INTERNATIONAL SOCIETY FOR
SOIL MECHANICS AND
GEOTECHNICAL ENGINEERING
This paper was downloaded from the Online Library of
the International Society for Soil Mechanics and
Geotechnical Engineering (ISSMGE). The library is
available here:
https://www.issmge.org/publications/online-library
This is an open-access database that archives thousands
of papers published under the Auspices of the ISSMGE and
maintained by the Innovation and Development
Committee of ISSMGE.
1/35
Residual Pore Pressures in Compacted Clay
Pression de l ’eau interstitielle résiduelle dans l ’argile compactée
by T. W illiam L am b e, P rofessor a n d H ead , Soil E ngineering M assach u setts In stitu te o f T ech n o lo g y C am bridge,
M ass. U .S .A .
Summary
Sommaire
This paper presents values o f negative pore water pressure
measured in laboratory com pacted samples of clay. Tensions as
large as 14 pounds per square inch below atm ospheric pressure
were recorded.
The results obtained show th at the pore pressures existing in
clay following com paction are very sensitive to the following
details of the com paction process : ( 1) molding water content ;
(2) type and am ount of com paction ; (3) m ethod of attaining
the molding water content ; (4) tem perature ; and (5) confinement.
The measured pore pressures are consistent with the com paction
theory proposed by the auth o r (1958), and supported and extended
by Seed and C han (1959). Residual pore pressure is considered
an im portant and useful m anifestation of soil structure.
Cet article présente des valeurs de la pression négative de l’eau
interstitielle sur des échantillons de laboratoire d ’argile compactée.
Des pressions atteignant 14 livres p ar pouce carré au-dessous de la
pression atm osphérique ont été enregistrées.
Les résultats obtenus m ontrent que les pressions interstitielles
existant dans l’argile après com pactage sont très sensibles aux
éléments suivants de la m éthode de com pactage : (1) teneur en
eau de moulage ; (2) mode et intensité de com pactage ; (3) mé­
thode d ’obtention de la teneur en eau de moulage ; (4) tem péra­
ture ; (5) mode de confinement.
Les pressions interstitielles mesurées sont en accord avec la
théorie du com pactage proposée p ar l’auteur (1958), confirmée
et développée p ar S e e d et C h a n (1959). La pression interstitielle
résiduelle est considérée comme un im portant et utile élément de
structure du sol.
I. Introduction
T h e g reat influence o f co m p ac tio n o n th e engin eering
p ro p ertie s o f c lay is well k n o w n . T h e stren g th , p erm eab ility ,
a n d volum e chan g e ch aracteristics o f co m p acted clay can
d ep en d very co n sid erab ly o n : ( 1) m o ld in g w ater c o n ten t,
(2) a m o u n t o f co m p ac tio n , (3) ty p e o f co m p ac tio n , a n d (4)
d etails o f c o m p ac tio n p ro ced u re, such as th e m eth o d used
to get th e soil to th e m o ld in g w ater co n ten t.
In tw o p a p e rs p u blish ed in th e A .S .C .E . proceedings
( L a m b e , 1958) th e a u th o r p ro p o sed a m ech an istic th eo ry
to ac c o u n t fo r the efFects o f co m p ac tio n o n th e b e h a v io r o f
clay . T h e th e o ry atte m p te d to ex p lain th e k n o w n p ro p ertie s
o f c lay in term s o f “ stru c tu re ” — th e arran g em en t o f soil
particles a n d the electrical forces betw een ad ja c e n t particles.
T h e th e o ry also ex p lain ed th e efFects o f c o m p ac tio n o n clay
stru ctu re. T est d a ta w ere p resen ted to help su b sta n tia te th e
th eo ry . S e e d an d C h a n (1959) p resen ted a w ealth o f e x p er­
im ental d a ta w hich co n firm ed a n d ex ten d e d facets o f the
stru c tu re concepts. T he th ree p ap ers show ed th a t, in g en eral :
(1) In creasin g th e m o ld in g w ater ten d ed to resu lt in a
m o re p arallel a rra n g e m e n t o f p a rtic le s ;
(2) In creasin g th e co m p ac tiv e effort ten d ed to resu lt
in a m o re p arallel arran g em en t o f p a rtic le s ;
(3) K n ead in g co m p ac tio n ten d ed to give a m o re p arallel
arran g em en t o f particles th a n d id static co m p actio n .
