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John Dunnicliff
GEOTECHNICAL
INSTRUMENTATION
FOR MONITORING
FIELD PERFORMANCE
GEOTECHNICAL INSTRUMENTATION
FOR MONITORING
FIELD PERFORMANCE
A Wiley-Interscience Publication
John Wiley & Sons
New York / Chichester / Brisbane / Toronto / Singapore
II
WILEY
Geotechnical Engineer
Seattle, Washington
Gordon E. Green
With the assistance of:
John Dunnicliff
Geotechnical Instrumentation Consultant
Lexington, Massachusetts
GEOTECHNICAL INSTRUMENTATION
FOR MONITORING
FIELD PERFORMANCE
1 0 9 8 7 6 5 4 3 2 1
P r i n t e d i n t h e U n i t e d S t a t e s o f A m e r i c a
L i b r a r y o f C o n g r e s s C a t a l o g i n g i n P u b l i c a t i o n D a t a :
D u n n i c 1 i f f , J o h n .
G e o t e c h n i c a l i n s t r u m e n t a t i o n f o r m o n i t o r i n g f i e l d p e r f o r m a n c e /
J o h n D u n n i c 1 i f f w i t h t h e a s s i s t a n c e o f G o r d o n E . G r e e n .
" A W i l e y - I n t e r s c i e n c e p u b l i c a t i o n . "
I S B N 0 - 4 7 1 - 0 9 6 1 4 - 8
1 . E n g i n e e r i n g g e o l o g y - I n s t r u m e n t s . 2 . S o i l s - T e s t i n g .
I . G r e e n , G o r d o n E . I I . T i t l e .
T A 7 0 5 . D 8 6 1 9 8 8
6 2 4 . 1 ' 5 1 ' 0 2 8 - d c I 9 8 7 - 2 7 7 0 9
C o p y r i g h t © 1 9 8 8 b y J o h n W i l e y & S o n s , I n c .
A l l r i g h t s r e s e r v e d . P u b l i s h e d s i m u l t a n e o u s l y i n C a n a d a .
R e p r o d u c t i o n o r t r a n s l a t i o n o f a n y p a r t o f t h i s w o r k
b e y o n d t h a t p e r m i t t e d b y S e c t i o n 1 0 7 o r 1 0 8 o f t h e
1 9 7 6 U n i t e d S t a t e s C o p y r i g h t A c t w i t h o u t t h e p e r m i s s i o n
o f t h e c o p y r i g h t o w n e r i s u n l a w f u l . R e q u e s t s f o r
p e r m i s s i o n o r f u r t h e r i n f o r m a t i o n s h o u l d b e a d d r e s s e d t o
t h e P e r m i s s i o n s D e p a r t m e n t , J o h n W i l e y & S o n s , I n c .
To the manufacturers of
geotechnical instruments,
without whom there would be no
geotechnical instrumentation
for monitoring field performance
vii
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 impendingfailure. Part
of the criticism directed at Teton Dam followingits
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 shouldjudge 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
Every geotechnical design is to some extent hypo-
thetical, and every construction job involvingearth
or rock runs the risk of encountering surprises.
These circumstances are the inevitable result of
workingwith 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 significantproperties 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
mightbe 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, fieldinstrumentation 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 colleaguesin other fields,must have more than
casual knowledgeof instrumentation: to them it is a
workingtool, 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
FOREWORD
s u r e t h a t d e p e n d a b l e d a t a c a n b e o b t a i n e d t h r o u g h -
o u t t h e p e r i o d w h e n t h e o b s e r v a t i o n s a r e n e e d e d .
U s u a l l y , t h e m o s t d e p e n d a b l e d e v i c e s a r e t h e s i m -
p l e s t .
I f o n e c a n w i t h s u f f i c i e n t a c c u r a c y m a k e a d i r e c t
v i s u a l o b s e r v a t i o n w i t h a g r a d u a t e d s c a l e , t h e n a
m i c r o m e t e r s h o u l d n o t b e u s e d . I f o n e c a n u s e a
m i c r o m e t e r , a m e c h a n i c a l s t r a i n g a g e s h o u l d n o t b e
u s e d . I f o n e c a n u s e a m e c h a n i c a l s t r a i n g a g e , a n
e l e c t r i c a l o n e s h o u l d n o t b e u s e d . M e c h a n i c a l i n -
s t r u m e n t s a r e t o b e p r e f e r r e d t o e l e c t r i c a l d e v i c e s
a n d s i m p l e e l e c t r i c a l d e v i c e s d e p e n d i n g o n s i m p l e
c i r c u i t s a r e t o b e p r e f e r r e d t o m o r e c o m p l e x e l e c -
t r o n i c e q u i p m e n t . T h a t i s , w h e r e a c h o i c e e x i s t s ,
t h e s i m p l e r e q u i p m e n t i s l i k e l y t o h a v e t h e b e s t
c h a n c e f o r s u c c e s s .
N e v e r t h e l e s s , s i m p l e i n s t r u m e n t s a r e s o m e t i m e s
i n a p p r o p r i a t e a n d m o r e c o m p l e x o n e s m u s t b e
u s e d . A n o p e n s t a n d p i p e m a y b e t h e s i m p l e s t d e -
v i c e f o r o b s e r v i n g a p i e z o m e t r i c l e v e l , b u t t h e p o i n t
a t w h i c h t h e p o r e p r e s s u r e n e e d s t o b e m e a s u r e d
m a y b e l o c a t e d w h e r e d i r e c t a c c e s s i s i m p o s s i b l e . A
m o r e s o p h i s t i c a t e d a r r a n g e m e n t i s t h e n n e c e s s a r y .
I f o n e w i s h e s t o d e t e r m i n e t h e s t a t e o f s t r e s s i n a
m a s s o f r o c k , t h e r e i s n o c h o i c e b u t t o i n s t a l l
s o p h i s t i c a t e d e q u i p m
e n t t o m a k e m e a s u r e m e n t s a t
a c o n s i d e r a b l e d i s t a n c e f r o m t h e p o s i t i o n o f t h e o b -
s e r v e r , a n d t h e s t r a i n s l i k e l y t o b e o b s e r v e d w i l l b e
t o o s m a l l t o d e t e c t b y a n y m e c h a n i c a l d e v i c e . T h u s ,
n o t h i n g b u t a s o p h i s t i c a t e d s y s t e m w i l l s e r v e .
N o t a l l s o p h i s t i c a t e d s y s t e m s a r e e q u a l l y r e l i -
a b l e . E q u i p m e n t t h a t h a s a n e x c e l l e n t r e c o r d o f p e r -
f o r m a n c e c a n b e r e n d e r e d u n r e l i a b l e i f a s i n g l e
e s s e n t i a l b u t a p p a r e n t l y m i n o r r e q u i r e m e n t i s o v e r -
l o o k e d d u r i n g t h e i n s t a l l a t i o n . T h e b e s t o f i n s t r u c -
t i o n m a n u a l s c a n n o t p r o v i d e f o r e v e r y f i e l d c o n d i -
t i o n t h a t m a y a f f e c t t h e r e s u l t s . T h e r e f o r e , e v e n
s l a v i s h a t t e n t i o n t o i n s t r u c t i o n s c a n n o t g u a r a n t e e
s u c c e s s . T h e i n s t a l l e r m u s t h a v e a b a c k g r o u n d i n
t h e f u n d a m e n t a l s o f g e o t e c h n i c s a s w e l l a s k n o w l -
e d g e o f t h e i n t r i c a c i e s o f t h e d e v i c e b e i n g i n s t a l l e d .
S o m e t i m e s t h e i n s t a l l e r m u s t c o n s c i o u s l y d e p a r t
f r o m t h e i n s t a l l a t i o n m a n u a l .
