Geotechnical Instrumentation for Monitoring Field Performance

GEOTECHNICAL INSTRUMENTATION

FOR MONITORING

GEOTECHNICAL INSTRUMENTATION

FOR MONITORING

FIELD PERFORMANCE

John Dunnicliff

Geotechnical Instrumentation Consultant Lexington, Massachusetts

With the assistance of:

Gordon E. Green

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Copyright © 1988 by John Wiley & Sons, Inc.

All rights reserved. Published simultaneously in Canada. Reproduction or translation of any part of this work beyond that permitted by Section 107 or 108 of the 1976 United States Copyright Act without the permission of the copyright owner is unlawful. Requests for permission or further information should be addressed to the Permissions Department, John Wiley & Sons, Inc. Library of Congress Cataloging in Publication Data: Dunnic1iff, John.

Geotechnical instrumentation for monitoring field performance/ John Dunnic1iff with the assistance of Gordon E. Green.

"A Wiley-Interscience publication." ISBN 0-471-09614-8

1. Engineering geology-Instruments. 2. Soils-Testing.

I. Green, Gordon E. II. Title. TA705.D86 1988

624.1'51'028-dcI9 87-27709 Printed in the United States of America 10987654321

To the manufacturers of geotechnical instruments, without whom there would be no

geotechnical instrumentation for monitoring field performance

FOREWORD

Every geotechnical design is to some extent hypo-thetical, and every construction job involving earth or rock runs the risk of encountering surprises. These circumstances are the inevitable result of working with materials created by nature, often be-fore the advent of human beings, by processes sel-dom resulting in uniform conditions. The inability of exploratory procedures to detect in advance all the possibly significant properties and conditions of natural materials requires the designer to make as-sumptions that may be at variance with reality and the constructor to choose equipment and construc-tion procedures without full knowledge of what might be encountered.

Field observations, including quantitative mea-surements obtained by field instrumentation, pro-vide the means by which the geotechnical engineer, in spite of these inherent limitations, can design a project to be safe and efficient, and the constructor can execute the work with safety and economy. Thus, field instrumentation is vital to the practice of geotechnics, in contrast to the practice of most other branches of engineering in which people have greater control over the materials with which they deal. For this reason geotechnical engineers, unlike their colleagues in other fields, must have more than casual knowledge of instrumentation: to them it is a working tool, not merely one of the components of research.

Notwithstanding its vital role, instrumentation is not an end in itself. It cannot guarantee good design or trouble-free construction. The wrong instru-ments in the wrong places provide information that may at best be confusing and at worst divert atten-tion from telltale signs of trouble. Too much in-strumentation is wasteful and may disillusion those who pay the bills, while too little, arising from a

desire to save money, can be more than false econ-omy: it can even be dangerous.

Every instrument installed on a project should be selected and placed to assist in answering a specific question. Following this simple rule is the key to successful field instrumentation. Unfortunately, it is easier to install instruments, collect the readings, and then wonder if there are any questions to which the results might provide an answer. Instrumenta-tion is currently in vogue. Some design agencies and many regulatory bodies mandate instrumenta-tion whether the results might be useful or not. It is a widely held dogma, for instance, that every earth dam should be instrumented, in the hope that some unsuspected defect will reveal itself in the observa-tions and give warning of an impending failure. Part of the criticism directed at Teton Dam following its failure was paucity of instrumentation. Yet, it is extremely doubtful that any instrumental observa-tions could have given timely warning of the partic-ular failure that occurred. Instruments cannot cure defective designs, nor can they indicate signs of im-pending deterioration or failure unless, fortuitously, they happen to be of the right type and in the right place.

The engineer should bring the best knowledge and judgment to bear on every geotechnical prob-lem that arises and should analyze the quality of the information on which a design is based. The en-gineer should judge not only the way the design will function if the information is essentially correct, but how the gaps or shortcomings might influence the performance of the project. Then, and only then, can specific items be identified that will reveal whether the project is performing in accordance with design assumptions or, if not, in what signifi-cant way the performance differs. Then the critical

viii FOREWORD questions can be framed-the answers to which will

fill the gaps or correct the errors in the original de-sign assumptions-and the engineer can determine what instruments, at what locations, can answer those questions.

Of course, not all instruments are installed to monitor the safety of a structure or construction operation or to confirm design assumptions. Some are used to determine initial or background condi-tions. Observations of groundwater prior to con-struction, or in situ stresses in rock masses, or of elevations of structures before the start of adjacent construction are examples. Certain types of con-struction, such as the installation of tiebacks, are inherently dependent on instrumentation. Further-more, advancements in the state of our knowledge require large-scale or full-scale observations of an extent and complexity far beyond the requirements of the practicing engineer. Yet, in all these applica-tions, it is equally true that every instrument should be selected and located to assist in answering a specific question.

Instrumentation needs to be kept in perspective.

It is one part of the broader activities of observation and surveillance. Trained people, using the best of all instruments, the human eye, can often provide all the information necessary and are always an es-sential part of the field observations on any project. Even when instruments are used because the neces-sary quantities are too small to be observed by eye, or the events are taking place out of the reach of a human being, the findings must be related to other activities. Without good records of the progress of excavation and of details of the excavation and bracing procedures, for example, the results of measurements of deformations or earth pressure associated with a braced cut become almost mean-ingless. Among the most valuable uses of instru-mentation are empirical correlations between con-struction procedures and deformations or pres-sures, correlations that can be used immediately to improve the procedures so as to reduce the move-ments or pressures. Thus, highly sophisticated, fully automated installations for obtaining and pre-senting data, sometimes held in high favor by those intrigued with gadgetry, may fail to serve a useful purpose because the simple visual observations of what may be affecting the readings are overlooked.

Not only is instrumentation not an end in itself, but neither is sophistication or automation. The two prime requirements are sensitivity sufficient to pro-vide the necessary information and reliability to

en-sure that dependable data can be obtained through-out the period when the observations are needed. Usually, the most dependable devices are the sim-plest.

If one can with sufficient accuracy make a direct visual observation with a graduated scale, then a micrometer should not be used. If one can use a micrometer, a mechanical strain gage should not be used. If one can use a mechanical strain gage, an electrical one should not be used. Mechanical in-struments are to be preferred to electrical devices and simple electrical devices depending on simple circuits are to be preferred to more complex elec-tronic equipment. That is, where a choice exists, the simpler equipment is likely to have the best chance for success.

Nevertheless, simple instruments are sometimes inappropriate and more complex ones must be used. An open standpipe may be the simplest de-vice for observing a piezometric level, but the point at which the pore pressure needs to be measured may be located where direct access is impossible. A more sophisticated arrangement is then necessary. If one wishes to determine the state of stress in a mass of rock, there is no choice but to install sophisticated equipment to make measurements at a considerable distance from the position of the ob-server, and the strains likely to be observed will be too small to detect by any mechanical device. Thus, nothing but a sophisticated system will serve.

