3). The Indian Standard Classification System (lS 1498) is simii3C in some respeccs except that the fine-grained soils are divided into three ranges of Jjquid limit as opposed to only two in the unified soil classification system. 1.6 RELATIVE DENSITY OF GRANULAR SOIL The engineering properties of grnnular soil primarily depend upon its relative density, grain0 s ize distribution. and shape of grains. The relative density determines the compactness 1 which the solid grains arc assembled in a soil skeleton and is expressed as enw;- e x 100 emu - C~nin (1 .4) Copyrighted material
  26. 26. Soil as an Engi,eeri11g Material • 7 1.5 SOIL CLASSIFICATION Density. void ratio. and water content are fundamental soil parameters which help to identify ·and assess-at least qualitatively, the nature of the soil deposit. For example, a high void ratio of a sandy soU would indicate a loose state of compaction while a clayey soil with high water content is likely to be more compressible than one with low water content Table 1.2 gives the density, void ratio, and the water content of some common soil deposits. Table l.l Density, void ratio, and water content of some soils - Soil Gmiogk: Mexioo City BW.t dms#y (8/<c) 9.0 1.6 0.7 0.4 1.3 1.0 1.8 1.9 2.0 1.7 350 o.s 2.0 30 2. 1 0.8 1.0 1.5 80 2.0 30 1.9 38 Volcanic Eslultrine Marine Glacio! Alluvlal AlluvW Marine Marine Morine Shd lhav~n clay. En~and London Clay. Sdsee Boulder Clay. England Nonnal Calcutta CJay (UJ>Pef) Nonnal Colcuua Clay (Lower) Bangkok Cwy (Soft) B:;mgkok Clay (Still) Norwegian Quick CWy WaJ~I' Void m1io contmt (Sf!) 60 35 20 so Soil consisJs of solid grains that have various sizes ranging from coarse grained particles such as boulder. gravel and sand down to the fine g.rajned particle$ like, siJt and clay. The grains are classified according to their sizes. The most common system of classification is the M.I.T. system as illustrated in Fig. 1.7. 2.0 CoMU 0.6 0.06 0.2 Medium Sand Fine 0.006 0.02 Coarse Medium 0.002 Flnt $lit 0.0006 eo.roe . . 00002 Medium Flnt (co/IOhJIII) Clay Fig. 1.7 M.I.T. dassifk:ation system. Natural soil generally consists of mixture of several groups and the soil, in such cases, is named after the principal constituent present. For example. a soil that is predominantly clay but also contains some silt is called silty clay. The grain-size distribution of a soil is best represented by the grain-size distribution curves. refer Fig. 1.8. The shape of the curve indicates whether the soil is uniform or poorly graded. or well-graded. llte unifonnily coefficient of the soil is defined as D c., = !:12. D,o ( 1.3) Copyrighted material
  27. 27. Soil as an Engineering MateriDI • 9 where emu = void ratio in loosest stale emiD = void ratio in densest state e = in-situ void ratio The properties of granular soil an: also dependent on their particle size distribution, that is, whether the soil is well-graded or poorly-graded. In well-~ soils, the smaller grains ~ to fill the void$ between the larger grains and thus, malte the soil more compact. The shape of grains (angular sub-rounded or rounded) may also have some effect on the propenies of granular soil. 1.7 SOliE SPECIAL son. TYPES Apart from the common soil types that may be identified by the different soil classification sY1(ems, cenain natum soils are characterized by the properties of their chemical and mineral constituents. Such soils exhibit characteristic features with regard to their streJigth and compressibility and need particular care when used to support a foundation. Or1anic soilJ are those which contain large quantity of organic/vegetable matter in various stages of decomposition. Natural soils may contain varied percentages of organic matter and only a small percentage may be sufficient to affect its properties. In organic clay, vegetable matter is intermixed with the predominant clay mineral while P<Qr consists almost wholly of vegetable matter. Peat is characterized by high liquid limit and sponzy mucture with low specific gravity. The top 10-12 m of the normal Calcutta deposit is a common organic clay of the Bengal basin. A thin layer of peat is often encountered in this depoait at many locations as shown in Fig. 1.9. Dosa1ption ol strata II Glwy/doltl - silty day, ~ lilt - semi· 2... 55 12 28 2.5 0.005 lilly day will calcareous ..-..s 10 30 60 25 8.0 0.002 20 38 20 30 4.5 ~ 25 65 25 10.0 ~-pieooa Ill Bluilll - IV -.ysllowioh brown sandy slltlallty tine sand with OCCISk»naa tenses of brQ'M'I and grey silty day V Moltled - - silly etay wl1h lomlnallono - 0.001 wi1h rusty brown spots VI Brown.1igllt brown lilly 11M lo medium sand ' >> 4(Y Fig. 1.8 Notmal Colc:ulla ~ Copyrighted material
  28. 28. 10 • Theory and Praclice of Foundation DeJ·ign Expamive soils are found in many pans of India, Africa, and the middle-east. The black cotton soils of India and Africa are lhe most common types of expansive clays. These soils have high expansive potential because of the predominant presence of montmorillonite minerals. They are apparently stiff when dry but undergo swelling when saturated with water. e.g .• due to seasonal fluctuation of ground water table. The Atterberg limits along with the percentage of solid particles less than 0.001 mrn is taken as criteria for identification and classification of the expansive soils. Table 1.4 gives the classification of expansive clays. Table 1.4 Classification of expansive clays IJ, of Panicl~s f*rllum Jndu r~sl Data Pkulicily ind<x ('If>) Probabl~ o:pa~rsion. Shri>Wge limit (CJ,) 0.001 mm Dtgrtt of apaJUiolt S41tlrvttd cond;IIOII) 28 20-31 13- 23 3S II 2S-41 7-12 IS-41 IS 1.8 undtr prtuure of 0.07 kg/cm2 (Dry 10 18 10-16 IS 30 20-30 10-20 10 Vesy hish Hlsh Low Low GROUNDWATER The water which is available below the ground surface is termed as groundwatu or subsurface water. Practically all ground water originates from the surface water. The process by which the s~rface water infiltrates into the ground surface and percolates deep into the ground is termed as natural ruluJrge and artificial recharge. Main sources of natural recharge of groundwater include precipitation, rivers, lakes, and other natural water bodies. Artificial recharge of groundwater occurs from excess irrigation, seepage from canals, leakage from reservoirs or tanks, or from water purposely applied on the ground surface to augment groundwater storage. Water from any of these sources infiltrates into the ground and percolates downwards under the action of gravity through soil pores and, rock crevices until further movement is prevented by an impermeable stratum. It is then stored as groundwater. The groundwater exposed to atmospheric pressure beneath the ground surface constitutes the wat~r table. Water table rises and falls based on the amount of precipitation, the rate of withdrawal or recharge. and climatic conditions. Groundwater held by the geolog.ical formation is. however, not static but moves slowly in the lateraJ ctirecdon towards some point of escape and appear as springs, infiltration galleries or wells, or reappears to join the river, or lake. or the sea. 1.8 . 1 Types of Water-bearing Formations Groundwater occurs in most geological formations, of whjch the most important ones are aquifers. An aquifer is defined as a geological formation that permits storage as well as transmission of water through it. Thus. an aquifer contains saturated soil which yields si.gnificanl quantity of water to wells and springs. Sands and gra.vels typical examples of formations which serve as aquifers. Copyrighted material
  29. 29. Soli as an Eugiuttrlng Mnttriol • II Other geological form.ations include aquiclude, aquitard. and aquifuge. An aquiclude may be defined as a geological format.ion of relatively impermeable material which permiiS storage of water but is not capable of transmitting it easily. Thus. an aquiclude contains. saturated soil which does not yield appreciable quantities of water to wells. Clay is an example of such a formation. An aquitard is defined as a geologic formation of poorly permeable or semipervious materiaJ which permits storage of water but does not yield water freely to wells. However, it may transmit appreciable quantity of water to or from adjacent aquifers. A sufficiently thick aquitard may constitute an imponant groundwater storage zone. A formation of sandy clay belongs to this category. An aquifuge is a geological formation of relatively impermeable material which neither contains nor transmits water, for exampJe, solid rocks. 1.8.2 Di'rialon of Subeurface Water As shown in Fig. 1.10, subsurface water can be divided into the following zones: (a) (b) (c) (d) Soil-water zone Intermediate zone Capillary zone, and Zone of saturation ! Ground aurfaca o ~- """' ln~a1e zone ! Cooilary zone f I' Soil water Pellicular and grv....tional water cap111ary wale< Wolef .:le 1- Ground water 1lmpermeoble FJg. 1.10 Zones of subsutface water. Soil-water zone The soil- water zone extends from the ground surface to the major root zone. The soil in this zone becomeS saturated either during irrigation or rainfall. The water in the soil- water zone is gradually depleted by evaporation from within the soil and by transpiration by vegetal growth on the ground surface and if it is not R:plenished. the water content may be reduced to such an extent that only thin fi1m of moisture known as hygroscopic warer remains adsorbed on the surface of the soil particles. Copyrighted material
