Foundation failure of the Vankleek Hill tower silo

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

Foundation failure of the Vankleek Hill tower silo

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=43e602af-1821-48ab-aa75-4c4ae3323b70

https://publications-cnrc.canada.ca/fra/voir/objet/?id=43e602af-1821-48ab-aa75-4c4ae3323b70

m

I

02-I

t-2

_{NATIONAL RESEARCH COUNCIL O F CANADA}

"

*

530

CONSEIL NATIONAL D E RECHERCHES DU CANADA

FOUNDATION FAILURE O F T H E VANKLEEK H I L L TOWER S I M by M. Bozozuk ANALYZED R e p r i n t e d f r o m P r o c e e d i n g s of t h e ASCE S p e c i a l t y C o n f e r e n c e o n P e r f o r m a n c e of 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 held a t P u r d u e U n i v e r s i t y , 11 - 1 4 J u n e 1972 Vol. 1 , P a r t 2, p.

L

RESEARCH>

-

Lfi3R.!i,R)'-

i

R e s e a r c h P a p e r No. 530 of the D i v i s i o n of Building R e s e a r c h OTTAWA J u l y 19 72 P r i c e 25 c e n t s NRCC 12730

RUINE DE LA FONDATION DE LA TOUR-SILO DE VANKLEEK HILL

S OMMAIRE

L a r u i n e de la f o r c e portante de l a fondation d'un silo en bkton m e s u r a n t 70 pieds (21 m) de hauteur et 20 pieds (6 m ) de d i a m k t r e a kt6 une excellente occasion d'analyser l a r k s i s t a n c e du sol au cisaillement et d e vkrifier l e s t h e o r i e s actuelles concernant l a capa- cite portante. L a r k s i s t a n c e du s o l d 7 a r g i l e m a r i n e , m e s u r k e in situ peu a p r k s l a ruine au moyen d'un s c i s s o m k t r e , Btait d'au moins 0.16 kg/cm2 'a une profondeur de 12 pieds (3.7 m ) et augmentait graduellement selon l a profondeur, Des e s s a i s de consolidation s u r d e s Bchantillons non r e m a n i k s de c a r o t t i e r 3 piston ont montrk que l e s p r e s s i o n de prkconso comprennen i n t e r s t i t i e l l e e inclinaisons de tance. L'analys l a capacitk p o r cisaillement d

FOUNDATION FAILURE O F THE VANKLEEK HILL TOWER SILO by

M . Bozozuk:g

ABSTRACT

T h e bearing capacity f a i l u r e of the foundation supporting a 70-ft ( 2 1 - m ) high, 20-ft ( 6 - m ) d i a m e t e r c o n c r e t e s i l o provided a n e x c e l l e n t opportunity t o analyse the s h e a r s t r e n g t h of the soil and t o check existing bearing c a p a c - i t y t h e o r i e s . The s t r e n g t h of the m a r i n e clay s o i l m e a - s u r e d i n situ w i t h a field vane s h o r t l y a f t e r the f a i l u r e o c c u r r e d had a m i n i m u m value of 0 . 1 6 k g / c m 2 a t a depth of 12 ft ( 3 . 7 m ) , then i n c r e a s e d g r a d u a l l y with depth. Consolidation t e s t s c a r r i e d out on u n d i s t u r b e d piston tube s a m p l e s showed t h a t the foundation p r e s s u r e s had g r e a t l y exceeded the p r e c o n s o l i d a t i o n p r e s s u r e of the s o i l .

L a b o r a t o r y s t r e n g t h t e s t s included t r i a x i a l CAU t e s t s with p o r e p r e s s u r e m e a s u r e m e n t s and s m a l l vane t e s t s p e r f o r m e d a t v a r i o u s inclinations f r o m the v e r t i c a l t o investigate s t r e n g t h a n i s o t r o p y . P r e l i m i n a r y a n a l y s i s showed a good c o r r e l a t i o n between the u l t i m a t e b e a r i n g c a p a c i t y of the s o i l and t h e r e d u c e d s h e a r s t r e n g t h due t o a n i s o t r o p y . On 30 S e p t e m b e r 1970 a 70-ft ( 2 1 - m ) high, 20-ft ( 6 - m ) d i - a m e t e r c o n c r e t e t o w e r s i l o o v e r t u r n e d suddenly 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 of t h e foundation c l a y s o i l , just a f t e r i t w a s f i l l e d t o c a p a c i t y f o r the i i r s t t i m e with c o r n s i l a g e . T h e s i l o w a s ' % R e s e a r c h O f f i c e r , G e o t e c h n i c a l S e c t i o n , Division of Building R e s e a r c h , National X e s e a r c h Council of C a n a d a , Ottawa, C a n a d a .

886

EARTH STRUCTURES

d e s t r o y e d and the e s t i m a t e d 600 tons of feed spilled onto the ground. Although the d i s a s t e r was costly t o the f a r m e r , i t did provide an excellent opportunity t o check bearing capacity t h e o r i e s . It w a s a l s o possible t o compare ( a ) the strength of the soil m e a s u r e d in situ with a field vane, ( b ) the anisotropic strength m e a s u r e d with a l a b o r a t o r y vane on undisturbed soil specimens, and ( c ) t r i a x i a l anisotropically consolidated undrained (CAU) strength t e s t s on undisturbed soil s p e c i m e n s , a s the failure provided a full s c a l e field t e s t for the s h e a r s t r e n g t h of the soil.

