Ice forces on the Nanisivik wharf

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Ice forces on the Nanisivik wharf

Frederking, R. M. W.; Sayed, M.

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National Research

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Ice Forces on the Nanisivik Wharf

by R.M.W. Frederking and M. Sayed

Appeared in

Proceedings of the 9th International Symposium on Ice

Sapporo, Japan, August 23

-

27,1988

IAHR Committee on Ice Problems, Vol. 1

p. 463-472

(IRC Paper No. 1610)

Reprinted with permission

NRCC 30754

N R C

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C4STI

I

R

C

L I B R A R Y

o l

r

24 1289

_-

B I B L I O T H ~ Q U E

I R C

CNRC

-

ICiST -I

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Le present document examine les contraintes exerc6es par la glace et les facteurs

environnementaux au quai de Nanisivik en bordure du dktroit de Strathcona, dans

1'Arctique canadien, pendant l'hiver de

1985-

1986.

Le

&@me

glaciaire

tst

caractkrisi

par

la formation de glace de rive de premibre

annke

qui atteint une 6paisseur maximale de

1,6

m

en mai. Le mecanisme premier de 1'apparitj.on de charges engendrees par la glace

i

cet

endroit semble

Stre

les deformations de la couvertute de glace attribuables

aux

variations de

temphture. Une analyse thermique basCe sut le comportement du fluage secondah p+&t

des contraintes therrniques inferieures

i

celles mesurees.

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IAHR Ice

Symposium 1988

Sapporo

ICE FORCES ON THE NANISIVIK WHARF

R. Frederking and M. Sayed Research O f f i c e r s

Geotechnical Section I n s t i t u t e f o r Research i n Construction

National Research Counci 1 Canada CANADA

ABSTRACT

I c e stresses and environmental f a c t o r s a t N a n i s i v i k wharf on Strathcona Sound i n t h e Canadian A r c t i c are reported f o r t h e w i n t e r 1985/86. The i c e regime i s t h a t o f l a n d f a s t f i r s t - y e a r sea i c e t h a t a t t a i n s a maximum t h i c k n e s s o f 1.6 m i n May. The primary mechanism of generating i c e 1 oads a t t h i s 1 o c a t i o n appears t o be temperature-i nduced s t r a i n s i n t h e i c e cover. A thermal a n a l y s i s based on secondary creep behaviour p r e d i c t s thermal s t r e s s e s lower than those measured.

INTRODUCTION

I c e pressure measurements a t l o c a t i o n s i n t h e A r c t i c , g e n e r a l l y a t o f f s h o r e s t r u c t u r e s , a r e u s u a l l y designed t o generate i n f o r m a t i o n r e g a r d i n g t h e s a f e t y o f o f f s h o r e d r i l l i n g a c t i v i t i e s - ( H a w k i ns e t al., 1983,

P i l k i n g t o n e t al., 1983). The primary concern was w i t h t h e extreme o r maximum loads. I n t h i s study i n t e r e s t i s focused on a near shore

s t r u c t u r e , a sheet p i l e c e l l wharf constructed a t Nani s i v i k on Strathcona Sound (Girgrah and Shah, 1978). E f f o r t was focused on o b t a i n i n g a r e c o r d o f loads and associated environmental f a c t o r s over a complete w i n t e r season. This paper w i l l p r e s e n t t h e r e s u l t s o f measurements made by NRC over t h e w i n t e r 1985-86, p r o v i d e an i n t e r p r e t a t i o n o f t o t a l l o a d on t h e wharf, and discuss t h e i c e loads i n r e l a t i o n t o environmental factors. A

more complete record of t h e r e s u l t s i s a v a i l a b l e (Frederking and Sayed, 1987).

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SITE

IAHR Ice Symposium

1988

Sapporo

The Nanisivik Wharf i s located on t h e south shore o f Strathcona Sound (73'04'N-84'33'W) about 20 km from i t s o u t l e t . Strathcona Sound i s a f j o r d - l i k e f e a t u r e a t t h e n o r t h end o f B a f f i n I s l a n d i n the eastern Canadian Arctic. The width o f t h e Sound a t t h e wharf s i t e i s about 5 km; maximum t i d a l range i s 2.5 m. The wharf comprises t h r e e sheet p i l e c e l l s 21 m i n diameter, 38 m on centres, about 50 m offshore (Fig. 1). The t h r e e c e l l s are joined by an apron on the shore side, w i t h a causeway t o t h e shore. Maximum water depth a t the outer edge o f t h e c e l l s i s 13.5 m.

