Effect of dynamic loads on piles in frozen soils

Publisher’s version / Version de l'éditeur:

Vous avez des questions? Nous pouvons vous aider. Pour communiquer directement avec un auteur, consultez la première page de la revue dans laquelle son article a été publié afin de trouver ses coordonnées. Si vous n’arrivez pas à les repérer, communiquez avec nous à PublicationsArchive-ArchivesPublications@nrc-cnrc.gc.ca.

Questions? Contact the NRC Publications Archive team at

PublicationsArchive-ArchivesPublications@nrc-cnrc.gc.ca. If you wish to email the authors directly, please see the first page of the publication for their contact information.

https://publications-cnrc.canada.ca/fra/droits

L’accès à ce site Web et l’utilisation de son contenu sont assujettis aux conditions présentées dans le site LISEZ CES CONDITIONS ATTENTIVEMENT AVANT D’UTILISER CE SITE WEB.

Proceedings of the 3rd International Specialty Conference on Cold Regions

Engineering. Northern Resource Development, 1, pp. 41-52, 1984-04-04

READ THESE TERMS AND CONDITIONS CAREFULLY BEFORE USING THIS WEBSITE. https://nrc-publications.canada.ca/eng/copyright

NRC Publications Archive Record / Notice des Archives des publications du CNRC : https://nrc-publications.canada.ca/eng/view/object/?id=2c573ed1-e168-42f4-a49f-969f86bca2b7 https://publications-cnrc.canada.ca/fra/voir/objet/?id=2c573ed1-e168-42f4-a49f-969f86bca2b7

NRC Publications Archive

Archives des publications du CNRC

This publication could be one of several versions: author’s original, accepted manuscript or the publisher’s version. / La version de cette publication peut être l’une des suivantes : la version prépublication de l’auteur, la version acceptée du manuscrit ou la version de l’éditeur.

Access and use of this website and the material on it are subject to the Terms and Conditions set forth at

Effect of dynamic loads on piles in frozen soils

~ 2 1 d

National Research

Conseil national

no. 1190

1

1

Council Canada

de rechercha Canada

c. 2 1

B m

EFFECT OF DYNAMIC LOADS ON PILES IN FROZEN SOILS

by V.R. Parameswaran

ANALYZED

Appeared in

Proceedings Third International Specialty Conference

Cold Regions Engineering, "Northern Resource Development" Edmonton, Alberta, April 4,5 and 6, 1984

Volume I, p. 41

-

52

Reprinted with permission

DBR Paper No. 1190

Division of Building Research

On a constat'e qu'une charge a l t e r n k a u s s i f a i b l e que 5% d e l a c o n t r a i n t e s t a t i q u e acc'elsre l e taux de d6placement d e s pie- enfonc'es dans l e s s o l s g e l & e t supportant d e s charges de longue dur'ee, a i n s i que l e taux de f l u a g e d e s d c h a n t i l l o n s d e s o l g e l 6 soumis au chargelaent u n i a x i a l e n compression. Cet e f f e t r a u i t l a capacit'e p o r t a n t e d e s pieux enfonc'es dans l e s s o l s gel'es e t d o i t B t r e p t i s en c o n s i d k - a t i o n pour l e s c a l c u l s de fondations dans l e s regions de p e r g d l i s o l .

EFFECT OF DYNAMIC LOADS ON

PILES I N FROZEN SOILS

V.R. Parameswaran, D i v i s i o n of Building Research,

N a t i o n a l Research Council of Canada, Ottawa, O n t a r i o , K1A OR6.

ABSTRACT

An a l t e r n a t i n g s t r e s s a s s m a l l a s 5% of t h e s t a t i c s t r e s s was found t o a c c e l e r a t e t h e displacement r a t e s of p i l e s s u b j e c t e d t o long-term l o a d s i n f r o z e n s o i l s , a s w e l l a s t h e c r e e p r a t e of f r o z e n s o i l samples s u b j e c t e d t o u n i a x i a l l o a d i n g under compression. This e f f e c t

e s s e n t i a l l y reduces t h e b e a r i n g c a p a c i t y of p i l e s i n f r o z e n s o i l s , and has t o be taken i n t o c o n s i d e r a t i o n i n t h e d e s i g n of foundations i n permafrost a r e a s .

