Effect of moisture content on frozen ground strength

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Effect of moisture content on frozen ground strength

Shusherina, E. P.; Bobkov, Y. P.; National Research Council of Canada.

Division of Building Research

NATIONAL RESEARCH COUNCIL OF CANADA CONSEIL NATIONAL DE RECHERCHES DU CANADA

TECHNICAL TRANSLATION

91

TRADUCTION TECHNIQUE

A N A L Y Z E D

T i tl e/Ti t r e : E f f e c t o f m o i s t u r e c o n t e n t on f r o z e n ground s t r e n g t h ( 0 v l i y a n i

i

v l a z h n o s t i merzlykh gruntov na i kh prochnost)

Au thors/Au t e u r s : E.P. Shusherina and Yu. P. Bobkov

Reference/Reference: Merzl otnye I s s 1 edovaniya, 9: 122-1 37, 1969 Trans1 ator/Traducteur: V . _Poppe

Canada I n s t i t u t e f o r I n s t i t u t canadien de

S c i e n t i f i c and Technical 1 ' i n f o r m a t i o n s c i e n t i f i q u e I n f o r m a t i o n e t technique

Ottawa, Canada KIA OS2

PREFACE

S t r e n g t h p r o p e r t i e s o f f r o z e n s o i l s have been shown b y many i n v e s t i g a t o r s t o be a f u n c t i o n o f t h e t o t a l m o i s t u r e c o n t e n t , expressed as a r a t i o o f t h e w e i g h t o f w a t e r c o n t a i n e d i n t h e f r o z e n s o i l t o t h e d r y w e i g h t o f t h e s o i l m a t r i x . The a u t h o r s o f t h i s paper have i n v e s t i g a t e d t h e e f f e c t o f t o t a l m o i s t u r e c o n t e n t o n t h e u n i a x i a l compressive s t r e n g t h o f t h r e e f r o z e n s o i l s (supes, s u g l i n o k and c l a y ) under s h o r t - t e r m l o a d i n g a t t e m p e r a t u r e s between -1 OOC and - 5 5 ' ~ . P o l y c r y s t a l l i n e i c e specimens were

a l s o t e s t e d under s i m i l a r c o n d i t i o n s . \

The compressive s t r e n g t h o f a l l f r o z e n s o i l s i n c r e a s e s a t m o i s t u r e c o n t e n t s s h o r t o f c o n i p l e t e w a t e r s a t u r a t i o n and decreases when t h e s o i l i s f u l l y s a t u r a t e d o r o v e r s a t u r a t e d w i t h m o i s t u r e g r a d u a l l y a p p r o a c h i n g t h e compressive s t r e n g t h o f p u r e i c e .

The d i v i s i o n i s g r a t e f u l t o M r . V . Poppe, T r a n s l a t i o n S e r v i c e s , Canada I n s t i t u t e f o r S c i e n t i f i c and T e c h n i c a l I n f o r m a t i o n , N a t i o n a l Research C o u n c i l o f Canada, f o r t r a n s l a t i n g t h i s paper and t o M r . T.H.W. Baker o f t h i s D i v i s i o n , f o r c h e c k i n g t h e t r a n s 1 a t i o n f o r t e c h n i c a l a c c u r a c y . C.B. Crawford, D i r e c t o r , D i v i s i o n o f B u i l d i n g Research. Ottawa, O n t a r i o , J a n u a r y 1978

EFFECT OF MOISTURE CONTErlT ON FROZEN GROUND STRENGTH

The m o i s t u r e c o n t e n t o f f r o z e n s o i l s ( t h e t o t a l c o n t e n t o f i c e and u n f r o z e n w a t e r ) i s one of t h e most i m p o r t a n t f a c t o r s w h i c h d e t e r m i n e t h e s t r e n g t h o f f r o z e n s o i l s . However, t h e r o l e o f t h i s f a c t o r has ~ o t been a d e q u a t e l y s t u d i e d , a l t h o u g h t h e e f f e c t of t h e m o i s t u r e c o n t e n t on t h e s t r e n g t h o f f r o z e n s o i l s was i n v e s t i g a t e d b y s e v e r a l a u t h o r s 1 $3-10)

The a v a i l a b l e d a t a r e f e r t o d i f f e r e n t s o i 1 s . ( c o m p o s i t i o n , s t a t e ) and d i f f e r e n t e x p e r - i ~ i i e n t a l c o n d i t i o n s ( t e n i p e r a t u r e , t y p e o f d e f o r m a t i o n , d u r a t i o n o f l o a d i n g ) . The t e s t s were performed m a i n l y on specinlens w i t h a d i s t u r b e d s t r u c t u r e . The g r a i n s i z e c o m p o s i t i o n and t h e m o i s t u r e c o n t e n t v a r i e d w i t h i n a w i d e r a n g e o f v a l u e s . I n most e x p e r i m e n t s t h e r a n g e o f t h e m o i s t u r e c o n t e n t s extended beyond t h e A t t e r b e r g l i m i t s ( t h e upper and t h e l o w e r ) . I n some e x p e r i m e n t s , t h e m o i s t u r e c o n t e n t amounted t o a f e w p e r c e n t , w h i l e i n o t h e r i t exceeded t h e w a t e r r e t a i n i n g c a p a c i t y o f t h e s o i l . Most e x p e r i m e n t s were c a r r i e d o u t a t t e ~ i p e r a t u r e s r a n g i n g f r o m

- l 0 c

t o - 1 0 ' ~ 93-8)

,

much f e w e r were performed a t - 1 0 ' ~ t o - 2 0 ' ~ 4 y 7 y 9 )

,

and s t i l l f e w e r

O 4,9) o r l o w e r ( - 5 5 0 ~ ) ~ ) . The d e f o r m a t i o n a t t e m p e r a t u r e s o f - 2 0 ' ~ t o -40 C t e s t s i n c l u d e d u n i a x i a l compression and t e n s i o n , s h e a r , p e n e t r a t i o n t e s t i n g , e t c . As a r u l e , l o a d i n g was of s h o r t d u r a t i o n ( f r o m f r a c t i o n s o f a second6) ,, t o 20

-

30 m i n u t e s ) . E x a m i n a t i o n o f a v a i l a b l e d a t a y3-10) shows t h a t t h e e f f e c t o f t h e m o i s t u r e c o n t e n t o n t h e s t r e n g t h o f f r o z e n s o i l s may v a r y .

