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Ice stress measurements in a rubble field surrounding a
caisson-retained island
Sayed, M.; Frederking, R. M. W.; Croasdale, K. R.
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Ice Stress Measurements in a
Rubble Field Surrounding a
Caisson-Retained Island
by M. Sayed, R.M.W. Frederking and K.R. CroasdaleANALYZED
Reprinted fromIce Technology
Proceedings of the 1st International Conference Cambridge, Mass.
U.S.A.,
June 1986p. 255
-
262(IRC Paper No. 1439)
Price $3.00 NRCC 27175 N R C
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IC-On a mesure' l e s c o n t r a i n t e s q u i s ' e x e r ~ a i e n t dans l e champ de fragments e n t o u r a n t un I l o t - c a i s s o n dans l e Sud de l a mer de Beaufort. Les mouvements, l a gComBtrie e t l a composition des fragments o n t C t C examinBs. La c o n t r a i n t e maximale, prSs du bord du champ de fragments, B t a i t de 200 kPa, c e q u i B t a i t a t t r i b u a b l e , semble-t-il, B l ' e x p a n s i o n thermique de l a g l a c e s e t r o u v a n t e n t r e l e r i v a g e e t l ' t l o t . Les c o n t r a i n t e s mesurees prSs du c a i s s o n B t a i e n t n 6 g l i g e a b l e s , c e q u i semble i n d i q u e r que l'amoncellement de fragments ancrCs au s o l a b s o r b a i e n t t o u t e s l e s f o r c e s exercCes p a r l a g l a c e f l o t t a n t e . Les mouvements d e s fragments pendant l a pgriode d ' o b s e r v a t i o n s e s o n t a u s s i a v e r & nkgligeables.
"
Ice Stress Measurements in a Rubble Field Surrounding a Caisson-
Retained Island
'
M.
Sayed,
R.M.
W. Frederking
Division ofBuilding Research, National Research Council Canada, Ottawa,
Ontario K I A OR6
K . R .
Croasdale
K . R . Croasdale
&
Assoc.,
334
40th Avenue S.
W . , Calgary, Alberta
T2S
OX4
ABSTRACT
S t r e s s e s were measured i n t h e r u b b l e f i e l d surrounding a c a i s s o n - r e t a i n e d i s l a n d i n t h e s o u t h e r n Beaufort Sea. The r u b b l e movements, geometry and composition were examined. The maximum s t r e s s , n e a r t h e edge of t h e r u b b l e f i e l d , was
approximately 200 kPa, a p p a r e n t l y caused by thermal expansion of t h e i c e between t h e s h o r e and t h e i s l a n d . S t r e s s e s measured n e a r t h e c a i s s o n were n e g l i g i b l e , which s u g g e s t s t h a t a l l
i
f o r c e s a p p l i e d by t h e f l o a t i n g i c e were r e s i s t e d by groundingof t h e rubble. Movements of t h e r u b b l e d u r i n g t h e o b s e r v a t i o n p e r i o d were a l s o n e g l i g i b l e .
F l o a t i n g i c e i n t e r a c t i o n w i t h c a i s s o n - r e t a i n e d i s l a n d s i n t h e Beaufort Sea g i v e s r i s e t o complex p r o c e s s e s both i n t h e i c e and i n t h e caisson-berm system. Monitoring of i c e s t r e s s e s and s t a b i l i t y of t h e c a i s s o n s t r u c t u r e s i n t h e l a s t few y e a r s has improved o u r knowledge of t h e s e p r o c e s s e s , however, some a s p e c t s of i c e behaviour s t i l l remain poorly understood. The formation of r u b b l e f i e l d s and t h e i r r o l e i n t r a n s f e r r i n g l o a d s from f l o a t i n g i c e t o s t r u c t u r e s , f o r example, a r e s t i l l t h e
*
s u b j e c t of much s p e c u l a t i o n .Most f i e l d s t u d i e s r e l e v a n t t o t h e above problems a r e p r o p r i e t a r y . Published d a t a i n c l u d e measurements of s t r e s s e s
'
i n a r u b b l e f i e l d a d j a c e n t t o an a r t i f i c i a l beach i s l a n d( S t r i l c h u k , 1977). Observations of i c e i n t e r a c t i o n w i t h n a t u r a l i s l a n d s were r e p o r t e d by Frederking e t a l . (1984) and by Kovacs and Sodhi (1981). S t r e s s e s i n t h e f l o a t i n g i c e around Esso's c a i s s o n - r e t a i n e d i s l a n d were measured d u r i n g t h e 1983/84 w i n t e r a t Kadluk l o c a t i o n (Johnson e t a l . , 1985).