THE TOWER SILO

The tower silo was constructed i n M a y 1970. The s u p e r - s t r u c t u r e consisted of a 70-ft (21 - m ) high, 20-ft ( 6 - m ) inside d i a m - e t e r reinforced concrete c i r c u l a r tube with walls 6$ i n . (16. 5 c m ) thick. A 4-in. (10-cm) thick concrete loading chute, c a s t m o n o - lithically with the m a i n walls on the outside, extended f o r the full height of the silo. The concrete tube was centred on a ring founda- tion; no floor was provided. Density t e s t s on s e v e r a l s a m p l e s of the concrete gave an a v e r a g e 141.0 l b j c u ft (2258 kg/m3) f r o m which the weight of the s u p e r s t r u c t u r e was calculated t o be 183 tons.

The ring foundation consisted of concrete placed d i r e c t l y in an excavated t r e n c h 3 ft ( 1 m ) wide and about 4 ft ( 1 . 2 m ) deep. I t s weight was e s t i m a t e d a t 54 tons.

The s i l o was constructed within one c o r n e r of a 70-ft (21 -m) wide, 100-ft ( 3 0 - m ) long, 4-in. ( 1 0 - c m ) thick concrete apron placed on 8 in. (20 c m ) of f i l l approximately 25 f t ( 8 m ) away f r o m the m a i n b a r n . When the s i l o collapsed, i t toppled in the direction of the loading chute onto the paved apron. The soil heaved beyond the apron in the opposite direction of the fall.

During the s u m m e r the silo was loaded with about 50 ft (15 m ) of hay silage that was used for s u m m e r feeding of beef cattle. Ap- proximately 20 f t ( 6 m ) of hay remained when the s i l o was f i r s t filled to capacity with corn silage late in September.

The owner e s t i m a t e d that the total weight of silage at the time of f a i l u r e was 600 t o n s . McCalrnont (1964) indicated that a s i l o of this s i z e could contain 582 tons of corn silage provided that the silo was refilled once a f t e r the silage had settled and the top had been well tramped. The owner had not had the opportunity to r e f i l l the

silo due to the sudden f a i l u r e , nor had the silage been t r a m p e d . The actual weight of silage, t h e r e f o r e , m u s t have been l e s s than the above e s t i m a t e s .

SILO FAILURE

T o obtain a r e a s o n a b l e e s t i m a t e of t h e a c t u a l load, a joint r e s e a r c h p r o j e c t w a s u n d e r t a k e n with t h e E n g i n e e r i n g R e s e a r c h S e r v i c e , C a n a d a A g r i c u l t u r e , in S e p t e m b e r 1971 t o m e a s u r e t h e a v e r a g e d e n s i t y of c o r n s i l a g e i n a n 82-ft ( 2 5 - m ) h i g h , 30-ft ( 9 - m ) d i a m e t e r s i l o a s i t w a s being f i l l e d . O v e r a n i n e - d a y p e r i o d t h e s i l o was f i l l e d with 1440 t o n s a f t e r which i t w a s topped up s e v e r a l t i m e s s o t h a t by the end of 30 d a y s i t contained 1600 t o n s . T h e a v e r a g e d e n s i t y a f t e r s e t t l e m e n t o v e r n i g h t i s plotted a g a i n s t t h e height of s i l a g e i n F i g u r e 1. T h i s f i g u r e a l s o shows t h e a v e r a g e d e n s i t y i m m e d i a t e l y a f t e r t h e s i l o was f i l l e d b e f o r e t h e s i l a g e s e t t l e d . D E N S I T Y , K G M 3 1 6 0 3 2 0 " 8 0 6 4 0 8 0 0 9 6 0 I I I I

-

- - E S T ~ M A T E ~ AVERAGE DENSITY DURING FILLING O F SILO - _{AVERAGE DENSITY} AFTER SETTLING O V E R N l G H l - -

-

- - N O T E - -

N O ALLOWANCE hWDE FOR JUICES LOST THROUGH DPnlNAGE FROM MSE

0 1 0 2 0 3 0 4 0 5 0 6 0 D E N S I T Y . P C F F ! G U R E I A V E R A G E ;'ill D E N S I T Y o r C O R N S I L A G E M E A S U R E D D U R I N G F I R S T F I L L I N G O F 2 0 FT ( P n l D I A M E T E R BY 8 2 FT l 2 5 r n ) H I G H S I L O , SEPT, 1 9 7 1 B R I B B B - l

EARTH STRUCTURES

McCalmont (1964) indicated that a t the s a m e m o i s t u r e con- t e n t hay silage i s a p p r o x i m a t e l y 5 to 10 p e r cent h e a v i e r than c o r n silage provided t h a t the silo i s refilled once a f t e r i t s e t t l e s and the top well t r a m p e d . B e c a u s e the hay was collected during the d r y period of the s u m m e r i t s m o i s t u r e content w a s probably m u c h l e s s than that of the c o r n t h a t was collected during the wet period e a r l y in the fall; n e i t h e r was the s i l o refilled with hay n o r t r a m p e d . C o n s e - quently, f o r d e t e r m i n i n g the weight of silage placed, i t was a s s u m e d t h a t the density of the h a y and c o r n s i l a g e w e r e the s a m e .

F r o m the information p r e s e n t e d on F i g u r e 1, t h e r e f o r e , the e s t i m a t e d weight of s i l a g e in the 70-ft (21 - m ) s i l o a t the t i m e of f a i l u r e w a s 533 tons; f o r 50 f t ( 1 5 m ) i t w a s 342 t o n s .

T h e 533-ton e s t i m a t e should be c o r r e c t e d f u r t h e r t o compen- s a t e f o r the amount of s i l a g e juices l o s t by d r a i n a g e f r o m p o r t s provided a t the b a s e of t h e tower silo. It i s known t h a t during load- ing a c o n s i d e r a b l e quantity of juice e s c a p e d , although the actual amount w a s not m e a s u r e d . Assuming that 2 l b s p e r cubic f t

(32 kg/m3) i s a r e a l i s t i c a v e r a g e l o s s , the net weight of silage in the 70-ft ( 2 1 - m ) s i l o was e s t i m a t e d t o be 511 tons a t the t i m e of f a i l u r e .

