Geotechnical Engineering: Principles and Practices of Soil Mechanics and Foundation Engineering i s a long titl e befitting a major work. I am pleased t o introduce this superb volume destined for a readership o f students, professors, an d consultants. What makes thi s text differen t from othe r books on these subjects that appear each year and why am I recommending i t to you? I have been workin g and teaching in the area of geotechnical engineering for 25 years. I have read and use d score s o f textbook s i n m y classe s an d practice . Dr . Murthy' s tex t i s b y fa r th e mos t comprehensive tex t I have found. You will find tha t his organization of the subject matter follows a logical progression. Hi s example problems are numerous and, like the text, start from fundamenta l principles an d progressivel y develo p int o mor e challengin g material . The y ar e th e bes t se t of example problems I have seen in a textbook. Dr . Murthy has included ample homework problem s with a rang e o f difficult y mean t t o hel p th e studen t ne w t o th e subjec t t o develo p his/he r confidence an d t o assis t th e experience d enginee r i n his/he r revie w o f th e subjec t an d i n professional development .
As the technical editor I have read the entire manuscript three times. I have been impressed by the coverage, th e clarity of the presentation, and the insights into the hows an d whys of soil and foundation behavior. Often I have been astonished at Dr. Murthy's near-conversational approac h to sharing helpfu l insights . You ge t th e impressio n he' s righ t ther e wit h yo u guidin g yo u along , anticipating your questions, and providing instruction and necessary informatio n as the next step s in the learning process. I believe you will enjoy this book and that it will receive a warm welcom e wherever it is used.
I thank Dr. Murthy for his commitment to write this textbook and for sharing his professional experience wit h us. I thank him for his patience in making corrections and considering suggestions. I thank Mr. B. J. Clark, Senior Acquisitions Editor at Marcel Dekker Inc., for the opportunity to be associated with such a good book. I likewise express my appreciation to Professor Pierr e Foray of 1'Ecole National e Superieur e d'Hydrauliqu e e t d e Mecaniqu e d e Grenoble , Institu t Nationa l Polytechnique de Grenoble, Franc e for his enthusiastic and unflagging support while I edited the manuscript.
MarkT. Bowers, Ph.D., P. E. Associate Professo r o f Civil Engineering University o f Cincinnati
FOREWORD
It give s m e grea t pleasur e t o writ e a forewor d fo r Geotechnical Engineering: Principles an d Practices o f Soil Mechanics an d Foundation Engineering. This comprehensive, pertinen t an d up-to-date volum e i s wel l suite d fo r us e a s a textboo k fo r undergraduat e student s a s wel l a s a reference boo k fo r consultin g geotechnica l engineer s an d contractors . Thi s book is well writte n with numerous example s on applications o f basic principles to solve practical problems.
The early history of geotechnical engineerin g and the pioneering wor k of Karl Terzaghi in the beginning of the last century are described i n Chapter 1 . Chapters 2 and 3 discuss methods of classification of soil and rock, the chemical and the mechanical weathering of rock, and soil phase relationships an d consistenc y limit s fo r clay s an d silts . Numerou s example s illustrat e th e relationship between th e differen t parameters . Soi l permeability and seepage ar e investigated in Chapter 4 . Th e constructio n o f flo w net s an d method s t o determin e th e permeabilit y i n th e laboratory and in the field ar e also explained.
The concept o f effective stres s and the effect o f pore water pressure o n effective stress ar e discussed in Chapter 5. Chapter 6 is concerned with stress increase in soil caused by surface load and methods to calculate stress increase caused by spread footings, rafts, and pile groups. Several examples are given in Chapter 6 . Consolidation of soils an d the evaluation of compressibility in the laborator y b y oedomete r test s ar e investigate d in Chapte r 7 . Determinatio n o f draine d an d undrained shea r strengt h by unconfine d compression, direc t shea r o r triaxia l test s i s treate d i n Chapter 8.
The importan t subject o f soi l exploratio n i s discusse d i n Chapter 9 , includin g the us e of penetration tests such as SPT and CPT in different countries. The stability of slopes is investigated in Chapte r 10 . Methods usin g plain and circular slip surfaces to evaluate stability are describe d such a s th e method s propose d b y Bishop , Fellenius , Morgenstern , an d Spencer . Chapte r 1 1 discusses method s t o determine activ e an d passive earth pressures actin g on retaining and sheet pile walls.
Bearing capacity an d settlement of foundation an d the evaluation of compressibility in the laboratory by oedometer test s are discussed in Chapters 12 , 13, and 14 . The effec t o f inclination and eccentricity of the load o n bearing capacity is also examined. Chapter 1 5 describes differen t pile types , th e concep t o f critica l depth , method s t o evaluat e th e bearin g capacit y o f pile s i n cohesive and cohesionless soils , and pile-driving formulae. The behavior of laterally loaded pile s is investigated in Chapter 1 6 for piles in sand and in clay. The behavior of drilled pier foundation s VII
and the effect of the installation method on bearing capacity and uplif t ar e analyzed in Chapter 17. Foundations on swelling and collapsible soils are treated i n Chapter 1 8 as are methods tha t can be used t o reduc e heave . Thi s i s a n importan t subject, seldo m treate d i n textbooks . Th e desig n o f retaining wall s i s covere d i n Chapte r 19 , as wel l a s th e differen t factor s tha t affec t activ e an d passive eart h pressures. Differen t applications of geotextiles are covered in this chapter a s well as the topic o f reinforced earth. Cantilever, anchored, and strutted sheet pil e walls are investigated in Chapter 20 , a s ar e method s t o evaluat e stabilit y an d th e momen t distribution . Different soi l improvement methods , suc h a s compactio n o f granula r soils , san d compactio n piles , vibroflotation, preloading , an d ston e columns , ar e describe d i n Chapte r 21 . Th e chapte r als o discusses lim e and cemen t stabilization . Appendix A provides a lis t o f S I units , an d Appendi x B compares methods tha t have been proposed .
This textbook b y Prof . V. N. S . Murthy i s highly recommended fo r student s specializin g i n geotechnical engineerin g an d fo r practicing civil engineers i n the United State s an d Europe . Th e book include s recen t development s suc h a s soi l improvemen t an d stabilizatio n method s an d applications o f geotextiles t o control settlement s an d lateral earth pressure . Numerou s graph s an d examples illustrat e th e mos t importan t concept s i n geotechnica l engineering . Thi s textboo k should serve as a valuable reference book fo r many years to come.
BengtB.Broms, Ph.D. Nanyang Technical University , Singapore (retired) .
PREFACE
This boo k has the following objectives:
1. T o explain the fundamentals of the subject from theor y to practice in a logical wa y 2. T o be comprehensive an d meet th e requirements o f undergraduate student s
3. T o serve as a foundation course for graduate students pursuing advanced knowledge in the subject
There ar e 21 chapters i n this book. The first chapte r traces the historical background o f the subject and the second deal s with the formation and mineralogical composition o f soils. Chapter 3 covers th e index properties an d classification of soil. Chapter s 4 and 5 explain soi l permeability , seepage, an d th e effec t o f wate r o n stres s condition s i n soil . Stresse s develope d i n soi l du e t o imposed surfac e loads , compressibilit y an d consolidatio n characteristics , an d shea r strengt h characteristics o f soi l ar e deal t wit h i n Chapters 6,7 , an d 8 respectively. Th e firs t eigh t chapter s develop the necessary tool s for computing compressibility an d strength characteristics o f soils.
Chapter 9 deals with methods for obtainig soil samples in the field for laboratory tests and for determining soi l parameter s directl y b y us e o f fiel d tests . Chapter s 1 0 to 2 0 dea l wit h stability problems pertaining to earth embankments, retaining walls, and foundations. Chapter 2 1 explains the various methods by which soil in situ can be improved. Many geotechnical engineer s hav e not appreciated th e importanc e o f thi s subject . N o amoun t o f sophisticatio n i n th e developmen t o f theories will help the designers if the soil parameters used in the theory are not properly evaluated to simulate field conditions . Professors wh o teach this subject should stress thi s topic.
The chapter s i n this book are arranged i n a logical wa y for the development o f the subject matter. There is a smooth transition from on e chapter to the next and the continuity of the material is maintained. Each chapter starts with an introduction to the subject matter, develops th e theory, and explain s it s applicatio n t o practica l problems . Sufficien t example s ar e wor1:ed ou t t o hel p students understand th e significanc e of the theories. Man y homework problem s ar e given a t the end of each chapter .
