READ THESE TERMS AND CONDITIONS CAREFULLY BEFORE USING THIS WEBSITE. https://nrc-publications.canada.ca/eng/copyright
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.
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
Evaluating strength tests from foundation failures/ resistance au
cisaillement: evaluation des essais
Bozozuk, M.
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.
NRC Publications Record / Notice d'Archives des publications de CNRC:
https://nrc-publications.canada.ca/eng/view/object/?id=61d971f7-a04d-4fea-ad05-4ba6c7a5c083
https://publications-cnrc.canada.ca/fra/voir/objet/?id=61d971f7-a04d-4fea-ad05-4ba6c7a5c083
Ser
TH1
N21d
no.
747
c.
2
$
Council Canada
National Research
Conseil national
de
recherches
Canada
4 N 4 ~ ~ ~ E D
EVALUATING STRENGTH TESTS FROM
FOUNDATION FAILURES
Reprinted, with permission, from
11
BUILD!P!G
RESEARCH
Proceedings,
IXth
International Conference on
-
LiBRARY
-
Soil
Mechanics and Foundation Engineering
i
I
B
VOL
I,
1977
1;DEC
12
1977
Ip.
55
-
59
'#DBR Paper No.
747
Division of Building Research
SO MMAIRE
L a r u p t u r e de l a capacitd portante des s o u s - s o l s de t r o i s s i l o s -
t o u r s a 6th analysde & l1aide dlun scissom'etre in s i t u e t d l e s s a i s
de r e s i s t a n c e triaxiale s u r d e s dchantillons d l a r g i l e obtenus avec
d e s dprouvettes cylindriques 'aparois minces. Les enqu8tes effec-
tuds en Ontario, au Canada, s u r un s i l o dlune hauteur de 15m
reposant s u r de l ' a r g i l e stratifide
'a
New L i s k e a r d , puis s u r un
s i l o dlune hauteur de 21m reposant s u r de l l a r g i l e marine con-
solidde presque normalement
'a
Vankleek Hill, e t s u r u n s i l o dlune
hauteur de 32m reposant s u r d e l 1 a r
gile surconsolid6e pr'es de
Richmond.
Une historique de c a s p e r m e t l'dvaluation de c e s
mdthodes d l e s s a i pour m e s u r e r l a r e s i s t a n c e au cisaillement
d e s s o l s de fondations.
L e s analyses ont montrd que l e scissom'etre d e llInstitut norvdgien
de gdotechnique a mesurd l e s r e s i s t a n c e s au cisaillement moy-
ennes i n s i t u addquatement e t que c e s r d s i s t a n c e s peuvent S t r e
utilisdes pour p r d d i r e l a capacitd portante ultime. En compa-
r a i s o n l e s e s s a i s de r d s i s t a n c e triaxiale sous-estimaient l e s
r d s i s t a n c e s m e s u r d e s avec l e scissom'etre dans l l a r g i l e s t r a -
tifide e t dans l l a r g i l e marine f i s s u r d e ou dessech6e.
Sous l e
s o l dessdchd l e s r e s i s t a n c e s d e pointe t r i a x i a l e s excddaient l e s
r d s i s t a n c e s du scissom'etre, mais lee valeurs dlapr'es pointe
dtaient considdrablement plus b a s s e s . L e s e s s a i s d e r 6 s i s t a n c e
'a l a compression sans contrainte laterale ont sous-estimh con-
sequemment l e s m e s u r e s i n s i t u du cisaillement. Les r a i s o n s
pour le comportement du scissom'etre qui apparait satisfaisant
pour p r d d i r e l a capacitd portante ultime s o n t discutdes.
Evaluating Strength Tests from Foundation Failures
Resistance au Cisaillement', Evaluation des Essais
.hr4
v z ~ ~
M.BOZOZUK
Geotechnical Section, Division of Building Research, National Research Council of Canada, CanadaSYNOPSIS Three b e a r i n g c a p a c i t y f a i l u r e s o f tower s i l o s have been analysed using i n s i t u vane and t r i a x i a l s t r e n g t h t e s t s on thin-wall p i s t o n tube samples o b t a i n e d from t h e foundation c l a y s . The i n v e s t i g a t i o n s were c a r r i e d o u t i n O n t a r i o , Canada, on a 15-m high s i l o on varved c l a y a t New Liskeard, on a 21-m high s i l o on n e a r - normally c o n s o l i d a t e d marine c l a y a t Vankleek H i l l , and on a 32-m high s i l o on o v e r c o n s o l i d a t e d marine c l a y n e a r Richmond. Case records p e r m i t t e d e v a l u a t i o n o f t h e s e t e s t methods f o r measuring s h e a r s t r e n g t h o f t h e foundation s o i l s .