Due to the configuration of the foundation i t was i m p o s s i b l e to d e t e r m i n e the d i s t r i b u t i o n of s t r e s s e s imposed on the soil by t h e above loads. A s s u m i n g , t h e r e f o r e , t h a t t h e loads w e r e d i s t r i b u t e d u n i f o r m l y over a c i r c u l a r a r e a i m m e d i a t e l y below the footings, the net applied v e r t i c a l p r e s s u r e s due t o the t o t a l weight of the s i l o and i t s contents f o r the v a r i o u s e s t i m a t e d loads a r e given i n Table 1. SOIL CONDITIONS

The s i l o was located on a l e v e l clay plain consisting of m a r i n e d e p o s i t s of the Champlain S e a (Gadd, 1963). The c l a y s a r e g e n e r a l l y v e r y weak and highly c o m p r e s s i b l e . M a n y landslide f a i l u r e s have o c c u r r e d i n gullies and r a v i n e s t h a t e x i s t i n t h i s region.

Two weeks a f t e r the f a i l u r e o c c u r r e d continuous undisturbed 2-in. ( 5 0 - m m ) dia. piston tube soil s a m p l e s w e r e taken with a Norwegian (NGI) s o i l s a m p l e r ( B j e r r u m , 1954), about 6 0 _{f t (18 m )}

f r o m the s i l o to a depth of 34 f t (10 m ) . The in s i t u s h e a r s t r e n g t h w a s m e a s u r e d at the s a m e location with a 55- x 110-mm Geonor vane (Anderson and B j e r r u m , 1956) t o a depth of 64.2 f t (19.6 m ) , the depth of r e f u s a l . A s u m m a r y of the engineering t e s t s i s shown in F i g u r e 2.

The s o i l profile c o n s i s t s of 1 f t (0. 3 m ) of o r g a n i c topsoil o v e r 10 f t ( 3 m ) of d e s i c c a t e d reddish-brown s i l t y clay. F r o m 11 f t

SILO FAILURE

889

T A B L E 1. - NET A P P L I E D VERTICAL FOUNDATION PRESSURES DUE T O THE T O T A L WEIGHT O F THE SILO AND SILAGE

Height of Silage E s t i m a t e d Weight ( T o n s ) 1 . 8 3 1. 68 1. 63 1 . 2 4 (a) 70 f t ( 2 1 m ) ( b ) 70 it (21 m ) ( c ) 70 i t (21 m ) ( d ) 5 0 f t ( 1 5 m ) Applied V e r t i c a l P r e s s u r e ( T S F , kg/cm2) 600 533 51 1 342

SILO

FAILURE

89

₁

( 3 . 4 m ) t o 34 ft ( 1 0. 3 m ) , the m a x i m u m depth of sampling, the s o i l s w e r e m a i n l y soft g r e y silty clays except f o r a thin silt l a y e r a t 14. 6 f t (4. 5 m ) . The g r e y s i l t y c l a y contained s o m e black mottling c o m - m o n l y found in the m a r i n e c l a y s of t h i s region. Although no s a m p l e s w e r e obtained f r o m the d e e p e r f o r m a t i o n s , i t a p p e a r s that the m a r i n e clays extended t o a depth of 64. 2 ft ( 1 9. 6 m ) .

A t t e r b e r g l i m i t t e s t s w e r e conducted on soil t r i m m i n g s ob- tained f r o m the tube s a m p l e s . G e n e r a l l y the p l a s t i c l i m i t was f a i r l y uniform a t about 25 p e r cent. The liquid l i m i t of the soil n e a r the b a s e of the s i l o footings was approximately 82 p e r cent. It d e c r e a s e d t o about 60 p e r cent below a depth of 10 f t ( 3 m ) , giving a n a v e r a g e P l a s t i c i t y Index of 36 p e r cent. The n a t u r a l w a t e r content was approximately 55 p e r cent n e a r the footings and it i n c r e a s e d with depth to a l m o s t 95 p e r cent exceeding the liquid l i m i t by n e a r l y 30 p e r cent. Consequently the d e e p e r soil f o r m a t i o n s would be v e r y sensitive and would have low remoulded s t r e n g t h s .

The g r a i n s i z e analysis conducted on the s o i l t r i m m i n g s showed that 8 5 t o 90 p e r cent of the s o i l p a r t i c l e s f e l l within the c l a y s i z e f r a c t i o n with the exception of the s i l t l a y e r a t 14. 6 f t ( 4 . 5 m ) . T h e r e was a complete absence of sand s i z e p a r t i c l e s in the soil profile.

Consolidation t e s t s conducted on 20 - c m 2 s p e c i m e n s , using a

tp/p r a t i o of

fr,

showed that the s o i l profile was n o r m a l l y consol-

idated ( F i g u r e 2 ) . The t e s t s indicated that the m a x i m u m depth of the groundwater table w a s about 11 f t ( 3 . 4 m ) although i t was only 2 f t ( 0 . 6 m ) deep a t the t i m e of the investigation. The a v e r a g e initial void r a t i o was approximately 1. 5 f o r the d e s i c c a t e d zone and about 2. 5 f o r the soft c l a y f o r m a t i o n below.