  30. 30. 12 • Th.ory and Practice of Foundntion D.sign IAtermedlate zone 1be intermediate zone occupies the space between the lowe.r edge of soli-water zone and upper limit of the capillary zone. This zone usually contains static water which is held by molecular and surface tension forces in the form of hygroscopic and capillary water. Temporarily. though. this zone may also contain some excess water which moves downward as gravitational water. The thickness of this zone may vary from zero when the water table is high to more than 100 m under deep water table conditions. Capillary zone The capillary zone extends from the water table upto the limit of capillary rise of water. In this case, the pore space may be considered to represent a capillary and hence, just above the water table almost all pores contain capillary water. Zoae of oaturattou In the zone of saturation, all the interstices are filled wi!f! water under hydrostatic pressure. The zone of saturation is bounded at the top either by the ground water table or an overlying impermeable stratum, and stntches upto underlying impermeable strata (or bed rock). Generally, all soils below ground water table are fully satunoted. 1.9 ENG~G PROPERTIES OF SOU. 1.9 .1" PermeabWty The flow of water through the pores of a soil under a pressure gradient or under a differential head is a common engineering phenomenon. ~epage of water through earth dams and consolidation of clay under a building foundation are some instances where percolation of water through the soil plays an imponant role on the performance of the foundalion. The ease with which water can flow through a soil, called permeability, is therefore. of fundamental importance in soil mechanics. Ducy'alaw Darcy' s law which governs the flow of fluid through porous media is also found to be applicable to soils when now is due to a combination of pressure and positional gradient. With the exception of flow through coarse gnovels, the flow of water through soils is streamiined, and can be expressed as: v = ki ( 1.5) where. "' = average rate of now of water in unit time. i = hydraulic gradient. i.e., head loss per unit length of soil measured in the direction of flow. and k = coefficient of penneability of the soil. Copyrighted material
  31. 31. Scil as an Engineering MateriiJl • 13 * For any given soil, depends on the porosity of the soil. the struCtural amngement of panicles. the size of panicles. the propenies of pore fluid (e.g. density and viscosity) ciA:. (Taylor 1948). The temperature of the pore fluid should theorelically have some effect but for practical purposes. the variation of k with the range of temperature normally encountered in the soil is small. The coefficient of permeabili[)' of a soil can be meuured from the laboratory constant head test. variable head test. or the field pumping test. In view of the hetrogeneity and nonhomogeneity of natural soi~ field pumping tesiS give a better measure of the permeability of the soil in the field. Typical values of permeability for different types of soil are p ven in Table 1.5. Tillie 1.5 Pameobility of cliff.- !ypes of soil Soill)p< Grsvd C>ane sand Medium sand -Fine 5aDCl Slllysand Sllllweuta.d clays lntiCl days t t- to-' tcr•- ur' tO"'-to-' tcr'-tcr' to-'- tcr' tcr'-tcr' Oood Poor Very poo< Darcy's Jaw is valid only for laminar flow. Since Reynolds number serves as a criterion to distinguish between laminar and turbulent flow. the same may be employed to establish the limit up<o which Darcy's law holds good. Reynolds number for this case Is expressed as .- R _ pvd (1.6) Jl where p is mass density of fluid. v is the discharge veloc-ity. and Jl is the dynamic viscosity of the Ouid. Most of the natural groundwater Oow occurs with R, < I and hence. Darcy's law is valid. However. Darcy's law is not applicable in aquifers containing coarse gravels. rocklills. and also in the immediate vicinity of wells where the flow may not be laminar due to steep hydraulic gradients. 1.9.2 The Principle of Effective Stress In a multi-phase system composed of solids and voids. the behaviour of the material under applied stresses depends on how total stress is distributed amongst several components in the soil aggregate, namely the intergranular pressure that acts between the soil grains at their points of contact and the pore pressure which acts in lhc pore fluid. The normal stress on any plane is, in general. the sum of two components. namely the stress carried by lhc solid pan_clcs and the pressure in the Ouid in the void space. The principle of effective stress i Copyrighted material
  32. 32. 14 • 111eory and Pmctice of Foundation Design provides a Slltisfactory basis for understanding the deformation and strength characteristics of a soil under an applied load. This can be simply be stAted as: (a) The volume change of a soil is controlled not by the total normal stress applied on the soil, but by the difference between the total normal stress and tile pressure in the nuid in lhe void space. termed the pore pressure. For an all-round pressure increase. this can be expressed by the relationship. 6V -- v = C (6<7. - 6u) ' (1.7) • where. 6 V/V = volume change per unit volume of soil 6.G,. = change in total normal stress/pressure 6u = change in pore pressure ct' = compressibility of soil skeleton This relationship can be illustrated by a simple teSt where a Sllturated soil sample is subjected to undrained loading followed by drained compression as shown in Fig. 1.11. Not until tilere is a change of effective stress. is there a change of volume of the soil. This is the primary cause of long term consolidation se-ttlemenl of foundation on clay. This also explains the settlement or an area due to ground water tabJ Jowering, either for construction work or for e water supply. Undrained Total •1ress l (A<») -·- .-·- . ,pi; ' , 'l<i>·<;r.---·-· ,,... Streu ! .' . , !• . , · ... ' r:- AV -v Pore -- t ......._ • Pressure (.Au) Time - ........ -- -+ Fill, 1.11 Pnndplo ol olloeiMo ...... (b) The shear strengtil of a soil is determined by tile frictional forces between tile solid panicles. These are clearly a function of the component of normal stress that is carried by the solid grains rather than the total nonnal stress on the plane considered. This ·may be expressed by tile equation. r = c' + (cr. - u) tan f (1.8) where, c' = apparen1 cohesion (l = angle of shearing resistance o;. = total normal pressure u = pore-pressure. Copyrighted material
  33. 33. Soil as an Engineering Matuial • IS In Eqs. (1.7) and (1.8). lhe tenn (u,- u ) is tenned lhe effective stress and is denoted by the symbol u', that is, o' = o - u ( 1.9) In most engineering problems, the magnitude of total normal stre..c;s can be estimated from considerations of statics while the magnitude of pore pressure depends on the hydraulic boundary conditions. Bishop (1955) has shown that the effective stress in a soil can be related to the intergranular pressure at the points of contact. For a unit area perpendicular to the plane X- X through a soil rru~ss, the total s tress, u acting on lhe plane of contact can be divided into two components, (refer Fig. 1.1 2) . lll l l l l l u 1 ll l l "' 1 1 t Fig. 1.12 EffectlYe stress and lntergranular pressure. a = o' + u ( 1.10) where. u = Pore water pressure and u• = Effective stress If a = effective contact area per unit area of the plane and pressure, then, for a unit area u or x I = u1 x I + (I - o-1 = ( U- u) + au O; = average intergranular a)u (1.1 I) But a is small (!hough not equal to 0) and hence. o-1 = (u- u) ( 1.12) From Eqs. ( 1.11) and ( 1.12) o1 = a' (1.13) Thus, for practical purposes, effective stress may be considered equal to the intergranular pressure. the average pressure between the solid gains. In a natural soil deposit, (refer Fig. 1.13), the total stress at any depth is given by the overburden pressure at that depth while the pore pressure. in the absence of any artesian condition, is given by lhe hydrostatic head of water at that deplh. The distribution of total and effective stress in the soil is shown in Fig. 1.13. Copyrighted material
