Components of Pore Water Pressure

8/12/2019 Components of Pore Water Pressure

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C O M P O N E N T S O F P O R E W T E R P R E S S U R E

N D T H E I R E N G I N E E R I N G S I G N I F I C N C E

y

J K MITCHELL 1

U n i v e r s i t y o f C a l i f o r n i a , B e r k e l e y

A B S T R A C T

P o r e f l u i d p r e s s u r e s t h a t d e v e l o p w i t h i n s o i l m a s s e s a s a r e s u l t o f b o t h m e c h a n i c a ] a n d

p h y s i c o - c h e m i c a l e f f e ct s i n f l u e n c e t h e m a g n i t u d e o f th e i n t e r g r a n u l a r o r e f f e c t i v e s t r e ss e s .

T h e i n t e r g r a n u l a r s t r e s s es c o n t r o l, i n m a n y c a se s, s o il b e h a v i o r i n s h e a r a n d c o m p r e s si o n .

T h e p h y s i c a l s i g n i f i c a n c e o f p o r e w a t e r p r e s s u r e i n a c o h e s i v e s o i l i s e x a m i n e d i n t e r m s o f

s e v e r a l c o m p o n e n t s w h i c h c o m b i n e t o g i v e t h e t o t a l p r e s s u r e . A n a n a l y s i s o f fl u id p r e s s u r e s

a t v a r i o u s p o i n t s w i t h i n a s o i l m a s s b a s e d o n a c o n d i t i o n o f n o f lo w a t e q u i l i b r i u m s h o w s

t h a t c h a n g e s i n a n y o n e c o m p o n e n t o f t h e t o t a l p r e s s u r e f r o m p o i n t t o p o i n t a r e o f fs e t b y

c h a n g e s in o t h e r c o m p o n e n t s .

T h e a n a l y s i s s h o w s t h a t a p o r e p r e s s u r e m e a s u r e m e n t r e fl ec ts in t e r p a r t i c l e r e p u l si v e

p r e s s u re s a n d w a t e r a d s o r p t i v e f o r c es a s w e l l a s p u r e l y h y d r o s t a t i c p r e s s u r e s a r is i n g t r o m

m e c h a n i c a l ef fe c ts . T h e i n d i v i d u a l c o m p o n e n t s o f t o t a l p o r e p r e s s u r e a r e n o t d i r e c t l y m e a s u r -

a b l e e x c e p t i n s p e c ia l s y s te m s . D a t a a r e p r e s e n t e d w h i c h i n d i c a t e t h a t a s i g n if i ca n t p o r t i o n

o f t h e s w e l l o f a c o m p a c t e d s o i l i s a t t r i b u t a b l e t o w a t e r p r e s s u r e d e f i c i e n c i e s c a u s e d b y

m e c h a n i c a l a n d c a p i ll a r y c o m p o n e n t s w h i c h a c t i n a d d i t i o n t o o s m o t i c p r e ss u r e c o m p o n e n t s .

C o m p o n e n t w a t e r p r e s s u r e s a r e r e l a t e d t o s t r e s s e s b e t w e e n p a r t i c l e s . I t i s s h o u a a t h a t

i n t e r g r a n u l a r p r e s s u r e s a r e d e p e n d e n t o n o s m o t i c a n d a d s o r p t i v e c o m p o n e n t s o f t h e t o t a l

w a t e r p r e s s u r e .

I N T R O D U T I O N

T h e p r i n c i p l e o f e f f e c t i v e s t r e s s i s o n e o f t h e m o s t i m p o r t a n t c o n c e p t s o f

m o d e r n s o i l m e c h a n i c s . I t h a s b e e n f o u n d u s e f u l a s a b a s i s f o r t h e u n d e r -

s t a n d i n g o f s t r e s s a n d s t r a i n c h a r a c t e r i s t i c s o f s o i l s a n d h a s b e c o m e i n -

c r e a s i n g l y i m p o r t a n t i n p r a c t i c a l e n g i n e e r i n g p r o b l e m s . T c r z h a g i 1 9 2 5 ) a p -

p e a r s t o h a v e b e e n t h e f i r s t t o r e c o g n i z e t h e i m p o r t a n c e o f e f f e c t i v e s t r e s s e s

w i t h i n s o il m a s s e s . B i s h o p 1 9 6 0 b ) h a s s u m m a r i z e d t h e h i s t o r i c a l d e v e l o p -

m e n t o f t h e c o n c e p t o f e f f e c t iv e s t r e s s e s i n s o i l m a s s e s a n d h a s c o n s i d e r e d

t h e t h e o r e t i c a l a s p e c t s o f t h e p r i n c i p l e i n d e t a i l .

A c c o r d i n g t o t h e p r i n c i p l e o f e f f e c t i v e s t r e s s t h e s t r e n g t h a n d c o m p r e s s i -

b i l i t y p r o p e r t i e s o f a so i l d e p e n d n o t o n t h e t o t a l s t r e s s a p p l i e d t o t h e s o i l

1 A s s i s t a n t P r o f es s o r o f C iv il E n g i n e e r i n g a n d A s s i s t a n t R e s e a r c h E n g i n e e r , I n s t i t u t e o f

T r a n s p o r t a t i o n a n d T r a f f i c E n g i n e e r i n g , U n i v e r s i t y o f C a l i f o r n i a , B e r k e l e y .

162


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COMPONENTS OF PORE WATER

163

mass, but rather on the difference between the total stress and the stress

carried by the pore fluid. This difference is termed the effective or inter-

granular 1 stress and is given, for a sat ura ted soil, by

- : ~ u 1 )

where a = the total normal stress,

and u = the hydr ostati c pressure in the pore fluid.

Analysis of the forces acting within a soil mass across a surface which

approximates a plane, but passes through the pore space and points of

interparticle contact, indicates that the average intergranular force per

unit area of the horizontal projection of the plane is given by

a i = a - -

(1

- - a c U

,

(2)

where a c is t he con tac t area of the soil particles per uni t area of the plane.

Bishop (1960b) has demonstra ted, however, th at a - u controls the volume

changes of a granular soil independently of the contact area. Bishop also

points out that whether shear strength depends on a - u alone and not

on

a c

is still a matter for conjecture. In most soils, however, the contact

area is probably of very small magnitude; thus eqs. (1) and (2) could be

expected to yield very nearly the same result.

The validity of the effective stress principle in satura ed cohesionless

soils has been demonstrated by Bishop and Eldin (1950). The principle has

been found extremely useful in saturated clays as well; however, as pointed

out by Lambe (1960), Lambe and Whitman (1959), Seed, Mitchell and Chan

(1960), Rosenqvist (1959) and others, forces in addition to those arising

from applied loads and hydrost atic water pressures come into play owing

to the surface activity of the clay particles. In spite of these additional

forces, the effective stress principle and information obtained through pore

water pressure m~asurements c~rrelate well with observed behavior of

many fine-grained soils. At the same time, pore water pressure studies have

yielded valuable information relative to soil structure and other physico-

chemical aspects of soil behavior.

Recent studies have shown effective stress as stated by eq. (1) to be in-

adequate to account for the behavior of partially saturated soils. Modi-

fications of eq. (1) have been made by Bishop (1960a, 1960b, 1960c),

Jennings (1960), Aitchison (1960), and Croncy and Coleman (1953). The

expression suggested by Bishop is the most general in that it accounts for

pore air pressures different from 1 atm, a condition that may easily arise in

practice. His expression is:

= a - u , + x ( u a i , . - U w a t e , . ) ,

(3)

x It should be noted that intergr~nular stress defined in this manner is not the actual

stress transmitted between grain contacts, but represents the force carried by the solid

structure per unit area of the mass.


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64 NINTH NATIONAL CONFERE NCE ON CLAYS AND CLAY MINERALS

w h e r e x is a p a r a m e t e r r a n g i n g b e t w e e n 0 in a d r y s o i l a n d 1 i n a s a t u r a t e d

s o il. S u c h m o d i f i c a t i o n s o f e q . 1 ) a r e n e c e s s a r y s in c e , i n p a r t i a l l y s a t u r a t e d

s oi ls t h e w a t e r p r e s s u r e a c t s o v e r o n l y a p a r t o f t h e a r e a o f a n y p l a n e t h r o u g h

the so i l .

A s i n d i c a t e d b y e q s . 1 ) a n d 3 ) t h e v a l u e o f t h e p o r e p r e s s u r e , u, u s u a l l y

d e t e r m i n e d b y d i r e ct m e a s u r e m e n t , p l a y s a n e s s en t ia l p a r t i n t h e e v a l u a t i o n

o f e ff e c ti v e st re s s. I n s y s t e m s w h e r e f o r c es o f a p h y s i c o - c h e m i e a l n a t u r e

a r e a c t i v e , t h e p h y s i c a l si g n if ic a n c e o f p o r e w a t e r p r e s s u r e m a y b e s o m e w h a t

d i f f e r e n t t h a n i n s y s t e m s f r e e f r o m p a r t i c l e s u r f a c e f o r c e s . W a t e r l o c a t e d

w i t h i n t h e c l a y p a r t i c l e f o r c e f i e l d s m a y b e e x p e c t e d t o b e h a v e i n a m a n n e r

d i f f er e n t f r o m f r e e w a t e r . I t w o u l d s e e m l o g i ca l, t h e r e fo r e , t h a t p o r e w a t e r

p r e s s u r e s s h o u l d r e f l e c t th e i n f lu e n c e o f t h e s e f o r c e f i e ld s a s w e l l a s s tr e s s e s

i n d u c e d b y m e c h a n i c a l s t r a in o f t h e s y s t e m . E x p r e s s i o n s h a v e b e e n d e v e l o p e d

r e l a t in g t h e c h a n g e i n p o r e w a t e r p r e s s u r e t o a c h a n g e i n a p p l i e d t o t a l s t r e s s

i n t e r m s o f t h e r e l a t i v e c o m p r e s s i b i li t ie s o f t h e s o i l s t r u c t u r e a n d w a t e r

B i s h o p a n d E l d i n , 1 9 5 0), a n d t o s h e a r a n d v o l u m e t r i c s t r a i n s M a r s a l a n d

R e s i n e s , 1 9 6 0 ) . F u n d a m e n t a l l y , h o w e v e r , t h e s t r e s s - s t r a i n a n d c o m p r e s s i -

b i l i t y c h a r a c t e r i st i c s o f a c l a y s o il a r e f u n c t i o n s o f i ts c o m p o s i t i o n a n d i n t e r -

p a r t i c l e f o r c e s y s t e m s .