The subjec t matte r deal t wit h i n eac h chapte r i s restricte d t o th e requirement s o f undergraduate students. Half-baked theories an d unconfirmed test results are not developed in this book. Chapter s ar e up-to-dat e a s pe r engineerin g standards . Th e informatio n provide d i n Chapter 1 7 on drilled pier foundations is the latest available at the time of this writing. The design
of mechanically stabilized earth retaining walls is also current . A new method for predicting the nonlinear behavior of laterally loaded vertica l and batter piles i s described in Chapter 16.
The boo k i s comprehensive , rational , an d pertinen t to th e requirement s o f undergraduate students. It serves as a foundation course for graduate students, and is useful as a reference book for designers an d contractors in the field o f geotechnical engineering .
ACKNOWLEDGEMENTS
It i s my pleasure t o than k Marcel Dekker, Inc., fo r accepting m e as a single author for the publication of my book. The man who was responsible for this was Mr. B.J. Clark, the Executive Acquisition Editor . I t wa s m y pleasur e t o wor k unde r hi s guidance . Mr . Clar k i s a refine d gentleman personified , polished , an d clea r sighted . I thank hi m cordiall y fo r th e courtesie s an d help extended to me during the course of writing the manuscript. I remain ever grateful t o him.
Writing a book fo r American Universities by a nonresident o f America is not an easy task. I neede d a n America n professo r t o edi t m y manuscrip t an d guid e m e wit h regard s t o th e requirements of undergraduate students in America. Dr. Mark T. Bowers, Associate Professor o f Civil Engineering , Universit y o f Cincinnati, accepte d t o become m y consultant an d chief editor . Dr. Bowers i s a man of honest y and integrity. He is dedicated t o the cause of his profession. He worked hard for over a year in editing my book and helped me to streamline t o make it acceptable to the undergraduate student s o f American Universities. I thank Dr. Bowers fo r the help extende d
to me.
There ar e man y i n Indi a wh o helpe d m e durin g th e cours e o f writin g thi s book . Som e provided m e usefu l suggestion s an d other s wit h references . I acknowledg e thei r service s wit h thanks. The members are :
Mr. S. Pranesh Managin g Directo r
Prism Books Pvt Ltd Bangalore
Dr. K.S.Subba Ra o Professo r of Civil Engineering
Indian Institute of Science Bangalor e
Dr. T.S. Nagaraj Professo r of Civil Engineering
(Emeritus), Indian Institute o f Science, Bangalore
Professor of Civil Engineering
Dr. C. Subba Rao India n Institute of Technology
Kharagpur
Chaitanya Graphics , Bangalore , provide d th e artwor k fo r th e book . I than k M r S.K . Vijayasimha, the designer, for the excellent job don e by him.
My so n Prakas h wa s associate d wit h th e boo k sinc e it s inception . H e carrie d o n correspondence wit h th e publisher s o n m y behalf and sen t referenc e books a s needed. M y wif e Sharadamani wa s mainl y responsibl e fo r keepin g m y spiri t hig h durin g th e year s I spen t i n writing the book. I remain grateful t o my son and my wife fo r all they did.
I sincerely thank Mr. Brian Black for his continuous effort s i n the production of this book. I immensely thank Mr. Janardhan and Mr. Rajeshwar, compute r engineers of Aicra Info Mate s Pvt Ltd., Hyderabad, for their excellent typesetting work on this book.
CONTENTS
Foreword Mar k T . Bowers v
Foreword Beng t B. Broms vi
i
Preface i
x
CHAPTER 1 INTRODUCTIO N 1
1.1 Genera
l Remark s 1
1.2 A
Brie f Historica l Developmen t 2
1.3 Soi
l Mechanic s an d Foundatio n Engineerin g 3
CHAPTER 2 SOI L FORMATION AND CHARACTERIZATION 5
2.1 Introductio
n 5
2.2 Roc
k Classificatio n 5
2.3 Formatio
n o f Soil s 7
2.4 Genera
l Type s o f Soil s 7
2.5 Soi
l Particl e Siz e an d Shape 9
2.6 Compositio
n o f Cla y Mineral s 1
1
2.7 Structur
e of Cla y Mineral s 1
1
2.8 Cla
y Particle-Wate r Relation s 1
4
2.9 Soi
l Mas s Structur e 1
7
CHAPTER 3 SOI L PHASE RELATIONSHIPS, INDEX PROPERTIES
AND CLASSIFICATION 1
9
3.1 Soi l Phas e Relationship s 1 9
3.2 Mass-Volum e Relationships 2 0
3.3 Weight-Volum e Relationship s 2 4
3.4 Comment s o n Soi l Phas e Relationship s 2 5
3.5 Inde x Propertie s o f Soil s 3 1
3.6 Th e Shap e an d Siz e o f Particle s 3 2
3.7 Siev e Analysi s 3 3
3.8 Th e Hydromete r Metho d o f Analysis 3 5
3.9 Grai n Siz e Distributio n Curves 4 3
3.10 Relativ e Densit y o f Cohesionles s Soil s 4 4
3.11 Consistenc y o f Cla y Soi l 4 5
3.12 Determinatio n o f Atterberg Limit s 4 7
3.13 Discussio n o n Limit s an d Indices 5 2
3.14 Plasticit y Char t 5 9
3.15 Genera l Consideration s fo r Classificatio n o f Soil s 6 7
3.16 Fiel d Identificatio n of Soil s 6 8
3.17 Classificatio n o f Soil s 6 9
3.18 Textura l Soi l Classificatio n 6 9
3.19 AASHT O Soi l Classificatio n System 7 0
3.20 Unifie d Soi l Classificatio n Syste m (USCS ) 7 3
3.21 Comment s o n th e System s o f Soi l Classificatio n 7 6
3.22 Problem s 8 0
CHAPTER 4 SOI L PERMEABILITY AND SEEPAGE 8
7
4.1 Soi l Permeabilit y 8 7
4.2 Darcy' s La w 8 9
4.3 Discharg e an d Seepag e Velocitie s 9 0
4.4 Method s o f Determinatio n of Hydrauli c Conductivity of Soil s 9 1 4.5 Constan t Hea d Permeabilit y Tes t 92
4.6 Fallin g Hea d Permeabilit y Tes t 9 3
4.7 Direc t Determinatio n of k o f Soil s i n Plac e b y Pumpin g Test 9 7
4.8 Borehol e Permeabilit y Test s 10 1
4.9 Approximat e Value s o f th e Hydrauli c Conductivity of Soil s 10 2 4.10 Hydrauli c Conductivit y in Stratifie d Layer s o f Soil s 10 2
4.11 Empirica l Correlation s fo r Hydrauli c Conductivity 10 3
4.12 Hydrauli c Conductivit y of Rock s b y Packe r Metho d 11 2
4.13 Seepag e 11 4
Contents xii i
4.15 Flo w Ne t Constructio n 11
6
4.16 Determinatio n o f Quantit y of Seepag e 12
0
4.17 Determinatio n o f Seepag e Pressur e 12
2
4.18 Determinatio n o f Uplif t Pressure s 12
3
4.19 Seepag e Flo w Throug h Homogeneou s Eart h Dam s 12
6
4.20 Flo w Ne t Consistin g o f Conjugat e Confocal Parabola s 12
7
4.21 Pipin g Failur e 13
1
4.22 Problem s 13
8
CHAPTER 5 EFFECTIV E STRESS AND PORE WATER PRESSURE 14 3
5.1 Introductio
n 14
3
5.2 Stresse
s whe n N o Flo w Take s Plac e Through th e
Saturated Soi l Mas s 14
5
5.3 Stresse
s Whe n Flo w Take s Plac e Through th e Soi l
from To p to Botto m 14
6
5.4 Stresse
s Whe n Flo w Take s Plac e Throug h th e Soi l
from Botto m t o To p 14
