soil mechanics and foundations

781 Pages·2011·21.47 MB·English

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THIRD EDITION
SOIL MECHANICS
AND FOUNDATIONS
MUNI BUDHU
Professor, Department of Civil Engineering & Engineering Mechanics
University of Arizona
JOHN WILEY & SONS, INC.
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Library of Congress Cataloging-in-Publication Data
Budhu, M.
Soil mechanics and foundations / Muni Budhu.—3rd ed.
p. cm.
Includes bibliographical references.
ISBN 978-0-470-55684-9 (hardback)
1. Soil mechanics. 2. Foundations. I. Title.
TA710.B765 2010
624.1'5136—dc22
2010023265
Printed in the United States of America
10 9 8 7 6 5 4 3 2 1
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PREFACE
This textbook is written for an undergraduate course in soil mechanics and foundations. It has three pri-
mary objectives. The fi rst is to present basic concepts and fundamental principles of soil mechanics and
foundations in a simple pedagogy using the students’ background in mechanics, physics, and mathematics.
The second is to integrate modern learning principles, teaching techniques, and learning aids to assist
students in understanding the various topics in soil mechanics and foundations. The third is to provide
a solid background knowledge to hopefully launch students in their lifelong learning of geotechnical
engineering issues.
Some of the key features of this textbook are:
• Topics are presented thoroughly and systematically to elucidate the basic concepts and fundamental
principles without diluting technical rigor.
• A large number of example problems are solved to demonstrate or to provide further insights into
the basic concepts and applications of fundamental principles.
• The solution of each example is preceded by a strategy, which is intended to teach students to
think about possible solutions to a problem before they begin to solve it. Each solution provides a
step-by-step procedure to guide the student in problem solving.
• A “What you should be able to do” list at the beginning of each chapter alerts readers to what
they should have learned after studying each chapter, to help students take responsibility for
learning the material.
• Web-based applications including interactive animations, interactive problem solving, interactive
step-by-step examples, virtual soils laboratory, e-quizzes, and much more are integrated with this
textbook.
With the proliferation and accessibility of computers, programmable calculators, and software,
students will likely use these tools in their practice. Consequently, computer program utilities and
generalized equations that the students can program into their calculators are provided rather than
charts.
The content of the book has been signifi cantly enhanced in the third edition:
• Reorganization of chapters—Several chapters in the second edition are now divided into mul-
tiple chapters for ease of use.
• Enhancement of content—The content of each chapter has been enhanced by adding
updated materials and more explanations. In particular, signifi cant improvements have been
made not only to help interpret soil behavior but also to apply the basic concepts to practical
problems.
• Examples and problems—More examples, with more practical “real-world” situations, and more
problems have been added. The examples have been given descriptive titles to make specifi c
examples easier to locate.
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iv PREFACE
ACKNOWLEDGMENTS
I am grateful to the many reviewers who offered many valuable suggestions for improving this textbook.
The following persons were particularly helpful in reviewing the third edition: Juan Lopez, geotechnical
engineer, Golder Associates, Houston, TX; Walid Toufi g, graduate student, University of Arizona, Tucson,
AZ; and Ibrahim Adiyaman, graduate student, University of Arizona, Tucson, AZ.
Ms. Jenny Welter, Mr. Bill Webber, and the staff of John Wiley & Sons were particularly helpful
in getting this book completed. Additional resources are available online at www.wiley.com/college/
budhu.
Also available from the Publisher: Foundations and Earth Retaining Structures, by Muni Budhu
ISBN: 978-0471-47012-0
Website: www.wiley.com/college/budhu
A companion lab manual is available from the Publisher: Soil Mechanics Laboratory Manual, by
Michael Kalinski
The soil mechanics course is often accompanied by a laboratory course, to introduce students to com-
mon geotechnical test methods, test standards, and terminology. Michael Kalinski of the University of
Kentucky has written a lab manual introducing students to the most common soil mechanics tests, and
has included laboratory exercises and data sheets for each test. Brief video demonstrations are also
available online for each of the experiments described in this manual.
Soil Mechanics Laboratory Manual, by Michael Kalinski
Website: www.wiley.com/college/kalinski
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NOTES for Students and Instructors
PURPOSES OF THIS BOOK
This book is intended to present the principles of soil mechanics and its application to foundation analy-
ses. It will provide you with an understanding of the properties and behavior of soils, albeit not a perfect
understanding. The design of safe and economical geotechnical structures or systems requires consider-
able experience and judgment, which cannot be obtained by reading this or any other textbook. It is
hoped that the fundamental principles and guidance provided in this textbook will be a base for lifelong
learning in the science and art of geotechnical engineering.
The goals of this textbook in a course on soil mechanics and foundation are as follows:
1. To understand the physical and mechanical properties of soils.
2. To determine parameters from soil testing to characterize soil properties, soil strength, and soil
deformations.