INTRODUCTION

P i l e foundations s u p p o r t i n g b u i l d i n g s i n permafrost a r e a s a r e s u b j e c t t o s t a t i c l o a d s due t o t h e superimposed s t r u c t u r e , and t o

dynamic l o a d s from o p e r a t i n g machinery w i t h i n t h e b u i l d i n g s . Shocks and v i b r a t i o n s from e x t r a n e o u s s o u r c e s a r e a l s o t r a n s m i t t e d t o t h e

foundations through t h e ground. Conventional d e s i g n s of f o u n d a t i o n s i n permafrost a r e a s u s u a l l y t a k e i n t o c o n s i d e r a t i o n only s t a t i c l o a d s and an allowable s e t t l e m e n t of t h e foundation during t h e a n t i c i p a t e d s e r v i c e l i f e of t h e s t r u c t u r e . The b e a r i n g c a p a c i t y of a p i l e i s e s t i m a t e d from t h e a d f r e e z e bond s t r e n g t h between t h e p i l e and v a r i o u s s o i l s (which i s determined e x p e r i m e n t a l l y from short-term and a few long-term p i l e l o a d t e s t s ) and from t h e behaviour of e x i s t i n g s t r u c t u r e s i n permafrost a r e a s . The end-bearing c a p a c i t y f o r a p i l e i n f r o z e n s o i l s i s a

f u n c t i o n of t h e compressive s t r e n g t h of t h e s o i l m a t e r i a l . For i c e - r i c h s o i l s t h i s i s very s m a l l and u s u a l l y neglected. The u n c e r t a i n t i e s

caused by v a r i a b i l i t i e s i n t h e type of s o i l and t h e m o i s t u r e c o n t e n t and i t s d i s t r i b u t i o n i n t h e s o i l , and t h o s e due t o t h e s u r f a c e

c h a r a c t e r i s t i c s of t h e p i l e a r e normally compensated f o r by a f a c t o r of s a f e t y i n c o r p o r a t e d i n t o t h e d e s i g n c r i t e r i a .

A t p r e s e n t , t h e e f f e c t s of dynamic s t r e s s e s a r e n o t t a k e n d i r e c t l y i n t o c o n s i d e r a t i o n i n design. The l a r g e s a f e t y f a c t o r normally used i s probably s u f f i c i e n t t o compensate f o r t h e s e e f f e c t s . Although i t i s known t h a t t h e mechanical p r o p e r t i e s of f r o z e n s o i l s and i c e a r e a f f e c t e d by dynamic loading on t h e s e m a t e r i a l s (Kaplar (1969

1,

Stevens (1975), Vinson e t . a l . (1978) ( a ) and ( b ) , Czajkowski and Vinson (1980)) no f i e l d s t u d i e s have a s y e t been r e p o r t e d on t h e e f f e c t of a l t e r n a t i n g l o a d s on t h e r a t e of s e t t l e m e n t of p i l e s i n f r o z e n s o i l s . Laboratory s t u d i e s were undertaken t o o b t a i n information on t h e behaviour of p i l e s i n f r o z e n s o i l s under v a r i o u s c o n d i t i o n s of l o a d ( s t a t i c and dynamic) and temperature. P r e l i m i n a r y r e s u l t s of t h e s e i n v e s t i g a t i o n s showed

t h a t an a l t e r n a t i n g s t r e s s a s small as 3 t o 5% of t h e s t a t i c s t r e s s caused a doubling of t h e displacement r a t e of wood and c o n c r e t e p i l e s i n f r o z e n s o i l s a t -2.2OC (Parameswaran (1982)). This i n d i c a t e d t h a t

dynamic l o a d s reduce t h e l i f e expectancy of a s t r u c t u r e t o h a l f t h a t which i s p r e d i c t e d from s t a t i c s t r e s s alone.

The p r e s e n t paper r e p o r t s t h e r e s u l t s of more r e c e n t s t u d i e s on t h e e f f e c t of dynamic s t r e s s e s on t h e r a t e of s e t t l e m e n t of p i l e s embedded i n f r d z e n s o i l s w i t h f r e e ends, without any end-bearing. S i n c e t h e t o t a l b e a r i n g c a p a c i t y of a p i l e i s a combination of t h e a d f r e e z i n g s t r e n g t h a t t h e p i l e - s o i l i n t e r f a c e and t h e end-bearing c a p a c i t y

e s t i m a t e d from t h e compressive s t r e n g t h of t h e s o i l under t h e p i l e , i t

w a s a l s o decided t o study t h e e f f e c t s of dynamic s t r e s s e s on t h e c r e e p of c y l i n d r i c a l samples of f r o z e n s o i l s under u n i a x i a l compression.