7
5.5 Effectiv
e Pressur e Du e t o Capillar y Wate r Ris e i n Soi l 14
9
5.6 Problem
s 17
0
CHAPTER 6 STRES S DISTRIBUTIO N IN SOILS
DUE TO SURFACE LOADS 17
3
6.1 Introductio
n 17
3
6.2 Boussinesq'
s Formul a fo r Poin t Load s 17
4
6.3 Westergaard'
s Formul a fo r Point Load s 17
5
6.4 Lin
e Load s 17
8
6.5 Stri
p Load s 17
9
6.6 Stresse
s Beneat h th e Corne r o f a Rectangula r Foundation 18
1
6.7 Stresse
s Unde r Uniforml y Loade d Circula r Footin g 18
6
6.8 Vertica
l Stres s Beneat h Loade d Area s o f Irregula r Shap e 18
8
6.9 Embankmen
t Loading s 19
1
6.10 Approximat e Method s fo r Computin g c r 19
7
6.11 Pressur e Isobar s 19
8
6.12 Problem s 20
3
CHAPTER 7 COMPRESSIBILIT Y AN D CONSOLIDATION 20
7
7.1 Introductio
n 20
7
7.2 Consolidatio
n 20
8
7.4 Th
e Standar d One-Dimensiona l Consolidatio n Tes t 21
3
7.5 Pressure-Voi
d Rati o Curve s 21
4
7.6 Determinatio
n of Preconsolidatio n Pressur e 21
8
7.7 e-logp
Fiel d Curves for Normally Consolidate d and
Overconsolidated Clay s of Low to Medium Sensitivit y 21
9
7.8 Computatio
n of Consolidatio n Settlemen t 21
9
7.9 Settlemen
t Du e t o Secondar y Compressio n 22
4
7.10 Rat e o f One-dimensiona l Consolidation Theor y o f Terzagh i 23
3
7.11 Determinatio n of th e Coefficien t o f Consolidatio n 24
0
7.12 Rat e o f Settlemen t Du e t o Consolidatio n 24
2
7.13 Two - and Three-dimensional Consolidatio n Problem s 24
3
7.14 Problem s 24
7
CHAPTERS SHEA R STRENGTH O F SOIL 25
3
8.1 Introductio
n 25
3
8.2 Basi
c Concep t o f Shearin g Resistanc e an d Shearin g Strengt h 25
3
8.3 Th
e Coulom b Equatio n 25
4
8.4 Method
s o f Determinin g Shea r Strengt h Parameters 25
5
8.5 Shea
r Tes t Apparatu s 25
6
8.6 Stres
s Conditio n at a Point i n a Soil Mas s 26
0
8.7 Stres
s Condition s in Soi l Durin g Triaxial Compressio n Tes t 26
2
8.8 Relationshi
p Between th e Principal Stresse s an d Cohesio n c 26
3
8.9 Moh
r Circl e o f Stres s 26
4
8.10 Moh r Circl e o f Stres s When a Prismatic Elemen t i s Subjecte d t o
Normal an d Shea r Stresse s 26
5
8.11 Moh r Circl e o f Stres s fo r a Cylindrical Specime n
Compression Tes t 26
6
8.12 Mohr-Coulom b Failur e Theor y 26
8
8.13 Moh r Diagra m fo r Triaxial Compressio n Tes t a t Failure 26
9
8.14 Moh r Diagra m fo r a Direct Shea r Test a t Failure 27
0
8.15 Effectiv e Stresse s 27
4
8.16 Shea r Strengt h Equatio n i n Terms o f Effectiv e Principa l Stresse s 27
5
8.17 Stress-Controlle d an d Strain-Controlled Test s 27
6
8.18 Type s o f Laborator y Tests 27
6
8.19 Shearin g Strengt h Test s o n San d 27
8
8.20 Unconsolidated-Undraine d Test 28
4
8.21 Unconfine d Compressio n Test s 28
6
8.22 Consolidated-Undraine d Tes t o n Saturate d Cla y 29
4
8.23 Consolidated-Draine d Shea r Strengt h Tes t 29
6
8.24 Por e Pressur e Parameter s Unde r Undraine d Loadin g 29
8
Contents x v
8.26 Othe r Method s fo r Determinin g Undraine d Shea r Strengt h
of Cohesiv e Soil s 30
2
8.27 Th e Relationshi p Between Undraine d Shear Strengt h an d
Effective Overburde n Pressure 30
4
8.28 Genera l Comment s 31
0
8.29 Question s an d Problem s 31
1
CHAPTERS SOI L EXPLORATION 31
7
9.1 Introductio
n 31
7
9.2 Borin
g o f Hole s 31
8
9.3 Samplin
g in Soi l 32
2
9.4 Roc
k Cor e Samplin g 32
5
9.5 Standar
d Penetration Tes t 32
7
9.6 SP
T Values Related t o Relative Densit y o f Cohesionles s Soil s 33
0
9.7 SP
T Values Related t o Consistenc y o f Cla y Soi l 33
0
9.8 Stati
c Con e Penetratio n Tes t (CPT ) 33
2
9.9 Pressuremete
r 34
3
9.10 Th e Flat Dilatomete r Tes t 34
9
9.11 Fiel d Van e Shear Test (VST) 35
1
9.12 Fiel d Plat e Loa d Test (PUT ) 3
51
9.13 Geophysica l Exploratio n 35
2
9.14 Plannin g of Soi l Exploratio n 35
8
9.15 Executio n o f Soi l Exploratio n Progra m 35
9
9.16 Repor t 36
1
9.17 Problem s 36
2
CHAPTER 1 0 STABILIT Y O F SLOPES 36
5
10.1 Introductio n 36
5
10.2 Genera l Consideration s an d Assumptions in th e Analysis 36
7
10.3 Facto r o f Safet y 36
8
10.4 Stabilit y Analysi s of Infinit e Slope s i n San d 37
1
10.5 Stabilit y Analysis of Infinit e Slope s i n Cla y 37
2
10.6 Method s o f Stabilit y Analysis of Slope s o f Finit e Heigh t 37
6
10.7 Plan e Surfac e of Failur e 37
6
10.8 Circula r Surface s of Failure 37
8
10.9 Failur e Unde r Undraine d Condition s ((f>
u= 0 ) 38
0
10.10 Friction-Circl e Metho d 38
2
10.11 Taylor' s Stabilit y Numbe r 38
9
10.12 Tensio n Crack s 39
3
10.14 Bishop' s Simplifie d Metho d o f Slice s 40 0 10.15 Bisho p an d Morgenster n Metho d fo r Slop e Analysi s 40 3 10.16 Morgenster n Metho d o f Analysi s for Rapi d Drawdow n Conditio n 40 5
10.17 Spence r Metho d o f Analysis 40 8
10.18 Problem s 41 1
CHAPTER 1 1 LATERA L EARTH PRESSURE 41
9
11.1 Introductio n 41 9
11.2 Latera l Eart h Pressur e Theor y 42 0
11.3 Latera l Eart h Pressur e fo r a t Res t Conditio n 42 1
11.4 Rankine' s State s o f Plasti c Equilibriu m for Cohesionles s Soil s 42 5 11.5 Rankine' s Eart h Pressur e Agains t Smoot h Vertica l Wall wit h
Cohesionless Backfil l 42 8
11.6 Rankine' s Activ e Earth Pressur e wit h Cohesiv e Backfil l 44 0 11.7 Rankine' s Passiv e Eart h Pressur e wit h Cohesiv e Backfil l 44 9 11.8 Coulomb' s Eart h Pressur e Theor y fo r San d fo r Active Stat e 45 2 11.9 Coulomb' s Eart h Pressur e Theor y fo r San d fo r Passiv e Stat e 45 5 11.10 Activ e Pressur e b y Culmann' s Metho d fo r Cohesionles s Soil s 45 6 11.11 Latera l Pressure s b y Theor y o f Elasticit y fo r Surcharg e Load s
on th e Surfac e of Backfil l 45 8
11.12 Curve d Surface s o f Failur e fo r Computin g Passiv e Eart h Pressur e 46 2 11.13 Coefficient s o f Passiv e Eart h Pressur e Table s an d Graph s 46 4 11.14 Latera l Eart h Pressur e o n Retainin g Walls Durin g Earthquake s 46 7
11.15 Problem s 47 6
CHAPTER 1 2 SHALLO W FOUNDATION I:
ULTIMATE BEARING CAPACITY 48
1
12.1 Introductio n 48 1
12.2 Th e Ultimat e Bearing Capacit y o f Soi l 48 3
12.3 Som e o f th e Term s Define d 48 3
12.4 Type s o f Failur e i n Soi l 48 5
12.5 A n Overvie w o f Bearin g Capacit y Theorie s 48 7
12.6 Terzaghi' s Bearin g Capacit y Theor y 48 8
12.7 Skempton' s Bearin g Capacit y Facto r NC 49 3
12.8 Effec t o f Water Tabl e o n Bearin g Capacit y 49 4
12.9 Th e Genera l Bearin g Capacit y Equatio n 50 3
12.10 Effec t o f Soi l Compressibilit y o n Bearin g Capacit y o f Soi l 50 9 12.11 Bearin g Capacit y o f Foundation s Subjecte d t o Eccentri c Load s 51 5 12.12 Ultimat e Bearin g Capacit y o f Footing s Base d o n SP T Value s (N) 51 8 12.13 Th e CP T Metho d o f Determinin g Ultimat e Bearin g Capacit y 51 8
