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FOUNDATION ENGINEERING FOR DIFFICULT SUBSOIL CONDITIONS

FOUNDATION ENGINEERING FOR DIFFICULT SUBSOIL CONDITIONS Leonardo Zeevaert Second Edition

Inii5I

VAN NOSTRAND REINHOLD COMPANY

~

New York

Cincinnati

Toronto

London

Melbourne

Copyright © 1983 by Van Nostrand Reinhold Company Inc. Library of Congress Catalog Card Number: 82-1877 ISBN: 0-442-20169-9 All rights reserved. Certain portions of this work copyright © 1972 by Van Nostrand Reinhold Company Inc. No part of this work covered by the copyright hereon may be reproduced or used in any form or by any means-graphic, electronic, or mechanical, including photocopying, recording, taping, or information storage and retrieval systems-without permission of the publisher. Manufactured in the United States of America Published by Van Nostrand Reinhold Company Inc. 135 West 50th Street, New York, N.Y. 10020 Van Nostrand Reinhold Publishing 1410 Birchmount Road Scarborough, Ontario MIP 2E7, Canada Van Nostrand Reinhold Australia Pty. Ltd. 17 Queen Street Mitcham, Victoria 3132, Australia Van Nostrand Reinhold Company Limited Molly Millars Lane Wokingham, Berkshire, England 15 14 13 12 II 10 9 8 7 6 5 4 3 2 I

Library of Congress Cataolging in Publication Data Zeevaert, Leonardo, 1914Foundation engineering for difficult subsoil conditions. Includes bibliographies and index. I Foundations. 2. Soil mechanics. 1. Title. TA775.z45 1982 624.1'5 82-1877 ISBN 0-442-20169-9 AACR2

PREFACE TO FIRST EDITION

Throughout thirty years of professional practice in such difficult subsoil conditions as those encountered in the seismic area of Mexico City, the author has had the benefit of observing and designing many large foundations. The new concepts and working hypotheses given in this book are based on this experience, in order to achieve better designs on a rational basis, reducing practical rules that in the past have resulted in poor performance of building foundations. In the engineering profession it is necessary to investigate continuously the physical laws of soil behavior and soil masses, to be able to eliminate the guesswork supported by empirical generalizations. Statistics, however, is a valuable research tool in investigating the general trend of the phenomena and an aid to establish theories and working hypotheses when deviations from the statistical laws established are understood and carefully observed. Several good books on soil mechanics, foundations and engineering geology have been written, in which the foundation engineer can study the general aspects of design and construction in foundation engineering. The scope of this book is to supplement this literature with basic technical fundamentals, pointing out the problems that may be encountered in practice when the foundation is involved with difficult subsoil conditions. Therefore, the writer assumes the reader is acquainted with the current literature on this subject. Foundation engineering is not an exact science. Nevertheless, sufficient precision is required to assure a successful foundation design and construction. This goal is achieved when the behavior in the field complies within the predictions and factors of safety used, thus obtaining a satisfactory performance without sacrificing economy. Difficult subsoil conditions may be defined as those encountered in soil sediments of medium to very high compressibility and medium to very low shear strength extending to great depth, and in those where the hydraulic conditions play v

