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Laboratory tests on downdrag loads developed by floating ice covers
on vertical piles
Ref
Ser
National Research Conseil national
T H ~
I
+
Council Canada
de recherches Canada
N21d no.
874
I(
IRC
PUB
I
: [
I
,BORATORY TESTS ON DOWNDRAG LOADS
DEVELOPED BY FLOATING ICE COVERS
I
ON VERTICAL PILES
by
R.M.W.
FrederkingReprinted, with permission, from Vol. 2, POAC 79 Proceedings
=fth International Conference on Port and Ocean Engineering under Arctic Conditions
held at the Norwegian Institute of Technology 1 3 . 1 8 August 1979
p. 1097.1110
DBR Paper No. 874
Division of Building Research
SO
MMAIRE
L ' a u t e u r effectue d e s e s s a i s s y s t 6 m a t i q u e s e n l a b o r a t o i r e
a f i n d e d 6 t e r m i n e r l e s e f f e t s d e l a v i t e s s e d e f l b c h i s s e m e n t ,
d e 1 1 6 p a i s s e u r d e l a g l a c e e t du diam'etre d e s p i e u x s u r l e s
c h a r g e s v e r t i c a l e s d e l a glace s u r c e s pieux. L ' a u t e u r
utilise d e s pieux de bois d l u n diam'etre d e
SO
'a
145 m m
d a n s d e s 6 p a i s s e u r s de g l a c e d e
2 5
i
200m m . L e s e s s a i s
s o n t effectu6s 'a d e s v i t e s s e s de f l k c h i s s e m e n t n o m i n a l e s
c o n s t a n t e s . L ' a u t e u r c o n s t a t e une a u g m e n t a t i o n d e l a c h a r g e
s u r l e pieu 'a
m e s u r e q u e s l a c c r o i t l a v i t e s s e de f l 6 c h i s s e -
ment.
L a c h a r g e u n i t a i r e s u r l e s pieux augmente a v e c
l l a c c r o i s s e m e n t d e 1 1 6 p a i s s e u r d e l a g l a c e e t d i m i n u e a v e c
l ' a c c r o i s s e m e n t du d i a m g t r e du pieu.
PORT AND OCEAN ENGINEERING UNDER ARCTIC CONDITIONS. At the Nonvegian Institute og 'fech~~ology.
LABORATORY TESTS ON DOWNDRAG LOADS DEVELOPED BY I'LO/\TlNG ICI COVL.IIS ON VL'IVI ICAI, 1'ILL.S
R . M . W . I r e d e r k i n g , N a t i o n a l R e s e a r c h Canada Research O f f i c e r , C o u n c i l of Canada D i v i s i o n o f B u i l d i n g R e ~ e n r c h ABSTIZACT S y s t e m a t i c laboratory t e s t s wcre c a r r i e d o u t t o d c t e r m i n c t h e c f f c c t s o f d c f l c c t i o n r a t e , i c e t h i c k n e s s and p i l e d i a m e t e r on v e r t i c a l l y a c t i n g i c e l o a d s on p i l e s . Wooden p i l e s w i t h d i a m e t e r s r a n g i n g from 50 t o 145
mm
were u s e d i n i c e t h i c k n e s s e s from 25 t o 200 m m . T e s t s were done a t c o n s t a n t nominal d e f l e c t i o n r a t e s . P i l e l o a d was o b s e r v e d t o i n c r e a s e w i t h i n c r e a s i n g d e f l e c t i o n r a t e . Unit p i l e load 1:)creased w i t h i n c r e a s i n g i c e t h i c k n e s s and d e c r e a s e d w i t h i n c r e a s i n g p i l e d i a m e t e r C o n t c n t s ---
I n t r o d u c t i o n D e s c r i p t i o n o f a p p a r a t u s T e s t p r o c e d u r e T e s t r e s u l t s A n a l y s i s o f r e s u l t s D i s c u s s i o n o f r e s u l t s ~ o r i c l u s i o n sI n t r o d u c t i o n
A f l o a t i n g i c e c o v e r can d e v e l o p s u b s t a n t i a l v e r t i c a l l o a d s on a s t r u c t u r e t o which i t i s f r o z e n a s a r e s u l t o f c h a n g e s i n w a t e r l e v e l . F o r example, drarvdown o f a r e s e r v o i r would g e n e r a t e downward l o a d s w h i l e t h e w a t e r l e v e l i n c r e a s e r e s u l t i n g Eron~ a w l n t e r r a i n f a l l can g e n c r a t c upward-acting l o a d s . S c l c h c s i n l a r g e l a k c s and s t o r m t i d e s i n c o a s t a l w a t e r s may a l s o l e a d t o v e r t i c a l i c e l o a d s . I s o l a t e d p i l e s , p i l e g r o u p s , and long s t r u c t u r e s s u c h a s r e t a i n i n g w a l l s and wharves a r e a l l s u s c e p t ~ b l e t o t h i s s o r t of l o a d i n g .