Contents xvi i
12.14 Ultimat e Bearin g Capacit y o f Footing s Restin g o n Stratifie d
Deposits o f Soi l 52
1
12.15 Bearin g Capacit y o f Foundation s o n Top of a Slop e 52
9
12.16 Foundation s o n Roc k 53
2
12.17 Cas e Histor y o f Failur e o f th e Transcona Grai n Elevato r 53
3
12.18 Problem s 53
6
CHAPTER 13 SHALLO W FOUNDATION II:
SAFE BEARING PRESSURE AND SETTLEMENT CALCULATION 54
5
13.1 Introductio n 54
5
13.2 Fiel d Plat e Loa d Test s 54
8
13.3 Effec t o f Siz e o f Footing s o n Settlemen t 55
4
13.4 Desig n Chart s fro m SP T Values for Footing s o n San d 55
5
13.5 Empirica l Equation s Base d o n SP T Values for Footing s o n
Cohesionless Soil s 55
8
13.6 Saf e Bearin g Pressur e fro m Empirica l Equation s Based o n
CPT Values for Footings o n Cohesionles s Soi l 55
9
13.7 Foundatio n Settlemen t 56
1
13.8 Evaluatio n o f Modulu s of Elasticit y 56
2
13.9 Method s o f Computin g Settlement s 56
4
13.10 Elasti c Settlemen t Beneat h th e Corne r o f a Uniforml y Loade d
Flexible Are a Base d o n the Theor y o f Elasticit y 56
5
13.11 Janbu , Bjerrum an d Kjaernsli' s Metho d o f Determinin g
Elastic Settlemen t Unde r Undrained Conditions 56
8
13.12 Schmertmann' s Metho d o f Calculatin g Settlemen t i n Granula r
Soils b y Usin g CPT Values 56
9
13.13 Estimatio n o f Consolidatio n Settlemen t b y Usin g Oedometer
Test Dat a 57
5
13.14 Skempton-Bjerru m Metho d o f Calculatin g Consolidatio n
Settlement (1957 ) 57
6
13.15 Problem s 58
0
CHAPTER 14 SHALLO W FOUNDATION III:
COMBINED FOOTINGS AND MAT FOUNDATIONS 58
5
14.1 Introductio n 58
5
14.2 Saf e Bearin g Pressure s fo r Ma t Foundation s o n San d an d Clay 58
7
14.3 Eccentri c Loadin g 58
8
14.4 Th e Coefficien t o f Subgrad e Reactio n 58
8
14.6 Desig n o f Combine d Footing s b y Rigi d Metho d (Conventiona l
Method) 59 2
14.7 Desig n o f Ma t Foundatio n b y Rigi d Metho d 59 3
14.8 Desig n o f Combine d Footing s b y Elasti c Lin e Metho d 59 4 14.9 Desig n o f Ma t Foundation s by Elasti c Plat e Metho d 59 5
14.10 Floatin g Foundatio n 59 5
14.11 Problem s 60 3
CHAPTER 15 DEE P FOUNDATION I :
PILE FOUNDATION 60
5
15.1 Introductio n 60 5
15.2 Classificatio n of Pile s 605
15.3 Type s o f Pile s Accordin g to th e Metho d o f Installatio n 60 6
15.4 Use s o f Pile s 60 8
15.5 Selectio n o f Pil e 60 9
15.6 Installatio n of Pile s 61 0
PART A-VERTICAL LOAD BEARING CAPACITY OF A SINGLE VERTICAL PILE 61
3
15.7 Genera l Consideration s 61 3
15.8 Method s o f Determinin g Ultimat e Load Bearin g Capacit y o f a
Single Vertical Pile 61 7
15.9 Genera l Theor y fo r Ultimat e Bearing Capacit y 61 8
15.10 Ultimat e Bearin g Capacit y i n Cohesionles s Soil s 62 0
15.11 Critica l Dept h 62 1
15.12 Tomlinson' s Solutio n fo r Qbin Sand 62 2
15.13 Meyerhof' s Metho d o f Determinin g Qbfor Pile s i n San d 62 4
15.14 Vesic' s Metho d o f Determinin g Qb 62 5
15.15 Janbu' s Metho d o f Determinin g Qb 62 8
15.16 Coyl e an d Castello's Metho d o f Estimating Qbin San d 62 8 15.17 Th e Ultimat e Skin Resistanc e of a Singl e Pile i n Cohesionles s Soi l 62 9 15.18 Ski n Resistanc e Qfby Coyl e an d Castell o Metho d (1981 ) 63 1 15.19 Stati c Bearin g Capacit y o f Pile s i n Cla y Soi l 63 1 15.20 Bearin g Capacit y of Pile s i n Granula r Soil s Base d o n SP T Value 63 5 15.21 Bearin g Capacit y o f Pile s Base d o n Stati c Con e Penetratio n
Tests (CPT ) 65 2
15.22 Bearin g Capacit y o f a Singl e Pil e b y Loa d Tes t 66 3 15.23 Pil e Bearing Capacity fro m Dynamic Pile Driving Formulas 66 6 15.24 Bearin g Capacit y o f Pile s Founde d o n a Rock y Be d 67 0
Contents xi x
PART B-PILE GROUP 67
4
15.26 Numbe r an d Spacin g o f Pile s i n a Group 67
4
15.27 Pil e Grou p Efficienc y 67
6
15.28 Vertica l Bearin g Capacity o f Pil e Group s Embedde d i n
Sands an d Gravel s 67
8
15.29 Settlemen t o f Piles an d Pile Group s i n Sand s an d Gravels 68
1
15.30 Settlemen t o f Pil e Group s i n Cohesive Soil s 68
9
15.31 Allowabl e Loads o n Groups o f Pile s 69
0
15.32 Negativ e Frictio n 69
2
15.33 Uplif t Capacit y o f a Pile Grou p 69
4
15.34 Problem s 69
6
CHAPTER 16 DEE P FOUNDATION II :
BEHAVIOR OF LATERALLY LOADED VERTICAL AND
BATTER PILES 69
9
16.1 Introductio n 69
9
16.2 Winkler' s Hypothesi s 70
0
16.3 Th e Differentia l Equatio n 70
1
16.4 Non-dimensiona l Solution s fo r Vertical Piles Subjecte d t o
Lateral Load s 70
4
16.5 p- y Curve s fo r th e Solutio n o f Laterally Loade d Pile s 70
6
16.6 Broms ' Solution s fo r Laterall y Loade d Pile s 70
9
16.7 A Direc t Metho d fo r Solvin g th e Non-linea r Behavio r o f
Laterally Loade d Flexibl e Pil e Problem s 71
6
16.8 Cas e Studie s fo r Laterally Loade d Vertical Pile s in San d 72
2
16.9 Cas e Studie s fo r Laterally Loade d Vertica l Pile s i n Clay 72
5
16.10 Behavio r o f Laterall y Loade d Batte r Pile s i n San d 73
1
16.11 Problem s 73
9
CHAPTER 17 DEE P FOUNDATION III :
DRILLED PIER FOUNDATIONS 74
1
17.1 Introductio n 74
1
17.2 Type s o f Drille d Pier s 7
41
17.3 Advantage s and Disadvantage s o f Drilled Pie r Foundation s 74
3
17.4 Method s o f Constructio n 74
3
17.5 Desig n Consideration s 75
1
17.6 Loa d Transfe r Mechanis m 75
2
17.7 Vertica l Bearing Capacit y o f Drilled Pier s 75
4
17.8 Th e Genera l Bearin g Capacit y Equatio n fo r th e Bas e Resistanc e
17.9 Bearin g Capacit y Equation s fo r th e Bas e i n Cohesiv e Soi l 75 6 17.10 Bearin g Capacit y Equatio n fo r th e Bas e i n Granula r Soi l 75 6 17.11 Bearin g Capacit y Equation s fo r th e Bas e i n Cohesiv e IG M o r Roc k 75 9 17.12 Th e Ultimat e Ski n Resistanc e o f Cohesiv e an d
Intermediate Material s 76 0
17.13 Ultimat e Ski n Resistanc e i n Cohesionles s Soi l an d Gravell y Sand s 76 3
17.14 Ultimat e Side an d Tota l Resistanc e i n Roc k 76 4
17.15 Estimatio n o f Settlement s of Drille d Pier s a t Working Load s 76 5
17.16 Uplif t Capacit y o f Drille d Pier s 77 7
17.17 Latera l Bearin g Capacit y o f Drille d Pier s 77 9
17.18 Cas e Stud y o f a Drille d Pie r Subjecte d t o Latera l Load s 78 7
17.19 Problem s 78 7
CHAPTER 18 FOUNDATION S ON COLLAPSIBLE AND
EXPANSIVE SOIL S 79
1
18.1 Genera l Consideration s 79 1
PART A-COLLAPSIBLE SOILS 79
3
18.2 Genera l Observation s 79 3
18.3 Collaps e Potentia l an d Settlemen t 79 5
18.4 Computatio n o f Collaps e Settlemen t 79 6
18.5 Foundatio n Desig n 79 9