Analyses showed t h a t t h e NGI vane measured t h e average s h e a r s t r e n g t h s adequately i n s i t u and t h a t t h e s e s t r e n g t h s can be used t o p r e d i c t u l t i m a t e b e a r i n g c a p a c i t y . I n comparison, t r i a x i a l s t r e n g t h t e s t s underestimated t h e vane t e s t s t r e n g t h s i n t h e varved c l a y s and i n t h e f i s s u r e d o r d e s i c c a t e d marine c l a y . Below t h e d e s i c c a t e d s o i l t h e t r i a x i a l peak s t r e n g t h s exceeded t h e vane s t r e n g t h s , b u t t h e post-peak values were considerably lower. The unconfined t e s t s c o n s i s t e n t l y underestimated t h e i n s i t u measurements. The reasons f o r t h e apparent s a t i s f a c t o r y performance o f t h e vane f o r p r e d i c t i n g u l t i m a t e b e a r i n g c a p a c i t y a r e d i s c u s s e d .
INTRODUCTION NEW LISKEARD
For s o i l t o s u p p o r t a s t r u c t u r e , a s h e a r i n g r e s i s t a n c e equal t o t h e a p p l i e d s t r e s s e s must be mobilized with- i n t h e s o i l mass. The maximum r e s i s t a n c e t h a t can b e mobilized a t any p o i n t w i t h i n t h e mass i s equal t o t h e peak s h e a r s t r e n g t h . Both t h e peak s t r e n g t h and t h e s t r a i n a t which i t occurs u s u a l l y vary from p o i n t t o p o i n t and with d i r e c t i o n o f a p p l i e d s h e a r s t r e s s . To mobilize the r e q u i r e d s h e a r i n g r e s i s t a n c e , t h e r e - f o r e , d i f f e r i n g s t r a i n s must develop i n p r o g r e s s i o n along t h e c r i t i c a l p l a n e . The s o i l may then b e s t r e s s e d t o post-peak, peak, and pre-peak v a l u e s , depending upon t h e magnitude o f t h e a p p l i e d load and l o c a t i o n along t h e c r i t i c a l s h e a r p l a n e . Consequently,
i t i s impossible t o develop t h e peak s h e a r i n g r e s i s t a n c e simultaneously a l o n g the e n t i r e s h e a r i n g s u r f a c e . ?he u l t i m a t e b e a r i n g c a p a c i t y o f t h e s o i l i s n o t r e l a t e d t o t h e peak s t r e n g t h b u t t o some "average" s t r e n g t h i n t e r m e d i a t e between t h e peak and post-peak o r residua2 s t r e n g t h o f t h e s o i l .
In a n a l y s i n g t h r e e b e a r i n g c a p a c i t y f a i l u r e s o f tower s i l o s c o n s t r u c t e d on d i f f e r e n t c l a y s o i l s i n O n t a r i o , s h e a r s t r e n g t h s were measured i n s i t u with a
55 x 110 mm NGI vane (Anderson and Bjerrum, 1956) and i n t h e l a b o r a t o r y on undisturbed s o i l samples o b t a i n e d with a 54-mm d i a t h i n - w a l l p i s t o n tube sampler
(Bjerrum 1954). These t e s t r e s u l t s were used t o p r e d i c t u l t i m a t e b e a r i n g c a p a c i t y , which could then be compared with t h e known b e a r i n g c a p a c i t y of t h e s o i l a t f a i l u r e . The f i e l d vane t e s t , s e v e r e l y c r i t i c i z e d by Schmertmann (1975), LaRochelle e t a l . (1973, 1974) and Roy (1975), h a s provided t h e most reasonable value o f i n s i t u "average" s h e a r s t r e n g t h f o r
determining t h e u l t i m a t e b e a r i n g c a p a c i t y o f t h e s o i l s . This paper compares t h e r e s u l t s o f t h e s e s t r e n g t h t e s t s and d i s c u s s e s t h e reasons f o r t h e apparent s a t i s f a c t o r y performance o f t h e vane.
%
In J u l y 1961 a 15.24-111 h i g h , 6.10-m d i a p r e c a s t c o n c r e t e s t a v e s i l o 6 km n o r t h e a s t o f New Liskeard had j u s t been f i l l e d f o r t h e f i r s t time with hay s i l a g e when i t f a i l e d suddenly as a r e s u l t o f a b e a r i n g c a p a c i t y f a i l u r e (Eden and Bozozuk, 1962) . The r e s u l t s o f f i e l d and l a b o r a t o r y t e s t s on s o i l s from t h e s i t e a r e summarized i n Figure 1. The s o i l p r o f i l e (Figure l a ) showed a weathered c r u s t t o a depth o f 1 . 5 m, u n d e r l a i n by l a y e r s o f s o f t , s e n s i t i v e brown c l a y s and grey varved c l a y s t o a depth o f 6.7 m. Below t h i s t h e sample b o r i n g t r a v e r s e d grey varved c l a y s t o a depth of 14 m. These c l a y s a r e d e p o s i t s o f g l a c i a l Lake Barlow.