EXPERLMENTAZ, PROCEDURE

The experimental s e t u p t o measure t h e displacement r a t e s of p i l e s i n f r o z e n s o i l s under s t a t i c l o a d s and superimposed a l t e r n a t i n g l o a d s was described e a r l i e r (Parameswaran (1982)). The same c r e e p frame was a l s o used t o study t h e c r e e p of c y l i n d r i c a l f r o z e n s o i l samples, a s shown i n Figure 1.

Samples of f r o z e n s o i l , 75 mm i n diameter and about 175 mm i n l e n g t h , were prepared from a n a t u r a l s i l t y c l a y o b t a i n e d from permafrost a r e a s i n t h e Northwest T e r r i t o r i e s . The d r i e d s o i l was mixed w i t h water (50% by weight of d r y s o i l ) and t h e r e s u l t i n g s l u r r y was packed i n t o c y l i n d r i c a l c a v i t i e s c u t i n t o a Styrofoam block w i t h a f l e x i b l e p o l y e t h y l e n e s h e e t l i n i n g . The samples were allowed t o f r e e z e

u n i d i r e c t i o n a l l y by exposing t h e open end t o t h e a i r i n s i d e a c o l d room k e p t a t -2.2OC. A f t e r complete f r e e z i n g , t h e c y l i n d r i c a l samples were removed from t h e Styrofoam block and t h e i r ends f a c e d i n a l a t h e . The f i n i s h e d samples were 150 mm long. The i c e l e n s e s i n t h e f r o z e n s o i l samples were d i s t r i b u t e d uniformly, but randomly o r i e n t e d . The average m o i s t u r e c o n t e n t of t h e samples was about 45%.

For a creep t e s t ( a s shown i n Figure I ) , a c y l i n d r i c a l sample ( A ) w a s mounted on t h e s t e e l p e d e s t a l

(B),

and t h e l e v e r arm (Dl was a d j u s t e d h o r i z o n t a l l y , w i t h t h e l o a d c e l l (C) above t h e sample. A h y d r a u l i c j a c k

(J)

was used t o r a i s e and lower t h e l e v e r arm. The combined weight of t h e beam

(D),

t h e shaker ( E l , and t h e l o a d i n g

p l a t f o r m

(P)

provided t h e s t a t i c l o a d on t h e specimen. Weights could b e placed, i f necessary, on t h e l o a d i n g p l a t f o r m (P) t o i n c r e a s e t h i s

s t a t i c load. Dynamic l o a d was a p p l i e d on t h e f r o z e n s o i l sample by t h e electrodynamic s h a k e r (E). The frequency and amplitude of t h e dynamic l o a d w a s c o n t r o l l e d by f e e d i n g a s u i t a b l e wave form s i g n a l t o t h e s h a k e r from a wave g e n e r a t o r and an a m p l i f i e r . For most t e s t s a s i n u s o i d a l wave of 10 Hz w a s used, a s t h i s was c l o s e t o t h e c r i t i c a l frequency of v i b r a t i o n f o r p i l e s i n f r o z e n s o i l determined e a r l i e r i n t h e l a b o r a t o r y

(Parameswaran (1982)) and observed i n t h e f i e l d ( P e r n i c a e t . a l . (1984)). (The term ' c r i t i c a l frequency' i s used h e r e t o d e n o t e t h e frequency a t which t h e amplitude of v i b r a t i o n , measured by a n

Figure 1. Schematic diagram of t h e experimental s e t u p t o s t u d y c r e e p of c y l i n d r i c a l samples of f r o z e n s o i l s . A

-

f r o z e n s o i l sample, B

-

s t e e l p e d e s t a l , C

-

l o a d c e l l , D

-

l e v e r arm, E

-

e l e c t r o d y n a d c shaker, J

-

h y d r a u l i c jack, L

-

l i n e a r v a r i a b l e d i f f e r e n t i a l t r a n s d u c e r (LVDT)

,

P

-

l o a d i n g p l a t f o r m

accelerometer on t h e p i l e , i s a maximum.) The s t a t i c and dynamic conrponents of t h e l o a d a p p l i e d on t h e specimen were monitored

continuously from t h e o u t p u t of t h e load c e l l ( C ) on a c h a r t r e c o r d e r . The deformation of t h e sample ( a x i a l compression) was a l s o recorded on t h e c h a r t r e c o r d e r , measured by a l i n e a r v a r i a b l e d i f f e r e n t i a l

t r a n s d u c e r (LVDT) 'L'

.

