AN INTRODUCTION·.TO .·
GEOTECHNICAL ENGINE.ERING
Second Edition
Robert D. Holtz, Ph.D., P.E., D.GE
University of Washingt()n
William D. Kovacs, Ph.D., P.E., D.GE
. University of Rhode Island ..
·
Thomas C.
Sheahan~
Sc.D.,' P.E.
Northeastern University ·
·
PEARSON
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Library of Congress Cataloging-in-Publication Data Holtz, R. D.. (Robert D.)
An introduction to geotechnical engineering I Robert D. Holtz, William D. Kovacs, Thomas C. Sheahan.-- 2nd ed. p.cm.
Includes bibliographical references and index. ISBN 978-0-13-031721-6 (alk. paper)
1. Soil mechanics--Textbooks. 2. Rock mechanics--Textbooks. I. Kovacs, William D. II. Sheahan, Thomas C. III. Title. TA710.H564 2011 624.1'51--dc22 2010028254 Prentice Hall is an imprint of
PEARSON
· www.pearsonhighered.com 10 9 " 8 7 6 5 4 3 2 1 ·rsB~-13: 978-d-13-249634-6 ISBN~10: 0-13-249634-8(p
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Contents
Prefaceviii
Chapter 1 1.1 1.2 1.3 1.4 1.5 1.6 1.7 Chapter 2 2.1 2.2 2.3 2.4 2.5 2.6 2.72.8
2.9
2.10 Chapter 3 3.1 3.2 3.3 3.4Introd~ction to:<Jeotec~nicalEngineering.
1 Geotechnical Engineering. 1The Unique Nature of Soil and Rock Materials 3 Scope of this Book · · '4 : 1
: • ' : ''; 1 ' • .• · • •
Historical Development of Geotechnical' Engineering... ,
5 .
Suggested Approach to the Study of Geotechnical Engineering .Notes on Symbols and Units 6, ·· .·.
Some Comments on.How to Study in GeneraL ..
.7; ..
Problems 8 · · · · · · · · · .
Index and Classification Properties of Soils .· ·· · 9. • • " ' • . . ~ ' l • . ' •• ,' . ' ' ' • ' ~ • • ':. ' ; ' • ' ' -•
Introduction • '· 9 :l: ·;
Basic Definiti~ns and Phase Relations for Soils · ·10''
S~lution of Ph~se Problems 15
SOil Texture 31-' :·;'
Grain Size and Grain Size Distribution .. · 32 Particle Shape 39
Atterberg Limits
@) · :
Introduction to Soil Classification .47
Unified Soil Classification System(USCS). 48·.· AASHTO Soil Classification System·. 61 Problems · 61 ' · · .· · · · · ·. · ·
. . ' Geology, Landforms, and the Origin of Geom.aterials , Importance of Geology to Geotechnical Engineering · · 69 The Earth; Minerals, Rocks, and Rock Structure 71 · · , Geologic Processes and Landforms 76
Sources of Geologic Information 117
Problems 119
6
69
iv Contents
Chapter 4
Clay Minerals, Soil and Rock Structures, ·
and Rock Classification
122
4.1 Introduction 122
4.2 Products of Weathering 122 4.3 Clay Minerals 123
4.4 Identification of Clay Minerals and Activity 131 4.5 Specific Surface 133
4.6 . Interaction Between Water and Clay Minerals 134 4.7 Interaction of Clay Particles 138
4.8 Soil Structure and Fabric of Fine-Grained Soils ·
139
4.9 Granular Soil Fabrics 1474.10 Soil Profiles, Soil Horizons, and Soil Taxonomy 150. 4.11 Special Soil Deposits 151
4.12 Transitional Materials: Hard Soils Versus Soft Rocks 152
4.13 Propertie~, Macrostnicture, a~d Classification of Rock Masses 154
Problems 161
Chapter 5
Compaction and Stabilization of Soils • 163
5.1 Introduction · 163 '. 5.2 5.3 5.4 5.5 5.6 5.7 5.8
Compaction and Densification ·. ' 164 .
Theory of Compaction for Fine~Grairied Soils · · 164 · Structure. of Compacted Fine~GraineciSoils · · 172 Compaction of Granular Soils 173
Field Compaction Equipment and Procedures 178 Specifications and Compaction Control' · '190 ··' Estimating Performance of Compacted Soils .. • 206 ·
Problems 210 .... :.
Chapter 6
Hydrostatic Water in Soils and Rocks
214,
6.1 Introduction 214 6.2 Capillarity 215
6.3 Groundwater Table and the Vadose Zone .· · 227 6.4 Shrinkage Phenomena in Soils 230. · · · · 6.5 Expansive Soils and Rocks· · · · 236 ' . 6.6 Engineering Significance'of Shrinkage and Swelling 6.7 Collapsible Soils and Subsidence 246
6.8 . Frost Action 249 · .
6.9 ' Intergranular 'or Effective Stress 257 I ; ~ 6.10 · Vertical Stress Profiles 262
244
6.11 RelationshipBetween Horizontal and Vertical Stresses , · · 266
/
Contents v
Chapter· 7
Fluid Flow in Soils and Rock. · · 272
7.1 Introduction 2727.2 Fundamentals ofFluidHow 273
7.3 Darcy's Law for Flow Through Porous Media· · .. 275
7.4 Measurement of Permeability or Hydraulic Conductivity 277 7.5 · , Heads and One-Dimensional Flow · 285 ·. · •·. · 7.6 Seepage Forces, Quicksand, and Liquefaction '294 7.7 Seepage and Flow Nets: Two-Dimensional Flow 306 · ·
7.8 Seepage Toward Wells. 321 .
7.9 'seepage Through Dams and Embankments · 325 7.10 Control of Seepage and Filters 327 ·
Problems · 338 ,
Chapter 8
Compressibility of Soil
an~Rock
345
8.1 Introduction 345• 8.2 Components of Settlement 347 8.3 · Compressibility of Soils . 347 ·
8.4 One-Dimensional Consolidation Testing 350 8.5 Preconsolidation Pressure and Stress History 352
8.6 · Consolidation Behavior
of
Natural and Compacted Soils · 357 8.7 Settlement Calculations 3648.8 Tangent Modulus Method .· 377 " , . , ..
8.9 Factors Affecting the Determination of a~
'·
380. · 8.10 Prediction of Field Consolidation Curves · · 380 8.11 ' • Soil Profiles . 388 • . · ' . · · ,'8.12
Approximat~
Methodsa~d Typi~al
Values ofComp~ession
Indices 8.13 Compressibility of Rock andTransitionalMaterials , . 395 8.14 Burland's Intrinsic Compressibility Properties 395 8.15 In,Situ Determination.of,Compr~ssibil~W.·. ·· .. 398 · ... ·.·Problems 399 ·
Chapter 9 ·
Time R~ti
of Corisolidatib~
· :.·, 404
9.1 ' Introduction . · 4049.2 The Consolidation Process 405
9.3 Terzaghi's One-Dimensional Consolidation Theory · 407 9.4 Determination of the Coefficient of Consolida'tion Cv. •• 427 :
9.5 , Determination of the Coefficient of Permeability ·. • · 432 9.6 Typical Values of the CoeffiCient of Consolidation c~ 433 .· 9.7 In Situ Determination of Consolidation Properties -434 9.8 Evaluation of Secondary Settlement · •• • 435 · · '
Problems 442
vi Contents
Chapter 10
Stress Distribution and Settlement Analysis, •
450 . ·
10.1 Introduction 450 ,, •
10.2 Settlement Analysis of Shallow Foundations .· · • 451 10.3 Stress Distribution .. 454 • :·: , .; . 10.4 Immediate Settlement : 472;: .. • ..