C l a s s i f i c a t i o n t e s t s (Figure l b ) on t h e s o i l a t depths from 2 t o 6 m show a p l a s t i c i t y index of about 40 p e r c e n t , a l i q u i d l i m i t o f about 60 p e r c e n t , and a n a t u r a l w a t e r c o n t e n t o f about 80 p e r c e n t . These v a l u e s decreased below 7 m.
The s h e a r s t r e n g t h s shown on Figure l c a r e from i n - s i t u vane t e s t s , l a b o r a t o r y unconfined undrained
(UU)
,
and c o n s o l i d a t e d i s o t r o p i c a l l y undrained (CIU) t r i a x i a l s t r e n g t h t e s t s on u n d i s t u r b e d s o i l samples. The UU and CIU t e s t s were performed a t s t r a i n r a t e s of 1 p e r c e n t p e r minute and 2 p e r c s n t p e r hour, r e s p e c t i v e l y . The c o n f i n i n g p r e s s u r e s f o r t h e CI Ut e s t s were made equal t o t h e e f f e c t i v e overburden p r e s s u r e . Proving r i n g s were used t o measure t h e a p p l i e d a x i a l l o a d s .
The e s t i m a t e d t o t a l weight o f t h e s i l o and i t s c o n t e n t s when f i l l e d was 449 tonnes (hay s i l a g e 314 tonnes, s t r u c t u r e and foundation 135 t o n n e s ) . Using t h e e q u a t i o n f o r u l t i m a t e b e a r i n g c a p a c i t y (Skempton, 1951):
1/12 5 0 1 1 P R O F I L E W A T E R C O N T E N T , % S H E A R S T R E N G T H TI, k P a T R l A X l A L C I U S T R E N G T H T E S T S A X I A L f T R & I N e l , Rb ( d l F i g u r e 1 Engineering P r o p e r t i e s o f S o i l a t New L i s k e a r d where q U = u l t i m a t e b e a r i n g c a p a c i t y o f t h e s o i l c = a v e r a g e s h e a r s t r e n g t h o f t h e s o i l t o a d e p t h below t h e f o u n d a t i o n e q u a l t o 2 / 3 o f t h e o u t s i d e d i a m e t e r o f t h e f o u n d a t i o n N = s h a p e f a c t o r P = overburden p r e s s u r e above t h e b a s e o f t h e f o u n d a t i o n and u s i n g a l s o t h e a v e r a g e s h e a r s t r e n g t h o f t h e s o i l measured i n s i t u w i t h t h e vane, t h e u l t i m a t e b e a r i n g c a p a c i t y o f t h e s o i l was determined t o b e 4 4 7 t o n n e s . T h i s gave a f a c t o r o f s a f e t y o f 1 . 0 0 a t f a i l u r e . The two l a b o r a t o r y s t r e n g t h t e s t methods produced s h e a r s t r e n g t h s c o n s i d e r a b l y below t h o s e measured
i n
s i t u w i t h t h e vane. Using t h e s e r e s u l t s t h e f a c t o r o f s a f e t y a t f a i l u r e was a b o u t 0 . 8 ; t h a t i s , t h e u l t i m a t e b e a r i n g c a p a c i t y would have b e e n c o n s i d e r a b l y u n d e r e s t i m a t e d . The r e a s o n f o r t h e low s t r e n g t h s mayS O I L P R O F I L E i L A l E R C O N T E N T , % S H E A R S T R E N G T H T , , k P o T R l A X l A L C A U S T R E N G T H T E S T S
be deduced from a s t u d y o f t h e CIU s t r e s s - s t r a i n c u r v e s ( F i g u r e I d ) . The d e v i a t o r s t r e s s i n c r e a s e d r a p i d l y t o a maximum v a l u e t h e n l e v e l l e d o f f . The a b s e n c e o f s h a r p p e a k s from t h e s t r e s s - s t r a i n c u r v e s may b e due t o d i s t u r b a n c e from s a m p l i n g and t r a n s i t t o t h e l a b o r a t o r y . Consequently t h e s e t e s t s d i d n o t measure t h e peak s t r e n g t h o f t h e s o i l b u t produced a measure o f t h e p o s t - p e a k r e s i d u a l s t r e n g t h .
VANKLEEK HILL
On 30 September 1970 a 21.34-111 h i g h , 6.10-m d i a c o n c r e t e tower s i l o a b o u t 5 km s o u t h o f Vankleek H i l l
o v e r t u r n e d s u d d e n l y due t o a b e a r i n g c a p a c i t y f a i l u r e j u s t a f t e r i t had been f i l l e d f o r t h e f i r s t time w i t h 492 t o n n e s o f c o r n s i l a g e (Bozozuk, 1 9 7 2 ) . The s o i l s a r e n e a r - n o r m a l l y c o n s o l i d a t e d marine d e p o s i t s of t h e Champlain S e a .