A c c o r d i n g t o W .A. T s y t o v i c h and O.M. ~ u m e n s k a ~ a ~ s 7 ) , who p e r f o r m e d

u n i a x i a l compression t e s t s on sand, s u g l i n o k * , supes**, and c l a y a t - 5 ' ~ and - 1 ~ O C , t h e s t r e n g t h o f f r o z e n s o i l i n c r e a s e s w i t h i n c r e a s i n g ~ n o i s t u r e c o n t e n t

b u t t h e n f a l l s t o a c e r t a i n 1 i m i t . As a r u l e , t h e a s c e n d i n g 1 imb o f t h e c u r v e c o r r e s p o n d s t o an i n c o m p l e t e and r i s i n g s a t u r a t i o n o f f r o z e n s o i l

,

whi 1 e t h e descending 1 imb corresponds t o corr~pl e t e s a t u r a t i o n .

*

S u g l i n o k

-

c l a y e y s i l t w i t h some sand, c o n t a i n s 10 t o 30% c l a y s i z e b y w e i g h t w i t h p a r t i c l e s l e s s t h a n 0.005 mm i n s i z e , 0.005 b e i n g t h e d i v i d i n g

l i n e between c l a y s i z e and s i l t s i z e i n t h e S o v i e t s o i l c l a s s i f i c a t i o n ( T r a n s l

. ) .

**

_Supes

_-

_{s i l t y sand w i t h some c l a y , c o n t a i n s 3 t o 10% c l a y s i z e by w e i g h t} w i t h p a r t i c l e s l e s s t h a n 0.005 mm i n d i a m e t e r ( T r a n s l . )

.

Similar r e s u l t s were obtained by E . P .

Shusherina and

Y u .

P .

~obkov" in uniaxial compression t e s t s

o n

super

and

suglinok a t temperatures

0

ranging from - 1 0 ' ~ t o -55 C .

I n

the case of supes, an increase in the

moisture content a t incomplete and r i s i n g saturation was accompanied by an

increase in the strength.

The l a t t e r f e l l when saturation was complete. I n

the case of suglinok, the pores were almost completely f i l l e d with i c e and

unfrozen water and a reduction in the moisture content was accompanied by a

reduction in strength. I n the case of clay, however, t h e same experimental

conditions led t o d i f f e r e n t r e s u l t s : when saturation was complete, the

strength of frozen clay increased or remained practically the same as the

moisture content was r i s i n g .

Apart from the aforemen t i oned experiments

3 9 7 9 9 ) ,

we should also

mention the work of

G . M .

Veselov, A . I . Zelenin and A . P . stepanovl

~ ~ 1 ,

who too

observed a peak on the curves i l l u s t r a t i n g the dependence of the strength of

frozen s o i l s on t h e i r moisture content. The l a t t e r authors experimented with

a wide range of temperatures

(-1°c

t o

-40°c), d i f f e r e n t s o i l s (sand, supes,

suglinok, clay) and d i f f e r e n t types of deformation (cutting, f a i l u r e under an

impact

load,

uniaxial

crushing

and rupture, penetration

t e s t i n g ) .

Unfortunately, no allowances were made f o r the f i l l i n g - i n of pores with i c e

and unfrozen water.

I n

the experimental data of N . K . Pekarskaya and

G . A .

Shloido 5,6)

o n

d i f f e r e n t s o i l s (sand, supes, clay) a t - l O c

t o

-loOc, the maximum

o n

the

curves was present only in the case of uniaxial rupture.

In the case of

uniaxial compression, shear and penetration, the strength of frozen s o i l s

increased with t h e i r moisture content.

Water saturation was determined by

these authors only in the uniaxial compression t e s t s on supes, where i t was

p r a c t i c a l l y compl e t e .

The data of

M . L .

~ h e i

kov8) f o r completely saturated c l a y , supes

and s i l t show t h a t a t about

-zOc,

the shear strength i s practically

independent of the moisture content. However, a t - 5 ' ~ ~

the shear strength

increases with the moisture content. This was also noted by R .

on^")

who

performed shear t e s t s

on

frozen sand, supes and clay between - 5 ' ~ and -18'~.

The l a t t e r author did

n o t

provide any information

o n

the water saturation of

the specimens.

Therefore, t h e a v a i l a b l e e x p e r i m e n t a l m a t e r i a l i n d i c a t e s t h a t t h e e f f e c t o f t h e m o i s t u r e c o n t e n t on t h e s t r e n g t h of f r o z e n s o i l s v a r i e s , which i s due t o a number of f a c t o r s , and f i r s t of a l l t h e s a t u r a t i o n w i t h w a t e r , t h e t e m p e r a t u r e , t h e c o m p o s i t i o n o f t h e s o i l s k e l e t o n , and t h e m o i s t u r e c o n t e n t . U n f o r t u n a t e l y , s e v e r a l a u t h o r s d i d n o t pay enough a t t e n t i o n t o t h e s e f a c t o r s , w h i c h makes i t d i f f i c u l t t o a n a l y s e t h e i r d a t a .

E x a m i n a t i o n o f a v a i l a b l e d a t a on t h e dependence of t h e s t r e n g t h o f f r o z e n s o i l s o n t h e i r m o i s t u r e c o n t e n t i n d i c a t e s t h a t i t i s n o t p o s s i b l e t o s u g g e s t any g e n e r a l p a t t e r n s t h i s dependence may f o l l o w f o r t h e main t y p e s o f f r o z e n s o i l s u n d e r c o n d i t i o n s o f b o t h c o m p l e t e and p a r t i a l s a t u r a t i o n w i t h w a t e r .

A t t h e same t i m e , i t i s i m p e r a t i v e t o know t h e s e p a t t e r n s b o t h f o r p r a c t i c a l purposes and f o r t h e s t u d y o f c e r t a i n a s p e c t s of t h e p h y s i c s and mechanics o f f r o z e n s o i l s .