  34. 34. 16 • Tlr~ory and Practlce of Foundntion Design VertiCal stress Ground surface G.~t __j'___ _ z A Depth Poro pressure T- tlrMS Fig. 1.13 OltOI>utlon ol lOIII and . - , .... In soils. Therefore, the vertical total stress at the point A is given by a. = llt + r<z - :,) where, (1.14) r. = unit weight of soil within depth lt r = unit weight of soil below deplh lt The pore water p.essure u = r.<z - l t ) (1.1~) whete Yw = unit weight of water. Hence, the effective stress at A is given by a; = Ytlt + y(z - :,) - = when: r' = submerged density ll t r.<z - :,) + y '(t - : ,) (1.16) of soil below water table 1.9.3 Pore-preuure lD SoU due to Applied Load The application of structural load causes an incn:ase in the total stresses in the ground, the magnitudes of which can be delennined from the theory of elasticity. If the subsoil consists of clay of low permeability and construction is sufficieo~y rapid, these changes in total stress occur under conditions of no volume change and are associated with simultaneous development of excess pore water pressure. The concept of pore pressure coefficient is utilized to obtain a clear picture of how the po"'"pn:ssure in a soil n:sponds to diffen:nt combinations of applied stress (Skempton 1954). This concept not only explains the relationship between different types of triaxial test, but also provides a basis for estimating the ll}ignitude of pore-pressure to be encountered in practical problems. Copyrighted material
  35. 35. SoU as an Enginuring Martrial • 17 Skempton (1954) expressed the change of pore-pressure in a soil under axi-symmetric stress changes in terms of two empirical parameters A and 8 where ( 1.17) where. Au = change in pore-pressure Aa1 = change in total vertical pressure Aa3 = change in total lateral pressure A and B = pore-pressure parameters By putting Aa3 and (Aq1 - Aa3) equal to zero successively, it can be shown that the parameters B and A represent the effect of the aJI round stress increase and the deviatoric stress increase respectively on the pore-pressure developed in a soil element. Accordingly, Au = B·Aa3 when Aa1 - Aa3 = 0 when Aa3 = 0 and ( 1.18) The parameter B depends on the degree of saturation of soil (for fully saturated soil B = 1). 1be parameter A. however, depends upon a number of factors such as srress history of the soH. stress level. strain level. and so on, the most influential being the stress history. that is. whether the soil is nonnally consolidated or overconsolidated (Lambe 1962). Table 1.6 gives typical values of pore-pressure parameter A for different stress history of the soil. Table 1.6 l'on:·pru$U,. parameter A of differont soil types VQ/u of A aJ failun Sensiti~ clay 1.2-2.S 0.7-1.2 Normally consolidated clay 0'erconsolidakd day Heavily overconsolidatcd cLay 1.9 .4 0.~.7 -0.~.0 Shear Strength of SoU. The shear strength of a soil under any given condition of drainage is defined as the maximum shear stress which the soil can withstand. When a structure is erected on a soil, the soil elements beneath the foundation are subjected to increased shear stresses. The capacity of the foundation to bear load is a function or the shear strength of the soil. The maximum shear stress a soil can withstand depends to an appreciable extent on the manner of loading and the boundary conditions. A soil specimen does not. therefore. have a unique shear strength and it depends on factOrs such as rote of sttain, drainage condition, and size of sample. The faj lure. criterion most commonly used for defining the shear strength of a sqil along a plane is expressed by the Mohr-Coulomb equation. as illustrated in Fig. 1.14, r = c + a. tan; (1.19) where c is lhe cohesion intercept. a" is the normal pressure on the plane considered, and ; is the angle or shearing resistance Copyrighted material
  36. 36. 18 • Theory and Practice of Foundalio, DeJign ~ .!! • " • • eli 4 ' c Normal stress Ffg:. 1.14 Mohr-Coulomb faJiure criterion. In Eq. (1.19). the shear strength parameters c and 9 for any panicular soil depend on several factors, the most important being the condition of drainage. Therefore, Eq. (1.19) expressed in tenns of the total normal s-tress on the plane considered may be used to study the shear behaviour of soiJ under undrained condition. A more general expression may be written in terms of effective stress (Bishop 19SS) as: r: c' + (a. - u ) 9' -r' = c' + a' tanf' or where c' and tan (1.20) f are the shear strength parameters in terms of effective stress. lleuuremeut of ahear IOtteueth While the Vane shear test (Cadling and Odenstnd 1950) or the Pressuremcter test (Menard 1956, 1969) may be used to determine the undrained shear strength or soils in the field, the shear behaviour of soils is best understood from the laboratory triaxial test. figure 1.15 Luclte chamber Chamber fluid Sol specimen To drainage and!Of' pcwe watet preuure Copyrighted material
  37. 37. Soil 41 an Mat~n·al + 19 shows triaxial test set-up. Here, a cylindrical soil specimen is enclosed in a thin rubber membrane with rigid caps at top and bouom. The soiJ is placed inside a triax.ial cell which is then filled with water. Pressure is applied to the specimen through water and deviator stress is applied through the end caps until lhe specimen fails in compression. Drainage or waler from the pores or the soil may be conttolled through suitable valves at the base or lhe triaxiaJ ceiJ. If required~ volume change and/or pore water pressure can be measured. Tile details of the triaxial apparatus and various tests that can be performed with it have been described by Bishop and Henkel (1962). Typeo of trlulaJ teot There are, in general, three conditions of drainage under which the triaxial lest is performed, namely unconsolidaled undrained (UU). consolidaled undrained (CU), and consolidated drained (CD). In the unconsolidated undrained (UU) test, dte sample is first subjected to an all round pressure a3 and !hen to a deviator sttess (a 1 - a 3) under undrained condition. The deviator stress is applied rapidly-usually at a rate of strain of 1-2% per minute, ana failure is achieved in 10:.20 minutes. For saturated soil, lhe Mohr envelope remains hOriZ9ntal giving ;, = 0. The shear strength obtained from lhe UU test is called lhe undrained shear strtntrh or the soil c., see Fig. 1.16. The sbear sttength obtained from lhe UU lest is used in the study of bearing capacity of foundations on clay or in the rapid construction of embankments on clay. The unconfined compression test is a special case or UU test, where no confining pressure is applied to the specimen prior to shear. She• Sire» Total S.b'ess failure · · - (,•0) Normal stress Fig. 1.1S Unoonsolictated undrai'led (UU) test on sah.nled clay. In the consolidoled undrained (CU) leS~ the sample is allowed to consolidale under an all round pres..~ure a3 but no drainage is allowed during shear. If pore pressure is measured during the test, both the pore pressure parameter A and 8, and the shear strength parameters c' and , can be determined. Figure 1.17 depicts consolidated undrained (CU) test on saturated clay. The data from CU test may be used in the analysis of stability for stage conslruction of embankments on clay. Copyrighted material
  38. 38. 2t • nu~ory and Praclice of FoUJuiaticn Design •. u· ______ ... -- --- ~ -~-- -- . __,_ ··7.5" ' I u _., Namal - ... 3.01P"q u_.,. •.o .,,, ." (lcQicm') Fig. 1.17 Consolidaled ..-!nod {CU) - on oalura1ed day. The consolidated drnined (CD) test diffen from the cOIISOiidated undrained test in the way that both, the initial all round pressure and the subsequent shear stresses are applied under fully drnined condition. The test, therefore, gives the shear strength parameteiS of a soil in terms of effective stress, as given by Fig. 1.18. The results of this test can be used in the study of long tenn stability problems. { L--":!,o,. -,a~.-----~~=;:".:,;4 ai Normal stresS Fla. 1.11 Consclklaled dtalnod (COl - r..- Facton affectmc andral.ned ohear atre..,U. of clay ID-eltu Different types of triaxial test find their applications in specific field problems but, the most important strength parameters required for the analysis of foundations on clay are those obtained from the quick unconsolidated undrained (UU) triaxial test. For saturated clays, this shear strength is expressed by the undrained cohesion c. · '" should theoretically be zero. if the soil is fully saturated (Skempton 1948). Therefore, the shear strength of the clay in this condition may be expressed as ( 1.21) Although the UU test is easy to perfonn in the laboratory. application of the test data to field problem has to be done with care. Facton such as anisotropy, rate of shear, sample size. and so on affect the test results significantly (Skempton and L. Rochelle 1955). For stiff-fissured c lays, in particular. the shear s tre ngth of the clay mass in the field may be Copyrighted malerial