10.5 Vertical Effective Overburden and Preconsolidation Stress Profiles . . . 477 10.6 Settlement Analysis Examples i. :· 479,: , .
Problems 492 ·
Chapter 11
The.Mohr Cirde, Failure
Theories;'~~cl Str~engthTesting
of Soil and Rocks
497:
l, · ·,,11.1 Introduction 497
11.2 Stress at a Point · 498 . . . · · .· ..
11.3 Stress-Strain Relationships·and Failure Criteria : · 507 · 11.4 The Mohr-Coulomb Failure Criterion · 508
11.5 Laboratory Tesfs for the Shear Strength of Soils and Rocks · 516 11.6 In Situ Tests for the Shear Strength of Soils and Rocks . 526
Problems. . . 536 : ,, ·. : :: ·. ·
.-.,
~ /'- ~ ' " ' ' ' '
Chapter 12
An Introduction to ShearStrengthof Soils and Rock.
540
12.1 Introduction 540 :.: ' · :
12.2 Angle of Repose of Sands • 542 · . .
12.3 Behavior of Saturated Sands During Drained Shear 543
12.4 Effect of Void Ratio and Confining Pressure
on
Volume Chan.ge 545 12.5 . Factors that Affect the Shear Strength ofSands ·. . 553 . . . • 12.6 Shear Strengthof SandsU sing In SituTests : .· 1.558 ··. . .12.7 The CoeffiCient of Earth Pressure at'Restfor Sands' : ,· 560 12.8 BehaviorofSatunited'coh.'esive Soils Du'rin'g 'shear·· ··•·.'563 12.9 Consolidated-Drained Stress~ Deformation .and Strength ·
Characteristics 564 · · · ' ; '
12.10 Consolidated-Undrained Stress-Deformation,and Strength .. Characteristics ·' 570 ·• '· • · ' '' · · ::• · 12.11 Unconsolidated-Undrained Stress-Deformation and Strength;, .
Characteristics 578 . ·. · , , 12.12 Sensitivity • 591 .
12.13 ·.The Coefficient of Earth Pressure at Rest for Clays.; .. , 592 12.14 Strength of Compacted Clays . · 596 ·
12.15 . Strength of Rocks and Transitional Materials . . 600 12.16 Multistage Testing 601, .. , <
12.17 Introduction to Pore Pressure Parameters . 606 • . Problems 610
Contents vii Chapter 13
Advanced Topics in Shear Strength of Soils and Rocks
614
13.1 Introduction 614 13.2 Stress Paths 616
13.3 Pore Pressure Parameters for Different Stress Paths 627 13.4 Stress Paths During Undrained Loading_:Normally and Lightly
Overconsolidated Clays · 629' · ·
13.5 Stress Paths During Undrained Loading-Heavily
Overconsolidated Clays 644 · ·
13.6 Applications of Stress Paths to Engineering Practice 647 13.7 Critical State Soil Mechanics · 652 ·
13.8 Modulus and Constitutive Models for Soils 663
13.9 Fundamental Basis of the Drained Strength of Sands 675 13.10 Behavior of Saturated Sands in Undrained Shear 682 13.11 Plane Strain Behavior of Sands 696
13.12 Residual Strength of Soils 702 · · ·
13.13 · Stress-Deformation and Shear Strength of Clays: Special Topies · 705
13.14 · Strength of Unsaturated Soils 731 ·
13.15 Properties of Soils Under Dynamic Loading 737 · 13.16 Failure Theories for Rock 750
Problems 754
Appendix A ·
Application of the SI System of Units
to Geotechnical Engineering
765
.Appendix 8.1
Derivation ofLaplace's Equation
778
Appendix 8.2
Derivation arid Solution of Terzaghi's .
One-Dimensional Corisolidatiori TheorY
· 780
· Appendix 8.3 · ·
Pore Pressure Parameters
786 ·
Appendix( .· ·
The .Method of
Fragme~ts
795 ·
References
806
•. .
viii
Preface
, • ' t _ ,
It has been thirty years since the publication of the first edition of An Introduction to Geotechnical Engineering. During those years, the practice of geotechnical engineering has greatly changed, but the fundamentals of soil mechanics and soil properties have remained essentially the same. Engineering ·education also has changed during that time, mostly for the better. On the other hand, reduced gradu~ ation requirements and the increased use of computations instead of the laboratory experience have often resulted in a tendency toward reduced rigor arid over-simplification of some undergraduate
edu-cation and textbooks. · · ·
. We still believe that there is.· a need for more detailed. and moder~· coverage. of the engineering properties of geo-materials than is found in most undergraduate texts. This applies to students who concentrate in geotechnical engineering as well as the general civil engineering undergraduate student. Our students will be involved in increasingly more complex projects, esp_ecially those in transportation, structural, construction and 'environmental engineering.' Those projects will increasingly involve envi-ronmental, economic and political constraints that will demand innovative solutions to civil engineer-ing problems. Modern analytical techniques usengineer-ing digital computers have had a revolutionary effect on engineering design practice. However, the validity of the results from these computational procedures (which typically include 'striking graphical output) is highly dependent on the quality of the geotechni-. cal engineering design parameters as well as the geology and site conditionsgeotechni-. geotechni-. geotechni-. geotechni-. geotechni-.
Like the first edition, this ~dition is intended for use in the first of a two-~ourse sequence in geotech-nical engineering usuallytinight
to
third~ and fourth~yea~ undergraduate civil engineering students. We · assume the students have a working knowledge of undergraduate mechanics, especially statics and mechanics of materials, including fluids. In the first course we introduce the ''language" of geotechnical engineering-that is, the classification and engineering properties of soils and rocks. Once the student has a working knowledge of the behavior'of geo-materials, he/she can begin to 'predict soil behavior and, in the second course, carry out the design of simple foundations and earth structures.We have tried to make the text easily readable by the average undergraduate. To this end, An Introduction to Geotechnical Engineering is written at a rather elementary level, although the material covered may at times be quite sophisticated and complex. Involved derivations are relegated to appen-dices, where they are available to the interested student.
·The emphasis throughout is on the praCtical, and admittedly empirical, knowledge of soil and rock behavior required by geotechnical engineers for the design and. construction of foundations, embankments, and underground structures. Most of the material in the text is descriptive, since most of the engineering design applications are usually left to the second course in foundation engineering. Consequently, in order to strengthen this connection between the fundamental and applied, we have tried to indicate wherever possible the engineering significance of the property being discussed, why the property is needed, how it is determined or measured, and, to some extent, how it is actually used in spe-cific design applications. We illustrate some simple geotechnical designs-for example, determining the
;.;
Preface ix
··flow, uplift pressures;and exit gradients in•2-D seepage problems, and estimating the settlement of
shallow foundations on sands and saturated clays. . ' ' .