T he existence o f these tendencies w as p ro v ed b y actu a l
m easurem ents o f p article arran g em en t. Since such m easu r­
em ents a re very difficult to m ak e o th e r m eth o d s o f d e te r­
m ining stru c tu re w ere so u g h t. O ne g o o d in d icatio n o f th e
stru c tu re o f a soil sam ple w as fo u n d to be th e a m o u n t o f
sh rin k ag e th e sam ple u n d erw en t u p o n d ry in g . T h e th eo ry
suggested a n o th e r m an ifestatio n o f soil stru c tu re w ould be
th e p o re w ater pressu re existing in th e co m p acted clay p rio r
to th e ad d itio n o f m o re w ater o r o f sh ear stresses. Such p o re
pressures are herein term ed , “ resid u al p o re p ressu res” .
T h e rem ain in g p o rtio n o f th is p a p e r presen ts m easu rem ents
o f resid u al p o re p ressu res in tw o clays. T h e effects o f m olding
w ater, a m o u n t a n d ty p e o f co m p ac tio n , o th e r co m p actio n
variab les an d te m p eratu re a re show n an d discussed.
II. Soils used in tests
T w o soils w ere used in th e in v estig atio n , n am ely :
(1) A silty clay fro m V icksburg, M ississippi ( “ V icksburg
L o ess” ), h av in g a liq u id lim it o f 34 p e r cent a n d a plastic
lim it o f 26 p er c e n t ;
(2) A p u re k ao lin ite fro m B ath , S o u th C aro lin a, h av in g
a liq u id lim it o f 55 p er cen t, a n d a p lastic lim it o f 33 p er cent.
T hese a re essen tially th e sam e “ silty c lay ” an d “ k a o lin ite”
u sed in th e Seed an d C h an (1959) te s ts ; i.e., th ey are from
th e sam e sources, b u t d ifferent batches.
III. Pore water pressure measurements
P o re w ater p ressu re m easu rem en ts in soil-w ater system s
w ere first m ad e m an y years ago. In a review o f th e d ev elo p ­
m en ts in p o re p ressu re m easu rem en t tech n iq u e B i s h o p (1960)
described tests b y O sb o rn e R e y n o l s in a p p ro x im ately 1886.
R e y n o l d s m easu red w ith a m ercu ry m an o m eter a neg ative
p o re p ressu re o f 13 lb. p er sq. in. in a sam ple o f sa tu ra te d
san d su b jected to a sh ear stress. T a y l o r (1943) describes
tests by L. R e n d u l i c in ap p ro x im a te ly 1937 in w hich pore
p ressu res w ere m easu red w ith in a sam ple o f clay d u ring
shear. R e n d u l i c used a co re o f san d -m ica as his sensing
elem ent. M o re recen t p io n eerin g w o rk in p o re p ressu re
m easu rem en ts w as d o n e b y th e B u reau o f R eclam atio n (at
th e en d s o f triax ial specim ens) a n d b y T a y l o r a t M .I.T .
(w ithin triax ial specim ens b y m eans o f pro b es). In an o th er
p a p er ( W h i t m a n , R i c h a r d s o n a n d H e a l y , 1961) to this
207
C onference is described a system u sed to m easu re a n eg ativ e
p o re pressu re o f 20 lb. p er sq. in. T h is system em p lo y ed
an electrical pressu re tra n sd u c e r co n n ected to a n oscilloscope.