T h e i n s t a l l e r m u s t a l s o w a n t d e s p e r a t e l y t o d o
t h e j o b w e l l a n d m u s t o f t e n w o r k u n d e r d i f f i c u l t a n d
u n p l e a s a n t c o n d i t i o n s , t r y i n g t o d o p r e c i s i o n w o r k
w h i l e s u r r o u n d e d b y w o r k e r s w h o s e t e a m w o r k o r
o p e r a t i o n o f e q u i p m e n t i s b e i n g i n t e r r u p t e d , o r
w o r k i n g t h e g r a v e y a r d s h i f t i n a n a t t e m p t t o r e d u c e
s u c h i n t e r r u p t i o n s . D e d i c a t i o n o f t h i s s o r t i s t h e
p r i c e o f s u c c e s s , a n d i t i s r a r e l y f o u n d a t t h e p r i c e
t e n d e r e d b y t h e l o w e s t b i d d e r . M o r e o v e r , t h e i n -
q u e s t i o n s c a n b e f r a m e d - t h e a n s w e r s t o w h i c h w i l l
f i l l t h e g a p s o r c o r r e c t t h e e r r o r s i n t h e o r i g i n a l d e -
s i g n a s s u m p t i o n s - a n d t h e e n g i n e e r c a n d e t e r m i n e
w h a t i n s t r u m e n t s , a t w h a t l o c a t i o n s , c a n a n s w e r
t h o s e q u e s t i o n s .
O f c o u r s e , n o t a l l i n s t r u m e n t s a r e i n s t a l l e d t o
m o n i t o r t h e s a f e t y o f a s t r u c t u r e o r c o n s t r u c t i o n
o p e r a t i o n o r t o c o n f i r m d e s i g n a s s u m p t i o n s . S o m e
a r e u s e d t o d e t e r m i n e i n i t i a l o r b a c k g r o u n d c o n d i -
t i o n s . O b s e r v a t i o n s o f g r o u n d w a t e r p r i o r t o c o n -
s t r u c t i o n , o r i n s i t u s t r e s s e s i n r o c k m a s s e s , o r o f
e l e v a t i o n s o f s t r u c t u r e s b e f o r e t h e s t a r t o f a d j a c e n t
c o n s t r u c t i o n a r e e x a m p l e s . C e r t a i n t y p e s o f c o n -
s t r u c t i o n , s u c h a s t h e i n s t a l l a t i o n o f t i e b a c k s , a r e
i n h e r e n t l y d e p e n d e n t o n i n s t r u m e n t a t i o n . F u r t h e r -
m o r e , a d v a n c e m e n t s i n t h e s t a t e o f o u r k n o w l e d g e
r e q u i r e l a r g e - s c a l e o r f u l l - s c a l e o b s e r v a t i o n s o f a n
e x t e n t a n d c o m p l e x i t y f a r b e y o n d t h e r e q u i r e m e n t s
o f t h e p r a c t i c i n g e n g i n e e r . Y e t , i n a l l t h e s e a p p l i c a -
t i o n s , i t i s e q u a l l y t r u e t h a t e v e r y i n s t r u m e n t s h o u l d
b e s e l e c t e d a n d l o c a t e d t o a s s i s t i n a n s w e r i n g a
s p e c i f i c q u e s t i o n .
I n s t r u m e n t a t i o n n e e d s t o b e k e p t i n p e r s p e c t i v e .
I t i s o n e p a r t o f t h e b r o a d e r a c t i v i t i e s o f o b s e r v a t i o n
a n d s u r v e i l l a n c e . T r a i n e d p e o p l e , u s i n g t h e b e s t o f
a l l i n s t r u m e n t s , t h e h u m a n e y e , c a n o f t e n p r o v i d e
a l l t h e i n f o r m a t i o n n e c e s s a r y a n d a r e a l w a y s a n e s -
s e n t i a l p a r t o f t h e f i e l d o b s e r v a t i o n s o n a n y p r o j e c t .
E v e n w h e n i n s t r u m e n t s a r e u s e d b e c a u s e t h e n e c e s -
s a r y q u a n t i t i e s a r e t o o s m a l l t o b e o b s e r v e d b y e y e ,
o r t h e e v e n t s a r e t a k i n g p l a c e o u t o f t h e r e a c h o f a
h u m a n b e i n g , t h e f i n d i n g s m u s t b e r e l a t e d t o o t h e r
a c t i v i t i e s . W i t h o u t g o o d r e c o r d s o f t h e p r o g r e s s o f
e x c a v a t i o n a n d o f d e t a i l s o f t h e e x c a v a t i o n a n d
b r a c i n g p r o c e d u r e s , f o r e x a m p l e , t h e r e s u l t s o f
m e a s u r e m e n t s o f d e f o r m a t i o n s o r e a r t h p r e s s u r e
a s s o c i a t e d w i t h a b r a c e d c u t b e c o m e a l m o s t m e a n -
i n g l e s s . A m o n g t h e m o s t v a l u a b l e u s e s o f i n s t r u -
m e n t a t i o n a r e e m p i r i c a l c o r r e l a t i o n s b e t w e e n c o n -
s t r u c t i o n p r o c e d u r e s a n d d e f o r m a t i o n s o r p r e s -
s u r e s , c o r r e l a t i o n s t h a t c a n b e u s e d i m m e d i a t e l y t o
i m p r o v e t h e p r o c e d u r e s s o a s t o r e d u c e t h e m o v e -
m e n t s o r p r e s s u r e s . T h u s , h i g h l y s o p h i s t i c a t e d ,
f u l l y a u t o m a t e d i n s t a l l a t i o n s f o r o b t a i n i n g a n d p r e -
s e n t i n g d a t a , s o m e t i m e s h e l d i n h i g h f a v o r b y t h o s e
i n t r i g u e d w i t h g a d g e t r y , m a y f a i l t o s e r v e a u s e f u l
p u r p o s e b e c a u s e t h e s i m p l e v i s u a l o b s e r v a t i o n s o f
w h a t m a y b e a f f e c t i n g t h e r e a d i n g s a r e o v e r l o o k e d .
N o t o n l y i s i n s t r u m e n t a t i o n n o t a n e n d i n i t s e l f ,
b u t n e i t h e r i s s o p h i s t i c a t i o n o r a u t o m a t i o n . T h e t w o
p r i m e r e q u i r e m e n t s a r e s e n s i t i v i t y s u f f i c i e n t t o p r o -
v i d e t h e n e c e s s a r y i n f o r m a t i o n a n d r e l i a b i l i t y t o e n -
F O R E W O R D
v i i i
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 field-instrumentation 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 of the capabilities and
shortcomings of the instruments themselves, and an
appreciation of the practical problems of installa-
tion. Italso 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.
Smallwonder that the need exists for a book dealing
comprehensively with this subject.
RALPH B. PECK-
ixFOREWORD
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
xi
JOHN DUNNICLIFF
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!
This is intended to be a practical book for use by
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
PREFACE
Arild Andresen Herbert J. Dix Thomas K. Liu
WilliamR. Beloff James Dorsey C. Leroy McAnear
Douglas J. Belshaw Edward J. Drelich Verne C. McGuffey
Bradford P. Boisen Charles N. Easton John B. McRae
Jean-Louis Bordes Alex I. Feldman P. Erik Mikkelsen
Jean Boucher Bengt H. Fellenius Anthony Minnitti
Michael Bozozuk Rainer Glotzl Dewayne L.Misterek
Ed Brylawski Charles W. Hancock, r-. Ian Mitchell
Georgi A. Buckley Leo D. Handfelt John G. Morrison
Thomas G. Bumala Richard K. Harris MichaelW. O'Neill
Roy W. Carlson David G. F. Hedley Walter Nold
Pierre Carrier Anwar Hirany Ralph B. Peck
David J. Clements Robert D. Holtz Arthur D. M. Penman
J. Barrie Cooke Gary R. Holzhausen Edward C. Pritchett
Christopher B. H. Cragg Robert G. Horvath Red Robinson
J. Clive
P. Dalton Bob Joy Arthur Ross
Richard R. Davidson Kunsoo Kim Birger Schmidt
Brian J. Dawes Peter Lang Ernest T. Selig
George B. Deardorff Pierre LeFrancois Dale Shoup
Elmo B. DiBiagio Jeffrey M. Lingham Tony Simmonds
Walter Dieden G. Stuart Littlejohn Patrick D. K. Smith
xiii
I would not have been able to write this book without the help of many
people.
First, Gordon E. Green, Associate, Golder Associates, Inc., Seattle, WA
(formerlywith Shannon & Wilson, Inc., Seattle, WA), has made an extraor-
dinary commitment of time and enthusiasm to this book. He has givenmany
hours to guide me with content and format, to provide a second opinion on
numerous issues ofjudgment, and to make detailed reviews of the chapters
in Parts 2, 3, and 4. For this untiring dedication, I can only express my great
gratitude. No author could hope for more.