I I : ~ ~ i : ~ l y ! ; i s v e t I i I I i I s I i s [ 1 1 Ilc ;ISS~IIIIC:J t11;it i c t . h a s a v i s c o p l a s t i c b e l l a v i o u r w i t h d e f o r n l a t i o n p r o p o r t i o n a l t o t h e cube r o o t of t i m e . A means f o r p r e d i c t i n g v e r t i c a l l o a d was g i v e n , b u t no d i r e c t f i e l d o r l a b o r a t o r y measurements were a v a i l a b l e t o c o n f i r m t h e p r e d i c t i o n s . S t e h l e [2] h a s r e p o r t e d measurements of s h o r t - and l o n g - t e r m h o l d i n g s t r e n g t h of wood p i l e s i n i c e . S h o r t - t e r m t a n g e n t i a l a d f r e e z e s t r e n g t h s o f a b o u t 2 MPa were r e p o r t e d f o r p i l e s 70 nun i n d i a m e t e r . Measurements of v e r t i c a l i c e l o a d s developed on i n s t r u m e n t e d p i l e s have been r e p o r t e d b y Doud [ 3 ] . A maximum l o a d o f 80 kN on a 356 mm d i a m e t e r s t e e l p i l e was d e v e l o p e d by a 350 mm t h i c k i c e c o v e r . R e c e n t l y Parameswaran [4] c a r r i c d o u t a l a b o r a t o r y s t u d y of t h e a d f r e e z e bond between v a r i o u s p i l e m a t e r i a l s and f r o z e n ground. An a d f r e e z e s t r e n g t h o f 1 MPa was o b t a i n e d a t a d i s p l a c e m e n t r a t e of 10-5 mm.s f o r B r i t i s h Columbia f i r p i l e s 75 mm i n d i a m e t e r . T h i s b r i e f review i n d i c a t e s t h a t a l t h o u g h t h e r e a r e some u s e f u l d a t a i n t h e l i t e r a t u r e t h e r e i s a need f o r f u r t h e r s y s t e m a t i c i n v e s t i g a t i o n o f v e r t i c a l i c e l o a d s on p i l e s . Some f i e l d t e s t s w i t h c y l i n d r i c a l p i l e s were u n d e r t a k e n b y t h e a u t h o r s e v e r a l y e a r s ago [ 5 ] , b u t w e a t h e r c o n d i t i o n s were s o v a r i a b l e t h a t u n i f o r m t e s t c o n d i t i o n s were d i f f i c u l t t o o b t a i n and it was i m p o s s i b l e t o r e p e a t t e s t s . I t was d e c i d e d , t h e r e -
f o r e , t o s i m u l a t e t h e f i e l d l o a d i n g c o n d i t i o n i n t h e l a b o r a t o r y . Working i n t h e laboratory, s y s t e m a t i c i n v e s t i g a t i o n s o f p i l e d i a m e t e r , i c e t h i c k n e s s , l o a d i n g r a t e , p i l e m a t e r i a l , c t c . , c o u l d be u n d e r t a k e n .
I n n a t u r e a w a t e r l e v e l change i s u s u a l l y t h e i n d e p e n d e n t v a r i a b l e s o i n t h e s e e x p e r i m e n t s t h e r e l a t i v e r a t e o f movement between t h e p i l e and t h e i c e c o v e r was n~ade t h e c o n t r o l l e d v a r i a b l e . T h i s r e l a t i v e movement i s comprised o f bending o f t h e i c e c o v e r and s h e a r i n t h e i c e a d j a c e n t t o t h e p i l e . A t low l o a d i n g r a t e s , however, s h e a r p r e d o m i n a t e s . I t i s t h e r e f o r e p o s s i b l e t o model i n t e r a c t i o n
between a p i l e and t h e i c e i n a t a n k of r e l a t i v e l y s m a l l d i a m e t e r . A c i r c u l a r t a n k was d e s i g n e d i n which a p i l e c o u l d be f r o z e n i n t o a uniform i c e c o v e r . A tempcra- t u r e g r a d i e n t was m a i n t a i n e d i n t h e i c e c o v e r d u r i n g n t e s t . A l o a d i n g frame p l a c e d o v e r t h e t a n k c o n t a i n e d a mechanism f o r ' drawing t h e p i l e up a t a c o n s t a n t r a t e . T h i s p a p e r r e p o r t s t h e r e s u l t s o f l a b o r a t o r y t e s t s on t h e e f f e c t of i c e t h i c k n e s s , p i l e d i a m e t e r and d i s p l a c e m e n t r a t e on downdrag l o a d s developed on a v e r t i c a l wood p i l e .