T h e system used to o b ta in th e d a ta p resen ted in Figs. 1-6
consisted essentially o f a sensing elem en t co n n ected to a
m easuring elem ent. T w o sensing elem ents w ere u sed , n am ely :
a p o ro u s ceram ic disk (G ra d e 0-6 m an u factu red b y th e Selas
C o rp o ra tio n , h av in g an a ir e n tran ce p ressu re o f 60 lb. p er
sq. i n . ) ; a n d a 3/32 in. diam . p ro b e o f stainless steel tu b e
co n tain in g a 200 m esh screen, tip p ed w ith clay an d th en
fired (after H i l f , 1956). T w o m easu rin g elem ents w ere u sed :
an electrical pressu re tra n sd u c e r (D y n isco P T 25, m a n u ­
factu red by th e D y n a m ic In s tru m e n t C o.) co n n ected to
an oscilloscope ( H e w l e t t - P a c k a r d 130 A ); a n d a nu ll
section connected to a pressu re an d v acu u m source. T he
tran sducer-oscilloscope m easu rin g elem en t is m u ch m o re
co n v en ie n t th a n the n u ll section. T he tra n sd u cer gives an
in stan tan eo u s pressu re read in g an d req u ires essentially n o
fluid flow to a ctu a te - th e en tire d iap h rag m deflection, c o rres­
p o n d in g to a pressu re ran g e o f 100 psi, is o n ly 0-00035 inches.
As sho w n by th e d a ta in Fig. 1 g o o d ag reem en t w as o b ta i­
n ed w ith th e various arran g em en ts o f sensing an d m easu re­
m en t elem ents. T h e g reat m a jo rity o f th e test d a ta re p o rte d
i s h o p , 1960; H i l f , 1956 ; L a m b e , 1948 ; T a y l o r , 1944, etc.).
A ll p o re p ressu re m easu rem en ts w ere m ad e o n clay sam ples
c o m p acted in th e H a rv a rd M in iatu re m o ld (2.816 inches
high b y 1.312 inches in diam eter) u sin g o n e o f th e th ree
follow ing m eth o d s :
( 1 ) 3 layers, 25 tam p s p e r la y e r w ith 40 lb. sp rin g (a p p ro x ­
im ately eq u al to sta n d a rd A A S H O effort).
(2) 5 layers, 25 tam p s p er lay er w ith 40 lb. spring.
(3) S tatic lo ad n ecessary to give th e desired d ensity.
(B
IV. Test results
T h e resu lts o f th e p o re p ressu re m easu rem en ts are show n
in Figs. 1 th ro u g h 6 ; th e resu lts show th e fo llo w in g :
A . M easurement Technique — Fig. 1 p resen ts p o re pressures
m easu red in V icksburg L oess b y v ario u s ex p erim en to rs
u sing sev eral different ty p es o f eq u ip m en t. T h e close ag reem ent
o f th e d a ta in d icates th a t h u m a n facto rs an d eq u ip m en t
setu p s h ad relativ ely m in o r influence o n th e accu racy o f
th e m easu rem en ts. I t w as n o t, how ev er, a n o b jectiv e o f this
in v estig atio n to ev a lu a te v ario u s techn iq u es-in fact, all o f
th e d a ta in Figs. 3 th ro u g h 6 w ere o b tain ed by th e sam e
p erso n using th e sam e technique. A s n o te d in th e p receding
section, th e tran sd u cer-o scillo sco p e (o r X -Y reco rd er) system
is m uch faster an d m o re co n v en ie n t th a n a h a n d -b alan ced
n u ll system .
B. M olding Water Content— A ll th e d a ta (Figs. 1-6) show
th a t th e h ig h er th e m o ld in g w ater c o n ten t, th e less n eg ativ e
is th e resid u al p o re pressu re in th e c o m p ac ted sam ple.
C. A m ount o f Compaction — T h e d a ta o n th e effects o f
“ a m o u n t o f c o m p a c tio n ” a re scan t. T h ey (Fig. 2) suggest a
h ig h er resid u al p o re p ressu re fro m h ig h er co m p ac tiv e effort.
D . M ethod o f Compaction — F o r th e sam e m o ld in g w ater
co n te n t a n d th e sam e co m p ac ted d en sity , k n ead in g co m p ac­
tio n gives h ig h er resid u al p o re p ressu res th a n does static
co m p a c tio n (Figs. 2 a n d 3).