Second, some colleagues have coauthored or assisted with sections or
chapters. When I felt that my own experience of a subject has been too
limited, I asked one or more engineers to help me with the text. Their names
and affiliations are given in footnotes on appropriate pages. To these col-
leagues I say, thank you-for enduring my persistent questioning and for
helping me to convert my shaky drafts into texts fit for a book. Your roles
have been crucial.
Third, some colleagues have helped me by providing facts or opinions on
a multitude of subjects. These include the following:
ACKNOWLEDGMENTS
T o t h e s e c o l l e a g u e s - t h a n k y o u f o r p a s s i n g o n y o u r e x p e r i e n c e a n d f o r
g u i d i n g m e a s I t r i e d t o r e s o l v e s o m a n y u n c e r t a i n t i e s .
F o u r t h , I t h a n k J u d y G r a n d e a n d S a r a h M a t t h e w s , w h o s e w o r d p r o c e s s -
i n g s k i l l s a n d r e s p o n s i v e n e s s t o d e a d l i n e s h a v e b e e n o u t s t a n d i n g .
L a s t , b u t n e v e r l e a s t , m y w i f e , M a r g a r e t , a n d m y c h i l d r e n , C h r i s t o p h e r ,
J o n a t h a n , a n d T a n y a . T h e i r t o l e r a n c e a n d u n d e r s t a n d i n g d u r i n g t h e 5 y e a r s
o f w r i t i n g h a v e m a d e i t a l l p o s s i b l e .
J a m e s R . W h e e l e r
S t a n l e y D . W i l s o n
A n w a r E . Z . W i s s a
S t e p h e n P . W n u k
J o h n R . W o l o s i c k
P e t e r R . V a u g h a n
J a m e s W a r n e r
R o b e r t C . W e e k s
W i l l i a m A . W e i l e r , J r .
C l a r k W e l d e n
T e r r y S t e v e n s
K a l m a n S z a l a y
D u n c a n T h a r p
P e t u r T h o r d a r s o n
A r n o T h u t
B e n g t - A r n e T o r s t e n s s o n
A C K N O W L E D G M E N T Sx i v
xv
Chapter 4 Systematic Approach to Planning Monitoring Programs Using
Geotechnical Instrumentation / 37
4.1 Define the Project Conditions, 37
4.2 Predict Mechanisms that Control Behavior, 38
4.3 Define the Geotechnical Questions that Need to Be Answered, 38
4.4 Define the Purpose of the Instrumentation, 38
4.5 Select the Parameters to Be Monitored, 38
4.6 Predict Magnitudes of Change, 38
4.7 Devise Remedial Action, 39
4.8 Assign Tasks for Design, Construction, and Operation Phases, 39
4.9 Select Instruments, 40
4.10 Select Instrument Locations, 42
Chapter 3 Benefits of Using Geotechnical Instrumentation / 33
3.1 Benefits During Design, 33
3.2 Benefits During Construction, 34
3.3 Benefits After Construction Is Complete, 36
3.4 General Considerations, 36
PART 2 PLANNING MONITORING PROGRAMS
Chapter 2 Behavior of Soil and Rock / 13
2.1 Behavior of Soil, 13
2.2 Behavior of Rock, 23
Chapter 1 Geotechnical Instrumentation: An Overview / 3
1.1 What Is Geotechnical Instrumentation?, 3
1.2 Why Do We Need to Monitor Field Performance?, 3
1.3 What Capabilities Must the People Have?, 5
1.4 What Capabilities Must the Instruments Have?, 5
1.5 Where Have We Been?, 5
1.6 Where Are We Now?, 5
1.7 Where Are We Going?, 10
1.8 The Key to Success, 12
PART 1 INTRODUCTION
CONTENTS
C h a p t e r 8 I n s t r u m e n t a t i o n T r a n s d u c e r s a n d D a t a A c q u i s i t i o n
S y s t e m s / 7 9
8 . 1 M e c h a n i c a l I n s t r u m e n t s , 7 9
8 . 2 H y d r a u l i c I n s t r u m e n t s , 8 0
8 . 3 P n e u m a t i c I n s t r u m e n t s , 8 7
8 . 4 E l e c t r i c a l I n s t r u m e n t s , 9 2
C h a p t e r 7 M e a s u r e m e n t U n c e r t a i n t y / 7 5
7 . 1 C o n f o r m a n c e , 7 5
7 . 2 A c c u r a c y , 7 5
7 . 3 P r e c i s i o n , 7 5
7 . 4 R e s o l u t i o n , 7 6
7 . 5 S e n s i t i v i t y , 7 6
7 . 6 L i n e a r i t y , 7 6
7 . 7 H y s t e r e s i s , 7 6
7 . 8 N o i s e , 7 7
7 . 9 E r r o r , 7 7
P A R T 3 M O N I T O R I N G M E T H O D S
C h a p t e r 6 C o n t r a c t u a l A r r a n g e m e n t s f o r F i e l d I n s t r u m e n t a t i o n
S e r v i c e s / 5 7
6 . 1 G o a l s o f C o n t r a c t u a l A r r a n g e m e n t s , 5 7
6 . 2 D e f i n i t i o n o f T e r m s , 5 7
6 . 3 C o n t r a c t u a l A r r a n g e m e n t s f o r I n s t r u m e n t I n s t a l l a t i o n , 5 9
6 . 4 C o n t r a c t u a l A r r a n g e m e n t s f o r R e g u l a r C a l i b r a t i o n a n d
M a i n t e n a n c e , 6 1
6 . 5 C o n t r a c t u a l A r r a n g e m e n t s f o r D a t a C o l l e c t i o n , P r o c e s s i n g ,
P r e s e n t a t i o n , I n t e r p r e t a t i o n , a n d R e p o r t i n g , 6 1
6 . 6 C o n t e n t o f S p e c i f i c a t i o n s f o r F i e l d I n s t r u m e n t a t i o n S e r v i c e s , 6 3
C h a p t e r 5 S p e c i f i c a t i o n s f o r P r o c u r e m e n t o f I n s t r u m e n t s / 4 5
5 . 1 T a s k A s s i g n m e n t f o r P r o c u r e m e n t , 4 5
5 . 2 S p e c i f y i n g M e t h o d , 4 6
5 . 3 B a s i s f o r D e t e r m i n i n g P r i c e , 4 7
5 . 4 C o n t e n t o f S p e c i f i c a t i o n s f o r P r o c u r e m e n t o f I n s t r u m e n t s , 4 9
P l a n R e c o r d i n g o f F a c t o r s t h a t M a y I n f l u e n c e M e a s u r e d D a t a , 4 3
E s t a b l i s h P r o c e d u r e s f o r E n s u r i n g R e a d i n g C o r r e c t n e s s , 4 3
L i s t t h e S p e c i f i c P u r p o s e o f E a c h I n s t r u m e n t , 4 3
P r e p a r e B u d g e t , 4 3
W r i t e I n s t r u m e n t P r o c u r e m e n t S p e c i f i c a t i o n s , 4 4
P l a n I n s t a l l a t i o n , 4 4
P l a n R e g u l a r C a l i b r a t i o n a n d M a i n t e n a n c e , 4 4
P l a n D a t a C o l l e c t i o n , P r o c e s s i n g , P r e s e n t a t i o n , I n t e r p r e t a t i o n ,
R e p o r t i n g , a n d I m p l e m e n t a t i o n , 4 4
W r i t e C o n t r a c t u a l A r r a n g e m e n t s f o r F i e l d I n s t r u m e n t a t i o n
S e r v i c e s , 4 4
U p d a t e B u d g e t , 4 4
C O N T E N T S
4 . 2 0
4 . 1 9
4 . 1 1
4 . 1 2
4 . 1 3
4 . 1 4
4 . 1 5
4 . 1 6
4 . 1 7
4 . 1 8
x v i
Chapter 13 Measurement of Load and Strain in Structural Members I 297
13.1 Instrument Categories and Applications, 297
13.2 Load Cells, 297
Chapter 12 Measurement of Deformation / 199
12.1 Instrument Categories, 199
12.2 Surveying Methods, 199
12.3 Surface Extensometers, 209
12.4 Tiltmeters, 216
12.5 Probe Extensometers, 219
12.6 Fixed Embankment Extensometers, 233
12.7 Fixed Borehole Extensometers, 237
12.8 Inclinometers, 250
12.9 Transverse Deformation Gages, 268
12.10 Liquid Level Gages, 275
12.11 Miscellaneous Deformation Gages, 292
Chapter 11 Measurement of StressChange in Rock / 185
11.1 Applications, 185
11.2 Instrument Categories, 185
11.3 Soft Inclusion Gages, 186
\11.4 Rigid Inclusion Gages, 191
11.5 Recommended Procedures for Measurement of Stress Change in
Rock, 195
Chapter 10 Measurement of Total Stress in Soil / 165
10.1 Instrument Categories and Applications,
165
10.2 Embedment Earth Pressure Cells, 165
10.3 Contact Earth Pressure Cells, 177
Chapter 9 Measurement of Groundwater Pressure / 117
9.1 Instrument Categories and Applications, 117