D e s c r i p t i o n o f a p p a r a t u s
The t a n k , which had a l i n e r o f g a l v a n i z e d s t e e l , was 660
mm
i n d i a m e t e r and 610 mm d r c p . llcnt i n g t : ~ l , c s ( e l c r t r i c : ~ I r c s i s t n n c c t y ] ) c ) wcrc coi l r t l :1ro111id tllc l)oi ton1 . 1 1 1 r l s i d e s o f t h e t a n k ( F i g u r e 1 ) . T h r e e s e p a r a t e t a p e s , e a c h 18 m long and r a t e d a t a t 420 W , were u s e d . Thermocouple t e m p e r a t u r e s e n s o r s f i x e d d i r e c t l y t o t h e bottom and lower s i d e h e a t i n g t a p e s a l l o w e d t h e t e m p e r a t u r e o f e a c h t o b ei n d e p c n d c n t l y c o n t r o l l e d . The b o t t o m and s i d e s of t h e t a n k were i n s u l a t e d t o a t h i c k n e s s o f a b o u t 100 mm w i t h f o a m e d - i n - p l a c e u r e t h a n e . A p r e s s u r e r e l i e f t u b e , c o n t a i n i n g a n e l e c t r i c a l h e a t i n g e l e m e n t , was c o n n e c t e d t o t h e b o t t o m o f t h e t a n k t o e n s u r e t h a t e x c e s s p r e s s u r e d i d n o t b u i l d u p under t h e i c e d u r i n g t h e f r e e z i n g p r o c e s s . A l o a d frame s e t up o v e r t h e t a n k p u l l e d t h e p i l e upwards r e l a t i v e t o t h e i c e b y r e a c t i n g a g a i n s t t h e l i n e r . The o u t e r p e r i m e t e r of t h e i c e c o v e r was f i x e d t o t h e t a n k b y a d h e s i o n t o t h e l i n e r . V e r t i c a l motion was a p p l i e d t o t h e p i l e t h r o u g h a 50 kN c a p a c i t y screw j a c k a c t u a t o r . A v a r i a b l e speed DC motor d r o v e t h e screw jack
V A R I A B L E S P E E D D . C . M O T O R U P P H E A L O W H E A G E A R B O X D l S P L A C E M E N T T R A N S D U C E R S R E L I E F T U B E I N S U L A T I O N 0
-
200 4 0 0 S C A L E , m mF i g . 1 Schematic of i c e growing tank and p i l e loading a p p a r a t u s
through one of two double-reduction speed-reducing g e a r boxes. With t h e low speed gear box, a c t u a t o r speed could b e v a r i e d i n f i n i t e l y o v e r t h e range 7 x mm*s-l
t o 4 x mm-s-l. With t h e high speed g e a r box t h e range was 1.35 x mm-s-I t o 3.5 x mmes-l. C a l i b r a t i o n t e s t s confirmed t h e s e r a t e s under b o t h no-load and load c o n d i t i o n s . Because of f r i c t i o n i n t h e screw jack a c t u a t o r a t o r q u e was ,imposed on t h e p i l e . A t a load of 10 kN t h e t o r q u e was 45 N * m which corresponds
t o 2 kN a t t h e p i l e r a d i u s o f 25 nun. To reduce t h i s e f f e c t a t o r q u e r o d was used t o r e s t r a i n r o t a t i o n of t h e p i l e .
Loads were measured by means of a load c e l l placed between t h e p i l e and t h e screw jack a c t u a t o r (Figure 1)
.
Four displacement t r a n s d u c e r s ( l i n e a r v a r i a b l ed i f f e r e n t i a l t r a n s f o r m e r s ) , p l a c e d on a c r o s s arm a t r a d i i of 100 mm and 250 mm, measured r e l a t i v e displacement between t h e p i l e and t h e t o p s u r f a c e of t h e i c e c o v e r . Load and displacement s i g n a l s were recorded on analogue s t r i p c h a r t r e c o r d e r s and a l s o f e d i n t o a d i g i t a l d a t a a c q u i s i t i o n system f o r r e d u c t i o n and p l o t t i n g of t h e r e s u l t s . I c e growth and i c e temperature during t h e t e s t were monitored v i a a thermocouple probe. I c e t h i c k n e s s e s d u r i n g growth were checked with a r e s i s t a n c e wire t h i c k n e s s gauge [ 6 ] .
The p i l e s were made o f d r y B . C . f i r t u r n e d t o a uniform diameter on a l a t h e . They
were n o t t r e a t e d i n any way and had a s u r f a c e roughness o f k0.1 mm. A threaded r o d
through t h e c e n t r e of t h e p i l e connected i t t o a c o u p l e r on t h e bottom of t h e load c e l l .
Test procedure '
The i c e growth and subsequent t e s t i n g were c a r r i e d out i n a cold room which was
maintained a t -10°C. To model n a t u r e t h e p i l e was s e t i n t h e tank and t h e i c e
grown around i t . Before i n i t i a t i n g i c e growth t h e p i l e was f i x e d t o t h e loading frame and c a r e f u l l y a l i g n e d v e r t i c a l l y .