I n t h i s p a p e r t h e p h y s i c a l s ig n i fi ca n c e o f p o r e w a t e r p r e s s u r e is e x a m i n e d

i n t e r m s o f t h e v a r i o u s c o m p o n e n t s in d u c e d b y m e c h a n i c a l a n d p h y s i c o -

c h e m i c a l p h e n o m e n a . C o n c l u si o n s a r e d r a w n p e r t i n e n t t o s t u d ie s o f t h e

e n g i n e e r i n g b e h a v i o r o f s oi l. T h e a n a l y s i s r e li e s h e a v i l y o n t h e s t u d y o f

t o t a l a n d c o m p o n e n t p o t e n t i a l s o f m o i s t u r e in s o il d e v e l o p e d b y B o l t a n d

Mil ler 1958) .

A C K N O W L E D G M E N T S

T h e a u t h o r i s i n d e b t e d t o h i s c o l l e a g u e s P r o f e s s o r H . B . S e e d , P r o f e s s o r

C . L . M o n i s m i t h a n d M r . R . B . K r o n e o f t h e U n i v e r s i t y o f C a l i fo r n i a f o r

t h e i r v a l u a b l e c r it ic i sm s a n d s u g g e s ti o n s .

T h e d r a w i n g s w e r e p r e p a r e d b y M r . G . D i e r k in g .

T O T A L A N D C O M P O N E N T P O R E W A T E R P R E S S U R E S

Total ressure

I t is c o n v e n i e n t t o c o n s id e r t h e t o t a l p r e s su r e a s t h e s u m o f s e v e r a l

c o m p o n e n t s m e a s u r e d w i t h r e s p e c t t o a s p e c i f i e d r e f e r e n c e s t a t e , u s u a l l y a

b o d y o f f re e p u r e w a t e r w i t h a f l a t su r f ac e e x p o s e d t o a t m o s p h e r i c p r e s s u r e

a n d a t t h e s a m e t e m p e r a t u r e a s t h e w a t e r u n d e r i n v e s t i g a ti o n . I t i s i m -

p o r t a n t t o n o t e t h a t a t e q u i l i b r i u m t h e t o t a l p r e s s u r e i s t h e s a m e a t e v e r y

p o i n t a n d f l o w c a n o c c u r o n l y b e t w e e n p o i n t s o f d i f f e r e n t t o t a l p r e s s u r e .

P o s i t i v e p r e s s u r e s c a u s e a f lo w f r o m t h e p o i n t u n d e r i n v e s t i g a t i o n t o -

w a r d s t h e r e f e r e n c e p o o l .


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COMPONENTS O~ PORE W TER

165

C o m p o n e n t s o / T o t a l P r e s s u r e

A n u m b e r o f i n v e s ti g a to r s , e.g. E d l e f s o n a n d A n d e r s o n (1 94 3), B a y e r

( 1956 ) , Bo l t a nd M i l l e r ( 1958 ) , L ow a nd D e m i ng ( 1953 ) , L ow ( 1958 ) , M a r -

s h a l l ( 1 9 5 9 ) , h a v e c o n s i d e r e d t h e t o t a l s o i l m o i s t u r e o r p r e s s u r e i n t e r m s o f

s e v e ra l c o m p o n e n t s . B o t h m e c h a n i c a l a n d t h e r m o d y n a m i c a n a l y se s h a v e

b e e n u se d . T h e r e s u l t s a r e g e n e r a l l y s i m i l a r i n f o r m , a l t h o u g h c e r t a i n d if -

f e r e n c e s i n d e f i n i t i o n a n d d e t a i l h a v e b e e n p o i n t e d o u t b y B o l t a n d F r i s s e l

(1 96 0) w h o r e v i e w e d t h e p e r t i n e n t l i t e r a t u r e o n s o il m o i s t u r e t h e r m o -

d y n a m i c s a n d d e r i v e d a g e n e r a l e q u a t i o n o f s o i l m o i s t u r e e q u i l i b r i u m .

T h e a n a l y s is o f t o t a l a n d c o m p o n e n t p o t e n t i a ls p r e s e n t e d b y B o l t a n d

Mil ler ( 1 9 5 8 ) i s e a s i l y a d a p t e d f o r t h e p r e s e n t c o n s i d e r a t i o n s o f p o r e w a t e r

p r e s s u r e a n d i s s u m m a r i z e d e x t e n s i v e l y in a l a t e r s ec t io n . I n t h e p r e s e n t

p a p e r p r e s su r e s r a t h e r t h a n p o t e n t i a l s a r e c o n s id e r e d , s in c e p r e s s u r e s h a v e

w i d e a p p l i c a t i o n i n e n g i n e e r i n g p r a c t i c e .

T h e f o u r c o m p o n e n t s o f t h e t o t a l f lu i d p r e s s u r e ( o r t o t a l h e a d ) u n d e r

c o n d i t io n s o f c o n s t a n t t e m p e r a t u r e a r e a s s u m e d d u e t o g r a v i t y f o r ce f ie ld s,

h y d r o s t a t i c p r e s s u r e e f f ec t s, o s m o t i c o r io n i c c o n c e n t r a t i o n d i f f e re n c e e f fe c ts ;

a n d a d s o r p t i v e f o r c e f ie ld s . A s p o i n t e d o u t b y B o l t a n d F r i s s e l (1 96 0), i n

a n y b r e a k d o w n o f t h e t o t a l p r e s s u re s in t o c o m p o n e n t s t h e r e i s t h e d a n g e r

o f c o u n t i n g c e r t a i n e f f e ct s t w i c e a s t h e y m a y b e h i d d e n i n o t h e r t e r m s .

I t m a y b e , fo r e x a m p l e , t h a t a ri g o r o u s d i v i si o n b e t w e e n o s m o t i c p r e s s u r e s

a r i s i n g f r o m d o u b l e l a y e r i n t e r a c t i o n s a n d a d s o r p t i v e e f f e c t s c a n n o t b e

m a d e , t h u s i n v a l i d a t i n g t h e u s e o f s e p a r a t e c o m p o n e n t s f o r t h e s e tw o e ff ec ts .

T h i s p o s e s n o s e r i o u s p r o b l e m i n t h e p r e s e n t p a p e r , h o w e v e r , s i nc e q u a li -

t a t i v e r e l a t i o n s a n d c o n c e p t s r a t h e r t h a n r i g o r o u s m a t h e m a t i c a l r e l a t i o n -

s h i p s a r e de s i r e d .

(1) Po s i t iona l or e l eva t ion head i .e . grav i t y pres su re z a r i s e s f r o m t h e

d i ff e re n c e s i n e l e v a t i o n b e t w e e n t w o p o i n t s .

(2) H y d r os t a t i c p r e s s ur e p a r i s e s f r o m s u c h f a c t o r s a s i n c o m p l e t e s a t u -

r a t i o n w h i c h l e a d s t o c u r v e d a i r - w a t e r i n t e r f a c e s a n d i n r e a c t i o n t o e x -

t e r n a l l y a p p l i e d s tr es se s. T h e h y d r o s t a t ic c o m p o n e n t is t h e p u s h i n g o r

p u l l i n g c o m p o n e n t b e tw e e n p a r ti c le s . I t is t h e p re s s u re t h a t w o u l d be

r e f l e c t e d b y a p r e s s u r e g a g e i n s e r t e d a t t h e p o i n t i n q u e s t i o n . A s w i l l b e

s h ow n , th e m a g n i t u d e o f t h e h y d r o s t a t i c c o m p o n e n t is d e p e n d e n t o n t h e

m a g n i t u d e s o f th e o t h e r p r e s s u r e c o m p o n e n t s a s w e ll a s th e m e c h a n i c a l

s t re s ses .

(3) O s m ot i c p r e s s ur e ~ , a r i se s f r o m d i f f e re n c e s i n i o n ic c o n c e n t r a t i o n f r o m

p o i n t t o p o i n t . T h e c o n c e p t o f o s m o t i c p r e s s u r e i n s o i l s h a s b e e n d e s c r i b e d

e l s e w he r e , e.g. Bol t a nd M i l l e r ( 1958 ) , L ow ( 1959 ) , L a dd ( 1959 ) .

I t i s i m p o r t a n t t o k e e p in m i n d t h a t w h e n c o n si d er in g t w o p o i n ts a t d i f-

f e r e n t i o n ic c o n c e n t r a t i o n s , i f t h e i o n s a r e n o t f r e e t o m o v e , w a t e r w i l l t e n d


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166 NINTH NATIONAL CONFERENCE ON CLAYS AN]) CLAY MINERALS

to flow from the point of lower concentration towards the point of higher

concentration.

The osmotic pressure between two points of different concentration is

approximated for dilute solutions by the Van't Hoff equation,

zt = R T (2: ca -- 22 cl) , (4)

where R = universal gas constant,

T = absolute temperature,

Z , Xc~ = sum of concentra tions of all ionic const ituen ts at points 1

and 2, respectively.

It is assumed in the development of eq. (4) that ionic force fields do not

mutually influence each other.

(4)

T h e ads or p t ion p r e s s ur e a

arises from the attraction of water mole-

cules by clay surfaces. Bolt and Miller (1958) place attractive soil-water

forces in two categories:

a) Short ,range chemical forces, extending only a few molecular layers from the surface,

caused by local ionic interactions, hydrogen bonds and London van der Waals forces.

b) Long-range forces, which may be effective beyond 100 ]~, caused by interac tion of

a water dipole with the electros tatic field originating in a charged soil surface.

Bolt and Miller consider it reasonable to ignore the effects of short-range

forces in usual soil-water systems as particle spacings exceed the range of

influence of these forces. In the view of the author the more usual condition

for soils encountered in engineering problems is one of essentially particle-to-

particle (or particle-thin film of water and cations-particle) contact and

therefore short-range forces should not be neglected. If the short-range

forces are neglected the adsorptive pressure at any point is given by

8 - - ]

a - - d v ) ~ , 5 )

z

where e = dielectric constant,

~o _- electrical potential at the point.

The potential ~ may be approximated by means of the Gouy-Chapman

theory of the double layer. Adequate theories for expression of the short-

range forces have not yet been developed.

The total pressure P is given by

P = z + p + ~ + a , (6)

where z and p may be either positive or negative and n and a are usually

negative in soils.