9.2 Observation Wells, 118
9.3 Open Standpipe Piezometers, 118
9.4 Twin-Tube Hydraulic Piezometers, 123
9.5 Pneumatic Piezometers, 126
9.6 Vibrating Wire Piezometers, 127
9.7 Electrical Resistance Piezometers, 128
9.8 Miscellaneous Single-Point Piezometers, 130
9.9 Multipoint Piezometers, 136
9.10 Hydrodynamic Time Lag, 139
9.11 Types of Filter, 141
9.12 Recommended Instruments for Measuring Groundwater Pressure
in Saturated Soil and Rock, 141
9.13 Recommended Instruments for Measuring Pore Water Pressure in
Unsaturated Soil, 144
9.14 Saturation of Filters, 146
9.15 Installation of Piezometers in Fill, 148
9.16 Installation of Piezometers by the Push-in Method, 148
9.17 Installation of Piezometers in Boreholes in Soil, 150
9.18 Installation of Piezometers in Boreholes in Rock, 163
xviiCONTENTS
C h a p t e r 1 8 C o l l e c t i o n , P r o c e s s i n g , P r e s e n t a t i o n , I n t e r p r e t a t i o n , a n d
R e p o r t i n g o f I n s t r u m e n t a t i o n D a t a / 3 6 7
1 8 . 1 C o l l e c t i o n o f I n s t r u m e n t a t i o n D a t a , 3 6 7
1 8 . 2 P r o c e s s i n g a n d P r e s e n t a t i o n o f I n s t r u m e n t a t i o n D a t a , 3 7 4
1 8 . 3 I n t e r p r e t a t i o n o f I n s t r u m e n t a t i o n D a t a , 3 8 2
1 8 . 4 R e p o r t i n g o f C o n c l u s i o n s , 3 8 4
C h a p t e r 1 7 I n s t a l l a t i o n o f I n s t r u m e n t s / 3 4 7
1 7 . 1 C o n t r a c t u a l A r r a n g e m e n t s f o r I n s t a l l i n g I n s t r u m e n t s , 3 4 7
1 7 . 2 L o c a t i o n s o f I n s t r u m e n t s , 3 4 7
1 7 . 3 D e t a i l e d I n s t a l l a t i o n P r o c e d u r e s , 3 4 8
1 7 . 4 I n s t a l l a t i o n a t t h e G r o u n d S u r f a c e , 3 4 8
1 7 . 5 I n s t a l l a t i o n i n B o r e h o l e s , 3 4 8
1 7 . 6 I n s t a l l a t i o n i n F i l l , 3 5 8
1 7 . 7 I n s t a l l a t i o n i n U n d e r g r o u n d E x c a v a t i o n s , 3 6 3
1 7 . 8 P r o t e c t i o n f r o m D a m a g e , 3 6 3
1 7 . 9 A c c e p t a n c e T e s t s , 3 6 4
1 7 . 1 0 I n s t a l l a t i o n R e c o r d s , 3 6 4
1 7 . 1 1 I n s t a l l a t i o n S c h e d u l e , 3 6 4
1 7 . 1 2 C o o r d i n a t i o n o f I n s t a l l a t i o n P l a n s , 3 6 5
1 7 . 1 3 F i e l d W o r k , 3 6 5
1 7 . 1 4 I n s t a l l a t i o n R e p o r t , 3 6 6
C h a p t e r 1 6 C a l i b r a t i o n a n d M a i n t e n a n c e o f I n s t r u m e n t s / 3 4 3
1 6 . 1 I n s t r u m e n t C a l i b r a t i o n , 3 4 3
1 6 . 2 I n s t r u m e n t M a i n t e n a n c e , 3 4 5
C h a p t e r 1 5 A R e c i p e f o r R e l i a b i l i t y o f P e r f o r m a n c e M o n i t o r i n g / 3 4 1
1 5 . 1 I n s t r u m e n t I n g r e d i e n t s i n a R e c i p e f o r R e l i a b i l i t y , 3 4 1
1 5 . 2 P e o p l e I n g r e d i e n t s i n a R e c i p e f o r R e l i a b i l i t y , 3 4 2
P A R T 4 G E N E R A L G U I D E L I N E S O N T H E E X E C U T I O N O F
M O N I T O R I N G P R O G R A M S
C h a p t e r 1 4 M e a s u r e m e n t o f T e m p e r a t u r e / 3 3 1
1 4 . 1 A p p l i c a t i o n s , 3 3 1
1 4 . 2 M e r c u r y T h e r m o m e t e r , 3 3 2
1 4 . 3 B i m e t a l T h e r m o m e t e r , 3 3 2
1 4 . 4 T h e r m i s t o r , 3 3 3
1 4 . 5 T h e r m o c o u p l e , 3 3 3
1 4 . 6 R e s i s t a n c e T e m p e r a t u r e D e v i c e ( R T D ) , 3 3 4
1 4 . 7 F r o s t G a g e s , 3 3 5
1 4 . 8 O t h e r T r a n s d u c e r s f o r M e a s u r e m e n t o f T e m p e r a t u r e , 3 3 6
1 4 . 9 C o m p a r i s o n A m o n g T r a n s d u c e r s f o r R e m o t e M e a s u r e m e n t s , 3 3 6
1 4 . 1 0 I n s t a l l a t i o n o f T r a n s d u c e r s f o r M e a s u r e m e n t o f T e m p e r a t u r e , 3 3 8
1 3 . 3 S u r f a c e - M o u n t e d S t r a i n G a g e s , 3 0 6
1 3 . 4 E m b e d m e n t S t r a i n G a g e s , 3 2 0
1 3 . 5 D e t e r m i n a t i o n o f E x i s t i n g S t r e s s , 3 2 6
1 3 . 6 C o n c r e t e S t r e s s C e l l s , 3 2 7
x v i i i C O N T E N T S
Chapter 26 The Key to Success: The Chain with 25 links / 493
PART 6 THE KEY TO SUCCESS
Chapter 25 Drilled Shafts / 483
25.1 General Role of Instrumentation, 483
25.2 Principal Geotechnical Questions, 483
25.3 Overview of Routine and Special Applications, 489
25.4 Selected Case Histories, 489
Chapter 24 Driven Piles / 467
24.1 General Role of Instrumentation, 467
24.2 Principal Geotechnical Questions, 467
24.3 Overview of Routine and Special Applications, 479
24.4 Selected Case Histories, 479
Chapter 23 Underground Excavations / 453
23.1 General Role of Instrumentation, 453
23.2 Principal Geotechnical Questions, 455
23.3 Overview of Routine and Special Applications, 461
23.4 Selected Case Histories, 461
Chapter 22 Excavated and Natural Slopes / 443
22.1 General Role of Instrumentation, 443
22.2 Principal Geotechnical Questions, 443
22.3 Overview of Routine and Special Applications, 448
22.4 Selected Case Histories, 448
Chapter 21 Embankment Dams / 417
21.1 General Role of Instrumentation, 417
21.2 Principal Geotechnical Questions, 418
21.3 Long-Term Performance Monitoring of Embankment Dams, 423
21.4 General Guidelines on the Execution of Monitoring Programs for
Embankment Dams, 432
21.5 Selected Case Histories, 435
Chapter 20 Embankments on Soft Ground / 407
20.1 General Role of Instrumentation, 407
20.2 Principal Geotechnical Questions, 407
20.3 Overview of Routine and Special Applications, 410
20.4 Selected Case Histories, 410
Chapter 19 Braced Excavations / 389
19.1 General Role of Instrumentation, 389
19.2 Principal Geotechnical Questions, 390
19.3 Overview of Routine and Special Applications, 400
19.4 Selected Case Histories, 400
PART 5 EXAMPLES OF INSTRUMENTATION APPLICATIONS
xixCONTENTS
A . C h e c k l i s t f o r P l a n n i n g S t e p s / 5 0 1
B . C h e c k l i s t f o r C o n t e n t o f S p e c i f i c a t i o n s f o r P r o c u r e m e n t o f
I n s t r u m e n t s / 5 0 5
C . C h e c k l i s t f o r C o n t e n t o f S p e c i f i c a t i o n s f o r F i e l d I n s t r u m e n t a t i o n
S e r v i c e s / 5 0 7
D . C o m m e r c i a l l y A v a i l a b l e G e o t e c h n i c a l I n s t r u m e n t s / 5 1 1
E . D e t a i l s o f T w i n - T u b e H y d r a u l i c P i e z o m e t e r S y s t e m / 5 1 9
F . D i m e n s i o n s o f D r i l l R o d s , F l u s h - J o i n t C a s i n g , D i a m o n d C o r i n g B i t s ,
H o l l o w - S t e m A u g e r s , a n d U . S . P i p e / 5 2 7
G . E x a m p l e o f I n s t a l l a t i o n P r o c e d u r e , w i t h M a t e r i a l s a n d E q u i p m e n t
L i s t / 5 3 3
H . C o n v e r s i o n F a c t o r s / 5 3 9
R e f e r e n c e s / 5 4 1
I n d e x / 5 6 3
P A R T 7 A P P E N D I X E S
x x C O N T E N T S
GEOTECHNICAL INSTRUMENTATION
FOR MONITORING
FIELD PERFORMANCE
E v e r y i n s t r u m e n t o n a p r o j e c t s h o u l d b e
s e l e c t e d a n d p l a c e d t o a s s i s t w i t h a n s w e r i n g
a s p e c i f i c q u e s t i o n : i f t h e r e i s n o q u e s t i o n ,
t h e r e s h o u l d b e n o i n s t r u m e n t a t i o n .