18.6 Treatmen t Method s fo r Collapsibl e Soil s 80 0
PART B-EXPANSIVE SOILS 80
0
18.7 Distributio n of Expansiv e Soil s 80 0
18.8 Genera l Characteristic s o f Swellin g Soil s 80 1
18.9 Cla y Mineralog y an d Mechanis m o f Swellin g 80 3
18.10 Definitio n o f Som e Parameter s 80 4
18.11 Evaluatio n o f th e Swellin g Potentia l o f Expansiv e Soil s b y Singl e
Index Metho d 80 4
18.12 Classificatio n o f Swellin g Soil s b y Indirec t Measuremen t 80 6
18.13 Swellin g Pressur e b y Direc t Measuremen t 81 2
18.14 Effec t o f Initia l Moistur e Conten t an d Initia l Dry Densit y o n
Swelling Pressur e 81 3
18.15 Estimatin g the Magnitud e of Swellin g 81 4
18.16 Desig n o f Foundation s i n Swellin g Soil s 81 7
18.17 Drille d Pie r Foundation s 81 7
18.18 Eliminatio n o f Swellin g 82 7
Contents xx i
CHAPTER 19 CONCRET E AND MECHANICALLY STABILIZED
EARTH RETAINING WALLS 83
3
PART A-CONCRETE RETAINING WALL S 83
3
19.1 Introductio n 83
3
19.2 Condition s Unde r Whic h Rankin e an d Coulom b Formula s Ar e
Applicable t o Retainin g Walls Unde r th e Active Stat e 83
3
19.3 Proportionin g o f Retainin g Walls 83
5
19.4 Eart h Pressur e Chart s fo r Retainin g Walls 83
6
19.5 Stabilit y o f Retainin g Walls 83
9
PART B-MECHANICALLY STABILIZED EART H RETAINING WALL S 84
9
19.6 Genera l Consideration s 84
9
19.7 Backfil l an d Reinforcin g Material s 85
1
19.8 Constructio n Detail s 85
5
19.9 Desig n Consideration s fo r a Mechanically Stabilize d Eart h Wal l 85
7
19.10 Desig n Metho d 85
9
19.11 Externa l Stabilit y 86
3
19.12 Example s o f Measure d Latera l Eart h Pressure s 87
5
19.13 Problem s 87
7
CHAPTER 20 SHEE T PILE WALLS AND BRACED CUTS 88
1
20.1 Introductio n 88
1
20.2 Shee t Pil e Structure s 88
3
20.3 Fre e Cantileve r Shee t Pil e Wall s 88
3
20.4 Dept h o f Embedmen t o f Cantileve r Walls i n Sand y Soil s 88
5
20.5 Dept h o f Embedmen t o f Cantileve r Walls i n Cohesiv e Soil s 89
6
20.6 Anchore d Bulkhead : Free-Eart h Suppor t Method—Dept h o f
Embedment o f Anchored Shee t Pile s i n Granula r Soil s 90
8
20.7 Desig n Chart s fo r Anchored Bulkhead s i n San d 91
3
20.8 Momen t Reductio n fo r Anchored Shee t Pil e Walls 91
6
20.9 Anchorag e o f Bulkhead s 92
5
20.10 Brace d Cut s 93
1
20.11 Latera l Eart h Pressur e Distributio n o n Braced-Cut s 93
5
20.12 Stabilit y o f Brace d Cut s i n Saturate d Clay 93
8
20.13 Bjerru m an d Eid e Metho d o f Analysis 94
0
20.14 Pipin g Failure s i n San d Cut s 94
5
CHAPTER 21 SOI L IMPROVEMENT
21.1 Introductio n
21.2 Mechanica l Compaction
21.3 Laborator y Test s o n Compaction
21.4 Effec t o f Compactio n o n Engineerin g Behavio r
21.5 Fiel d Compactio n an d Contro l
21.6 Compactio n fo r Deepe r Layer s o f Soi l
21.7 Preloadin g
21.8 San d Compactio n Pile s an d Ston e Column s
21.9 Soi l Stabilizatio n b y th e Us e o f Admixtures
21.10 Soi l Stabilizatio n by Injectio n of Suitabl e Grout s
21.11 Problem s
951
951 952 953 959 962 973 974 980 981 983 983APPENDIX A S I UNITS IN GEOTECHNICAL ENGINEERING
987
APPENDIX B SLOP E STABILITY CHART S AND TABLES
993
REFERENCES
1007
CHAPTER 1
INTRODUCTION
1.1 GENERA L REMARK S
Karl Terzaghi writing in 1951 (Bjerrum, et. al., 1960), on 'The Influence of Modern Soil Studies on the Design and Construction of Foundations' commente d on foundations as follows:
Foundations can appropriately be described as a necessary evil. If a building is to be constructed on an outcrop of sound rock, no foundation is required. Hence, in contrast to the building itself which satisfies specific needs, appeals to the aesthetic sense, and fills its matters with pride, the foundations merely serve as a remedy for the deficiencies of whatever whimsical nature has provided for the support of the structure at the site which has been selected. On account of the fact that there is no glory attached to the foundations, and that the sources of success or failures are hidden deep in the ground, building foundations have always been treated as step children; and their acts of revenge for the lack of attention can be very embarrassing.
The comment s mad e b y Terzaghi ar e ver y significan t an d shoul d b e take n note o f by al l practicing Architects and Engineers. Architects or Engineers who do not wish to make use of the growing knowledge of foundation design are not rendering true service to their profession. Sinc e substructures are as important as superstructures, persons wh o are well qualified in the design of substructures shoul d alway s b e consulted an d the old proverb that a 'stitc h in tim e save s nine' should always be kept in mind.
The design of foundations is a branch of Civil Engineering. Experience has shown that most of these branches have passed i n succession throug h two stages, th e empirical an d the scientific, before they reached th e present one which may be called the stage of maturity.
The stage of scientific reasoning in the design of foundations started with the publication of the book Erdbaumechanik (means Soil Mechanics) by Karl Terzaghi in 1925. This book represents the first attempt to treat Soil Mechanics on the basis of the physical properties o f soils. Terzaghi' s
contribution for the development of Soil Mechanics and Foundation Engineering is so vast that he may trul y be called th e Father o f Soil Mechanics, Hi s activity extended over a period o f about 50 years starting from th e year 1913 . He was born on October 2, 188 3 in Prague and died on October 25, 196 3 i n Winchester, Massachusetts, USA. His amazing career is well documented in the book
'From Theory t o Practice in Soil Mechanics' (Bjerrum , L., et. al., 1960) .
Many investigator s in th e fiel d o f Soi l Mechanics wer e inspired by Terzaghi . Som e o f th e notable personalitie s wh o followe d hi s footstep s ar e Ralp h B . Peck , Arthu r Casagrande , A. W. Skempton, etc. Because of the unceasing efforts of these and other innumerable investigators, Soil Mechanics and Foundation Engineerin g ha s come to stay as a very important par t of the Civil Engineering profession.
The transitio n of foundatio n engineering fro m th e empirica l stag e t o tha t of th e scientific stage started almos t a t the commencement o f the 20th century . The design o f foundations durin g the empirical stage wa s based mostly on intuition and experience. There use d to be many failures since the procedure o f design was only by trial and error .