-0 0 0 -

f

, NO. 5

/e-

-

N0.4 No. l

-

N0.3

\

'No. 2 -0- o-o- I C E MOVEMENT POINTS . \ - 0

- -

0 - W H A R F 0

-

m ? O J O i O S C ~ L I - m

FIGURE 1 Schematic showing l o c a t i o n s o f i c e s t r e s s measuring instruments . i n s t a l l e d adjacent t o N a n i s i v i k wharf, w i n t e r 1985-86.-

MEASUREMENT PROGRAM

The f o l l o w i n g elements were measured: i n s i t u stresses i n t h e i c e cover adjacent t o t h e wharf, h o r i z o n t a l movements of t h e i c e cover, i c e temperatures, and meteor01 ogical data. I c e conditions were observed by means of a series o f s i t e v i s i t s and t h e use o f unattended data loggers.

Instruments were i n s t a l le d i n November, t h e i r operation v e r i f i e d i n March, and they were recovered i n May.

I n e a r l y November, f i v e panel -type transducers were i n s t a l l e d i n t h e l e v e l i c e cover adjacent t o t h e wharf t o measure i n s i t u stresses. Four panels were aligned p a r a l l e l t o t h e dock and one was placed normal t o i t

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IAHR Ice

Symposium

1988

Sapporo

.

1 ). Panels No. 1 t o 4 were 1 m x 1 m, sectioned h o r i z o n t a l l y so t h a t t h e average s t r e s s i n t h e upper 0.5 m and lower 0.5 m c o u l d be measured independently. Panel 5, 1

m

x 2 m, was h o r i z o n t a l l y sectioned i n f o u r s e c t i o n s each 0.5 m high. This allowed v e r t i c a l d i s t r i b u t i o n o f s t r e s s i n t h e i c e cover t o be measured. The f i v e panels provided a t o t a l o f 12 channels o f data. Output was recorded a t 15-minute i n t e r v a l s u s i n g a programmable battery-powered data l o g g e r able t o operate a t temperatures down t o -40'C. As problems were encountered i n i t i a l l y w i t h t h e logger, no

re1 i able recordings were obtained u n t i 1 January 1986.

An e l e c t r o n i c distance-measuring

(EDM) instrument coupled t o a

t h e o d o l i t e ' w a s used t o measure movements o f reference p o i n t s on t h e i c e cover a t a number o f s e l e c t l o c a t i o n s . One a r r a y comprised n i n e reference p o i n t s arranged i n t h r e e l i n e s p a r a l l e l t o t h e face o f t h e wharf and 5, 25 and 55 m from i t (Fig. 1 ) t o determine h o r i z o n t a l movements . r e l a t i v e t o t h e wharf. A second a r r a y was e s t a b l i s h e d about 200 m t o t h e west o f t h e wharf and about 100 m from shore where t h e water depth was 5 m. This area was considered t o be r e p r e s e n t a t i v e o f a n a t u r a l beach and served as a b a s i s o f comparison w i t h h o r i z o n t a l i c e cover movements adjacent t o t h e wharf.

I n November an automatic meteorological s t a t i o n was s e t up on t h e i c e s u r f a c e half-way across t h e Sound t o measure wind speed and d i r e c t i o n , a i r temperature, and temperatures i n t h e i c e . Data were recorded a t half-hour

i n t e r v a l s on a b a t t e r y powered logger. The thermocouple probe f o r measuring i c e temperatures had i n d i v i d u a l sensors a t depths o f 0.0, 0.25, 0.5, 0.75, 1.0, 1.5 and 2.0 m i n t h e ice.

RESULTS

AND

DISCUSSION

Results from i c e s t r e s s panel No. 3 f o r t h e p e r f o d 20 January t o 16 May, 1986 are shown i n Fig. 2. The r e s u l t s f o r a l l t h e panels were s i m i l a r . Panel No. 2, which was normal t o t h e face o f t h e wharf, showed stresses s u b s t a n t i a l l y l e s s than 100 kPa, except f o r a s h o r t p e r i o d a t t h e end o f March when they s l i g h t l y exceeded 100 kPa. The zero s t r e s s l e v e l e s t a b l i s h e d f o r these readings was taken i n November j u s t a f t e r t h e panels had been placed i n t h e i c e cover. I n e a r l y March s l o t s were c u t around t h e panels t o r e l i e v e t h e s t r e s s on them, thereby checking t h e zeros. A f i n a l s e t o f zero readings was obtained i n May when t h e panels were removed. The

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IAHR Ice

Symposium

1988

Sapporo

w x i m u m v a r i a t i o n i n the 'zero' stress l e v e l s was l e s s than 40 kPa, i n d i c a t i n g t h a t t h e panels were r e l a t i v e l y f r e e from zero d r i f t . The

concentration o f t h e l a r g e s t i c e stresses i n t h e t o p 0.5

m

o f the i c e cover was t y p i c a l f o r a l l panels. The maximum measured pressure was about

425 kPa on 18 February, 1986 and occurred on t h e upper 0.5 m sector of panel 3.