A l l t e s t s were c a r r i e d o u t a t a temperature of

-

2.Z0C

+

o.z0c.

DISPCACEMENT RATES, mm/h

1

L C i l O N I ALTERNATING STRESS I I I WITHOUT WITH 1 I 1 L B.C. FIR I N

I

T H O M P S O N SILTY CLAY 12;/ 9 y " 8

\*----

7 r--

'

1

= 0.076 MPa

----

r = 0.1053 MPa A

is

0 & - - I 1 I I 0 250 500 750 1000 1250 1500 1750 TIME, h

F i g u r e 2. T y p i c a l creep curve showing t h e displacement of a wood p i l e i n f r o z e n s i l t y c l a y w i t h time

RESULTS AND DISCUSSIONS

Displacement of P i l e s i n Frozen S o i l s

Figure 2 shows a t y p i c a l creep curve g i v i n g t h e displacement w i t h time of a n uncoated wood p i l e (made from Douglas f i r ) i n a f r o z e n s i l t y clay. The dashed l i n e s show t h e displacement under s t a t i c l o a d a l o n e , and t h e s o l i d l i n e s i n d i c a t e t h e r e g i o n s where a n a l t e r n a t i n g l o a d (of about 5% of t h e s t a t i c l o a d ) was superimposed on t h e s t a t i c load. The arrows i n d i c a t e t h e p o s i t i o n s where t h e s t a t i c l o a d was i n c r e a s e d by adding weights t o t h e l o a d i n g pan

(P

i n Figure

I).

I n t h e e a r l y p a r t of t h e t e s t , t h e displacement r a t e of t h e p i l e i n t h e f r o z e n s o i l decreased continuously w i t h time, i n d i c a t i n g primary creep behaviour of t h e f r o z e n s o i l under s h e a r a t t h e p i l e s o i l i n t e r f a c e . A f t e r t h e second l o a d increment (B i n F i g u r e 2 ) , t h e i n i t i a l displacement r a t e w a s s t i l l of t h e primary c r e e p type, a s s e e n i n r e g i o n 6 of F i g u r e 2. A marked i n c r e a s e i n t h e displacement r a t e of t h e p i l e w a s observed a f t e r t h e s h a k e r was t u r n e d on t o impose t h e a l t e r n a t i n g load. The displacement r a t e decreased soon a f t e r t u r n i n g t h e shaker o f f . I n r e g i o n s 7,

9

and 11, t h e e f f e c t of t h e superimposed a l t e r n a t i n g l o a d i n enhancing t h e displacement r a t e of p i l e i n t h e f r o z e n s o i l i s c l e a r l y seen. The

v a l u e s of t h e displacement r a t e s w i t h and w i t h o u t t h e a l t e r n a t i n g s t r e s s , under a c o n s t a n t s t a t i c s t r e s s of 0.1325 MPa

a t

t h e a d f r e e z i n g i n t e r f a c e between t h e p i l e and t h e s o i l , a r e a l s o g i v e n i n F i g u r e 2. The a l t e r n a t i n g s t r e s s causes t h e displacement r a t e t o be doubled from r e g i o n 6 t o 7, and t o be i n c r e a s e d more t h a n 3 times from r e g i o n 8 t o 9, and from r e g i o n 10 t o 11. This would imply t h a t a s t r u c t u r e designed only on t h e b a s i s of s t a t i c l o a d s f o r a c e r t a i n allowable displacement i n a given time p e r i o d w i l l , under combined s t a t i c and dynamic l o a d s ,

F i g u r e 3. Schematic r e p r e s e n t a t i o n of t h e displacements w i t h and w i t h o u t a superimposed a l t e r n a t i n g l o a d

a t t a i n t h a t displacement i n 113 of t h e d e s i g n l i f e , i f s u f f i c i e n t s a f e t y f a c t o r s a r e n o t i n c o r p o r a t e d i n t h e design.

D i f f e r e n t k i n d s of p i l e s such a s uncoated wood (B.C. f i r ) p i l e s , c o n c r e t e , and s t e e l p i p e p i l e s were t e s t e d i n v a r i o u s k i n d s of s o i l s , i n c l u d i n g a uniform sand (of average g r a i n diameter 0.2 t o 0.6 mm)

c o n t a i n i n g 14% m o i s t u r e (by weight of d r y s o i l ) , a s i l t y s o i l w i t h 20% m o i s t u r e , and a s i l t y c l a y with 50% moisture. From t h e displacement r a t e s observed u n d e r s t a t i c l o a d s and w i t h a superimposed a l t e r n a t i n g l o a d , t h e i n c r e a s e i n p i l e displacement p e r c y c l e of a l t e r n a t i n g l o a d was c a l c u l a t e d a s shown i n F i g u r e 3, f o r v a r i o u s s t a t i c stress l e v e l s

and displacements.