The Department o f Permafrost S t u d i e s , Moscow S t a t e U n i v e r s i t y and t h e VNIIPROMGAS* I n s t i t u t e c a r r i e d o u t e x p e r i m e n t a l s t u d i e s t o d e t e r m i n e t h e r e l a t i o n s h i p between t h e s t r e n g t h o f f r o z e n s o i l s and t h e i r m o i s t u r e c o n t e n t . The e x p e r i m e n t s were conducted on d i f f e r e n t s o i l s w i t h i n a w i d e r a n g e o f t e m p e r a t u r e s and m o i s t u r e c o n t e n t s . E x p e r i m e n t a l d e t a i 1 s. S i n c e i t was q u i t e i n i p o s s i b l e t o d u p l i c a t e even t h e m a i n s i t u a t i o n s w h i c h o c c u r i n n a t u r e , we had t o l i m i t o u r e x p e r i m e n t s t o t h e f o l l o w i n g cases. I n t h e f i r s t s t a g e o f o u r i n v e s t i g a t i o n s we s t u d i e d f r o z e n s o i l s w h i c h were c o m p l e t e l y s a t u r a t e d w i t h w a t e r . O b v i o u s l y , we c o u l d e x p e c t t h a t a t c o n d i t i o n s r e m a i n i n g t h e same ( t h e d e n s i t y o f t h e s o i l s k e l e t o n , t e m p e r a t u r e ) , a decrease i n s a t u r a t i o n w o u l d l e a d t o a r e d u c t i o n i n t h e s t r e n g t h o f f r o z e n s o i l s and t h a t t h e l a t t e r would depend o n t h e m o i s t u r e c o n t e n t .

The e x p e r i m e n t a l s o i l s were supes, s u g l i n o k and c l a y 9 ) , w h i c h were s u b j e c t e d t o u n i a x i a l c o r r ~ p r e s s i o n te s t s u n d e r a s h o r t - t e r m 1 oad. The t e m p e r a t u r e ranged from - 1 0 ' ~ t o - 5 5 ' ~ . The m o i s t u r e c o n t e n t v a r i e d between v a l u e s c o r r e s p o n d i n g t o a minimum p o r o s i t y and t h o s e e x c e e d i n g t h e w a t e r r e t a i n i n g c a p a c i t y o f t h e s o i l . The specimens had a d i s t u r b e d s t r u c t u r e .

*

A l l - U n i o n Research I n s t i t u t e of Gas U t i l i z a t i o n i n t h e N a t i o { , a l Economy and of Underground S t o r a g e of Petroleum, P e t r o l e u m P r o d u c t s and L i q u e f i e d Gas (Trans1

.

)

.

T h e i r s a t u r a t i o n w i t h w a t e r was t a k e n as 1 , t h e t e x t u r e was massive. The specimf?ns had a c y l i n d r i c a l form and were a b o u t 36 mm i n d i a m e t e r and 8 5

-

8 6 mm i n h e i g h t . P r e p a r a t i o r o f t h e specimens and t h e physico-mechanical

9 p r o p e r t i e s o f t h e e x p e r i m e n t a l s o i l s were d e s c r i b e d e a r l i e r

.

I n a d d i t i o n t o f r o z e n s o i l s we a l s o t e s t e d i c e under i d e n t i c a l c o n d i t i o n s , s i n c e i c e i s t h e main component o f f r o z e n s o i l and d e t e r m i n e s t h e s p e c i f i c mechanical p r o p e r t i e s o f t h e 1 a t t e r . Furthermore, i c e may be r e g a r d e d a s t h e u l t i m a t e s t a t e o f f r o z e n s o i l ( t h e m o i s t u r e c o n t e n t equal t o i n f i n i t y ) .

Experiments were c a r r i e d o u t on 1 ayered, f i n e - c r y s t a l 1 in e i c e ( a v e r a g e u n i t w e i g h t 0.90 g/cm3, p o r o s i t y a b o u t 2 % ) . The average volume o f

3

c r y s t a l s was a b o u t 1 mm

,

t h e t o r t u o s i t y f a c t o r was a b o u t 4.2, and t h e 2 3 s p e c i f i c i n t e r f a c e 61 cm /cm

.

The p r e d o m i n a n t o r i e n t a t i o n o f t h e m a i r o p t i c a l c r y s t a l axes was c h a o t i c b u t i n a b o u t 20% of c r y s t a l s t h e m a i n o p t i c a l axes were o r i e n t a t e d a l m o s t p e r p e n d i c u l a r l y t o t h e i c e l a y e r s . To p r e p a r e t h e specimens, 2 mm l a y e r s o f d i s t i l l e d w a t e r were f r o z e n l a y e r b y l a y e r a t a b o u t - 2 0 ' ~ i n c y l i n d r i c a l s t e e l t u b e s 60

_mm

i n d i a m e t e r and 200 mm i n h e i g h t . The l a t e r a l s u r f a c e s o f t h e specimens were m e l t e d down t o g i v e them a c y l i n d r i c a l f o r m . The ends o f t h e i c e specimens were t h o r o u g h l y p o l i s h e d w i t h sand p a p e r . The dimensions o f i c e specimens were i d e n t i c a l t o t h o s e o f f r o z e n s o i l .

The u n i a x i a l compression s t r e n g t h o f f r o z e n s o i l s and i c e was d e t e r m i n e d i n a 30 t o n u n i v e r s a l ZDMK d e v i c e ( f r o m E a s t Germany) a t

-loOc,

-20°c, -40°c, and - 5 5 ' ~ . B o t h l o n g i t u d i n a l and t r a n s v e r s e d e f o r m a t i o n s were measured. The t e s t s were s t o p p e d when t h e l o a d began t o f a 1 1 ( a s i n d i c a t e d b y t h e manometer), w h i c h was u s u a l l y accompanied b y f o r m a t i o n o f v i s i b l e c r a c k s . I n t h e cases where no c r a c k f o r m a t i o n was o b s e r v e d and t h e r e was no decrease i n t h e l o a d , t h e t e s t s were s t o p p e d when l o n g i t u d i n a l d e f o r m a t i o n was o f t h e o r d e r o f 30

-

35%.

The u l t i m a t e s t r e n g t h o f f r o z e n s o i l s and i c e was d e t e r m i n e d from t h e t r u e compression diagrams ( w i t h a l l o w a n c e s f o r t h e a r e a of t h e specimen) as t h e maxinlum r e s i s t a n c e t o u n i a x i a l compression. T h i s method was d e s c r i b e d i n d e t a i l i n a n e a r l i e r s t u d y 9 ) .