The s o i l p r o f i l e ( F i g u r e 2a) shows a d e s i c c a t e d s i l t y c l a y t o a d e p t h o f 3 . 3 m o v e r l y i n g g r e y , s o f t , s i l t y c l a y w i t h some b l a c k m o t t l i n g . T h i s e x t e n d s t o a d e p t h o f a t l e a s t 20 m. C l a s s i f i c a t i o n t e s t s ( F i g u r e 2b) i n d i c a t e d a l i q u i d l i m i t o f 82 p e r c e n t n e a r t h e b a s e o f t h e s i l o f o o t - i n g s , d e c r e a s i n g t o a b o u t 60 p e r c e n t below a d e p t h o f 3 m . The a v e r a g e p l a s t i c i t y i n d e x was a b o u t 36 p e r c e n t . N a t u r a l w a t e r c o n t e n t was a b o u t 55 p e r c e n t a t f o u n d a t i o n l e v e l and i n c r e a s e d w i t h d e p t h t o a l m o s t 95 p e r c e n t , e x c e e d i n g t h e l i q u i d l i m i t a t each l e v e l by n e a r l y 30 p e r c e n t . G r a i n s i z e a n a l y s i s i n d i c a - t e d t h a t 85 t o 90 p e r c e n t o f t h e s o i l p a r t i c l e s were c l a y s i z e .
I n s i t u vane s h e a r t e s t s and l a b o r a t o r y t r i a x i a l CAU t e s t s performed on u n d i s t u r b e d s o i l samples a r e p l o t t e d on F i g u r e 2c. The CAU t e s t s were performed a t a s t r a i n r a t e o f 2 p e r c e n t p e r h o u r , and t h e a x i a l l o a d s were measured w i t h a n e l e c t r i c f o r c e t r a n s d u c e r . C o n s o l i d a t i o n p r e s s u r e s f o r t h e s e t e s t s were o l =
PA,
t h e c e l l p r e s s u r ea,
= KOPA
where Po i s t h e e f f e c t i v e o v e r b u r d e n p r e s s u r e , and KO = 0 . 5 . A t t h e time o f f a i l u r e t h e combined w e i g h t o f t h e s t r u c t u r e , s i l a g e and f o u n d a t i o n s was 672 t o n n e s . Using t h e a v e r a g e i n s i t u s h e a r s t r e n g t h o f t h e s o i l measured w i t h t h e vane t h e e s t i m a t e d f a c t o r o f s a f e t y was 1 . 1 0 . A d d i t i o n a l t e s t s p e r f o r m e d w i t h a s m a l l vane on u n d i s t u r b e d s o i l samples a t p r e - s e l e c t e d i n c l i n a t i o n s from t h e v e r t i c a l p r o v i d e d c o r r e c t i o n s f o r t h e s l o p e o f t h e f a i l u r e s u r f a c e (Bozozuk, 1972). When a p p l i e d t o t h e i n s i t u f i e l d vane s t r e n g t h s , t h e f a c t o r o f s a f e t y a g a i n s t a b e a r i n g c a p a c i t y f a i l u r e was r e d u c e d t o 0 . 9 9 . T h i s c o r r e c t i o n a g r e e d w i t h B j e r r u m ' s (1972) f a c t o r b a s e d on t h e p l a s t i c i t y i n d e x o f t h e s o i 1 .CAU t r i a x i a l s t r e n g t h s a r e compared w i t h i n s i t u vane s11ear s t r e n g t h s on F i g u r e 2c. I n t h e d e s i c c a t e d c r u s t t h e CAU v a l u e s f a l l below t h o s e measured i n s i t u w i t h t h e v a n e . The s t r e s s - s t r a i n c u r v e s i n t h i s s o i l f o r m a t i o n ( c u r v e s 1 and 2 i n F i g u r e 2d) do n o t d i s p l a y a s h a r p peak b u t l e v e l o f f a t t h e maximum d e v i a t o r s t r e s s , g i v i n g some measure o f t h e " r e s i d u a l " s t r e n g t h o f t h i s s o i l . I n t h e n o r m a l l y - c o n s o l i d a t e d s i l t y c l a y below t h e d e s i c c a t e d f o r m a t i o n , t h e CAU s h e a r s t r e n g t h s v a r i e d from 100 t o 130 p e r c e n t o f t h e i n s i t u vane s h e a r s t r e n g t h s . The s t r e s s - s t r a i n c u r v e s ( c u r v e s 3, 4 , 5 i n F i g u r e 2d) d i s p l a y a s h a r p peak a t f a i l u r e , f o l l o w e d by a r a p i d d r o p - o f f w i t h i n c r e a s i n g a x i a l s t r a i n . U n f o r t u n a t e l y t h e t e s t s were n o t f o l l o w e d t h r o u g h t o l a r g e s t r a i n s when t h e y were performed and c o n s e q u e n t l y t h e minimum p o s t - p e a k s t r e n g t h s r r were n o t d e t e r m i n e d . The t r e n d i n F i g u r e 2c, however, i n d i c a t e s t h a t