3. To apply the principles of soil mechanics to analyze and design simple geotechnical systems.
LEARNING OUTCOMES
When you complete studying this textbook you should be able to:
• Describe soils and determine their physical characteristics such as grain size, water content, and
void ratio.
• Classify soils.
• Determine compaction of soils.
• Understand the importance of soil investigations and be able to plan a soil investigation.
• Understand the concept of effective stress.
• Determine total and effective stresses and porewater pressures.
• Determine soil permeability.
• Determine how surface stresses are distributed within a soil mass.
• Specify, conduct, and interpret soil tests to characterize soils.
• Understand the stress–strain behavior of soils.
• Understand popular failure criteria for soils and their limitations.
• Determine soil strength and deformation parameters from soil tests, for example, Young’s modulus,
friction angle, and undrained shear strength.
• Discriminate between “drained” and “undrained” conditions.
• Understand the effects of seepage on the stability of structures.
• Estimate the bearing capacity and settlement of structures founded on soils.
• Analyze and design simple foundations.
• Determine the stability of earth structures, for example, retaining walls and slopes.
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vi NOTES FOR STUDENTS AND INSTRUCTORS
ASSESSMENT
You will be assessed on how well you absorb and use the fundamentals of soil mechanics. Three areas
of assessment are incorporated in the Exercise sections of this textbook. The fi rst area, called “Theory,”
is intended for you to demonstrate your knowledge of the theory and extend it to uncover new rela-
tionships. The questions under “Theory” will help you later in your career to address unconventional
issues using fundamental principles. The second area, called “Problem Solving,” requires you to apply
the fundamental principles and concepts to a wide variety of problems. These problems will test your
understanding and use of the fundamental principles and concepts. The third area, called “Practical,” is
intended to create practical scenarios in which you can use not only the subject matter in the specifi c
chapter but also prior materials that you have encountered. These problems try to mimic some aspects
of real situations and give you a feel for how the materials you have studied so far can be applied
in practice. Communication is at least as important as the technical details. In many of these “Practi-
cal” problems you are placed in a situation in which you must convince stakeholders of your technical
competence. A quiz at the end of each chapter is at www.wiley.com/college/budhu to test your general
knowledge of the subject matter.
SUGGESTIONS FOR PROBLEM SOLVING
Engineering is, foremost, about problem solving. For most engineering problems, there is no unique
method or procedure for fi nding solutions. Often, there is no unique solution to an engineering problem.
A suggested problem-solving procedure is outlined below.
1. Read the problem carefully; note or write down what is given and what you are required to fi nd.
2. Draw clear diagrams or sketches wherever possible.
3. Devise a strategy to fi nd the solution. Determine what principles, concepts, and equations are
needed to solve the problem.
4. When performing calculations, make sure that you are using the correct units.
5. Check whether your results are reasonable.
The units of measurement used in this textbook follow the SI system. Engineering calculations
are approximations and do not result in exact numbers. All calculations in this book are rounded, at the
most, to two decimal places except in some exceptional cases, for example, void ratio.
WEBSITE
Additional materials are available at www.wiley.com/college/budhu. The National Science Digital
Library site “Grow” (www.grow.arizona.edu) contains a collection of learning and other materials on
geotechnical engineering.
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NOTES for Instructors
I would like to present some guidance to assist you in using this book in undergraduate geotechnical
engineering courses based on my own experiences in teaching this material.
DESCRIPTION OF CHAPTERS
The philosophy behind each chapter is to seek coherence and to group topics that are directly related
to each other. This is a rather diffi cult task in geotechnical engineering because topics are intertwined.
Attempts have been made to group topics based on whether they relate directly to the physical char-
acteristics of soils or mechanical behavior or are applications of concepts to analysis of geotechnical
systems. The sequencing of the chapters is such that the preknowledge required in a chapter is covered
in previous chapters.
Chapter 1 sets the introductory stage of informing the students of the importance of geotechnical
engineering. Most of the topics related to the physical characteristics of soils are grouped in Chapters
2 through 5. Chapter 2 deals with basic geology, soil composition, and particle sizes. Chapter 3 is about
soils investigations and includes in situ and laboratory tests. The reasons for these tests will become clear
after Chapters 4 through 10 are completed. In Chapter 4, phase relationships, index properties, and soil
classifi cation and compaction are presented. Chapter 5 describes soil compaction and why it is impor-
tant. One-dimensional fl ow of water and wellpoints are discussed in Chapter 6.
Chapter 7 deals with stresses, strains, and elastic deformation of soils. Most of the material in this
chapter builds on course materials that students would have encountered in their courses in statics and
strength of materials. Often, elasticity is used in preliminary calculations in analyses and design of geo-
technical systems. The use of elasticity to fi nd stresses and settlement of soils is presented and discussed.
Stress increases due to applied surface loads common to geotechnical problems are described. Students
are introduced to stress and strain states and stress and strain invariants. The importance of effective
stresses and seepage in soil mechanics is emphasized.