· One thing that has not changed in thirty years is tliat units remain a problem with U.S. geotech-nical'engineers. In line with the rest· of the world, the 'American Society of Civil Engineers, and the American Society for Testing and Materials, we have used the S.l. System of Units in the text. Most stu-, dents are conversant in both the U.S. customary (or British) engineering units and S.l.stu-, but readers
unfamiliar with S.l. may find Appendix A helpful. We have examples and problems in both systems, and we have been careful to use the correct definition of density (mass/unit volume) in phase relation-ships as well as in geostatic and hydrostatic pressure computations. ·
We consider the laboratory component of the first course to be an essential part of the neophyte . engineer's experience with soils as a unique engineering material. How else is the young engineer to begin :' to develop a "feel" for soils and soil behavior, so essential for the successful practice of geotechnical
engi-.. neering? An emphasis on laboratory and field testing is found throughout the text. The organization and development of the material iri the text is traditional and generally follows the order of the laboratory por-tion of our first courses. The early chapters introduce the discipline of geotechnical engineering, phase rela-tionships, index and classification properties of soils and rocks, geology, landforms, and the origin of geo-materials, clay minerals, soil and rock structures, and rock classification. Chapter 3, "Geology, Land~ forms, and the Origin of Geo-Materials," has been added to this edition because these topics are so critical ·. to understanding the properties and subsequent behavior of geo~materials under. various loading condi-.. tions. These chapters provide the background and terminology for the remainder of the textcondi-..
· .. ·Following a very practical discussion of compaction in Chapter 5, Chapters 6 and 7 describe how water influences. and affects soil behavior. Topics presented in Chapter 6 include groundwater and vadose water, capillarity, shrinkage, swelling, and. collapsing soils; frost· action, and effective stress. Chapter 7 discusses permeability, seepage, and seepage control.· , . · · .
. The last six chapters deal with the compressibility and shear strength of soils and rocks. The treatment of these topics is quite modern and has been updated considerably. We now have stress dis-tribution and settlement analyses, including immediate settlement, in. a: new ChapterlO to separate these practical procedures from the more basic time-rate and compressibility behavior of natural and · , , compacted soils and rock,masses described in Chapters 8 and 9. In these latter chapters we have . ,included new material on Janbu's tangent modulus method, in situ determination of compressibility of soil and rock, Burland's intrinsic compressibility of soils, and finite difference solution to the Terzaghi . consolidation equation. We have extended. the. Schmertmann method for prediction of field
compres-sion curves to overconsolidated soils, and we have updated Mesri's work on secondary comprescompres-sion. We received much criticism about the length of Chapter 11 on shear strength in the first edition, so now shear strength properties of soils and rocks are discussed in three new chapters. New Chapter 11 . on the Mohr circle, failure theories; and strength testing of soil and rocks has new material on the
obliq-uity relations and in situ tests for shear strength. Chapter 12 is an introduction to shear strength of soils and rock and is primarily suitable for undergraduate students. More advanced topics in shear strength of soils and rocks are discussed in Chapter 13, which graduate students and practicing geotechnical engi-.neers should find useful. New material in Chapter·12jncludes multistage testing, in situ tests for the . shear strength of sands and the strength of compacted clays, rocks, and transitional materials. We now .. have the stress-path method in Chapter 13, which also includes sections on critical-state soil mechanics and an introduction to constitutive models. We then discuss some advanced topics on the shear strength : .· of sands that start with the fundamental basis of.their drained, undrained, and plane-strain. strengths . • . The residual shear strength of sands and clays provides a transition into the stress-deformation and . 'shear strength of clays, where we discuss failure definitions, Hvorslev strength parameters, stress history, :the·· Jurgenson-Rutledge hypothesis, consolidation methods.· to overcome sample . disturbance, anisotropy, plane-strain strength, and strain-rate effects. We .. end. Chapter 13 with sections on the . strength of unsaturated soils, properties of soils up.der dynamic loading, and failure theories for rock.
x Preface
Even though it is primarily for the beginning student in geotechnical engineering, advanced stu-dents in other disciplines and engineers desiring a refresher in engineering properties may find the book helpfuL Because of the many fully worked example problems, the book is almost "self-teaching." This aspect of the text also potentially frees the instructor in a formal· course from the necessity of working example problems during lectures. It allows the instructor to concentrate on explaining basic principles and illustrating specific engineering applications of the points in question. From the first edi-tion, we know that many practicing geotechnical engineers will find this book useful as a refresher and for the typical v~lues given for classification and engineering properties for a wide variety of soils; we have found such a compendium very useful in our own engineering practice. ·
The solutions manual and test manual as well as PowerPoint figures of all images and tables from this book can be downloaded electronically from our Instructor's Resource Center located at www.pearsonhighered.com. The material available through the Instructor Resource Center is provided solely for the use of instructors in teaching their courses and assessing student learning. All requests for instructor access are verified against our customer database and/or through contacting the requestor's institution. Contact your local sales representative for additional assistance or support.
ACKNOWLEDGMENTS
To acknowledge all who have contributed to this book is a formidable task. We have tried whenever possible to indicate by references or quotations, concepts and ideas originating in the literature or with our former teachers, especially Profs. B. B. Broms, A. Casagrande, R. J. Krizek, C. C. Ladd, J. K. Mitchell, J. 0. Osterberg, and H. B. Seed. Others have made helpful suggestions or reviewed portions of the text, resulting in improvements to the final pr'oduct. These include Roy Borden, David Chang, Herbert Einstein, Milt Harr, Vic Kaliakin, Jerry Leonards, Bill Likos, Harry Stewart, Dayakar Penumadu, Siva Sivakugan, and Tom Zimmie, and countless others who have made comments about the first edition over the years. The comments of Dick Galster; Teresa Taylor, and Hank Waldron significantly improved · early drafts of Chapter 3. .. · · ·
· We'are grateful to our Production Project Manager, Clare Romeo, for the patience, diligence and humor she exhibited in the face of many challenges, imd for her help in ensuring the quality of the fmal product. We also acknowledge those who assisted in the development of the 2nd edition through contribu-tions of figures, reports, and administrative assistance. Figures and other resources were graciously supplied by John Burland, Don DeGroot, and Paul Mayne, among others·from the 1st edition. At·Northeastern, ·Joan Omoruyi, Ed Stevens, and Brett McKiernan provided research and administrative assistance.
Thank you to the reviewers of this edition: Kamal Tawfiq, Florida State University; Monica Prezzi, Purdue University; Jay DeNatale; California Polytechnic State University; Robert Mokwa, Montana State University; Balasingam Muhunthan, Washington State University; Trevor Smith, Port-land State University; Tom Zimmie; Rensselaer Polytechnic Institute; Scott Ashford, University of California:...san·Diego; Robert D'Andrea, Worcester Polytechnic Institute; Samuel Clemence, Syracuse University; Dave Elton, Auburn University; and Khaled Sob han, Florida Atlantic University.
· On a personal note, we wish to thank our respective spouses, ·cricket, Eileen and Maryrose, who endured from a few to several years of delays, sporadic periods of stress, and many evenings and week-ends that should have been spent with their husbands instead of sharing their marriages with this book.
R.D.HOLTZ
SEATTLE, WASHINGTON W.D.KOVACS KINGSTON, RHODE ISLAND
T.
C. SHEAHAN BOSTON, MASSACHUSETTS1.1 ' I ' ) ' • ., '
·C · H A · P ·. T · E R
1 ·.
• I ,. 'lntrod'uction to·.Geote:chnical
Enginee~ing.
, ' ! ; GEOTECHNICAl ENGINEERING ,. Geotechnical engineering is concerned with .the application of civil· engineering technology to some
aspect of the earth, usually the naturalmaterials found on.ornear the'earth's surface. Civil engineers call these materials soil and rock. Soil, in an engineering sense,·is.the:relativelyloose agglomerate of mineral and organic materials and sediments found above the .bedrock. Soils can be relatively easily broken down into their constituent mineral or organic particles. Rock, on the other hand, has very ;strong internal cohesive and molecular forces which hold its constituent mineral grains together. This is true for massive bedrock as well as for a piece of gravel found in a clay soil. The dividing line between . soil and rock is arbitrary, and many.natural materials encountered in engineering practice cannot be · ;easily classified. They may be either a "very soft rock",or a "very hard soil.'.'.. . .