Let 111 be t h e displacement and

il

be t h e displacement r a t e d u r i n g p e r i o d t l under s t a t i c s t r e s s a l o n e , and l e t L2 and

i2

be t h e

corresponding v a l u e s f o r a p e r i o d t 2 w i t h t h e superimposed a l t e r n a t i n g stress. Assuming t h a t t h e displacement r a t e s

t l

and

I2

remain unchanged d u r i n g t h e p e r i o d s t l and t 2 , and e x t r a p o l a t i n g t h e c r e e p curve beyond

t l t o t 2 a s shown i n F i g u r e 3, t h e a d d i t i o n a l displacement (AL) caused

by t h e a l t e r n a t i n g l o a d a l o n e i s given by:

The i n c r e a s e i n displacement p e r c y c l e of l o a d i n g i s g i v e n by: d ~ l l ( i 2 - i , ) t 2

,

(

i2-ill

- =

dN t 2 x f f (2)

where N i s t h e number of c y c l e s of a l t e r n a t i n g s t r e s s a p p l i e d d u r i n g t h e t i m e p e r i o d t 2 , and f i s t h e frequency of loading.

1 0 . 0 L I 1 1 1 1 ! ! I 1 1 1 1 1 1 1 1 1 1 1 I l l l ~ l ~ 4

-

- C O N C R E T E P l L E I N F R O Z E N S A N D

-

-

B . C . F I R I N S A N D

-

-

-

-

C O N C R E T E I N S I L T Y S O I L

-

0 S T E E L P l L E I N S I L T Y C L A Y

-

-

m a V , 0 B . C . F I R I N S I L T Y C L A Y F i g u r e 4 . V a r i a t i o n of t h e increment i n p i l e displacement p e r c y c l e of a l t e r n a t i n g l o a d w i t h mean s t r e s s F i g u r e 4 shows t h e v a l u e s of dAll/dN c a l c u l a t e d f o r v a r i o u s p i l e s p l o t t e d on a log-log s c a l e a g a i n s t t h e mean s t r e s s r a t t h e p i l e s o i l i n t e r f a c e . For p i l e s i n f r o z e n sand and s i l t y s o i l , t h e r e i s some i n d i c a t i o n ( a s shown by l i n e A i n F i g u r e 4 ) t h a t dAll/dN i n c r e a s e s w i t h mean s t r e s s

r

as:

d All

a : ?"

dN

where t h e v a l u e of n o b t a i n e d from t h e s l o p e of t h e s t r a i g h t l i n e A was 6.5. I n t h e c a s e of p i l e s i n f r o z e n f i n e g r a i n e d c l a y e y s o i l s

c o n t a i n i n g about 50% moisture by weight of d r y s o i l , t h e v a l u e s of dALldN v a r i e d between 10-lo and loe7 mm/min f o r a very s m a l l change i n s t r e s s about t h e mean v a l u e of 0.13 MPa ( l i n e

B

i n F i g u r e 4 ) .

I I ,

-

l l i l I I 1 i I ll ' ] I I 1 1 1 I l l 2

-

-

_-

-

C O N C R E T E P I L E I N S A N D

-

-

A B . C . F I R I N S A N D

-

-

-

A B . C . F I R I N S I L T Y C L A Y

-

.

-

S T E E L P l L E I N S I L T Y C L A Y

-

-

-

-A r - 1 , A & - 2

-

03

-

-

,d'

1

-

0

-

-

_/'

-

-

_'a,

-

-

_-

-

4 a m - 3 '5

-

-

_-

-

-

a- --\

-

- \

-

=-

--

--

.--.