However, in the present scientific age, the design of foundations based on scientific analysis has received a much impetus. Theories hav e been develope d base d o n fundamental properties of soils. Still one can witness unsatisfactory performance of structures constructed even on scientific principles. The reasons for such poor performance are many. The soil mass on which a structure is to be buil t i s heterogeneou s i n characte r an d n o theor y ca n simulat e fiel d conditions . Th e fundamental propertie s o f soi l whic h w e determin e i n laboratorie s ma y no t reflec t trul y th e properties o f th e soi l in-situ. A judicial combination o f theor y an d experienc e i s essentia l fo r successful performance o f any structure built on earth. Another method that is gaining popularity is the observational approach. Thi s procedur e consist s i n makin g appropriat e observation s soo n enough during construction to detect signs of departure of the real conditions from those assume d by th e designe r an d i n modifying either th e design o r th e method o f construction in accordanc e with the findings.
1.2 A BRIE F HISTORICA L DEVELOPMEN T
Many structure s that were buil t centurie s ago ar e monument s of curiosit y eve n today . Egyptia n temples buil t three or four thousand years ago still exist though the design of the foundations were not based o n any presently known principles. Romans built notable engineering structures such as harbors, breakwaters, aqueducts, bridges, large public buildings and a vast network of durable and excellent roads . Th e leaning tower o f Pisa i n Ital y complete d durin g th e 14t h centur y is stil l a center of tourist attraction. Many bridges were also built during the 15t h to 17t h centuries. Timber piles were used for many of the foundations.
Another marvel of engineering achievement i s the construction o f the famed mausoleum Taj Mahal outsid e th e city of Agra. This was constructed in the 17t h century by the Mogul Emperor of Delhi, Shahjahan, to commemorate his favorite wife Mumtaz Mahal. The mausoleum is built on the bank of the river Jamuna. The proximity of the river required special attention in the building of the foundations. I t i s reported tha t masonry cylindrical wells have been use d fo r th e foundations . It goes to the credit of the engineers who designed and constructed this grand structure which is still quite sound even afte r a lapse o f about three centuries.
The firs t rationa l approach fo r th e computatio n o f eart h pressure s o n retainin g wall s wa s formulated by Coulomb (1776), a famous French scientist. He proposed a theory in 1776 called the "Classical Eart h Pressur e Theory" . Poncele t (1840 ) extende d Coulomb' s theor y b y givin g a n elegant graphica l metho d fo r findin g th e magnitud e of eart h pressur e o n walls . Later, Culmann (1875) gave th e Coulomb-Poncelet theor y a geometrical formulation , thus supplying the metho d with a broad scientifi c basis. Rankine (1857) a Professor o f Civil Engineering in the University of
Introduction
Glasgow, propose d a new earth pressur e theory , whic h is also called a Classical Earth Pressure Theory.
Darcy (1856), on the basis of his experiments on filter sands, proposed a law for the flow of water in permeable materials and in the same year Stokes (1856) gave an equation for determining the terminal velocity of solid particles falling in liquids. The rupture theory of Mohr (1900) Stres s Circles ar e extensivel y used i n th e stud y of shea r strengt h of soils . On e o f th e mos t importan t contributions to engineering scienc e wa s made by Boussinesq (1885) wh o proposed a theory for determining stres s distributio n under loaded area s i n a semi-infinite, elastic, homogeneous, an d isotropic medium .
Atterberg (1911), a Swedish scientist, proposed simpl e tests for determining the consistency limits o f cohesiv e soils . Felleniu s (1927 ) heade d a Swedis h Geotechnica l Commissio n fo r determining the causes of failure of many railway and canal embankments. The so-called Swedish Circle method or otherwise termed as the Slip Circle method was the outcome of his investigation which was published in 1927 .
The developmen t o f th e scienc e o f Soi l Mechanic s an d Foundatio n Engineerin g fro m th e year 1925 onwards was phenomenal. Terzaghi laid down definite procedures in his book published in 192 5 for determinin g propertie s and the strengt h characteristic s of soils . The moder n soi l mechanics wa s born i n 1925 . Th e present stag e o f knowledge i n Soil Mechanics an d the design procedures o f foundation s ar e mostl y du e t o th e work s o f Terzagh i an d hi s ban d o f devote d collaborators.
1.3 SOI L MECHANIC S AN D FOUNDATIO N ENGINEERIN G
Terzaghi define d Soil Mechanics a s follows:
Soil Mechanics is the application of the laws of mechanics and hydraulics to engineering problems dealing with sediments and other unconsolidated accumulations of solid particles produced by the mechanical and chemical disintegration of rocks regardless of whether or
not they contain an admixture of organic constituents.
The ter m Soil Mechanics i s no w accepte d quit e generall y t o designat e tha t disciplin e of engineering science which deals with the properties an d behavior of soil as a structural material.
All structures have to be built on soils. Our main objective in the study of soil mechanics is to lay down certain principles, theories and procedures for the design of a safe and sound structure. The subjec t of Foundation Engineering deal s wit h the desig n o f variou s types o f substructures under different soi l an d environmental conditions.
During th e design , th e designe r ha s t o mak e us e o f th e propertie s o f soils , th e theorie s pertaining t o th e desig n an d hi s ow n practica l experienc e t o adjus t th e desig n t o sui t fiel d conditions. H e has t o deal wit h natural soil deposit s whic h perform th e engineering functio n o f supporting th e foundatio n an d th e superstructur e abov e it . Soi l deposit s i n natur e exis t i n a n extremely errati c manne r producin g thereb y a n infinit e variet y o f possible combination s whic h would affect th e choice and design of foundations. The foundation engineer must have the ability to interpret the principles of soil mechanics to suit the field conditions. The success or failure of his design depends upo n how much in tune he is with Nature.
CHAPTER 2
SOIL FORMATION AND CHARACTERIZATION
2.1 INTRODUCTIO N
The word 'soil' has different meaning s for different professions . To the agriculturist, soil is the top thin layer of earth withi n which organic forces ar e predominant and which is responsible fo r the support of plant life. To the geologist, soi l is the material in the top thin zone within which roots occur. Fro m th e poin t o f vie w o f a n engineer , soi l include s al l eart h materials , organi c an d inorganic, occurring in the zone overlying the rock crust.
The behavior of a structure depends upon the properties o f the soil materials o n which the structure rests. The properties o f the soil materials depend upon the properties o f the rocks fro m which they are derived. A brief discussion of the parent rocks is, therefore, quite essential in order to understand the properties o f soil materials.
2.2 ROC K CLASSIFICATIO N
Rock can be defined as a compact, semi-hard to hard mass of natural material composed of one or more minerals. The rocks that are encountered at the surface of the earth or beneath, are commonly classified into three groups according to their modes of origin. They are igneous, sedimentary and metamorphic rocks.
Igneous rock s ar e considere d t o b e th e primar y rock s forme d b y th e coolin g o f molte n magmas, or by the recrystallization of older rocks under heat and pressure great enough to render them fluid. They have been formed on or at various depths below the earth surface. There are two main classes of igneous rocks. They are:
1. Extrusiv e (poured out at the surface), and
Initially both classes of rocks were in a molten state. Their presen t state results directly from the way in which they solidified. Due to violent volcanic eruptions in the past, some of the molten materials wer e emitte d int o the atmospher e wit h gaseous extrusions . These coole d quickl y and eventually fell on the earth's surface as volcanic ash and dust. Extrusive rocks are distinguished, in general, by their glass-like structure.
Intrusive rocks , coolin g an d solidifyin g a t grea t depth s an d unde r pressur e containin g entrapped gases, are wholly crystalline in texture. Such rocks occur in masses of great extent, often going to unknown depths. Some of the important rocks that belong to the igneous group are granite and basalt. Granite i s primarily composed of feldspar, quart z and mica an d possesses a massive structure. Basal t i s a dark-colored fine-graine d rock. I t is characterized b y th e predominance o f plagioclase, th e presence of considerable amounts of pyroxene and some olivine and the absence of quartz. Th e colo r varie s fro m dark-gre y t o black . Bot h granit e an d basal t ar e use d a s building stones.