vi

PREFACE TO FIRST EDITIOhl

an important role, as well as when the soil deposits are found in areas subjected to strong ground motions induced by earthquakes. Under these environmental conditions, the foundation engineer is compelled to use all the knowledge and experience he has gained in soil and foundation engineering, sampling and testing of materials. The aspects of engineering geology in recognizing the engineering characteristics of the subsoil used for foundations are of primary importance, since it is recognized that the behavior of a small soil sample is not representative of that of the entire deposit or strata encountered. It should be kept in mind that the foundation engineer has to work with soil deposits that are far from being isotropic and homogeneous. Therefore, his understanding of the behavior of the subsoil can only be complete after considering the real conditions that may be expected from a geological point of view. Allowance should be given in all engineering designs, using a factor of safety to cover the deviations of the theories and working hypotheses, the mechanical properties of the material, and construction procedures that may also deviate to a certain degree from design considerations. The selection of a factor of safety should be based on the knowledge the foundation engineer has obtained from the environmental conditions and forces involved, namely, the geological and physiographical conditions, hydraulic and mechanical properties of the sediments, as well as the functional requirements of the project for which the foundations should be designed. All these elements should be made compatible with the economy of the design; therefore, the precision required in the calculations is summarized by the ability of the foundation engineer to manipulate the laws, theories and working hypotheses that may be available in soils and foundation engineering to a degree to which he has gained confidence from experience. This book specially emphasizes this approach as strictly necessary to be able to perform a rational and successful design. In order to avoid mentioning "approximate method" throughout this book, the author wishes to point out that actually in civil engineering and mostly in foundation engineering, there is not such a thing as an "exact method or theory." All the methods proposed in this book have a degree of accuracy, or shall we say, an uncertainty acceptable from the practical engineering point of view. Nevertheless, it is true that some methods are more reliable than others for the problems encountered in practice. The uncertainty of a particular method is covered by the corresponding factor of safety, which as mentioned before, should also cover not only the so-called theory, but also the deviations of any other environmental forces found under field conditions. Therefore, foundation engineering requires experience of field behavior and of the deviations obtained from the theoretical design calculations. Moreover, one should not forget that theories and methods of design in civil engineering are subjected to further investigations, as more experience is gained with time. Therefore, theories have to be established under simplified assumptions covering, in the best possible manner, the mechanics expected under real conditions. Often, because of the nonuniform characteristics encountered, it would be a waste of time-or rather an illusion-to try to approximate the solution of a problem to an unreal accuracy. The decision depends on the ability of the foundation engineer to visualize the problem and make a good estimate that will enable

PREFACE TO FIRST EDITION

vii

him to obtain sufficient precision and economy in the design. Nevertheless, it should be kept in mind tilat during construction the design expectation may be somewhat altered. Construction methods should go together with theoretical design, and the factor of safety selected accordingly. Chapter II has been prepared as a review of the mechanical properties of difficult soils, advancing some concepts of approach, mainly in the field of fine sediments exhibiting intergranular viscosity. The methods exposed have been used by the author satisfactorily for several years. They have suffered theoretical adjustments since first published to obtain better correlations with behavior observed in the field. In deformation problems, the soil should be considered a two-phase material. The solid phase represented by the skeleton structure and the liquid phase represented by the water should be studied separately. This implies knowledge of the stress-strain-time properties of the materials and of the stress dissipation in the soil mass due to load application, as well as of the state of hydraulic pressures and their changes imposed during construction or other environmental conditions. Chapters II and III have been prepared to review these concepts, providing the practicing foundation engineer, in Chapter III, with stress nets to facilitate estimates of stress changes. The theoretical background to trace flow nets in different foundation problems is also reviewed. The use of well systems to dewater excavations is treated. At the end of Chapter III, the problem on stability and bearing capacity is discussed. Bearing capacity factors for deep foundations are given based on current theoretical considerations; the result given, however, is not more than another theoretical essay on bearing capacity complying with the experience of the author. In Chapters IV, VI and VII an attempt is made to introduce the foundation engineer to the complex field of sub grade reactions. This may be considered where the foundation and structural engineers meet. Furthermore, the author believes, from his experience, that soil mechanics and foundations cannot be divorced from design of the foundation structure, since there must exist compatibility between these two branches of civil engineering. The unit foundation modulus, also called the "coefficient of subgrade reaction," is a variable function of the geometry of the loaded area, the subgrade reaction distribution, and the mechanical properties of the subsoil for the stress level applied. The foundation structural problem becomes very complicated when the foundation structure is in itself a statically indeterminate structure. The only means to solve these complicated problems in a practical manner is by means of simplified working assumptions, reducing the unknowns to a number that may be handled by current methods. The methods given in the book may be used by the experienced foundation engineer. Nevertheless, since all of them give only particular solutions, they will only serve as a guide to establish a school of thOUght. The final assumptions and methods of calculation, however, call for the skill and experience of the foundation and structural engineers involved in the solution of the particular problem, to establish the best and most practical procedures. Computer programs may be written to facilitate and speed up the calculations. The ground surface subsidence occurring in difficult subsoil conditions and the