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N E G A T I V E P O I ~ E - W A T E I ~ P R E S S U R E S I N S O I L S

Before analyzing component and to tal pressures at various points within

a soil, a few comments and data relative to negative pore pressures will be

considered as they aid in an understanding of osmotic effects and the in-

fluence of capillary effects on hydrostatic pressure. It is well known that

water pressures less than atmospheric exist in soils. Whet her or not pressures

less than zero absolute i . e . a state of tension in the water) can exist has been

the subject of considerable dispute in the literature. Aitehison (1960) has

considered the relative points of view and concludes that there is no satis-

factory argumen t against the use of a capillary model and the development

of large wate r tensions fo r describing the behavior of pore water a t pressures

less than atmospheric.

The fundamental causes of negative pressures appear to be osmotic and

adsorptive effects and the surface tension of water. Mechanical factors such

as a tendency towards straightening of bent particles on load release or a

tendency towards dilation of the soil structure on shear may also cause

negative water pressures but, in the final analysis, these effects are all de-

pendent on the adsorptive forces between water and soil.

Negative pressures may exist in both saturated and partially saturated

soils. The principal causes of negative pressures in saturated soils are prob-

ably osmotic effects, hydrostatic stresses resulting from a dilating tendency

on shear, and hydrostatic effects resulting from the stresses carried by bent

particles and distorted particle groups.

In partially saturated systems the surface tension of water comes into

play in conjunction with the adsorptive forces at particle surfaces, leading

to curved air-water interfaces which, in turn, result in additional pressures

either positive or negative, depending on whether curvature is coneave or

convex across the interface. It is most convenient to think of this pressure

as a capillary hydrostatic pressure given by

2 z / r

where T is the surface

tension and r is the radius of curvature of the meniscus.

It may be readily demonstrated that in partially saturated systems of

nominal salt content (say 0.05-0.5 N) the osmotic component and the hydro-

static co mponent due to eapillary effects are of the same order of magn itude

for menisci of about 0.5 radius of curvatu re. Capillary stresses become

increasingly effective at smaller pore diameters and for radii of curvature

of 0.1 or less the y ma y exceed the osmotic pressures by ten times or more.

Data illustrating a condition where osmotic and hydrostatic tension due

to mechanical effects are of the same order of magnitude in a partially

saturated soil are presented in Fig. 1. Samples of an expansive sandy-clay

soil were prepared to a water content of 17.3 percent, a dry density of

111.3 lb per ft a and degree of saturation of 90.6 percent, with distilled water

as the pore fluid. Two methods of eompaetion were used: kneading, which


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68 NINT~NATIONAL CONFERENCE ON CLAYS AND CLAY V[INERALS

at this water content induces a dispersed structure, and static, which leads

to a more floceulent struc ture as dem ons tra ted b y Seed and Chan 1959).

Samples were then permitted to expand und er a surcharge of 0.1 kg per cm 2

in solutions of calcium acetate of different concentration. The relation

between the amoun t of swell and solution concentration in Fig. 1 shows

A l l s a m p l e s c o m p a c t e d a t a w a t e r o n le n t

o f 1 7 . 3 t o o d r y d e n $ i t y f l l. 3_ + . 3 I b

5 O q ~ ~ - S a m p l e s p r e p a r e d y s ta ti c o m p a c t i o n

~ / / ~ ( F l oc c u le n t t r uc f ur e )

4 0

S a m p l e s p r e p a r e d y k n e a d in g o m p a c ti o n

/ ~ ( i s p e r s e d s t ru c tu r e

2

I O

S u r c h a r g e pr es su re -- , / k g p e r s q c m

0 o 4 o 8 / 2 1 6 2 0

C o n c e n t r a t i o n o f C a l c i u m A c e t a t e S o l u t i o n ] /7

w h i c h S a m p l e s A l l o w e d t o S w e l l - m o l e s p e r l/ t e r

Fm~RE 1.--Effect of structure and electrolyte concentration of absorbed solution

on swell of compacted sandy clay.

that as concentration increases, swell decreases. This is a result of the

decrease in the difference between the osmotic pressure in the double layers

between particles and the osmotic pressure of the calcium acetate solution

with increasing concentration of calcium acetate. Similar results have been

obtained b y La dd 1959) for anothe r soil. Also evident in Fig. 1 is the

significant difference between the amount of swell of samples prepared by

different methods of compaction.

It may be noted that the present percent swell approaches a constant

value at the high salt contents, suggesting that osmotic or ionic concen-

trat ion difference effects have ceased to be an im por tant fact or in influencing

the amount of swell. It is doubtful that any significant swell would be

observed in samples immersed in solutions more concentra ted than 2 N

if osmotic effects were the soIe cause of expansion. Thus, as indicated by


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C O M P O N E N T S O P O R E W A T E R

169

t h e s h a d e d a r e a s i n F ig . 1 , o s m o t i c e f fe c ts p r o b a b l y c a n a c c o u n t f o r s o m e

0 to 28 9 p e r c e n t s w e ll d e p e n d i n g o n t h e c o n c e n t r a t i o n o f t h e s w e ll in g

so lu t ion .

T h e r e m a i n d e r o f t h e s w e l l - - a b o u t 1 p e r c e n t fo r th e s a m p le s p r e p a r e d

b y k n e a d i n g c o m p a c t i o n a n d 3 + p e r c e n t fo r t h e s a m p le s p r e p a r e d b y

s ta t ic c o m p a c t i o n - i s p r o b a b l y a t t r i b u t a b l e t o i n i ti a l h y d r o s t a t i c p re s su r e

d e fi ci en c ie s a r is i n g f r o m a c o m b i n a t i o n o f c a p i ll a r y a n d p a r t i c le d e f o r m a t i o n

e f fe c t s a n d f r o m w a t e r a d s o r p t i v e f o r c e s a t t h e c l a y p a r t i c l e s u r fa c e s . M e a -

s u r e m e n t s h a v e c o n s i s t e n t l y s h o w n , e g L a m b e ( 1 9 6 1 ) , t h a t a s - c o m p a c t e d

p o r e w a t e r t e n s i o n s a r e s i g n if i c an t l y g r e a t e r f o r s t a t i c a l ly c o m p a c t e d t h a n

f o r k n e a d i n g c o m p a c t e d s a m p l e s o f t h e s a m e s oi l. T h e f l o c c u la t e d s t ru c -

t u r e s a s so c i a t ed w i t h s t a t i c c o m p a c t i o n l e a d t o m o r e p r o n o u n c e d e f fe c ts

f r o m b e n t p a r t i c l e s a n d c a p i l l a r y s tr e s s e s ( w h i c h h o l d b e n t p a r t i c l e s i n p l a n e

u n t i l r e l i e v e d b y e x p o s u r e t o w a t e r ) a s a r e s u l t o f t h e e d g e - t o - f a c e p a r t i c l e

a s s o c i a t i o n s . I n t h e m o r e d i s p e r s e d k n e a d i n g s t r u c t u r e s t h e l a r g e s h e a r

s t r a i n s d u r i n g c o m p a c t i o n e n a b l e p a r t i c l e s t o s l i d e i n t o a m o r e p a r a l l e l

a s s o c i a t io n w i t h a r e s u l t a n t c o n d i t i o n o f l es s p a r t i c l e d i s t o r t i o n . A s a

r e s u l t , t h e r e l e a s e o f c a p i l l a r y s t r e s s e s i s n o t a c c o m p a n i e d b y l a r g e v o l u m e

e x p a n s i o n s .

W a t e r a d s o r p t i v e f o r c e s a t p a r t i c l e s u r f a c e s m a y h a v e c o n t r i b u t e d

s o m e w h a t t o t h e s w e ll o f t h e s e s a m p l e s s i n ce i t h a s b e e n o b s e r v e d t h a t a n

i n i t ia l l y a i r- d r i e d s a m p l e w i ll fr e e l y a b s o r b w a t e r f r o m a 9 8 : k p e r c e n t

r e la t iv e h u m i d i t y a t m o s p h e r e t o a w a t e r c o n t e n t o f a b o u t 2 0 p e rc e n t .

A s p r e v i o u s l y n o t e d t h e c o m p a c t i o n w a t e r c o n t e n t w a s 1 7 . 3 p e r c e n t f o r

t h e s e t e s t s .

A N A L Y S I S O F C O M P O N E N T A N D T O T A L W A T E R

P R E S S U R E S A T V A R I O U S P O I N T S I N T H E

S O I L - W A T E R S Y S T E M

F o l l o w i n g t h e a n a l y s i s o f B o l t a n d M i l l e r ( 19 5 8 ), i t is c o n v e n i e n t t o

c o n si d er fi rs t a n " i d e a l " c l a y - w a t e r - e l e c t r o l y t e s y s t e m a t e q u il ib r iu m .

I n F i g . 2 i s sh o w n a s c h e m a t i c r e p r e s e n t a t i o n o f a p a r t o f a s a t u r a t e d

s o il m a s s c o m p o s e d o f e q u a l l y s p a c e d p a r a l l e l fi a t p l a t e s o f u n i f o r m t h i c k n e ss .

H y d r o s t a t i c p r e s s u r es d u e t o e x t e r n a l l y a p p l i e d s tr e ss e s m a y b e a c t i n g .

C o n s i d e r a p i e z o m e t e r i n s e r t e d i n t o t h e m a s s a s s h o w n i n F i g . 2 . T h e

p i c z o m e t e r c o n t a i n s f l u id t h a t is in e q u i l i b r iu m w i t h , b u t o u t s i d e t h e f o r c e

f ie ld o f, t h e w a t e r i n t h e d o u b l e l a y e rs . T h e f lu i d le v e l i n t h e p i e z o m e t e r is

a d j u s t e d s o t h a t a n o - f lo w c o n d i t i o n e x i s ts .

I f a ll p r e s s u re s a r e m e a s u r e d r e l a t i v e t o a d a t u m o f p u r e w a t e r a t t h e

sa me e l e v a t ion , t h e n a t p o i n t 1 , F ig . 2 : z --- 0 ; a = 0 , s i nc e th e po in r i s ou t

o f t h e r a n g e o f a d s o r p t i v e f o r c e f i el d s ; P l = h Y l, w h e r e y ! i s t h e u n i t w e i g h t

CC ~ 12


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170 Nr~ TH NATIONAL

CONFERENCE ON CLAYS AND CLAY :ViINERALS

o f t h e f lu i d i n t h e p i e z o m e t e r ; a n d ~ : =

RTfi

T h u s t h e t o t a l p r e s s u r e a t

p o i n t 1 i s

P1 = P : + z~l. 7)

P o i n t 2 l i e s m i d w a y b e t w e e n t w o c l a y s u r f a c e s s e p a r a t e d b y a d i s t a n c e

2 d . T h e o r e t i c a l e q u a t i o n s B o l t , 1 95 5) h a v e b e e n d e v e l o p e d e n a b l i n g t h e

c o m p u t a t i o n o f o s m o t i c p r e s s u re s b e t w e e n i n t e r a c t i n g p l at e s , a n d s o lu -

t i o n s h a v e b e e n t a b u l a t e d B o l t a n d M i ll er , 1 9 5 5 ; B o l t , 1 9 56 ). T h u s ~ 2

c a n , i n t h e o r y a t l e a s t , b e e v a l u a t e d .