For e a s e of r e p r o d u c t i o n a type S-2 [ 7 ] columnar-grained i c e cover was grown f o r
; I I I thc. t e s t s . T h i s l y p c of i c r i.; r c p r c s c n t a t i v c of tlic t i : ~ t ~ ~ r : ~ l i c e covcr wlircli
g e n e r a l l y forms on a l a k e . S a t i s f a c t o r y r e p r o d u c t i o n o f t h e same i c e f o r each t e s t
r e q u i r e d c a r e f u l a t t e n t i o n t o growth c o n d i t i o n s . The procedure followed was a s
f o l lows :
The w a t e r i n t h e tank was mixed u n t i l i t had cooled t o a uniform temperature of 4"C, t h e temperature o f maximum d e n s i t y o f w a t e r .
With c o o l i n g below 4 " ~ , w a t e r becomes l e s s dense s o t h a t a tem-
p e r a t u r e g r a d i e n t develops. When t h e water temperature 10 mm
below t h e s u r f a c e reached 1°C t h e t a n k was seeded. Seeding was
done by s i e v i n g ( 0 . 8 mm opening) f i n e - g r a i n e d snow a t - 1 0 " ~ onto
t h e w a t e r s u r f a c e . T h i s i n i t i a t e d t h e formation of a f i n e - g r a i n randomly-oriented s u r f a c e l a y e r of i c e which w i t h i n a couple of c e n t i m e t r e s was transformed i n t o a columnar s t r u c t u r e i n which t h e c r y s t a l l o g r a p h i c symmetry a x e s were randomly o r i e n t e d t e n d i n g
t o be i n t h e h o r i z o n t a l p l a n e . The i c e cover grew a t a r a t e of
about 25 mm p e r day. To maintain a continuous columnar s t r u c t u r e ,
h e a t was a p p l i e d through t h e h e a t i n g t a p e s t o keep t h e water
temperature a t t h e bottom of t h e tank a t between 3.5 and 4°C. I c e
of r e p r o d u c i b l e c h a r a c t e r i s t i c s could be formed i n t h i s f a s h i o n .
A t 15 mm depth t h e average g r a i n s i z e was 1 . 5 mm and a t 80 mm, 4 mm. The i c e
t h i c k n e s s i n t h e tank was q u i t e uniform w i t h no t h i c k e n i n g a d j a c e n t t o t h e p i l e . The c o l d room used f o r t h e t e s t s had a f a i r l y low humidity so s e v e r a l m i l l i m e t r e s of i c e sublimated o f f t h e s u r f a c e d u r i n g t h e ice-growing p e r i o d . The normal i c e
t h i c k n e s s f o r t h e m a j o r i t y of t h e t e s t s was 80 mm so about two t e s t s could b e r u n
each week.
Before running a t e s t t h e displacement t r a n s d u c e r s , load c e l l and e l e c t r o n i c s were
s e t up a11d allowed t o s t a b i l i z e f o r a t l e a s t an hour. T e s t s were run a t a nominal
c o n s t a n t r a t e .
T e s t r e s u l t s
Over 20 t e s t s were c a r r i e d o u t t o i n v e s t i g a t e e f f e c t s o f d e f l e c t i o n r a t e , i c e t h i c k n e s s and p i l e diameter on v e r t i c a l p i l e l o a d s . Continuous r e c o r d s o f load and d e f l e c t i o n s v e r s u s time were made f o r a l l t e s t s . Temperature p r o f i l e s of t h e
i c e cover were a l s o measured i n each c a s e . A l l t h e t e s t c o n d i t i o n s and r e s u l t s a r e
~ ) t - ( ; \ c n t c J i l l 'l'nhlc 1 . I ) c ~ t : r i l s o f tlic iriEornratjon i n t h c t ; l l ) l c w i l l Ilc disc-ussctl
l a t e r i n t h i s p a p e r .
To i l l u s t r a t e t h e l o a d - and displacement-time behaviour t h e r e s u l t s from t e s t No. 7
a r e p l o t t e d i n F i g u r e 2 . The f i r s t p o i n t e v i d e n t from t h e s e r e s u l t s i s t h a t t h e
a c t u a l displacement r a t e o f t h e p i l e w i t h r e s p e c t t o t h e i c e cover i s not c o n s t a n t . In t h e example i l l u s t r a t e d i n Figure 2 t h e displacement r a t e i n c r e a s e d c o n t i n u o u s l y
reaching a v a l u e of 1 . 4 3 x N O I I - S - ~ a t y i e l d and o n l y approached t h e nominal
r a t e of 2 . 4 x mm*s-l i n t h e p o s t y i e l d r e g i o n . C h a r a c t e r i z i n g t h e t e s t w i t h a
s i n g l e displacement r a t e i s q u e s t i o n a b l e . Examining t h e load-time curve i n
TABLE I : RESULTS OF LABORATORY TESTS WITH WOODEN PILES
Nominal
D e f l e c t i o n P i l e I c e Maximum
Test Rate, Diameter, Thickness, Load,
-
1 No.mm.
s mm mm kN Time t o Maximum Load Load, Rate, - 1 s N-s*Test a b o r t e d due t o i c e s l i p p a g e on tank w a l l .