E . E ffect o f M oisture Attainm ent — T h e p o re pressu re
c u rv e o n th e rig h t in F ig. 4 w as o b tain ed o n soil b ro u g h t
to th e desired m o ld in g w ater co n te n t, e q u ilib rated , an d th e n
co m p ac ted . T h e cu rv e o n th e left w as o b tain ed o n soil m ois­
ten ed , eq u ilib rated , d ried b ack a p p ro x im a te ly 2 p e r c en t
o f m o istu re co n te n t, eq u ilib rated a n d th e n co m p ac ted . T h e
d a ta show th a t sam ples w hich h a v e once been w etter th a n the
m o ld in g w ater c o n te n t h a v e h ig h e r resid u al p o re p ressures.
F . Effect o f Temperature — T h e d a ta in Figs. 5 an d 6 show :
(1) Sam ples co m p acted co o l h av e h ig h er resid u al p o re
p ressures th a n th o se c o m p ac ted w a r m ;
(2) C o o lin g th e sam p le a n d m o ld causes a n increase in
p o re pressu re ; h eatin g th e sam p le a n d m o ld causes a decrease
in p o re p re s s u re ;
(3) C o o lin g a n u n c o n fin e d sam p le causes a decrease in
p o re pressure.
G . E ffect o f Confinement — R em o v in g a sam p le o f k ao lin ite
fro m th e m old in w hich it w as co m p ac ted causes a red u ctio n
in p o re p ressu re (F ig. 6). N o ch an g e in p o re p ressu re o ccu rred
in th e silty clay u p o n rem o v al fro m th e m old.
Fig.
1
Pore pressures measured with various setups.
Pressions interstitielles
mesurées
avec différents
appareillages.
h erein (in p a rticu lar, th e d a ta in Figs. 3-6) w ere o b tain ed
using th e H ilf-type p ro b e an d a n u ll section. Several p ro b es
(necessitated by breakage) w ere used. A ir e n tran ce pressures
fo r th e pro b es varied from 6 to 10 lb. p er sq. in. T h e a rra n g e ­
m en t o f elem ents is sim ilar to th o se show n in the literatu re
20 8
V. Discussion of test results
T h ree co n cep ts will b e u sed in discussing th e test r e s u lts ;
these a r e : (1) water deficiency, (2) prestressing a n d (3) stress
equilibrium. T hese concepts will be co n sid ered befo re dis­
cussing th e d a ta presented in Figs. 1-6.
F o r several reaso n s (h y d ra tio n o f clay surface, h y d ra tio n
o f ex ch an g eab le ions, etc.) a clay p article h as th e ab ility
to a ttra c t w ater. T h u s a soil elem ent has a certain cap acity
a
m = ra tio m in eral-m in eral c o n ta c t a re a to to ta l a r e a ;
= a ir p ressu re w here c o n ta c t is a ir-m in e ra l;
= p o re w ater p ressu re ;
= ra tio o f a rea o f w ater-m in eral c o n ta c t to to ta l a r e a ;
= electrical rep u lsiv e p ressu re betw een p a rtic le s ;
= electrical a ttra c tiv e p ressu re betw een p articles.
pa
u
aw
R
A
A
'
Vicksburg Loess
+
1
Static compaction
3 layer, kneading comp.
-4/
i
—
aa.
/
-------
r
Kaolinite
+
O
I
/
Static compaction
3 lager, kneading comp. /
1
f+
O
/
/
/
/
\
k
3>.
\ *
X
%
^
o
N-
V
+
V
\ N
k
\
4+ /
;/
\
A
A
100
\
98
\
It
H
!i
M o ld in g
16
10
w afer content %
ZZ
0
■ 96
14
N
'
Fig.
2
Pore pressures for different types and am ounts of
com paction.
Pressions interstitielles po u r différents modes et inten­
sités de compactage.
n
X
N v ' î H ;+
+
ZZ
o f w ater w hich it can im b ib e ; th is cap a c ity d ep en d s o n th e
co m p o sitio n o f th e soil in th e elem ent, o n th e stru c tu re o f
th e elem ent a n d th e e x te rn a l pressu res actin g o n th e elem ent.