T h i s paper d e s c r i b e s i c e s t r e s s measurements c a r r i e d o u t a t E s s o ' s c a i s s o n - r e t a i n e d i s l a n d from March t o May 1985. The
KENALOOAK
NEKTORALl K K-59
m FIERKLERK M-Pf
W(VILRUK 0-3 KOPANOAR M-13
F i g u r e 1. Arnerk a r t i f i c i a l i s l a n d , Beaufort Sea
c a i s s o n was a t t h e Amerk 0-09 l o c a t i o n (shown i n F i g u r e 1). S t r e s s s e n s o r s were i n s t a l l e d a t d i f f e r e n t p o s i t i o n s i n t h e r u b b l e f i e l d surrounding t h e c a i s s o n t o examine r a d i a l s t r e s s d i s t r i b u t i o n s . S e v e r a l types of s e n s o r s were used i n o r d e r t o improve t h e confidence i n t h e measurements. I n a d d i t i o n t o s t r e s s measurements, r u b b l e displacements and p r o f i l e s were monitored t o determine t h e r u b b l e f i e l d response t o impinging f l o a t i n g i c e . A comprehensive d i s c u s s i o n of t h e f i e l d p r o j e c t was given by Croasdale (1985).
SITE AND INSTRUMENTATION
Esso's c a i s s o n - r e t a i n e d i s l a n d i s o c t a g o n a l i n p l a n , 100 m a c r o s s t h e f l a t s . The c a i s s o n ' s d e s i g n was d i s c u s s e d by Mancini e t a l . (1983). It was placed on an e l e v a t e d submarme berm a t Amerk 0-09. The water d e p t h was 26 m and t h e berm came
t o within 9 m of Ehe surf ace. By February, a r u b b l e f i e l d
surrounded t h e c a i s s o n and remafned s t a b l e through the 4
abservation period t h a t ended i n May, Floating i c e around the c
r u b b l e f i e l d was mobile d u r i n g the first f i e l d t r i p in
Febr~isryIMarch. It became s t a b l e by the time of the second A
t r i p i n May.
It was decided t o c o n c e n t r a t e t h e o b s e r v a t i o n s on t h e e a s t and s o u t h s e c t o r s of t h e r u b b l e f i e l d s i n c e t h e f l o a t i n g i c e was expected t o move predominantly from t h e e a s t . Also thermal expansion of t h e i c e cover i n t h e s p r i n g could e x e r t s t r e s s e s i n t h e d i r e c t i o n of t h e n e a r e s t s h o r e l i n e , t o t h e south. The stress s e n s o r s c o n s i s t e d of two Exxon Production Research (EPR) p a n e l s , t h r e e Hexpack ( A r c t e c ) p a n e l s , an I d e a l ( T e r r a s c i e n c e )
p a n e l and a b i a x i a l Canada Marine Engineering s e n s o r (CMEL-IV). The CMEL s e n s o r can r e s o l v e t h e magnitude and d i r e c t i o n of t h e p r i n c i p a l s t r e s s e s i n t h e p l a n e of t h e i c e cover. The o t h e r p a n e l s measure o n l y t h e s t r e s s component a c t i n g normal t o t h e i r f a c e s . Approximate l o c a t i o n s of t h e s e n s o r s a r e shown i n F i g u r e 2. An ARCDATS d a t a l o g g e r was used t o r e c o r d a l l
,SURVEY BASE THERMISTOR
U N I T S I N M E T R E S
I
L/
10f
EDGEF i g u r e 2. Sensor l o c a t i o n s and i d e n t i f i c a t i o n numbers
channels a t five-minute i n t e r v a l s . Eight s u r v e y r e f l e c t o r s were a l s o placed i n t h e e a s t e r n and s o u t h e r n s e c t o r s of t h e r u b b l e f i e l d . An e l e c t r o n i c d i s t a n c e measuring (EDM)
e
i n s t r u m e n t (Wild DI-20) coupled w i t h a t h e o d o l i t e (Wild T2) was .,
used t o determine p o s i t i o n s of t h e r e f l e c t o r s r e l a t i v e t o a f i x e d base l i n e from t h e h e l i d e c k . Displacements were
9
determined from r e p e a t e d surveys. The EDM h a s a n accuracy of f5 mm f o r d i s t a n c e s up t o 1000 m. H o r i z o n t a l and v e r t i c a l a n g l e s had an accuracy of f5 seconds of a r c .
