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Page i THE FOUNDATION ENGINEERING HANDBOOK Page ii This page intentionally left blank. Page iii THE FOUNDATION ENGINEERING HANDBOOK Edited by Manjriker Gunaratne Taylor & Francis Taylor & Francis Group Boca Raton London New York A CRC title, part of the Taylor & Francis imprint, a member of the Taylor & Francis Group, the academic division of T&F Informa plc. Page iv Published in 2006 by CRC Press Taylor & Francis Group 6000 Broken Sound Parkway NW, Suite 300 Boca Raton, FL 33487–2742 © 2006 by Taylor & Francis Group, LLC CRC Press is an imprint of Taylor & Francis Group This edition published in the Taylor & Francis e-Library, 2006. To purchase your own copy of this or any of Taylor & Francis or Routledge’s collection of thousands of eBooks please go to www.eBookstore.tandf.co.uk. No claim to original U.S. Government works 10 9 8 7 6 5 4 3 2 1 ISBN 0-203-48441-X Master e-book ISBN ISBN 0-203-61133-0 (OEB Format) International Standard Book Number-10: 0-8493-1159-4 (Print Edition) (Hardcover) International Standard Book Number-13: 978-0-8493-1159-8 (Print Edition) (Hardcover) Library of Congress Card Number 2005050886 This book contains information obtained from authentic and highly regarded sources. Reprinted material is quoted with permission, and sources are indicated. A wide variety of references are listed. Reasonable efforts have been made to publish reliable data and information, but the author and the publisher cannot assume responsibility for the validity of all materials or for the consequences of their use. No part of this book may be reprinted, reproduced, transmitted, or utilized in any form by any electronic, mechanical, or other means, now known or hereafter invented, including photocopying, microfilming, and recording, or in any information storage or retrieval system, without written permission from the publishers. For permission to photocopy or use material electronically from this work, please access www.copyright.com (http://www.copyright.com/) or contact the Copyright Clearance Center, Inc. (CCC) 222 Rosewood Drive, Danvers, MA 01923, 978–750–8400. CCC is a not-for-profit organization that provides licenses and registration for a variety of users. For organizations that have been granted a photocopy license by the CCC, a separate system of payment has been arranged. Trademark Notice: Product or corporate names may be trademarks or registered trademarks, and are used only for identification and explanation without intent to infringe. Library of Congress Cataloging-in-Publication Data The foundation engineering handbook/edited by Manjriker Gunaratne. p. cm. Includes bibliographical references and index. ISBN 0-8493-1159-4 1. Foundations—Handbooks, manuals, etc. 2. Soil mechanics—Handbooks, manuals, etc. I.Gunaratne, Manjriker. TA775.F677 2006 624.1'5–dc22 20050508 Taylor & Francis Group is the Academic Division of T&F Informa plc. Visit the Taylor & Francis Web site at http://www.taylorandfrancis.com and the CRC Press Web site at http://www.crcpress.com Page v Preface A genuine need existed for an updated foundation engineering handbook that incorporates, in addition to classical principles of foundation designs, significant contributions made to the art of foundation design by practitioners and researchers during the last two decades. Of special significance in this regard is the knowledge of (1) innovative in situtesting and site improvement techniques that have evolved recently; (2) cost-effective design methods that make use of geogrids for mechanically stabilized earth retaining structures; (3) concepts involved in ground deformation modeling using finite elements; and (4) latest modifications in the ACI codes applicable to structural design of foundations. This handbook largely fulfills the above needs, since the editor and the contributors have focused on discussing the state of the art of theoretical and applied foundation engineering and concrete design in a concise and simple fashion. Reliability-based design concepts that have been incorporated in most up-to-date structural and pavement design guidelines are making inroads into foundation engineering as well. Hence, the editor decided to include reliability-based design and LRFD (load resistance factor design) concepts along with relevant illustrative examples in this handbook. This step not only makes this handbook somewhat unique among other currently available foundation engineering literature, but also it provides an opportunity for practitioners and students alike to familiarize themselves with the basics of limit state design applied to foundation engineering. Furthermore, the editor’s extensive experience as an engineering educator has constantly indicated that, in spite of the availability of a number of excellent textbooks in foundation engineering, a quick reference that mostly focuses on significant and commonly-used foundation engineering principles and illustrative examples has been in demand. This handbook also addresses such a need, since it can be adopted conveniently as a textbook, both at the undergraduate and graduate levels. It is indeed my pleasure to have worked with a distinguished set of contributors who took time off of their extremely busy professional careers and produced their best in keeping with their usual professional performance. My appreciation is conveyed to Ingrid Hall of the Civil and Environmental Engineering Department, University of South Florida’s civil engineering graduate students Alex Mraz, Ivan Sokolic, Mathiyaparanam and Kalyani Jeyisankar, Dumina Randeniya, and undergraduate student Mercedes Quintas for their help in preparing the manuscript. The support of my children, Ruwan and Aruni, and my wife, Prabha, during the arduous task of making this project a reality is also gratefully acknowledged. I wish to extend my special thanks to Cindy Renee Carelli, former engineering acquisitions editor; Matt Lamoreaux, current engineering acquisitions editor; Elizabeth Spangenberger; and other staff of Taylor & Francis for their meticulous work on publishing this handbook. Thanks are also due to the relevant publishers who permitted the use of material