r r
c o u l d b e s m a l l e r t h a n t h e vane s h e a r s t r e n g t h s . RICHMOND ROAD On 30 September 1975 a 32.3-m h i g h , 9.14-m d i a tower s i l o w i t h 152-mm t h i c k c o n c r e t e w a l l s o v e r t u r n e d d u r i n g f i l l i n g w i t h c o r n s i l a g e when t h e l o a d r e a c h e d 1797 t o n n e s . I t was s u p p o r t e d on a c i r c u l a r c o n c r e t e r i n g f o u n d a t i o n 61 mm t h i c k , w i t h o u t s i d e and i n s i d e d i a m e t e r s o f 11.89 and 7.62 m, r e s p e c t i v e l y . The f o u n d a t i o n c o n t a i n e d no s t e e l r e i n f o r c i n g . The t o t a l w e i g h t o f t h e s t r u c t u r e , i t s c o n t e n t s and f o u n d a t i o n a t t h e t i m e o f f a i l u r e was a b o u t 2258 t o n n e s . The s i l o was e r e c t e d a b o u t 7 km n o r t h o f Richmond on a c l a y p l a n e o f o v e r c o n s o l i d a t e d marine d e p o s i t s o f t h e Champlain S e a . The s o i l p r o f i l e ( F i g u r e 3a) shows brown d e s i c c a t e d c l a y e y s i l t t o a d e p t h o f 2.4 m, changing t o g r e y c l a y e y s i l t t o 3 . 1 m, and u n d e r l a i n by g r e y s i l t y c l a y s w i t h b l a c k m o t t l i n g . A t 15.5 m t h i s changed t o a g r e y , b r i t t l e , s i l t y c l a y . The c l a s s i f i c a t i o n t e s t s shown on F i g u r e 3b i n d i c a t e t h a t t h e p l a s t i c i t y i n d e x and l i q u i d l i m i t s were a b o u t c o n s t a n t f o r t h e s o i l p r o f i l e a t 20 and 40 p e r c e n t r e s p e c t i v e l y . The n a t u r a l w a t e r c o n t e n t was a b o u t e q u a l t o t h e l i q u i d l i m i t s a t a d e p t h o f 2 m, b u t i t g r a d u a l l y i n c r e a s e d t o 60 p e r c e n t below a d e p t h o f 9 m. G r a i n s i z e a n a l y s i s i n d i c a t e d 37 p e r c e n t c l a y s and 6 3 p e r c e n t s i l t s a t 2 m, changing a l m o s t l i n e a r l y w i t h d e p t h t o 5 8 p e r c e n t c l a y and 42 p e r c e n t s i l t a t a d e p t h o f 16 m. The s h e a r s t r e n g t h o f t h e s o i l p l o t t e d on F i g u r e 3c was measured i n s i t u w i t h t h e vane, and i n t h el a b o r a t o r y w i t h u n c o n f i n e d u n d r a i n e d (UU) and c o n s o l i - d a t e d i s o t r o p i c a l l y u n d r a i n e d (CIU) t r i a x i a l s t r e n g t h t e s t s on u n d i s t u r b e d s o i l samples. The UU and CIU t e s t s were l o a d e d a t c o n s t a n t r a t e s o f s t r a i n o f 1 p e r c e n t p e r minute and 1 p e r c e n t p e r h o u r , r e s p e c - t i v e l y . A c e l l p r e s s u r e o f o c = KO PA where
KO = 0.75 was used i n t h e CIU t e s t s .
The u l t i m a t e b e a r i n g c a p a c i t y o f t h e s o i l f o r t h e s i l o , b a s e d on i n s i t u a v e r a g e vane s h e a r s t r e n g t h , was 241 kPa. The f a c t o r o f s a f e t y a t t h e time o f
f a i l u r e was d i f f i c u l t t o c a l c u l a t e b e c a u s e t h e non- r e i n f o r c e d c o n c r e t e f o o t i n g had c r a c k e d d u r i n g f i l l i n g o f t h e s i l o , r e d u c i n g t h e c o n t a c t a r e a f o r d i s t r i - b u t i o n o f t h e l o a d t o t h e f o u n d a t i o n s o i l . For t h e o r i g i n a l c o n t a c t a r e a o f 111.0 m2, t h e f a c t o r o f s a f e t y was 1 . 2 ; f o r a c o n t a c t a r e a e q u a l t o t h a t of t h e 9.14-111 d i a tower, i . e . , 70.1 m2, i t was 0 . 8 . A f a c t o r o f s a f e t y o f 1 . 0 0 would have b e e n o b t a i n e d i f t h e a c t u a l c o n t a c t a r e a a t t h e t i m e o f f a i l u r e had been 91 m 2 . Consequently, i t i s r e a s o n a b l e t o assume t h a t t h e u l t i m a t e b e a r i n g c a p a c i t y o f t h e s o i l was a d e q u a t e l y d e t e r m i n e d u s i n g t h e a v e r a g e s h e a r s t r e n g t h measured i n s i t u w i t h t h e vane.