R e s u l t s and d i s c u s s i o n . The maximum v a l u e s o f r e s i s t a n c e o f f r o z e n s o i l s t o u n i a x i a l compression a r e g i v e n i n t h e T a b l e and i n F i g s . 1-3.

The h i g h e s t u n i ? x i a l compression s t r e n g t h o f f r o z e n , c o m p l e t e l y s a t u r a t e d supes was observed a t a m o i s t u r e c o n t e n t o f a b o u t 26% t h r o u g h o u t t h e i n v e s t i g a t e d t e m p e r a t u r e range. The s t r e n g t h decreased as t h e m o i s t u r e c o n t e n t i n c r e a s e d . The l o w e r t h e t e m p e r a t u r e , t h e h i g h e r was t h e e f f e c t o f t h e m o i s t u r e c o n t e n t .

A t m o i s t u r e c o n t e n t s below 26% t h e s t r e n g t h o f supes decreased a t a l l t e m p e r a t u r e s . I t s h o u l d be noted, however, t h a t a t t h e s e w a t e r c o n t e n t s i t was i m p o s s i b l e t o a c h i e v e c o m p l e t e s a t u r a t i o n w i t h w a t e r . Hence t h e d a t a f o r m o i s t u r e c o n t e n t s below 26% s h o u l d n o t b e c o n s i d e r e d .

The t e s t s a t -lo°C showed t h a t t h e r e l a t i o n s h i p between t h e compression s t r e n g t h o f f r o z e n supes and t h e m o i s t u r e c o n t e n t i s n o t l i n e a r : a s t h e m o i s t u r e c o n t e n t i n c r e a s e s , t h e s t r e n g t h decreases a t a s l o w e r r a t e . We may assume t h a t t h e same o c c u r s a t t e m p e r a t u r e s below - l o ° C .

Comparison of t h e s t r e n g t h o f c o m p l e t e l y s a t u r a t e d f r o z e n supes w i t h t h a t o f i c e shows t h a t w i t h i n t h e i n i t i a l r a n g e o f m o i s t u r e c o n t e n t s f r o z e n s o i l i s s t r o n g e r t h a n i c e a t a l l t e m p e r a t u r e s . The l a r g e s t d i f f e r e n c e i n t h e s t r e n g t h o f f r o z e n supes and i c e o c c u r s a t n i o i s t u r e c o n t e n t s o f a b o u t 26%; t h i s d i f f e r e n c e decreases, a s t h e m o i s t u r e c o n t e n t i n c r e a s e s . I t i s a1 so dependent o n t h e t e m p e r a t u r e : as t h e t e m p e r a t u r e i ncreases, t h e d i f f e r e n c e decreases. The d a t a o b t a i n e d a t - 1 0 ' ~ i n d i c a t e t h a t t h e compression s t r e n g t h s o f supes and i c e become i d e n t i c a l a t c e r t a i n n i o i s t u r e c o n t e n t s . On f u r t h e r i n c r e a s e i n t h e m o i s t u r e c o n t e n t , t h e compression s t r e n g t h o f i c e surpasses t h a t o f supes, and t h e d i f f e r e n c e between t h e s t r e n g t h s o f i c e and supes i n c r e a s e s i n accordance w i t h t h e r e l a t i o n s h i p between t h e s t r e n g t h o f supes and i t s m o i s t u r e c o n t e n t ( w i t h d i m i n i s h i n g i n t e n s i t y ) . The same w i l l e v i d e n t l y a p p l y a t t e m p e r a t u r e s be1 ow -1 o°C.

The u n i a x i a l compression s t r e n g t h o f f r o z e n , c o n ~ p l e t e l y s a t u r a t e d s u g l i n o k depends on t h e m o i s t u r e c o n t e n t i n t h e same way a s i n t h e case o f supes: t h e s t r e n g t h of s u g l i n o k decreases a s i t s m o i s t u r e c o n t e n t i n c r e a s e s . I n a g e n e r a l case, t h i s dependence i s n o n l i near, i . e . t h e s t r e n g t h decreases a t a s l o w e r r a t e as t h e m o i s t u r e c o n t e n t i n c r e a s e s .

'The aforementioned r e 1 a t i o n s h i p s o c c u r a t a1 1 temperatures, and a s t h e temperature decreases, t h e e f f e c t o f t h e m o i s t u r e c o n t e n t on t h e s t r e n g t h i n c r e a s e s ( a s i n t h e case o f supes). I n t h e i n i t i a l m o i s t u r e c o n t e n t rangE, t h e compression s t r e n g t . h o f s u g l i n o k a t - 5 5 ' ~ and - 4 0 ' ~ i s g r e a t e r t h a n t h a t o f i c e , and as t h e m o i s t u r e c o n t e n t i n c r e a s e s , t h e d i f f e r e n c e i n t h e s t r e n g t h s o f s u g i i n o k and i c e decreases. For each t e m p e r a t u r e t h e r e i s a d e f i n i t e m o i s t u r e c o n t e r l t f o r w h i c h t h e cgmpression s t r e n g t h s of f r o z e n s u g l i n o k and i c e become i d e n t i c a l , b u t o n f u r t h e r i n c r e a s e i n t h e m o i s t u r e c o n t e n t t h e c o n ~ p r e s s i o n s t r e n g t h o f s u g l i n o k f a l l s below t h a t o f i c e . As t h e m o i s t u r e c o n t e n t i n c r e a s e s , t h e d i f f e r e n c e between t h e compression s t r e n y t h s of i c e and sugl i n o k i n c r e a s e s a t a s l o w e r r a t e .

A t - 2 0 ' ~ and

-loOc,

t h e s t r e n g t h o f i c e i s h i g h e r t h a n t h a t o f

s u g l i nok t h r o u g h o u t t h e i n v e s t i g a t e d range o f moi s t u r e c o n t e n t s . As a t - 4 1 ~ ~ ~ and -55Oc, t h e d i f f e r e n c e between t h e s t r e n g t h s o f i c e and s u g l i n o k i n c r e a s e s

a t a s l o w e r r a t e as t h e m o i s t u r e c o n t e n t i n c r e a s e s .