0 ~ m m ~

O

m

1 2 d

t ~ / P o r o u s T ip

FIOUZ~E 2.--Sch ematic diagram of idealized satu rate d clay -wat er syste m.

S i n ce V ~ ~ m u s t e q u a l 0 ff s h o r ~ -r a n g e f o r ce s a r e n e g l e c t e d b e c a u s e t h e

d o u b l e l a y e r is s y m m e t r i c a l a b o u t t h e m i d - p l a n e , t h e n a 2 = 0 , a c c o r d i n g

t o eq . 5 ). A t e q u i li b r iu m , t h e c o n d i t i o n P ~ = P : m u s t e x i s t , a n d a s s u m i n g

t h a t t h e e l e v a t i o n o f p o i n t 2 is t h e s a m e a s t h a t o f p o i n t 1 :

P2 = :r~ P2 = ~: + P l . 8 )

E q u a t i o n 8) s h ow s t h a t t h e h y d r o s t a t i c p re s s u re c o m p o n e n t s p : a n d ~o2

m u s t d i ff er b y a n a m o u n t e q u a l t o t h e d i ff e re n c e i n o s m o t i c p re s su r e .

P o i n t 3 r e p r e s e n t s t h e g e n e r a l c a se o f a p o i n t l o c a t e d b e t w e e n t w o p a r a l l e l

i n t e r a a t i n g p l a t e s b u t n o t a t t h e c e n t e r l in e . F o r s i m p l i c i ty l e t

Z3 ~ Z1 ~ 0.


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C O M P O NE N T S O F P O R E W T E R 171

T h e a d s o r p t i v e c o m p o n e n t is g i v e n b y e q . 5 ), a n d ~vS a n d ~ s a r e g i v e n

t h e o r e t i c a l l y b y t h e d o u b l e la y e r e q u a t i o n s . S in c e P s = P x ,

a s q - ~ s q - ~ ~ = ~ q- 39a 9 (9 )

E q u a t i o n 9 ) i n d i c a t e s t h a t s i nc e t h e t o t a l p r e s s u r e r e m a i n s t h e s am e

a t a ll p o i n t s , t h e i n d i v i d u a l c o m p o n e n t s a t p o i n t 3 a s s u m e v a l u e s s u c h t h a t

t h e i r s u m e q u a l s P z . I f t h e o s m o t i c a n d a d s o r p t i v e c o m p o n e n t s a r e g i v e n

c o r r e c t l y b y t h e t h e o r e t i c a l e x p r e s s io n s , t h e n t h e h y d r o s t a t i c c o m p o n e n t 10s

a s s u m e s a v a l u e a p p r o p r i a t e t o s a t i s f y t h e c o n d i t i o n P s = P x .

FIGURE 3.--Schematic diagram of pa rtly saturated soil containing

coarse part ic les

a n d i m p e r f e c t c l a y orientation.

A n i d e a l c l a y - w a t e r s y s t e m s u c h a s c o n s i d e r e d t o t h i s p o i n t i s r a r e l y , i f

e v e r , e n c o u n t e r e d in n a t u r e . I n t h e m o r e g e n e r a l ea s e t h e p r e s e n c e o f

c o a rs e p a r t i c le s a n d t h e i n f lu e n c e o f i n c o m p l e t e s a t u r a t i o n m u s t b e c o n -

s i de r ed . I n F i g . 3 , i n c o m p l e t e s a t u r a t i o n a n d t h e p r e s e n c e o f c o a rs e p a r -

t ic le s a r e i n d i c a t e d ; a p i e z o m e t e r w i t h w a t e r l e v e l a d j u s t e d f o r e q u i l i b r i u m

is s h o w n . A s i n t h e p r e c e d i n g a n a l y s i s, l e t i t b e a s s u m e d t h a t a l l p o i n t s a r e

a t e s s e n t i a l l y t h e s a m e e l e v a t i o n s o t h a t z c o m p o n e n t s m a y b e n e g l e c t e d .

P o i n t 4 r e p r e s e n t s a p o i n t i n t h e f l u id i n e q u i l i b r iu m w i t h t h e s a m p l e

a n d , a s f o r a n i d e a l c l a y w a t e r s y s t e m ,

P 4 = n 4 + P 4 , ( 1 0 )

b o t h o f w h i c h a r e m e a s u r a b l e .

12"


11/23


12/23

COMPONENTS OF PoRE W TER

173

d o u b l e l a y e r a n d a p o i n t i n t h e f ie ld - fr e e r e g io n s . I t w a s a s s u m e d i n t h e

d e v e l o p m e n t o f e q . 1 6) t h a t t h e a i r p re s s u r e in a l l v o i d s i s t h e s a m e . T h i s

m a y b e e x p e c t e d i n p a r t i a l l y s a t u r a t e d g r a n u l a r s o il s w h e r e p a r t i c le s u r fa c e

p h e n o m e n a a r e n e g li g ib l e a n d i n p a r t i a l l y s a t u r a t e d c l ay s c o n t a i n i n g in t e r -

c o n n e c t e d a i r v o i d s . I f a i r v o i d s a r e i s o l a t e d , h o w e v e r , t h e a i r p r e s s u r e s

n e e d n o t b e t h e s a m e f r o m p o i n t t o p o i n t . I s o l a t e d a i r v o i d s w o u l d b e

e x p e c t e d a b o v e s o m e r e l a t i v e l y lo w d e g r e e o f s a t u r a t i o n . T h u s , t h e p r e s su r e

c o n d it io n s w i t h i n a p a r t i a l ly s a t u r a t e d c l a y a r e u n d o u b t e d l y m u c h m o r e

c o m p l i c a t e d t h a n t h e p r e c e d i n g a n a l y s i s w o u l d i n d i c a t e .

S i m i la r r e l a ti o n s m a y b e d e v e lo p e d f o r p o i n t 7 , w h i c h h a s n e g a t iv e

m e n i s c u s c u r v a t u r e , a n d p o i n t 8 , w h i c h l i e s m i d w a y b e t w e e n p a r a l l e l c l a y

p l a te s . T h e c u r v a t u r e a n d c o n c e n t r a t i o n a t a n y p o i n t s a r e , o f c o u rs e , d i c-

t a t e d b y a n u m b e r o f f a c t o r s s u c h a s a r r a n g e m e n t o f p ar t ic l e s , si ze o f p o r e s,

d e g r ee o f s a t u r a t i o n , m a g n i t u d e o f t h e e l e c t r o l y t e c o n c e n t r a t i o n i n t h e

s y s t e m a s a w h o l e, a n d w a t e r s t r u c t u r e . I t i s lo g i c al t o a s s u m e t h a t t h e

a c t u a l v a r i a t i o n s o f r a d i i o f c u r v a t u r e , o s m o t i c p r es s u r e, e t c . , a re s u c h

t h a t t h e e n e r g y o f t h e s y s t e m i s a m i n i m u m a t e q u i l ib r iu m .

L I M I T A T I O N S O F T H E A N A L Y S I S

B o l t a n d M i l le r 1 95 8) p o i n t o u t t h a t t h e e x p a n d i n g l a t t ic e c la y s , e g

m o n t m o r i l lo n o i d m i n e ra ls , a r e t h e o n l y o n e s t h a t a p p r o a c h t h e p r o p e r ti e s

a s s u m e d f o r i d e a l c l a y - w a t e r s y s t e m s . O t h e r c l a y m i n e r a l s , s u c h a s t h e

i ll it e s a n d k a o l i n i t e s , a r e m u c h t h i c k e r , h a v e a m u c h l o w e r s p e c if ic s u r f a c e ,

m a y d e v i a t e w i d e l y f r o m t h e a s s u m p t i o n o f t h i n f l a t p l a te s , a n d m a y e x h i b i t

b e h a v i o r t y p i c a l o f b o t h c o a r s e a n d f i n e p a r t i c l e s . I n a d d i t i o n t e r r a c e d

r a t h e r t h a n p l a n a r s u r f a c e s m a y e x i s t , c o m p l i c a t i n g t h e d e t e r m i n a t i o n o f

in t e r pa r t i . c le sp a c ing .

T h e G o u y - C h a p m a n t h e o r y o f th e d o u b l e l a y e r is i ts e lf s e v e r el y l im i t e d

i n a p p l i c a t i o n b e c a u s e o f t h e r e s t r i c t i n g a s s u m p t i o n s r e q u i r e d f o r t h e

s o l u t io n o f t h e d i f f e re n t i a l e q u a t i o n s . A l t h o u g h i t h as b e e n r a t h e r c o n c lu -

s i v e ly d e m o n s t r a t e d t h a t c e r t a i n c l a y p a r t ic l e s m a y b e . p o si ti v el y c h a r g e d a t

t h e i r e d g e s , t h e e f f e c t o f t h i s r e v e r s a l o f c h a r g e f r o m s u r f a c e t o e d g e i s n o t

c o n s i d e re d i n t h e t h e o r y . T h e f a c t s t h a t m o s t n a t u r a l s o fts a r e m o r e o r l es s

r a n d o m m i x t u r e s o f s e v e ra l e l e c tr o l y te s a n d m i n e r a ls a n d t h a t t h e c l a y

p l a t es a r e n o t l i k e l y t o b e a r r a n g e d i n a p r e c i s e l y u n i f o r m p a r a l le l a r r a y

a r e f u r t h e r c o m p l i c a t i n g f a c t o r s . I n a d d i t i o n , i n t e r p a r t i c l e s p a c i n g s a r e

g e n e r a l l y s m a l l , p o s s i b l y i n t r o d u c i n g p r e s s u r e s i n t h e w a t e r n o t a c c o u n t e d

f o r b y e x i s t in g t h e o r i es .

T h e a d s o r p t i v e c o m p o n e n t s w e r e f o u n d in m o s t c a s es t o b e z e ro . I n a c t u -

a l i ty t h is w o u l d p r o b a b l y n o t b e t h e c as e. I t i s m o r e l i k e ly t h a t a c o m p o n e n t

o f t h e a d s o r p t i v e f o r c e f ie ld s w o u l d b e a c t i v e a t a l l p o i n t s b e t w e e n t w o

a d j a c e n t p a r t i c le s , e v e n a t p o i n t s w h e r e t h e e l e c t r ic f i el d s t r e n g t h i s z e r o .