  39. 39. Soil as an Eng;neering Material • 21 considerably less tlw1 the strength obtained from conventional labonltory tests on small size samples (Biihop 1966). Seu!ttnty Most clays have been found to lose a part of their strength when remoulded. This remoulding may be caused by physical or mechanical means such as pile driving. However, with time, the clay may regain this strength either wholly or partly, by a phenomenon known as thixotropic hanl•ning (Skempton and Northey 1952). From the point of view of sensitivity towards remoulding, clays may be classified as fol lows (Table 1.7): T•ble 1.7 Sens.itivity of cl1ys (Skempton and Northey 19S2) Less than 2 2-4 4-8 8-16 16 loscnsitivc Moderalety sensltivc Sensitive Very sensitive Quiet clays SensHivity. in chis context. is defined as lbe ratio of tbe undrained shear strength of lhe undisturbed soil to that of the fully remoulded soil. 1.9 .5 ConsoUdatlon The gradual squeezing out of water from the pore space of a soil skeleton under the influence of externally awUed load or gravity is called co•solidation. The process results in a net change in volume of the soil and is time-dependent When an element of soil is subjected to an increase of rotal suess under undrained condition, the pressure is distributed among the solid grains and water depending on the rclarive comprcssibililies of the two phases and lbeir boundary conditions. For a confined saturated clay- water system with no drainage, the compressibility of the mineral skeleton is so large compared to that of water alone that virtually all the applied pre&&ure is transmitted as an excess pore water pressure. If drainage is now permitted. the resulting hydraulic gradient initiates a flow of water out of the clay and the soil consolidates. There is a consequent transfer of the applied load from the water 10 the mineral skeleton. The mechanism of consolidation and the factOI'S that govern lhe process of consolidation of c layey soils are studied experimentally in the laboratory in the consolidation test or the oedometer test The arrangement for nedometer test is shown in Fig. 1.19. A sample. usually 76 mm dia x 20 mm thick, is enclosed in a mel2l ring and sandwiched between two porous stones placed at top and bottom. A load is awlied to the sample through the porous stones using a lever anangemenl. As the sample consolidates., thickness of the sample decreases. Being laterally confined within the metal ring, the sample is prevented from expanding laterally and the entire volume change takes place in the venical direction. The flow is, therefore, one dimensional and the tate of consolidation is governed by the permeability of Copyrighted material
  40. 40. l l • T1reory and Pracrict of Foundation Dt.sign the soil in the vertical direction only. However, consolidation is not always oneoodimensional in rhe field and the consolidation test as performed in the laboratory represents the field condition only under certain boundary conditions. Olal -1----..jf+- Po"'"s sle>ne Pre.eure-vol4 ratio relatloaolllpe 111e change of volume of a sample as measured in the consolidation test is a function of increment in applied stress and is generally expressed in tenns of the pressure-void ratio relationship of the type shown in Fig. 1.20. For a normally consolidated soil, this relationship is found to be linear on a semi-log ploL .. ~ .... 0. ' 1! > ~ ... ~ --·----~----- .,: p,, ... ;- 6P : Po: ,P, Pressure Pressure Fig. 1.20 p,_...-wlcl ratio retotionohlp. p, : (log scate) The coefficient of volume decrease is defined as the volumetric strain per unit increase of effective pressure and this can be expressed. in terms of void ra6o, as m, = . de I dp 1 + •o (1.22) where. t.o = in.itia.l void ratio for dte s.tress increment considered. Copyrighted material
  41. 41. Soil as an Engineering Material • 23 For the linear void ratio versus effective stress relationship on the semi-log plot, the compressibility of the soil is gjven by the compression index. C, defined as de d(logp) c, : dV and - v : c, I+~ (1.23) log 12. (1.24) Po The compressibility of a soil is a measure of its consistency. The greater 1he compressibility, the softer is the soil and vice versa. Again. the compressibility of a soil is not a constant propeny. lt decreases with increasing effeetive stress, as is evident from the decreasing slope of the pressure-void ratio relationship with increasing pressure. But for the range of stresses usually encountered in practice~ Table I .8 gives one dimensional compressibility of some representative clays. Table 1.8 <Joe.< compmsibifity of some cl.tys Szrtn His.tory Qay Goopon Clay. England Shellhavm Clay Nonnal Calcuua Clay Cochin Morine Clay Brown London Clay Blue London Clay Non&.~ Calcuua Clay Nonnally Consolidated Over Consolidated m. (m2/lcN) C,AI + •ol 0.0015 0.0025 0.0005 0.0006 0.017 0.010 0.1~.20 0.20 o.os (desiccaled) (Dau from St cmptoa aDd Bitbop (19S4). Onammcr (1976) and Author's fikl) !'formally eonoolldated aDd onrcoaooJJdated cJaya Let us consider an element of soH during deposition under water, Fig. 1.21. As more and more soil is deposited on lhe elemen~ the overburden pressure on the element increases and its void ratio decreases along the curve AB. When the maximum height of deposition~ H,. is reached. the pressure increases to Pe and the void ratio decreases to ~r The soil·anywhere on the curve AB is called nomrally consolidated, to indicate thai it has never in its past, been subjected to a pressure grester than that corresponding to the curve AB. Marine clays and alluvial soils of India are typical examples of nonnally consolidated clays. Ground Erosion '---~--~-- log p Fig. 1.21 Po stresses in en etemenl soil below ground surface. p, p Copyrighted material
  42. 42. 24 t Theory Olld Practice of Founda1io11 Design Now, if some pan of the overburden is removed by, say. erosion and the remaining height of deposition is only H. conuponding to which the pressure is Po- the e lement of soil will undergo swelling and the void ratio will increase to e. This means that the soil in its past has been subjected to a pressure greater than that exists now. Many <:lays and clay shales. for example, London clay and Bearpaw shale, are heavily ovcrconsolidated in nature. The strength and deformation characteristics of a soil depend to a large extent on it being normally consolidated or overconsolidated. Soft clays are normaJiy consolidated and their behaviour from overconsolidated c lays and clay shales. Structures founded on normally consolidated clays. in general, experience much more settlement than those founded on overconsolidated clays. Although erosion of overburden has been identified as one of the causes of overconsolidation. many residual and alluvial soils near the ground surface are rendered over.. consolidated by desiccation. Alternate wetting and drying due to seasonal Ouctuation of water table and changes of temperature intrOduce capillary forces in the soil and the latter develops a pseudo overconsolidation effect. This results in increased strength and decreased compressibility of the soil. Depending on the severity of changes during desiccation, the over consolidation effect may be quite appreciable. Most residual soils of India and occasionally the alluvial soils near the ground surface appear overconsolidatcd due to desiccation. Rate of eo.....Uclatl on The rate of consolidation of clayey soils is governed by the theory of one-dimensional consolidation (Terzaghi 1923), as shown in Fig. 1.22: ....,. Z Ground z• z H - --- -- =0 (b) (a) Ftg. 1.22 On&-<Jimenslonal oon1011da1ion. The one-dimensional consolidation rate is expressed as 2 ou ; - k ou St r.m., sr.z (1.2S) where u is ihe pore pressure at a depth z from the free drainage surface, at a time 1 after the pressure increment; and k and m,. are the permeability and compressibility of the soil for a particular pressure increment. Both k and m,, may vary with pressure but their ratio remains Copyrighted material