For f r o z e n c l a y , t h e dependence o f t h e compression s t r e n g t h on t h e m o i s t u r e c o n t e n t i s a t f i r s t t h e same as f o r supes and s u g l i n o k . A t h i g h e r m o i s t u r e c o n t e n t s , t h e r e may be 170 d i s c e r n i b l e r e l a t i o n s h i p between t h e

s t r e n g t h o f f r o z e n c l a y and i t s m o i s t u r e c o n t e n t (e.g., a t -IO~C), b u t a t s t i l l h i g h e r m o i s t u r e c o n t e n t s t h e s t r e n g t h o f c l a y i n c r e a s e s . T h i s p a t t e r n has been observed a t a l l i n v e s t i g a t e d t e m p e r a t u r e s . However, t h e s t r e n g t h o f c l a y becomes more dependent on t h e m o i s t u r e c o n t e n t as t h e temperature d r o p s .

I n a l l i n v e s t i g a t e d cases, t h e s t r e n g t h o f f r o z e n c l a y i s l o w e r t h a n t h a t o f i c e . A t 1 ow m o i s t u r e c o n t e n t s t h e d i f f e r e r ~ c e between t h e s t r e n g t h s o f i c e and c l a y i n c r e a s e s a t a s t e a d i l y d e c r e a s i n g r a t e , as t h e m o i s t u r e c o n t e n t i ncreases, and a t c e r t a i n m o i s t u r e c o n t e n t s becomes p r a c t i c a l l y c o n s t a n t . T h i s d i f f e r e n c e decreases, as t h e m o i s t u r e c o n t e n t i n c r e a s e s s t i l l f u r t h e r , and t h e s t r e n g t h o f c l a y approaches t h a t o f i c e .

B e f o r e e x p l a i n i n g t h e aforementioned r e l a t i o n s h i p s , 1 e t us examine some g e n e r a l concepts o f t h e s t r e n g t h o f f r o z e n s o i l .

As i s known, f r o z e n s o i l i s m e c h a n i c a l l y n o n u n i f o r m due t o d i f f e r e n t mechanical p r o p e r t i e s o f i t s components ( s o i l g r a i n s , i c e , u n f r o z e n w a t e r , gases) and t o d i f f e r e n t s t r e n g t h s o f t l - ~ e bonds between i t s s t r u c t u r a l elements ( s o i l g r a i n s , s o i l aggregates, s o i l p a r t i c l e s , i c e i n c l u s i o n s , e t c

.)

.

The s t r e n g t h o f s o i l g r a i n s i s c o n s i d e r a b l y h i g h e r t h a n t h e s t r e n g t h o f o t h e r s o i l components and t h a t o f t h e s t r u c t u r a l bonds.

The s t r e n g t h o f f r o z e n s o i l depends on t h e complex i n t e r a c t i o n o f a l l i t s components and s t r u c t u r a l bonds. The r o l e o f each component depends o n i t s dimensions, i t s mechanical p r o p e r t i e s , and i t s arrangement w i t h r e s p e c t t o o t h e r components.

The dimensions o f t h e components c a n b e d e t e r m i n e d q u a n t i t a t i v e l y from t h e r a t i o o f s p e c i f i c w e i g h t t o u n i t w e i g h t , t h e t o t a l m o i s t u r e c o n t e n t , and t h e m o i s t u r e c o n t e n t due t o u n f r o z e n w a t e r . Some q u a l i t a t i v e d a t a a r e a v a i l a b l e o n t h e mechanical p r o p e r t i e s o f f r o z e n s o i 1 ~ 3 ~ ) .

A t p r e s e n t t h e o n l y a v a i l a b l e d a t a o n t h e m u t u a l arrangement o f components and o n t h e s t r u c t u r a l bonds o f f r o z e n s o i l a r e i n t h e f o r m o f qua1 i t a t i v e d e s c r i p t i o n s . There a r e p r a c t i c a l l y no q u a n t i t a t i v e c h a r a c t e r i s t i c s . S i n c e t h e s t r u c t u r a l bonds a r e g r e a t l y a f f e c t e d by t h e c o m p o s i t i o n o f f r o z e n s o i l , t h e q u a n t i t a t i v e c h a r c t e r i s t i c s of t h e l a t t e r may b e used t o examine t h e s t r u c t u r a l bonds.

The u n i a x i a l compression of f r o z e n s o i l r e s u l t s i n l o c a l shear phenomena, i . e . i n f o r m a t i o n o f f i s s u r e s w h i c h d e v e l o p p r e d o m i n a n t l y i n t h e d i r e c t i o n o f t h e l a r g e s t s h e a r i n g s t r e s s e s . The l a t t e r may be s t r o n g e r t h a n t h e f o r c e s o f i n t e r a c t i o n between t h e components and t h e r e s i s t a n c e o f i c e and u n f r o z e n w a t e r . I f t h e d e n s i t y o f t h e s o i l s k e l e t o n i s s u f f i c i e n t l y h i g h , t h e s o i l p a r t i c l e s a r e n o t o n l y i n c o n t a c t w i t h each o t h e r b u t may adhere t o each o t h e r as w e l l . I n o r d e r t o move d u r i n g shear, t h e p a r t i c l e s must disengage and t h i s may l e a d t o d e s t r u c t i o n o f i c e , r u p t u r e o f f i l m s of u n f r o z e n w a t e r , and f a i l u r e o f s t r u c t u r a l bonds.

To dete,rmine t h e s t r e n g t h o f f r o z e n s o i l i t i s e s s e n t i a l t o know t h e magnitude o f each component ( s o i l s k e l e t o n , i c e , u n f r o z e n w a t e r , a i r ) and t h e i r r a t i o s . I t s h o u l d be borne i n mind t h a t a t a l l o t h e r c o n d i t i o n s b e i n g equal

,

t h e amount o f i c e and u n f r o z e n w a t e r ( a n d hence t h e i r r a t i o ) , as we1 1

as t h e i r s t r e n g t h , v a r y depending on t h e temperature. Because o f t h i s , t h e s t r e q g t h o f bonds between t h e components o f f r o z e n s o i l i s a l s o dependent on t h e temperature.