Wind and temperature d a t a a t t h e s i t e were o b t a i n e d from Esso Resources Canada.
DISCUSSION OF THE RESULTS
Rubble Displacements and P r o f i l e s
Displacements of t h e survey r e f l e c t o r s a r e given i n Table 1.
he-coordinates
a r e re1ati;e t o t h e s o u t h e a s t c o r n e r of t h e Table 1 Ice Rubble Displacements24 Feb. t o 13 May 13 t o 15 May
Coordinates L
(m) d i s p l a c e - d i s p l a c e -
Survey ment d i r . ment d i r .
marker E a s t North (m) (deg) (m) (deg
l ~ e l a t i v e t o s o u t h e a s t c o r n e r of h e l i d e c k ( s e e F i g u r e 2) 2Angle clockwise from n o r t h
h e l i d e c k ( s e e F i g u r e 2). A l l displacements were l e s s t h a n 0.01 m, which is w i t h i n t h e accuracy of measurement. Some movements a t t h e edges of t h e r u b b l e f i e l d could be d e t e c t e d , though not surveyed. For example t h e p i l e u p s o u t h of s e n s o r #7 appeared t o have deformed s l i g h t l y between February and May. The s a i l t i l t e d t o cover t h e l o c a t i o n of t h e s e n s o r by May.
Rubble s a i l p r o f i l e s were o b t a i n e d from a e r i a l
photographs. A p r o f i l e a l o n g a l i n e r a d i a t i n g southwards and p a s s i n g n e a r s t r e s s s e n s o r 117 is shown i n Figure 3. This i s t y p i c a l of most of t h e r u b b l e f i e l d except f o r t h e n o r t h e a s t p a r t , where s a i l s r e a c h i n g 10 m above water l e v e l w e r e * observed. E s t i m a t e s of grounding r e s i s t a n c e t o t h e f o r c e s a p p l i e d by f l o a t i n g i c e can be made by assuming t h a t t h e s h e a r s t r e s s a t t h e bottom of t h e r u b b l e i s e q u a l t o t h e normal
D I S T A N C E F R O M C A I S S O N . rn
s t r e s s m u l t i p l i e d by a f r i c t i o n c o e f f i c i e n t . This c o e f f i c i e n t may be e q u a l t o t h e t a n g e n t of t h e i n t e r n a l f r i c t i o n a n g l e of
t h e berm's s o i l . This simple method i m p l i e s t h a t t h e f r i c t i o n
c o e f f i c i e n t a c c o u n t s f o r s l i d i n g f r i c t i o n and s c o u r of t h e
berm. For example, t h e s a i l h e i g h t of 4 m and k e e l depth of
26 m n e a r t h e t i d a l c r a c k a s i n F i g u r e 3 would g i v e a grounding r e s i s t a n c e of 4 kPa (assuming t h a t s a i l and k e e l voids
r a t i o = 0.3, u n i t weight of i c e = 8.8 k ~ / m ~ and f r i c t i o n
a n g l e = 30'). S i m i l a r e s t i m a t e s could be made along any r u b b l e
p r o f i l e by c o n s i d e r i n g wdter d e p t h t o vary l i n e a r l y from 9 m a t t h e c a i s s o n t o 26 m a t t h e t i d a l crack.
S t r e n g t h and t h i c k n e s s of t h e f r o z e n l a y e r of r u b b l e were
examined by d r i l l i n g s e v e r a l 50 mrn ( 2 i n . ) d i a m e t e r boreholes.
Those were d r i l l e d on 16 May i n v a l l e y s where t h e i c e s u r f a c e was c l o s e t o t h e water l e v e l . R e s i s t a n c e t o t h e auger u s u a l l y
dropped a f t e r 0.5 t o 1.5 m, i n d i c a t i n g a decrease of s t r e n g t h . .