viii

PREFACE TO FIRST EDITION

implications of this phenomenon in civil engineering works cannot be vv..:rlooked, since in most occasions, difficult and complex problems may be encountered. The illustration and deduction of working hypotheses to evaluate these problems and their effects in foundation engineering may be explained more simply by means of a case history, as used by the author in Chapter V. The behavior of friction piles is an important item in foundation engineering, mainly in those problems related with negative skin friction in piles and piers. Chapter VIn has been devoted to explain the mechanics and use of friction piles, based on an ultimate skin friction theory. The methods of calculation are also given; their applications are studied in Chapter IX for the friction pile compensated foundation, and in Chapter X for negative friction on point bearing piles and piers. These methods of calculation have been used extensively by the author with satis· factory results, and are published for the first time to their full extent in this book. The process of performing excavations is an important factor in the future be· havior of foundations requiring deep excavations. The water flow induced by deep pumping produces changes in the effective stresses in the soil mass, affecting the stability and deformation during excavation. The approach to these problems is treated in Chapter XI; however, the reader should be acquainted first with Chapters III and VII. There are many places in the world with difficult subsoil conditions subjected to destructive earthquakes, where it is necessary to investigate the behavior of foundations to be able to perform a rational and safe design. For this purpose, the foundation engineer should investigate the probable behavior of the subsoil mass under strong ground motions. Chapter XII was prepared with the aim of introducing the foundation engineer to seismic foundation engineering. With this in mind, the author has taken the case history of Mexico City, where field information on strong earthquakes is available. The contents of sections 3, 4, 5 and 6 of Chapter XII are given for the first time in this book. They may be taken as an advance and guidance from investigations carried on in this subject. Although the foundation engineer is compelled to generalize the subsoil conditions to be able to produce workable and practical methods of computation, this generalization should be made on a sound and rational basis using all the power of soil mechanics he has at his disposal, considering, moreover, that in nature there is no such thing as an isotropic subsoil condition. The mechanical properties of soils are more complex than any other engineering material. Therefore, the only means is to use the closest representative theories and working hypotheses that may be compatible with the behavior observed in the field, and from there establish the most simple correlation satisfying the statics of the problem. The development of theories is necessary to establish the basis of comparison with real behavior in the field, and accordingly, screen out inconsistencies with the aim of obtaining more reliable and technical methods of approach. The bibliography in soil mechanics is very extensive at present, and has grown considerably in each country where basic research is carried on. The obtention of published material and the thorough study and selection of its contents, with the

PREFACE TO FIRST EDITION

ix

barrier of languages, is becoming a gigantic task beyond the possibilities of an individual. Therefore, the author wishes apologize if some important publications on the subject treated in this book have escaped his attention. The selected bibliography given to each chapter is intended only to contribute in the understanding of the corresponding chapter. The main content of this book is the compilation of the work of the author during his professional practice, which has been gradually added to by experienced colleagues in the field to whom the author is greatly indebted, mainly on the intergranular viscosity of soils, the critical stress in preconsolidated soils and hardening, the plastic theory to estimate friction in piles, the dewatering of excavations to reduce heave, the injection of water outside excavations to reduce settlements, and the drifting forces on underground elements, motivated by strong ground motions due to earthquakes. The author is highly indebted to his nephew, Mr. Adolfo E. Zeevaert, C. E., M.Sc., for his great help and interest during the preparation of the manuscript, in the calculation of graphs and tables, checking formulas and practical illustrative examples used in the text, and in the Appendices. The author wishes ~lso to extend his appreciation to his secretary, Mrs. Diana A. de Balseca, for the arduous task she has taken in typing the manuscript, and finally, to the editor, whose interest in this book contributed in a presentation beyond the aim of the author.

to

Mexico, D. F.