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.
Introduction
Part 1
3
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
1.2. WHY DO WE NEED TO MONITOR
FIELD PERFORMANCE?
The engineeringpractice 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
duringthe 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 monitoringof 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
successfulutilization may at first appear simple and
straightforward, considerable engineering and plan-
ning are required to obtain the desired end results.
1.1. WHAT IS GEOTECHNICAL
INSTRUMENTATION?
GEOTECHNICAL INSTRUMENTATION:
AN OVERVIEW
CHAPTER 1
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.
4
F i g u r e 1 . 1 . E x a m p l e s o f m e a s u r i n g i n s t r u m e n t s f o r i n s i t u d e t e r m i n a t i o n o f s o i l o r r o c k p r o p e r t i e s : ( a )
P i e z o c o n e : c o m b i n e d s t a t i c c o n e a n d p o r e p r e s s u r e p r o b e ( c o u r t e s y o f G e o t e c h n i q u e s I n t e r n a t i o n a l .
I n c . , M i d d l e t o n , M A l ; ( b ) v a n e s h e a r e q u i p m e n t ( c o u r t e s y o f G e o n o r A I S , O s l o , N o r w a y ) ; ( c ) s e l f - b o r i n g
p r e s s u r e m e t e r ( c o u r t e s y o f C a m b r i d g e l n s i t u , C a m b r i d g e , E n g l a n d ) ; a n d ( d ) b o r e h o l e d e f o r m a t i o n g a g e
( c o u r t e s y o f G e o k o n , l n c . , L e b a n o n , N H ) .
( d )
C c l
( b )
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
1.6. WHERE ARE WE NOW?
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 1930sand 1940s.During the first 50 years of its
life, a general trend can be observed. In the early
years, simplemechanical 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 significantnumber of the failures can
be attributed to an unsound marriage between in-
struments and people.
1.5. WHERE HAVE WE BEEN?
instrument procurement on a low-bid basis will re-
main a stumbling block to good field performance.
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 highquality can be specifiedadequately,
1.4. WHAT CAPABILITIES MUST THE
INSTRUMENTS 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 motivation.
1.3. WHAT CAPABILITIES MUST THE
PEOPLE HAVE?
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
comparisonwith 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-
gineeringproperties 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
willperform as designed. There will generally be no
need to monitor fieldperformance. 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.
5WHERE ARE WE NOW?
6
F i g u r e 1 . 2 . E x a m p l e s o f m e a s u r i n g i n s t r u
m e n t s f o r m o n i t o r i n g f i e l d p e r f o n n a n c e : ( a ) t w i n - t u b e h y -
d r a u l i c p i e z . o m e t e r ( c o u r t e s y o f G e o t e c h n i c a l I n s t r u m e n t s ( U . K . ) L t d . , l e a m i n g t o n S p a , E n g l a n d ) ; ( b )
v i b r a t i n g w i r e p i e z o m e t e r ( c o u r t e s y o f T e l e m a c , A s n i e r e s , F r a n c e ) ; ( c ) v i b r a t i n g w i r e s t r e s s m e t e t
( c o u r t e s y o f G e o k o n , I n c . , l e b a n o n , N H ) ; ( d ) l o a d c e J l ( c o u r t e s y o f P r o c e q S A , Z O r i c h , S w i t z e r l a n d ) ;
( e ) e m b e d m e n t e a r t h p r e s s u r e c e l l ( c o u r t e s y o f T h o r I n t e r n a t i o n a l , I n c . , S e a t t l e , W A ) ; ( f ) s u r f a c e -
m o u n t e d v i b r a t i n g w i r e s t r a i n g a g e ( c o u r t e s y o f l r a d G a g e , a D i v i s i o n o f K l e i n A s s o c i a t e s , I n c . , S a l e m ,
N H ) ; ( 8 ) m u l t i p o i n t f i x e d b o r e h o l e e x t e n s o m e t e r ( c o u r t e s y o f S o i l l n s t r u m e n t s l t d . , U c k f i e l d , E n g l a n d ) ;
a n d ( h ) i n c l i n o m e t e r ( c o u r t e s y o f S l o p e I n d i c a t o r C o m p a n y , S e a t t l e , W A ) .
( 0
( e )
( d )
C c )
( a )
( b )
7
(h)
(g)
F i g u r e 1 . 6 . I n s t a l l i n g f i x e d e m b a n k m e n t e x t e n s o m e t e r w i t h v i b r a t -
i n g w i r e t r a n s d u c e r . B a l d e r h e a d D a m , E n g l a n d , 1 9 6 3 ( c o u r t e s y o f
A r t h u r D . M . P e n m a n ) .
r a t i o n a l a n d s y s t e m a t i c m a n n e r . a n d t h e r e f o r e p l a n -
n i n g p r o c e d u r e s a r e e m p h a s i z e d i n t h i s b o o k .
U s e r s o f g e o t e c h n i c a l i n s t r u m e n t a t i o n o f t e n h a v e
a m i s c o n c e p t i o n o f t h e s i z e o f t h e i n d u s t r y t h a t
m a n u f a c t u r e s i n s t r u m e n t s f o r p e r f o r m a n c e m o n i -
t o r i n g . I t i s n o t a l a r g e i n d u s t r y : i t i s i n f a c t v e r y
s m a l l . T h e m a n u f a c t u r i n g i n d u s t r y e m p l o y s b e -
t w e e n 3 0 0 a n d 4 0 0 p e o p l e w o r l d w i d e , a n d t h e t o t a l
a n n u a l v o l u m e o f s a l e s i s a b o u t 3 0 m i l l i o n U . S . d o l -
l a r s . T h i s m i s c o n c e p t i o n s o m e t i m e s l e a d s t o u n -
r e a s o n a b l e e x p e c t a t i o n s o n t h e p a r t o f u s e r s : w e
c a n n o t e x p e c t t h e m a n u f a c t u r e r s t o m a k e l a r g e e x -
p e n d i t u r e s o n r e s e a r c h , d e v e l o p m e n t . a n d t e s t i n g o f
s p e c i a l i n s t r u m e n t s , u n l e s s j u s t i f i e d b y t h e s i z e o f
t h e m a r k e t J f t h e m a r k e t i s s m a l l , s p e c i a l f u n d i n g i s
n e e d e d .