Not all sophisticated systems are equally reli-able. Equipment that has an excellent record of per-formance can be rendered unreliable if a single essential but apparently minor requirement is over-looked during the installation. The best of instruc-tion manuals cannot provide for every field condi-tion that may affect the results. Therefore, even slavish attention to instructions cannot guarantee success. The installer must have a background in the fundamentals of geotechnics as well as knowl-edge of the intricacies of the device being installed. Sometimes the installer must consciously depart from the installation manual.

The installer must also want desperately to do the job well and must often work under difficult and unpleasant conditions, trying to do precision work while surrounded by workers whose teamwork or operation of equipment is being interrupted, or working the graveyard shift in an attempt to reduce such interruptions. Dedication of this sort is the price of success, and it is rarely found at the price tendered by the lowest bidder. Moreover, the

in-FOREWORD ix

staller can hardly be motivated to be dedicated to the task of installing instruments of inferior quality that are likely to fail prematurely or to produce questionable data. Rugged, reliable instruments are not necessarily expensive, but lowest cost of the hardware is rarely a valid reason for its choice. No arrangement for a program of instrumentation is a candidate for success if it sets cost above quality of instruments or fee above experience and dedication of the installer.

Instruments are discontinuities, nonrepresenta-tive objects introduced into soil/rock structure sys-tems. Their presence or the flows or displacements required to generate an observation alter the very quantities they are intended to measure. The alter-ation may be significant or negligible; its extent de-pends on the nature of the phenomenon being ob-served, on the design of the instrument, and on the operations required for installation. The engineer who embarks on a program of field instrumentation needs to understand the fundamental physics and mechanics involved and how the various available instruments will perform under the conditions to which they will be subjected. In addition, the en-gineer needs to know whether corrections can be made by calibration or by theoretical calculations, or whether under the circumstances no valid result is possible. Perhaps the classic examples of the lat-ter eventuality were the attempts in the early 1920s to measure earth pressures against braced cuts by observing deflections of the wales between struts, a procedure made futile because of the then unsus-pected phenomenon of arching. The same phenom-enon affects to greater or lesser degree the results of all earth-pressure cells.

Finally, there must be enough instruments not only to allow for the inevitable losses resulting from malfunction and damage by construction activities, but also to provide a meaningful picture of the scat-ter in results inherent in geotechnics as a conse-quence of variations in geology and in construction procedures. For example, in the early days of con-struction of the Chicago subways, when the loads in

the struts for bracing open cuts were measured, it was observed that struts at the same elevation in the same cut carried widely different loads. To some extent the difference was the result of slight differ-ences in soil properties. To a much greater extent, however, the differences were associated with con-struction procedures. When a strut was placed as soon as possible, it always carried a load substan-tially greater than the load in another strut at the same elevation but not placed until excavation had advanced considerably beyond its future location. If the strut loads had been measured on only a few struts, for example, along a single vertical line, the influence of the construction procedure on strut loads could not have been detected, and entirely erroneous conclusions might have been drawn about the magnitude of the earth pressures resisted by the bracing. Since the variability of the quan-tities measured by geotechnical instrumentation de-pends not only on the kind of measurement itself-whether it be pore pressure, displacement, or load in a structural member-but also on the geology and on details of the construction procedures, the design of a system of measurements requires ma-ture judgment based on experience and understand-ing of the geotechnical problems at hand.

Use of field1nstrumentation therefore requires a thorough grounding in geotechnical principles, a de-tailed conception of the variations that may be ex-pected in the natural or artificial deposit in which the observations are to be made, a realistic notion of the construction procedures likely to be fol-lowed, a thorough knowledge ofthe capabilities and shortcomings ofthe instruments themselves, and an appreciation of the practical problems of installa-tion. It also requires a clear perception of the way in which the results of the observations will be ob-tained, recorded, digested, and used on the particu-lar project for which the design is being prepared. Small wonder that the need exists for a book dealing comprehensively with this subject.

PECK-PREFACE

practitioners. There is information for all those who plan or implement geotechnical instrumentation programs: owners, project managers, geotechnical engineers, geologists, instrument manufacturers, specialty geotechnical contractors, civil engineers, and technicians. The book should also be helpful to students and faculty members during graduate courses in geotechnical engineering.

A practical book about geotechnical instrumenta-tion must go beyond a mere summary of the techni-cal literature and manufacturers' brochures: it must hold the hands of readers and guide them along the way. This need has created two difficulties for me. First, my own practical experience is that of one person and does not arm me to write a comprehen-sive guide on my own. I have tried to fill this gap by drawing on the experience and opinions of many colleagues, who are identified elsewhere.

Second, it is certain that, soon after publication of this book, I will alter some of my opinions as my experience increases. I am well aware that the sub-ject of geotechnical instrumentation is a contentious

subject, made so by strongly held views among practitioners and by vested commercial interests. The guidelines in this book are an attempt to convey the "best ways" as I see them today. You, the reader, will have your own experience and your own best ways, which may differ from mine. I therefore have a plea: when you see possibilities for improving the content of this book, send me rea-sonable evidence. My address is in the Directory of the American Society of Civil Engineers. Not only will I learn from you, but I will try to disseminate the improvements, perhaps ultimately in a second edition of this book.

Length restrictions have strongly influenced the contents. There is no attempt to describe every in-strument, either currently available or described in published papers. Some available instruments are not well suited to their intended purpose, and the

literature abounds with descriptions of prototype gadgets that have found little real use in practice. In selecting information to be included, I have been guided by the title of the book, and thus there is nothing on geotechnical instrumentation for in situ measurement of soil and rock properties. Detailed case histories have been excluded in favor of guide-lines directed toward the problem-oriented reader. However, summaries of selected case histories are included in Part 5.

Finally, a few words about the organization of the book and how it may be used. The book is di-vided into seven parts, each with a self-explanatory title. Readers looking for an overview may start with the Foreword and Chapter 1, then scan through Chapter 26, The Key to Success. In my view, the greatest shortcoming in the state-of-the-practice is inadequate planning of monitoring pro-grams, and therefore problem-oriented readers should give their first concentrated attention to Chapter 4, Systematic Approach to Planning

Moni-toring Programs Using Geotechnical Instrumenta-tion. The various steps in this chapter lead readers

to each of the chapters in Parts 2, 3, and 4: Chapter 4 is therefore the hub of the book. The chapters in Part 5, Examples of Instrumentation Applications, are intended as supplementary chapters to open the minds of readers to the possible role of geotechnical instrumentation 'on various types of construction projects and to guide them toward implementation. They are not intended as exhaustive summaries, state-of-the-art papers, or "cookbooks." If a reader uses this book by (1) turning to the chapter in Part 5 that discusses his or her type of project, (2) noting the types of instruments suggested in Part 5, (3) noting the sketched layouts in Part 5, (4) studying Part 3, Monitoring Methods, for details of the in-struments, and (5) proceeding with a monitoring program, that reader is misusing the book. Turn back to Chapter 4!