I f th e existing w ater c o n te n t o f a sam p le is less th a n its c a p a c ­
ity , a “ w ater deficiency” exists. In o th er w ords, th e w ater
deficiency is the “ th irst” th e sam p le has.
T h e co m p ac tio n o f a clay sam p le can be co n sid ered a
“ p restressin g ” o f the soil. T h e co m p ac tio n p rocess rearran g es
soil particles an d forces th em in to p o sitio n s closer th a n
eq u ilib riu m fo r th e co n d itio n o f n o ex tern al loads. U p o n
rem o v al o f th e co m p ac tin g force th e soil sk eleto n ten d s to
ex p a n d to a n eq u ilib riu m volum e. If sufficient w ate r is in th e
soil a n d a w ater deficiency exists, th e ten d en cy o f th e soil
sam ple to ex p a n d is op p o sed b y n eg ativ e p o re pressures.
T h u s fo r m old in g w ater co n ten ts rea so n a b ly n e a r o p tim u m , a
c o m p acted clay sam ple h as a co m p ressiv e prestress w hich is
held by a n eg ativ e p ressu re in th e p o re w ater.
In a n o th e r p a p e r ( L a m b e , 1 9 6 0 ) th e a u th o r p ro p o sed th e
follow ing e q u a tio n to express a n eq u ilib riu m o f n o rm a l
stress in soil,
. . . . ( 1)
w here,
ct
= com bined stress betw een ad ja c e n t p a rtic le s ;
a = c o n ta c t stress w here th e particles a re in m in eralm in eral c o n ta c t;
x
Fig.
3
24
Zb
ZB
Molding water content
%
Effect of com paction m ethod on residual pore
pressures.
Effet des différentes méthodes de com pactage sur les
pressions interstitielles résiduelles.
I f an y th in g is d o n e to a soil elem ent to ch an g e a n y term in
E q. 1, som e o th e r term (s) m u st ch an g ed to re-estab lish
p ressu re eq u ilib riu m . F o r ex am ple, if an electro ly te is leached
fro m a dispersed soil sam ple (th ereb y in creasing R), the
sam p le will ex p an d (red u cin g R -A ) o r th e co n fin in g p ressure a
need ed to m ain ta in th e v olum e will increase.
C a p illa rity play s an im p o rta n t ro le in soils w hen th e p o re
w ater has an interface w ith a i r ; such a co n d itio n exists in
u n sa tu ra te d soils a n d in u n co n fin ed sam ples. B ecause o f
its w ater deficiency, the soil sam ple tries to suck in w ater,
m u ch like a p erso n draw in g a d rin k th ro u g h a straw . Since
w ater “ w ets” a m in eral surface an d since a “ surface ten sio n ”
can exist at th e air-w ater interface, cap illary m enisci can
d evelop. T hese m enisci ten d to resist th e so il’s su ction,
ju s t as an o b stru c tio n can sto p flow into o u r straw . In b o th
cases th e fluid is su b jected to a ten sio n , relativ e to a tm o s­
p h eric pressure. In v ery sm all p o res, as exist in fine-grained
soils, th e p o re fluid ten sio n can far exceed ab so lu te zero
p ressu re.
T h u s c a p illarity can help m obilize neg ativ e p o re pressures
b y resisting soil suctio n . It should n o t be co n sid ered th e
209
■ 1
K a o l in
it e
1
1
+
/
O So il m o ist en ed , equ ilibra t ed ,
+
d r ie d b o ck , eq u ilib ra t ed , compa t ed.
So il m o ist en ed , eq u ilib ra t ed ,
co m p a
j
/
,+
J
r
+/
/
/
!
t
Fig.
4
V
/
As noted in the preceding subsections, the flocculated structure
has a higher capacity for prestressing. Thus increased com ­
pactive effort an d kneading com pactive action tend to destroy
the prestressing, thereby resulting in higher pore w ater
pressures.
C. M ethod o f M oisture A ttainm ent —W etting a soil sample
above the m olding w ater content tends to give the sample
a dispersed structure, some of which is retained on drying
the sample. T hus “ overw etting” the sample tends to give
higher residual pore pressures.