F i g u r e 1 . 5 . I n s t a l l i n g t w i n - t u b e h y d r a u l i c p i e z o m e t e r s . U s k D a m ,
E n g l a n d , 1 9 5 2 ( c o u r t e s y o f A r t h u r D . M . P e n m a n ) .
F i g u r e 1 . 4 . D e t e r m i n a t i o n o f l o a d i n a s t e e l s t r u t . u s i n g a m e c h a n i -
c a l s t r a i n g a g e . O p e n c u t f o r s t a t i o n i n c l a y . C h i c a g o S u b w a y ,
1 9 4 8 ( c o u r t e s y o f R a l p h B . P e c k ) .
F i g u r e 1 . 3 . M e a s u r i n g l o a d i n a t i m b e r s t r u t , u s i n g a h y d r a u l i c
j a c k . O p e n c u t f o r s t a t i o n i n c l a y . C h i c a g o S u b w a y , 1 9 4 0 ( c o u r -
t e s y o f R a l p h B . P e c k ) .
G E O T E C H N I C A L I N S T R U M E N T A T I O N : A N O V E R V I E W
8
Figure 1.9. Bonded resistance strain gages on segmented steel
liner for soft ground tunnel. Port Richmond Water Pollution Con-
trol Project, Staten Island, NY, 1974.
• TotaJ stress in soil and stress change in rock
• Groundwater pressure
• Load and strain in structural members
• Deformation
• Temperature
This book includes chapters that describe avail-
able methods for monitoring various geotechnically
related parameters. The followingis a summary rat-
ingof our current ability to obtain reliable measure-
ments of these parameters, in order of increasing
reliability:
Figure 1.8. Installing fixed embankment extensorneters in em-
bankment dam. ludington Pumped Storage Project, ludington,
MI,1972.
Figure 1.7. Manometer panels for twin-tube hydraulic piezometers. Plover Cove Main Dam, Hong
Kong, 1965 (after Dunnidiff, 1968). Reprinted by permission of Institution of Civil Engineers, london.
9WHERE ARf WE NOW?
A s w e l o o k a h e a d , t h e r e i s n o r e a s o n t o b e l i e v e i n a
d e c r e a s i n g r o l e f o r g e o t e c h n i c a l i n s t r u m e n t a t i o n .
G e o t e c h n i c a l d e s i g n a n d c o n s t r u c t i o n w i l l a l w a y s
b e s u b j e - C tt o u n c e r t a i n t i e s , a n d i n s t r u m e n t a t i o n w i l l
c o n t i n u e t o b e a n i m p o r t a n t i t e m i n o u r t o o l b o x .
H o w e v e r , s e v e r a l c u r r e n t t r e n d s c a n b e i d e n t i f i e d ,
e a c h o f w h i c h w i l l c o n t i n u e i n t h e f u t u r e a n d c h a n g e
t h e s t a t e o f t h e p r a c t i c e .
F i r s t , t h e r e i s t h e a d v e n t o f a u t o m a t i c d a t a a c q u i -
s i t i o n s y s t e m s a n d c o m p u t e r i z e d d a t a p r o c e s s i n g
a n d p r e s e n t a t i o n p r o c e d u r e s . C l e a r l y , t h e s e s y s -
t e m s a n d p r o c e d u r e s h a v e m a n y a d v a n t a g e s , y e t w e
m u s t r e m a i n a w a r e o f t h e i r l i m i t a t i o n s . N o a u t o -
m a t i c s y s t e m c a n r e p l a c e e n g i n e e r i n g j u d g m e n t .
W h e n a u t o m a t i c d a t a a c q u i s i t i o n s y s t e m s a r e u s e d ,
1 . 7 . W H E R E A R E W E G O I N G ?
f i g u r e 1 . 1 1 . P n e u m a t i c p i e z o m e t e r a n d e a r t h p r e s s u r e c e l l o n
o p p o s i t e f a c e s o f p r e c a s t c o n c r e t e p i l e , p r i o r t o c o n c r e t i n g . K e e h i
I n t e r c h a n g e , H o n o l u l u , H I , 1 9 7 7 .
R e l i a b i l i t y i s s t r o n g l y i n f l u e n c e d b y t h e e x t e n t t o
w h i c h m e a s u r e m e n t s a r e d e p e n d e n t o n l o c a l c h a r -
a c t e r i s t i c s o f t h e z o n e i n w h i c h t h e i n s t r u m e n t s a r e
i n s t a l l e d . M o s t m e a s u r e m e n t s o f p r e s s u r e , s t r e s s ,
l o a d , s t r a i n , a n d t e m p e r a t u r e a r e i n f l u e n c e d b y c o n -
d i t i o n s w i t h i n a v e r y s m a l l z o n e a n d a r e t h e r e f o r e
d e p e n d e n t o n l o c a l c h a r a c t e r i s t i c s o f t h a t z o n e .
T h e y a r e o f t e n e s s e n t i a l l y p o i n t m e a s u r e m e n t s ,
s u b j e c t t o a n y v a r i a b i l i t y i n g e o l o g i c o r o t h e r c h a r -
a c t e r i s t i c s , a n d m a y t h e r e f o r e n o t r e p r e s e n t c o n d i -
t i o n s o n a l a r g e r s c a l e . W h e n t h i s i s t h e c a s e , a l a r g e
n u m b e r o f m e a s u r e m e n t p o i n t s m a y b e r e q u i r e d b e -
f o r e c o n f i d e n c e c a n b e p l a c e d i n t h e d a t a . O n t h e
o t h e r h a n d , m a n y d e f o r m a t i o n m e a s u r i n g d e v i c e s
r e s p o n d t o m o v e m e n t s w i t h i n a l a r g e a n d r e p r e s e n -
t a t i v e z o n e . D a t a p r o v i d e d b y a s i n g l e i n s t r u m e n t
c a n t h e r e f o r e b e m e a n i n g f u l , a n d d e f o r m a t i o n m e a -
s u r e m e n t s a r e g e n e r a l l y t h e m o s t r e l i a b l e a n d l e a s t
a m b i g u o u s .
f i g u r e 1 . 1 0 . I n s t a l l i n g m u l t i
p o i n t f i x e d b o r e h o l e e x t e n s o m e t e r
a b o v e a l i g n m e n t o f r o c k t u n n e l . E a s t 6 3 r d S t r e e t S u b w a y , N e w
Y o r k , N Y , 1 9 7 6 .
G E O T E C H N i C A l I N S T R U M E N T A T I O N : A N O V E R V I E W
1 0
Figure1.14. Heads of multipoint fixed borehole extensometers,
installedto monitor rock movementsduring full-scaleheater test
for studies relating to disposal of high-levelnuclear waste. Basalt
Waste Isolation Program, Hanford, WA, 1980 (courtesy of De-
partment of Energy).
figure 1.13. Installinggage with induction coil transducers, for
monitoring convergence of slurry trench test panel in soft clay,
AlewifeStation, Cambridge,MA,1978.
there is a real possibility that 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 wiJI be given second pLace and
that correlations between causes and effects will
not be made. We should take aU 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
Figure1.12. Electricaltransducers for monitoringmovement of
multipletelltalesduring load test of precast concrete pile. Keehi
Interchange,Honolulu,HI, 1977.
11WHERE ARE WE GOING?
F u U b e n e f i t c a n b e a c h i e v e d f r o m g e o t e c h n i c a l i n -
s t r u m e n t a t i o n p r o g r a m s o n l y i f e v e r y s t e p i n t h e
p l a n n i n g a n d e x e c u t i o n p r o c e s s i s t a k e n w i t h g r e a t
c a r e . T h e a n a l o g y c a n b e d r a w n t o a c h a i n w i t h
m a n y p o t e n t i a l w e a k l i n k s : t h i s c h a i n b r e a k s d o w n
w i t h g r e a t e r f a c i l i t y a n d f r e q u e n c y t h a n i n m o s t
o t h e r g e o t e c h n i c a l e n g i n e e r i n g e n d e a v o r s . T h e
l i n k s i n t h e c h a i n a r e d e f i n e d i n C h a p t e r 2 6 : t h e i r
s t r e n g t h d e p e n d s b o t h o n t h e c a p a b i l i t i e s o f m e a s u r -
i n g i n s t r u m e n t s a n d t h e c a p a b i l i t i e s o f p e o p l e . T h e
s u c c e s s o f p e r f o r m a n c e m o n i t o r i n g w i l l b e m a x -
i m i z e d b y m a x i m i z i n g t h e s t r e n g t h o f e a c h l i n k .