500

-

LEGCNO

PANEL NO. 3 (EAST)

UPPEP 0.5 r

..CO!tZ il:?.I!--

100

-

500

-

-1W ,

ao 27 1 1 0 1 7 2 I ID 17 n 31 r 1 4 21 a s 1 1 13

JANUARY FEBRUARY RARCH RPRIL MAY

1986

Figure 2 Results o f i c e pressure panel No. 3 f o r t h e period 1986 January

20 t o May 16.

Horizontal movements o f a l l nine reference p o i n t s next t o the wharf and t h e array o f p o i n t s 200 m t o the west o f i t were remarkably s i m i l a r , i n d i c a t i n g t h a t t h e i c e cover i n t h i s area e s s e n t i a l l y moved as a body. The movement r a t e averaged over the f i r s t h a l f of t h e w i n t e r was 40 mnlday; f o r t h e l a t t e r h a l f o f t h e winter i t had slowed t o 20 mnlday. The

d i r e c t i o n was n o r t h e r l y . Such movements have been measured previously a t t h i s l o c a t i o n (Frederki ng and Nakawo, 1984).

Close examination of two l i n e s o f reference p o i n t s 25 and 55 m n o r t h of the outer face o f the wharf showed t h e i c e cover t o be undergoing

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fve s t r a f fis f n a d i r e c t i o n normal t o

the

outer face o f t h e wharf. For the f i r s t h a l f o f t h e w i n t e r the t o t a l s t r a i n change i n a n o r t h e r l y d i r e c t i o n was -1.6 x o r a s t r a i n r a t e o f 1.7 x 10 l"sl. That. i n t h e second h a l f of t h e winter, was -2.0 x o r a s t r a i n r a t e o f 2.9 x 10 -los-1

.

A i r temperature and i c e temperatures a t various depths measured a t t h e centre o f the Sound f o r t h e period 20 January t o 21 May, 1986 are p l o t t e d i n Fig. 3. This time i n t e r v a l was selected t o f a c i l i t a t e comparison o f meteorological data w i t h i c e stress results.

10

AIR AND ICE TEMPERATURE

A I R

-sc J

XI n 1 10 I? 24 1 10 17 n 11 7 1 4 ZI n 5 11 19

JANUARY FEBRUARY IlARCH APR 1 L MAY

! 986

Figure 3 A i r temperature and teniperature a t various depths i n t h e i c e cover f o r the period 1986 January 20 t o May 21.

The i c e conditions on t h e Sound could be characterized as landfast f i r s t - y e a r sea i c e having a very l e v e l surface.

In

November i c e thickness was 0.3 m. By t h e beginning o f March i t was 1.2 m, increasing t o 1.5 m by May. Ice cores taken i n March indicated an average s a l i n i t y o f 5.5

*/..

3

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IAHR Ice

Symposium 1988

Sapporo

t u r e t o be columnar over t h e e n t i r e thickness o f t h e i c e cover. A zone o f i c e about 4 m t h i c k and 8 m wide and hinged t o t h e l e v e l fi rst-year i c e accomodated r e l a t i v e v e r t i c a l movements o f t h e i c e and the wharf due t o tide. This feature, termed t h e ' a c t i v e zone' has been observed previously a t the s i t e (Frederking and Sinha, 1978; Frederking and Nakawo, 1984).

LOAD INTERPRETATION

The r e s u l t s f o r t h e f o u r s t r e s s panels aligned p a r a l l e l t o the wharf were combined t o determine an average e f f e c t i v e l i n e a r load a c t i n g on the wharf. For each panel t h e stress was m u l t i p l i e d by t h e area of the sector on which i t was acting. These loads were summed t o get t h e t o t a l load a c t i n g through t h e depth o f the i c e cover f o r t h e panel width, i n a l l cases 1 m. These r e s u l t s were then averaged t o o b t a i n an average i c e force per u n i t width o f t h e structure. The r e s u l t s o f t h i s c a l c u l a t i o n f o r the period 20 January t o 16 May, 1986 are p l o t t e d i n Fig. 4. The maximum i c e load (averaged over t h e width o f t h e wharf) was 175 kN/m and the average 1 oad was 50 kN/m.