T h e total v e r t i c a l s t r e s s e s under the c e n t r e line of the s i l o f o r 342 tons of hay and 533 tons of c o r n s i l a g e using the Boussinesq s t r e s s distribution ( J u m i k i s , 1971) r e l a t i v e to the consolidation t e s t s , a r e shown on F i g u r e 2. It i s evident that t h e s e loads would have caused v e r y l a r g e s e t t l e m e n t s had the s i l o not failed.

DESCRIPTION O F THE FAILURE

The attitude of the ring foundation relative t o i t s o r i g i n a l position following the b e a r i n g capacity f a i l u r e i s shown on F i g u r e 3. I t s final slope was 50 d e g r e e s f r o m the h o r i z o n t a l which was identical to that m e a s u r e d on a s i m i l a r s i l o that failed at New L i s k e a r d (Eden and Bozozuk, 1962). One end heaved approximately 6 f t ( 1 . 8 m ) while the opposite end sank about 13 f t ( 4 . 0 m ) . The amount of soil heaved could m t be m e a s u r e d a s s o m e of i t had been

EARTH STRUCTURES

F I G U R E 3 A T T I T U D E . O F S I L O F O U N D A T I O N A F T E R F A I L U R E

BII ,Bad- l

r e m o v e d p r i o r t o t h e i n v e s t i g a t i o n , but i t c e r t a i n l y exceeded 6 f t ( 1 . 8 m ) and could have b e e n a s m u c h a s 8 f t ( 2 . 4 m ) .

T h e e x a c t location of t h e sliding s u r f a c e w a s not d e t e r m i n e d . B y studying t h e r o t a t i o n of t h e foundation, however, i t w a s p o s s i b l e t o r e c o n s t r u c t t h e c i r c u l a r f a i l u r e s u r f a c e shown on t h e f i g u r e . The f a i l u r e s u r f a c e extended t o a d e p t h of 23 ft ( 7 . 0 m ) delineating t h e

s o i l whose s t r e n g t h g o v e r n e d the b e a r i n g c a p a c i t y f o r t h e foundation. T h i s a g r e e s with S k e m p t o n l s (1951) s u g g e s t i o n t h a t the s t r e n g t h of t h e s o i l t o a d e p t h below t h e foundation e q u a l t o t w o - t h i r d s of i t s d i a m e t e r , ( i . e .

,

5 f t ( 1 . 5 m ) t o 21 f t ( 6 . 4 m ) ) , be u s e d f o r d e t e r - m i n i n g t h e b e a r i n g c a p a c i t y .

SHEAR STRENGTH O F THE SOIL

T h e i n s i t u u n d i s t u r b e d and r e m o u l d e d s h e a r s t r e n g t h s of t h e s o i l m e a s u r e d with t h e f i e l d vane t o a d e p t h of 34 f t ( 1 0 . 4 m ) a r e plotted on F i g u r e 2, and a s u m m a r y of a l l the m e a s u r e m e n t s t o a d e p t h of 64 f t ( 1 9 . 6 m ) i s given i n T a b l e 2. T h e u n d i s t u r b e d s t r e n g t h v a r i e s f r o m a high of about 1 T S F ( 1 k g / c m 2 ) i n t h e d e s i c c a t e d c r u s t a t a d e p t h of 5 f t ( 1 . 5 m ) , t o a m i n i m u m of 0 . 1 6 T S F ( 0 . 1 6 k g / c m 2 ) i n t h e soft g r e y s i l t y c l a y a t 12 f t (3. 7 m ) . T h e s t r e n g t h i n c r e a s e s g r a d u a l l y with i n c r e a s i n g d e p t h t o about 0 . 8 T S F ( 0 . 8 k g / c m 2 ) a t 60 f t ( 1 8 m ) . T h e a v e r a g e u n d i s t u r b e d s h e a r s t r e n g t h of t h e s o i l f r o m 5 f t ( 1 . 5 m ) t o 23 f t ( 7 . 0 m ) i s about 565 psf ( 0 . 2 7 6 k g / c m 2 ) .

SILO FAILURE

893

T A B L E 2.

-

IN SITU SHEAR STRENGTH O F SOIL, MEASURED

WITH 55- X 110-MM FLELD VANE

Depth ( f t ) 4 . 6 5. 6 6. 6 7 . 6 8 . 6 9 . 6 l o . 6 11. 6 12.6 1 3 . 6 1 4 . 6 15. 6 1 6 . 6 1 7 . 6 1 8 . 6 1 9 . 6 20. 6 2 1 . 6 22.6 2 3 . 6 24. 6 25. 6 Depth ( f t ) 26. 6 2 7 . 6 2 8 . 6 29. 6 3 0 . 6 3 1 . 6 3 2 . 6 3 3 . 6 3 4 . 6 35. 6 36. 6 3 9 . 6 4 2 . 6 45. 6 48.6 51. 6 53. 6 5 7 . 6 6 0 . 6 63. 6 64. 2 S h e a r Undisturbed 1563 1350 1058 710 639 48 6 410 289 272 320 262 360 390 359 414 325 397 392 499 420 40 1 3 54 S t r e n g t h ( p s f ) Remoulded 2 50 170 138 6 5 54 22 22 11 7 0 0 0 0 0 11 0 0 0 4 4 7 17 S h e a r Undisturbed 531 438 590 603 690 499 694 547 743 636 790 80 1 1000 1025 1196 970 8 59 712 1671 750 R e f u s a l S t r e n g t h ( p s f ) Remoulded 9 11 2 2 2 2 1 6 2 0 1 6 1 3 54 4 8 4 5 4 0 4 7 1 3 17 0 3 5 11

894

EARTH STRUCTURES

The remoulded s t r e n g t h m e a s u r e d in the d e s i c c a t e d c r u s t v a r i e d f r o m 22 psf (0.01 k g / c m 2 ) t o 250 psf ( 0 . 12 k g / c m 2 ) indicating a sensitivity of 6 t o 22 f o r t h i s l a y e r . Below t h i s soil f o r m a t i o n t o a depth of 24 f t (7. 3 m ) , the remoulded s t r e n g t h dropped t o p r a c t i c a l l y nothing, indicating an infinite sensitivity. F o r the soil below 24 f t (7. 3 m ) the sensitivity v a r i e d f r o m 20 to o v e r 100.