  43. 43. Soil •s an Engintering Mauritll + 25 approximately constant (Skempton and Bishop 1954). Conse<juently, Eq. (1.25) may be writte.n as. liu ~ C li u lit • liz' 1 (1.26) where C" is defined as the coefficient of consolidation. Solving Eq. (1.26) for the appropriate boundary conditions, we get the distribution of excess pore pressure with depth at a given time t as shown in Fig. 1.22(b). Then, integrating the area of the pore pressure dissipation diagram at a given time and expressing it as a ratio of the initial pore pressure diagram, we get the average degree of comolidation, U of the soil as a function of the time factor T~ Thus, the degree of consolidation, U can be conveniently expressed as, ( 1.27) U = /(T,) where. ( 1.28) and H = length of the drainage path, to be taken as full depth of c lay when drainage is from one end and half the depth of clay when drainage is from both ends. The coefficient of consolidation is governed primarily by the size and nature of particles as reflected by tbe water content or whether the soil is nonnally consolidated or overconsolidated. C" is determined from laboratory consolidation test by curve fitting methods (Taylor I 948). The relationship between the U and T. for tbe most common boundary conditions of single or two-way drainage is ploned in Fig. 1.23. 1.0 Send 0.8 2H = IT·41 I Send H,}H 0 10 Rod< j+-4u,.j --- 0.2 v v ... ~- ~~;:, Sand / / 30 50 60 A""rage degree of consolidation, U{%) 70 80 90 This g.ives the C., value for one-dimensional consolidation or small size specimens. U.S. Department of Navy (1971) proposed nn empirical relationship between C11 and liquid limit ror determining che field c" for proclical use and lhis is shown in Fig. 1.24. Copyrighted material
  44. 44. l6 • Theory and Practice of Foundation Design • ' .... • • • • ". ' ] ~ ' .... ' <5 • "-< " / ' . ~" / Undisturbed samples; Cv in range or >nrgin ccmpres.sion _ c., in range of rec::ompression lies aboYe this lower limit / ' • • ['.. • ~ I'.. '-.. ' ~lely remouldod.L . . . . f"-~:t'--~ lies below this Uml • ....... • • ., '"' LIQuid limit I.L • ' . "' ....... .. .. .. , ... Fig. 1.24 Yal'iation of C., with lquid limit (after U.S. department of Navy (1971)). 1.9.6 Properties of SoU Coane·p-a.IDecl ooiJa The relative density of granular soils is measured from in·situ standard penetraticn test. This test involves counting the number of blows required 10 drive a standard split-spoon sampler to a depth of 30 em by means of a 65 kg hammer falling from a height of 15 em. An empirical comlation between standard penetration resistance. N (blows per 30 em), the relative density. and shear strength of granular soils is shown in Table 1.9 (Terzaghi and Peck 1967). Table 1.9 Relative density of granular soils (Tcrzoghi and Peck 1967) Rtldl1tt dtnsity (%) 0-15 IS-35 JS-65 65-35 7-85 Nott': N (Btows/30 em} ().4 4-10 10-30 30-50 so ~values are tO be increased by Anglt' of shroring ruislana (f) u• Very Loose 28-30" Loosc Medium Dense Very Omse 30-36' 3~ ) 0 41. s• for soils containjng less material
  45. 45. Soil as an £nginuring Matt!rial • 17 • Pine-grained eoUa Soils containing clay-size particles and large proportion of silt have low permeability and their propenies vary wilh the rate of load application. Under undrained condition. their sttength is derived almost exclusively from cohesion. These soils oflen possess low shear strength and high compressibility, lhus mal<ing !hem poor foondation materiaL A cohesive soil is described as ve:ry soft. soft. medium. stiff, and so on according to its shear s1rength as determined from the unconfined compression test or from !he undrained uiaxial test. Attempts have also been made to correlate thC standard penetration resistance (as above) with the shear strength of cohesive soils. Table 1.10 gives the classification of cohesive soils on !he basis of their shear strength (ferzaghi and Peck 1967). Table 1.10 Shear $lTCngth of cohesive soils (l'erughl and Peck 1967) Undrained $hMt' <AnsUttncy strtngth. c. N (Blows ptr 30 em) (lfm:) ~1.25 ~2 1.25-2.50 2.5()-S.OO 2-4 Very Son Son Medium Sliff S.~IO.OO Very Stiff 10.~20.00 4-8 8-16 16-32 > 20.00 32 Hllld Stiff clay often possesses cracks and fissures which affect the shear slrCngth of 1 clay he mass. These fissures are planes of weakness and an: prone to softening by water. Laboratory tests on small specimens do not often give lhe propenies of the soils in~·situ (Bishop 1966, Burland et al. 1966, Man;land 1971). Elaatlc: parametera Young's modulus and Poisson's ratio are impOrtant soil parameters thai are required to study the defonnation behaviour of a soil. When a saturated clay is loaded rapidly. no volume change of the c lay occurs during loading and Poisson's ratio can be taken as 0.5. When there is volume change. typical values of Poisson 's ratio may be 1aken as those in Table: 1.11 (Barkan 1962). Tablt 1.11 Poisson's nuio or di.fferent soils (Barkan 1962) Soil f)'Pt Salurated clay (undrained) CJ:ay with sand and silt Un.sawrated clay Lo<.. Sand P<'iSJ()n ·s ralio 0.30-0A2 0.35- 0.40 0.44 0.30-0.35 Copyrighted material
  46. 46. 28 • Theory mui Practice of Foundation Design Young' s modulus of a soil can be determined from the stress-strain relationship obtained from laboratory triax.ial tests. However, these relationships are highly susceptible to sampling disturbances and the E value thus obtained is generally much lower than the in-siru modulus. In ca.iie of homogeneous deposits, detennination of £ by back calculation from field plate load test gives reliable data. lndirec.t estimate of E can also be made from empirical relation with the shear strength measured from the unconsolidated undrained triuial lest. Bj errum (1964), and Bozuzuk and Leonards (1972) suggest the following approximate correlation: (1.29) E = (500 - IOOO)c. where c., is the undrained shear strength of the clay. ln·eltu compreselbllity The pre.ssure versus void ratio relationships of natural clays are vuy sensitive to sampling disturbanc.e s and the linear e versus Jog p relationship is not always obtained even for a nonnally consolidated clay. Also, there is a pseudo overconsolidation effect on the sample because of the removal of in-s itu stresses by sampling. Consequently, e versus )og p relationship obtained from laboratory consolidation tests on undisturbed samples geniraiJy takes the shapes as shown in Fig. 1.25. The curves move downwards as the sampling disturbances are increased. Schmertman (1953) observed that. irrespective of sampling disturbances. the straight Jine portions of all the curves meet at a void ratio equal to 0.42 ~0• where e0 is the in-situ void ratio of the sample. Then joining this point with that corresponding to t 0 and Po (where Po in·situ effective overburden pressure) would give the virgin consolidation curve. For norma11y consolidated soil, the e.0 -p0 point lies to the right of the extension of the straight line in the laboratory e versus log p curve while for overconsolidated samples. the point would lie to the left. The field • versus log p relationship for the overconsolidated range is obtained by drawing a line parallel to the laboratory rebound curve and the point of intersection of this line with extended straight line of the laboratory curve would give the pre-consolidation pressure (Fig. 1.25). = Void ratio, e ---- ' •• LabOratory ., Po=Pt; '' -'~~,~ VW'gin ~ression curve. slope c f: oonsoCidalion 1 curve • ---- -- ~- 1 I I I I ' OA2~;to ------~ -+1 I I I _ _,__;__~ Pressure, p, P2 p (tog scale} Fig. 1.25 Laboratory e versus log p relationship. Copyrighted material
  47. 47. 1. 10 SOJL DEPOSITS OF INDIA Tile soil deposita of lncha m:~y be classified under most of lhe ~minant aeological formations (dcscnbed earlier). namely (b) Marine depositS (c) Dcsen ooll (a) Alluvial oolls (d) Laterite soil (c) Sleek cotton ooll (f) Boulder depositS Figure 1.26 shows the di01ribution of predominant soil deposits in India (Ranjan and Rao 2000). 32° 2$. z•· 2&· 23.$. zo• ~AA.MIIt 18" .-a•ll mo.-.- m--- tr r;m~on~~- 12" IZ3-~ & ' ! ] - dopoOIII 54· aa· 1'2" 16· eo- 54· aa· 92" ga· 1ocr Fig. 1.26 Soil deposits of lndta. Allu..W aoUa Large parts o( northern and CIStern India lyina; in the lndo-ao.naetic plains and the Brahmaputra valley arc covered by the sedimentary deposits of the river> and their tributaries. Tiley often hove thickness greater than 100 m above lhe bed rock. Tile deposits ..-ly constitute layer> of sand. sdt. and clay depending on lhe pDOJtion of lhe river away from lhe soun:c. llarlDe depoe.IU India has a long coast line extending along lhe Arabian sea. Indian ocean, ond lhe Bay of Bengal. n.e deposits along the coast are mostly laid down by lhe sea. 11leiC marine c lays of India are generally soft and often contain organic matter. 'They pOCSC$$ low shear strength and high compresSibility. <..opynghted matenal