D . E ffect o f Tem perature —There is considerable experi­
m ental evidence th a t cooling a soil element tends to increase
R -A in Eq. 1. Increasing R perm its the soil to be com pacted
in a m ore dispersed structure. A reduction in tem perature
acts as a reduction in salt or an increase in m olding w ater
content. W hile the theoretical reasons for this tem perature
effect are n ot clearly established, the experim ental facts
(heat tends to flocculate a soil-water su sp en sio n ; cooling
an element of soil in contact with w ater causes an expansion)
are convincing.
The three effects of tem perature show n in Figs. 5 an d 6 can
thus be explained :
(1) The colder the soil at tim e o f com paction, the more
dispersed the structure — therefore, the higher the residual
pore pressures.
(2) Cooling a sam ple m old decreases the size of the m old
and thus increases tr in Eq. 1. A n increase a is accom panied
by an increase in the pore pressure u. This increase in a
tends to occur before (and tends to outw eight) the R -A
increase.
Effect of method of obtaining molding water content
on residual pore pressures.
Effet de la m éthode d ’intégration de l’eau de moulage
sur les pressions interstitielles résiduelles.
o
S o il eq u ilib ra t ed at 40'
co m p a ct ed a t 40°F.
Soil eq u il ib r a t ed at 8 0
co m p a ct ed at 80° F. , t h en
m old cooled t o approx.
40°F.
Ç
fundam ental cause of, or an essential ingredient to, negative
pore pressures. The tru th o f this statem ent is indicated by
the fact that W h it m a n , et al (1961) m easured negative pore
pressures below absolute zero pressure in a soil sample
which was saturated and had no surface exposed to air ;
i.e., no capillarity existed. As illustrated by the W h it ma n
tests, the soil suction of pore w ater can be resisted by seepage
forces.
The concepts described in the preceding paragraphs will
now be used to explain the trends shown in Figs. 1-6.
sr
5 -4
&
210
y
~5
to
C -8
c.
Q.
u
2
u
to -12
A. M olding W ater Content—The higher the m olding w ater
content, the less is the w ater deficiency. Increasing m olding
w ater tends to give a m ore dispersed structure (as discussed in
detail in Lambe, 1958) : the dispersed structure (particles
tending tow ard a parallel array) is m ore com pressible (in the
low pressure range). The low w ater deficiency m eans a low
tendency to generate negative pore pressures. T he higher
skeleton com pressibility means a lower capacity for prestress.
These two factors com bine to give higher pore pressures, i.e.,
smaller tensions. T hat is to say, the smaller the w ater deficiency
the lower the pore tensions ; and the m ore nearly parallel
the particles, the lower the pore tensions.
B. Am ount an d M eth od o f Com paction —T he greater the
com pactive effort, the m ore nearly dispersed the structure ;
kneading com paction gives a m ore dispersed structure th an
does static com paction. K needing introduces large shear
strains between particles and, in effect, “ rem olds” the clay
during the com paction process. These tendencies are discussed
a t length in the Lambe (1958) and Seed and Chan (1959) papers.
T he larger shrinkage upon drying for dispersed th an for
flocculated structures is illustrated a t the tops of Figs. 2 and 3.
Ko oh n ite
S t a t ic co m p a ct io n
So il eq u il ib r a t ed a t 40°F. ,
co m p a ct ed at 40°F., t h en
m o l d h ea t ed t o 80 ° F.
So il eq u il ib r a t ed at 80 ° F and
co m p a ct ed a t 8 0 ° F - - - - - - - - -
-16
=9
c
L.
Q>
22
24
26
M o l d in g
Fig.
5
28
30
N o t er co n t e n t
%
32
34
Effect of tem perature on residual pore pressures.
Effet de tem pérature sur les pressions interstitielles
résiduelles.