. , . . Other scientific disciplines have different meanings .for the terms soil. and rock; In geology, for example, rock means all the materials found in the earth's crust, including what most of us would call soil. Soils to a geologist are just decomposed and disintegrated rocks found in the very thin upper part of the crust and usually capable of supporting plant life. Similarly, pedology (soil science) and agron-. '•, • omy are concerned with only tlie very uppermost layers of soil- that is, those agron-.materials important to
·.agriculture and forestry. Geotechnical engineers can learn much· from both. geology and pedology . . Geotechnical engineering has considerable overlap with.these fields, especially with engineering geol- .. ,
ogy and geological engineering. But beginning students should remember that these fields may have different terminology, approaches; and objectives than geotechnical engineering .• · .. ·
·Geotechnical engineering has several different aspects or emphases. Soil mechanics is concerned . i with the engineering mechanics and properties of soil, whereas rock mechanics is concerned with the : •. engineering mechanics and properties of rock---: usually, but not limited to, the bedrock. Soil mechanics applies to soils the basic principles of mechanics including kinematics, dynmpics, fluid mechanics, and the mechanics of materials. In other words, soil-rather than.water, steel, or concrete, for example-is the engineering material whose properties and behavior we niust understand in order to build with it or upon it. A similar statement could also be made for rock mechanics. However, because in significant ,ways soil masses behave differently from rock masses, in practice thereis not much overlap between
2 Chapter 1 Introduction to Geotechnical Engineering
the two disciplines. This divergence is unfortunate from the viewpoint of the practicing civil engineer. Inconveniently, the world does not consist only of soft or loose soils and hard rock, but rather, most geo-materials fall somewhere between those extremes. In your professional practice you will have to learn to deal with a wide range of material properties and behaviors. '
Foundation engineering applies engineering geology, soil mechanics, rock mechanics, and tural engineering to the design and construction of foundations for civil engineering and other struc-tures. The foundation engineer must be able to predict the performance or response of the foundation soil or rock to the loads the structure imposes. Examples include foundations for industrial, commer-cial, and residential buildingS, bridges, towers, ~nd retaining walls, as well as foundations for oil and other kinds of tanks and offshore structures~ Ships must have a drydock during construction or repairs, and the drydock must have. a foundation. During construction and launch, rockets and appurtenant structures must be safely supported. Related geotechnical engineering problems the foundation engi-neer faces are the stability of natural and excavated slopes, the stability of permanent and temporary earth-retaining structures, problems of construction, control of water movement and water pressures, and even the maintenance and rehabilitation of old buildings. Not only must the foundation safely sup-port static structural and construction loads, but it must also adequately resist dynamic loads due to wind, blasting, earthquakes, and the like.
If you think about it, we cannot design or construct any civil engineering structure, whether built on the earth or extraterrestrial, without ultimately considering the foundation soils and rocks. The per-formance, economy, and safety of any civil engineering structure ultimately are affected or even con-trolled by its foundation.
Earth materials are often used as a construction material because they are the cheapest possible building material. However, their engineering properties such as strength and compressibility are often naturally poor; and measures must be taken to densify, strengthen, or otherwise stabilize and reinforce soils so that they will perform satisfactorily. Highway and railway embankments, airfields, earth and rock dams, levees,· and aqueducts are· examples of earth structures, and the geotechnical engineer is responsible for their design and construction. Dam safety and rehabilitation of old dams are important aspects of this phase· of geotechnical engineering. A related consideration, especially for highway and airfield engineers, is the design of the surface layer on the earth' structure-the pavement. Here the · overlap between the transportation and geotechnical disciplines is apparent.
Rock engineering, analogous to foundation engineering for soils, is concerned with rock as a foundation and construction material. Because most of the earth's·surface is covered with soil (or water), rock engineering usually occurs underground (tunnels, underground power houses, petroleum storage rooms, mines; yours, and so on). But some rock engineering problems occur at the surface, such as in the case of building and dam foundations carried to bedrock, deep excavations to bedrock, stabil-ity of rock slopes, and the like.
In recent years, geotechnical engineers have become increasingly involved in the solution of environmental problems involving soil and rock. This developing interdisciplinary field is called geoenvironmental engineering or environmental geotechnics. Especially challenging are problems of polluted groundwater, proper disposal arid containment of municipal and industrial wastes, design and construction of nuclear waste repositories, and remediation of hazardous· waste repositories (aka dumps) and other contaminated sites. Although all these problems have a major geotechnical engi-neering component, they are interdisciplinary in nature, and their solutions require that geotechnical engineers work together with environmental and chemical engineers, environmental and public health . specialists, geohydrologists, and regulatory agency personnel. · · ·
In presenting some of the typical problems facing the geotechnical erigineer, we wanted you to see, first, how broad the field is and, second, how important it is to the design and construction of civil engineering structures, as well as to the basic health and safety of society. In a very real sense, geotech-. nical engineering combines the basic physical and mathematical sciences, geology, and pedology, with
1.2 The Unique Nature of Soil. and Rock Materials 3
~nviron~ental, hydraulic, structural, transportation, construction, and mining engineering. It truly is an . exciting and challenging field.
1.2 THE UNIQUE NATURE OF SOIL AND ROCK MATERIALS
. '
"
• , · · · . f , " - ;
We mentioned earlier that soil.,..-:from a civil engineering point of view-is the relatively loose agglom-eration of mineral and organic materials found above the bedrock. In a broader sense; of course, even shallow bedrock is of interest to geotechnical engineers, as illustrated by examples given above.