-

4 I 1 1 11 11 1 1 I 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

Figure 5. V a r i a t i o n of t h e increment i n p i l e displacement p e r c y c l e of a l t e r n a t i n g l o a d w i t h displacement

F i g u r e 5 shows t h e v a l u e s of dAII/dN p l o t t e d a g a i n s t displacement of t h e p i l e on a l o r l o g s c a l e . The behaviour of p i l e s i n f r o z e n sand and s i l t ( l i n e s 1, 2 and 3 i n Figure 5) showed t h a t dAg/dN i n c r e a s e d s h a r p l y w i t h displacement II. L i n e s 4 and 5 f o r a wood p i l e and s t e e l

c y l i n d r i c a l p i l e , r e s p e c t i v e l y , i n a f r o z e n clayey s o i l show d e f i n i t e d e v i a t i o n s from t h i s behaviour. The l i m i t e d amount of d a t a o b t a i n e d s o f a r does not allow one t o draw any conclusion o r propose a g e n e r a l i z e d -

behaviour of p i l e s i n c l a y e y s o i l s . F u r t h e r t e s t s a r e underway t o study t h e behaviour of p i l e s i n f r o z e n f i n e g r a i n e d s o i l s under dynamic

5 I I 7 I ₁ I

. .

-

_{2 6 '} / # / / / * # 0 / 4

-

1/

0.742 MPa 0 *#

-

/ / 0/ 00. I

,

_{0.64 MPa}

-

/' 0' /

-

/ Be / / I 0

,'

/"

40#0 5

i

3 - / /

-

/ ,/e4 u / / a I- /RO/

-

V) 0

:

/

/ ' / ' , , ,

,/'

H ~ ~ ,/// 0.63 M P a 0, W 2

-

U _t I 0 #/-'

i

,/'

,*

-

/

/'

'

/'

/"

1

-

I / ,/*

-

//'

_-

0 1 I 1 I I ! I 0 50 100 150 200 TIME, h

F i g u r e 6. Typical c r e e p curves of c y l i n d r i c a l samples of a f r o z e n s i l t y

c l a y

Creep of C y l i n d r i c a l S o i l Samples

Figure 6 shows t y p i c a l c r e e p curves f o r t h r e e d i f f e r e n t samples of

s i l t y c l a y w i t h 50% m o i s t u r e t e s t e d a t -2.2OC. The dashed l i n e s show

the dependence of c r e e p s t r a i n on time under s t a t i c s t r e s s a l o n e , and

t h e s o l i d l i n e s show t h e c r e e p s t r a i n under t h e combined s t a t i c and

dynamic s t r e s s e s . The peak-to-peak amplitude of t h e a l t e r n a t i n g s t r e s s

was about 5% of t h e s t a t i c s t r e s s . F i g u r e 7 shows a n o t h e r sample of t h e

same m a t e r i a l t e s t e d a t a lower mean s t r e s s , b u t f o r a l o n g e r t i m e . The

e f f e c t of dynamic s t r e s s i n i n c r e a s i n g t h e c r e e p r a t e of t h e samples i s

TIME, h

TIME, h

F i g u r e 7. A t y p i c a l long-term c r e e p curve of a f r o z e n sample of s i l t y c l a y

Creep r a t e s c a l c u l a t e d f o r v a r i o u s samples w i t h and w i t h o u t a l t e r n a t i n g stress a r e p l o t t e d a g a i n s t t i m e on a log-log s c a l e i n F i g u r e 8. The shaded symbols g i v e t h e r a t e s under combined s t a t i c and dynamic s t r e s s e s , and t h e blank symbols, t h e r a t e s under s t a t i c stress

alone. The c r e e p r a t e s

( k )

under s t a t i c a s w e l l a s dynamic stresses

d e c r e a s e c o n t i n u o u s l y w i t h t i m e ( t ) , following a power law:

When a = 0, = c o n s t a n t , a s observed i n t h e e a r l y p a r t of t h e c r e e p curves ( f o r t < 100 hours) f o r samples 5 and 8 t e s t e d under mean

s t r e s s e s of 0.63 and 0.52 m a , r e s p e c t i v e l y . For t h e dynamic tests

as

w e l l a s f o r sample 1 t e s t e d under a mean s t a t i c s t r e s s of 0.66 MPa, t h e v a l u e of t h e exponent ' a ' was 0.8. T h i s s u g g e s t s t h a t t h e deformation behaviour of t h e sample i s c l o s e t o t h a t of l o g a r i t h m i c c r e e p ( a = 1 ) i n t h e primary p a r t of t h e c r e e p curve, which i s s i m i l a r t o t h e behaviour observed i n many m e t a l l i c m a t e r i a l s a t e l e v a t e d temperatures. For

sample 8, t e s t e d f o r a l o n g e r d u r a t i o n a t a low mean s t a t i c s t r e s s v a l u e of 0.52 MPa, t h e s l o p e f o r s t a t i c c r e e p changes from a v a l u e of z e r o f o r

t < 100 h o u r s , t o a v a l u e of a = 2 o r more f o r t > 200 h o u r s , which e v e n t u a l l y l e a d s t o a v e r y ' l o w v a l u e of c r e e p r a t e .