  48. 48. 30 • Theory and Practice of Fomtdation Design Black cottOD ooU The central p:lrt of India has extensive deposits of the exp:msive soil known as black couon soil. This covers wide areas of Maharas tra, Madhya Pradesh, Karnataka, Andhra Pradesh, Tamil Nadu. and U11ar Pradesh. The soil contains montrnOrillonile clay mineral which has high swelling potential. Laterite IOU This soil covers wide areas of Kerala. Karnati.lka. Maharasua, Orissa. and pans of West Bengal. Lalerites are residual soils fonned by decomposition of rock which fonns oxides of iron and aluminium. Desert IOU Large areas of Rajasthan in lhe Thar desert are composed of wind blown deposits of desert soil. like loess. The sand dunes are often 15 m high and are formed under highly arid conditions. Boulder depoetto Boulders are deposited is hilly rerrains where the rivers flow wilh high velociry and carry large size boulders. These deposits are found in the sub-Himalayan regions of Uttar Pradesh and Himachal Pradesh. Barkan. 0.0 (1962), Dynamics of Bases and Foundations, McGraw Hill Book Co., New York. Bishop. A.W . (1959), The Principle of Effective Stress, Teknisk Ukebald, Vol. 106, No. 39, pp. 859-863. Bishop, A.W. ( 1966), Strength of Soils pp. 89-130. as Engineering Materials, Geoteclrnique, Vol. 16, . Bishop, A.W. and OJ. Henkel ( 1964), The Meilsuremeni of Soil Prop<rtus in the Triaxial Tw, Edward Arnold, London. Bjerrum. L. (1964). R~lasjon M~lom Malte og Berengnede Serninger av Byggverk pa Leire og Sa11d, Norwegion Geotechnical Institute, Oslo, Norway. BoWZ<~k ond G.A. Leonards (1972), The G/oucnt<r Test Fill. Proeee&ngs ASCE Speciality Conference on performance of earth and eanh retaining Sln.lctures, Lafayette. Indiana. USA. Burlond, L., F.G. Buller. and P. Ouniean ( 1966), The Behaviour and Design of lArge Diometu Bored Piles in Stiff Clay, Proceedings SympOsium on Large Bored Piles. The lnslilUtion of Civil Engineers. London, pp. 51- 71. Copyrighted material
  49. 49. Soil as arr fngineerirrg Material • 3 1 Chummar, A.V. (1976), Foundation Problems b1 Cochln, Proceedings Symposium in Foundations and Excavations in Weak Soil, Calcuua, Vol. I, Paper No. C 4 . IS 1498 (1970). Classification and Identification of Soils. Bureau of lodian SWldards, New Delhi. Lambe, T.W. (1962), Por<-pr<ssur<s ilr a Foundation Clay 1962, Proceedings ASCE. Soil Mechanics and Foundation Division, Vol. 88, pp. 19-47. Marsland. A. ( 197 I). The Shear Strengths of Stiff Fissur.d Clays, Proceedings Roscoe Memorial Symposium, Cambridge, U.K., pp. 59~8. Menard, L. (1956), An Apparatus for Measuring tire Strength of Soils in P/Qce, MS. Thesis, University of lllinois. Urbana, lllinois, USA. Ranjan. G. and A.S.R. Rao (2000), Basic and Applied Soil Mechanics, 2nd edition, New Age International (P) Ltd.. New Delhi. Schmertmann, J.H. (1953). Undisturbed Consolidation Behaviour of ClayJ. Transactions. ASCE, Vol. 120, p. 1201. Scott, R.F. (1965), Principles of Soil Mechanics. Addison Wesley. Boston, USA. Skempton, A.W. (1948), The ~ = 0 At~alysis of Stability and its Theoretical Basis, Proceedings 2nd International Conference on SMFE, Vol. 2. Skempton, A.W. and A.W. Bishop ( 1954), Soils, their Elasticity and Inelasticity. North Holland Publishing Co. Amsterdam. Skempton, A.W. and R.D. Northey (1952), 17re Sensitivity of Clays, Geotechnique. Vol 3, pp. 30-54. Taylor, D.W. ( 1948), Fundamentals of Soil Mechanics, John Wiley & Sons, New York. Terzaghi, K. and R.B. Peck (1967). Soil Mechanics in Ei1ginuring Practiu, 2nd edition, John Wiley aod Sons, Inc. N.Y. U.S. Depanment of Navy (1971), Design Manual-$oil Mechanics, Foundations and Earth Structures, NAVFAC. DM- 7. U.S. Government Printing Office. Washington, D.C. Copyrighted material
  50. 50. II II Site In estigation 2 .1 INTRODUCTION ll is essential to carry OUt site investigation preparing the design of civil engineering works. The investigation may range in from simple examination of the surface soils. with or without a few shallow lrial pits. a detailed s1udy of the soil and ground water conditions for a considerable depth below ground ·surface by means of boreholes and in· situ and/or laboratory tests on lhe soils :~~~~~!. 1be extent of the investigation depends on the imponance of the stNCture. the c• of the soil conditions. and the infonnation already available on the behaviour of foundations on similar soils. Thus, it is not the normal practice to sink boreholes and soil leSLS for single or two-storey dwelling houses since normally. there is adoquale of the safe bearing pressure of the soil in any particular locality. Only in soils such as peat or loose fill would it be necessary to sink deep boreholes. supplemented · by soil tests. More extensive investigation for light st~tures is needed structures are buih on filled·up soil or in ground conditions where there is no available on foundation behaviour of similar structures. A detailed s ite investigation deep boreholes and laboratory testing of soils is always a necessity for heavy such as bridges~ multi-storeyed buildings or industrial plants. Thus. the major objectives of site inveftigatuon are: 1 (a) Knowing the general suitability of site for proposed works. (b) Assessing local conditions and proj>lems likely to be encountered in foundation construction. (c) Acquiring data for adoqua1e and ecxf>otmic design of foundlltion. 2 .2 INFORMATION EJJ::TB:A.C:TJI;.> FROM SITE INVESTIGATION A lot of information is extracted from site iqv·estigattion to facilitate foundation design. This includes I. General topography of the site whii h affects foundatio n design and construction, presence of water courses. and so on. e .g .. s urface c.o nfiguration, adjacent Copyrighted material
  51. 51. Sile lnvesn'gation t 33 2. Location of buried services such as power lines, telephone cables, water mains, sewers pipes and so on. 3. General geology of the area with particular reference to the principal geological formations underlying the site. 4. Previous history and use of the site including information of any defects and failures of struc.turcs built on the site. 5. Any special features such as possibility of earthquake, flooding, seasonal swelling etc. 6. Availability and quality of local construction materials. 7. A detailed record of soil or rock strata, ground· water conditions within the zone affected by foundation loading and of any deeper strata affecting the site conditions in any way. 8. Design data which comprises strength and compressibility characteristics of the different strata. 9. Results of chemical analysis on soil or ground water to detennine pos.'ible deleterious effects on foundation structures. 2 .S STAGES OF SITE INVESTIGATION Different stages of site investigation for a major civil engineering project may be summarised as shown in Table 2.1. T able 2.1 Stages or site investigation ReconMissance Study (I) Geological - (b) Ptdologlcal data (c) An:al photographs (d) Geophysical investigaaion Dt-roild JnwstigatWtt (I) Boring (b) Sampling (C) Ttstioa (i) Lab test (ij) l"!dd ""' (d) Aerial phococnphs (e) Geophysical melhods P~tjOnrt(JM, Study (I) further testing lnsuwn<nwion (c) Paformance evaluation (b) 2 .S . l ReeonnaisaaDee Study Roconnajssancc study involves the preliminary feasibility study that is undertaken before any detailed planning is done-mainly for the purpose of selection of site. This is to be done at minimum cost and no large scale exploratory work is usually undertllken at this stage. The required data may be obtained from: Copyrighted material