CIU a n d UU s t r e n g t h t e s t r e s u l t s a r e compared w i t h t h e i n s i t u measurements on F i g u r e 3c. I n t h e d e s i c c a t e d c r u s t t h e CIU v a l u e s were lower t h a n t h o s e measured i n s i t u and t h e t r i a x i a l t e s t s c u r v e s ( 1 and 2
,
F i g u r e 3d) do n o t d i s p l a y a s h a r p peak. Below t h e c r u s t t h e t r i a x i a l t e s t c u r v e s ( 3 , 4, 5 ) developed a s h a r p peak a t f a i l u r e . The measured maximum1/12
S O I L PROFILE W A T E R C O N T E N T , OD S H E A R S T R E N G T H r,. k P . T R I A X I A L C I U S T R E N G T H T E S T S G R E Y S I L T Y G R E Y B R I T T L E 1 I 3 4 5 6 7 A X I A L S T R A I N el, %Figure 3 Engineering P r o p e r t i e s o f S o i l a t Richmond
F A C T O R O F S A F E T Y FOR L O A D I N G C O N D I T I O N S 1 , F S >, 3 2 , 1 < F S < 3
.
-&
-C O M P R E S S I O N S T R E S S E S E X T E N S I O N S T R E S S E S C R I T I C A L S H E A R P L A N E I / P E A K S H E A R S T R E N G T H OF T H E SOIL, T, P O S T P E A K " R E S I D U A L * S H E A R S T R E N G S H E A R P L A N E A B C O M P R E S S I O N E X T E N S I O N 1 l b ) T H .i n s i t u values. The post-peak s t r e n g t h s , T,, measured
a t l a r g e a x i a l s t r a i n s were generally 70 t o 90 p e r
c e n t of t h e vane s t r e n g t h s . The maximum unconfined
(UU) s t r e n g t h s measured on undisturbed s o i l samples from below a depth o f 8 m were c o n s i s t e n t l y lower t h a t t h e f i e l d vane s t r e n g t h s .
DISCUSSION
The bearing capacity o f a s o i l r e q u i r e d t o support a s t r u c t u r e (Figure 4a) i s r e l a t e d t o the shearing r e s i s t a n c e t h a t can be mobilized along a c r i t i c a l s h e a r p l a n e . The shearing r e s i s t a n c e must equal t h e sum o f t h e a p p l i e d s h e a r s t r e s s e s , which vary with
d i r e c t i o n along t h e c i r c l e o f shear. Beneath the
s t r u c t u r e t h e a p p l i e d s t r e s s e s a r e i n compression, and a t t h e o t h e r end o f t h e c r i t i c a l c i r c l e they a r e
i n extension. Thus the peak shear s t r e n g t h o f the
s o i l , T ~ ,along t h e k r i t i c a l plane may vary a s shown
on Figure 4b.
To mobilize shearing r e s i s t a n c e , a s h e a r s t r a i n (which
i s a function of applied s t r e s s ) i s required. Applied
s t r e s s e s a r e maximum beneath the s t r u c t u r e , s h e a r s t r a i n s a r e t h e r e f o r e a l s o g r e a t e r . A s t h e a p p l i e d
Figure 4 Shearing Resistance Mobilized Along C r i t i c a l
Shear Plane a t Various Factors of S a f e t y f o r Bearing Capacity
l o a d s i n c r e a s e , t h e s t r a i n s p r o g r e s s non-uniformly a l o n g t h e c r i t i c a l p l a n e , s o t h a t when t h e f a c t o r o f
s a f e t y a g a i n s t a b e a r i n g c a p a c i t y f a i l u r e i s
>
3, t h em o b i l i z e d s h e a r i n g r e s i s t a n c e a l o n g t h e p l a n e may b e r e p r e s e n t e d by c u r v e 1 ( F i g u r e 4 b ) ; when 1 < FS < 3, by c u r v e 2; and when FS 4 1 , by curve 3. Because o f t h e p r o g r e s s i v e development o f t h e s t r a i n s t h a t must t a k e p l a c e , i t i s i m p o s s i b l e t o m o b i l i z e .rf s i m u l - t a n e o u s l y a l o n g t h e whole c r i t i c a l p l a n e . The minimum s h e a r i n g r e s i s t a n c e t h a t c a n b e m o b i l i z e d i s t h e p o s t - p e a k " r e s i d u a l " s h e a r s t r e n g t h ,
-rr.