Figure 2 s u g g e s t s t h a t t h e t e s t c o n d i t i o n could b e r e a s o n a b l y approximated a s a c o n s t a n t loading r a t e . The displacement- and load-time behaviour of t e s t No. 7 was g e n e r a l l y t y p i c a l o f a l l t e s t s i n t h e s e r i e s .
I c e s u b j e c t e d t o u n i a x i a l compressive loading a t c o n s t a n t nominal cross-head r a t e s on a conventional t e s t m a ~ h i n e shows load- and displacement-time behaviour [ 8 ] s i m i l a r t o t h a t e x h i b i t e d
ip
< t h i s s e r i e s . This behaviour i s c o n t r o l l e d by t h e r e l a t i v e s t i f f n e s s o f t h e t e s t machine and t h e i c e specimen and i s t h e s u b j e c t of a n o t h e r paper t o b e p r e s e n f e d a t t h i s conference [ 9 ] .~ ! ~ u ; e 2 a l s o s e r v e s t o i l l u s t r a t e t h e f a i l u r e process o f t h e i c e s h e e t and t h e maximum i c e load developed on t h e p i l e . As t h e p i l e i s d i s p l a c e d upward w i t h r e s p e c t t o t h e i c e t h e load i n c r e a s e d monotonically and t e n s i l e r a d i a l and
T I M E , s h = 103 m m L O A D R A T E 6 . 3 N - s-l I I I
I
P I L E D E F L E C T I O N M E A S U R E D R E L A T I V E T O I C E S U R F A C E A T 100 m m R A D I U S- - -
-
250 m m R A D I U S-
-
T I M E , sFiPg. 2 R e s u l t s o f t e s t No. 7 . Nominal d e f l e c t i o n r a t e : 2 . 4 x mm-s-l; p i l e d i a m e t e r : 50 mm; i c e t h i c k n e s s : 103
mm
observed t o g r a d u a l l y s e p a r a t e from t h e t o p of t h e wood p i l e . A t a load l e v e l o f about 1 kN t h e f i r s t v e r t i c a l micro c r a c k s ( 1 mm o r s m a l l e r ) appeared w i t h i n 10 t o
20 mm of t h e p i l e . They t e n d t o b e v e r t i c a l l y a l i g n e d , i . e . , p a r a l l e l t o t h e L O I I I I I . I % C ~ W C C I I LL 10;lil oi- 3 ; i r ~ J 4 k N sm:il 1 "p1:itc" c r a c k s , o r i e n t c ~ l pasallel t o t h e i c e s u r f a c e began t o develop on a p l a n e midway through t h e i c e cover a d j a c e n t t o t h e p i l e . The s e p a r a t i o n of t h e i c e from t h e p i l e a t t h i s time extended down about 10 mm. The next event i n t h e f a i l u r e p r o c e s s was t h e a b r u p t development o f r a d i a l c r a c k s which extend through t h e f u l l depth of t h e i c e and r a n from t h e p i l e o u t t o t h e tank w a l l . An a b r u p t drop i n p i l e load was a s s o c i a t e d w i t h t h e formation of t h e s e r a d i a l c r a c k s . There i s , however, no corresponding i n f l e c t i o n i n t h e d e f l e c t i o n - t i m e curve. As t h e p i l e c o n t i n u e s t o move t h e load a g a i n i n c r e a s e s while a t t h e same time t h e " p l a t e " c r a c k s g r a d u a l l y c o a l e s c e i n t o a c o n i c a l f a i l u r e s u r f a c e . A c h a r a c t e r i s t i c s y i e l d - t y p e maximum load o c c u r s with t h e e x t e n s i v e development o f t h e c o n i c a l f a i l u r e s u r f a c e . A c r o s s - s e c t i o n of t h e c o n i c a l f a i l u r e s u r f a c e i s shown i n Figure 3 . In t h e c a s e of t e s t No. 7 t h e
F i g . 3 C r o s s - s e c t i o n of c o n i c a l f a i l u r e s u r f a c e
maximum p i l e load was a s s o c i a t e d with y i e l d ; i n some t e s t s , however, t h e subsequent y i e l d load was lower t h a n t h e i n i t i a l c r a c k l o a d .
The f a i l u r e p r o c e s s i n each t e s t i s i n d i c a t e d b y t h e p r e f i x and s u f f i x codes i n t h e Maximum Load column of Table I . The p r e f i x c i n d i c a t e s t h a t r a d i a l c r a c k s o c c u r r e d b e f o r e t h e maximum load was r e a c h e d . The s u f f i x y i n d i c a t e s t h a t maximum load was a s s o c i a t e d w i t h y i e l d behaviour; t h e s u f f i x c i n d i c a t e s maximum l o a d corresponded t o r a d i a l c r a c k o c c u r r e n c e . The prime on t h e s u f f i x c i n d i c a t e s t h a t t h e l o a d d r o p s a b r u p t l y t o n e a r zero a f t e r t h e c r a c k a t maximum load o c c u r s .