from other references. I also express my profound gratitude to late Professor Alagiah Thurairajah, former dean of the Faculty of Engineering, Peradeniya University, Sri Lanka, and prominent member of the Cambridge University’s Cam Clay group for introducing me to North America and postgraduate studies in geotechnics. Page vi Finally, it is to my mother, Jeannette Gunaratne, and my late father, Raymond Gunaratne, that I dedicate this book. Manjriker Gunaratne University of South Florida Tampa Page vii Abstract This handbook contains some of the most recent developments in theoretical and applied foundation engineering in addition to classical foundation design methods. The inclusion of recent developments mostly enriches the classical design concepts in Chapters 3–7, 10 and 11. It also enables the reader to update his or her knowledge of new modeling concepts applicable to foundation design. Most recently developed in situtesting methods discussed in detail in Chapter 2 certainly familiarize the reader with state-of-the-art techniques adopted in site testing. In addition, modern ground stabilization techniques introduced in Chapter 12 by an experienced senior engineer in Hayward-Baker Inc., a leading authority in site improvement work across North America, provides the reader with the knowledge of effective site improvement techniques that are essential for foundation design. Innovative and widely used methods of testing pile foundations are introduced with numerical illustrations in Chapters 2 and 7. LRFD designs in Chapters3 and 6 and the design of retaining structures with geogrids included in Chapter 10 are unique features of this foundation engineering handbook. For the benefit of the reader, the basic and advanced soil mechanics concepts needed in foundation design are elaborated with several numerical examples in Chapter 1. Page viii This page intentionally left blank. Page ix Editor Manjriker Gunaratne is a professor of civil engineering at the University of South Florida. He completed his pre-engineering education at Ananda College, Colombo, Sri Lanka, receiving the S.A.Wijetileke prize for the highest ranking student. Thereafter, he obtained his bachelor of science in engineering (Honors) degree from the Faculty of Engineering, University of Peradeniya, Sri Lanka, in 1978. In 1977, he was awarded the Professor E.O.E.Pereira prize for the highest ranking student at the Part (II) examination in the overall engineering class. Subsequently, he pursued postgraduate education in North America, earning master of applied science and doctoral degrees in civil engineering from the University of British Columbia, Vancouver, Canada, and Purdue University, West Lafayette, Indiana, respectively. During his 18 years of service as an engineering educator, he has authored 25 papers in a number of peer-reviewed journals, such as the American Society of Civil Engineering (geotechnical, transportation, civil engineering materials, and infrastructure systems) journals, International Journal of Numerical and Analytical Methods in Geomechanics, Civil Engineering Systems, and others. In addition, he has made a number of presentations at various national and international forums in geotechnical and highway engineering. He has held fellowships at the United States Air Force (Wright-Patterson Air Force Base) and the National Aeronautics and Space Administration (Robert Goddard Space Flight Center) and a consultant’s position with the United Nations Development Program in Sri Lanka. He has also been a panelist for the National Science Foundation and a member of the task force for investigation of dam failures in Florida, U.S.A. Page x This page intentionally left blank. Page xi Contributors Dr. Austin Gray Mullins is an associate professor of civil engineering at the University of South Florida, Tampa, Florida, who specializes in geotechnical and structural engineering. He obtained B.S., M.S., and Ph.D. degrees in civil engineering from the University of South Florida. Prior to joining USF’s department of civil and environmental engineering, he worked as an engineer at Greiner Inc. Roadway Group, Tampa. His most recent research work has been in the areas of statnamic testing of building foundations and drilled shafts as well as structural testing of bridges. He is a professional engineer registered in the state of Florida. Dr. Alaa Ashmawy is an associate professor of civil engineering at the University of South Florida, Tampa, Florida, with specialization in geotechnical and geoenvironmental engineering. He obtained the B.S. degree in civil engineering from Cairo University, Egypt, and M.S. and Ph.D. degrees from Purdue University. Prior to joining USF’s department of civil and environmental engineering, he was a postdoctoral research associate at the Georgia Institute of Technology. His most recent research work has been in the areas of hydraulic and diffusion characteristics of surface amended clays, evaluation of the Purdue TDR Method for soil water content and density measurement, and discrete element modeling of angular particles. He is a professional engineer registered in the state of Florida. Dr. Panchy Arumugasaamy graduated with first class honors bachelor degree in civil engineering from the University of Sri Lanka, Katubedde Campus, and is the recipient of the 1973 gold medal from the UNESCO Team for ranking first in the Faculty of Engineering and Architecture of that year. He earned his Ph.D. degree in structural engineering in 1978 from the University of Sheffield, England. In 1998, he earned his Executive M.B.A. graduate degree from Ohio University. He has over 25 years of extensive experience in engineering consulting (civil and structural engineering), project management, teaching, advanced research, and product development. He is well respected by his peers for his competencies in the analysis and design of complex structural systems for buildings, bridges, and other structures for different types of applications, and assessment of behavior of elements using both classical and computer aided methods. He is familiar with many codes of practices including American Codes (ACI, AISC, ASCE, SEAOC, and AASHTO), CEP-FIP