Hence, t h e u l t i m a t e b e a r i n g c a p a c i t y o f t h e s o i l must b e r e l a t e d t o some "average" s h e a r s t r e n g t h t h a t i s l e s s than -rf and g r e a t e r t h a n rr.When a vane i s i n s e r t e d i n t h e ground i t d i s p l a c e s
some o f t h e s o i l . I n s o d o i n g i t d e v e l o p s s h e a r s t r a i n s t h a t d i s t u r b t h e s o i l and a f f e c t i t s s t r e n g t h (Law e t a l * ) . Furthermore, a t t h e s t a r t o f t h e t e s t when a t o r q u e i s f i r s t a p p l i e d , t h e s h e a r s t r e s s e s a r e g r e a t e s t j u s t ahead o f t h e b l a d e s and z e r o i m m e d i a t e l y b e h i n d them. The s h e a r s t r a i n s , t h e r e - f o r e , a r e n o t uniform around t h e c y l i n d r i c a l s h e a r i n g s u r f a c e . As t h e vane i s r o t a t e d f u r t h e r , s h e a r s t r a i n s and, h e n c e , s h e a r i n g r e s i s t a n c e i s m o b i l i z e d p r o g r e s s i v e l y ; p a r t o f t h e s o i l may b e a t p o s t - p e a k , peak and p r e - p e a k s h e a r s t r e s s e s . The maximum s h e a r s t r e n g t h t h a t c a n t h e r e f o r e b e measured by t h e vane i s some " i n t e r m e d i a t e " v a l u e between t h e peak and
p o s t - p e a k s t r e n g t h s . These " i n t e r m e d i a t e 1 ' s h e a r s t r e n g t h s , u s e d w i t h Skempton's (1951) b e a r i n g c a p a c i t y e q u a t i o n , c o r r e l a t e d w e l l w i t h t h e o b s e r v e d u l t i m a t e b e a r i n g c a p a c i t y f o r t h e f a i l e d s i l o s . T r i a x i a l s t r e n g t h t e s t s on b o t h varved c l a y s and d e s i c c a t e d c l a y s gave s o i l s t r e n g t h s s m a l l e r t h a n
t h o s e measured i n s i t u w i t h t h e vane. The s t r e s s -
s t r a i n c u r v e s had no s h a r p p e a k , p r o b a b l y b e c a u s e t h e
s o i l s were d i s t u r b e d d u r i n g sampling. The s t r e n g t h s
d e t e r m i n e d from t h e s b t e s t s a p p e a r t o b e i n d i c a t i v e o f t h e r e s i d u a l v a l u e s .
The peak s t r e n g t h s o f t h e s o i l s below t h e d e s i c c a t e d c r u s t , d e t e r m i n e d from CIU and CAU t e s t s , w e r e h i g h e r
t h a n t h e i n s i t u vane t e s t s t r e n g t h s . These s t r e n g t h s , however, would n o t n o r m a l l y c o r r e l a t e w i t h t h e u l t i m a t e b e a r i n g c a p a c i t y o f t h e s o i l a t f a i l u r e b e c a u s e t h e y c a n n e v e r b e s i m u l t a n e o u s l y m o b i l i z e d a l o n g t h e c r i t i c a l s h e a r i n g s u r f a c e . I f peak s t r e n g t h s a r e t o b e u s e d f o r d e s i g n p u r p o s e s , t h e b e a r i n g c a p a c i t y t h e o r i e s s h o u l d b e r e f i n e d t o
accommodate them. The maximum u n c o n f i n e d t e s t
s t r e n g t h s were c o n s i d e r a b l y lower t h a n i n s i t u v a l u e s and h e n c e p r o v i d e v e r y c o n s e r v a t i v e e s t i m a t e s o f u l t i m a t e b e a r i n g c a p a c i t y o f t h e s e c l a y s o i l s . CONCLUSIONS A n a l y s i s o f t h r e e b e a r i n g c a p a c i t y f a i l u r e s on d i f f e r e n t c l a y s o i l s h a s l e d t o t h e f o l l o w i n g c o n c l u s i o n s : 1. The u l t i m a t e b e a r i n g c a p a c i t y o f t h e c l a y s o i l s a g r e e d w i t h t h e c a l c u l a t e d v a l u e b a s e d on "average" s t r e n g t h a s measured i n s i t u w i t h t h e NGI f i e l d v a n e . 2. The f i e l d vane does n o t measure t h e peak s t r e n g t h b u t r a t h e r some i n t e r m e d i a t e v a l u e between t h e peak
*
Measured S t r e n g t h s under F i l l s on S e n s i t i v e Clay.9 t h I n t e r n a t i o n a l Conference o n S o i l Mechanics and Foundation E n g i n e e r i n g . Tokyo, J a p a n , 11-15 J u l y 1977 and p o s t - p e a k s t r e n g t h o f t h e s o i l . 3 . Peak s t r e n g t h c a n n o t b e u s e d t o p r e d i c t u l t i m a t e b e a r i n g c a p a c i t y w i t h e x i s t i n g t h e o r i e s b e c a u s e i t c a n n o t b e s i m u l t a n e o u s l y m o b i l i z e d a l o n g t h e e n t i r e c r i t i c a l s h e a r i n g s u r f a c e . 4 . The u n c o n f i n e d s t r e n g t h t e s t c o n s i s t e n t l y under- e s t i m a t e d i n s i t u s h e a r s t r e n g t h o f t h e s o i l . These m e a s u r e t e n t s would p r o v i d e v e r y c o n s e r v a t i v e e s t i m a t e s o f t h e u l t i m a t e b e a r i n g c a p a c i t y . 5 . B e a r i n g c a p a c i t y i s r e l a t e d t o some average s t r e n g t h t h a t i s l e s s t h a n t h e peak and g r e a t e r t h a n t h e r e s i d u a l v a l u e . ACKNOWLEDGEMENTS Many c o l l e a g u e s i n t h e G e o t e c h n i c a l S e c t i o n , DBR/NRC, were i n v o l v e d i n t h e s a m p l i n g and a n a l y s i s o f t h e b e a r i n g c a p a c i t y f a i l u r e s , and t h e i r a s s i s t a n c e i s g r a t e f u l 1 y acknowledged. 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 Division. o f
B u i l d i n g Research, N a t i o n a l Research Council o f 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 o f t h e D i r e c t o r o f t h e D i v i s i o n .