The f r o z e n l a y e r was c o n s i s t e n t l y 2.5 m thick. Water was found
i n t h e pores below t h a t depth. The t h i c k n e s s of f l o a t i n g i c e
o u t s i d e t h e r u b b l e f i e l d was 1.85 m on t h e same d a t e . S t r e s s e s
S t r e s s s e n s o r /I7 (EPR f l a t panel 0.5 x 2 m ) , which was placed
n e a r t h e s o u t h e r n edge of t h e r u b b l e f i e l d , was t h e o n l y s e n s o r
t o g i v e a p p r e c i a b l e s i g n a l s . The maximum s t r e s s averaged over
t h e a r e a of t h e p a n e l was approximately 200 kPa about 10 m from t h e edge of t h e r u b b l e f i e l d . Assuming t h a t t h e s t r e s s i s uniform o v e r t h e f r o z e n r u b b l e l a y e r , t h e maximum f o r c e p e r
u n i t l e n g t h would be 500 kN/m. The f o r c e a p p l i e d by t h e
f l o a t i n g i c e would be s l i g h t l y l a r g e r due t o t h e grounding r e s i s t a n c e of t h e r u b b l e between t h e s t r e s s s e n s o r and t h e
t i d a l crack. These r e s u l t s a r e i n good agreement w i t h t h e
measurements of Johnson e t a l . (1985), which were conducted i n t h e f l o a t i n g i c e around t h e r u b b l e f i e l d a t Kadluk ( s e e
F i g u r e 1) d u r i n g t h e s p r i n g of 1984.
D e t a i l s of t h e s t r e s s r e c o r d on s e n s o r #7 a r e shown i n
F i g u r e 4. Maximum s t r e s s e s were recorded on 3 A p r i l and
23 A p r i l ( F i g u r e s 4a,b). The s t r e s s record showed a n e a r
semi-diurnal cycle. The p e r i o d of t h e s t r e s s wave was
approximately 14 h o u r s , probably caused by t h e t i d e , which h a s a semi-diurnal c y c l e .
1 The o t h e r s e n s o r s i n s t a l l e d i n s i d e t h e r u b b l e f i e l d a l l
recorded z e r o s t r e s s . Obviously t h e grounding r e s i s t a n c e of
t h e r u b b l e was l a r g e enough t o t r a n s m i t a l l of t h e f l o a t i n g i c e
f o r c e s t o t h e berm. The n e g l i g i b l e movements of t h e r u b b l e
confirm t h i s r e s u l t .
S t r e s s e s measured a t s e n s o r /I7 over t h e f u l l period of
o b s e r v a t i o n a r e compared t o wind and temperature r e c o r d s i n F i g u r e 5. High s t r e s s e s appear t o c o i n c i d e w i t h l a r g e r a t e s of
i n c r e a s e of temperature. This i s expected s i n c e t h e s t r e s s
s e n s o r was on t h e s i d e of t h e r u b b l e f i e l d f a c i n g t h e shore. I c e cover expansion would be r e s p o n s i b l e f o r t h e s t r e s s
M A R C H A P R I L 1985
Figure 4. Ice s t r e s s r e s u l t s a t the a r t i f i c i a l island
Amerk 0-09
high s t r e s s e s because of the creep of the i c e cover. Some
southerly wind a l s o coincides with high s t r e s s e s recorded i n
early April. S t i l l , no strong correlation between wind and
DAY NUMBER
F i g u r e 5. Comparison of s t r e s s e s w i t h wind speed and a i r temperatures
CONCLUSION
S t r e s s measurements were c a r r i e d o u t i n t h e r u b b l e f i e l d a t Esso's Amerk c a i s s o n - r e t a i n e d i s l a n d from March t o May 1985. Rubble p r o f i l e s were o b t a i n e d from a e r i a l photos. Repeated surveys of markers showed no movement of t h e r u b b l e d u r i n g t h a t period.
S t r e s s e s n e a r t h e s o u t h e r n edge of t h e r u b b l e f i e l d had a maximum of 200 kPa and showed s e m i - d i u r n a l c y c l i n g . High
? s t r e s s e s appeared t o correspond t o i n c r e a s e s i n temperature
(and consequent expansion of t h e i c e cover between t h e i s l a n d and t h e shore). The t h i c k n e s s of t h e f r o z e n r u b b l e l a y e r , which probably t r a n s m i t t e d most of t h e s t r e s s e s , was 2.5 m. *
The maximum f o r c e p e r u n i t l e n g t h t r a n s m i t t e d t o t h e o u t e r edge of t h e r u b b l e f i e l d would be approximately 500 kN/m, assuming t h a t s t r e s s e s were uniform a c r o s s t h e f r o z e n rubble. S t r e s s e s measured i n t h e r u b b l e f i e l d n e a r t h e c a i s s o n were n e g l i g i b l e . This s u g g e s t s t h a t a l l t h e f o r c e a p p l i e d by f l o a t i n g i c e was r e s i s t e d by grounding of t h e rubble.
More d e t a i l e d s t r e s s measurements n e a r t h e p e r i p h e r y of r u b b l e f i e l d s a r e s t i l l needed t o determine t h e manner i n which s t r e s s e s a r e t r a n s f e r r e d through grounded rubble.