JOHN DUNNICLIFF

GEOTECHNICAL INSTRUMENTATION

FOR MONITORING

Every instrument on a project should be selected and placed to assist with answering

a specific question: if there is no question, there should be no instrumentation.

Part 1

Introduction

Part 1 is intended to serve as a general introduction. Chapter 1 sets the stage for the book, describing the role of geotechnical instrumentation and giving a historical perspective and a look into the future. It is hoped that Chapter 1 will motivate the reader toward a deeper study of the subject. Chapter 2 presents an overview of key aspects of soil and rock behavior, targeted for the practitioners who become in-volved with geotechnical instrumentation programs and who do not have formal training in soil or rock mechanics.

CHAPTER 1

GEOTECHNICAL INSTRUMENTATION:

AN OVERVIEW

1.1. WHAT IS GEOTECHNICAL INSTRUMENTATION?

The engineering practice of geotechnical instrumen-tation involves a marriage between the capabilities of measuring instruments and the capabilities of people.

There are two general categories of measuring instruments. The first category is used for in situ determination of soil or rock properties, for ex-ample, strength, compressibility, and permeability, normally during the design phase of a project. Ex-amples are shown in Figure 1.1. The second cate-gory is used for monitoring performance, normally during the construction or operation phase of a proj-ect, and may involve measurement of groundwater pressure, total stress, deformation, load, or strain. Examples are shown in Figure 1.2. This book is concerned only with the second category.

During the past few decades, manufacturers of geotechnical instrumentation have developed a large assortment of valuable and versatile products for the monitoring of geotechnically related parame-ters. Those unfamiliar with instrumentation might believe that obtaining needed information entails nothing more than pulling an instrument from a shelf, installing it, and taking readings. Although successful utilization may at first appear simple and straightforward, considerable engineering and plan-ning are required to obtain the desired end results.

The use of geotechnical instrumentation is not merely the selection of instruments but a com-prehensive step-by-step engineering process begin-ning with a definition of the objective and ending with implementation of the data. Each step is criti-cal to the success or failure of the entire program, and the engineering process involves combining the capabilities of instruments and people.

1.2. WHY DO WE NEED TO MONITOR FIELD PERFORMANCE?

The term geotechnical construction can be used for construction requiring consideration of the en-gineering properties of soil or rock. In the design of a surface facility, the ability of the ground to sup-port the construction must be considered. In the design of a subsurface facility, consideration must also be given to the ability of the ground to support itself or be supported by other means. In both cases, the engineering properties of the soil or rock are the factors of interest. The designer of geotechnical construction works with a wide vari-ety of naturally occurring heterogeneous materials, which may be altered to make them more suitable, but exact numerical values of their engineering

4

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figure 1.1. wmpks of measuring instrumenU IOf in situ determin<ltion of soil or rock properties: (ol) Pie:zocone: combined static cone ,lind pore pressure probe (courtesy of Geotechniques International, Inc., Middleton, MAl; (b) V.lne sM;1Ir equipment (courtesy of Ceonor AIS, Oslo, Norway); Ie) self-boring pressu~ter (courtesy of Cambridg~ Insilu, CamtN-idse. Engl.lnd);.OO (d) borehole deformation gage (courtesy of Geokon, Inc., lebanon, NH).

WHERE ARE WE NOW? 5

properties cannot be assigned. Laboratory or field tests may be performed on selected samples to ob-tain values for engineering properties, but these tests will only provide a range of possible values.

The significance of these statements about geotechnical construction can be demonstrated by comparison with steel construction. A designer of a steel structure works with manufactured materials. The materials are specified, their manufacture is controlled, and fairly exact numerical values of en-gineering properties are available for design. An ac-curate analysis can be made and design plans and specifications prepared. Then, provided construc-tion is in accordance with those plans, the structure will perform as designed. There will generally be no need to monitor field performance. Similar remarks apply to reinforced concrete. In contrast, the design of geotechnical construction will be based on judg-ment in selecting the most probable values within the ranges of possible values for engineering prop-erties. As construction progresses and geotechnical conditions are observed or behavior monitored, the design judgments can be evaluated and, if neces-sary, updated. Thus, engineering observations dur-ing geotechnical construction are often an integral part of the design process, and geotechnical in-strumentation is a tool to assist with these obser-vations.

1.3. WHAT CAPABILITIES MUST THE PEOPLE HAVE?

Basic capabilities required for instrumentation per-sonnel are reliability and patience, perseverance, a background in the fundamentals of geotechnical en-gineering, mechanical and electrical ability, atten-tion to detail, and a high degree of motivaatten-tion.

1.4. WHAT CAPABILITIES MUST THE INSTRUMENTS HAVE?

Reliability is the overriding desirable capability for instruments. Inherent in reliability is maximum simplicity, and in general the order of decreasing simplicity and reliability is optical, mechanical, hydraulic, pneumatic, electrical. Also inherent in reliability is maximum quality. Lowest cost of an instrument is rarely a valid reason for its choice, and unless high quality can be specified adequately,

instrument procurement on a low-bid basis will re-main a stumbling block to good field performance.

1.5. WHERE HAVE WE BEEN?

Figures 1.3-1.15 show examples of past uses of geotechnical instrumentation.

The birth of geotechnical instrumentation, as a tool to assist with field observations, occurred in the 1930s and 1940s. During the first 50 years of its life, a general trend can be observed. In the early years, simple mechanical and hydraulic instruments predominated, and most instrumentation programs were in the hands of diligent engineers who had a clear sense of purpose and the motivation to make the programs succeed. There were successes and failures, but the marriage between instruments and people was generally sound. In more recent years, as technology has advanced and the role of geo-technical instrumentation has become more secure, more complex devices with electrical and pneu-matic transducers have become commonplace. Some of these devices have performed well, while others have not. At the same time, the technology has attracted an increasingly large proportion of the geotechnical engineering profession, and an in-creasing number of instrumentation programs have been in the hands of people with incomplete motiva-tion and sense of purpose. There have continued to be successes and failures but, in contrast to the early years, a significant number of the failures can be attributed to an unsound marriage between in-struments and people.

1.6. WHERE ARE WE NOW?

The state of the art of instrument design is now far ahead of the state of the practice by users, and many more imperfections in current instrumenta-tion programs result from user-caused people prob-lems rather than from manufacturer-caused instru-ment problems. As users we are fortunate in having access to such a wide variety of good instruments.