codes, BSI (for bridges), and CSA. He has hands-on experience in computer modeling, computer aided design including 2D and 3D frame analysis, grillage analysis for bridges, 2D and 3D finite element analysis, and plate analysis to optimize the structural system (steel and concrete structures). He is also proficient in 3D computer modeling. He has also specialized in optical engineering and holds many patents for his inventions. He has published many papers on national and international journals as a coauthor and has received the following awards for the best designs and research papers. He is currently working with MS Consultants Inc. as the head of the structural division in Columbus, Ohio. Page xii He has been a research scholar and senior adjunct faculty at University of West Indies, St. Augustine, Trinidad and Tobago (WI), Florida Atlantic University, Boca Raton, and research associate professor at the University of Nebraska, Lincoln-Omaha. James D.Hussin received his B.S. in civil engineering from Columbia University and M.S. in geotechnical engineering from California Institute of Technology (CalTech). Dr. Hussin has been with Hayward-Baker Inc. for 20 years and in his current position of director is responsible for the company’s national business development and marketing efforts and oversees engineering for the southeast U.S. and the Caribbean. Before joining Hayward- Baker, Dr. Hussin was a geotechnical consultant in Florida and South Carolina. Dr. hussin is a member and past chairman of the American Society of Civil Engineers (ASCE) Geoinstitute National Soil Improvement Committee and is a current board member of the National ASCE Technical Coordination Council that oversees the technical committees. Dr. Hussin has over 20 publications, including associate editor of the ASCE Special Publication No. 69, “Ground Improvement, Ground Reinforcement, Ground Treatment, Developments 1987–1997.” Page xiii Contents 1. Review of Soil Mechanics Concepts and Analytical Techniques Used in 1 Foundation Engineering Manjriker Gunaratne 2. In Situ Soil Testing 47 Gray Mullins 3. Spread Footings: Analysis and Design 87 Manjriker Gunaratne 4. Geotechnical Design of Combined Spread Footings 145 Manjriker Gunaratne 5. Structural Design of Foundations 179 Punchy Arumugasaamy 6. Design of Driven Piles and Pile Groups 235 Manjriker Gunaratne 7. Design of Drilled Shafts 299 Gray Mullins 8. Design of Laterally Loaded Piles 327 Manjriker Gunaratne 9. Construction Monitoring and Testing Methods of Driven Piles 363 Manjriker Gunaratne 10. Retaining Walls: Analysis and Design 427 Alaa Ashmawy 11. Stability Analysis and Design of Slopes 485 Manjriker Gunaratne 12. Methods of Soft Ground Improvement 529 James D.Hussin 13. Impact of Groundwater on the Design of Earthen Structures 567 Manjriker Gunaratne Index 595 Page xiv This page intentionally left blank. Page 1 1 Review of Soil Mechanics Concepts and Analytical Techniques Used in Foundation Engineering Manjriker Gunaratne CONTENTS 1.1 Introduction 2 1.2 Soil Classification 2 1.2.1 Mechanical Analysis 2 1.2.2 Atterberg Limits 4 1.2.3 Unified Soil Classification System 6 1.3 Effective Stress Concept 6 1.4 Strength of Soils 9 1.4.1 Drained and Undrained Strengths 9 1.4.2 Triaxial Tests 10 1.4.2.1 Triaxial Testing of Rocks 11 1.4.2.2 Selection of Triaxial Test Type Based on the Construction Sit11tion 1.4.2.3 Computation of Strength Parameters Based on Triaxial Tests 13 1.4.3 Unconfined Compression Test 15 1.5 Compressibility and Settlement 16 1.5.1 Estimation of Immediate Settlement in Soils 17 1.5.1.1 Elastic Properties and In Situ Test Parameters 19 1.5.2 Estimation of Foundation Settlement in Saturated Clays 20 1.6 Soil Densities and Compaction 25 1.6.1 Bulk Unit Weight 25 1.6.2 Dry Unit Weight 26 1.6.3 Saturated Unit Weight 26 1.6.4 Submerged (Buoyant) Unit Weight 27 1.6.5 Soil Compaction 27 1.6.5.1 Laboratory Compaction 27 1.6.5.2 Evaluation of Field Compaction 29 1.7 Finite Element Concepts Used in Modeling of Earthen Structures 31 1.7.1 Finite Element Approach 31 1.7.2 Finite Element Formulation 31 1.7.3 Equilibrium and Compatibility Conditions 33 1.8 Common Methods of Modeling the Yielding Behavior of Soils 35 1.8.1 Modified Cam-Clay Model 36 1.8.1.1 Isotropic Consolidation of Clays 36 1.8.1.2 Critical State of Deformation of Clay 37 Page 2 1.8.1.3 Stress-Strain Relations for Yielding Clays 39 1.8.2 Cap Model 39 1.8.3 Nonlinear Elastic Stress-Strain Relations 42 1.8.3.1 Evaluation of Nonlinear Elastic Parameters 42 1.8.3.2 Evaluation of Gmax from Standard Penetration Tests 43 1.8.4 Concepts of Stress Dilatancy Theroy for Granular Soils 43 References 44 1.1 Introduction Geotechnical engineering is a branch of civil engineering in which technology is applied in the design and construction of structures involving geological materials. Earth’s surface material consists of soil and rock. Of the several branches of geotechnical engineering, soil and rock mechanics are the fundamental studies of the properties and mechanics of soil and rock, respectively. Foundation engineering is the application of the principles of soil mechanics, rock mechanics, and structural engineering to the design of structures associated with earthen materials. On the other hand, rock engineering is the corresponding application of the above-mentioned technologies in the design of structures associated with rock. It is generally observed that most foundation types supported by intact bedrock present no compressibility problems. Therefore, when designing common foundation types, the foundation engineer’s primary concerns are the strength and compressibility of the subsurface soil and, whenever applicable, the strength of bedrock. 