The components o f i n v e s t i g a t e d f r o z e n s o i l s a r e shown i n F i g s . 1-3 ( s o i 1 s k e l e t o n

,

i c e , unfrozen water, and a i r )

.

To understand t h e dependence o f t h e u n i a x i a l compression s t r e n g t h on t h e m o i s t u r e c o n t e n t , we s h o u l d b e a r i n mind t h a t t h e mechanism o f d e f o r m a t i o n and f a i l u r e changes w i t h t h e m o i s t u r e c o n t e n t .

A t low m o i s t u r e c o n t e n t s , t h e s k e l e t o n o f frozen supes forms a dense framework whose s t r e n g t h i s g r e a t l y dependent on t h e adhesion o f s o i l p a r t i c l e s . F i l m s o f unfrozen water* occupy a small volume and a r e p r o b a 5 l y i n t e r r u p t e d by s o i l p a r t i c l e s and i c e . I c e f i l l s t h e pores i n t h e framework i n t h e for111 o f s c a t t e r e d i n c l u s i o n s . L o c a l shear, which o c c u r s d u r i n g u n i a x i a l compression i n t h e weakest s p o t s (i .e. voids**, f i l m s o f unfrozen w a t e r , and i c e ) , c a n n o t develop as 1 ong as t h e i n i t i a l d e n s i t y o f t h e s o i 1 s k e l e t o n remains unchanged.

As t h e m o i s t u r e c o n t e n t i n c r e a s e s , t h e d e n s i t y o f t h e s o i l s k e l e t o n and t h e s o i l volume decrease ( F i g . 1

) .

The adhesion between s o i l p a r t i c l e s weakens and t h e v o i d s i n t h e framework i n c r e a s e i n s i z e . The volume o f i c e i n c l u s i o n s i n c r e a s e s a c c o r d i n g l y ( F i g . 1 ) . I n t h e i n i t i a l p e r i o d , l o c a l shear develops a l o n g f i l m s o f unfrozen w a t e r and on i c e . The o v e r a l l r e s u l t o f t h i s i s a decrease i n t h e s t r e n g t h o f frozen supes ( F i g .

1 ) .

As l o n g as t h e t o t a l f o r c e r e q u i r e d t o overcome t h e r e s i s t a n c e o f t h e s o i l s k e l e t o n and f i l m s o f unfrozen w a t e r d u r i n g shear i s g r e a t e r t h a n t h e f o r c e r e q u i r e d t o develop shear i n t h e i c e i n c l u s i o n s , t h e o v e r a l l s t r e n g t h o f f r o z e n s o i l w i l l exceed t h e s t r e n g t h o f i c e . As t h e m o i s t u r e c o n t e n t i n c r e a s e s , t h e s t r e n g t h o f f r o z e n s o i l approaches t h a t of i c e , and when t h e s t r e n g t h e n i n g e f f e c t o f t h e s o i 1 s k e l e t o n i s c o m p l e t e l y balanced o f f

by t h e weakening e f f e c t o f t h e water f i l m s and gas, t h e s t r e n g t h of f r o z e n s o i l becomes equal t o t h a t o f i c e . A t s t i l l h i g h e r m o i s t u r e c o n t e n t s , t h e weakening e f f e c t o f u n f r o z e n w a t e r exceeds t h e s t r e n g t h e n i n g e f f e c t of the s o i l s k e l e t o n . As a r e s u l t o f t h i s , f r o z e n s o i l becomes weaker t h a n i c e . As

*

_{Unfrozen w a t e r c o n t a i n e d i n frozen supes cannot be d e t e r ~ i . ~ e u by t h e}

c a l o r i m e t r i c method b u t i t i s present.

**

_{I n most cases, t h e v o i d s occupied 1 t o}

₅₂

_{o f t h e volume of frozen}

_supes

( F i g . 1 ) .

t h e r o l e o f t h e s o i l s k e l e t o n d i m i n i s h e s , t h e d i f f e r e n c e between t h e s t r e n g t h o f i c a and t h a t o f s o i l i n c r e a s e s . I t was mentioned e a r l i e r t h a t as t h e m o i s t u r e c o n t e n t o f f r o z e n supes i n c r e a s e s , i t s s t r e n g t h decreases a t a d i m i n i s h i n g r a t e . T h i s r e f 1 e c t s t h e processes which t a k e p l a c e d u r i n g f a i l u r e o f t h i s s o i l a t d i f f e r e n t m o i s t u r e c o n t e n t s . The r a p i d decrease i n t h e s t r e n g t h i n t h e i n i t i a l r a n g e o f m o i s t u r e c o n t e n t s i s e v i d e n t l y r e l a t e d t o a r a p i d decrease i n t h e s t r e n g t h e n i n g e f f e c t of t h e s o i l s k e l e t o n . The l e v e l l i n g o f f o f t h e a-w c u r v e a t h i g h e r m o i s t u r e c o n t e n t s can be e x p l a i n e d b y t h e d i m i n i s h i n g r o l e o f t h e s o i l s k e l e t o n and a n i n c r e a s e i n t h e volume o f i c e .

Temperature a f f e c t s t h e dependence o f t h e u n i a x i a l compression s t r e n g t h o f f r o z e n supes on t h e m o i s t u r e c o n t e n t because i t causes changes i n t h e amount and s t r e n g t h o f unfrozen w a t e r and i c e , w h i c h i n t u r n a f f e c t s t h e r e s i s t a n c e o f s o i l p a r t i c l e s t o d i s p l a c e m e n t .

I n t h e case of f r o z e n s u g l i n o k , a s w e l l a s f r o z e n c l a y ( b u t o n l y i n t h e i n i t i a l r a u g e o f m o i s t u r e c o n t e n t s ) , t h e r e l a t i o n o f t h e u n i a x i a l compression s t r e n g t h t o t h e m o i s t u r e c o n t e n t a t f u l l s a t u r a t i o n i s g e n e r a l l y t h e same as f o r supes. However, t h e ~ e a k e n i n g e f f e c t o f unfrozen w a t e r i n t h e s e s o i l s i n c r e a s e s ( t h e q u a n t i t y o f t h i s w a t e r i n c r e a s e s ) . T h i s i s e s p e c i a l l y t r u e i n t h e case o f c l a y .