The nature and behavior. of soil and rock are discussed in greater detail throughout this text. For now, we want just to set the.stage for what you are about to study. We assum(! you understand that rock refers to any hard solid aggregate or mass of mineral matter found in the earth's crust. You also already have· a layperson's idea about soil. At lea~t you know in general ~hat sand and gravel are, and perhaps you even have an idea about fine-grained soils such as silts and days. These terms have quite precise engineering definitions, as we shall later see, but for now the general concept that soils are particles will
: suffice.' · · · · ·
Soils are particles of ~hat? Well; usually particles of mineral niatt~r or, more simply, broken-up · · pieces of rock that result from weathering and other geologic processes ( desciibed in Chapter 3) acting on massive rock deposits and layers. If we talk for the moniellt about the 'size of the particles, gravels are small pieces ofrock and typically contain several minerals, whereas sands are even smaller pieces, ·· and each grain usually consists of only a single mineraL If you cannot se~ each individual grain of a soil, .:thenthe soil is either a silt or a clay or a'mixture of each. In fact, natural soils generally are a mixture of several different particle sizes and may even contain organic' matter. Some soils, such as peat, may be almost entirely organic: Furthermore; bee~ use soils are a particulate material, they have voids, and the ·voids _are usiuilly filled with water and air; The physical and chemical interaction of the water and air in the voids with the partiCles of soil, as well as the interaction of the 'particles themselves,'makes soil's behavior complicated and leads to some of its unique propeities. . ' . · ·' · ' ·
. . Because' o(the nature of soil and rock materials an·d the' complexity of the geological environ-. illent, geotechnical engineering is highly empiricalenviron-. It is_ perhaps much more of an "art" than the other
disciplines within civil engineering:' Soils and rocks are often highly variable, even within a distance of . a few millimeters. Iri other words, soils and rocks are h~tero'geneous rather thanhomoge~ious
materi-als. That is, their material or engineering properties may vary widely from point to point within a soil or 'rock mass. Furthermore,· these materials in general-are nonlinear; their stress-strain curves are not · straight lines. To furthefcompliciitethirigs (as well asi:llake them interesting!); soils especially are non-- conservative materials: That is, they have' a fantastic meinoiynon-- they remember almost everything that · ever happened to thein; a~d this fact strongly affects their engineering behiwior. Instead of being ' isotropic; soils and rocks are typically anisotropic, which means that their material or engineering
prop-erties are not the same in all directions. ' . ' .. _ '
· Most of our theories about the mech~mi6al behavior of engineering materials assume that they are homogeneous and isotropic and. obey linear stress-strain laws. Common. engirieeriilg 'materials such as steet'_and concrete do notaeviate too significantly from these ideals, so' we can,use, with dis-cretion, simpldinear, theories to predictthe response of these materials_ to engineering loads. With soilsand rock, we are riot so fortunate. Weinay· assume a linear stress-stniin response, but then we ''must apply large empirical correction or ~·safety" factors to our designs to accoundor the real
mate-rials' behavior. Furthermore, the behayior of soil androck-·inaterials in.siti1 is often controlled by ·joints Gust dori't inhale), fractures, weak layers and zones, and other .','defects" in the material, which ' ' ) . ; our laboratory tests 'and simplified methods of analysis often do not. or are unable to take into account. That is why the practice of geotechnical engineeiing is more an "art" than a science. Success-ful practice depends on the good judgment and expedence of the designer, constructor, or consultant.
4 Chapter 1 Introduction to Geotechnical Engineering
1.3
. •
Put another way, the successful geotechnical engineer must develop a "feel" for soil and rock behav-ior before a safe and economic foundation or tunnel design can be made, 'an earth structure can be safely built, or an environmentally sound waste containment and disposal system or a site remedia-tion plan can be developed.
In summary, because of their nonlinear, nonconservative, and anisotropic mechanicaL behavior, plus the variability and heterogeneity of natural deposits due to' the capriciousness of nature, soils and rocks are indeedcomplex engineering and construction materials. Helping you find some order in this potential chaos is our primary objective in this book. ·
SCOPE OF THIS BOOK
In this intr~ductory text, rather than attempt an all-inclusive appr~ach to geotechnical engineering, we ' ·. put primary emphasis. ori th~ cla~sification and engi~eering belz.avior of soil and rock materials. The
rea-son is that successful practiCe of geotechnical engine~ring requires a thorough knowledge and under-standing of the engineering properties and behavior of soils and rocks in situ.,- that is, when they are ' subjected to engineering loads and environinent~l con.diiions. Therefore the beginning student must first develop an appreCiation. for the engineering properties of geo-materials as distinct from other common civil engineerhig materials b'efore 'learning how to analyze and design foundations,
earth-works, tunnels, imd the like. ; ' ' . .· ' . ' . '
Actually, this first pa~t isthe hard part. Most engineering students (and e_ngineers) are very good at analysis and performing design calculations. But these are worthless if an incorrect picture of the site geology has been assumedorthe:wrong.ellgineering properties assumed for the design. ·
. As much ofthe practice of geotechnical engineering depends on the site geology, landforms, and the nature of the soil and rock deposits at a site, we have included an 'optional Chapter 3 on geology and landforms--:'-just in case you haven;t had a basic courseiil geology. If you have had such a course, you can skip this chapter. If you haven't, you are strongly encouraged to take a physical geology or an 'engineering geology C()UrSe in ,connection With your studies of geotechnical engineering .
In the. early chapters; we introduce some. of the basic definitions, index properties, and
classifica-.. tion schemes for geo-matedals which are used throughout the book. Classification of soils and rocks is
important because it is the ''language" engineers use to communicate certain general knowledge about ' the engineering behavior of the materials at a particular site.' . ' ' · The rest of the book is concerned with the' engineering properties of soils and rocks-properties ' that are' necessary for the design offoundations, earth and underground structures, and
geoenviron-mentarsystems. We describe how water affects soil and rock behavior, including hydraulic-conductivity . and seepage characteristics. Then 'we get ~nto compressibility, the important engineering property we · need to understand in order to p~edict the settlement of structures constructed on soil and rock masses . . Finally, we describe some elementary strength characteristics of both soils and rocks. Strength is very important for the stability of, for exampleifoundations, retaining walls, slopes, tunnels, and waste
con-. • tainment systems: . . · · · · . · · · ·
. Keep in mind that this is an elementary text that emphasizes the fundamentals, but with an eye toward the practical applications that you as a civil engineer are likely to encounter. Having studied this text, you will be well prepared for follow~up courses in foundations and earthwork engineering, environmental geotechnics, ~ock' mechanics, and engineering geology. You should have a fairly good idea of what to look for at a site and how to obtain the soil and rock properties required for most designs. If you are able to accurately classify the materials, you will know the probable range of values for a given soil or rock property. Finally, we hope you will learn enough about soils and rocks to be aware of your own limitations, and to avoid costly and dangerous mistakes in those aspects of your pro-fessional career that involve soils and rocks as engineering materials.
1.4 .. Historical Development of Geotechnical Engineering 5
1.4. HISTORICAL DEVELOPMENT.OF GEOTECHNICAL EN(jiNEERING
As long as people have been building things, they have us~d.soils androcks as a foundation or con-struction material: The ancient Egyptians, Babylonians, Chinese, and Indians knew about constructing .. dikes and levees out of the soils found in river flood plains. Ancient temples and monuments built all around the world involved soil and rock in some way: The Azie~s constructed temples and cities on the very poor soils in the Valley of Mexico long before the Spaniards arrived in the so~called New World. European architects a~d builders during the Middle Ages learned about the problems 'cif ~ettlements of cathedrals and large buildlngs. The most noteworthy example is,' of course, the LeanihgTower of . Pisa. Vikings in Scandinavia used timber piles to support houses. and wharf stru~ttiies' on. their soft clays~ The "desig~" offoundations and otn~r constructions involving: soil and rock; was by rule of thumb, arid very little theory as such was developed until the mid~1700s.'·' ' .