1.2 Soil Classification 1.2.1 Mechanical Analysis According to the texture or the “feel,” two different soil types can be identified. They are: (1) coarse-grained soil (gravel and sand) and (2) fine-grained soil (silt and clay). While the engineering properties (primarily strength and compressibility) of coarse-grained soils depend on the size of individual soil particles, the properties of fine-grained soils are mostly governed by the moisture content. Hence, it is important to identify the type of soil at a given construction site since effective construction procedures depend on the soil type. Geotechnical engineers use a universal format called the unified soil classification system (USCS) to identify and label different types of soils. The system is based on the results of common laboratory tests of mechanical analysis and Atterberg limits. In classifying a given soil sample, mechanical analysis is conducted in two stages: (1) sieve analysis for the coarse fraction (gravel and sand) and (2) hydrometer analysis for the fine fraction (silt and clay). Of these, sieve analysis is conducted according to American Society for Testing and Materials (ASTM) D421 and D422 procedures, using a set of U.S. standard sieves (Figure 1.1) the most commonly used sieves are U.S. Standard numbers 20, 40, 60, 80, 100, 140, and 200, corresponding to sieve openings of 0.85, 0.425, 0.25, 0.18, 0.15, 0.106, and 0.075mm, respectively. During the test, the percentage (by weight) of the soil sample retained on each sieve is recorded, from which the percentage of soil (R%) passing through a given sieve size (D) is determined. On the other hand, if a substantial portion of the soil sample consists of fine-grained soils (D<0.075mm), then sieve analysis has to be followed by hydrometer analysis Page 3 FIGURE 1.1 Equipment used for sieve analysis. (Courtesy of the University of South Florida.) (Figure 1.2). The hydrometer analysis test is performed by first treating the “fine fraction” with a deflocculating agent such as sodium hexametaphosphate (Calgon) or sodium silicate (water glass) for about half a day and then allowing the suspension to settle in a hydrometer jar kept at a constant temperature. As the heavier particles settle, followed by the lighter ones, a calibrated ASTM 152H hydrometer is used to estimate the fraction (percentage, R%) that is still settling above the hydrometer bottom at any given stage. Further, the particle size (D) that has settled past the hydrometer bottom at that stage in FIGURE 1.2 Equipment used for hydrometer analysis. (Courtesy of the University of South Florida.) Page 4 time can be estimated from Stokes’ law. Then, it can be seen that R% is the weight percentage of soil finer than D. Complete details of the above-mentionedtests such as the correction to be applied to the hydrometer reading and determination of the effective length of the hydrometer are provided in Bowles (1986) and Das (2002). For soil samples that have significant coarse and fine fractions, the sieve and hydrometer analysis results (R% and D) can be logically combined to generate grain (particle) size distribution curves such as those indicated in Figure 1.3. As an example, from Figure 1.3, it can be seen that 30% of soil type A is finer than 0.075mm (U.S. Standard no. 200 sieve), with R%=30 and D=0.075mm being the last pair of results obtained from sieve analysis. In combining sieve analysis data with hydrometer analysis data, one has to convert R% (based on the fine fraction only) and D(size) obtained from hydrometer analysis to R% based on the weight of the entire sample in order to ensure continuity of the curve. As an example, let the results from one hydrometer reading of soil sample A be R%=90 and D=0.05 mm. To plot the curve, one requires the percentage of the entire sample finer than 0.05 mm. Since what is finer than 0.05 mm is 90% of the fine fraction (30% of the entire sample) used for hydrometer analysis, the convertedR% for the final plot can be obtained by multiplying 90% by the fine fraction of 30%. Hence, the converted data used to plot Figure 1.3 are R% =27 and D=0.05mm. 1.2.2 Atterberg Limits As mentioned earlier, properties of fine-grained soils are governed by water. Hence, the effect of water has to be considered when classifying fine-grained soils. This is achieved FIGURE 1.3 Grain (particle) size distribution curves. (FromConcrete Design Handbook, CRC Press. With permission.) Page 5 FIGURE 1.4 Variation of the fine-grained soil properties with the water content. by employing the Atterberg limits or consistency limits. The physical state of a fine-grained soil changes from brittle to liquid state with increasing water content, as shown in Figure 1.4. Theoretically, the plastic limit (PL) of a soil is defined as the water content at which the soil changes from “semisolid” to “plastic” (Figure 1.4). For a given soil sample, this is an inherent property of the soil that can be determined by rolling a plastic soil sample into a worm shape to gradually reduce its water content by exposing more and more of an area until the soil becomes semisolid. This change can be detected by cracks appearing on the sample. According to ASTM 4318, the PL is the water content at which cracks develop on a rolled soil sample at a diameter of 3 mm. Thus, the procedure to determine the PL is one of trial and error. Although the apparatus (ground glass plate and moisture cans) used for the test is shown in Figure 1.5, the reader is referred to Bowles (1986) and Das (2002) for more details. On the other hand, the liquid limit (LL), which is visualized as the water content at which the state of a soil changes from “plastic” to “liquid” with increasing water content, is determined in the laboratory using the Casagrande liquid limit device (Figure 1.5). This device is specially designed with a standard brass cup on which a standard-sized soil paste is applied during testing. In addition, the soil paste is grooved in the middle by a standard grooving tool thereby creating a “gap” with standard dimensions. When the brass cup is made to drop through a distance of 1 cm on a hard rubber base, the number of drops (blows) required for the parted soil paste to come back into contact through a FIGURE 1.5 Equipment for the plastic limit/liquid limit tests. (Courtesy of the University of South Florida.) Page 6 distance of 0.5 in. is counted. Details of the test procedure can be found in Bowles (1986) and Das (2002). ASTM 4318 specifies the LL as the water content at which the standard-sized gap is closed in 25 drops of the cup. Therefore, one has to repeat the experiment for different trial water contents, each time recording the number of blows required