  52. 52. 34 t TI1eory and Practice of Fou11dation Design Geolo~cal ourny reporto and mape Geological interpctation of land forms and underlying strata give !he .sequence of events leading to the formation of subsoil deposits. They help to define the propenies of the material in a general way. · Pedolo~cal dot& Many areas have been surveyed for agricultural purposes-usually to depths of 2 or 3 m. Materials arc often classified according to colour, texture, chemical composition, and so on. Aerial photocrapha/aateWte lmageo Photographic representation of a portion of earth· s surface taken from the air or space. Geophya.lcal methodo Applica6on of the methods and principles of physics to detennine the propenies of s ubsurface materials. These methods are particularly useful for identifying bed rock. Seismic refraction method or electrical resistivity tests are usually done for this purpose. 2.4 BORING (DETAILED SOIL INVESTIGATION) Detailed soil investigation is done through a series of boring, sampling, and testing to obtain the en.gineering properties of soil. The following subsections are devoted 10 boring which discuss different methods of boring in detail. 2.4.1 ~al Pit• Trial phs arc the cheapest way of site eltploration and do not require any specialized equipmenl. A pic is manually excavated to get an indication of the soiJ stratification and obtain undisturbed and disturbed samples. TriaJ pits allow visuaJ inspection of any .change of strata and facilitate in-situ tesling. They are. however, suitable for exploration of shallow d4:plh only. Figure 2.1 is a diagrammatic representation of trial pitS. Test pit Brownish grey silly (jay Soil Strata F1Q. 2.1 2.4.2 Wash Trial pit BoriDC A hole, usually 150-200 mm diameter, is advanced into the soil through a suitable cutter at the bonom of a drill rod. The soil is loosened and removed from the borebole by a stream of water or drilling mud, issuing from the lower end of the wash pipe which is worked up and Copyrighted m3erial
  53. 53. Sitt' Juvestigtaion • 35 down or romted by hJind in the borehole. Water or mud now carries the soil up the onnular space between the wosh pipe ond the cosing. ond it overflows ot ground level where the soil in suspension is allowed to settle in a tank and the fluid is re·circulated or discharged to waste as required. Samples of the settled soil can be retained for identification purpOses. Figure 2.2 shows the arrangement for wash boring. Waktr Winch._ awiveJ Pump Engine Sump Wash pipe ' Flg. 2.2 Wash boring, The method is simple and cheap. The structure of the soil below the boring apparatus is not disturbed and thus, both disturbed and undisrurt>ed samples can be obtained. 2.4.3 Auger Boring In this method. the borehole is advanced by turning a.n auger into the soil, withdrawing it and removing the soil for examination and tes1. The auger is re· inserted for further boring. The auger may be manually or mec.hanically operated. Extensions are added to reach the desired depth. Disrurt>ed samples may be obtained from the soil brought up by the auger while undisturbed samples nrc obtained by pushing sampling tubes at suimble intervals in the borehole. The apparatus for auger boring is shown in Fig. 2.3. ~Handle 1- Dril - -~ .... I I I I I I Post.flole auger- ' rod t I I I I '' : • I Fig. 2.3 Auger bof'lng, Copyrighted material
  54. 54. 36 • Tlreory and Practice of Fowu/n1ion Design 2 .4 .4 Rotary Drllling Rotnry drilling is done by rapidly rotating drilling bits attached to the bottom of the drill rod to cut and advnnce chc borehole. Rmary drilling can be used in sand, clay or inl,act rocks wilh warer or drilling mud being circulnted through che drill rod to remove the cuttings as the mud returns u pwards chrough che annular space between the drill rod and the side of the hole. Core bnrcls wilh diamond bits may be used in r01ory drilling co obtain rock cores. 2 .4.5 Percussion Drilling llle soil is loosened by repeated blows of a heavy c hisel or spud and the resulting slurry is removed by c-rculating wacer. This rnelhod is recommended for boring in rocks and hard soil. i The hole is advanced with o cuuing edge using sceel shots. lung:scen carbide. or djamond bits. 2.4.6 Stabi.Uzatlon of Boreholes Boreholes need 10 be stabilized while being odvanced for prevenring caving in of sides :md bottom or hole and to avoid disturbance to the soil to be sampled. Stabiliuuion may be done by c irculation of woter or drilling flu id or by using steel casing. " Stabilit.ation bJ water: Stabiliz.ation by water is not suitable in partly-saturated soils above G. W.T. because free water destroys the capillary forces and causes increase in w:.ter content. It is genemlly used in rock and stiff clays. Stabilization by drilling fluid: Borehole is fi lled with drilling fl uid or mud which, when circulated. removes the loose material from the bottom of the hole. Drilling mud is obtained by mixing locally available fat clays wilh water or by using commercially available bentonite. effect of drilling mud is improved by higher spe-cific gravity of the mud in c,omparison with water. Also. there is fonnation of a relatively impc.rivous layer on the side· or che borehole which getS liquefied again by resuming the agitation. SlabiliUJilon by ca.ti11g: Casing or lining a borehole with steel pipes provides the safest. though relalively expensive method of stabilization. After a ce11ain depth or when difficult ground condition is reached, it is often d ifficult to adv::mce the original cas-ing. A smaller casing is 1hen inserted through the one nlready in place. Lower end of casing is generally protected by a shoe or hardened steel w-ith inside bevel so that the soil enrers the casing :md c.on be removed. This nrrangel'nenl is depicted in Fig. 2.4. :::r- t--Outer casing r- f- f't- Inner casing ll-+Fig, 2.4 Cutting shoe Stabilizaclon or t>orehose by castng. Copyrighted material
  55. 55. Sit~ l11~srigation + 37 Exccpr when undisrurbed samples ore required in ~nsitive clays. !he casing is genernlly driven by repeUicd blows of a drop hammer. Casing prevcnrs side caving, bur nor always bottom caving. This can be achieved by filling the casing with water or driiJjng fluid. However. casing should nor be filled wirh warer if bonom of casing is above ground warer table and undisturbed samples are required. 2.5 SAMPLING The different types of sample obtained from boreholes are shown in Table 2.2. Table l..l Types of sample SampJ~s Undisrurbed OiSIWI>cd I RtmdUidcd R~ve (The structure of the soil is disturbed to a consjderable degrte by che action of boring toots and ucavation equipment.) 2 .5.1 (Retains as closely as practicable. lhe true in-shu structure and water cornen1 of the soiJ.) SampUng from Trial Pits Block samples (refer Fig. 2.5) are hand cur from trial piiS or open cxcavarions. A block of clay is c--arefully trimmed with a sharp knife. taking care that no water comes into contact wilh rhe sample. Good quolity samples can be obtained by this merhod. but if the soil does nor possess ony cohesion. ir may be dirticuh, if 001 impossible. 10 obi:Un block samples. Fig. 2.5 2.5.2 Bloc:k sanplc.-of clay, SampUng from Boreholes Undisturbed samples may be obtained from boreholes by open drive samplers or piston samplers. An open drive sampler is shown in Fig. 2.6. Drift rod Fig. 2.6 Open drive sampler. Copyrighted material
  56. 56. 38 • Theory alld Practice of Foundation Design Open drive samplers consist of thin-walled tubes which are pushed or driven into the soil at the bottom of the hole and then rotated to detach the lower end of the sample from the soil as shown in Fig. 2 .6. Most soft or moderately stiff cohesive soil can be sampled without extensive disturbance in thin·walled seamless steel tubes having diameter not less than 50 mm. The lower end of the tube is sharpened to from a cutting edge and the other end is machined for attachment to drill rods. The entire tube is pushed or driven into the soil at the bonom of the hole and is removed with the sample inside. The two ends of the tube are then sealed and the sample shipped to the laborntory. Good quality undisturbed samples are obtained from piston samplers which use thinwalled sampling tubes with a piston inside. While the tube is being lowell:d to the bouom of the drill hole, the piston rods and the piston are held at the boltom of the sampler by means of a drill rod which rises to the top of the borehole. A pislOn sampler ls shown in Fig. 2.7. The presence of the piston prevents excess soil from sequeezing into the tube and thus. maintains the integrity of the sample. -Fi9- 2.7 f'lsiDn sampler. Table 2.3 lists the requirements of a good sampling tube (ll:fer Fig. 2.8) as follows: Flog. 2.8 Sampling tube. Copyrighted material