It is our responsibility to develop an adequate level of understanding of the instruments that we select and to maximize the quality of our own work if we are to take full advantage of instrumentation tech-nology. The greatest shortcoming in the state of the practice is failure to plan monitoring programs in a

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Fillure 1.2. Enmpla of mnSllrins instruments for m<woitorinll flCld perlorTJUlRCt: (ill) twin-luM hr-dr"l.Ilic piezometer (coortesy of CMIKhnkaJ Instruments (U.K.) ltd., leamington SpJi, Engl,lnd); (bl vibr.ting win! pie:zomete... (cour1~ of TtlemK, Asn~rti, Fr.nct); Ie) vibrililinl wire slressmeler (courtesy of Geokon, Inc., ~non. NH); (d) ~ cell (courtesy of Proceq 5A, Zurich, 5wilzm.nd);

It) embedment t.llrth prnstJre celt (courtesy of Thof Intenwotioml, loc., Seatde, WA); (0

surface-mounted vibtatins wire strain PIe (courtesy of 'rad Gage, a Division of Klein As.soci.JIles, Inc., Salem, NH); (g) multipoint fiKed borehole exlensometer (courtesy of Soillnslrumenls ltd., Ucklield, England);

,h)

8 GEOTKHNICAL INSTRUMENTATION: AN OVERVIEW

Figu,!!' 1.3. Mrasuring Io.ad in a tim~' strut, using a hydraulic jack. Open cui for slalion in clay. Chic~o Subway, 1940 (cou r-lesy of Ralph 8. Peck),

figure 1.4. Delennination of load in a steel strut, using a mKh.ani-cal str.llin gage. Open cui for station in day. Chiago Subw.JIy, 1948 (courtesy of R.J.1!)h B. Peck).

figure 1.5. Installing twin·lube: hydraulic piezometers. Usk Dam, England, 1952 (courtesy of Arthur D. M. Penman).

rational and systematic manner. and therefore plan-ning procedures are emphasiz.ed in this book.

Users of geotechnical instrumentation oOcn have a misconception of the size of the industry that

manufactures instruments for performance

moni-toring. It is not a large industry: it is in faCI very

small. The manufacturing industry employs be·

tween 300 and 400 people worldwide. and the total annual volume of sales is about 30 million U.S. dol·

lars. This misconception sometimes leads to un-reasonable expectations on the part of users: we

cannot expect the manufacturers to make large

ex-penditures on research. development. and testing of

special instruments, unless justified by the size of

the market. (fthe market is small. special funding is

needed.

Figur~ 1.6. Ilt§tollling fixed em~nkment extensometer with vibr.t -ing wire tr.nsducer. Balderkead Dam, England, 1963 (courtesy of Arthur D. M. Penm.Jn).

WHERE ARE WE NOW~

,

Figure 1.7. Manomeler panels for twin-tube hydraulic piezometers. Plover Cove Main Dam, Hong ICong, 1965 (.lifter Dunnicliff. 1968). Reprinted by permission of tnstitulion of Civil Engioeers, London.

f,&ure I.B. Inst.lllling fixed embankment u tensometers in em -boIInkment cbm. ludinglon Pumped Storage Projed, ludington, MI,1972.

This book includes chapters that describe avail

-able methods for monitoring various gcotcchnically

related parameters. The following is a summary rat -ing of our current ability to obtain reliable measur

e-ments of these parameters. in order of increasing

reliability:

Total stress in soil and stress change in rock Groundwater pressure

Load and strain in structural members Deformalion

Temperature

figure 1.9. Bonded resistance strain &-lgts on ~ted steel

liner for sofl grtltmd tunnel. POr1 Richmond Woller Pollution Con -lrol Project, Slalen Island, NY, 1974.

10 GEOTECHNICAL INSTRUMENTA.TlON: AN OVERVIEW

figure 1.10. Insl~lIin8 multipoint filed borehole Il'Xtensometer

;l00ve iIIlignment of rock lunnel. Us! 6lrd Street S4.1bway. New

Yori;, NY. 1976.

Reliability is strongly influenced by the extent to which measurements are dependent on local ch ar-acteristics of the zone in which the instruments arc installed. Most measurements of pressure, stress,

load, strain, and temperature are influenced by con-ditions within a very small z.one and are therefore dependent on local characteristics of that zone. They are often essentially point measurements. subject to any variability in geologic or other char -acteristics. and may therefore not represent condi-tions on a larger scale. When this is the case, a large number of measurement points may be required be

-fore confidence can be placed in the data. On the other hand, many deformation measuring devices respond to movements within a large and represen

-tative zone. Data provided by a single instrument can therefore be meaningful, and deformation mea-surements are generally the most reliable and least ambiguous.

filure 1.11. Pocurmlic piuomet~ .lind e.rth preuure ctll on

oppositt f.Jl«S of precut concrete pile, prior 10 concretins. Keehi

InttKhanse, Honolutu, HI, 1977.

1.7. WHERE ARE WE GOING!

As we look ahead, there is no reason to believe in a decreasing role for geotechnical instrumentation. Geotechnical design and construction will always

be subject to uncertainties. and instrumentation will continue to be an important item in our tool box.. However, several current trends can be identified. each of which will continue in the future and change the state of the practice.

First, there is the advent of automatic data acqui-sition systems and computerized data processing and presentation procedures. Clearly, these

sys-tems and procedures have many advantages. yet we must remain aware of their limitations. No auto-matic system can replace engineering judgment. When automatic data acquisition systems are used,

WHERE ARE WE GOING? 11

Figure 1.12. Electric.ltr.nsducen fOf monitoring moVemeflt of multiple tellt.1fl; during ~ test of prec~t concrete pile. Keehi Inten:hl.nge, Honolulu, HI, 1977.

there is a real possibililY Ihat visual observations will not be made, that other factors influencing

measured data will not be recorded, and that in-formation will therefore not be available for relat

-ing measured effects to their likely causes. When

computerized data processing and presentation pro-cedures are used, there is a real possibility that en-gineering judgment will be given second place and that correlations between causes and effects will not be made. We should take all possible advantage of this exciting new technology but should never

forget that judgment plays an important and often overriding role in the practice of geotechnical en-gineering.

Second, increasing labor costs in many countries

have sharply reduced the availability of competent personnel. This trend of course encourages the use of automatic systems and procedures. yet reduces

figure 1.ll. Inst.lling pge with induction coil tr.nsducen, for

monitoring convergence of slurry trench test pa.neI in soh ct.y,

Alewife Station, C.mbridge, MA, 1978.

figure t.14. Ht..ds of multipoint fbed borehole txten§OO1eters,

insulted to monitor rock rIIOVflT1tflts during full-sc.1e ke.tff test

for studies relating to dispowl of high.level nucleu w~te. 8asalt Wnte Iw'ation Progr.m, H.nford, WA, 1980 (courtesy of De-partment of EMf'lY).

12 GEOTECHNICAL INSTRUMENTATION: AN OVERVIEW

Figu~ 1.15. Re.ling inclinometer during construction of under

-w,ter test fill, Chell: Up kok Replacement Airport. Hong Kong.