to fulfill the closing condition of the soil gap. Finally, the water content corresponding to 25 blows (or the LL) can be interpolated from the data obtained from all of the trials. The plasticity index (PI) is defined as follows: PI=LL▯ PL (1.1) 1.2.3 Unified Soil Classification System In the commonly adopted USCS shown in Table 1.1, the aforementioned soil properties are effectively used to classify soils. Example 1.1illustrates the classification of the two soil samples shown in Figure 1.3. Definitions of the following two curve parameters are necessary to accomplish the classification: where D ii the diameter corresponding to the ith percent passing. Example 1.1 Classify soils A and B shown in Figure 1.3. Solution Soil A. The percentage of coarse-grained soil is equal to 70%. Therefore, A is a coarse- grained soil. The percentage of sand in the coarse fraction is equal to (70–30)/70×100 =57%. Thus, according to the USCS (Table 1.1), soil A is sand. If one assumes a clean sand, then 2 Cc(0.075) /(2×0.013)=0.21 does not meet criterion for SW (well-graded) Cu=(2)/(0.013)=153.85 meets criterion for SW Hence, soil A is a poorly graded sand, or SP (poorly graded). Soil B. The percentage of coarse-grained soil is equal to 32%. Hence, soil B is a fine- grained soil. Assuming that LL and PL are equal to 45 and 35, respectively (then PI is equal to 10 from Equation (1.1)), and using Casagrande’s plasticity chart (Table 1.1), it can be concluded that soil B is a silty sand with clay (ML or lean clay). 1.3 Effective Stress Concept Pores (or voids) within the soil skeleton contain fluids such as air, water, or other contaminants. Hence, any load applied on a soil is partly carried by such pore fluids in addition to being borne by the soil grains. Therefore, the total stress at any given location Page 7 TABLE 1.1 Unified Soil Classification System Division Description Group Identification Laboratory Classification Criteria Symbol More than More Clean GW Well graded Cu>4, 1<C c3 50% soil than 50% gravels gravels US 200 US No 4 GP Poorly graded Not meeting GW criteria sieve (4.75mm) (0.075 mm gravels Gravel with GM Silty gravel Falls below A line in the plasticity chart, or PI fines less than 4 GC Clayey gravel Falls above A line in the plasticity chart, or PI greater than 7 More Clean sand SW Well graded Cu>4, 1<C c3 than 50% sand passing SP Poorly graded Not meeting SW criteria US N 4 sand (4.75mm) Sand with SM Silty sand Falls below A line in the plasticity chart, or PI fines less than 4 SC Clayey sand Falls above A line in the plasticity chart, or PI greater than 7 More than Fine grained soils ML Inorganic silts 50% soil (LL<50) with low passing plasticity US 20 CL Inorganic clays sieve (0.075mm) with low plasticity OL Organic clays/silts with low plashcity Fine grained soils MH Inorganic silts (LL>50) with high plasticity Use the Casagrande Plasticity chart shown above CH Inorganic clays with high plasticity OH Organic clays/silts with low plasticity Highly organic soils Pt Page 8 within a soil mass can be expressed as the summation of the stress contributions from the soil skeleton and the pore fluids as ▯=▯ '+up (1.2) where ▯is the total stress (above atmospheric pres' is the stress in the soil skeleton (above atmospheric pressure), apd u is the pore (fluid) pressure (above atmospheric pressure). The stress in the soil skeleton or the intergranular stress is also known as the effective stress since it indicates that portion of the total stress carried by grain to grain contacts. In the case of dry soils in which the pore fluid is primarily air, if one assumes that all pores anywhere within the soil are open to the atmosphere through interporous connectivity, from Equation (1.2) the effective stress would be the same as the total stress: ▯'=▯ (1.3) On the other hand, in completely wet (saturated) soils, the pore fluid is mostly water and the effective stress is completely dependent on the pore water pwessure (u ). Then, from Equation (1.2): ▯'=▯▯ uw (1.4a) Using the unit weights of so▯ and waterw▯▯), Equation (1.4a) can be modified to a more useful form as shown in Equation (1.4b): (1.4b) where z is the depth of the location from the ground surface (Figurew1.6) and d is the depth of the location from the groundwater table (Figure 1.6). A detailed discussion of the unit weights of soil is provided in Section 1.6. Finally, in partly saturated soils, the effective stress is governed by both the pore water and pore air pressures (u ). For unsaturated soils that contain both air and water with a high degree a of saturation (85% or above), Bishop and Blight (1963) showed that ▯=▯ '+ua▯▯▯▯a▯uw) (1.5) where (ua▯uw) ▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯ parameter in the range of 0 to 1.0 that depends on the degree of saturation. One can verify the applicability of Equation (1.4a) for saturated soils based on Equation ▯▯▯▯▯▯▯▯▯▯▯▯▯▯ completely saturated soils. FIGURE 1.6 Illustration of in situ stresses. Page 9 1.4 Strength of Soils The two most important properties of a soil that a foundation engineer must be concerned with are strength and compressibility. Since earthen structures are not designed to sustain tensile loads, the most common mode of soil failure is shearing. Hence, the shear strength of the foundation medium constitutes a direct input to the design of structural foundations. 