F i g u r e 8. V a r i a t i o n of creep s t r a i n r a t e w i t h time f o r s i l t y c l a y under s t a t i c and dynamic l o a d s

10 -6

The behaviour d e p i c t e d i n Figure 8 is very s i m i l a r t o t h e long-term c r e e p behaviour of p o l y c r y s t a l l i n e i c e and sand-ice m a t e r i a l s (Butkovich and Landauer (1959), Mellor and Testa (1969), Hooke e t . a l . (1972)), which f o l l o w s t h e e m p i r i c a l e q u a t i o n (Weertman (1 973)

1:

-

-

S T A T I C D Y N A M I C \ \

-

-

a m ₁

-

_{0.66 M P a}

-

-

_{a A}

-

5

-

0.63 M P a

-

o 8

-

0.52 M P a

-

4 I I I q 1 1 1 1 I ! I r I 1 1 1 1 I 1 1 r ( l 1 '

where E i s t h e s t r a i n , t t h e time and A and B a r e c o n s t a n t s f o r a

p a r t i c u l a r m a t e r i a l . D i f f e r e n t i a t i n g (5) w i t h r e s p e c t t o time:

10 100 1000 10 000

TIME, h

The v a l u e of m o b t a i n e d by Butkovich and Landauer w a s 3 , which ave

i

_{a t-0*66. T h i s i s c l o s e t o o u r p r e s e n t o b s e r v a t i o n ,} E a t-~". Gold

samples. I f m = 1,

2

i n e q u a t i o n ( 6 ) becomes a c o n s t a n t , which i s t h e behaviour observed on samples ( 5 ) and (8) ( F i g u r e 8 ) f o r t < 100 hours. T h i s s u g g e s t s t h a t t h e c r e e p of f r o z e n s o i l s a l s o f o l l o w s t h e same e m p i r i c a l e q u a t i o n used t o analyze c r e e p of p o l y c r y s t a l l i n e i c e .

CONCLUSIONS

A dynamic stress of small amplitude amounting t o a f r a c t i o n of t h e s t a t i c stress, enhances t h e r a t e s of displacement of f r e e ended p i l e s i n f r o z e n s o i l s . This i n d i c a t e s t h a t t h e s h e a r a d f r e e z e s t r e n g t h a t t h e p i l e s o i l i n t e r f a c e i s decreased by dynamic loading. The c r e e p r a t e of c y l i n d r i c a l samples of f r o z e n clayey s o i l s ( c o n t a i n i n g about 45%

moisture) under u n i a x i a l compression was a l s o i n c r e a s e d by a

superimposed a l t e r n a t i n g s t r e s s . A s t h e t o t a l b e a r i n g c a p a c i t y f o r p i l e s i n f r o z e n s o i l s i s a f u n c t i o n of both t h e a d f r e e z e s t r e n g t h a t t h e p i l e - s o i l i n t e r f a c e and t h e end-bearing s t r e n g t h of t h e m a t e r i a l under t h e p i l e , t h e s e e f f e c t s s h o u l d be t a k e n i n t o c o n s i d e r a t i o n f o r s a f e and optimal d e s i g n of s t r u c t u r e s i n permafrost a r e a s . S i n c e t h e c r e e p r a t e i s a f u n c t i o n of t h e mean s t a t i c s t r e s s , one way t o e n s u r e s a f e t y i n t h e d e s i g n when a l t e r n a t i n g l o a d s a r e p r e s e n t i s t o reduce t h e s t a t i c l o a d on e a c h p i l e by u s i n g a l a r g e r number of p i l e s under t h e s t r u c t u r e .

I n g e n e r a l , t h e c r e e p of i c e - r i c h f r o z e n s o i l s was found t o f o l l o w t h e same e m p i r i c a l e q u a t i o n t h a t was used e a r l i e r f o r p o l y c r y s t a l l i n e i c e . This i n d i c a t e s t h a t t h e c r e e p of f r o z e n s o i l s i s e s s e n t i a l l y governed by t h e c r e e p of i c e i n t h e s o i l matrix.