The e f f e c t o f t h e g r a i n s i z e c o m p o s i t i o n on t h e a-w r e l a t i o n s h i p i s shown i n F i g . 4. It f o l l o w s from F i g . 4 t h a t t h i s e f f e c t m a n i f e s t s i t s e l f s t r o n g e s t i n t h e case o f supes and l e a s t i n t h e case o f c l a y ( i n t h e i n i t i a l r a n g e o f m o i s t u r e c o n t e n t s ) . The e f f e c t o f t h e g r a i n s i z e c o m p o s i t i o n decreases as t h e m o i s t u r e c o n t e n t i n c r e a s e s , which i s due t o t h e r i s i n g i m p o r t a n c e o f i c e .

F i g . 4 shows f u r t h e r t h a t t h e e f f e c t o f t h e g r a i n s i z e c o m p o s i t i o n i s s t r o n g e s t a t - 5 5 ' ~ and decreases a s t h e t e m p e r a t u r e i n c r e a s e s .

The e f f e c t o f t h e g r a i n s i z e c o m p o s i t i o n manifests i t s e l f a l s o i n t h e f a c t t h a t a t - 2 0 ' ~ and

-loOc

i n t h e case o f sugl-inok, and a t a l l i n v e s t i g a t e d temperatures

i

n t h e case o f c l a y , t h e u n i a x i a l compression s t r e n g t h i s never h i g h e r t h a n t h a t o f i c e , a s i n d i c a t e d by t h e a-w c u r v e s (see F i g s . 2, 3 and 4 ) . The e x p l a n a t i o n i s t h a t a t t h e s e t e m p e r a t u r e s t h e

s t r e n g t h e n i n g e f f e c t of t h e s k e l e t o n o f s u g l i n o k and c l a y i s l e s s t h a n t h e weakeninp e f f e c t o f unfrozen water. The s t r e n g t h o f s u g l i n o k exceeds t h a t o f

i c e a t - 4 0 ' ~ and - 5 5 ' ~ ( F i g . 2), which p o i n t s t o a r e d u c t i o n i n t h e weakening e f f e c t o f unfrozen water. E v i d e n t l y t h i s may a l s o a p p l y t o c l a y below -55'~. As was p o i n t e d o u t e a r l i e r , t h e a-w curves f o r supes c o n t a i n segments

i n d i c a t i n g a h i g h e r s t r e n g t h t h a n t h a t o f i c e a t a l l i n v e s t i g a t e d temperatures ( f r o m - 5 5 ' ~ t o - 1 0 ' ~ ) .

Reduction i n t h e s t r e n g t h o f f r o z e n s o i l w i t h i n c r e a s i n g m o i s t u r e c o n t e n t cannot go on i n d e f i n i t e l y . As t h e i c e i n c l u s i o n s i n c r e a s e .in number and s i z e , t h e y j o i n u p forming an i c e framework whose c e l l s a r e t h e n f i l l e d w i t h frozen s o i l c o n t a i n i n g ice-cement and unfrozen water. The volume o f unfrozen water decreases* and t h e r o l e o f t h i s water diminishes ( F i g . 3 ) . The l a r g e s t number o f displacements occurs i n an i c e framework t h e volume o f which i s i n c r e a s i n g ( F i g . 3 ) . The s t r e n g t h o f f r o z e n s o i l begins t o i n c r e a s e and approach t h a t o f i c e .

Therefore, t h e a n a l y s i s o f t h e experimental d a t a shows t h a t under c o n d i t i o n s of complete s a t u r a t i o n , t h e dependence o f t h e u n i a x i a l compression s t r e n g t h o f f r o z e n s o i l s on t h e i r water c o n t e n t i s g e n e r a l l y t h e same, i r r e s p e c t i v e o f t h e g r a i n s i z e composition. The general form o f t h i s dependence i s shown i n F i g . 5. The c u r v e i n F i g . 5 can be d i v i d e d i n t o t h e f o l l o w i n g segments: AB

-

a decreases a s w increases;

u

> uice. A t B, o = uice. BC

-

0 decreases as w increases; a 4 uice. CD

-

a i s p r a c t i c a l l y

independent o f

w .

DE

-

0 increases as w increases; a t E, = i c e ' W m i n i s

t h e m o i s t u r e c o n t e n t which corresponds ( a t f u l l s a t u r a t i o n ) t o t h e minimum p o r o s i t y o f f r o z e n s o i l ( t h e maximum d e n s i t y o f t h e s k e l e t o n ) . The o r d i n a t e o f p o i n t A corresponds t o t h e u n i a x i a l compression s t r e n g t h a t wmin.

The 0 - w r e l a t i o n s h i p s a l s o depend on t h e temperature and t h e

composition of t h e s k e l e t o n of f r o z e n s o i l .

*

Merging o f i c e i n c l u s i o n s w i l l reduce t h e s u r f a c e o f i c e . Because o f t h i s , t h e amount o f unfrozen water i n

w t . %

per d r y sample should decrease and hence V should decrease t o a g r e a t e r e x t e n t than shown i n Table 3, which was combiled on t h e assumption t h a t t h e amount o f unfrozen water

1

oras

independent o f t h e t o t a l moisture c o n t e n t . There a r e some data i n d i c a t - i n g t h a t t h e amount o f unfrozen water increases as t h e t o t a l m o i s t u r e con- t e n t increases. However, some authors b e l i e v e t h a t t h i s i s not so.

We should n o t e t h a t F i g . 5 a p p l i e s t o s o i l s completely s a t u r a t e d w i t h w a t e r . If s a t u r a t i o n i s n o t complete b u t i s c o n s t a n t f o r a l l m o i s t u r e c o n t e n t s , t h e a-w c u r v e w i l l f l a t t e n o u t and t h e A B segment w i l l tend t o g e t s h o r t e r and may disappear a1 t o g e t h e r . If t h e degree o f s a t u r a t i o n v a r i e s w i t h t h e m o i s t u r e c o n t e n t , t h e n a t u r e o f t h e a-w r e l a t i o n s h i p may be e n t i r e l y d i f f e r e n t . For example, if t h e degree o f s a t u r a t i o n increases w i t h t h e m o i s t u r e c o n t e n t , o n l y t h e s t r e n g t h o f f r o z e n s o i l w i l l i n c r e a s e . F i n a l l y ,

F i g . 5 r e f e r s t o c o n d i t i o n s o f short-term l o a d i n g . Under a long-term load, t h e 0 - w r e l a t i o n s h i p may be d i f f e r e n t .