1.4.1 Drained and Undrained Strengths The shear strength of soils is assumed to originate from the strength properties of cohesion (c) and internal friction Using Coulomb’s principle of friction, the shear strength of a soil, can be expressed as (1.6) where ▯ n is the effective normal stress on the failure plane. More extensive studies on stress- strain relations of soils (Section 1.8) indicate that more consistent and reliable strength parameters are obtained when Equation (1.6) is expressed with respect to the intergranular or the effective normal stress. Hence, c and are also known as the effective strength parameters and sometimes indicated as cN and NN. It is obvious that the strength parameters obtained from a shear strength test conducted under drained conditions would yield effective strength parameters due to the absence of pore water pressure. Hence, the effective strength parameters cN and NN are also termed the drained strength parameters. Similarly, failure loads computed based on effective or drained strength parameters are applicable in construction situations that either do not involve development of pore water pressures or where an adequate time elapses for dissipation of any pore pressures that could develop. Effective strength parameters can also be obtained from any shear strength test conducted under undrained conditions if the pore water pressure developed during shearing is monitored accurately and Equation (1.6) is applied to estimate the shear strength in terms of the effective normal stress ▯.nOn the other hand, during any shear strength test conducted under undrained conditions, if Equation (1.6) is applied to estimate the shear strength in terms of the total normal stress ▯ , one would obtain an entirely different set of strength parameters c and N, which are called the total stress-based strength parameters. Using the concepts provided in the Section 1.7 and relevant stress paths, it can be shown that the total stress-based strength parameters are generally lower in magnitude than the corresponding effective stress parameters. From the discussion of soil strength it is realized that the measured shear strength of a soil sample depends on the extent of pore pressure generation and therefore the drainage condition that prevails during a shearing test. Hence, the type of soil and the loading rate expected during construction have an indirect bearing on the selection of the appropriate laboratory drainage condition that must be set up during testing. A wide variety of laboratory and field methods is used to determine the shear strength parameters of soils, c and The laboratory triaxial and discrete shear testing, the in situ standard penetration testing (SPT), static cone penetration testing (CPT), and vane shear testing (VST) are the most common tests used to obtain foundation design parameters. The determination of the strength parameters using SPT and CPT is addressed in detail in Chapter 2. Hence, only method of evaluating strength parameters based on the triaxial test will be discussed in this chapter. Page 10 1.4.2 Triaxial Tests In this test, a sample of undisturbed soil retrieved from a site is tested under a range of pressures that encompasses the expected field stress conditions imposed by the building foundation. Figure 1.7(a) shows the schematic of the important elements of a triaxial setup; the actual testing apparatus is shown in Figure 1.7(b). The pore pressure increase that can be expected during triaxial loading of a soil can be expressed using Skempton’s pore pressure parameters, A and B, for that particular soil as ▯ u=B▯ ▯3A▯▯ ▯1 ▯ ▯3 (1.7) ▯▯▯▯▯▯▯ ▯1▯▯▯▯▯ ▯3re the increments of the major and the minor principal stresses, respectively. When A and B for a given soil type are determined using a set of preliminary triaxial tests, one would be able to predict the magnitude of the pore pressure that would be generated in that soil under any triaxial stress state. It can be shown that, for saturated soils, B=1.0. An alternative way of expressing the pore pressure increase due to triaxial loading is as follows: (1.8) where a is the Henkel pore pressure parameter anoct and are octahedral normal and shear stresses defined, respectively, as ▯oct▯+▯1▯]23 3 (1.9a) (1.9b) FIGURE 1.7 (a) Schematic diagram of triaxial test. (From Concrete Design Handbook, CRC Press. With Permission.) (b) Triaxial testing apparatus for soils. (Courtesy of the University of South Florida.) Page 11 where ▯i2 the intermediate principal stress. Under the triaxial state of stress, Equations (1.9a) and (1.9b) simplify to ▯oct▯+21]/3 3 (1.10a) (1.10b) With respect to the drainage condition that is employed during testing, three types of triaxial tests can be conducted: (1) consolidated drained tests (CD), (2) consolidated undrained tests (CU), and (3) unconsolidated undrained tests (UU). In CU and CD tests, prior to applying the axial compression, the pressure of the cell fluid is used to consolidate the soil sample back to the in situ effective stress state that existed prior to sampling. On the other hand, in the UU tests, the cell pressure is applied with no accompanying drainage or consolidation, simply to provide a confining pressure. 1.4.2.1 Triaxial Testing of Rocks When foundations are designed on rocks, as in the case of pile foundations driven to bedrock and pile and drilled shaft foundations cast on bedrock, an accurate estimate of the shear strength of the in siturock is essential. A variety of methods is available in the literature (Goodman, 1989) to determine the shear strength of rock. Of them, the most accurate method of shear strength estimation is perhaps through triaxial testing. Triaxial testing is even more reliable for rock samples than in soils since sample disturbance is not a major issue in the case of rocks. Moreover, correlations that have been developed between the shear strength of rock and the unconfined compression strength (Section 1.4.3) and the rock quality designation (RQD) also provide convenient means of estimating the shear strength parameters of rocks. Further details of such correlations are provided in Section 6.10. Triaxial testing of rock samples is performed using a special apparatus that can sustain the relatively large confining pressures and deviator stresses that must be applied on rock samples to induce shear failure. A set of such apparatus is illustrated in Figure 1.8(a) and (b). 