1'81 (courtesy of leo O. Handfel!),

the number of personnel available for exercising en-gineering judgment.

Third, the use of design tools such as finite and

boundary element modeling techniques leads to a

need for field verification. Geotechnical instrum

en-tation is likely to play an increasing role in pr ovid-ing a check on these advanced analytic predictions.

Founh, there is a trend to develop and improve transducers and to include built-in features that create redundancy and that provide direct output in engineering units. The trend includes provisions for calibrations and zero checks.

Fifth, there is a trend toward use of new con-struction methods. Examples of innovations in the recent past include earth reinforcement, lateral

sup-port. and ground modification. These innovations often require field verification before they become widely accepted. and geotechnical instrumentation

will always playa role.

The above five trends. good or bad, are inevi ta-ble. There is a sixth trend, which the author views as wholly bad: the procurement of instruments and the awarding of field instrumentation service co

n-tracts on the basis of the lowest bid. If an

in-strumentation program sets cost above quality of

instruments, or fcc above experience, dedication. and motivation of people. it deserves to be a failure.

We must work hard to reverse this trend.

1.8. THE KEY TO SUCCESS

Full benefit can be achieved from geotechnical in-strumentation programs only if every step in the planning and execution process is taken with great care. The analogy can be drawn to a chain with

many potential weak links: this chain breaks down with greater facility and frequency than in most other geotechnical engineering endeavors. The links in the chain are defined in Chapter 26: their strength depends both on the capabilities of mea sur-ing instruments and the capabilities of people. The

success of performance monitoring will be

CHAPTER 2

BEHAVIOR OF SOIL AND ROCK

geotechnical instrumentation programs do not have formal training in soil or rock mechanics. The pur-pose of this chapter is to present a brief and simple overview of the key aspects of soil and rock behav-ior that relate to the use of geotechnical instrumen-tation. For a thorough treatment of soil behavior readers are referred to Holtz and Kovacs (1981) and Terzaghi and Peck (1967). McCarthy (1977) pres-ents similar material oriented for studpres-ents at techni-cal colleges. Rock behavior is well described by Blyth and DeFreitas (1974) and Franklin (1988). 2.1. BEHAVIOR OF SOil

2.1.1. Constituents of Soil

Soil is composed of solid particles with intervening spaces. As shown in Figure 2.1, the particles are referred to as the mineral skeleton and the spaces as

pore spaces, pores, or voids. The pore spaces are

usually filled with air andlor water. A soil in which the pore spaces are completely filled with water is called a saturated soil. If any gas is present in the pore spaces, the soil is called an unsaturated soil. The term partially saturated is sometimes used, but because it's either saturated or it isn't, this is not a satisfactory term.

2.1.2. Basic Types of Soil

Soil can be categorized into two broad groups:

cohesionless soil and cohesive soil. Cohesionless

soils include sand and gravel, which consist

offrag-ments of rocks or minerals that have not been al-tered by chemical decomposition. Inorganic silt is a fine-grained soil with little or no plasticity and can generally be classified as cohesionless. Organic silt is a fine-grained soil with an admixture of organic particles and behaves as a plastic cohesive soil. Clay is a cohesive soil consisting of microscopic and submicroscopic particles derived from the chemical decomposition of rock constituents. 2.1.3. Stress and Pressure