Coulomb is the most famous 'engineering name of that era. He investigated the problems of earth pressures against retaining walls, al,ld some of his calculation procedures are still in use today. The most common theory for the shear strengtli ofsoils is"rianied after hini (Coulomb, 1776rDuring ilie next century, the French engineers Collin and Darcy and the Scotsman Rankine madeimportant dis-coveries. Collin (1846) was the first engineer to systematically examine failures in clay slopes as well as the measurement of the' shear strength of clays. Darcy (1856) established his law for the flow of water through sands. Rankine (1857) developed a method for estimating the earth pressure against retaining walls. In England, Gregory (1844) utilized horizontal sub drains and compacted eaith-fill buttresses to
stabilizerailroadcut slopes. · · · ·
. f By the turn of the century, important developments in the field' were occurring in Scandi-navia, primarily in Sweden.Atterberg (1911) defined consistency limits for days that are still in use today. During the period 1914-1922, in connection .with investigation~ of failures in harbors and railroads, the Geotechnical Commission of.the Swedish State Railways (Statens Jarnvagers Geot-ekniska Kommission, 1922) developed manyimport!mt concepts arid apparatuses in geotechnical engineering. They developed methods for calculating the stability of slopes as· well as subsurface investigation techniques such as weight sounding and piston and other types of siunplers. They •· understood important concepts such as sensitivity of clays and consolidation, whiCh is the squeezing of water out of the pores of the clay. Atthat time, clays were thought to be absolutely impervious, but the Swedes made field measurements to show they weren't. The Commission was the first to ·· use the word geotechnical (Swedish: geotekniska) in todaY's sense: the combination of geology and
civil engineering technology. · . . · . . · . ·
Even with these early developments in Sweden'; the true father of soil mechanics is an Austrian, . Prof. Karl Terzaghi. He pubiished the first modern textbook on soil mechanics in 1925, and in fact the name "soil· mechanics" is a translation of the German word Erdbaumechanik; which was part of the title of that book (Terzaghi, 1925a). Terzaghi was an outstanding and very creative' engineer. He wrote several other important books (for example, Terzaghi, 1943;Terzaghi and Peck, 1967; aridTerzaghi, Peck, and Mesri, 1996) and over 250 technical papers and articles. His mime will appear often in this book. He was a professor at Robert College in Istanbul, at TechnischeHochschule in Vienna, at M.I.T., and atHarvard University from 1938 until his retirement in 1956. He continued tcibeactive as a con~ sultant until his death in 1963 at. the age of 80. An excellent ref~rence about his life and e~gineering ,.careei-is thatof Goodman (1999) arid is well worthreading._ . · ·. • , · .. ··· ' ·. · ·, ,
, . . Another importantfigure is Prof. Arthur Casagrande,' who. was .at Harvard University from '.1932 'until1969. You will see his name often in this book, because he· made many import1mt
contri-. • buti~ns to the
aft
and schince of soil mechanics and fo~~dation· engineering. Since-the 1950~ the fieldhas grown substantially, and rmul.y peopl~ha~e beeri responsible for its rapid advancement. Impor-tant contributors to the field irid.ude Taylor, Peck, Tschebotarioff, Skempton, Bjerrum, Seed, Ladd,
and Leonards. · · · · ·. · · ·
6 Chapter 1 · Introduction to Geotechnical Engineering
Both Terzaghi and Casagrande began the teaching of soil mechanics and engineering geology in North America. Before the Second World War, the subject was offered only at a very few universities, mostly as a graduate course. After the war, it became commori for at least one course in the subject to be required in most civil engineering curricula. In recent years graduate programs in geotechnical engi-neering have been implemented at many universities: Finally, there has been a real information explo-sion in the number of conferences, technical jounials, and textbooks published on this subject during
the past three decades. .
Important recent developments you should know about i~clude soil dynamics and geotechnical earthquake engineering,. the use of computers. for . the solution. of complex engineering problems, deformation-based analyses and designs, the introduction of probability and statistics irito geotechni-cal engineering analysis and design: and geo-enviro. nmental engineering and technology.··
' ! , ' ' ' ' • ' •
1.5 SUGGESTED. APPROACH TO THE STUDY OF GEOTECHNICAL ENGINEERING
Because of the nature of soil and rock materials, both laboratory and field testing are very important in geotechnical engineering. Student engineers can begin to develop a feel for soil and rock behavior in the laboratory by performing the standard tests for classification and engineering properties on many different types of soils and rocks. In this way the novice can begin building up a "mental data bank" of how certain soils and rocks actually look; how they might:behave with varying amounts of water in them and under different types of engineering loads, and the range of probable numerical values for the different tests. This is sort of a self-calibration process, so that when you are faced with a new soil . deposit or.rock'type,,y(m will in advance have some.idea as to the engineering problems you. will encounter at that site; You can also begin to judge, at least qualitatively; the validity of laboratory and field test results for the materials at that site .
. Also important is a knowledge of geology. Geology is; of.course, the "geo" part of geotechnical engineering, and you should get as much exposure to it as you can during your academic career. After · •, a basic course in physical geology, coursesin.geomorphology and engineering geology are
recom-mended. Geomorphology is concerned with landforms, which are important to geotechnical engineers because the soils androcks at a site (and therefore the engineering problems) are strongly related to ·the particular landform. Engineering geology is concerned with the applications of geology to primar-, · ily civil engineering and has considerable interaction and overlap with geotechnical engineering.
The theoretical and analytical aspects of geotechnical engineering design also require a sound knowledge of engineering mechanics, including strength of materials and fluid mechanics. It also helps if you are familiar to some extent with basic structural analysis,reinforced concrete and steel design, hydraulic engineering and hydrology, surveying and engineering measurements, basic environmental . engineering, and civil engineering construction -in other words, just about all the courses in a typical
·undergraduate civil engineering curriculum. ·
1.6 . NOTES ON SYMBOLS AND UNITS
At the beginning of each chapter, we list the pertinent symbols introduced iri the chapter. As with most disciplines, a standard notation is not universal in geotechnical engineering, so we have tried to adopt the symbols most commonly· used. For ·example, the American Society for Testing and. Materials (ASTM, 2010) has a list of Standard Definitions oLTerms and Symbols Relating to Soil and Rock Mechanics, Designation D 653, which was' prepan!d jointly some years ago with the American Society
of
Civil Engineers (ASCE) and 'the International Society of Rock Mechanics (ISRM).·The Interna-. tional Society for Soil Mechanics and Foundation Engineering (ISSMFE, 1977) published an extensivelist of symbolS. Although we sometimes deviate from these recommendations because of our personal preference, we have generally tried to follow them.
1.7 Some Comments on how
to'
Study in Gener~l 7Units used in geotechnical engineering can be politely called a mess, and, less politely, several worse things. There has developed in practice, at least in the United States, a jumbled mixture of cgs-metric, Imperial or British Engineering units, arid hybrid Europe~m rrietricunits. With the introduction of the universal and consistent system ofunits;
"Le
Systeme Internation~l d'Unites',~ (SI) in the United Statesand Canada, the profession has a wonderful opportunity to bring some. coherence to units in geotechnical engineering practice. However, since British Engineering units are still rather commonly · used inthe United States, American students need to be familiar with the typicalyalues in both sets of ; units.To assist you with unit conversion where necessary, we have included a brief explanation of SI ··.:units as applied to geotechnical engineering in Appendix A.1.7
r
SOME COMMENTS ON HOW TO STUDY IN GENERALIt takes a while to learn how to study most effectively.' You are probabiy using the .study habits that you got by with in grade school and high school. As you progress professionally, things are going to get much harder, starting in your third year of university or college, when you take mostly preprofessional courses. We have all used the following methods to do homework assignments. (1) Just read the assign-ment to satisfy the moral obligation to do so. (2) Go further by underlining or highlighting passages to emphasize the main points. Consider what you are doing physically: the information goes through the eyes, down your neck and arm into the writing fingers, completely bypassing the brain! Both (1) and (2) are pretty much a waste of time unless you have a photographic memory. If we are really going to learn anything, most of us need to study a third way: (3) Read a few pages and then close the book. Write down in your own words what the main concepts are; a "bullet" format is OK. In order to do this, you must have the material in the brain to begin with. If youcan't write down anything about the pages you have just read, go back and read again, perhaps fewer pages this time. Repeat. Close the book and write in your own words the main points. Yes, this will take more time than "studying" using methods (1) and (2),but you will not be wasting your time.
A useful argument for doing it the recommended way is that you will have already started preparing for the exams, because'now you know the material. The rest of the time, you are brushing up
or reviewing the material, so you won't need to cram. · ·
. One big problem is that there may not be enough time in the.week to use method (3) when you are taking three or four other courses. However, follow it as much as you can. You have invested a lot in your education. Don't waste time with methods (1) and (2).