1 . 8 . T H E K E Y T O S U C C E S S
F o u r t h , t h e r e i s a t r e n d t o d e v e l o p a n d i m p r o v e
t r a n s d u c e r s a n d t o i n c l u d e b u i l t - i n f e a t u r e s t h a t
c r e a t e r e d u n d a n c y a n d t h a t p r o v i d e d i r e c t o u t p u t i n
e n g i n e e r i n g u n i t s . T h e t r e n d i n c l u d e s p r o v i s i o n s f o r
c a l i b r a t i o n s a n d z e r o c h e c k s .
F i f t h , t h e r e i s a t r e n d t o w a r d u s e o f n e w c o n -
s t r u c t i o n m e t h o d s . E x a m p l e s o f i n n o v a t i o n s i n t h e
r e c e n t p a s t i n c l u d e e a r t h r e i n f o r c e m e n t , l a t e r a l s u p -
p o r t , a n d g r o u n d m o d i f i c a t i o n . T h e s e i n n o v a t i o n s
o f t e n r e q u i r e f i e l d v e r i f i c a t i o n b e f o r e t h e y b e c o m e
w i d e l y a c c e p t e d , a n d g e o t e c h n i c a l i n s t r u m e n t a t i o n
w i l l a l w a y s p l a y a r o l e .
T h e a b o v e f i v e t r e n d s , g o o d o r b a d , a r e i n e v i t a -
b l e . T h e r e i s a s i x t h t r e n d , w h i c h t h e a u t h o r v i e w s
a s w h o l l y b a d : t h e p r o c u r e m e n t o f i n s t r u m e n t s a n d
t h e a w a r d i n g o f f i e l d i n s t r u m e n t a t i o n s e r v i c e c o n -
t r a c t s o n t h e b a s i s o f t h e l o w e s t b i d . I f a n i n -
s t r u m e n t a t i o n p r o g r a m s e t s c o s t a b o v e q u a l i t y o f
i n s t r u m e n t s , o r f e e a b o v e e x p e r i e n c e , d e d i c a t i o n ,
a n d m o t i v a t i o n o f p e o p l e , i t d e s e r v e s t o b e a f a i l u r e .
W e m u s t w o r k h a r d t o r e v e r s e t h i s t r e n d .
t h e n u m b e r o f p e r s o n n e l a v a i l a b l e f o r e x e r c i s i n g e n -
g i n e e r i n g j u d g m e n t .
T h i r d , t h e u s e o f d e s i g n t o o l s s u c h a s f i n i t e a n d
b o u n d a r y e l e m e n t m o d e l i n g t e c h n i q u e s l e a d s t o a
n e e d f o r f i e l d v e r i f i c a t i o n . G e o t e c h n i c a l i n s t r u m e n -
t a t i o n i s l i k e l y t o p l a y a n i n c r e a s i n g r o l e i n p r o v i d -
i n g a c h e c k o n t h e s e a d v a n c e d a n a l y t i c p r e d i c t i o n s .
F i g u r e 1 . 1 S . R e a d i n g i n c l i n o m e t e r d u r i n g c o n s t r u c t i o n o f u n d e r ·
w a t e r t e s t f i l l . C h e k l a p K o k R e p l a c e m e n t A i r p o r t , H o n g K o n g ,
1 9 8 1 ( c o u r t e s y o f l e o D . H a n d f e l t ) .
G E O T E C H N I C A L I N S T R U M E N T A T I O N : A N O V E R V I E W
1 2
13
Figure2.1. Constituentsof soil.
Pore spaces,
7~d'/.:;/>i:jo;:::,:;:~*~,---:::::::- pores, or
voids
2.1.3. Stress and Pressure
Stress and pressure are defined as force per unit
area, with typical units of pounds per square inch
(lb/in.") 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
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.
2.1.2. 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-
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. BEHAVIOR OF SOil
Many practitioners who become involved with
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 students at techni-
cal colleges. Rock behavior is well described by
Blyth and DeFreitas (1974) and Franklin (1988).
BEHAVIOR OF SOIL AND ROCK
CHAPTER 2
( f )
I e )
f i g u r e 2 . 3 .
S p r i n g a n a l o g y f o r s o i l b e h a v i o r .
I e )
1 0 l b
( d )
( b )
~ A r e a 1 i n , l - - - 1
( a )
~ A r e a 1 i n , l ~
L . . . - _ . . . . . . : : : ' - - _ _ . . . r - . . . . S p r i n g
w i t h
s t i f f n e s s
1 0 1 b l i n .
i n p l a n , o f 1 s q u a r e i n c h . F i g u r e 2 . 3 b s h o w s a n a n a l -
o g y i n w h i c h t h e r e s i s t a n c e o f t h e m i n e r a l s k e l e t o n
t o c o m p r e s s i o n i s r e p r e s e n t e d b y a s p r i n g , a n d t h e
p o r o u s p i s t o n i s r e p l a c e d b y a n i m p e r m e a b l e p i s t o n
w i t h a v a l v e d o r i f i c e . T h e o r i f i c e r e p r e s e n t s t h e r e -
s i s t a n c e t o t h e f l o w o f w a t e r t h r o u g h t h e s o i l . T h e
p i s t o n i s a s s u m e d t o b e w e i g h t l e s s , a n d t h e w a t e r i s
i n c o m p r e s s i b l e . I n i t i a U y , t h e v a l v e i s c l o s e d . T h e
s p r i n g h a s a s t i f f n e s s o f 1 0 I b / i n . , m e a n i n g t h a t a
f o r c e o f 1 0 p o u n d s i s r e q u i r e d t o p r o d u c e a n a x i a l
F i g u r e 2 . 2 . P o r e w a t e r p r e s s u r e c a u g h t i n 1 0 - 6 f i l l ( a f t e r P a r t i a l l y
I n t e g r a t e d , 1 9 6 2 ) .
e ' • • , • •
. ' . ' . . . ' . ,
: . ' : , ~ t u ~ a l ~ d ' . ' :
. s o i l .
. : : . .
• e "
W a t e r
s u r f a c e P o r o u s
2 . 1 . 5 . T o t a l a n d E f f e c t i v e S t r e s s e s
T o t a l s t r e s s i s t h e t o t a l f o r c e t r a n s m i t t e d a c r o s s a
g i v e n a r e a , d i v i d e d b y t h a t a r e a . T h u s , i f a 2 - f o o t
s q u a r e p i e c e o f w o o d i s p l a c e d o n t h e g r o u n d s u r f a c e
a n d a p e r s o n w e i g h i n g 2 0 0 p o u n d s s t a n d s o n t h e
w o o d , t h e t o t a l s t r e s s i n t h e g r o u n d i m m e d i a t e l y
b e l o w t h e w o o d i s i n c r e a s e d b y 5 0 I b / f t 2 .
E f f e c t i v e s t r e s s c a n b e e x p l a i n e d b y u s e o f a n
a n a l o g y . F i g u r e 2 . 3 a s h o w s s a t u r a t e d s o i l p l a c e d i n
a c y l i n d r i c a l c o n t a i n e r w i t h a c r o s s - s e c t i o n a l a r e a ,
2 . 1 . 4 . P o r e W a t e r P r e s s u r e
W h e n t h e p o r e s p a c e s a r e f i l l e d w i t h w a t e r ( s a t u -
r a t e d s o i l ) , t h e p r e s s u r e i n t h e w a t e r i s c a l l e d t h e
p o r e w a t e r p r e s s u r e ( F i g u r e 2 . 2 ) . I t a c t s i n a l l d i r e c -
t i o n s w i t h e q u a l i n t e n s i t y .
a r e a p p l i e d m o r e l o o s e l y ; f o r e x a m p l e , a s w i l l b e
d i s c u s s e d l a t e r , e a r t h p r e s s u r e a n d s o i l s t r e s s a r e
u s e d a s s y n o n y m s .
B E H A V I O R O F S O i l A N D R O C K
1 4
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.
As a practical example of consolidation, consider
2.1.6. Consolidation
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.