I t i s q u i t e p o s s i b l e t h a t w a t e r i s a d s o r b e d b y t h e f o r m a t i o n o f h y d r o g e n


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174 N r ~ NA~ONAL C O~ER~,~C E ON C r .~ys A~D C r ~Y MI ~m~ALS

b o n d s w i t h p a r t i c l e s u r f a c e s o r b y o t h e r m e c h a n i s m s n o t r e l a t e d t o t h e

e l e c tr i c f ie ld . N o w h e r e i n t h e a n a l y s i s h a s t h e e f fe c t o f i n t e r p a r t i c l e a t t r a c t i v e

f o r ce s o n w a t e r p r e s s u r e b e e n c o n s i d e r e 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 t h a t

v a n t i er W a a l s a n d o t h e r a t t r a c t i v e f o r c e s k r i o w n t o e x i s t b e t w e e n p a r t i c l e s

g o e n t i r e ly u n f e lt b y t h e w a t e r p h a s e .

T h u s , w h i l e q u a n t i t a t i v e a p p l i c a b i l i t y o f t h e r e l a t i o n s h i p s i n d i c a t e d b y

t h e a b o v e a n a l y s i s h a s b e e n d e m o n s t r a t e d f o r c e r t a i n h i g h l y i d e a l i z e d

s y s t e m s B o l t , 19 5 6) , d i r e c t a p p l i c a t i o n o f t h e r e s u l t s t o s o ft s o f t h e t y p e

e n e o u n t e r e d b y t h e e n g i n e e r h a s b e e n u n s u c c e s s f u l M i t c h el l, 19 6 0) . T h e r e

a r e, h o w e v e r, se v e r a l f u n d a m e n t a l c o n c e p t s t h a t e m e r g e fr o m s u c h a n

a n a ly s is t h a t m a y a i d i n t h e u n d e r s t a n d i n g o f c l a y b e h a v i o r w h e n c o n s i d e r ed

o n a p u r e l y q u a l i t a t i v e b a si s.

C O N C L U S I O N S F R O M T H E N L Y S I S

T h e a n a l y s i s o f c o m p o n e n t a n d t o t a l w a t e r p r e s s u r e s l e a d s t o t h e f o l l o w .

i n g c o n c l u s io n s r e l a t i v e t o t h e u n d e r s t a n d i n g o f t h e s i gn i fi c an c e o f p o r e

p r e s s u r e m e a s u r e m e n t s i n f i n e - g r a i n e d s o f t s .

1 ) T h e t o t a l w a t e r p r e s s u r e i n a s o f t - w a t e r s y s t e m a t e q u i l i b r i u m i s

e v e r y w h e r e t h e s a m e a n d m a y c o n s is t o f a n y o n e o r a c o m b i n a t i o n o f

s e v e r a l c o m p o n e n t p re s s u re s . A t a g i v e n d e n s i t y e x t e r n a l fo r c e s m u s t u s u a l l y

b e a p p l i e d t o t h e s o f t m a s s o r w a t e r p h a s e i n o r d e r t o m a i n t a i n a n o - fl o w

e q u i l i b r i u m c o n d i t i o n w h e n t h e l i q u i d p h a s e i s i n c o n t a c t w i t h e x t e r n a l

w a t e r .

2 ) T h e t o t a l p r e s s u r e i s m e a s u r a b l e a n d g i v e n b y e q . 7 ) f o r t h e c a se

o f n o e l e v a t i o n h e a d .

3) D o u b l e l a y e r i n t e ra c t i o n s a n d t h e c o n s e q u e n t r e p u l si o n b e t w e e n

o p p o s i n g p a r t i c l e s a r e r e fl e c t e d b y t h e o s m o t i c p re s s u r e .

4) A t e q u i l i b ri u m t h e c o m p o n e n t w a t e r p r es s u re s v a r y f r o m p o i n t t o

p o i n t i n s u c h a w a y t h a t t h e t o t a l w a t e r p r e ss u r e r e m a i n s c o n s t a n t .

5 ) A i r - w a t e r i n t e rf a c e s o f b o t h n e g a t i v e a n d p o s i t i v e c u r v a t u r e m a y

e x i s t i n t h e s a m e s o f t - w a t e r s y s t e m . I n t e r f a c e c u r v a t u r e t h a t r e s u lt s f r o m

a n e q u i l i b r i u m b e t w e e n s u r f a c e t e n s i o n , a d s o r p t i v e f o r c e s , a m o u n t o f

a v a i la b l e w a t e r a n d s y s t e m g e o m e t r y is re f le c t ed b y a c h a n g e i n h y d r o s t a t i c

p r e s s u r e f r o m t h e v a l u e e x i s t i n g b e n e a t h a f l a t a i r - w a t e r i n t e r f a c e .

T H E P H Y S I C L S I G N I F I C N C E O F P O R E

P R E S S U R E S S M E S U R E D I N S O I L M E C H N I C S

T E S T S

O n t h e b a s is o f t h e p r e s s u re c o n c e p t s d e v e l o p e d in t;h e p r e c e d i n g s e c t i o n

i t is o f i n t e r e s t t o a t t e m p t a n i n t e r p r e t a t i o n o f t h e p h y s i c a l s i gn i fi c an c e o f

p o r e w a t e r p r e s su r es a s u s u a l l y m e a s u r e d d u r i n g s t r e n g t h a n d c o m p r e ss i o n


14/23

C O M P ON E N T S O F P o R E W T E R 175

t e s ts o n c l ay s . S a t u r a t e d c l a y s o n l y w il l b e c o n s i d e r e d . P o r e p r e s s u r e s

c o m m o n l y a r e m e a s u r e d b y a s en s i n g e l e m e n t u s u a l ly e i t h e r a p o r o u s t i p

i n s e r t e d w i t h i n t h e s a m p l e o r a p o r o u s s t o n e a t t h e b a s e o f t h e s a m p l e )

w h i c h is c o n n e c t e d t o a m e a s u r i n g d e v i c e, a s s h o w n ~ c h e m a t i ca l l y i n F i g . 4 .

C o n n e c ti o n i s m a d e f r o m t h e s a m p l e t h r o u g h t h e p o r o u s s to n e t o a n u l l

= / ~ O 0 -3

I ~ IL ~ o - = O e v ] o fo r S t r e s s

.B

ample

s e a l e d t h / n

r u b b e rm e m b r o n e

I ~ - I / - N u l l

I ~ . ~ A ~ p o n t

~ T q p r e s s u r e c o n t r o /

- - ~ M e ~ u ~

FIaUR 4. Schematic diagram of trlaxlal test arrangement with null point po r

pressure measuring system.

b a l a n c e p o i n t , A . T h e w a t e r l ev e l is m a i n t a i n e d a t p o i n t A t h r o u g h o u t t h e

t e s t ; t h e p o s i t i v e o r n e g a t i v e h y d r o s t a t i c p r e s s u r e , ~ o x, r e q u i r e d t o m a i n t a i n

t h e l e v e l a t A i s t a k e n a s t h e p o r e w a t e r p r e s s u re w i t h in t h e s a m p l e ; t h i s

v a l u e o f p r e s s u r e i s s u b s t i t u t e d f o r u i n e q . 1 ) .

L e t p o i n t C b e s o m e r e p r e s e n t a t i v e p o i n t b e t w e e n p a r t i c l e s . I f t h e

s y s t e m i s i n e q u i l i b r iu m , t h e n t h e t o t a l p r e s s u r e a t a l l p o i n t s i s t h e s a m e

a n d P A = P c . T h e c o m p o n e n t s o f t o t a l p r e s su r e a t p o i n t A a r e p x g i v e n

b y t h e a p p l i ed b a c k p r e s su r e , a n d t h e o s m o t i c p r e s su r e a t A , ~ , w h i c h

d e p e n d s o n t h e c o m p o s i t i o n o f t h e f l u i d a t p o i n t A . T h e o s m o t i c p r e s s u re

a t C i s d e t e r m i n e d b y t h e d o u b l e l a y e r c o n d i t i o n s w h i c h , i n t u r n , a r e

d e p e n d e n t o n t h e e l e c tr i c f ie ld s t r e n g t h f r o m t h e p a r t i c l e s u rf a c es a n d t h e


15/23

176

I~INTH ~T TION L CONFERENCE ON CL YS ND CL Y

MINERALS

a m o u n t a n d t y p e o f e l e c tr o l y te i n t h e s a m p l e . T h e h y d r o s t a t i c p r e s s u r e P c

m u s t h a v e a v a l u e a p p r o p r i a t e t o s a t is f y th e c o n d i ti o n PA = Pc; t h u s

Pc + ~c T ac = PA + ~A , 17)

a n d i f p u r e w a t e r i s u s e d i n t h e m e a s u r i n g s y s t e m ,

7~ = 0 , a n d PA ~P c +7cc +ac 18)

I t w o u l d th u s a p p e a r t h a t m e a s u r e d p o r e w a t e r p r e s su r e i n cl u d es t h e

h y d r o s t a t i c p r e s s u r e , t h e o s m o t i c p r e s s u r e a n d t h e a d s o r p t i v e p r e s s u r e

b e t w e e n p a r t i c l e s . D u r i n g a s h e a r o r c o n s o l i d a t i o n t e s t , p o r e p r e s s u r e

m e a s u r e m e n t s s h o u l d r e fl e c t t h e n e t e f f e c t o f c h a n g e s i n t h e v a r i o u s c o m -

p o n e n t s . I t s h o u l d b e n o t e d a l so t h a t t h e p r e s s u r e c o m p o n e n t s m a y v a r y

f r o m p o i n t t o p o i n t w i t h i n t h e s a m p l e . T h u s t h e h y d r o s t a t i c c o m p o n e n t ,

w h i c h i s t h e o n e t h a t i s f e l t b y t h e p a r t i c l e s i n t h e s e n s e o f a t h r u s t w h i c h

i n fl u e nc e s t h e s t a t i c e q u i l i b r iu m o f t h e s t r u c t u r e , i s a v a r y i n g q u a n t i t y

t h r o u g h o u t t h e s a m p l e .