T r i a x i a l (CAU) s t r e n g t h t e s t s w e r e conducted on undisturbed t e s t s p e c i m e n s in the l a b o r a t o r y . The s p e c i m e n s w e r e t r i m m e d t o

1 . 4 in. (36 m m ) dia. by 3 . 0 in. (80 m m ) high, consolidated

anisotropically t o the i n s i t u field s t r e s s e s a s s u m i n g K = 0. 5, and loaded undrained t o f a i l u r e . The s h e a r s t r e n g t h s definzd a s

4 (Ao,

-

Ao,) a r e plotted on F i g u r e 2. The a v e r a g e s t r e n g t h of the s o i l f r o m 5 f t (1. 5 m ) t o 23 f t ( 7 . 0 m ) was 356 psf (0.174 k g / c m 2 ) , which was 63 p e r cent of the in s i t u s t r e n g t h m e a s u r e d with the field vane. The a v e r a g e slope of the f a i l u r e s u r f a c e m e a s u r e d on t h e t e s t

s p e c i m e n s was 53. 5 d e g r e e s .

The field vane and the l a b o r a t o r y t r i a x i a l t e s t s a r e not s u i t - able m o d e l s f o r d e t e r m i n i n g the s h e a r i n g r e s i s t a n c e of the s o i l supporting the silo. The field vane m e a s u r e d the s h e a r i n g r e s i s t a n c e along a v e r t i c a l plane, w h e r e a s the t r i a x i a l t e s t gave the s h e a r i n g r e s i s t = ~ c e along a 53. 5 - d e g r e e slope. The a c t u a l f a i l u r e under the s i l o probably took place along a curved s u r f a c e ( c i r c l e ) whose slope v a r i e d f r o m 0 t o 90 d e g r e e s f r o m the v e r t i c a l . It was n e c e s s a r y , t h e r e f o r e , t o d e t e r m i n e the s h e a r i n g r e s i s t a n c e of the s o i l along t h i s s u r f a c e .

The curved f a i l u r e s u r f a c e was r e p l a c e d with a s e r i e s of s t r a i g h t lines d r a w n tangent t o i t a t inclinations varying f r o m 0 t o 90 d e g r e e s ( F i g u r e 3). A t o t a l of 67 l a b o r a t o r y vane t e s t s w e r e con- ducted a t v a r i o u s inclinations on t e s t s p e c i m e n s jacked d i r e c t l y f r o m the s a m p l e t u b e s using a s m a l l 10 - m m dia-by 1 0 - m m high vane. The

s t r e n g t h s m e a s u r e d i n the n o r m a l position (0 d e g r e e s ) w e r e about the s a m e a s those m e a s u r e d in the field a s shown on F i g u r e 2. F o r other inclinations t h e r e was a m a r k e d reduction in the s h e a r i n g s t r e n g t h of t h e soil.

Fifty-one of the vane t e s t s w e r e conducted on t h e soil at depths f r o m 5 f t (1. 5 m ) t o 24 f t (7. 3 m ) a t inclinations of 0, 224, 45, 674 and 90 d e g r e e s ; 16 w e r e conducted on the soil a t depths f r o m 27 f t ( 8 . 2 m ) t o 34 f t ( 1 0 . 4 m ) a t inclinations of 0, 30, 60 and 90 d e g r e e s . F o r t h e s e angles the a v e r a g e p e r cent reduction i n s t r e n g t h was d e t e r m i n e d r e l a t i v e to the v e r t i c a l orientation

(0 d e g r e e s ) and the r e s u l t s plotted on F i g u r e 4. F r o m 5 ft (1. 5 m ) t o 24 f t ( 7 . 3 m ) the s t r e n g t h of the soil d e c r e a s e d up t o 20 p e r cent a s the inclination of t h e vane was i n c r e a s e d t o 90 d e g r e e s . F r o m

SILO FAILURE

INCLINATION OF LABORATORY VANE FROM VERTICAL, DEGREES

FIGURE 4

EFFECT O F I N C L I N A T I O N O F L A B O R A T O R Y V A N E O N THE SHEAR STRENGTH O F THE S O I L

BR4a8B-r

27 f t ( 8 . 2 m ) t o 34 f t ( 1 0 . 4 m ) , the l o s s w a s about 20 p e r c e n t a t 30 and 60 d e g r e e s , but only 11 p e r c e n t a t 90 d e g r e e s . T h i s could be a t t r i b u t e d t o t h e g r e a t e r v a r i a t i o n i n s o i l s t r e n g t h at' t h i s d e p t h and to t h e s m a l l e r n u m b e r of t e s t s p e r f o r m e d t o o b t a i n t h i s c u r v e .