Chapter 8 presents stress paths. Here basic formulation and illustrations of stress paths are discussed.
Drained and undrained conditions are presented within the context of elasticity. In Chapter 9, the basic
concepts of consolidation are presented with methods to calculate consolidation settlement. The theory of
one-dimensional consolidation is developed to show students the theoretical framework from which soil con-
solidation settlement is interpreted and the parameters required to determine time rate of settlement. The
oedometer test is described, and procedures to determine the various parameters for settlement calculations
are presented.
Chapter 10 deals with the shear strength of soils and the tests (laboratory and fi eld) required for
its determination. Failure criteria are discussed using the student’s background in strength of materials
(Mohr’s circle) and in statics (dry friction). Soils are treated as a dilatant-frictional material rather than
the conventional cohesive-frictional material. Typical stress–strain responses of sand and clay are
presented and discussed. The implications of drained and undrained conditions on the shear strength
of soils are discussed. Laboratory and fi eld tests to determine the shear strength of soils are described.
Some of the failure criteria for soils are presented and their limitations are discussed.
Chapter 11 deviates from traditional undergraduate textbook topics that present soil consolida-
tion and strength as separate issues. In this chapter, deformation and strength are integrated within the
framework of critical state soil mechanics using a simplifi ed version of the modifi ed Cam-clay model. The
emphasis is on understanding the mechanical behavior of soils rather than presenting the mathematical
vii
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viii NOTES FOR INSTRUCTORS
formulation of critical state soil mechanics and the modifi ed Cam-clay model. The amount of mathematics
is kept to the minimum needed for understanding and clarifi cation of important concepts. Projection
geometry is used to illustrate the different responses of soils when the loading changes under drained and
undrained loading. Although this chapter deals with a simplifi cation and an idealization of real soils, the
real benefi t is a simple framework, which allows the students to think about possible soil responses if
conditions change from those originally conceived, as is usual in engineering practice. It also allows them
to better interpret soil test results and estimate possible soil responses from different loading conditions.
Chapter 12 deals with bearing capacity and settlement of footings. Here bearing capacity and settle-
ment are treated as a single topic. In the design of foundations, the geotechnical engineer must be satisfi ed
that the bearing capacity is suffi cient and the settlement at working load is tolerable. Indeed, for most
shallow footings, it is settlement that governs the design, not bearing capacity. Limit equilibrium analysis
is introduced to illustrate the method that has been used to fi nd the popular bearing capacity equations
and to make use of the student’s background in statics (equilibrium) to introduce a simple but powerful
analytical tool. A set of bearing capacity equations for general soil failure that has found general use in
geotechnical practice is presented. These equations are simplifi ed by breaking them down into two cat-
egories—one relating to drained condition, the other to undrained condition. Elastic, one-dimensional
consolidation and Skempton and Bjerrum’s (1957) method of determining settlement are presented. The
elastic method of fi nding settlement is based on work done by Gazetas et al. (1985), who described prob-
lems associated with the Janbu, Bjerrum, and Kjaernali (1956) method that is conventionally quoted in
textbooks. The application of knowledge gained in Chapter 11 to shallow footing design is presented.
Pile foundations are described and discussed in Chapter 13. Methods for fi nding bearing capacity
and settlement of single and group piles are presented.
Chapter 14 is about two-dimensional steady-state fl ow through soils. Solutions to two-dimensional
fl ow using fl ownets and the fi nite difference technique are discussed. Emphases are placed on seepage,
porewater pressure, and instability. This chapter normally comes early in most current textbooks. The
reason for placing this chapter here is because two-dimensional fl ow infl uences the stability of earth
structures (retaining walls and slopes), discussion of which follows in Chapters 15 and 16. A student
would then be able to make the practical connection of two-dimensional fl ow and stability of geotechni-
cal systems readily.
Lateral earth pressures and their use in the analysis of earth-retaining systems and simple
braced excavations are presented in Chapter 15. Gravity and fl exible retaining walls, in addition to
reinforced soil walls, are discussed. Guidance is provided as to what strength parameters to use in
drained and undrained conditions.
Chapter 16 is about slope stability. Here stability conditions are described based on drained or
undrained conditions.
Appendix A allows easy access to frequently used typical soil parameters and correlations.
Appendix B shows charts to determine the increases in vertical stress and elastic settlement of
uniformly loaded circular foundation. Appendix C contains charts for the determination of the increases
in vertical stress for uniformly loaded circular and rectangular footings resting on fi nite soil layers.
A ppendix D contains charts for the determination of lateral earth pressure coeffi cients presented by
Kerisel and Absi (1990).
CHAPTER LAYOUT
The Introduction of each chapter attempts to capture the student’s attention, to present the learning
o bjectives, and to inform the student of what prior knowledge is needed to master the material. At
the end of the introduction, the importance of the learning material in the chapter is described. The
i ntention is to give the student a feel for the kind of problem that he or she should be able to solve on
completion of the chapter.