ACKNOWLEDGEMENTS
This p r o j e c t was funded by t h e f o l l o w i n g a g e n c i e s : The N a t i o n a l Research Council of Canada (NRCC); P u b l i c Works Canada; The U.S. Department of t h e I n t e r i o r (Minerals Management S e r v i c e s ) . T h e i r s u p p o r t i s g r a t e f u l l y acknowledged.
The p r o j e c t was a l s o supported by Esso Resources Canada
Ltd. (and t h e i r p a r t n e r s ) , who provided a c c e s s t o t h e l o c a t i o n ". of t h e s t u d y , l o g i s t i c a l s u p p o r t , and c e r t a i n d a t a c o l l e c t e d a s p a r t of t h e i r ongoing monitoring program f o r t h e Caisson-
Retained I s l a n d . The e n t h u s i a s t i c support by Esso, and * e s p e c i a l l y from t h e people i n v o l v e d , both i n t h e f i e l d and a t
t h e Research Laboratory, i s g r a t e f u l l y acknowledged. Charles Der i s s p e c i a l l y thanked f o r h i s v e r y v a l u a b l e s u p p o r t .
Valuable c o n t r i b u t i o n s t o t h e p r o j e c t were made by Moe Cheung, P u b l i c Works Canada. The t e c h n i c a l s u p p o r t of John Egan (PWC), and P a u l Anhorn and Joe N e i l (NRCC), i s g r a t e f u l l y acknowledged. This paper 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, N a t i o n a l Research Council Canada.
REFERENCES
Croasdale, K.R. (1985) I c e I n v e s t i g a t i o n s a t a Beaufort Sea Caisson. A r e p o r t by K.R. Croasdale A s s o c i a t e s Ltd., f o r t h e National Research Council of Canada and U.S. Department of t h e I n t e r i o r .
Frederking, R.M.W., Sayed, M., Wessels, E., Child, A.J., and Bradford, D.B. (1984) I c e I n t e r a c t i o n w i t h Adam I s l a n d , Winter 1982-83. IAHR I c e Symposium, Hamburg 1984, Proceedings 3, 187-201.
J o h n s o n , J.B., Cox, G.F.N., and Tucker W.B. (1985) Kadluk I c e S t r e s s Measurement Program. 8 t h I n t . Conf. on P o r t and Ocean Engineering under A r c t i c Conditions (POAC), Narssarssuaq, Greenland, 1985, 1 , 88-100.
Kovacs, A. and Sodhi, D. (1981) Sea I c e P i l i n g a t Fairway Rock, Bering S t r a i t , Alaska: Observations and T h e o r e t i c a l Analysis.
6 t h I n t . Conf. on P o r t and Ocean Engineering under A r c t i c l
Conditions (POAC), Quebec, 1981, 2, 985-1000.
Mancini, C.V., Dowse, B.E.W. and C h e v a l l i e r , J-M. (1983) b Caisson Retained I s l a n d f o r Canadian Beaufort Sea
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T h i s p a p e r i s b e i n g d l s t r i b u t e d i n r e p r i n t f o r m by t h e I n s t i t u t e f o r R e s e a r c h i n C o n s t r u c t i o n . A l i s t of b u i l d i n g p r a c t i c e and r e s e a r c h 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 I n s t i t u t e may be o b t a i n e d by w r i t i n g t o t h e ~ u b l i c a t l - o n s S e c t i o n , I n s t i t u t e f o r R e s e a r c h i n C o n s t r u c t i o n , N a t i o n a l R e s e a r c h C o u n c i l o f C a n a d a , O t t a w a , O n t a r i o , K1A 0R6. Ce document e s t d i s t r i b u B s o u s f o r m e d e t i r B - 8 - p a r t p a r 1 ' I n s t i t u t d e r e c h e r c h e e n c o n s t r u c t i o n . On p e u t o b t e n i r une l i s t e d e s p u b l i c a t i o n s d e 1 ' I n s t i t u t p o r t a n t s u r l e s t e c h n i q u e s ou les r e c h e r c h e s e n m a t i e r e d e b a t i m e n t e n B c r i v a n t B l a S e c t i o n d e s p u b l i c a t i o n s , I n s t i t u t d e r e c h e r c h e e n c o n s t r u c t i o n , C o n s e i l n a t i o n a l d e r e c h e r c h e s d u Canada, O t t a w a ( O n t a r i o ) , K1A 0R6.