Leonardo Zeevaert, Ph.D., C.E. Professor of Soil Mechanics and Foundations at the Faculty of Engineering, U.N.A.M. Consulting Civil Engineer.

PREFACE TO SECOND EDITION

In the eight years since the appearance of first edition, and through its use in the courses given by the author at the Graduate School of Engineering of the V.N.A.M., the author has improved the content of several chapters. These improvements have been included in the second edition to make it more explicit and practical for graduate courses and foundation engineering practice. All the chapters, however, have been revised. In Chapter II, new and more precise formulas are given to estimate vertical displacement due to the intergranular viscosity phenomenon. The basic principles, however, have been retained until future investigations may show a more accurate and practical method to be used. Chapter III has been extended to include, in the solid phase, formulas to calculate ground stresses for surface rectangular loaded areas and for different values of Frohlich's concentration factor. Also, theoretical methods of calculating the reduction of piezometric water levels in stratified subsoils and of estimating the depressed water table in well groups for excavation purposes have been added. A completely new Chapter VI has been written to include the most recent practical methods developed by the author regarding soil-structure foundation interaction considering the importance of knowing the approximate value of the subgrade reactions in foundation structural design. (See L. Zeevaert, 1980, ISE.) Chapters IV, V and VII to XI have been revised, and more on soil-structure interaction has been added to Chapter X. Chapter XII has been enlarged to include a practical and rational method of estimating the loss of bearing capacity in loose cohesionless soils during strong ground motions induced by earthquakes. A method is included for computing the seismic rocking phenomenon of box type foundations for tall buildings supported on stratified subsoil conditions. At the end of the chapter, a general method is given xi

xii

PREFACE TO SECOND EDITION

for estimating the seismic soil-pile interaction behavior, including illustrative numerical examples. Finally, in Appendix E, new numerical examples for Chapters VI and VII are presented with the purpose of illustrating the methods of computation for soilstructure interaction given in Chapter VI. The author has considered that nowadays the practicing foundation engineer is getting more and more involved in matrix algebra calculations he can perform with his desk computer, therefore more matrix algebra has been used in the book. With this in mind the author has given ready to use algorithms and methods of computation that will permit the practicing foundation engineer to write his own programs to expedite his calculations with an approximation compatible with the practical problems involved. Especially interesting along this line, are the calculations to estimate the ultimate skin friction in piles, subsoil seismic behavior, the soil-structure interaction of compensated mat foundations, the seismic rocking phenomenon and the behavior of piles, piers or vertical shafts subjected to strong ground motions. In the soil-structure interaction problems, the foundation engineer should carefully select the secant stress-strain parameters for the increment of stress and stress levels involved, as described in Chapters II and VII. The author is indebted to Miss Eloisa E. Rey, C. E., M.I., for her great help and interest in assisting the author to revise the new additions, formulas and examples for the second edition, and to the editor for his interest that this book should continue to be up-to-date, and serve the advanced student and professional practicing foundation engineer for consultation in his every day work. Mexico, D. F. Leonardo Zeevaert Professor of Soil Mechanics and Foundation Engineering Faculty of Engineering, U.N .A.M.

CONTENTS

v

Preface INTRODUCTION 1.1 Typical Foundations

1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8

Isolated Footings Continuous Footings Raft Foundation Compensated Foundations Compensated Foundations with Friction Piles Point Bearing Pile Foundations Pier Foundations Sand Pier Foundations

1.2 Subsoil Sediments 2.1 2.2 2.3 2.4 2.5 2.6 2.7

Residual Soils Eolian Deposits Alluvial Deposits Lacustrine and Marine Sediments Piemont Deposits Recent Volcanic Deposits Glacial Deposits