  57. 57. Site brvesagotiou • 39 T able 2.3 (a) Area ralio, C = . Requiremenls of sampling tube 1 1 o.. - oc o' • This represents the amount of soil that is displattd when the sampler is forttd into lhc: ground. Thicker the tube. more is tht diswrbanet". The ateil ratio of a good sampling tube should not cxct:cd ISIJt. (b) Inside clearance ratio, C1 = D, ~ D, For • lonJ sampl... 0.75% < C, < I.S% o.scx, For shon samples. 0 < C1 < The diameter of the sampling tube is kept slightly larger than the diameter of the cuuing edge to minimise: friction on the sampJe as it enters the: sampling tubt. (c) Outside clearance ratio, c. = D,.;, D, • :Z..J'h 'Tbis dearance is provided to mluce &he drivlog fOl"Cle requlttd to penetrate the sampler into the soil. DiametO" of samples should 001 be less !han 38 mm. In gcn<nl. SO. ISO mm diamelel' undUwrb<d samples are obtained from boreholes. 2.15.3 Preaervatlon of Samples Undisturbed samples which ""' to he tested after some time should he maintained in such a way that the natural water content is reraincd and no evaporation is allowed. Usually. two Co ats of 12 mm thick paraffin wax. and pelrolcum jelly are applied in · mohen state on either end of the sample to keep the water content unchanged for considerable time when the sample is preserved in o humidity controlled room. In the absence of such fa<:ili ties, the sampling tubes should he covered by hessian bags and sprinkled with water from time to time. Block samples may he coated with 6 mm thick paraffin wax and kept in air·tight box w:·h saw dust filli ng the annular space between the box and the sample. Figure 2.9 shows sol..e typical arrangements for preservation of samples. Cover Paraffin coat ~b'a<allin coat (Paraffin wax + P&ttOiel.wnjelly) $ample Wooden box (a) Block sample Saw dust (b) Tube sompte Fig. 2.9 Preservation of sam ples. Copyrighted material_
  58. 58. 40 + Theory a11d Practice of Foundation Design 2.6 TESTING OF SOIL Soil properties arc determined from appropriate laboratory and field restS. The specifications regarding laboratory tests and field tests for routine soil investigation are given in Tables 2.4 and 2.5. Tabl~ l .4 Laboratory cesting of soils T~ o/lt!St ProJHTt'JI of soil Qualiry of samplt C/4ssifo:4tlon I. 2. 3. l~ fication VisuaJ soil classification RID Grain size distribution (a) Sieve analysis } (b) Wet analysis D Consistency limits of (a) Li<jwd limit (b) Plastic U mit cohesive soils } RID (c) Shrink~t~e limit 4. S. Moisture content Unit weight UKit~tuing I. Moisture contctlt Spcdfic gravity UD 0 propntin Shear stren&th (a) Unconfined compression } (b) Din:ct .!lew 2. 3. 4. Cornpr=ibility UD (C) Triwal (UU/CU/CD) (a) Ocdomelerl<$1 1 UD (b) Tri>.xial .... (a) Constant head pcrmeabUil)' (b) Varioble head permeability test 1<$1) l'<rnle>bility (a) Proctor cesc ) Compaction characteristics {b) CBR teSI 5. Chemical and mlnetaloglcal composition (a) X-Ray diffracnon (b) D.T.A. UD I RID RID (<) 01emical test R-Reprts<:Dtative D-Dishubcd T able 2.5 UD-Undistu.rbed Field testin.a of soils TyfH of tat I. Relative densiry (granular soils) 2. Shear strength (cohes:ive soil) (a) Standard penetration lt$l (b) Dynamic cone cest (a) Vane test (b) Direct shear test (c) Sialic cone 3. 4. Bnring capacity and seu.Jement Permeabilily ln·situ strength and deformation Plo<e bearing teSI (a) Borehole/pumping tes1 (b) Pierometer teSI load cest (a) Moisture-density rel;nion (b) In-pioce density (c) C.B.R. test (a) Prts.suremeL test cr ch!ln•cteriscics (b) Dilotomctcr tt:sl S. Testing of piles 6. Compaction conU'OI 7. test or soil Copyrighted material
  59. 59. Siu lnvestigalion • 41 2 . 7 FJEI.D TESTS 2 .7 .1 StiiDdard Penetration Test (SPT) It is extremely difficult to obtain undisturbed samples of granular soils, so in-situ SPT is performed at frequent intervals along the depth of a borehole. A slllndard split spoon sampler (Fig. 2.10) is driven 45 em into the gnnund by means of a 6S kg hammer falling freely from a height of 75 em. The toW number of blows required to drive the second and third depth of IS em (i.e. total 30 em) is called the slllndard penetration resistance (N blows per 30 em). After the blow counts are recorded. the spoon is withdrawn and a representative sample is obtained for identification tests. To dril rod t SIOOI somplng . _ 2"(5(1).8) (split klngiludinally) Fig. 2.10 SPT - · For cohesive soils. a simple correlation between the slllndard penetration resistance (N) and the undrained shear strength, c, has been proposed by Stroud (1974), c, = kN (2.0) when: k is a constant having an average value of 4.5 kN/m2. Similar relationship has been obtained by SengupUI (1984) who studied the correlation for some cohesive soils and obUlined a value of 4.2 for the consUlnt k. Terugbi and Peek's relationship has been widely used to obtain the consistency of cohesive soils in terms of the undrained shear strength, as shown in Table 1.9 (Terzaghi and Peck, 1967). In granular soils, the SPT blow count is affected by the effective overburden pressure, u.;. So. N value obtained from the field should be corrected to correspond to a standard vaJue of a...'. Accordingly, (2.1) Copyrighted material
  60. 60. 42 • T1reory and Practice of Foundation Design where Ncvt = corrected N value for a standard value of a.,' (1 00 kN/m2) C,. = correction factor Nr = N value obtained from field The correction factor C,. may be taken from the empirical relationship given by Skempcon ( 1986). 2 (2.2) C, = I + O.Olcf, where a,' = vertical overburden pressure in kNim2 A dilatancy correction has been recommended for saturated fine sands and silts to account for the development of negative pore pressure. if any, during driving of the SPT sampler and consequent increase of shear strength and higher SPT blow count (Terz.aghi and Peck, 1967). For such soils that have N"" greater than 15 as per Eq. (2.1), a conection for dilatancy may be made as, N = 15 + 0.5(N.., - 15) (2.2a) The relative density and the degree of compaction of granular soil can be obtained from Tenaghi's empirical correlation, as in Table 2.6. Table l..6 Relative density from SPT blow oount No. of blows R'latiw tki&Sity (N/30 em) R0 = ( • • ., - ' ) x 1001& '-... - ~"min C)-4 Q-15~ 4-10 tQ-30 IS-35% 25-651& 6S-85'.6 > 85 3o-SO >so Very 10050 Loose Mcdlum Dense Very Dense Many attempts have been made to obtain empirical correlation between NC!OI and the angle of shearing resistance of sand. The most recent attempt by Halanakar and Uchida (1996) appears to agree well with laboratory test data, which gives ,z,j20N~ + 17 degrees (2.3) The modulus of elasticity is obtained by the relationship given by Mezenbach (1961) as E = C1 + C:/{ kg/em' (2.4) where C 1 and C2 are functions typical of the type of sand. Some C 1 and C 2 values corresponding to different soil types are given in Table 2.7. Copyrighted material
  61. 61. Sitt lm·estigotUm • 43 Table 2.7 Modulus or elasticity or sand C, (kstcm'l Soil t}JH C1 (tgtcm1/blow) 52 3.3 4.9 Fine sand (above G .W .T) Fine sand (below O .W.T) 3. Sand (Medium) 4. Coarst sand 5. Sand + cravel 6. Silty sand 7. Sih I. 2. 71 39 38 43 4.5 lOS 11.8 24 5.3 12 5.8 A similar correlation between the compression modulus E and the SPT blow count N has been obtained by Papadopoulos (1992). This is given by E = 75 + 8N (kglcm2) (2.4a) Bowles (1988) also gives useful relations to evaluate the srress~strain modulus of sand from SPT blow count, as shown in Table 2.8. But they generally give very conservative values. Table 2.8 Stress-strain tNXlulus of sand (Bowles. 1988) TyfH' of sond Sand (noonally oonsolidated) Sand (""'r31ed} Sand (ove"""solicbted) Sa.nd with gravel Silty sand E (kglem2) S(N + IS) 2.S(N + IS) 7.S(N + 24) 12(N + 6) for N > IS &_N + 6), N S IS 3(N + 6) Although standard penetration test is basically a qualitative test, correct interpretation of data gives good evaluation of soil properties particularly in granular soil. The main sources of error include inadequate cleaning of borehole. eccentric hammer blow, and presence of large boulders and gravels which give erratic results. 2 .7 .2 Dynamic Cone Penetration Test (DCPT) Dynamic cone penetration test is done by driving a standard 60° cone attached to a drill rod into the soil by blows of 65 kg hammer falling from a height of 750 mm. The blow count for every 30 e m penetration is made to get a continuous record of the variation of soil consistency with depth. The test does not need a bo"'hole. It can be done quici<Jy 10 cover a large area economically. The test helps to identify variability of subsoil profile and to locate soft pockets, such as filled up ponds. When DCPT is carried out c lose to a few bo<eholes, suitable correlations may be obtained for a particular s ite and the number of boreholes can be reduced. The dynamic cone penerrotion test is done either by using a 50 em diameter cone or a 65 mm diameter cone with circulation of bentonite slurry to eliminate friction on the drill rod. One of 1he proposed correlations between Ned obtained from DC.PT and Nt« obtained from SPT is Copyrighted material