I n t h e d e v e l o p m e n t o f e q s. 1 7) a n d 1 8) i t w a s a s s u m e d t h a t t h e s a m p l e

w a s i n e q u i l i b r i u m w i t h t h e m e a s u r i n g s y s t e m . T h i s i m p l i e s n o t o n l y a

c o n d i t i o n o f n o -f l o w b u t a ls o c o m p l e t e o s m o t i c e q u i l i b r iu m . S u c h a c o n -

d i t io n w i l l e x i s t o n l y if t h e e l e c t r o l y t e c o n c e n t r a t i o n i n t h e p o r o u s s t o n e

a n d m e a s u r i n g s y s t e m i s t h e s a m e a s th e f r ee e le c t r o ly t e c o n c e n t r a t io n o f

t h e s a m p l e . I f p u r e w a t e r is u se d i n t h e m e a s u r i n g s y s t e m t h e n e q u i l ib r i u m

w i l l e x i s t i n i t i a l l y i f t h e c l a y c o n t a i n s o n l y s u f f i c i e n t e l e c t r o l y t e t o j u s t

s a t i s fy d o u b l e l a y e r r e q u i r e m e n t s a n d n o e x c e ss . O t h e r w i s e t h e m e a s u r i n g

s y s t e m s h o u l d c o n t a i n e l e c t ro l y t e of t h e s a m e t y p e a n d c o n c e n t r a t i o n a s t h e

f r e e s a l t i n t h e c l a y f o r e q s . 1 7) a n d 1 8) t o b e v a l i d .

I f , a s i s u s u a l , a c l a y c o n t a i n i n g f r e e s a lt is c o n n e c t e d t o a m e a s u r i n g

s y s t e m c o n t a i n i n g S a l t - f r e e w a t e r t h e n e q u i l i b r i u m w i l l n o t b e r e a c h e d

w i t h in a n y c o n v e n i e n t t e s t p e r io d . T h e r e w i ll b e a n a b r u p t e l e c tr o l y t e

c o n c e n t r a t i o n c h a n g e i n p a s s i n g f r o m t h e s a m p l e to t h e p o r o u s s t o n e . S in c e

t h e s t o n e c a n n o t r e s t r i c t t h e m o v e m e n t o f t h e f r ee s a l t ~ on s t h e r e w i ll b e a

s lo w d if f us io n o f i o n s t h r o u g h t h e s t o n e u n t i l t h e f r ee s a l t c o n c e n t r a t i o n i n

b o t h t h e s a m p l e a n d t h e s y s t e m i s t h e s a m e . T h i s p r o ce s s is e x t r e m e l y s lo w

a n d m a y t a k e w e e k s o r m o n t h s t o r e a c h c o m p l e t io n . S e v e r a l t e s t s w e r e r u n

w h e r e i n 0 .5 N N a C 1 a n d p u r e w a t e r w e r e p l a c e d o n o p p o s i t e s id e s o f a

s a t u r a t e d f in e p o r o u s s t o n e o f a b o u t 0 . 4 in . t h ic k n e s s . N o s a l t w a s d e t e c t a b l e

o n t h e w a t e r s i de o f t h e s t o n e a f t e r a 2 4 h r p e r i o d . S i n c e t h e p o r o u s s t o n e

w a s n o t i m p e r m e a b l e t o i o n s , h o w e v e r , t h e r e w a s n o m e a s u r a b l e o s m o t i c

p r e s s u r e a c r o s s t h e s t o n e .

I n v i e w o f t h e s e c o n s i d e r a t i o n s , t h e r e f o r e , e q s . 1 7) a n d 1 8) w i ll n o t

b e c o r r e c t i n m o s t c a se s. T h e o s m o t i c p r e s s u r e t e r m , ~ c , m a y b e v is u a l i z e d

a s c o n s is t in g o f t w o c o m p o n e n t s , ~ c d u e t o t h e c o n c e n t r a t i o n o f f r ee

s a l t, a n d ~ lcr c a u s e d b y t h e a d d i t i o n a l i o n s n e e d e d a t p o i n t C i n o r d e r t o


16/23

COMPOnEnTS OF Po RE W TER

177

s a t i s fy d o u b l e l a y e r r e q u i r e m e n t s . T h e d o u b l e l a y e r io n s a r e n o t f r e e t o

d i ff u se i n t o t h e m e a s u r i n g s y s t e m ; t h e r e fo r e , o s m o t i c p r e s s u r e ~ w i ll b e

r e f le c t ed b y P A T h e o s m o t i c p r e s s u r e ~ d u e t o e x c e s s s a l t w i l l n o t in f l u e n c e

t h e v a l u e o f p A . T h u s , e q . ( 18 ) w o u l d b e

P A ~ - P c ~ 'c ' + a c = P B , (19)

w h e r e ~ i s a n o s m o t i c p r e s s u r e d u e t o a d i ff e re n c e i n s a lt c o n c e n t r a t i o n

f r o m p o i n t C a n d a p o i n t o u t o f th e p a r t i c le f o r ce fie ld s, b u t c o n t a i n i n g

f r e e s a l t, s u c h a s p o i n t B i n F i g . 4 . T h u s , i f p o i n t s A a n d B a r e a t t h e s a m e

e l e v a t i o n t h e p o r e p r e s s u r e m e a s u r e m e n t g i v e s t h e t o t a l p r e s s u r e a t C

r e l a t i v e t o a f l u i d c o n t a i n i n g f r e e s a l t i n e q u i l i b r i u m w i t h t h e c l a y ;

i . e .

t h e

p r e s s u r e a t O re l a t i v e t o B . T h i s i s p r e c i s e ly t h e p r e s s u r e d e s i re d i n p r a c t i c e

s i n c e t h e h y d r o s t a t i c p r e s s u r e s t h a t d e v e l o p r e l a t i v e t o t h e i n s i t u p o r e f l u id

w h e n a c l a y is s h e a r e d o r c o m p r e s s e d a r e t h e o n e s t h a t w i ll i n fl u en c e t h e

e f f e c t i ve s t r e s s e s .

T h e f o r e g o i n g c o n s i d e r a t i o n s s h o u l d h o l d t r u e r e g a r d l e s s o f t h e s a l t c o n -

t e n t o f t h e m e a s u r i n g s y s t e m s o l o n g as a n a p p r e c i a b l e q u a n t i t y o f s a l t

a n d w a t e r d o e s n o t d i f f us e i n o r o u t o f t h e s a m p l e b e f o r e o r d u r i n g a t e st .

I f a s i g n i f i c a n t a m o u n t o f f r e e s a l t d i f f u s e s o u t o f t h e s a m p l e s a n d i n t o t h e

m e a s u r i n g s y s t e m , f o r e x a m p l e , t h e r e w i l l b e a c h a n g e i n t h e d o u b l e l a y e r

o s m o t i c p r e s s u r e , g o , s i n c e d o u b l e l a y e r c o n c e n t r a t i o n s a r e e x t r e m e l y

s e n s it i v e t o f r e e s a l t c o n t e n t o f t h e c l a y . I t w o u l d a p p e a r d e s i r a b le , t h e r e -

f o r e , t o t e s t s a m p l e s a s s o o n a s p o s s i b l e a f t e r p l a c i n g t h e m i n c o n t a c t w i t h

t h e m e a s u r i n g s y s t e m , t o k e e p t h e v o l u m e o f w a t e r i n th e m e a s u r i n g s y s t e m

t o a m i n i m u m , a n d t o p r e v e n t t h e f lo w o f f lu id b e t w e e n s a m p l e a n d m e a -

s u r i n g s y s t e m , i n o r d e r t o m a i n t a i n t h e o r i g i n a l s a l t c o n c e n t r a t i o n s a t a l l

p o i n t s w i t h i n t h e s a m p l e . F o r l o n g t e r m t e s t i n g e q u a l c o n c e n t r a t i o n s o f

f r e e s a l t i n b o t h t h e m e a s u r i n g s y s t e m s a n d s a m p l e w o u l d b e d e s i r a b l e .

T e s t s a r e c u r r e n t l y in p r o g r e s s w h i c h a r e d e s i g n e d t o e s t a b li s h t h e v a -

l i d i ty o f t h e s e c o n c lu s io n s . T h e l i m i t e d d a t a o b t a i n e d t h u s f a r w o u l d t e n d

t o s u p p o r t t h e c o n c lu s io n t h a t t h e m e a s u r e d p o r e p re s su r e s a r e i n d e p e n d e n t

o f s a l t c o n c e n t r a t i o n i n t h e m e a s u r i n g s y s t e m .

P H Y S I C L I N T E R P R E T T I O N O F E F F E C T I V E

S T R E S S

I t w a s p o i n t e d o u t a t t h e o u t s e t o f t h i s p a p e r t h a t f o rc e s o t h e r t h a n t h o s e

d u e s o l e ly t o a p p l i e d s t re s s e s a n d p u r e l y h y d r o s t a t i c w a t e r p r e s s u r e s a r e

a c t i v e i n f i n e - g r a i n e d s o i l s . T h e a n a l y s i s o f c o m p o n e n t a n d t o t a l w a t e r

p r e s s u re s i n d i c a te s t h a t s o m e o f t h e s e a d d i t i o n a l f o r c e s ar e re f l e c te d t h r o u g h

t h e p r e s s u r e s d e v e l o p e d i n t h e f l u i d p h a s e . T h e c o n c e p t o f e f f e c t i v e s t r e s s

m a y b e e x a m i n e d i n t h e l i g h t o f t h e s e f in d i n g s. M e c h a n i s t i c a p p r o a c h e s

t o t h e e x p r e s s i o n o f e f f e c t i v e s t r e s s e s i n t e r m s o f i n t e r p a r t i c l e f o r c e s a s w e l l


17/23

178 NINTI~INATIONALCONFERENCE ON CLAYSAND CLA YMINERALS

a s a p p li e d f o rc e s h a v e b e e n p r e s e n t e d b y L a m b e (1 96 0), L a m b e a n d W h i t -

m a n ( 19 59 ) a n d S e e d , M i t c h e ll a n d C h a n ( 19 6 0), a m o n g o t h e r s , a l l l e a d i n g

t o f i n a l e x p r e s s i o n s o f t h e s a m e g e n e r a l f o r m . A n a l t e r n a t i v e a p p r o a c h b a s e d

o n t h e a n a l y s i s o f w a t e r p r e s s u r e i s p o s s ib l e .