300

-

AVERAGE FORCE

P U 1 10 17 24 1 10 17 24 11 7 I4 21 71 1 I I 13

JANUARY FEBRUARY MRRCH APR I L IlRY

1 9 8 6

- _Figure₄ _{Average i c e f o r c e per meter o f width a c t i n g on Nanisivik wharf} f o r p e r i o d 1986 January 20 t o May 16.

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The ave temperature

IAHR Ice

Symposium 1988

Sapporo

'rage i c e l o a d appears t o be r e l a t e d t o changes i n a i r (compare Fig. 3 and 4). There i s a general correspondence between a i r temperature and i c e load; i.e., an increase i n a i r temperature leads t o an increase i n i c e load and v i c e versa. There i s not, however, a d i r e c t correspondence between t h e magnitude o f t h e a i r temperature change and t h e magnitude o f t h e average i c e l o a d change. For example, t h e a i r temperature increase i n l a t e January was much l a r g e r than t h a t i n l a t e February, b u t t h e i c e l o a d increase i n l a t e February was more than t w i c e t h a t f o r January.

Sanderson (1984a) has provided suggestions f o r d e a l i n g w i t h thermal stresses i n f i r s t y e a r sea i c e covers. Stress, temperature, and time r a t e o f temperature change have a l l been p l o t t e d f o r t h e p e r i o d 16-23 February t o show t h e r e l a t i o n between s t r e s s and i c e temperature (Fig. 5). Time r a t e o f temperature change has been included, s i n c e i c e can behave as a v i s c o - e l a s t i c m a t e r i a l . I t i s t h e r e f o r e reasonable t o examine i c e stress v a r i a t i o n s i n terms o f i c e deformation r a t e also. Only t h e surface and 0.25 m temperatures have been included; Sanderson (1984a) states, and t h e s t r e s s measurements o f t h i s paper i n d i c a t e , t h a t t h e m a j o r i t y o f stress i s c a r r i e d i n t h e t o p 0.5 m o f t h e i c e cover. Taking a c o e f f i c i e n t o f thermal expansion o f 50 x 10 -60c-1 f o r sea i c e (Cox, 1983), s t r a i n changes and s t r a i n r a t e s I n t h e i c e cover can be calculated. The i c e surface

temperature increased by 4'C i n 2 days, which would correspond t o a s t r a i n increase o f

2

x 10'~. The maximum temperature r a t e s o f 4'C/day and P0C/day a t t h e i c e surface and 0.25 m depth correspond t o maximum s t r a i n rates o f 2.3 x 1 0 - ~ s - ' and 1.2 x 1 0 - ~ s - ~ . Taking an i c e temperature o f -12.C. a s a l i n i t y o f 5

'/..,

and applying Sanderson's (1984b) a n a l y s i s f o r columnar- grained ice, secondary creep stresses o f 110 and 90 kPa are obtained f o r each rate. These p r e d i c t e d stresses are much lower than those measured here. The t o t a l s t r a i n change, 2 x and time (about one day) are much smaller i n t h i s case than would be a p p l i c a b l e f o r t h e secondary creep behavi our assumed i n Sanderson's (1984b) analysis. A much more thorough treatment o f thermal stresses i s required.

The c r o s s - c o r r e l a t i o n between t h e s t r e s s from panel No. 3 versus t h e i c e surface temperature and r a t e o f change o f temperature i n t h e i c e cover a t a depth o f 0.25

rn

f o r t h e p e r i o d 16-23 February was determined (Fig. 6).

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IAHR Ice

Symposium 1988

Sapporo

- - - - - - - ..-.( O.25m DEPTH

0

F E B . 16 F E B . 2 3 TIME

Figure 5 I c e temperature and time r a t e o f i c e temperature change a t surface and 0.25 m depth and panel No. 3 stress f o r t h e period 1986 February 16-23.

There i s a high c o r r e l a t i o n c o e f f i c i e n t (about 0.8) f o r stress, lagging t h e 0.25

m

temperature r a t e by about 20 h. There i s a l s o a p o s i t i v e

c o r r e l a t i o n o f 0.6 between s t r e s s and i c e surface temperature, w i t h no phase lag. I n both cases t h e peaks are q u i t e broad, suggesting t h a t t h e r e may be several processes involved. It appears t h a t s t r e s s i s p r i m a r i l y a f u n c t i o n o f both surface temperature and i c e temperature r a t e a t a depth of 0.25 m.