1.4.2.2 Selection of Triaxial Test Type Based on the Construction Situation The CD strength is critical when considering long-term stability. Examples of such situations are: 1. Slowly constructed embankment on a soft clay deposit 2. Earth dam under steady-state seepage 3. Excavation of natural slopes in clay On the other hand, CU strength is more relevant for the following construction conditions: 1. Raising of an embankment subsequent to consolidation under its original height 2. Rapid drawdown of a reservoir of an earthen dam previously under steady-state seepage 3. Rapid construction of an embankment on a natural slope Page 12 FIGURE 1.8 (a) Triaxial cell and membrane used in testing of rock samples. (b) Triaxial testing of rocks. Page 13 TABLE 1.2 Measured CU Triaxial Test Data Test Cell Pressure (kPa) Deviator Stress at Failure (kPa) Pore Pressure at Failure (kPa) 1 20 20.2 5.2 2 40 30.4 8.3 Finally, the UU strength is applicable under the following conditions: 1. Rapid construction of an embankment over a soft clay 2. Large dam constructed with no change in water content in the clay core 3. Footing placed rapidly on a clay deposit 1.4.2.3 Computation of Strength Parameters Based on Triaxial Tests Computations involving CU and UU tests are given in Examples 1.2 and 1.3, and the reader is referred to Holtz and Kovacs (1981) for more details of the testing procedures. Example 1.2 Assume that one conducts two CU triaxial tests on a sandy clay sample from a tentative site in order to determine the strength properties. The applied cell pressures, deviator stresses, and measured pore pressures at failure are given in Table 1.2. The strength parameters can be estimated using the Mohr circle method as follows: Solution Total strength parameters. The total stresses (▯1nd ▯) 3cting on both test samples at failure are indicated in Figure 1.9(a). Accordingly, the Mohr circles for the two stress states can be drawn as shown in Figure 1.10. Then the total strength parameters (also referred to as the undrained strength parameters) can be evaluated from the slope of the direct common tangent, which is the Coulomb envelope (Equation (1.6)), plotted on the Mohr circle diagram as c=4.0 kPa and It is obvious that the generated pore pressure has been ignored in the above solution. The most appropriate applications of c and obtained above are cases where foundations are rapidly constructed on a well-consolidated ground. Effective strength parameters. The effective stresses on both (saturated) test samples at failure are computed by subtracting the pore pressure from the total stress (Equation (1.4a)), as indicated in Figure 1.9(b). The Mohr circles corresponding to the two stress FIGURE 1.9 Stress states at failure for Example 1.2: (a) total stress (kPa); (b) effective stress (kPa). (From Concrete Design Handbook, CRC Press. With permission.) Page 14 FIGURE 1.10 Mohr circle diagram for a CU test in Example 1.2. (From Concrete Design Handbook, CRC Press. With permission.) states are shown in Figure 1.10. The effective strength parameters (also referred to as the drained strength parameters) can be found from the slope of the Coulomb envelope for effective stresses plotted on the Mohr circle diagram as The most appropriate applications of the c' and are cases where found ati constructed rather slowly on a well-consolidated ground. Example 1.3 Assume that one wishes to determine the strength properties of a medium stiff clayey foundation under short-term (undrained) conditions. The most effective method for achieving this is to conduct a UU (quick) test. For the results presented in Table 1.3, estimate the undrained strength parameters. Solution In these tests, since the pore pressure generation is not typically monitored the total stresses can be plotted, as shown inFigure 1.11. From Table 1.3, it can be seen that the deviator stress at failure does not change with the changing cell pressure during UU tests. This is because, in UU tests, since no drainage is permitted the soil samples are not consolidated to the corresponding cell pressures. Therefore, the soil structure is largely unaffected by the change in cell pressure. Hence, the following strength parameters can be obtained from Figure 1.11: TABLE 1.3 Measured UU Triaxial Test Data Test Cell Pressure (kPa) Deviator Stress at Failure (kPa) Pore Pressure at Failure (kPa) 1 40 102.2 NA 2 60 101.4 NA Page 15 FIGURE 1.11 Mohr circle diagram for a UU test for Example 1.3. (From Concrete Design Handbook, CRC Press. With permission.) It should be noted that the subscript “u” is used to distinguish the UU test parameters. Under UU conditions, if Equation (1.6) is applied, then the undrained shear strength s =cu. u The most critical foundation design scenario presented by saturated, slow draining soils such as clays and silts involve undrained conditions prevailing immediately after the foundation is constructed. Therefore, the undrained shear strength (s ) us typically used to design foundations on soils where the predominant soil type is clay or silt. 1.4.3 Unconfined Compression Test Very often, it is convenient to use the unconfined compression strength to express the undrained shear strength of clayey soils especially when in situtests are used for such determinations. An unconfined compression test can be used to determine the c valueu based on the measured unconfined compression strength (q ). Sunce this test can be visualized as an undrained triaxial test with no confining pressure (hence unconsolidated), the Mohr circle for stress conditions at sample failure can be shown as in Figure 1.12. Then, it can be seen that (1.11) The same triaxial apparatus including the loading frame shown in Figure 1.8 can be used to test a clayey soil sample under unconfined compression conditions as well. Example 1.4 Determine the unconfined compression strength and the undrained shear strength of the soil tested in unconfined compression conditions as shown in Table 1.4. Solution The compression test data in Table 1.4 are plotted in Figure 1.13. From Figure 1.13, the unconfined compression strength is determined to be 320 kPa. Therefore, from Equation (1.11), the undrained strength of the clay is estimated to be 160 kPa. Page 16 FIGURE 1.12 Mohr circle plot for failure stress condition in unconfined compression test. 1.5 Compressibility and Settlement Soils, like any other material, deform under loads. Hence, even if the condition of structural integrity or bearing capacity of a foundation is satisfied, the ground supporting the structure can undergo compression, leading to structural settlement. In most dry soils, this settlement will cease almost immediately after the particles readjust in order to attain an equilibrium with the structural load. For convenience, this immediate settlement is evaluated