C l a y p a r t i c l e s a r e a s s u m e d t o b e e s s e n t i a l l y i n c o n t a c t w i t h t h e t y p i c a l

p a r t i c l e a s s o c i a ti o n b e i n g c o r n e r- o r e d g e - t o - fa c e a s s h o w n i n F i g . 5. W h e t h e r

t h e c o n t a c t s a r e m i n e r a l - t o - m i n e r a l o r t h e r e i s s o m e f e w A n g s t r 6 m s s e p a r a-

t i o n w i t h t h e i n t e r v e n i n g s p a c e f il le d w i t h a d s o r b e d w a t e r a n d c a t i o n s is

X ~ . . . . . . . . . . . . ~

p / , / , o f l e n g t h ~

FIauI~E 5. Idealize d representation of forces betwe en cla y particles.

n o t k n o w n . I t d o e s n o t s e em u n r e a s o n a b le , h o w e v e r , t h a t s o m e t y p e o f

s o li d o r s e m i s o li d c o n t a c t e x i s ts . A c o m p l e t e l y s a t u r a t e d s y s t e m i s as s u m e d .

I t is r e c o g n iz e d t h a t n o s h a r p b o u n d a r y , s u c h a s s h o w n b y t h e d o t t e d l i ne ,

s e p a r a t e s f r e e a n d b o u n d w a t e r . S u c h a li n e i s s h o w n o n l y t o a i d i n t h e

d e f i n i ti o n o f a s u r f a c e o f l e n g t h l t h r o u g h w h i c h s li p m u s t o c c u r if t h e

i n t e r p a r t i c l e b o n d i s t o f a i l i n s h e a r .

I n e x a m i n i n g t h e s t a t i c e q u i l i b r i u m o f t h i s c o n d i t i o n t h e f o r c e s t o b e

c o n s i d e r e d , F i g . 5 , a r e :

a ~ t h e a p p l i e d a v e r a g e e x t e r n a l st re s s.

A =

t h e n e t p h y s i c o - c h e m i c a l a t t r a c t i v e f o r c e b e t w e e n p a r t i c le s . I t i s

a s s u m e d t h a t a t t r a c t io n s p r e d o m i n a t e in t h e c o n t a c t z o n e a n d a r e

r e s p o n s i b le f o r t r u e c o h e s i o n i n c l a y s . T h e s e fo r c e s m a y a r is e f r o m

v a n d e r W a a l s a t t r a c t i o n , c e m e n t a t i o n , c a ti o n l in k a g es , h y d r o g e n

b o n d s a n d o t h e r m e c h a n is m s . R e p u l s i v e fo r c es m a y a ls o b e a c t iv e i n

t h e i n t e r p a r t ic a l c o n t a c t z o n e ; A i s t a k e n a s t h e e x c e ss o f a t t r a c t i o n

o v e r r e p u l s io n .

p = t h e h y d r o s t a t i c c o m p o n e n t of t h e t o t a l p o r e w a t e r p r es s u re .

T h e h y d r o s t a t i c p r e s s u r e 1o a t a n y p o i n t b e t w e e n c l a y p a r ti c l e s is r e l a t e d

t o t h e h y d r o s t a t i c p r es s u r e in t h e f lu i d a t a p o i n t o u t o f t h e p a r t i c l e f o r c e

f i e ld s P B ; a c c o r d in g to e q . ( 19 ),

PB = P + ~ + a , (20)


18/23

C o ~ o ~ v . ~ T S O r P o R WATER 179

w h e r e ~ i s t h e o s m o t i c p r e s s u r e a t t h e p o i n t b e t w e e n p a r ti c l e s r e l a t i v e t o

t h e o s m o t i c p r e s s u r e a t a p o i n t o u t s i d e o f t h e p a r t i c l e f o r c e fi el ds , a n d a i s

t h e a d s o r p t i v e c o m p o n e n t of w a t e r p r es s u re . B o t h ~ a n d a a r e n e g a t i v e

c o m p o n e n t s ; t h a t is , t h e h i g h e r c o n c e n t r a t i o n o f i o ns b e t w e e n c l a y p a r -

t ic l es r e s u l t i n g f r o m i n t e r a c t i n g d o u b l e l a y e r s a n d t h e a d s o r p t i v e p r e s su r e

t e n d t o d r a w f r e e w a t e r i n . T h e e f f e c t o f t h e s e p r e s s u r e s t h e n i s t o k e e p

p a r t i c l e s a p a r t . T h e y m a y b e c o n s i d e r e d a s o n e t e r m R , t h e i n t e r p a r t i c l e

r e p u l s i v e p r e s s u r e d u e t o t h e p h y s i c o - c h e m i c a l e f fe c ts i n t r o d u c e d b y t h e

s u r f ac e a c t i v i t y o f t h e c l a y p a rt i c le s . T h u s e q . (2 0) b e c o m e s :

p ~ = p + R ,

b u t s i n c e t h e p r e s s u r e IvB = ~o x, t h e m e a s u r e d p r e s s u r e , a n d p x i s c o n -

v e n t i o n a ll y te r m e d u w e o b t a i n

u = p

F u r t h e r , s in c e R i s a n e g a t iv e c o m p o n e n t w e m a y w r i te

u = 2 - - R . (21)

E x a m i n a t i o n o f t h e s t a t i c e q u i l i b r i u m o f t h e p a r t ic l e s , F i g . 5 , s h o w s

a a + A = T a + C ,

w h e r e a ' is t h e e f f e c ti v e a r e a c o v e r e d b y t h e p a r t i c l e s a n d G i s t h e c o n t a c t

f o r c e b e t w e e n p a r t i c l e s .

C o n v e r t i n g f o r c e s t o s t re s s e s g i v e s

a + A / a = p + C / a .

I t m a y b e n o te d t h a t C ] a i s n o w t h e a c t u a l s t r e s s t r a n s m i t t e d b e t w e e n

p a r t ic l e s p e r u n i t a r e a o f t h e m a s s ; i .e. t h e t r u e i n t e r g r a n u l a r s t r e s s , ~ .

l

L e t t i n g A ' / a ' = A a n d p = u + R , w e o b t a i n a + A = u + R + a , o r

= q + A - - u + R ) . 22)

T h i s e x p r e ss io n , d e v e l o p e d f r o m a c o n s i d e r a t io n o f p o r e w a t e r p r e s su r e

c o m p o n e n t s , is o f t h e s a m e g e n e r a l f o r m a s p r e v i o u s l y d e ri v e d e x p r e s s io n s

f o r i n t e r g r a n u l a r s t r e ss ( L a m b e a n d W h i t m a n , 1 9 59 ; L a m b e 1 9 6 0 ; S e ed ,

M i t c h e l l , a n d C h a n , 1 9 6 0 ) . R e l a t i o n s s u c h a s ( 2 2 ) u n f o r t u n a t e l y a r e q u i t e

l i m i te d in t h e i r a p p l i c a t i o n ; n o t o n l y b e c a u s e o f t h e a s s u m p t i o n s a n d

a p p r o x i m a t i o n s u s e d i n t h e i r d e v e l o p m e n t , b u t a l s o b e c a u s e m e t h o d s a r e

n o t a v a i l a b l e f o r t h e q u a n t i t a t i v e d e t e r m i n a t i o n o f t h e q u a n t i t ie s A , R

a n d a ' .

F i g u r e 6 h a s b e e n p r e p a r e d t o i l l u s tr a t e t h e v a r i o u s in t e r re l a ti o n s h ip s t h a t

e x i s t b e t w e e n a p p l i e d , p o r e w a t e r , a n d i n t e r g r a n u l a r p r e s s u r es . F o u r p o s s i b l e

s o il c o n d i t i o n s a r e i n d i c a t e d w i t h t h r e e c a s e s o f m e a s u r e d f l u id p r e s s u r e

s h o w n f o r e a c h c o n d i t i o n . T h e u = 0 c as e r e f e r s t o a m e a s u r e d p o r e w a t e r


19/23

18 NI~T~I ~ATIONAL CONFERENCE ON CLAYS AND CLAY MINERALS

i t ~ ~ r ~

~ ~ ~ .~

~ ~ ~ ~


20/23

COMPONENTS O PORE W TER

181

pressure of O, the a = 0 case refers to an equilibrium maintained by stressing

the fluid that communicates with the sample, and the general case refers

to the usual condition of both an applied normal stress and a measured

pore pressure. Complete saturation and the same void ratio are assumed

for each case.

Fig. 6(~) represents an ideal c lay-w ate r syst em composed of parallel

particles, not in contact, with no attractive forces active. Three possibilities

exist for main tain ing equilibrium. As indica ted by the u = 0 case the to tal

pressure of the pore fluid can be increased to zero by the application of a

normal stress a to the sample, which must be carried entirely by the pore

fluid since particles do not contact. The applied pressure induces a positive

hydrostatic component in the pore water which balances the negative

repulsive component. A second possibility is to decrease the total pressure

of the free wate r to a value of -- R as indica ted by the a = 0 case. The third

possibility consists of adjusting the total water pressures inside and outside

the sample to the same value by applying a combination of normal pressure,

aa, and stress to the free water, h y . Of course, an infinite number of aa

and h y combinations can be found so that equilibrium will be maintained,

and the u = 0 and a = 0 cases are actually special cases of the general case.

The values of the various pressure terms listed under the diagrams in

Fig. 6 can be deduced readily from a consideration of the pressures shown

on the diagram, the type of soil structure depicted, and an analysis of the

statics of equilibrium. It ma y be seen tha t for the ideal syst em of Fig. 6 (a)

R is measurable.

Fig. 6(b) represents a coarse system with insignificant R components

and contact between particles. Contact areas between particles have been

assumed to be insignificant. Conditions (a) and (b) are combined in Fig. 6 (c)

where a mixed system with particles in contact and electrical repulsion

forces active, but no attractive forces, is shown. It may be noted that a

portion of the applied normal stress a must be carried by the water in order

to bring the total water pressure to zero. Only the difference ~ -- R can be

effective in creating an intergranular pressure ~.

The mos t general case is shown in Fig. 6 (d). I t is evident from a con-

sideration of the relationship shown under the figure that the intergranular

stress is increased by the amount of the attractive pressure A. The general

equation for equilibrium for the general case, Fig. 6(d), is of the same

form as eq. (22) where h y corresponds to u.

There arises from the analysis a further point of considerable interest;

namely, what is the true meaning of effective stress in a clay? It is obvious,

as shown by eq. (22), t ha t in a soil with significant electrical repulsive

and attractive forces, the intergranular pressure is not simply the difference

between the total applied pressures and the measured pore water pressures.

Lambe and W hi tman (1959) have considered this point in considerable detail.