1.3 Total and Differential Allowable Settlements 1.4 Summary Bibliography

II MECHANICAL PROPERTiES OF SOIL

11.1 Introduction 112 Permeability

11.3 Stress-Strain-Time Relationships 3.1 General Concepts 3.2 The Elastic Unit

1 1 2 4 6 7 8 9 12 13

15 15 16 17 17 17 17 18

18 23 25 26 26 28 32 32 37 xiii

xiv CONTENTS

3.3 3.4 3.5 3.6

The Plastic Unit The Elasto-Plastic Unit The Strain Modulus The Compressibility of Fine Sediments 3.6a Normally Loaded and Preconsolidated-Type Sediments 3.6b Expansive or Swelling Soils 3.6c Collap,~ible Soils 3.6d Compaction and Desiccation 3.7 Linear Strain Modulus, Function of Confining Stress 3.8 Linear Strain Modulus, Function of Time 3.9 The Theory of Consolidation 3.10 Viscous Unit of Linear Fluidity 3.11 The Kelvin-Terzaghi Relationships 3.12 Theory of Consolidation When Load Increases Linearly with Time 3.13 The Intergranular Viscosity in Saturated Soil Sediments 3.13a The Z-Unit 3.13b The Strain-Time Behavior for Rapid Load Application 3.13c Strain-Time Behavior for Linear Load Application 3.14 Intergranular Viscosity in Saturated Soils with Cavities 3.15 Intergranular Viscosity in Nonsaturated Soils 3.16 The Use of Kv-Value in Soils Exhibiting Intergranular Viscosity 3.17 Parameter Determination: Fitting Methods 11.4 Shear Strength 4.1 Basic Concepts 4.2 Coulomb-Terzaghi's Equation 4.3 Coulomb-Mom's Failure Concept 4.3a The Drained Shear Strength 4.3b The Consolidated-Undrained Shear Strength 4.3c The Undrained Shear Strength 4.4 Determination of the Average Shear Parameters c and I/> 4.5 Coulomb-Mohr's Plasticity Condition 4.6 Rheological Considerations 4.7 Shear Strength Applications 4.8 Conclusions Bibliography III SOLID AND LIQUID PHASES OF SOIL 111.1 Basic Concepts 111.2 Solid Phase 2.1 Effective Stresses 2.2 Stress Distribution in Soil Mass 2.3 The Stress Nets 2.4 Stratified Soil Masses 2.5 Vertical Displacements of Rigid Footings

38 39 42 47 52 56 57 61 62 70 72 78 81 82 85 90 95 96 102 104 105 106

114 114 115 118 121 121 123 125 125 127 134 139

141 144 144 145 145 149 160 168 173

CONTENTS xv

111.3 Hydraulic Pressures: Liquid Phase

3.1 Water Flow Components 3.1 a Downward Flow 3.1b Upward Flow 3.2 The Flow Net 3.2a Isotropic Soil Mass 3.2b Stratified Soil Mass 3.3 Average Coefficients of Permeability 3.4 Vertical Flow in Stratified Soil Deposits 3.5 Dewatering by Wells 3.5a Study of a Single Well 3.5b Study of Well Groups 3.6 Ratio of the Discharge in One Well and in a System of Wells 111.4 Shear Strength Behavior in Soil Mass

205

206 206 207 207 209

Bibliography

214

4.1 Shear Correction 4.2 Moment Correction

V

176 179 180 181 183 186 187 189 193 194 197

4.1 Basic Considerations 4.2 Bearing Capacity 4.2a Shallow Footings 4.2b Deep Footings

IV SUBGRADE REACTION IV.1 General Considerations IV.2 Foundation Modulus IV.3 Rigid Foundations IV.4 Bending Moments and Shears in Rigid Foundation

IV.5

176

Recommended k-Values Bibliography

GROUND SURFACE SUBSIDENCE V.1 Introduction V.2 Mechanics of Ground Surface Subsidence V.3 Ground Surface Subsidence in Mexico City