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IAHR

Ice Symposium 1988

Sapporo

-

1

-

80 -

4

0

0 4 0

8

0

P H A S E

L A G ,

h o u r s

F i g u r e 6 R e s u l t s o f c r o s s - c o r r e l a t i o n o f i c e s t r e s s from panel No. 3 versus i c e surface temperature and 0.25 m i c e temperature and r a t e f o r t h e p e r i o d 1986 February 16-23.

CONCLUSION

The g l o b a l i c e loads a c t i n g on t h e wharf a t N a n i s i v i k are r e l a t i v e l y small, l e s s than 175 kN/m. Such low loads were expected, since t h e wharf i s l o c a t e d i n an area w i t h a s t a b l e l a n d f a s t i c e cover. There i s no evidence o f non-simultaneous i c e f a i l u r e i n t h i s instance, since a l l t h e l o a d panels gave s i m i l a r ( i n phase b u t various amplitudes) outputs. This i s n o t s u r p r i s i n g given t h e i ce behaviour o f re1 a t i v e l y u n i f o r m d u c t i l e deformation w i t h no cracks. The primary mechanism o f i c e load generation on t h e N a n i s i v i k wharf appears t o be temperature-induced s t r a i n s i n the i c e cover. A thermal a n a l y s i s based on secondary creep o f t h e i c e p r e d i c t s stresses s u b s t a n t i a l l y lower than those measured. An improved a n a l y t i c a l model i s required.

ACKNOWLEDGEMENTS

F i n a n c i a l support o f Pub1 i c Works Canada i s g r a t e f u l l y acknowledged, as i s t h e t e c h n i c a l assistance o f J.D. Neil, NRCC, and

J.

Egan, PWC.

F i n a l l y , t h e cooperation o f t h e management and s t a f f of N a n i s i v i k Mines Ltd. g r e a t l y f a c i l i t a t e d t h e f i e l d work.

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RENCES

IAHR Ice

Symposium 1988

Sapporo

Cox, G.F.N., 1983. Thermal expansion o f sea ice. Journal o f Glaciology, Vol. 29,

No.

103, pp. 425-432

Frederking, R.M.W. and Nakawo, M., 1984. I c e Action on N a n i s i v i k Wharf, Winter 1979-1980. Can, Geotech. J., Vol. 11, pp. 996-1003

Frederking, R. and Sinha, N.K., 1978. I c e a c t i o n on wharf a t Strathcona Sound. Proc. 4th I n t e r n a t i onal Conference on Port and Ocean Engineering under A r c t i c Conditions, St. John's, Nfld., Vo1. 2, pp. 707-717

Frederking R. and Sayed, M., 1987. N a n i s i v i k i c e pressure measurements

-

Winter 1985-86, Report G9, Geotechnical Section, I n s t i t u t e f o r Research i n Construction, National Research Council o f Canada. December, 1987

Girgrah, M. and Shah, V.K., 1978. Constructton o f a deep-sea dock i n t h e A r c t i c . Proc. 4th I n t e r n a t i o n a l Conference on Port and Ocean Engineering under A r c t i c Conditions, S t . John's, Nfld., Vol. 1, pp. 370-381

Hawkins, J.R., James, D.A., and Der, C.Y., 1983. Design, c o n s t r u c t i o n and i n s t a l l a t i o n o f a system t o measure environmental forces on a caisson r e t a i n e d i s 1 and. VTT Symposium 38, 7th I n t e r n a t i o n a l Conference on Port and Ocean Engineering under A r c t i c Conditions, H e l s i n k i , Finland, Vo1.4, pp. 770-779

P i l k i n g t o n , G.R., Blanchet, D, and Metge, M., 1983. F u l l s c a l e

miasurements o f i c e f o r c e s on an a r t i f i c i a l island. VTT Symposium 38, 7 t h I n t e r n a t i o n a l Conference on Port and Ocean Engineering under A r c t i c

Conditions, H e l s i n k i , Finland, Vol. 4, pp. 818-934

Sanderson, T.J.O., 1984a. Thermal i c e forces against i s o l a t e d structures. Proc IAHR Symposium on Ice, Hamburg, Germany, 27-31 August, 1984, Vol. 4,

pp. 289-299

Sanderson, T.J.O., 1984b. Theoretical and measured i c e f o r c e s on wide structures. Proc IAHR Symposium on Ice, Hamburg, Germany. 27-31 August, 1984, Vol. 4, pp. 151-207