T h e s h e a r s t r e n g t h of t h e s o i l along the f a i l u r e plane w a s d e t e r m i n e d by adjusting t h e i n s i t u s h e a r s t r e n g t h m e a s u r e d with t h e f i e l d vane by t h e a m o u n t s shown on F i g u r e 4 u s i n g t h e equation:

896

where

EARTH STRUCTURES

S = anisotropic s t r e n g t h f o r an angle a

dictated by the failure s u r f a c e S = in situ undrained s h e a r strength

v

of the soil m e a s u r e d with the field vane

R = reduction in s t r e n g t h determined f r o m F i g u r e 4 f o r the appropriate inclination of the f a i l u r e s u r f a c e , e x p r e s s e d a s a f r a c t i o n

The resulting s t r e n g t h v e r s u s depth curve coincided with the lower l i m i t of the undisturbed field vane strengths shown on F i g u r e 2. The a v e r a g e s t r e n g t h of the soil f r o m 5 ft (1. 5 m ) t o 2 3 ft ( 7 . 0 m ) was 500 psf (0.244 kg/cm2), which was about 12 p e r cent lower than the in situ vane s t r e n g t h s , and 40 p e r cent g r e a t e r than those m e a -

sured in the l a b o r a t o r y .

The difference in strength of 40 p e r cent obtained with the l a b o r a t o r y vane compared with the t r i a x i a l t e s t cannot be related t o the 20 p e r cent reduction due t o anisotropy o r t o disturbance f r o m

sampling (Bozozuk, 1971) but to s t o r a g e t i m e of the soil s a m p l e s and possible disturbance f r o m t r i m m i n g the t e s t specimens. The laboratory vane t e s t s w e r e performed on untrimmed t e s t specimens jacked d i r e c t l y f r o m sample tubes approximately two t o t h r e e months a f t e r the s a m p l e s w e r e obtained; the strengths c o r r e l a t e d with those m e a s u r e d in the field ( F i g u r e 2). The t r i a x i a l t e s t s w e r e performed on carefully t r i m m e d t e s t specimens that had been extruded f r o m the s a m e sample tubes and s t o r e d f o r 1 2 t o 14 months in a humid r o o m a t a constant t e m p e r a t u r e of 5 5 ° F . Although the s t o r a g e t e m p e r a - t u r e was s i m i l a r to the ground t e m p e r a t u r e in t h i s region, the reduction i n strength of the soil m u s t have been caused by the long period of s t o r a g e . T h e r e f o r e , f o r a c c u r a c y , engineering t e s t s on undisturbed soil s a m p l e s m u s t be p e r f o r m e d a s soon a s possible a f t e r they a r e obtained (Bozozuk 1971).

ANALYSIS OF BEARING CAPACITY

The bearing capacity of a soil i s r e l a t e d t o i t s s h e a r strength and to the s i z e , shape, and depth of the foundations. Skempton (1951) proposed the following equation f o r the ultimate bearing capacity f o r

SILO FAILURE

w h e r e

%

= u l t i m a t e b e a r i n g c a p a c i t y

c = a v e r a g e s h e a r s t r e n g t h of the s o i l t o a d e p t h below the foundation e q u a l t o two- t h i r d s of the d i a m e t e r

N = s h a p e f a c t o r =

6 . 6

f o r a c i r c u l a r foundation p = o v e r b u r d e n p r e s s u r e a t foundation level. E a r l i e r , Skempton (1942) had p r o p o s e d a n e q u a t i o n t h a t c o n - s i d e r e d the a d h e s i o n b e t w e e n the r o u g h s i d e s of t h e foundation and the s o i l . It had the f o r m

w h e r e L

% =

cN t p t - c ' c A L = o u t s i d e p e r i m e t e r of t h e foundation A = a r e a of t h e foundation

c' = a d h e s i o n between the s o i l and the foundation. I n t h i s a n a l y s i s , cr = c ; qU, C , Nc a n d p a r e t h e s a m e a s f o r e q u a - t i o n ( 2 ) .

M e y e r h o f (1951) c o n s i d e r e d t h e c a s e of a c i r c u l a r foundation with a r o u g h s h a f t b e a r i n g on p u r e l y c o h e s i v e s o i l . He p r o p o s e d the following f o r m u l a which t a k e s i n t o a c c o u n t t h e s o i l f r i c t i o n a c t i n g on the s i d e s of the foundation:

w h e r e

%

= u l t i m a t e b e a r i n g c a p a c i t y c = a v e r a g e s h e a r s t r e n g t h

N = shape f a c t o r

=

7 . 5 f o r c i r c u l a r foundation K = coefficient of e a r t h p r e s s u r e between t h e

s o i l a n d t h e s i d e s of t h e foundation

=

1

Y

= d e n s i t y of the s o i l above the b a s e of the foundation

D = depth t o t h e b a s e of t h e foundation.

Using t h e s e e q u a t i o n 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 w a s d e - t e r m i n e d f o r e a c h of t h e following t h r e e e s t i m a t e d a v e r a g e s h e a r

EARTH STRUCTURES

( a ) In situ undrained s t r e n g t h m e a s u r e d

with the field vane 565 psf (0. 276 kg/cm2)

( b ) Undrained field vane s t r e n g t h c o r r e c t e d f o r slope of f a i l u r e s u r f a c e

( a n i s o t r o p i c s t r e n g t h ) 500 psf ( 0 . 244 kg/cm2) ( c ) L a b o r a t o r y t r i a x i a l (CAU)

s t r e n g t h 356 psf ( 0 . 1 7 4 kg/crn2)

T h e calculated u l t i m a t e bearing c a p a c i t i e s w e r e c o m p a r e d with the a v e r a g e t o t a l foundation p r e s s u r e s f o r silage loads of 600,

533, 511 and 342 t o n s , and a f a c t o r of safety a g a i n s t f a i l u r e w a s d e t e r m i n e d . A s u m m a r y of the r e s u l t s i s given in Table 3.