using the theory of elasticity although it is very often nonelastic in nature. TABLE 1.4 2 Data for Example 1.8 (Height of Sample—7.5cm; Cross-Sectional Area of Sample—10.35cm ) Vertical Displacement (mm) Axial Force (N) Strain (%) Stress (kPa) 0.030 23.478 0.04 22.68 0.315 52.174 0.39 50.22 0.757 71.739 0.95 68.66 1.219 90.000 1.52 85.64 1.666 106.957 2.08 101.20 2.179 127.826 2.72 120.15 2.682 143.478 3.35 133.99 3.152 163.043 3.94 151.34 3.612 211.304 4.51 194.96 4.171 240.000 5.21 219.82 4.740 260.870 5.92 237.14 5.291 280.435 6.61 253.06 5.850 300.000 7.31 268.69 6.340 314.348 7.92 279.68 7.224 358.696 9.03 315.30 7.991 365.217 9.99 317.65 8.623 349.565 10.78 301.37 9.360 290.870 11.70 248.18 Page 17 FIGURE 1.13 Plot of the unconfined compression test results in Example 1.4. However, if the ground material consists of wet, fine-grained (low permeability) soil, the settlement will continue for a long period of time with slow drainage of water accompanied by the readjustment of the soil skeleton until the excess pore water pressure completely dissipates. This is usually evaluated by Terzaghi’s consolidation theory. In some situations involving very fine clays and organic soils, settlement continues to occur even after the pore water pressure in the foundation vicinity attains equilibrium with that of the far field. Secondary compression concepts introduced later in this chapter are needed to estimate this prolonged secondary settlement. 1.5.1 Estimation of Immediate Settlement in Soils The most commonly adopted analytical methods for immediate settlement evaluation in soils are based on the elastic theory. However, one must realize that reliable estimates of elastic moduli and Poisson ratio values for soils are not easily obtained. This is mainly because of the sampling difficulty and, particularly, the dependency of the elastic modulus on the stress state. On the other hand, reliable field methods for obtaining elastic moduli are also scarce. Very often, settlement of footings founded on granular soils or unsaturated clays is determined on the basis of plate load tests (Chapter 4). The following expression can be used to determine the immediate settlement (Bowles, 1896): (1.12) where ▯is a factor to be determined from Figure 1.14, B is the width of the foundation, L is the length of the foundation, q i0 the contact pressure (P/BL), s is ehe immediate settlement, E ss the elastic modulus of soil, v ss the Poisson ratio of soil, and f is equal to 0.5 or 1.0 (depending on whether s is evaluated at the corner or center of the foundation). Another widely used method for computing granular soil settlements is the Schmertmann and Hartman (1978) method based on the elastic theory as well: Page 18 FIGURE 1.14 Chart for obtaining the ▯factor. (1.13) where Izis the strain influence factor in Figure 1.15 (Schmertmann and Hartman, 1971), C is the foundation depth correction factor (0.5[q▯▯▯▯ q)]), 2 is the correction factor for ▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯is the stress at the foundation level (=P/BL), and q is the overburden stress at the foundation lev▯▯.▯▯ FIGURE 1.15 Strain influence factor. Page 19 TABLE 1.5 Poisson Ratios(▯) for Geomaterials Type of Soil ▯ Clay, saturated 0.4–0.5 Clay, unsaturated 0.1–0.3 Sandy clay 0.2–0.3 Silt 0.3–0.35 Sand, gravelly sand ▯0.1 to 1.00 Commonly used 0.3–0.4 Rock 0.1–0.4 (depends somewhat on type of rock) Loess 0.1–0.3 Ice 0.36 Concrete 0.15 Steel 0.33 Source: From Bowles, J.E., 2002, Foundation Analysis and Design, McGraw-Hill, New York. With permission. The elastic properties needed to manipulate the above expressions are provided in Tables 1.5 (Bowles, 1995) and Table 1.6, where the author, based on his experience, has extracted approximate values from Bowles (1995) for most common soil types. 1.5.1.1 Elastic Properties and In Situ Test Parameters The most commonly used in situtests that can be used to determine elastic properties of soil are the SPT and CPT tests (discussed in Chapter 2). Some useful relationships that can provide the elastic properties from in situtest results are given in Table 1.7. However, in TABLE 1.6Approximate Elastic Moduli of Geomaterials Soil Type Elastic Modulus (MPa) Soft clay 2▯ 25 Medium clay 15–50 Stiff clay 50–100 Loose sand 10–20 Medium dense sand 20–50 Dense sand 50–80 Loose gravel (sandy) 50–150 Dense gravel (sandy) 100–200 Silt 2–20 Page 20 TABLE 1.7Soil Elastic Moduli from In SituTest Data Soil SPT CPT Sand (normally consolidated) Sand (saturated) Es=250(N+15) E sFq c e=1.0, F=3.5 e=0.6, F=7.0 Sands, all (norm, consol.) ¶Es=(2,600–2900)N Sand (overconsolidated) E s(6–30)q c Gravelly sand E =1,200(N+6) s =600(N+6) N<15 =600(N+6)+2,000 N>15 Clayey sand Es=320(N+15) E s(3–6)q c Silts, sandy silt, or clayey silt E s(1▯ 2)qc Soft clay or clayey silt E =(3▯ 8)q s c *E selastic modulus) for SPT (Standard penetration test) and units c for CPT (Cone penetration test). Notes: E sn kPa for SPT and units of qcfor CPT; divide kPa by 50 to obtain ksf. The N values should be estimated as N55 and not N70 Source: From Bowles, J.E., 2002, Foundation Analysis and Design, McGraw-Hill, New York. With permission. foundation engineering, it is also common to assume the following approximate relations with respect to granular soils: E stsf)=8N (1.14a) E (kPa)=768N s (1.14b) where N is the SPT blow count, and E s2q c (1.15) where q is the cone resistance in CPT measured in units of stress; E and q have the same c s c units. A comprehensive example illustrating the use of the above relations is provided in Section 3.3. 1.5.2 Estimation of Foundation Settlement in Saturated Clays When the foundation load is applied on a saturated fine-grained soil, it is immediately acquired by the pore water, as illustrated in Figure 1.16(a). However, with the gradual dissipation of pore pressure accompanied by drainage of water, the applied stress (total stress, ▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯Figure 1.16b). The longterm rearrangement of the soil skeleton and the consequent foundation settlement that take place during this process is known as the phenomenon of consolidation settlement.

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