3.1 3.2 3.3 3.4

216

216 217 217 229 229

231 232 234 237 237 238 248

General and Historical Considerations Subsoil Conditions General Soil Properties Piezometric Pressure and Surface Subsidence Measurements 3.5 Foundation Problems 3.5a General Considerations 3.5b Case I: Water Wells 3.5c Case II: Shrinkage Problem 3.5d Case III: Buildings on Surface Foundations 3.5e Case IV: Buildings on Piles

248 250 255

Bibliography

273

260 263 263 264 264 267 269

xvi CONTENTS

VI

VII

SOIL·FOUNDATION STRUCTURE INTERACTION VL1 Introduction VL2 Soil-Structure Interaction VL3 Soil-Structure Interaction Matrix Equation Bibliography

COMPENSATED FOUNDATIONS VIL1 Basic Concepts Shear Strength VIL2 VIL3 Compressibility and Critical Stress Plastic Flow VilA Elastic Heave and Subsequent Settlement VIL5 VII.6 Lateral Contraction and Settlement Outside the Excavation VIL7 Methods to Reduce Heave VIL8 Overturning Moment and Base Shear

8.1 8.2 8.3 8.4

Introduction Elastic Response Permanent Tilt Base Shear

VII.9 Bending Moments and Shears in the Foundation Structure VII.10 Practical Considerations Bibliography VIII

ULTIMATE LOAD CAPACITY OF PILES AND PIERS

VII 1.1 Introduction VII 102 Point Bearing Capacity 2.1 Theory Review 2.2 Pile Groups 2.3 Point Vertical Displacements 2.4 Pile Group Bearing Capacity VII 1.3 Negative Friction 3.1 Basic Concepts 3.2 Effective Tributary Area 3.3 Use of Influence Charts 3.4 Confining Stress at the Pile Point Elevation 3.5 Allowable Point Bearing Load VII 104 Positive Friction on Piles 4.1 Basic Concepts 4.2 Skin Friction Considerations 4.3 Pile Group as a Single Unit VII 1.5 Behavior of Pile Fields Based on Mechanical Models 5.1 Basic Considerations 5.2 CASE I: Positive Friction, No Point Resistance 5.3 CASE II: Positive Friction and Point Resistance 5.4 CASE III: Negative Friction 5.5 CASE IV: Negative Friction Used for Building Support 5.6 CASE V: Restriction of Stress Relief in Soil Mass 5.7 CASE VI: Friction to Reduce Consolidation of Soil Mass

275 275 278 285 288 290 290 294 295 299 300 310 314 317

317 317 324 326 326 331 332 333 333 339

339 341 347 349 351

351 361 367 370 371 372

372 374 377 381

381 383 385 386 387 389 391

CONTENTS xvii

5.8 CASE VII: Friction Piles in Consolidating Mass Conclusions VIII.6 Field Loading Tests on Piles and Their Limitations 6.1 Basic Concepts 6.2 Friction Pik in Cohesive Soil 6.3 Point Bearing Piles in Sand 6.4 Vertical Displacement of Single Pile Tests, and Pile Groups VII1.7 Review on Pile Selection and Driving 7.1 Project Req uiremen ts 7.2 Structural Loads-Subsoil Exploration 7.3 Pile Foundation-Selection of Pile Type 7.4 Pile Types Most Commonly Used 7.5 Pile Driving and Control Bibliography IX DESIGN OF COMPENSATED FRICTION PILE FOUNDATIONS IX.1 General Considerations IX.2 Friction Pile Raft Foundations IX.3 Compensated Foundations With Friction Piles 3.1 General Considerations 3.2 Heave Problem 3.3 Load Reapplication 3.4 Importance of Point Resistance IX.4 Overturning Moments IX.5 Bending Moments and Shears Bibliography X DESIGN OF POINT BEARING PILES AND PIER FOUNDATIONS X.1 General Considerations X.2 Point Bearing Pile Foundations 2.1 Typical Cases of Point Bearing Piles 2.2 Case I 2.3 Case II 2.4 Case III X.3 Pier Foundations X.4 Overturning Moments and Base Shear 4.1 Tilting of Foundation 4.2 Tilting Control for Pile Foundations X.5 Shears and Bending Moments Bibliography XI STABILITY OF DEEP EXCAVATIONS FOR FOUNDATIONS XI.1 General Considerations XI.2 Sheet-Pile Wall 2.1 Lateral Support 2.2 Timber 2.3 Concrete 2.4 Steel XI.3 Pressures on the Sheet-Pile Wall XI.4 Dewatering of Excavations