Don't ask us to tell you how long it took for us to learn the correct way to study (it's too ·embarrassing).
Our suggested approach will help you prepare for the Fundamentals of Engineering (FE or EIT) exam and later the PE or PEng (professional engineer's exam). We strongly encourage you to take (and pass) the FE exam before you graduate and receive your engineering degree~·
I'M HAVING TROUBLE GETTING STARTED WITH
HOMEWORK ..
WELL, SOMETIMES VOU JUST !-\AVE TO OPEN THE BOOK,
AND GO RIGHT AT IT ..
8 Chapter 1 Introduction to Geotechnical Engin.eering
PROBLEMS
·
I 1L1 Interview a faculty member (other than your instructor) or a practicing engineer in Geotechnical Engineer-ing. Ask him or her how they became involved with this specialty and what education is necessary these days to practice. (You will be surprised how much help you will receive, because we all like to talk about our "life's work"!) Ask about the importance of taking the FE examination and obtaining the PE (or P.Eng.) license and their influence on one's salary and promotion.
1.2 Get on 'the WWW and, using a search engine, type in the following letters: USUCGER. Report on the mean-ing of the letters; list the various links that you find in terms of subject matter or key words. Comment on the
number of cross links found. Finally, select a web page and explore it; prepare a short summary of your find-ings in grammatically correct sentences (the hardest part of the question!).
1.3 Contact the Board of Registration for Professional Engineers in your state or province and find out the requirements for becoming
a
registered professional engineer. Start pl!inning to take the FE examination when it is given in your area next year.', [· ' ~ ,_
• • _l
Index- an·d. Cl-assification
ProJl'erti·es of Soi Is •
" r 1 ·
2.1 '·INTRODUcTION
In this chapter we introduce the basic terms and definitions used by geotechnical engineers to index
and classify soils. The following notation is used in this chapter. , ..
Symbol Cc Cu .. Dio · D3o D6o ',. e FB G ·a~ Gs Gw g Llor
h
LLorW£· M'·· Mt• Ms· ;. Mw N ,, .. ,. ·'• ' . '..
,; ·Dimension 'L .L . 'L:i·•. ,..· J• ··Mi :M· 'M ·M.·· Unit mm .mm mmc
decirri~l) N . g's :. DefinitionCoefficient of curvature - Eq. (2.36) Coefficient of uniformity - Eq. (2.35)
Diamet~r for 10% finer by weight
riiaineter 'for 30% ·finer by. weight • Diameter for 60% finerby weight
. Void ratio: Eq. (2.1)' ;· • . ..
· Buoyant force · · · · ·
'··specific gravity- Eq. (2.24) ...
Bulk specific gravity-Eq: (2.25)" ·'
.. Specific gra~ity' of solids - Eq. (2.26)'
Specific gravity of water- Eq. (2.27) Acceleration of gravity
. l.jquidity index- Eq:(2.40) · ; , Liquid limit - Eq. (2.38)
·Submerged (net) mass (Sec. 2.3.1)
Total mass · ·'
Mass of solids • ' · J
Mass of water ·· ,··: · ~
Blow count in liquid limit test- Eq. (2.38) Porosity.: Eq. (2.2)
; ...
(Continued)
10
2.2
Chapter 2 Index and Classification Properties of Soils
Symbol· Dimension Unit Definition
Pc - - Phnarg coefficient
PI or lp - - Plasticity index- Eq. (2.39)
PLorwp
-
- Plastic limit - Eq. (2.37)s
- (%) Degree of saturation- Eq. (2.4)SLorws - (%) Shrinkage limit
v,;
L3 m3 Volume of airv.
L3 m3 Volume of solidsVr
L3 m3 Total volume Vv L3 m3 Volume of voids Vw L3 M3 Volume of waterw
M kg Weight (Sec. 2.3.1)W' M kg Submerged (net) weight (Sec. 2.3.1)
w
-
(%) Water content- Eq. (2.5)Yd ML-21 2 , kN/m3 Dry unit weight- Eq. (2.28)
Ym ory1or y ML-Zr-2 kN/m3 Moist or total unit weight - Eqs. (2.20), (2.30)
Ys ML-2T-2 kN/m3 Solids unit weight- Eq. (2.22)
Ysat ML-Zr-2 kN/m3 Saturated unit weight- Eq. (2.33)
Yw ML-21 2 kN/m3 Water unit weight- Eq. (2.23)
y' ML.:..z12 kN/m3 Buoyant unit weight- Eq. (2.34)
p M/L3 kg/m3 Total, wet, or moist density - Eq. (2.6)
p' M/L3 kg/m3 Buoyant density- Eq. (2.11)
· Pd M/L3 ·kg/m3 Dry density- Eq. (2.9)
Ps M/L3 kg/m3 Density of solids- Eq. (2.7)
M/L3 kg/m3 Saturated density- Eq. (2.10)
M/L3 kg/m3 Density of water- Eq. (2.8)
In this list, L
=
length; M. = mass, and T = time. When densities of soils and water are expressed in kg!rp3, the numbers are rather large. For instance, the derisity of water Pw is 1000 kg/m3.Since 1000 kg = J Mg, to makethe numbers more manageable, we will often use Mg/m3 for densities. If you are 'unfamiliar with SI metric units and their conversion factors, it would be a good idea to read Appendix A before proceeding with the rest of this chapter.
For each of the p notations, there is a corresponding y notation, which denotes unit weight, rather than density. This y notatio~should be used when units of force (F) are used (for example, lb or kN) instead of units of mass. This is described further in Sec. 2.3.2. ·
BASIC DEFINITIONS AND PHASE RELATIONS FOR SOILS
In general, any mass of soil consists of solid particles with voids in between. The solids are small grains of different minerals, whereas the voids can be filled with either water or other fluid (for example, a contaminant) or with air (or other gas), or filled partly with some of each (Fig. 2.1). Also, as noted in . the introduction, while we can have units of either mass or weight, we will assume that our problems ·are in mass units.
So, the total volume
Vr
·of the soil mass consists of the volume of soil solidsV.
and the volume of .voids Vv. The volume of voids is in general made up of the volume of water Vw and the volume of air Va.· 2.2.; Basic Definitions arid Phase Relations for Soils 11
w
A phase diagram (Fig; 2.2) shows the three phases separately. It's as if we could "melt down" all the solids into a single layer at the bottom, then have the water sit on top of that, and finally h~ve the air in a single layer at the top. The phase diagram helps us solve problems involving soil phase relationships. On the left side we usually indicate the volumes of.the three phases; on the right side we show the corresponding masses. Even though the diagram is two dimensional, it is understood · ,. that the volume shown is in units of L3, such as cm3 or ft3• Also, since we're not chemists or physicists, we assume that the mass
, of air is zero. · ' , .· : ·
FIGURE 2.1 Soil skeleton containing solid particles (S) and voids with air (A)
and water (W).
1. The void ratio1 e is defined as
Volume
In engineering practice, we usually measure the total vol-ume
Vr,
the mass of water Mw, and the mass of dry solids Ms. Then we calculate the rest of the values and the mass-volume relationships that we need. Most of these relationships are independent of sample size, and they are often dimensionless. They are 'very simple and easyto
remember, especially if you draw the phase diagram.Three volumetric ratios that are very useful in geotech-nical engineering can be determined directly from the phase diagram (Fig. 2.2).