Stress and pressure are defined as force per unit

area, with typical units of pounds per square inch Ob/in.2) or pascals (Pa). Strictly, pressure is a gen-eral term meaning force per unit area, and stress is the force per unit area that exists within a mass. However, in geotechnical engineering, the terms

~~~" .. ::~lH!b~~4--.., Mineral skeleton

Pore spaces,

7~d·:::[:;~~"4,:.~I---=-pores, or

voids

Figure 2.1. Constituents of soil.

14 BEHAVIOR OF SOIL AND ROCK

are applied more loosely; for example. as will be

discussed later, earth pressure and soil stress arc

used as synonyms.

2.1.4. Pore Water Pressure

When the pore spaces are filled with water (satu

-rated soil), the pressure in the water is called the

pore water pressure (Figure 2.2), It acts in all di

rec-lions with equal intensity.

2.1.5. Total and Effective Stresses

Total stress is the total force transmitted across a

given area, divided by that area. Thus. if a 2-fool

square piece of wood is placed on the ground surface

and a person weighing 200 pounds stands on the

wood. the total stress in the ground immediately

below the wood is increased by 50 lb/ft2.

£JJective stress can be explained by use of an analogy. Figure 2.3a shows salumled soil placed in

a cylindrical container with a cross-sectional area.

Wtlltr surface Porous piston . Saturated soil . . Pinon

.'

.

f'!&ure 2.2. Port water pressure uught in 10-· fill (.dter Par1ially

lntqrated, 1962).

in plan, of 1 square inch. Figure 2.3b shows an an

al-ogy in which the resistance or the mineral skeleton

to compression is represented by a spring, and the

porous piston is replaced by an impenneable piston with a valved orifice. The orifice represents the re

-sistance to the flow of water through the soil. The

piston is assumed to be weightless. and the water is incompressible. Initially, the valve is closed. The spring has a stiffness of 10 Ib/in .. meaning that a

force of 10 pounds is required to produce an axial

Fine orifice. with valve 10lb L _ _ -""=--_-'=', Spring

I.'

'"

figure 2.3. Spring ;analogy for soil behavior.

I"

BEHAVIOR OF SOIL 15 Table 2.1. Sharing of Applied Force

Valve

Figure 2.3 Condition Position

(b) Initial Closed (c) 10 lb force Closed applied (d) Piston Open descended 0.4 in.

(e) Piston Open

descended 0.8 in.

(f) Piston Open

descended 1.0 in.

deflection of 1 inch. In Figure 2.3c, a 10-pound force has been applied to the piston. The water is not free to escape; therefore, the spring cannot compress and cannot carry the newly applied force. The wa-ter must therefore carryall the force, and the pres-sure gage will show an increase immediately as the force is applied. If the valve is now opened, water will pass through the orifice and the piston will de-scend. Figures 2.3d and 2.3e show intermediate steps, and Figure 2.3f shows the condition when the piston has descended 1 inch and there is no further flow of water. Because the spring has now been compressed 1 inch, it must be carrying a force of 10 pounds. The spring is now carrying all the force, and the pressure gage has returned to the same reading as in Figure 2.3b. Table 2.1 summarizes the steps and shows the sharing of applied force be-tween the spring and water. It can be seen from the table that the sum of the forces carried by the spring and the water is always equal to the force on the piston.

Effective stress is defined as the force acting

be-tween the points of the mineral skeleton per total area. Because a cross-sectional area of 1 square inch has been chosen in the above analogy, all the forces in Table 2.1 are numerically equal to stresses in Ib/in.2 if a real soil is considered. By thinking now in terms of stresses, it can be seen that the force on the piston represents the total stress, the force car-ried by the spring represents the effective stress, and the force carried by the water represents the pore water pressure. The following relationship al-ways applies:

Force on Force Carried Force Carried

Piston by Spring by Water

(lb) (lb) (lb) 0 0 0 10 0 10 10 4 6 10 8 2 10 10 0

total stress

=

+

This is Terzaghi's principle of effective stress. The

following symbols are normally used: Total stress, (J'

Effective stress, (J"

Pore water pressure, u

Thus,

(J' = (J"

+

Forces and stresses are plotted in Figure 2.4a. It

can be seen from the figure that the rates of pressure change decrease as time increases: this is consistent with the observation that the flow of water through the orifice in the piston decreases as the water pres-sure in the container decreases.

2.1.6. Consolidation

The process of gradual squeezing out of water, with the accompanying transfer of total stress to effec-tive stress and decrease in pore water pressure, is called consolidation. Figure 2.4b shows the

vol-ume change that occurs during consolidation. The amount by which the pore water pressure exceeds the equilibrium pore water pressure is called the

excess pore water pressure, and the gradual

de-crease of this pressure is often referred to as dissi-pation of pore water pressure.

,.

Force carried by waler, or pore witer pressure

Force carried by spring. Or effective $tre5S in soil

~

10~~1"

--

~~:::::o==="--

--

----

----~

1

o~---~~~---~

N,

•

,

"

figure 2.4. (al Sharing of applied force and stress. (b) Volume

change.

a layer of fill for a highway embankment, placed on

a clayey foundation soil. As the fill is placed, pore water pressure in the foundation immediately in

-creases and then starts to dissipate, resulting in ,~'el­ llement. The rate of settlement depends primarily on the permeability of the foundation soil. Perme-ability is a measure of the rate al which water can move through the soil. Cohesive soils have lower permeability than cohcsionlcss soils. and therefore consolidation and settlement of cohesive soils occur more slowly.

2.1.7. Shear Strength

It has been shown above that effective stresses in-crease as consolidation progresses. Because an in-crease of effective stress means that the grains within the mineral skeleton are pressing more tightly together, it becomes increasingly harder to cause sliding between the grains. As an analogy, a brick can be placed on a concrete floor and pushed sideways to cause it to slide. If a second brick is

now placed on top of the first brick, it takes more sideways force to cause sliding. It is therefore ev i-dent that the ability of a soil to resist sliding is

re-lated to the effective stress: the larger the effective

stress in a particular soil, the greater is its shear strength. The shear strength is a measure of the resistance to sliding between grains that are trying to move laterally past each other.

It can now be seen that the gain in shear strength during the consolidation process can be monitored by measuring pore water pressure.

2.1.8. Normally Consolidated and

Overconsolidated Soil

A normally consolidated soil is one that has never

been subjected to an effective stress greater than the existing overburden pressure. Examples include ocean and lake-bed clays. An overconsolidated soil is one that has been subjected to an effective stress

greater than the existing overburden pressure. Ex-amples include clays such as London clay, where thousands of feet of overburden have been eroded. 2.1.9. Difference Between Pore Water Pressure and Groundwater Level

The groundwater level is defined as the upper sur-face of a body of groundwater at which the pressure is atmospheric.

Figure 2.5 shows three penorated pipes installed in a soil within which there is no How of gr oundwa-ter; therefore, groundwater pressure increases uni· formly with depth. When such equilibrium condi-tions exist, the level of water within the pipe will

rise to the groundwater level, independent of the

location of the penorations.

Now consider what happens when a layer of fill is placed above the sand shown in Figure 2.5.

Fig-Groundwater ...J.:"":'" __ .:::._

level . ' .

_{, ,}

..

... 1 :

Three venical pipes.

with perforations near bottom

, ,

_,

, ,

,

,

,

,

_,

""'

Figure 2.5. Groundwater level when there is no now of gr

BEHAVIOR OF SOil 17 Original

ground

s u r f _ a c e I

~

'.

::>1 :.,::

,~, -C~-~"-! C~

Figure 2.6. Groundwater level and pore water pressure when there is flow of groundwater.

ure 2.6 shows the condition soon after fill place-ment, when consolidation is not yet complete; therefore, excess pore water pressures exist in the clay and the groundwater is no longer in equilib-rium. The four perforated pipes in Figure 2.6 are installed such that soil is in intimate contact with the outsides of the pipes. Pipe (b) is perforated throughout its length; the remaining pipes are perforated only near the bottom. Because of the high permeability of sand, excess pore water pres-sures in the sand dissipate almost immediately and do not exist there. As in Figure 2.5, pipe (a) indi-cates the groundwater level. Pipes (c) and (d) indi-cate the pore water pressures in the clay at loca-tions (1) and (2). The water level in pipe (c) is shown lower than in pipe (d) because more dissipa-tion of pore water pressure has occurred at level (1)

than at level (2): the drainage path for excess pore water pressure is shorter, and therefore the rate of dissipation is greater. In the case illustrated, pipe

(b) is likely to indicate the groundwater level be-cause the permeability of the sand is substantially greater than that of the clay: excess pore water pressures in the clay will cause an upward flow of water from the clay to the sand, via the pipe.

In the more general case of a perforated pipe installed through two or more strata, either with perforations throughout or surrounded with sand throughout, an undesirable vertical connection be-tween strata is created, and the water level in the

pipe will usually be misleading. This situation is dis-cussed further in Sections 9.1 and 9.12.2, and pipe

(b) should not be used in practice.

Pipe (b) is called an observation well. Pipes (a),

(c), and (d) indicate pore water pressure and its dissipation within the sand or clay and are called

piezometers. Details of both types of instrument are given in Chapter 9. As a general rule, piezometers are sealed within the soil so that they respond only to changes of pore water pressure at a local zone, whereas observation wells are not sealed within the soil, so that they respond to changes of groundwa-ter pressure throughout their length.

2.1.10. Positive and Negative Pore Water Pressures

All references to pore water pressures in earlier parts of this chapter have been to pressures that are above atmospheric pressure. These are called

posi-tive pore water pressures. As shown in Figures 2.4a and 2.6, pore water pressure can be increased by applying a compressive force to the soil. The ex-ample of placing fill for a highway embankment has been given. Pore water pressure can also increase when a shear force is applied to a soil in which the mineral skeleton is in a loosely packed state. When the array shown in Figure 2.7a is sheared, it de-creases in volume. When the pore spaces are filled with water, and water is prevented from leaving, pore water pressure will increase. As a practical example, consider a foundation failure of an em-bankment on soft ground, with a foundation of loose alluvial material. The material beneath the toe is subjected to lateral shear forces as the embank-ment is constructed. These shear forces cause formation, increased pore water pressure, and de-creased strength, and therefore increase the tendency toward failure.

Pore water pressure can also be negative, defined as pore water pressure that is less than at-mospheric pressure. Negative pore water pressure can sometimes be caused by removing a compres-sive force that has been applied to a soil. For ex-ample, when an excavation is made in clay, the soil below the base of the excavation is unloaded, caus-ing an initial decrease of pore water pressure, which may become negative. Pore water pressure can also decrease when a shear force is applied to a soil in which the mineral skeleton is in a densely packed state. When the array shown in Figure 2.7b is sheared, it increases in volume. If the pore spaces

18 BEHAVIOR OF SOil AND ROCK Shear_-. . . . force

~~~~)!"''''