Typical r e s u l t s f o r t h e i c e temperature p r o f i l e s from t e s t No. 13 a r e shown i n Figure 4 . The i n i t i a l average i c e temperature was -2.0°C and it i n c r e a s e d t o -1.7OC over t h e 5000 s i n t e r v a l o f t h e t e s t . In most c a s e s t h e t e s t time was s h o r t e r and t h e temperature i n c r e a s e was l e s s .
A n a l y s i s o f r e s u l t s
The t e s t r e s u l t s have been analysed i n terms of d e f l e c t i o n r a t e , i c e t h i c k n e s s and p i l e diameter e f f e c t s .
( i ) D e f l e c t i o n r a t e e f f e c t s
In c o n s i d e r i n g t h e e f f e c t o f d e f l e c t i o n r a t e on v e r t i c a l p i l e l o a d s t h e load r e s u l t s have been reduced i n terms of an adhesion s t r e n g t h , T, d e f i n e d a s
where P i s maximum v e r t i c a l i c e load on t h e p i l e , d i s p i l e diameter and h i s i c e t h i c k n e s s . The adhesion s t r e n g t h s measured i n t h e l a b o r a t o r y t e s t s a t c o n s t a n t nominal d e f l e c t i o n r a t e ,
8 ,
a r e p l o t t e d i n F i g u r e 5. Also shown i n t h e f i g u r e a r e t h e r e s u l t s o f f i e l d t e s t s [5] c a r r i e d o u t under a s t a t e of c o n s t a n t l o a d . The t e s t c o n d i t i o n s o f t h e s e d a t a cover q u i t e a broad range; i c e t h i c k n e s s 25 t o 500 mm and p i l e diameter 50 t o 150 mm; b u t a g e n e r a l t r e n d of i n c r e a s i n g s t r e n g t h with i n c r e a s i n g nominal d e f l e c t i o n r a t e i s c l e a r l y e x h i b i t e d . A r e g r e s s i o n a n a l y s i s gave t h e f o l l o w i n g b e s t f i t e q u a t i o n t o t h e l a b o r a t o r y t e s t d a t aT E M P E R A T U R E .
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I 1 I I 1 I I I F i g . 4 I c e temperature p r o f i l e , t e s t No. 1 3 ( i i ) I c e t h i c k n e s s e f f e c t sThe next s t a g e of t h e a n a l y s i s o f t h e d a t a was t o examine t h e l a b o r a t o r y t e s t r e s u l t s i n terms of i c e t h i c k n e s s . The adhesion s t r e n g t h s f o r a 50 mm d i a m e t e r p i l e have been c l a s s i f i e d i n t o two groups by i c e t h i c k n e s s and p l o t t e d v e r s u s
nominal d e f l e c t i o n r a t e i n Figure 6 . A l i n e i s drawn through each group of r e s u l t s t o h e l p c l a r i f y t h e d i f f e r e n c e between them. I t i s seen t h a t i n a d d i t i o n t o a d e f l e c t i o n r a t e e f f e c t t h e r e i s an i n c r e a s e i n adhesion s t r e n g t h w i t h i n c r e a s i n g i c e t h i c k n e s s .
A s p e c i f i c s e r i e s o f t e s t s t o examine i c e t h i c k n e s s e f f e c t s was c a r r i e d o u t a t a c o n s t a n t nominal d e f l e c t i o n r a t e of 2 . 4 x mm-s-l w i t h a 50 mm diameter p i l e . The r e s u l t s of t h i s s e r i e s , given i n F i g u r e 7 show t h a t t h e adhesion s t r e n g t h
i n c r e a s e s w i t h approximately t h e s q u a r e r o o t of t h e i c e t h i c k n e s s . A r e g r e s s i o n a n a l y s i s gave t h e f o l l o w i n g b e s t f i t t o t h e d a t a
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1F i g . 5 Adhesion s t r e n g t h v s nominal d e f l e c t i o n r a t e f o r l a b o r a t o r y and f i e l d t e s t s w i t h wooden p i l e s &----A 60 - 100 rnm
3 -
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0 150-
200 m m C1 ,. I I I 1 1 1 1 1 1 I 1 I 1 I 1 1 1 1 I I I I I I I l F i g . 6 D e f l e c t i o n r a t e dependence o f a d h e s i o n s t r e n g t h o f 50 mm d i a m e t e r wood p i l e i n i c e . (Note: numbers a d j a c e n t t o d a t a p o i n t s r e f e r t o t e s t number o f T a b l e I . )0 1 I I I I I
-
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P l L E D I A M E T E R 5 0 m m N O M I N A L D E F L E C T I O N R A T E 2 . 4 x m m .r
-
1 1 I I 1 1 1 1 I I I 1 1 I J I 10 100 1000 I C E T H I C K N E S S , m mF i g .