When th e product s o f th e disintegratio n an d decompositio n o f an y roc k typ e ar e transported, redeposited, an d partly or fully consolidate d o r cemented int o a new rock type , the resulting materia l i s classifie d a s a sedimentary rock. Th e sedimentar y rock s generall y ar e formed i n quite definitely arrange d beds, o r strata, which can be seen t o have been horizontal at one time although sometimes displace d throug h angles up to 90 degrees. Sedimentary rock s are generally classified on the basis of grain size, texture and structure. From a n engineering point of view, the most important rocks tha t belong to the group are sandstones, limestones, and shales. Rocks forme d b y th e complet e o r incomplet e recrystallizatio n o f igneou s o r sedimentar y rocks b y high temperatures, high pressures, and/or high shearin g stresse s ar e metamorphic rocks. The rock s s o produce d ma y displa y feature s varyin g from complet e an d distinc t foliatio n o f a crystalline structur e t o a fine fragmentary partially crystallin e stat e cause d b y direct compressiv e stress, including also the cementation of sediment particles by siliceous matter. Metamorphic rocks formed withou t intense shea r actio n have a massive structure . Some o f the importan t rock s tha t belong to this group are gneiss, schist, slate an d marble. Th e characteristic featur e of gneiss is its structure, th e minera l grain s ar e elongated, o r platy, and banding prevails. Generall y gneis s i s a good engineerin g material . Schist is a finely foliate d rock containin g a high percentage o f mica . Depending upo n the amount of pressure applied by the metamorphic forces, schis t may be a very good buildin g material. Slat e i s a dark colored, plat y roc k wit h extremely fin e textur e and eas y cleavage. Becaus e o f this easy cleavage , slat e i s split into very thin sheet s an d use d a s a roofing material. Marble is the end product of the metamorphism of limestone and other sedimentary rock s composed o f calciu m o r magnesiu m carbonate. I t i s ver y dens e an d exhibit s a wid e variet y o f colors. I n construction, marble is used for facing concrete o r masonry exterior an d interio r walls and floors .
Rock Mineral s
It i s essential t o examine th e properties o f the rock formin g minerals sinc e al l soil s ar e derive d through the disintegration or decomposition o f some parent rock. A 'mineral' is a natural inorganic substance of a definite structure and chemical composition. Som e o f the very important physical properties o f mineral s ar e crysta l form , color , hardness , cleavage , luster , fracture , an d specifi c gravity. Out of these only two, specific gravity and hardness, are of foundation engineering interest. The specifi c gravit y of th e mineral s affect s th e specifi c gravity of soil s derive d fro m them . Th e specific gravity of most rock and soil forming minerals varies from 2.50 (some feldspars) and 2.65 (quartz) to 3.5 (augite or olivine). Gypsum has a smaller value of 2.3 and salt (NaCl) has 2.1. Some iron minerals may have higher values, for instance, magnetite has 5.2.
It is reported tha t about 95 percent of the known part of the lithosphere consist s o f igneous rocks and only 5 percent of sedimentary rocks. Soil formation is mostly due to the disintegration of igneous rock which may be termed a s a parent rock.
Soil Formatio n and Characterization 7
Table 2. 1 Minera l compositio n of igneou s rock s
Mineral Percen t
Quartz 12-2 0
Feldspar 50-6 0
Ca, Fe and Mg, Silicates 14-1 7
Micas 4- 8
Others 7- 8
The average mineral composition of igneous rocks is given in Table 2.1. Feldspars ar e the most common rock minerals, which account for the abundance of clays derived from the feldspars on the earth's surface. Quartz comes next in order of frequency. Most sands are composed o f quartz.
2.3 FORMATIO N O F SOILS
Soil is defined a s a natural aggregate o f mineral grains, with or without organic constituents, that can b e separate d b y gentl e mechanica l mean s suc h a s agitatio n i n water . B y contras t roc k i s considered t o be a natural aggregate o f mineral grains connected by strong and permanent cohesive forces. Th e proces s o f weathering of the rock decrease s th e cohesive force s bindin g the mineral grains and leads t o the disintegration of bigger masses t o smaller an d smaller particles . Soil s ar e formed b y the process of weathering of the parent rock. The weathering of the rocks might be by mechanical disintegration, and/or chemical decomposition .
Mechanical Weatherin g
Mechanical weatherin g o f rock s t o smalle r particle s i s du e t o th e actio n o f suc h agent s a s th e expansive forces of freezing water in fissures, due to sudden changes of temperature or due to the abrasion o f roc k b y movin g water o r glaciers . Temperatur e change s o f sufficien t amplitud e an d frequency brin g abou t change s i n the volume of the rocks i n the superficia l layers o f the earth' s crust in terms of expansion and contraction. Such a volume change sets up tensile and shear stresse s in the rock ultimately leading to the fracture of even large rocks. This type of rock weathering takes place in a very significant manner in arid climates where free, extreme atmospheric radiation brings about considerable variatio n in temperature at sunrise and sunset.
Erosion by wind and rain is a very important factor and a continuing event. Cracking forces by growing plants and roots i n voids and crevasses of rock can force fragments apart.
Chemical Weatherin g
Chemical weatherin g (decomposition ) ca n transform hard rock minerals into soft, easily erodable matter. The principa l type s o f decomposition ar e hydmtion, oxidation, carbonation, desilication and leaching. Oxyge n and carbon dioxide which are always present in the air readily combine with the elements of rock in the presence of water.
2.4 GENERA L TYPES OF SOILS
It ha s bee n discusse d earlie r tha t soi l i s forme d b y th e proces s o f physica l an d chemica l weathering. The individua l size of th e constituent parts o f even th e weathere d roc k migh t range from th e smalles t stat e (colloidal ) t o th e larges t possibl e (boulders) . Thi s implie s tha t al l th e weathered constituent s of a parent rock cannot be termed soil . According t o their grain size , soil
particles are classified as cobbles, gravel, sand, silt and clay. Grains having diameters i n the range of 4.75 t o 76.2 mm are called gravel . If the grains are visible t o the naked eye , bu t are less than about 4.75 m m in size the soil is described a s sand. The lower limit of visibility of grains for the naked eyes is about 0.075 mm. Soil grains ranging from 0.075 to 0.002 mm are termed a s silt and those tha t are finer tha n 0.002 mm as clay. This classificatio n is purely based o n size which does not indicat e the properties o f fine grained materials .
Residual an d Transporte d Soil s
On the basis o f origin o f their constituents, soils can be divided into two larg e groups : 1. Residua l soils , an d
2. Transporte d soils .
Residual soils ar e thos e tha t remai n a t th e plac e o f thei r formatio n a s a resul t o f th e weathering o f parent rocks. The depth o f residual soil s depend s primaril y o n climatic condition s and th e tim e o f exposure . I n som e areas , thi s dept h migh t be considerable . I n temperat e zone s residual soils are commonly stif f an d stable. An important characteristic o f residual soil is that the sizes of grains are indefinite. Fo r example, whe n a residual sample i s sieved, th e amount passin g any given sieve siz e depends greatl y on the time and energy expended i n shaking, because o f the partially disintegrated condition.
Transported soils ar e soil s tha t ar e foun d a t location s fa r remove d fro m thei r plac e o f formation. The transporting agencies of such soils are glaciers, wind and water. The soils are named according t o th e mod e o f transportation . Alluvial soil s ar e thos e tha t hav e bee n transporte d b y running water. The soils that have been deposited i n quiet lakes, are lacustrine soils . Marine soils are thos e deposite d i n se a water. The soil s transporte d an d deposited b y win d are aeolian soils . Those deposite d primaril y through the action of gravitational force, a s in land slides, ar e colluvial soils. Glacial soil s ar e those deposited b y glaciers. Man y of these transported soil s ar e loose and soft t o a depth of several hundre d feet. Therefore, difficultie s wit h foundations and other types of construction are generally associated wit h transported soils .
Organic an d Inorgani c Soil s
Soils i n general ar e furthe r classifie d a s organic o r inorganic. Soil s of organic origi n ar e chiefly formed eithe r by growt h and subsequent decay o f plants such as peat, o r by th e accumulatio n of fragments o f the inorganic skeletons or shells of organisms. Hence a soil of organic origi n ca n be either organic or inorganic. The term organic soil ordinarily refers to a transported soil consisting of the products of rock weathering with a more or less conspicuous admixture of decayed vegetabl e matter.
Names o f Som e Soil s tha t ar e Generall y Use d i n Practic e
Bentonite i s a cla y forme d b y th e decompositio n o f volcani c as h wit h a hig h conten t o f
montmorillonite. It exhibits the properties o f clay to an extreme degree .
Varved Clay s consist of thin alternating layers of silt and fat clays of glacial origin. They posses s
the undesirable properties o f both silt and clay. The constituents of varved clays were transporte d into fresh wate r lakes b y the melted ice at the close o f the ice age .
Kaolin, Chin a Cla y are very pure forms of white clay used i n the ceramic industry.