21/23

8 2 N IN TI I N A T I O N A L C O ~ F E I~ E N C E Olq ~ K Y S A N D C L A Y M I~ E I~ A LS

T h e p o s s i b i l it y e x i s ts t h a t e f fe c t iv e s t re s s es a n d i n t e r g r a n u l a r s t re s s es a r e

n o t s y n o n y m o u s i n c l a y s. S ee d , M i t c h e ll a n d C h a n ( 19 6 0) s u g g e s t t h a t

s t r e n g t h s h o u l d b e a f u n c t i o n o f t r u e i n t e r g r a n u l a r p r e s s ur e , a n d L a m b e

a n d W h i t m a n (1 95 9) p o i n t o u t t h a t t h e o b s e r v e d b e h a v i o r of e x p a n s i v e c la y s

d o e s n o t c o r r e l a te w e ll w i t h e f fe c t iv e s tr e ss a s d e f i n e d b y t h e d i f fe r e n ce b e t w e e n

a p p l i e d p r e s s u re s a n d m e a s u r e d p o r e w a t e r p r e s s u r e. I t i s b y n o m e a n s s u r e ,

o n t h e o t h e r h a n d , t h a t t r u e i n t e r g r a n u l a r s t re s s c o m p l e t e l y c o n t ro l s b e-

h a v i o r i n f i n e - g r a i n e d s o i l s . F o r e x a m p l e , v o i d r a t i o - p r e s s u r e r e l a t i o n s h i p s

f o r e v e n h i g h l y p l a s t ic s o ft s, w h e r e R a n d A f o r c e s m a y b e o f a p p r e c i a b l e

m a g n i t u d e , h a v e b e e n f o u n d t o c o r r e la t e w e l l w i t h a n e f fe c t iv e s t re s s

d e f i n e d a s t h e d i f fe r e n c e b e t w e e n t o t a l a p p l i e d s t re s s es a n d o b s e r v e d p o r e

w a t e r p r e s s u r es . C o m p l e t e c l a r i fi c a t i o n o f t h e p r o b l e m o f t h e s i g n i fi c a n c e

o f i n t e r g r a m f l a r a n d e f fe c t iv e s t re s s e s i n f i n e - g ra i n e d so ft s m u s t a w a i t t h e

r e s u l t s o f f u t u r e r e s e a r c h .

S U M M A R Y A N D C O N C L U S I O N S

I t h a s b e e n t h e o b j e c t i v e o f t h i s p a p e r t o e x a m i n e t h e p h y s i c a l si g ni -

f ic a n c e o f p o r e w a t e r p r e s s u r e i n c la y s a n d t o p o i n t o u t s o m e i m p l i c a t io n s

p e r t i n e n t t o s oi l e n g in e e ri n g . T o t a l p o r e w a t e r p r e s s u re m a y b e c o n v e n i e n t l y

c o n s i d e r e d t o c o n s i s t o f t h e c o m b i n e d e f f e c t o f f o u r c o m p o n e n t s ; e le -

v a t i o n , h y d r o s t a t i c p r e s s u re , a p r e s s u r e d u e t o t h e a d s o r p t i v e f o r c e f ie ld s

o f c l a y p ar t ic l e s, a n d a n o s m o t i c p r e s su r e a r i s in g f r o m t h e p r e s e n c e o f di s-

s o l v e d sa l ts a n d i n t e r p a r t i c l e d o u b l e l a y e r e f fe c ts . D a t a a r e p r e s e n t e d w h i c h

i n d i c a t e a s i g n i fi c a n t p o r t i o n o f t h e s w e l l o f a c o m p a c t e d s o i l o n e x p o s u r e

t o w a t e r i s a t t r i b u t a b l e t o w a t e r p r e s s u r e d e fi c ie n c ie s c a u s e d b y m e c h a n i c a l

a n d c a p i l l a r y e f fe c t s w h i c h a c t i n a d d i t i o n t o t h e p r e s s u r e d e f ic i en c i es

a r is i n g f r o m d o u b l e l a y e r i n t e r a c ti o n s .

A n a n a l y s is o f t h e p r e s s u r e s a t v a r i o u s p o i n t s i n a c l a y , b a s e d o n t h e

c o n d i t i o n t h a t a t e q u i l i b r i u m ( no flo w ) t h e t o t a l p r e s su r e i s e v e r y w h e r e

t h e s a m e , s h o w s t h a t c h a n g es i n a n y o n e c o m p o n e n t fr o m o n e p o i n t t o

a n o t h e r a r e o f fs e t b y c h a n g e s i n t h e o t h e r s .

I t m a y b e c o n c l u d e d f r o m t h e a n a ly s is t h a t a p o r e w a t e r p r e s s u re m e a s -

u r e m e n t i n a s o il t e s t r e f l e ct s i n t e r p a r t i c l e re p u l s i v e p r es s u r e s ( o s m o t i c

p r e s s u r e s ) a n d w a t e r a d s o r p t i v e f o r c e s a s w e l l a s p u r e l y h y d r o s t a t i c p r e s -

s u r e s a r i s i n g f r o m m e c h a n i c a l e f f e c t s . T h e i n d i v i d u a l c o m p o n e n t s o f t h e

t o t a l p r e s s u re a r e n o t m e a s u r a b l e , h o w e v e r , e x c e p t i n s p e c ia l s y s t e m s , a n d

l i m i t a ti o n s i n t h e o r y p r e v e n t t h e d i r e c t c o m p u t a t i o n o f t h e s e co m p o n e n t s .

A p h y s i c a l i n t e r p r e t a t i o n o f e f f ec t iv e s t re s s is a t t e m p t e d b y r e l a t i n g

c o m p o n e n t w a t e r p r e s s u r e s t o s t r e s s e s b e t w e e n p a r t i c l e s . I t i s s h o w n t h a t

t h i s a p p r o a c h l e ad s t o a r e l a ti o n s h i p o f t h e s a m e g e n e r a l f o r m a s e x p r e s s io n s

f o r i n t e r g r a n u l a r p r e s s u re s p r e v i o u s l y d e v e l o p e d u s i n g d i f f e r e n t a p p r o a c h e s .

M o d e l s a r e p r e s e n t e d i l l u s t r a t i n g t h e i n t e r r e l a t i o n s h i p b e t w e e n c o m p o n e n t


22/23

C O M P O N E N T S O F 19 RE W A T E R

1 8 3

w a t e r p r e s s u r e s , t o t a l w a t e r p r e s s u re , t o t a l a p p l i e d p r e s s u r e a n d i n t e r -

g r a n u l a r p r e s s u r e i n c l a y s .

I t i s t o b e e x p e c t e d t h a t c o n t i n u e d r e s e a r c h w i l l l e a d t o i m p r o v e d u n d e r -

s t a n d i n g o f t h e f o r c e s a c t i v e i n a c l a y - w a t e r s y s t e m a n d a i d i n t h e d e t e r -

m i n a t i o n o f t h e t r u e i n t e r g r a n u l a r p r e s s u r e s i n a s o i l m a s s a n d t h e e v a l -

u a t i o n o f t h e t r u e e f f e c t i v e s t r e s s ; i .e . t h e s t r e s s c o n t r o l l i n g s o i l b e h a v i o r .

R E F E R E N E S

Aitch ison , G. D. (1960) Re la t ion sh ip o f mois tu re s t ress and e f fec t ive s t ress func t ions in un-

sa tu rd ted so i l s . P rec .

C o n f e r e n c e o n P o r e P r e s s u r e s a n d S u c t i o n i n 8 o i l s :

Bu t t e r wo r th s ,

L o n d o n , p p . 1 8 - 2 3 .

Baye r , L . D. (1956) S o i l P h y s i c s : J o h n W i l e y & S o n s, I n c . , Ne w Y o r k , 3 r d e d . , 48 9 p p .

B i s h o p, A .W . ( 19 60 a) T h e m e a s u r e m e n t o f p o r e p r e s s u re i n t h e t r i a x i a l t e s t . Prec . Con .

] er en ce o n P o r e P r e s s u r e a n d S u c t l o ~ i ~ S o i l s :

Bu t t e r w o r th s , L o n d o n , p p . 5 2- 6 0 .

Bishop , A. W . (1960b) Th e p r inc ip le o f e f fec t ive s t ress :

No rweg i an Geotechniea l Ins t i t u t e

Oslo, Pub. 32, pp. 1-5 .

Bishop , A. W . (1960e) Fa c to rs con t ro l l ing the s t reng th o f pa r t i a l ly sa tu ra t ed cohes ive soi ls .

Prec . A S C E Research Con/e rence on t he Shear S t rengt h e l Cohes ive So i l s : Bo ulder , Colorado.

In p ress .

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Geotechnlque

v. 2, pp. 13-32.

Bo l t , G . H . ( 1 9 55 ) An a ly s is o f t h e v a l i d i t y o f t h e Go u y - C h a p m a n th e o r y o f t h e e l e c t ri c

d o u b le l a y e r : J . Col l o i d Se i . v. 10, p. 206.

Bo l t , G . H . ( 1 9 5 6 ) P h y s i c a l - c h e m ic a l a n a ly s i s o f t h e c o m p r e s s ib i li t y o f p u r e c l a y :

Gee.

technique v. 6 , no. 2 , pp. 86-93.

Bol t , G. H . and Fr i s se l , ]~ I. J . (1960) The rm odyn am ics o f so i l m ois tu re : N e t h e r la n d s J . A g r .

S e i . v. 8 , no. 1 , pp. 57-78.

Bol t , G. H. and Mi l le r , R . D. (1955) Compress ion s tud ies o f i ll i t e suspens ions : Soi l Sc i . Soc .

A m e r . P r e c . v. 19, pp. 285-288.

Bo l t , G . H . a n d M i l l er , R . D . ( 1 9 58 ) Ca l c u l a ti o n o f t o t a l c o m p o n e n t p o t e n t i a l s o f wa t e r i n

so i l :

Tra ns . Am er . Geophys . Un .

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Croney, D. and Coleman , J . D. (1953) So i l m ois tu re suc t ion p roper t ie s and the i r bea r ing

o n t h e m o i s tu r e d i s t r i b u t i o n i n s o i ls :

P r e c . 3 r d I n t . C o n f . o n S o i l M e c h . a n d F o u n d . E n g .

v. 1 , pp. 13-18.

E d le f so n , N . E . a n d An d e r s o n , A . B . ( 1 9 43 ) T h e r m o d y n a m ic s o f s o i l m o i s tu r e : H i l g a r d i a

v. 15, pp. 31-298.

Hi]f , J .W . (1956) An inv es t iga t ion o f pore wa te r p ressu res in com pac ted cohes ive so i l s:

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Components of Pore Water Pressure

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