B a s e d on the a v e r a g e i n situ undrained s h e a r s t r e n g t h of the soil, the equations p r e d i c t e d f a c t o r s of safety g r e a t e r than unity f o r t h e 600-ton silage load. B e c a u s e the s i l o f a i l e d , i t i s a p p a r e n t that the vane o v e r e s t i m a t e d t h e s h e a r s t r e n g t h of the soil. Based on the I reduced a n i s o t r o p i c s t r e n g t h , however, equations ( 2 ) and ( 3 ) p r e -

dicted f a i l u r e , w h e r e a s equation ( 4 ) indicated t h a t the foundations w e r e stable.

Considering t h e a v e r a g e s h e a r s t r e n g t h d e t e r m i n e d f r o m the CAU t e s t s , equations ( 2 ) and ( 3 ) indicated t h a t even the 342-ton load (50 f t ( 1 5 m ) of hay s i l a g e ) should have c a u s e d f a i l u r e , w h e r e a s equation ( 4 ) p r e d i c t e d a f a c t o r of safety of 1.07. If the bearing capacity i s b a s e d on t h e in situ undrained s h e a r s t r e n g t h o r on t h e a n i s o t r o p i c s t r e n g t h , a l l equations p r e d i c t a v e r y high f a c t o r of safety f o r t h i s load. A s the s t r u c t u r e did not f a i l , i t c a n be con- cluded that the t r i a x i a l t e s t s u n d e r e s t i m a t e d the actual s h e a r s t r e n g t h of t h e soil.

F o r the 533- and 511 -ton silage l o a d s , equation ( 2 ) indicated f a c t o r s of safety of 1. 10 and 1 . 1 4 r e s p e c t i v e l y based on the a v e r a g e undrained field vane s t r e n g t h of the soil. B a s e d on the a n i s o t r o p i c

s t r e n g t h the f a c t o r s of safety w e r e 0.99 and 1 . 0 1 r e s p e c t i v e l y . B e - c a u s e the s t r u c t u r e did f a i l , the r e a l f a c t o r of safety had to be of t h i s o r d e r . Allowing f o r s o i l adhesion (equation ( 3 ) ) th e f a c t o r s of

safety w e r e i n c r e a s e d to 1. 01 and 1. 04 respectively. T h e s e values a r e s t i l l s o c l o s e t o unity that the design would have been considered unsafe. On t h e o t h e r hand, equation ( 4 ) p r e d i c t e d 1 . 2 0 and 1 . 2 4 r e s p e c t i v e l y , which would n o r m a l l y have been adequate f o r design. T h u s equations ( 3 ) and ( 4 ) which allow f o r s o i l adhesion t o the foundations o v e r e s t i m a t e t h e ultimate b e a r i n g capacity f o r the weak m a r i n e clay.

SILO FAILURE

T A B L E 3.

-

C O M P A R I S O N O F F A C T O R S O F S A F E T Y D E T E R M I N E D B Y T H R E E M E T H O D S USING D I F F E R E N T S O I L S T R E N G T H S F O R VARIOUS S I L A G E LOADS A n a l y s i s of U l t i m a t e B e a r i n g C a p a c i t y

-

S i l a g e L o a d s 1 F a c t o r of S a f e t y E q u a t i o n ( 2 ) ( 3 ) ( 4 ) 600 T 1 . 0 2 0 . 9 1 0. 67 1 . 0 4 0 . 9 3 0. 68 1 . 1 5 1 . 1 0 0 . 7 5 A v e r a g e S t r e n g t h of S o i l 5 6 5 p s f , ( 0 . 2 7 6 k g / c m 2 ) 500 p s f , ( 0 . 2 4 4 k g / c m 2 ) 356 p s f , ( 0 . 1 7 4 k g / c m 2 ) 565 sf, ( 0 . 2 7 6 k g / c m 2 ) 500 p s f , ( 0 . 2 4 4 k g / c m 2 ) 3 5 6 p s f , ( 0 . 1 7 4 k g / c m 2 ) 5 6 5 p s f , ( 0 . 2 7 6 k g / c m 2 ) 500 p s f , ( 0 . 2 4 4 k g / c m 2 ) 356 p s f , ( 0 . 1 7 4 k g / c m 2 ) 5 3 3 T 1 . 1 0 0 . 9 9 0 . 7 2 1 . 1 3 1. 01 0 . 7 4 1 . 2 4 1 . 20 0 . 8 1 51 1 T 1 . 1 4 1 . 01 0 . 7 4 1 . 1 6 1 . 0 4 0 . 7 6 1 . 2 8 1 . 24 0 . 8 4 3 4 2 T 1 . 4 5 1 . 30 0 . 9 6 1 . 4 9 1 . 3 3 0 . 9 8 1 . 6 4 1 . 4 6 1 . 0 7

EARTH STRUCTURES

In studying a n u m b e r of r e p o r t e d f a i l u r e s of f i l l s and e x c a - vations in clays f r o m a l l o v e r the world, Bjerrurn:x c o r r e l a t e d the calculated f a c t o r s of s a f e t y based on the undisturbed field vane s t r e n g t h with the P l a s t i c i t y Index

(T,)

( F i g u r e 5). In t h i s c a s e t h e calculated f a c t o r s of safety f o r t h e 3 3 3 - and 511-ton l o a d s based on the field vane s t r e n g t h of 565 psf (0.276 kg/cm2) using equation (2) w a s 1.10 and 1. 14 r e s p e c t i v e l y , giving an a v e r a g e of 1 . 12. A s

shown on F i g u r e 5 t h i s point supported B j e r r u m l s c o r r e l a t i o n . It a p p e a r s that t h i s r e l a t i o n s h i p m a y be a useful guide f o r establishing r e a l i s t i c f a c t o r s of safety i n design.