7
Effect ~ l f i c e t h i c k n e s s on adhesion strengthP l L E D I A M E T E R
-
2, m mF i g . 8 E f f e c t of p i l e d i a m e t e r on adhesion s t r e n g t h
where h i s i n mm and -r i s i n MPa. ( i i i) P i l e d i a m e t e r e f f e c t s
A v e r y l i m i t e d number o f t e s t s were c a r r i e d o u t t o i n v e s t i g a t e t h e n a t u r e of t h e p i l e d i a m e t e r i n f l u e n c e on a d h e s i o n s t r e n g t h . They were done a t a nominal d e f l e c - t i o n r a t c o f 2 . 4 x m m - s - l and an a v e r a g e i c e t h i c k n e s s of 80 m m . These r e s u l t s show t h a t t h e a d h e s i o n s t r e n g t h d e c r e a s e s w i t h i n c r e a s i n g p i l e d i a m e t e r ( F i g u r e 8 ) . A r e g r e s s i o n a n a l y s i s gave t h e f o l l o w i n g b e s t f i t t o t h e d a t a
where d i s i n mm and .r i s i n MPa. ( i v ) S t r e s s r a t e
A s a l r e a d y p o i n t e d o u t , t h e l a b o r a t o r y t e s t s c o u l d p r o b a b l y b e d e s c r i b e d more a c c u r a t e l y a s c o n s t a n t l o a d i n g r a t e t e s t s . With t h i s i n mind t h e r e s u l t s f o r t h e
50 mm d i a m e t e r p i l e were r e p l o t t e d i n t e r m s o f t h e l o a d i n g s t r e s s r a t e i n F i g u r e 9 .
A s t r a i g h t l i n e h a s been drawn t h r o u g h each of two i c e t h i c k n e s s g r o u p s .
I 1 I I 1 1 1 1 l I I I I I T I I I
-
-a A B R U P T F A I L U R E-
-
0-
I C E T H I C K N E S S 0 & - - - a 60 - 100 m m-
o 0 1 5 0-
200 m m I L I 1 1 1 1 1 1 I L t I 1 1 I l l I 1 I 1 I 1 1 1 L O A D S T R E S S R A T E , M P a - r - ' F i g . 9 S t r e s s r a t e dependence o f a d h e s i o n s t r e n g t h of 50 mm d i a m e t e r wood p i l e i n i c e . (Note: numbers a d j a c e n t t o d a t a p o i n t s r e f e r t o t e s t number of T a b l e I . )Discussion of r e s u l t s
The r e s u l t s of t h e l a b o r a t o r y t e s t s show a v e r y d e f i n i t e d e f l e c t i o n r a t e e f f e c t on adhesion s t r e n g t h . There i s a g e n e r a l i n c r e a s e i n s t r e n g t h with i n c r e a s i n g r a t e up t o about 5 x mm-s-l. Parameswaran [ 4 ] has observed s i m i l a r behaviour with 75 mm diameter B . C . f i r p i l e s i n f r o z e n sand. Over a deformation r a t e range
s i m i l a r t o t h a t of t h i s study he found t h e exponent of t h e power law r e l a t i o n t o be 0.22 compared w i t h t h e v a l u e of 0.175 found i n t h i s s t u d y . For deformation r a t e s g r e a t e r t h a n 5 x mm-s-l, i n t h e r e g i o n of a b r u p t f a i l u r e , t h e adhesion s t r e n g t h appears t o remain c o n s t a n t . T h i s i s s i m i l a r t o t h e s t r e n g t h behaviour of i c e s u b j e c t e d t o u n i a x i a l compressive loading [ 8 ] .
A g e n e r a l t r e n d o f d e c r e a s i n g a d h e s i v e s t r e n g t h with d e c r e a s i n g i c e t h i c k n e s s has been o1)served. T h i s i s t o be expected s i n c e f o r t h i n i c e t h e f l e x u r a l s t r e s s , which v a r i e s with 1 / h 2 , i s r e l a t i v e l y g r e a t e r t h a n t h e adhesion s t r e s s , which
v a r i e s w i t h l / h . T h e r e f o r e w i t h t h e t h i n n e r i c e f l e x u r a l f a i l u r e would predominate and t h c a s s o c i a t e d adhesion s t r e n g t h would be r e l a t i v e l y lower. The t h e o r e t i c a l
w o r k o f Mohaghcgh and Coon 1101 a l s o s u p p o r t s t h i s t r e n d of d e c r e a s i n g adhesive s t r e n g t h with d e c r e a s i n g i c e t h i c k n e s s f o r t h e c a s e of t h i c k c i r c u l a r p l a t e s . Adhesion s t r e n g t h was observed t o d e c r e a s e w i t h i n c r e a s i n g p i l e d i a m e t e r . T h i s same t r e n d was p r e d i c t e d t h e o r e t i c a l l y by Lofquist [ l ] f o r p i l e s f r o z e n i n t o i n f i n i t e i c e s h e e t s and by Mohaghegh and Coon [ l o ] f o r c i r c u l a r p l a t e s .