I. Acknowledgements
T h e tests described h erein w ere p erfo rm ed b y th ree o f
th e a u th o r’s research stu d en ts, n am ely : O liv er H . G ilb ert,
J r . ; R a u l S o lo r z a n o ; a n d D a rio F e rn a n d e z . S pecial credit
is d u e M r. F e rn a n d e z w ho ra n m o s t o f th e tests a n d review ed a
d ra ft o f th is p ap er.
Bibliography
[1]
Fig.
6
Effects of confinement and tem perature on residual
pore pressures.
Effets de confinem ent et de la tem pérature sur les
pressions interstitielles résiduelles.
Alan W., (1960). The M easurem ent of Pore Pressure
in the T riaxial Test. London Conference on Pore Pressures.
[2] C o l e m a n , J . D ., (April, J959). An Investigation of the
Pressure M em brane M ethod for M easuring the Suction
Properties of Soil. Road Research Laboratory, Research
N ote N o. R N/3464/JD C.
[3] C r o n e y , D ., C o l e m a n , J. D . a n d B r i d g e , P. M., (1952).
The Suction of M oisture Held in Soil and O ther Porous
M aterials. Road Research Technical, in Paper 24, London.
[4] F e r n a n d e z , D ario (June, 1960). Pore Pressures in Com ­
pacted K aolinite, C. E. Thesis, M .I.T.
[5]
G i l b e r t , O l i v e r H., Jr. (Sept. 1 9 5 9 ) . The Influence of
N egative Pore W ater Pressures on the Strength o f Com ­
pacted Clays, 5. M. Thesis, M .I.T.
[6] H i l f , Jack W. (Oct., 1956). An Investigation of PoreW ater Pressure in C om pacted Cohesive Soils, Tech.
M em orandum 654, Bureau of Reclam ation, Denver,
Colorado.
[7]
H v o r s l e y , M. J u u l ( N o v . 1949). Subsurface Exploration
and Sampling of Soils for Civil Engineering Purposes,
Waterways Experiment Station, Vicksburg, Mississipi.
[8] L a m b e , T. W. (1948). The M easurem ent o f Pore W ater
Pressures in Cohesionless Soils. Proc. of Second Interna­
tional Conference on Soil Mechanics
Engineering, R otterdam .
[9]
[10]
(3)
Cooling an unconfined sample causes an increase in
R-A which in turn causes a decrease in u.
[11]
The effects o f temperature are not as simple as the above
suggests. The influence o f temperature on surface tension at
air-water interfaces, pressure in air bubbles, and air-inwater solubility must also be considered. They are, however,
[12]
generally o f less importance, especially in plastic clays,
than the effects discussed in the preceding paragraphs.
E. Effect of Confinement—Removing a compacted sample
from its mold results in a reduction o f cr in Eq. 1 ; this reduc­
tion in turn causes a reduction in the pore pressure. (This
stress change is similar to that which occurs during clay
sampling below the water table. See, for example, Hvorslev,
1948.)
B is h o p ,
[13]
[14]
and
Foundation
— (M ay 1958). The Structure of C om pacted Clay and
The Engineering Behavior o f Com pacted Clay, Journal
o f Soil Mechanics and Foundation Div., Proc. of A.S.C.E.
— (June, 1960). A M echanistic Picture of Shear Strength
in Clays. A.S.C.E. Research Conference on Shear Strength
o f Cohesive Soils, C olorado.
S e e d , H. B. and C h a n , C . K. (Oct. 1 9 5 9 ) . Structure and
Strength Characteristics o f Com pacted Clays, Journal
o f Soil Mechanics and Foundations Division, Proc. of
A.S.C.E.
S o l o r z a n o , R. (June, 1959). Investigation o f the Influence
of C om paction M ethod on the Pore Pressures o f Com­
pacted Clays, S. M. Thesis.
T a y l o r , D.W ., Shear Research. Ninth Report (1943) ;
Tenth Report (1944) ; for U.S. Engineer Department,
M .I.T., Soil Mechanics L aboratory.
W h i t m a n , R . V . , R i c h a r d s o n , A. M., a n d H e a l y , K . A.
(1961). Time-Lags in Pore Pressure M easurements.
Proc. 5th International Conference o f Soil Mechanics and
Foundations Engineering, Paris.
211