REFERENCES
Anderson, A., and L. Bjerrum. ( 1 9 5 6 ) , "Vane T e s t i n g i n Norway", American S o c i e t y f o r T e s t i n g and M a t e r i a l s , S p e c i a l T e c h n i c a l P u b l i c a t i o n 193, p p . 54-60.
Bjerrum, L. (1954), " G e o t e c h n i c a l P r o p e r t i e s o f
Norwegian Marine Clays", go technique, Vol
.
4 ,No. 2, p p . 49-69.
Bjerrum,
L.
(1972), "Embankments on S o f t Ground1',P r o c e e d i n g s o f t h e ASCE S p e c i a l t y Conference on Performance o f E a r t h and E a r t h - S u p p o r t e d S t r u c t u r e s , Purdue U n i v e r s i t y , Vol. 11, p p . 1 - 5 4 .
Bozozuk, M. (1972), "Foundation F a i l u r e o f t h e
Vankleek H i l l Tower S i l o " , P r o c e e d i n g s , ASCE S p e c i a l t y Conference on Performance o f E a r t h and E a r t h - S u p p o r t e d S t r u c t u r e s , Purdue U n i v e r s i t y , Vol. I , P a r t 2 , pp. 885-902.
Eden, W. J . , and M. Bozozuk. (1962), "Foundation
F a i l u r e o f a S i l o on Varved Clay", The E n g i n e e r i n g J o u r n a l , Vol. 45, No. 9 , pp. 54-57.
LaRochelle, P . , M. Roy and F. Tavenas. (1973), " F i e l d
Measurements o f Cohesion i n Champlain C l a y s t 1 , P r o c e e d i n g s , E i g h t h ICSMFE, Moscow, Vol. 1 . 1 , pp. 229-236.
L a R o c h e l l e , P . , B . Trak, F. Tavenas and M. Roy. (1974),
" F a i l u r e o f a T e s t Embankment on a S e n s i t i v e Champlain Clay D e p o s i t " , Canadian C e o t e c h n i c a l
J o u r n a l , Vol. 11, No. 1, pp. 142-162. Roy, M. ( 1 9 7 5 ) , "Measurement o f I n S i t u S h e a r S t r e n g t h " , D i s c u s s i o n , P r o c e e d i n g s , ASCE Conference on I n S i t u Measurement o f S o i l P r o p e r t i e s , R a l e i g h , N.C., Vol. 11, pp. 164-167. Schmertmann, J . H . ( 1 9 7 5 ) , "Measurement o f I n S i t u S h e a r S t r e n g t h 1 ' , S t a t e - o f - t h e - a r t Report,
P r o c e e d i n g s , ASCE Conference on I n S i t u Measurement o f S o i l P r o p e r t i e s , R a l e i g h , N.C., Vol. 1 1 ,
pp. 57-138.
Skempton, A.W. ( 1 9 5 1 ) , "The B e a r i n g C a p a c i t y o f
c l a y s " , P r o c e e d i n g s , B u i l d i n g Research Congress, London, pp. 180-189.
This publication i s being distributed by the Division of Building R e s e a r c h of the National R e s e a r c h Council of Canada. I t should not be reproduced in whole o r in p a r t without permission of the original publisher. The Di- vision would be glad to b e of a s s i s t a n c e in obtaining such permission.
Publications of the Division may be obtained by m a i l - ing the appropriate remittance (a Bank, Express, o r P o s t Office Money Order, o r a cheque, made payable t o the Receiver General of Canada, c r e d i t NRC) to the National Research Council of Canada, Ottawa. K1A OR6. Stamps a r e not acceptable.
A l i s t of allpublications of the Division i s available and may be obtained f r o m the Publications Section, Division of Building Research, National Research Council of Canada, Ottawa. KIA OR6.