Mass
' '
, , ,
' ) '. (2.1)
1
Readers with British
backgrou~ds
will
note that the correct terminology is
voids ratio.I
12 Chapter 2 .• • · Index and Classification Properties of Soils
2. The porosity n is defined as ·
n
=~X
100(%) (2.2)where Vv = volume of voids, and ·
~ = total volume of soil sample.
. .
.
Porosity is traditionally expressed as a percentage. The. maximum· range of n ·is between
. 0 and 100%. •;.; · ,. · · ·
and
From Fig. 2.2 and Eqs. (2.1) and (2.2)1it can beshown that " ··
e
n=1+e
n
e =
-. 1 :-.-.-._ n 3 •.
Thedegre~ ofsat~ration
Sis defined asv:
.
S = V.w X 100(%) v (2.3a) (2.3b) (2.4) The degree of saturation tells us what percentage of the total void space contains water. If the soil is completely dry, then S = 0%, and if the pores are completely full of water, then the soil is fully satu-rated and S = 100%.Now let us look at the other side, the mass side, of the phase diagram in Fig. 2.2. First, we define a mass ratio that is probably the single most important thing we need to know about a soil. It is also the only strictly mass-based parameter that we'll define for phase relationships. We want to know how much water is present in the voids relative to the amount of solids in the soil, so we define a ratio called
the water content w as · · ..
where Mw
=
mass of water, and Ms = mass of soil solids.M .. , . w =·_____!£X 100(%)
Ms ' (2.5)
The ratio of the amount of water present in a soil .volume to the amount of soil grains is based on the dry mass of the soil and not on the total mass. The water content; which is usually expressed as a percentage, can range from zero (dry soil) to several hundred percent. The natural water content for most soils is well under 100%, although in some marine arid organic soils it can range up to 500% or
·higher.. . · · · .
The water content is easily determined iri the laboratory. The standard procedure is detailed in ASTM (2010) standard D 2216. A representative sample of soil is selected and its total or wet mass is determined. Then it is dried to constant mass iri.
a
convection oven at 110°C. Normally a con-. stant mass is obtained after the sample is left in the' oven overnightcon-. The mass of the drying dish must,. 2.2. :Basic.Oefinitions and Phase Relations for Soils 13 of course,.be subtracted from both the wet and dry masses .. Then the water.content is calculated according to Eq. (2.5). Example 2.1 illustrates how the calculations for water content are actually
done in practice. · · '
', t • : ; '
Example 2.1 Given:
A specimen of wet soil in a drying dish has a mass of 462 g. After drying in all oven at 110°C overnight, the sample and dish have a mass of:364 g. The mass of the dish alone is 39 g ...
Required:
Determine the water content of the soil.
Solution: Set up the following calculation scheme; fill in the "given'' or measured quantities a, b, and d, . and make the calculations as indicat~d for c, e, and f. ·
a. · Mass of total (wet) sample·:+- dish'=· 462 g
. b
..
Mass of dry sample+
dish = 364 gc;. Mass of water (a-b) = 98 g d. Mass of dish = 39 g
e:
Mass of dry soil (b - d) = 325 g f. Water c9nt~nt' ( c/e) X 100% ~30.2%. ·.:In the laboratory, masses are usually, determined in grams (g) on· an ordinary . balance. The required sensitivity of the balance depends on the size 9f the specimen, and ASTM D 2216 gives
· some recommendations. · '' • : ·
The water content may also be determined using an ordinary microwave oven. ASTM (2010) standard D .4643 explains the procedure. To avoid overheating the soil specimen, microwave energy is applied for only bdef intervals and repeated until the mass becomes nearly constant. A heat sink, such as a glass beaker filled with water, helps. to prevent overheating of the soil by absorbing microwave energy after water has been removed from the soil pores. Otherwise, the water content is determined exactly as indicated above. Note that the microwave water content is not a replacement for the oven dry(D 2216) water content but is used when the water content is needed quickly. Other methods sometimes used in the field for water content determination are described in Chapter 5,
. Sec. 5.7. , · , .
, : , .· It is easy. to be confused by the concepts of mass and weight. From physics, you know that the mass of an object is a measure of how much matter the object contains, while the weight of an object is'':-.. determined. by the gravitational force that causes its downward .acceleration. Recall that weight W
equals mass m times g, the acceleration due to gravity, or W · = mg; As noted in Appendix A, when we . weigh something in: the :laboratory,· we really are .determiningits mass-either by comparing two masses on a balance or by using a device calibrated against objects of known mass. It is basically an English-language.problem; we really should say ~'we massed it" when we determine the mass of an
14 Chapter 2. Index and Classification Properties of Soils
object in the laboratory.Another very useful concept in geotechnical engineering is density. You know from physics that density is mass per unit volume, so its units are kg/m3• (See Appendix A for
the corresponding units in the cgs and British engineering systems.) The density is the ratio that connects the volumetric side of the phase diagram with the mass side. Several densities are com-monly used in geotechnical engineering practice. First, we define the total, wet, or moist density p; the density of the particles, solid density Ps; and the density of water Pw· Or, in terms of the basic
masses and volumes of Fig. 2.2: · ·
Mt Ms
+
Mw p==-== ~ ~ , ·Ms Ps =V
s Mw Pw == Vw (2.6) (2.7) (2.8) In natural soils, the magnitude of the total density p will depend on how much water happens to be in the voids as well as the density of the mineral grains themselves. Thus, p can range from slightly above 1000 kg/m3 to as high as 2400 kg!m3 (1.0 to 2.4 Mg/m3).'!Ypical values of Ps for most soils range from 2500 to 2800 kg/m3 (2.5 to 2.8 Mg/m3). Most sands have p8ranging between 2.6 and 2'.7 Mg/m
3• For example, a common mineral in sands is quartz; its
Ps = 2.65 Mg/m3• Most clay soils have a value of Ps between 2.65 and 2.80 Mg/m3, depending on the
predominant mineral in the soil; whereas organic soils may have a Ps as low as 2.5 Mg/m3• Conse-quently, for most phase problems, unless a specific value of Ps is given, it is usually close enough for geotechnical work to assume a Ps of 2.65 or 2.70 Mg/m3• The density of water varies slightly, depending
on the temperature. At 4°C, when water is at its densest, Pw exactly equals 1000 kg/m3 (1 g/cm3), and
this density is sometimes designated by the symbol p;. For ordinary engineering work, it is sufficiently · accurate to take Pw :::; p0 == 1000 kg/m
3 == 1 Mg/m3•
Three other densities very useful in soils engineering are the dry density Pd, the saturated density
Psat' and the submerged or buoyant density p' or Pb·
Ms Pa ==-~ Ms.+, Mw(TT =
o'
s.
== 100%) - Ya ' Psat - . ~ ·. · ' " · p' == Psat ; Pw (2.9) (2.10) (2.11) Among other uses, the dry density Pais a common basis for judging a soil's degree of compaction after we have applied some mechanical energy to it, for example by using a roller or vibratory plate (ChapterS). The saturated density Psat' as the name implies, is the total density of the soil when 100% of its pores are filled with water; in this special case, p == Ps~t· The concept of submerged or buoyant density p; is often difficult for students to understand, so it is discussed later after we have done a few example problems. However, you may be familiar. with this concept from studying aggregates, where a "basket" of aggregate is weighed while it is submerged under water. Typical values of pa, Psat> and p' for several soil types are shown in Table 2.1.From the basic definitions provided in this section, other useful relationships can be derived, as we show in the examples in the next section.