393 395 395 395 398 401 404 406 406 408 409 413 416 420 422 422 424 431 431 431 435 438 439 439 440 441 441 444 444 444 446 448 452 455 455 456 458 460 461 461 467 467 468 469 470 470 475

xviii CONTENTS

XI.5 Stability of the Bottom of the Excavation Bibliography XII INTRODUCTION TO EARTHQUAKE PROBLEMS IN BUILDING FOUNDATIONS XI1.1 General Considerations XI1.2 Earthquake Engineering Characteristics 2.1 Introduction to Seismic Waves 2.2 Magnitude 2.3 Intensity 2.4 Earthquake Recording 2.5 Response Spectrum XI1.3 Subsoil Behavior 3.1 Basic Concepts 3.2 Resonant Periods of Vibration in Stratified Subsoil 3.3 Contribution of Vibration Modes in the Ground Motion 3.4 Problems Induced by Longitudinal Waves 3.5 Reduction of the Bearing Capacity Because of Seismic Action XII.4 Shear Modulus of Elasticity 4.1 Basic Concepts 4.2 The Free Torsion Pendulum 4.3 Results XII.5 Seismic Behavior of Building Foundations 5.1 Introduction 5.2 Foundation Response 5.3 Seismic Base Shear XI1.6 Seismic Behavior of Underground Structures 6.1 General Considerations 6.2 Vertical Shafts, Piles and Piers 6.3 Underground Pipes and Tunnels Bibliography APPENDIX A LIST OF SYMBOLS APPENDIX B INFLUENCE STRESS NETS AND CHARTS APPENDIX C INTEGRATION FORMULAS FOR SKIN FRICTION PROBLEMS IN PILE FIELDS APPENDIX D CONVERSION TABLES FROM METRIC (CGS) TO THE ENGLISH SYSTEM APPENDIX E NUMERICAL EXAMPLES TO CHAPTERS IV, VII, AND VIII Example A.IV Calculation of a Semiflexible Foundation Example B.IV Rigid Box Type Foundation Example A.VII Pontoon Strip Foundation Example B.VII Case History of Heave for Deep, Overcompensated Foundation Example VIII Calculation of Friction Piles INDEX

484 486 489 489 492 492

496 497 501 501 510 510 514 521 523 529 540 540 543 551 554 554 555

564 567 567 567

587 593 596 601 611 615 618 618 630 632

640 645 655

FOUNDATION ENGINEERING FOR DIFFICULT SUBSOIL CONDITIONS

~I~

INTRODUCTION

1.1 TYPICAL FOUNDATIONS The art of designing the best and most economical foundations for a project greatly depends on a careful investigation by the foundation engineer. A study should be made of the environmental factors and the compatibility of the subsoil engineering conditions with the type of foundation structure on which the loadings are to be supported. Hence, as a first approximation, the foundation engineer should consider the qualitative index and mechanical characteristics of the subsoil at the site at which the project will be constructed. This preliminary knowledge will permit him to judge the behavior of the subsoil under applied load, and after analyzing the probable behavior of different types of foundation structural systems in conjunction with the project requirements, he will be in the position to select the proper foundation. The purpose of this chapter is to visualize the selection of the type of foundation, reviewing the typical foundation structures that may be used in conjunction with the subsoil conditions to be encountered, to fulfill the requirements of total and differential settlements. It must be borne in mind, however, that in the design of a foundation there are two important mechanical items to be considered: first, the bearing capacity of the soil for the applied load; and second, whether the total and differential settlements are compatible with the foundation structure selected, type of superstructure and architectural demands of the project. As an example of total and differential settlements, the case of widely spaced footings used for light flexible roofs may be mentioned, where one may allow large differential settlements, in contrast with other problems like installation of machinery or equipment, where the differential settlemenfs are often restricted to very small values. Therefore, the foundation engineer should investigate the differential settlements that may be per-