References

1 .

Veselov, G . M. 0 kharaktere i zmeneniya prochnostnykh s v o i s t v merzlykh gruntov v z a v i s i m o s t i o t

i

kh v l a z h n o s t i . Sb. " F i z i k o - mekhanicheskie s v o i s t v a , d a v l e n i e

i

r a z r u s h e n i e gornykh porod"

(Changes i n t h e s t r e n g t h o f f r o z e n s o i l s i n r e l a t i o n t o t h e i r m o i s t u r e c o n t e n t . I n : "Physico-mechanical p r o p e r t i e s , pressure and f a i l u r e o f s o i l s 1 ' ) , No. 2, Moscow, Izd-vo AN SSSR, 1963. V o i t k o v s k i i

,

K.F. Mekhanicheskie s v o i s t v a 1 Ida (Mechanical p r o p e r t i e s of i c e ) , Moscow, I z d - v o AN SSSR, 1960.

Gumenskaya, 0 .M. V l i y a n i e v l a z h n o s t i i temperatury na s o p r o t i v l e n i e merzlykh gruntov. "Laboratornye i s s l edovaniya mekhanicheski kh

s v o i s t v merzlykh g r u n t o v " ( E f f e c t o f m o i s t u r e and temperature on t h e s t r e n g t h o f f r o z e n s o i l s . "Laboratory s t u d i e s o f mechanical p r o p e r t i e s o f f r o z e n s o i l s " ) , No. 1

,

Moscow-Leningrad, 1936.

Z e l e n i n , A.N., Veselov,

G.M.

and Stepanov, A.P. Rezul ' t a t y

l a b o r a t o r n y k h i s s l e d o v a n i i prochnostnykh s v o i s t v merzlykh gruntov. Sb. "Voprosy gornogo d e l a " ( R e s u l t s o f 1 a b o r a t o r y i n v e s t i g a t i o n s o f t h e s t r e n g t h o f frozen s o i l s . I n "Mining p r o b l e ~ i l s " ) ,

Moscow, U g l e t e k h i zdat, 1958.

Pekarskaya, N.K. Prochnost merzlykh gruntov p r i s d v i g e i ee z a v i s i m o s t o t t e k s t u r y (The shear s t r e n g t h o f f r o z e n s o i l s and i t s dependence o n t h e t e x t u r e ) , Moscow, Izd-vo AN SSSR, 1963. Pekarskaya, N.K., Vyalov, S.S., R o v i n s k i i , M . I . , and Shloido, G . A .

I s s l e d o v a n i y a parametrov p r o c h n o s t i merzlykh gruntov, opredelyayushchikh p r o t s e s s i k h razrusheniya zemleroinymi mashinami. " T r . I V soveshchaniya- seminara po obmenu opytom s t r o i t e l ' s t v a v surovykh k l im a t i c h e s k i kh

u s l o v i y a k h " ( I n v e s t i g a t i o n o f t h e s t r e n g t h parameters which determine t h e w o r k a b i l i t y o f f r o z e n s o i l s u s i n g e x c a v a t i n g equipment. "Proc. I V seminar on t h e exchange o f c o n s t r u c t i o n experience under severe c l i m a t i c c o n d i t i o n s " )

,

Vorkuta, 1966.

T s y t o v i c h , N .A. and Sumgin, M. I

.'

Osnovaniya mekhani k i merzlykh gruntov (Fundamentals o f mechanics o f f r o z e n s o i l ) , Moscow, I z d - v o AN SSSR, 1937. Sheikov, M. L. S o p r o t i v l e n i e merzlykh gruntov sdvigu. "Laboratornye i ssledovaniya mekhanicheski kh s v o i s t v rnerzlykh gruntov" (Shear s t r e n g t h o f frozen s o i l . I n "Laboratory i n v e s t i g a t i o n s of mechanical p r o p e r t i e s of frozen s o i 1 'I), No. 2, Moscow-Leni ngrad

,

1936.

Shusherina, E.P. and Bobkovy Yu. P. I s s l e d o - a n i e mekhanijiheskikh s v o i s t v merzlykh gruntov p r i n i z k i k h temperaturakh ( o t -10 do -55 C ) ( I n v e s t i g a t i o n o f mechanical p r o p e r t i e s o f f r o z e n s o i l s a t l o w temperature ( f r o m -10 t o -55%), "Merzlotnye i ssledovaniya"

,

i s s u e V I 11, Izd-vo MGU, 1968.

Jong, R.N. S o i l f r e e z i n g c o n s i d e r a t i o n i n f r o z e n s o i l s t r e n g t h . Proc. I n t . Conf. on Permafrost (November 1963). Washington, 1965.

F i g u r e 1

l a

-

o o f f r o z e n supes v s . w a t d i f f e r e n t t e m p e r a t u r e (1 t o 4): - 5 5 O ~ , -40°c, -20°c,

-loOc

r e s p e c t i v e l y .

l a , 2a, 3a, 4a

-

oice a t t h e same temperatures. I b

-

R e l a t i v e volumes o f components o f f r o z e n supes

v s . w a t t h e same temperatures a s above (1 t o 4 ) . v = c k

_-

' s k e l e t o n "A

-

-

-

' i c e

v~

'unfrozen w a t e r vr = v gas

Figure 2

2a

-

Same

as

Fig.

1

a

for frozen sugl inok

2 b

-

Same

as

Fig.

1

b

for frozen suglinok

2c

-

Changes in

v

in frozen suglinok

gas

F i g u r e

3

3 a

-

_Same

a s

_{F i g .}

_{l a}

_{f o r f r o z e n}

_{c l a y} 3 b

-

_Same

a s

_{f i g .}

1 b

_{f o r f r o z e n}

_{c l a y}

F i g u r e 4

G vs. w f o r d i f f e r e n t g r a i n s i z e compositions o f f r o z e n s o i l s

a t - 5 5 ' ~

(a),

- 4 0 ' ~ (b)

,

-20°c ( c )

,

-1 OOC

( d )

1

-

supes, 2

-

suglinok, 3

-

c l a y

F i g u r e 5