ACKNOWLEDGEMENTS

S i n c e r e thanks a r e extended t o Colin Hubbs f o r performing t h e t e s t s i n t h e p e r m a f r o s t l a b o r a t o r i e s of DBRINRCC. T h i s p a p e r i s a

c o n t r i b u t i o n from t h e D i v i s i o n of Building Research, National Research Council of Canada, and i s p u b l i s h e d w i t h t h e a p p r o v a l of t h e D i r e c t o r of t h e Division.

REFERENCES

Butkovich, T.R., Landauer, J.K. 1959. The flow law f o r i c e . U.S. Army Snow I c e and Permafrost Research E s t a b l i s h m e n t (now CRREL)

,

Research Report 56, 7 p.

Czajkowski, R.L., Vison, T.S. 1980. Dynamic p r o p e r t i e s of f r o z e n s i l t under c y c l i c l o a d i n g . J. Geotech. Eng. Div., ASCE, Vol. 106, No. GT9, pp. 963-980.

Gold, L.W. 1965. The i n i t i a l c r e e p of columnar-grained i c e . P a r t I: Observed behaviour, P a r t 11: Analysis. Canadian J o u r n a l of P h y s i c s , Vol. 43, pp. 1414-1434.

Hooke, R. LeB., Dahlin, B.B., Kauper, M.T. 1972. Creep of i c e c o n t a i n i n g d i s p e r s e d f i n e sand. J o u r n a l of Glaciology, Vol. 11, NO. 63, pp. 327-336.

Kaplar, C.W. 1969. Laboratory d e t e r m i n a t i o n of dynamic modulii of

f r o z e n s o i l s and i c e . U. S. Army Cold Regions R e s . Eng. Lab.

,

Research

Report 163, 48 p.

Mellor, M., T e s t a , R. 1969. E f f e c t of t e m p e r a t u r e on t h e c r e e p of

i c e . J o u r n a l of Glaciology, Vol. 8 , No. 52, pp. 131-145.

Parameswaran, V. R. 1982. Displacement of p i l e s under dynamic

l o a d s i n f r o z e n s o i l s . The Roger J.E. Brown Memorial Volume,

Proceedings of t h e F o u r t h Canadian Permafrost Conference, C a l g a r y , 1981, N a t i o n a l Research Council of Canada, Ottawa, pp. 555-559.

P e r n i c a , G., Pararneswaran, V.R., Johnston, G.H. 1984.

( a )

-

Measurement of v i b r a t i o n l e v e l s i n t h e Inuvik powerhouse,

( b )

-

V i b r a t i o n measurements on f r e e s t a n d i n g p i l e s i n permafrost.

To be published.

Stevens, H.W. 1975. The response of f r o z e n s o i l s t o v i b r a t o r y

loads. U.S. Army Cold Regions R e s . Eng. Lab., Technical Report 265,

103 p.

Vinson, T.S., Chaichanavong, T., Czajkowski, R.L. 1978(a).

Behaviour of f r o z e n c l a y under c y c l i c a x i a l loading. J. Geotech. Eng.

Div., ASCE, Vol. 104, No. GT7, pp. 779-800.

Vison, T.S., Chaichanavong, T. 1978(b). Dynamic behaviour of i c e

under c y c l i c a x i a l loading. J. Geotech. Eng. Div., ASCE, Vol. 104,

No. GT7, pp. 801-814.

Weertman, J. 1973. Creep of i c e . P h y s i c s and Chemistry of I c e ,

E d i t e d by E. Whalley, S.J. Jones and L.W. Gold, Royal S o c i e t y of Canada,

T h i s paper, w h i l e being d i s t r i b u t e d i n r e p r i n t form by t h e D i v i s i o n of B u i l d i n g Research, remains t h e c o p y r i g h t of t h e o r i g i n a l p u b l i s h e r . It should n o t be reproduced i n whole o r i n p a r t w i t h o u t t h e permission of t h e p u b l i s h e r . A l i s t of a l l p u b l i c a t i o n s a v a i l a b l e from t h e D i v i s i o n may be o b t a i n e d by w r i t i n g t o t h e P u b l i c a t i o n s S e c t i o n , D i v i s i o n of B u i l d i n g R e s e a r c h , N a t i o n a l R e s e a r c h C o u n c i l of Canada, O t t a w a , O n t a r i o ,

KIA

0R6.

Complete Proceedings a r e a v a i l a b l e from: CSCE

2050 Mansfield S t r e e t , S u i t e 7 0 0 , Montreal, Quebec. H 3 A 122