2

INTRODUCTION

mitted for different problems of building design, and also the magnitude of the total settlement not damaging adjacent construction. The specification of total and differential settlements is studied carefully for each problem in question, as the allowances can vary a great deal, depending on the mechanicallimitations of the project in question, as well as on adjacent buildings and public utilities. In other words, one could say that for a certain specific building, a total settlement of 30 cm may be allowed, provided that there is no damage and differential settlements for certain predetermined spans between columns do not exceed ~ cm. This specification appears to be bold, since one could say also that a total settlement of 30 cm is large, even if no damage takes place. If the total settlement, however, could be forecast and the building is isolated in an area away from other buildings and no damage of any property is expected, then there is no reason to allow large settlements in the design, provided also that the connections of public utilities going into the building are taken care properly, and the foundation structure is designed in such a way that differential settlements in the building will not produce damage to the construction. If such is the case, the functional requirements of the project are fulfilled and the foundation may be considered to work under satisfactory conditions. The foundation engineer experienced in soil mechanics and engineering geology, as well as with the behavior of foundation structures and building design, is able to visualize, as a first step, which foundation to select for the problem in question. Once he has selected the optimum type of foundation to be used, then he may investigate quantitatively its behavior. The selection should always be the most economical type of foundation that can be used, fulfilling the requirements of allowable total and differential settlements in conjunction with the subsoil condition encountered. In order to give the foundation engineer the first approach in the philosophy of selecting a foundation, the principal types of foundations will be discussed, and the relation they have with different subsoil deposits from which the probable behavior may be forecast. In this approach, the foundation engineer is assumed to be acquainted with the index and general mechanical properties of soils and with the general behavior of different types of foundation structures. 1.1 I solated Footings

Footings are understood formed by a rigid rectangular base of stone or concrete of dimensions: width B and length L, in which the ratio of LIB will not exceed 1.5. The foundation structure will support the column load. The bearing capacity of the footing may be estimated, and its dimensions selected; thereafter, a forecast of the settlement is made. To illustrate the case of footing foundations, consider a building with nine columns (Fig. loLl) supported on isolated footings. In this case, the footings will work independently of each other. Therefore, it is required that the differential settlements between footings will not exceed the allowable total and differential settlement requirements. The differential settlements may be reduced selecting

1.1 TYPICAL FOUNDATIONS 3

L

~

1.5B

L

Fig. 1-1.1 Single footings.

properly the area of the footings, and at times, using the stiffness of the superstructure. From the structural point of view, however, the superstructure should not be allowed to take high secondary stresses induced by the differential settlements of the footings, except in very special cases. Single footing foundations, in general, will be used only in soils of low compressibility and in structures where the differential settlements between columns may be controlled by the superstructure flexibility, or including in the design of the building joints or hinges that will take the differential settlements and/or rotations, respectively, without damaging the construction.

4

INTRODUCTION

1.2. Continuous Footings

When it is necessary to control within certain limits the magnitude of differential settlements between columns supported on footings, and when soil deposits of medium or low compressibility are encountered, it is recommended to use continuous footings. They may be defined as resisting elements joining columns together by foundation beams. Continuous footings are arranged by joining two or more columns together with beams. The vertical differential displacements may be controlled via beam stiffness (Fig. 2-1.1). The selection of the foundation beams, either running in one direction or the other along column rows, depends largely on the layout of the column loads,

Elevation

Cross section

(a)

3 A

(b)

B

c

Fig. 2-1.1 Continuous footings.

1.1 TYPICAL FOUNDATIONS 5 3

} }