Combining d e f l e c t i o n r a t e , i c e t h i c k n e s s and p i l e diameter e f f e c t s of t h e c u r r e n t t e s t s , i . e . , Equations ( 2 ) , ( 3 ) and ( 4 ) , t h e f o l l o w i n g composite e m p i r i c a l
equation can he formulated
[
.
0.175 -0.79 0.44-r
= 0.67 ti1
[A)
[15tmm]
1
~
~
~
(5)1
5 x 10-3 mm*s-1 I f o r t h e f o l l o w i n g ranges 1 0 - I ' mm-s-1-
< rf--
< 5 x 10-3 n~m-s-l 50 mm- -
< d < 150 mrn 25 mm- -
< h < 150mm
E x t r a p o l a t i n g beyond t h e s e ranges should o n l y be done with g r e a t c a u t i o n . The t e s t d a t a suggest t h a t f o r d e f l e c t i o n r a t e s g r e a t e r t h a n 5 x mm-s-l t h e r e i s no
f u r t h e r i n c r e a s e i n adhesion s t r e n g t h . I t should a l s o be pointed out t h a t t h e adhesion s t r e n g t h s given by Equation (5) a r e maximum o r upper bound v a l u e s which a r e o n l y achieved when s u f f i c i e n t d e f l e c t i o n has occurred.
Observations o f t h e f a i l u r e p r o c e s s f o r t h i s t e s t s e r i e s showed t h a t a combination of f l e x u r a l and s h e a r behaviour was p r e s e n t i n a l l c a s e s . The r e l a t i v e c o n t r i b u - t i o n s v n r i c d with t e s t c o n d i t i o n s , f l e x u r e predominating f o r t h e t h i n n e r i c e s h e e t s
i t l i ~ l s11c;11- f o r t h c t l ~ i c k e r i c e . I n t h e 150 n r l r ~ t h i c k i c e s h e c t s t h e c o n i c a l f a i l u r e
s u r f a c e s t a r t e d t o develop b e f o r e any r a d i a l c r a c k s formed. Radial c r a c k s were observed t o be a p a r t of t h e f a i l u r e p r o c e s s except f o r d e f l e c t i o n r a t e s l e s s than 4 x mm-s-l, i n which c a s e s h e a r predominated. I n t h e r e g i o n of abrupt f a i l u r e ,
6
> 5 x 10- mm-s- l , r a d i a l c r a c k i n g preceded t h e peak load, b u t f i n a l f a i l u r e was a s s o c i a t e d with i n s t a n t a n e o u s development of t h e c o n i c a l f a i l u r e s u r f a c e .I t i s u s e f u l t o compare t h e p r e d i c t i o n s of Equation (5) with information i n t h e l i t e r a t u r e . L o f q u i s t l s [ l ] t h e o r y i n d i c a t e s a maximum adhesive s t r e n g t h of 0.24 MPa f o r a 200 mm diameter p i l e and a200
mm
i c e t h i c k n e s s . For s i m i l a r c o n d i t i o n s Equation (5) p r e d i c t s an adhesion s t r e n g t h of 0.25 MPa. S t e h l e [2] measured r a p i dl o a d i n g adhesion s t r e n g t h of 1 . 7 MPa f o r 75 mm diameter p i l e s f r o z e n i n t o f r e s h water i c c of 250 mm t h i c k n e s s whereas Equation ( 5 ) p r e d i c t s an adhesion s t r e n g t h of
1 1 I t s l ~ o u l d 1)c notcd howcvc,r t h : ~ t S t c h l c ' s 1o:lding was pure s h e a r wit11 no
bending so h e r adhesion s t r e n g t h s would be expected t o be h i g h e r .
Conclusions
1 . Adhesion s t r e n g t h i n c r e a s e s w i t h d e f l e c t i o n r a t e up t o a p o i n t and then appears t o remain c o n s t a n t .
2 . Adhesion s t r e n g t h d e c r e a s e s w i t h d e c r e a s i n g i c e t h i c k n e s s .
3. Adhesion s t r e n g t h d e c r e a s e s with i n c r e a s i n g p i l e diameter.
4. The downdrag l o a d s developed on wood p i l e s by a f l o a t i n g i c e cover a r e a f u n c t i o n of b o t h f l e x u r a l and s h e a r f a i l u r e i n t h e i c e .
Acknowledgements
The, a s s i s t a n c e o f D . Wright a n d 'T'. Iloogcvcen, Technical O f f i c e r s . DRR/NRC, in d e s i g n i n g and c o n s t r u c t i n g t h e a p p a r a t u s , and c a r r y i n g o u t t h e t e s t s 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 o f B u i l d i n g Research, National Research Council o f Canada and i s published w i t h t h e approval of t h e D i r e c t o r of t h e D i v i s i o n .
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h e a t e d wire t e c h n i q u e . T tiland Waters Direc.torate, Environment Canada, Ottawa, Canada.Technical B u l l e t i n No. 88, 1975.
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E f f e c t of t e s t system s t i f f n e s s on s t r e n g t h of i c e . To b e p r e s e n t e d a t POAC
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