_--...!~~~~~~~~~~

Figure 2.7. (a) Positive and (b) negative pore water pressure caused by application of shear force.

are filled with water, and additional water is pre-vented from entering, pore water pressure de-creases. As a practical example, consider the exca-vation of a slope in overconsolidated clay. Pore water pressure decreases as a result of unloading, but significant additional decrease can be caused by the development of lateral shear forces. These shear forces cause deformation, a temporary de-crease in pore water pressure, and a temporary in-crease in strength.

2.1.11. Pore Gas Pressure

In an unsaturated soil, both gas and water are pres-ent in the pore spaces, and the pressure in the gas is called the pore gas pressure. As with pore water pressure, pore gas pressure act in all directions with equal intensity. Examples of unsaturated soil in-clude compacted fills for embankment dams and ganic soil deposits in which gas is generated as or-ganic material decomposes. When the gas is air, the term pore air pressure may be used.

The pore gas pressure is always greater than the pore water pressure. Consider the analogy of a straw placed in a container of water, as shown in

Meniscus Point B (pressure = P_w) Point A (pressure = Pa) Container Straw Water -+---Pa .:' ::. (a)

P_a = Pressure in air above meniscus

P_w = Pressure in water below meniscus

Pa>Pw

Air in pore space at pressure _ua

,'P-::~~oLWater in pore space at pressure U_w

(b)

Figure 2.8. Pore gas and pore water pressure: (a) straw in con-tainer of water and (b) element of unsaturated soil.

Figure 2.8a. The water level in the straw rises to a level higher than in the container and is "held up" by surface tension forces between the straw and water at the meniscus. The pressure in the air is atmospheric pressure Pa; therefore, the pressure at point A must also be P a: if it were not so, there would be a flow of water to create equality of pres-sures at the same level. The pressure at point A

must be greater than that at point B (Pw ), because the pressure in water increases as depth below the surface increases. Pa is therefore greater than Pw , and the pressure at the air side of the meniscus is greater than the pressure at the water side. It is well known that water in a smaller diameter straw rises to a greater height: the pressure difference across the meniscus is therefore greater, and the radius of curvature of the meniscus is smaller.

Now consider a meniscus between air and water in a pore space within soil, as shown in Figure 2.8b. The same rule applies as in the analogy: the pore air pressure U_a is greater than the pore water pressure

U_{w •} The smaller the pore space, the smaller the ra-dius of curvature of the meniscus, and therefore the difference between pore air pressure and pore water pressure is greater.

BEHAVIOR OF SOIL 19

2.1.12. Other Terms Relating to Behavior of Soil A perched groundwater level is above and not hy-draulically connected to the more general ground-water level. For example, groundground-water may be trapped above a clay layer at shallow depth, whereas the general groundwater level is in a lower layer of sand.

An aquifer is a pervious soil or rock stratum that contains water. An artesian aquifer is confined be-tween two relatively impervious layers and capable of carrying groundwater under pressure.

The piezometric elevation, or piezometric level, is the elevation to which water will rise in a pipe sealed within the soil, as shown for pipes (c) and (d)

in Figure 2.6.

2.1.13. Primary Mechanisms that Control Behavior of Soil

*

The primary mechanisms that control the engineer-ing behavior of soil may be categorized as hy-draulic, stress-deformation, and strength mecha-nisms.

When a soil is subjected to excess pore water pressure, the water flows through the pore spaces in the soil. These may be fairly large, as are the spaces

in openwork gravel, or microscopic, as in the spaces between the finest clay particles. As it flows, water reacts against the particles, causing friction and a resistance to flow. The amount of friction depends on both the velocity of flow and the size of the soil particles. In a very fine-grained soil, there is a larger area of contact between the water and soil particles than in a coarse-grained soil, so that there is more friction. The permeability of the soil is governed by the amount of friction and resistance to flow. Just as the soil acts on the water to retard the flow, so also does the water act on the soil. Water exerts a trac-tive force on the soil, in the direction of flow, owing to the friction. If the soil is saturated, water acts also with a buoyant force on the soil, whether or not the water is moving.

Stress-deformation characteristics of cohesive soil are governed by the time required for water to flow through the pore spaces in the soil and for consequent volume changes to occur. For cohesionless soils, most of the volume change is

* Written with the assistance of Norbert O. Schmidt, Professor of Civil Engineering, University of Missouri-Rolla, Rolla, MO.

caused by rearrangement of the relative positions of grains as shear deformation occurs.

Shear strength is governed by the nature, size and shape of the soil grains, packing density and effective stresses within the soil. Shear failure oc-curs when the stresses increase beyond those that can be sustained by the soil and the strength is ex-ceeded.

Footing Foundations

Figure 2.9 depicts a load applied to a footing foun-dation located on a soil mass. In the upper figure, the load is substantially less than that required to cause failure. The footing transfers the stress to the soil, causing the soil to compress in the vertical direction, and the footing settles. In the lower figure, a larger load is applied to the footing, stress-ing the soil and causstress-ing a rotation that tends to push the adjacent block of soil out of the ground, as if it were a rigid body. This movement is opposed by the shearing resistance of the soil along the potential failure surface. In a cohesionless soil, this shearing resistance is frictional, and the magnitude of friction depends directly on the normal stresses against the potential failure surface. In a cohesive soil, this shearing resistance is often purely cohesive, inde-pendent of the normal stresses during the shearing process but dependent on the history of previous effective stresses.

Deep Foundations

Figure 2.10 shows a loaded pile or drilled shaft, embedded in soil. The pile moves downward in re-sponse to the load and, as it moves with respect to the soil, it mobilizes the skin friction or shearing stress, which resists the sliding along the soil/pile interface. This accounts for part of the support that the pile receives from the soil. The rest of the sup-port is from point bearing, the same mechanism that supports a footing on soil.

When soil settles with respect to a pile or drilled shaft, the pile is loaded by frictional forces at the interface, causing an increase in stress within the pile and/or increased settlement. The loading is referred to as downdrag or negative skin

fric-tion. Downdrag loading may be caused by several events. First, when piles are driven through fill that overlies clay, the loading from the fill may cause continuing primary consolidation or second-ary time effects. Second, settlements are caused

20 BEHAVIOR OF SOil AND ROCK So i I co m presses (a) Load

~

Figure 2.9. Behavior of a footing foundation: (a) settlement of footing and (b) bearing failure.

when ground smface loading is increased after piles are driven, for example, by placing an approach fill alongside a pile-supported bridge abutment. Third, dewatering, site grading, and/or vibration during pile driving may cause settlement and downdrag loading.