Boulder Cla y i s a mixtur e o f a n unstratifie d sedimente d deposi t o f glacia l clay , containin g
unsorted rock fragments of all sizes ranging from boulders, cobbles, an d gravel to finely pulverize d clay material.
Soil Formation and Characterization 9
Calcareous Soi l i s a soil containing calcium carbonate. Such soil effervesces when tested wit h
weak hydrochloric acid.
Marl consists of a mixture of calcareous sands, clays, or loam.
Hardpan is a relatively hard, densely cemented soil layer, like rock which does not soften when
wet. Boulder clays or glacial till is also sometimes named as hardpan.
Caliche is an admixture of clay, sand, and gravel cemented by calcium carbonate deposited fro m
ground water.
Peat i s a fibrou s aggregat e o f fine r fragment s o f decaye d vegetabl e matter . Pea t i s ver y
compressible an d one should be cautious when using it for supporting foundations of structures.
Loam is a mixture of sand, silt and clay.
Loess is a fine-grained, air-borne deposit characterized by a very uniform grain size, and high void
ratio. The size of particles ranges between about 0.01 to 0.05 mm. The soil can stand deep vertical cuts because of slight cementation between particles. It is formed in dry continental regions and its color is yellowish light brown.
Shale is a material in the state of transition from clay to slate. Shale itself is sometimes considered
a rock but, when it is exposed to the air or has a chance to take in water it may rapidly decompose .
2.5 SOI L PARTICL E SIZ E AND SHAP E
The size of particles a s explained earlier, ma y range from grave l to the finest siz e possible. Their characteristics vary with the size. Soil particles coarser than 0.075 mm are visible to the naked eye or may be examined by mean s of a hand lens. They constitut e the coarser fraction s of the soils . Grains finer tha n 0.075 mm constitute the finer fraction s o f soils. It is possible to distinguish the grains lying between 0.075 mm and 2 \JL (1 [i = 1 micron = 0.001 mm ) under a microscope. Grain s having a size between 2 ji and 0.1 JLA can be observed under a microscope bu t their shapes cannot be made out . Th e shap e o f grain s smalle r tha n 1 ja ca n b e determine d b y mean s o f a n electro n microscope. The molecular structure of particles can be investigated by means of X-ray analysis.
The coarser fractions of soils consist of gravel and sand. The individual particles o f gravel, which ar e nothin g but fragments of rock, ar e composed o f one or more minerals , wherea s sand grains contain mostly one mineral which is quartz. The individual grains of gravel and sand may be angular, subangular , sub-rounded , rounde d o r well-rounde d a s show n i n Fig . 2.1. Grave l ma y contain grains which may be flat. Som e sands contain a fairly high percentage of mica flakes that give them the property of elasticity.
Silt and clay constitute the finer fractions of the soil. Any one grain of this fraction generally consists of only one mineral. The particles may be angular, flake-shaped or sometimes needle-like . Table 2. 2 give s th e particl e siz e classificatio n system s a s adopte d b y som e o f th e organizations i n th e USA . Th e Unifie d Soi l Classificatio n Syste m i s no w almos t universally accepted and has been adopted by the American Society for Testing and Materials (ASTM).
Specific Surfac e
Soil is essentially a paniculate system, that is, a system in which the particles are in a fine state of subdivision or dispersion. In soils, the dispersed or the solid phase predominates and th e dispersion medium, soil water, only helps to fill the pores between the solid particles. The significance of the concept of dispersion becomes mor e apparent when the relationship of surface to particle siz e is considered. In the case of silt, sand and larger size particles th e ratio of the area of surface of the particles to the volume of the sample is relatively small. This ratio becomes increasingl y large as
Angular Subangular Subrounded
Rounded Wel l rounded
Figure 2. 1 Shape s of coarse r fractions o f soil s
size decreases from 2 \JL which is the upper limit for clay-sized particles . A useful index o f relative importance o f surface effects i s the specific surface o f grain. The specific surface is defined as the total are a o f th e surfac e o f th e grain s expresse d i n squar e centimeter s pe r gra m o r pe r cubi c centimeter of the dispersed phase.
The shap e o f the clay particles is an important property fro m a physical poin t of view. The amount o f surfac e pe r uni t mas s o r volume varies with the shap e of the particles. Moreover , th e amount of contact area per unit surface changes with shape. It is a fact that a sphere has the smallest surface are a pe r uni t volum e wherea s a plat e exhibit s th e maximum . Ostwal d (1919 ) ha s emphasized the importance of shape in determining the specific surface of colloidal systems. Since disc-shaped particle s ca n b e brough t more i n intimat e contact wit h each other , thi s shape ha s a pronounced effect upo n the mechanical properties of the system. The interparticle forces between the surfaces of particles have a significant effect o n the properties of the soil mass if the particles in the media belong to the clay fraction. The surface activity depends not only on the specific surface but also on the chemical and mineralogical composition of the solid particles. Since clay particles
Table 2. 2 Particl e siz e classification b y various system s
Name o f the organizatio n Massachusetts Institut e of Technology (MIT )
US Department o f Agriculture (USDA) American Associatio n of State Highway an d Transportatio n Officials (AASHTO )
Unified Soi l Classificatio n System , US Bureau o f Reclamation, US Army Corps of Engineers an d American Society fo r Testing an d Material s
Particle size (mm)
Gravel San d Sil t Cla y
> 2 2 to 0.06 0.0 6 t o 0.002 < > 2 2 to 0.05 0.0 5 t o 0.002 < 76.2 t o 2 2 to 0.075 0.07 5 to 0.002 <
76.2 t o 4.75 4.7 5 t o 0.075 Fine s (silt s and clays) < 0.075
0.002 0.002 0.002
Soil Formatio n and Characterization 1 1 are th e activ e portion s o f a soi l becaus e o f thei r hig h specifi c surfac e an d thei r chemica l constitution, a discussion on the chemical composition and structure of minerals is essential.
2.6 COMPOSITIO N O F CLAY MINERAL S
The word 'clay' is generally understood to refer to a material composed of a mass of small mineral particles which, in association wit h certain quantities of water, exhibits the property of plasticity. According to the clay mineral concept, clay materials are essentially composed o f extremely small crystalline particle s o f on e o r mor e member s o f a smal l grou p of mineral s tha t ar e commonly known a s cla y minerals . Thes e mineral s ar e essentiall y hydrou s aluminu m silicates , wit h magnesium o r iro n replacing wholl y or in part for th e aluminum , in som e minerals . Many clay materials may contain organic material and water-soluble salts. Organi c materials occur either as discrete particles o f wood, leaf matter , spores, etc. , or they may be present a s organic molecule s adsorbed on the surface of the clay mineral particles. The water-soluble salts that are present in clay materials must have been entrapped in the clay at the time of accumulation or may have developed subsequently as a consequence of ground water movement and weathering or alteration processes. Clays can be divided into three general groups on the basis of their crystalline arrangement and i t i s observe d tha t roughl y simila r engineerin g properties ar e connecte d wit h al l th e cla y minerals belonging to the same group. An initial study of the crystal structure of clay minerals leads to a better understanding of the behavior of clays under different condition s of loading. Table 2.3 gives the groups of minerals and some of the important minerals under each group.
2.7 STRUCTUR E O F CLAY MINERAL S
Clay mineral s ar e essentiall y crystallin e i n natur e thoug h som e cla y mineral s d o contai n material which is non-crystalline (for example allophane). Two fundamental building blocks are involved in the formation of clay mineral structures. They are :
1. Tetrahedra l unit. 2. Octahedra l unit.
The tetrahedral unit consists o f four oxygen atoms (or hydroxyls, if needed t o balance the structure) placed a t the apices of a tetrahedron enclosing a silicon atom which combines together t o form a shell-like structur e with all the tips pointin g in the sam e direction . Th e oxygen at the bases of all the units lie in a common plane .
Each of the oxygen ions at the base is common to two units. The arrangement is shown in Fig. 2.2. The oxygen atoms are negatively charged with two negative charges each and the silicon with four positiv e charges. Eac h of the three oxygen ions at the base shares it s charges wit h the
Table 2.3 Cla y mineral s
Name of minera l Structura l formula
I. Kaoli n group
1. Kaolinite Al 4Si4O10(OH)g
2. Halloysite Al 4Si4O6(OH)16
II. Montmorillonit e grou p
Montmorillonite Al 4Si8O20(OH)4nH2O
III. Illit e group
Illite K y(Al4Fe2.Mg4.Mg6)Si8_y