(J' = (J" + u.
Thus,
Total stress, (J'
Effective stress, (J"
Pore water pressure, u
This is Terzaghi's principle of effective stress. The
following symbols are normally used:
total stress = effective stress + pore water pressure.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:
BEHAVIOROF SOIL 15
Table 2.1. Sharing of Applied Force
Force on Force Carried Force Carried
Valve Piston by Spring by Water
Figure 2.3 Condition Position (lb) (lb) (lb)
(b) Initial Closed 0 0 0
(c) 10 lb force Closed 10 0 10
applied
(d) Piston Open 10 4 6
descended
0.4 in.
(e) Piston Open 10 8 2
descended
0.8 in.
(f) Piston Open 10 10 0
descended
1.0 in.
F i g u r e 2 . 5 . G r o u n d w a t e r l e v e l w h e n t h e r e i s n o f l o w o f g r o u n d -
w a t e r .
T h r e e v e r t i c a l p i p e s ,
w i t h p e r f o r a t i o n s
n e a r b o t t o m
C l a y
. . S a n d
G r o u n d w a t e r " , : . . .: . : _ . : ' _ " ' : " _
l e v e l . : . . . . . ' : ' ,
• • • • • • : ' : 1
. . . . . . . . .
2 . 1 . 9 . D i f f e r e n c e B e t w e e n P o r e W a t e r P r e s s u r e
a n d G r o u n d w a t e r l e v e l
T h e g r o u n d w a t e r l e v e l i s d e f i n e d a s t h e u p p e r s u r -
f a c e o f a b o d y o f g r o u n d w a t e r a t w h i c h t h e p r e s s u r e
i s a t m o s p h e r i c .
F i g u r e 2 . 5 s h o w s t h r e e p e r f o r a t e d p i p e s i n s t a l l e d
i n a s o i l w i t h i n w h i c h t h e r e i s n o f l o w o f g r o u n d w a -
t e r ; t h e r e f o r e , g r o u n d w a t e r p r e s s u r e i n c r e a s e s u n i -
f o r m l y w i t h d e p t h . W h e n s u c h e q u i l i b r i u m c o n d i -
t i o n s e x i s t , t h e l e v e l o f w a t e r w i t h i n t h e p i p e w i l l
r i s e t o t h e g r o u n d w a t e r l e v e l , i n d e p e n d e n t o f t h e
l o c a t i o n o f t h e p e r f o r a t i o n s .
N o w c o n s i d e r w h a t h a p p e n s w h e n a l a y e r o f f i l l
i s p l a c e d a b o v e t h e s a n d s h o w n i n F i g u r e 2 . 5 . F i g -
2 . 1 . 8 . N o r m a l l y C o n s o l i d a t e d a n d
O v e r c o n s o l i d a t e d S o i l
A n o r m a l l y c o n s o l i d a t e d s o i l i s o n e t h a t h a s n e v e r
b e e n s u b j e c t e d t o a n e f f e c t i v e s t r e s s g r e a t e r t h a n
t h e e x i s t i n g o v e r b u r d e n p r e s s u r e . E x a m p l e s i n c l u d e
o c e a n a n d l a k e - b e d c l a y s . A n o v e r c o n s o l i
d a t e d s o i l
i s o n e t h a t h a s b e e n s u b j e c t e d t o a n e f f e c t i v e s t r e s s
g r e a t e r t h a n t h e e x i s t i n g o v e r b u r d e n p r e s s u r e . E x -
a m p l e s i n c l u d e c l a y s s u c h a s L o n d o n c l a y , w h e r e
t h o u s a n d s o f f e e t o f o v e r b u r d e n h a v e b e e n e r o d e d .
s t r e s s i n a p a r t i c u l a r s o i l , t h e g r e a t e r i s i t s s h e a r
s t r e n g t h . T h e s h e a r s t r e n g t h i s a m e a s u r e o f t h e
r e s i s t a n c e t o s l i d i n g b e t w e e n g r a i n s t h a t a r e t r y i n g
t o m o v e l a t e r a l l y p a s t e a c h o t h e r .
I t c a n n o w b e s e e n t h a t t h e g a i n i n s h e a r s t r e n g t h
d u r i n g t h e c o n s o l i d a t i o n p r o c e s s c a n b e m o n i t o r e d
b y m e a s u r i n g p o r e w a t e r p r e s s u r e .
2 . 1 . 7 . S h e a r S t r e n g t h
I t h a s b e e n s h o w n a b o v e t h a t e f f e c t i v e s t r e s s e s i n -
c r e a s e a s c o n s o l i d a t i o n p r o g r e s s e s . B e c a u s e a n i n -
c r e a s e o f e f f e c t i v e s t r e s s m e a n s t h a t t h e g r a i n s
w i t h i n t h e m i n e r a l s k e l e t o n a r e p r e s s i n g m o r e
t i g h t l y t o g e t h e r , i t b e c o m e s i n c r e a s i n g l y h a r d e r t o
c a u s e s l i d i n g b e t w e e n t h e g r a i n s . A s a n a n a l o g y , a
b r i c k c a n b e p l a c e d o n a c o n c r e t e f l o o r a n d p u s h e d
s i d e w a y s t o c a u s e i t t o s l i d e . I f a s e c o n d b r i c k i s
n o w p l a c e d o n t o p o f t h e f i r s t b r i c k , i t t a k e s m o r e
s i d e w a y s f o r c e t o c a u s e s l i d i n g . I t i s t h e r e f o r e e v i -
d e n t t h a t t h e a b i l i t y o f a s o i l t o r e s i s t s l i d i n g i s r e -
l a t e d t o t h e e f f e c t i v e s t r e s s : t h e l a r g e r t h e e f f e c t i v e
a l a y e r o f f i l l f o r a h i g h w a y e m b a n k m e n t , p l a c e d o n
a c l a y e y f o u n d a t i o n s o i l . A s t h e f i l l i s p l a c e d , p o r e
w a t e r p r e s s u r e i n t h e f o u n d a t i o n i m m e d i a t e l y i n -
c r e a s e s a n d t h e n s t a r t s t o d i s s i p a t e , r e s u l t i n g i n s e t -
t l e m e n t , T h e r a t e o f s e t t l e m e n t d e p e n d s p r i m a r i l y
o n t h e p e r m e a b i l i t y o f t h e f o u n d a t i o n s o i l . P e r m e -
a b i l i t y i s a m e a s u r e o f t h e r a t e a t w h i c h w a t e r c a n
m o v e t h r o u g h t h e s o i l . C o h e s i v e s o i l s h a v e l o w e r
p e r m e a b i l i t y t h a n c o h e s i o n l e s s s o i l s , a n d t h e r e f o r e
c o n s o l i d a t i o n a n d s e t t l e m e n t o f c o h e s i v e s o i l s o c c u r
m o r e s l o w l y .
F i g u r e 2 . 4 . ( a ) S h a r i n g o f a p p l i e d f o r c e a n d s t r e s s . ( b ) V o l u m e
c h a n g e .
( b l
( a l
T i m e
~ - - T - - - - f - - - - - - ~ ~ ~ - - - - - - - - - - - - , 1 0 N
c O
~
£
. j
"
( 5
u ,
F o r c e c a r r i e d b y w a t e r , o r p o r e
w a t e r p r e s s u r e
B E H A V I O R O F S O i l A N D R O C K1 6
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 alluvialmaterial. The material beneath the toe
is subjected to lateral shear forces as the embank-
ment is constructed. These shear forces cause de-
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
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.
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
Figure2.6. Groundwater level and pore water pressure when
there is flow of groundwater.
I I
U(2)
(d)
LJ
(b)
Clay
I I
UtI)
(e)
.... : Sand··
17BEHAVIOR OF SOil
F i g u r e 2 . 8 a . T h e w a t e r l e v e l i n t h e s t r a w r i s e s t o a
l e v e l h i g h e r t h a n i n t h e c o n t a i n e r a n d i s " h e l d u p "
b y s u r f a c e t e n s i o n f o r c e s b e t w e e n t h e s t r a w a n d
w a t e r a t t h e m e n i s c u s . T h e p r e s s u r e i n t h e a i r i s
a t m o s p h e r i c p r e s s u r e P a ; t h e r e f o r e , t h e p r e s s u r e a t
p o i n t A m u