Load

,

J

~

t

n

~,

Development of "point" bearing

Figure 2.10. Behavior of a driven pile or drilled shaft.

Excavated and Natural Slopes

Excavated and natural slopes often involve layered sediments, which may be parallel to the original sur-face of the slope. One of these layers is usually weaker than the others, so the potential surface of sliding is noncircular and tends to follow the weak-est layer as depicted in Figure 2.11. When the soil is relatively homogeneous, the potential surface of sliding may be more circular. The stability of slopes in soil is controlled by the ratio between available shearing resistance along a potential sur-face of sliding and the shear stress on that sursur-face. Any increase in pore water pressure along the po-tential surface of sliding decreases the shearing re-sistance and the factor of safety against sliding.

Retaining Walls

Soil is only partially capable of supporting itself. Sand requires a retaining structure if it is to stand vertically, but clay may support itself vertically for limited heights for some period of time. Figure 2.12 shows a retaining wall with forces against it. The earth pressure shown pushing the wall towards the

BEHAVIOR OF SOil 21

resistance opposes movement

Figure 2.11. Failure of a slope in layered soil.

left is that portion of the pressure that remains after the soil tries to support itself. It is known as the

active earth pressure and is reduced to its minimum value when the wall slides and tilts slightly to the left. The sliding also mobilizes the shearing resis-tance between the soil and the base of the wall. In front of the wall on the left, the passive earth

pres-sure is mobilized as the soil resists sliding.

Braced Excavations

When compared with retaining walls, braced exca-vations differ in their mobilization of earth resis-tance because significant sliding or tilting toward the excavation is not permitted. The wall of a braced excavation may be braced against the oppo-site wall with cross-lot bracing, braced against the bottom or the adjacent wall with rakers, or held in place by tieback anchors that pull against the soil outside the excavation. Stresses against a braced wall are greater than against a retaining wall be-cause the passive pressure is greater. However, if bracing is properly installed, deformation of the soil is small and buildings on adjacent sites may there-fore experience only small settlements and little dis-tress. When a deep braced excavation is made in soft soil, soil tends to move into the bottom of the excavation: this is called bottom heave, and special construction techniques may be required to control deformation.

Embankments

Embankments of relatively homogeneous soils overlying hard ground tend to fail by rotation along an almost circular arc. This is depicted in Figure 2.13. As with the bearing failure of a footing found

a-o· .' o· Force caused Force caused . . . . - by active earth ~ pressure pushing on wall by passive . . . :c:.:. p'. " ()' '.0. o. earth pressure _ _

f

• ---

f

f

f

Figure 2.12. Behavior of a retaining wall.

tion, shown in Figure 2.9, the soil behaves as if it were a rigid body. Rotation is resisted by mobiliza-tion of shearing resistance along the arc, as de-scribed for footing foundations.

Embankments on Soft Ground

The behavior of embankments on soft ground tends to be dominated by the properties of the soft ground. A potential circular failure surface may de-velop, with a large portion of the surface in the weak foundation material as shown in Figure 2.14a. However, the loading of the embankment may cause settlement and lateral bulging of the founda-tion, as shown in Figure 2.14b, long before the rota-tional failure occurs. The lateral bulging of the soft ground transfers horizontal tension to the embank-ment, which may experience tension cracking, since it is less deformable than the soft foundation.

Embankment Dams

Embankment dams may experience distress or fail-ure in a variety of ways.

Overtopping may result from incorrect

estima-Shearing resistance opposes movement

Figure 2.13. Behavior of an embankment of relatively homoge-neous soil.

22 BEHAVIOR OF SOil AND ROCK (a) ~ ~ ¢:J ¢:l

{J,

Q

c:::>

Figure 2.14. Behavior of an embankment on soft ground: (a) rota-tional slide along arc and (b) settlement and lateral bulging of soft foundation.

tion of storm water volume or duration. It may also be the result of slope failure on the reservoir rim, causing a large volume of material to slide into the reservoir and induce a wave, as occurred at Viaont Dam in Italy.

Internal erosion, or piping, can cause distress to an embankment dam. Erosion occurs where seep-age water is flowing at a velocity sufficient to carry soil particles along with the water. If the soil has some measure of cohesion, a pipe or small tunnel may form at the downstream exit of the seepage path as the soil is eroded. As the pipe lengthens in-side the embankment, there is less resistance to flow, the flow in the pipe increases, and piping ac-celerates. Silts and fine sands are most prone to piping, because they erode easily, and when moist are sufficiently cohesive to form the walls of a pipe without collapsing. In a well-designed embankment dam, piping is prevented by sizing downstream ma-terial such that the pore spaces of that mama-terial are just smaller than the larger sizes of the mineral skeleton in the adjacent upstream material, so that migration of particles is blocked. Such a down-stream material is known as a./ilter.

When the slopes of a dam are too steep, the dam may fail as described above in the discussion on embankments. Many river sediments consist of soft

ground, and a dam constructed over these materials may behave as shown in Figure 2.14.

If a dam is built on loose granular material,

espe-Original elevation of crest

After

Transverse crack

Figure 2.15. Transverse cracking of an embankment dam.

cially silty sand, an earthquake can cause the foun-dation to liquify and flow, as with the near failure of the San Fernando Dam in California.

Even if the design of the dam is adequate, the weight of the embankment dam on the underlying soil or rock must be considered. Heavily loaded soil under the dam may settle, and there will be down-ward and lateral movements of the base of the dam. Moreover, even well-compacted fill material will experience settlements when loaded with over-lying material. and poor compaction procedures will result in greater settlements. If the crest of the dam is initially level, with time it will settle, and the center of the dam will settle the most. If the abut-ments are steep, the settleabut-ments may put the crest of the dam in tension, as shown in Figure 2.15, pos-sibly causing cracks transverse to the axis of the dam.

Soft Ground Tunnels

When a tunnel is excavated in soft ground, it may be excavated to a diameter slightly larger than its lining so that the lining may be placed, and the soil may be unsupported for a short time period. Stresses are relieved and soil tends to move inwards toward the tunnel cross section as well as into the face. Although the space between the soil and lining is usually small and may be grouted, the ground surface may settle, with greatest settlements occur-ring directly over the tunnel as shown in Figure 2.16.

If the groundwater level was previously above the invert of the tunnel, the tunnel may drain the soil. In granular soils, the groundwater may have to be predrained to the level, of the invert, so that the tunnel can be constructed. The reduction in ground-water level will increase effective stresses in the soil