(Reproduced with the permission o f L . Bjerrum, Norwegian Geotechnical Institute) F I G U R E 5 R E L A T I O N S H I P B E T W E E N E S T I M A T E D F A C T O R O F S A F E T Y A T F A I L U R E A N D T H E P L A S T I C I T Y I N D E X BR 4 8 8 8

-

5 CONCLUSIONS

T h e foundation f a i l u r e of t h e tower s i l o supported on weak m a r i n e clay w a s a f u l l - s c a l e field t e s t of t h e s h e a r s t r e n g t h of t h e foundation soil. The d i m e n s i o n s of t h e s t r u c t u r e w e r e known and it

SILO

FAILURE

was possible t o obtain a good estimate of the applied load. The analysis led t o the following conclusions:

1. The ultimate bearing capacity calculated f r o m the anisotropic strength of the s o i l using equation ( 2 ) c o r r e l a t e d e x t r e m e l y well with the average bearing p r e s s u r e applied by the s t r u c t u r e that contained 511 to 533 tons of silage.

Two of the bearing capacity equations that allow f o r adhesion between the s i d e s of the foundation and the s o i l , overestimated the ultimate bearing capacity of the soil. It a p p e a r s that when the s o i l i s loaded rapidly t o f a i l u r e , adhesion does not contribute t o the bearing capacity.

The m a x i m u m depth of the reconstructed f a i l u r e s u r - f a c e supports Skemptonts (1951) rule that the strength of the soil to a depth below the footing equal t o two t h i r d s of i t s d i a m e t e r be used in design.

The field vane o v e r e s t i m a t e s the in situ undrained s h e a r strength of the soil for bearing capacity c a l - culations. The vane m e a s u r e s the shearing r e s i s t a n c e along a v e r t i c a l cylindrical s u r f a c e whereas the slope of the actual f a i l u r e s u r f a c e under the foundations v a r i e s with depth.

The reduction in s t r e n g t h of the soil along inclined shearing s u r f a c e s dictated by the reconstructed f a i l u r e s u r f a c e can be m e a s u r e d with a s m a l l lab- o r a t o r y vane. The strength reductions w e r e applied t o the in situ undrained s h e a r strengths m e a s u r e d with the field vane t o obtain the anisotropic strength of the soil.

The l a b o r a t o r y t r i a x i a l CAU t e s t s underestimated the actual s h e a r strength of the soil. The v e r y low s t r e n g t h s w e r e probably due t o the effects of 12 t o 14 months s t o r a g e in a humid room before the specimens w e r e t e s t e d .

This study supports B j e r r u m t s c o r r e l a t i o n between the calculated factor of safety based on undisturbed field vane s t r e n g t h s and the P l a s t i c i t y Index of the soil.

902

EARTH STRUCTURES

ACKNOWLEDGEMENTS

Many thanks a r e due t o R.G. Barton, owner of the tower silo, who p e r m i t t e d the investigations t o be c a r r i e d out, and to H.A. J a c k s o n , Development and Advisory Engineer, Canada Agriculture, who provided the data f r o m which the silage loads w e r e calculated.

T h i s p a p e r i s a contribution f r o m the Division of Building R e s e a r c h , National R e s e a r c h Council of Canada, and i s published with the approval of the D i r e c t o r of the Division.

REFERENCES

Anderson, A . , and B j e r r u m , L . , "Vane Testing i n Norway,

"

A m e r i c a n Society f o r Testing and M a t e r i a l s , Special Technical Publication 193, 1956, pp. 54-60.

B j e r r u m , L.

,

"Geotechnical P r o p e r t i e s of Norwegian M a r i n e Clays," Geotechnique, Vol. 4, No. 2, 1954, pp. 46-69.

Bozozuk, M . , "Effect of Sampling, Size and Storage on T e s t Results f o r M a r i n e Clay, I ' A m e r i c a n Society f o r Testing and M a t e r i a l s , Special Technical Publication 483, 1971, pp. 121 -131.

Eden, W . J . and Bozozuk, M .

,

"Foundation F a i l u r e of a Silo on Varved C l a y , " The Engineering J o u r n a l , Vol. 45, No. 9, September 1962, pp. 54-57.

Gadd, N. R . , "Surficial Geology of Ottawa M a p

-

A r e a Ontario and Quebec, 31 ~ / 5 , " Geological Survey of Canada, Department of Mines and Technical Surveys, P a p e r 62-16, 1963, 4 pages. J u m i k i s , A . R . , "Vertical S t r e s s T a b l e s f o r Uniformly Distributed

Loads on Soil, " College of Engineering, R u t g e r s University, Engineering R e s e a r c h Publication No. 52, 1971.

McCalmont, J . R.

,

" F a r m Silos, " Miscellaneous Publication No. 81 0, A g r i c u l t u r a l R e s e a r c h S e r v i c e , U. S. D e p a r t m e n t of Agriculture, A p r i l 1964, 26 pages.

Meyerhof, G. G.

,

"The Ultimate B e a r i n g Capacity of Foundations, " Geotechnique, Vol. 2, 1951, pp. 301-322.

Skempton, A. W .

,

"An Investigation of the Bearing Capacity of a Soft Clay Soil, J o u r n a l of the Institution of Civil E n g i n e e r s , P a p e r No. 5305, Vol. 18, 1942, pp. 307-321.

Skempton, A. W .

,

"The Bearing Capacity of C l a y s ,

"

P r o c e e d i n g s , Building R e s e a r c h C o n g r e s s , London, 1951, pp. 180-189.

T h i s publication i s being d i s t r i b u t e d by the Division of Building R e s e a r c h of t h e National R e s e a r c h Council of Canada. I t should n o t b e r e p r o d u c e d i n whole o r i n p a r t without p e r m i s s i o n of the o r i g i n a l p u b l i s h e r . The Di- v i s i o n would b e glad t o b e of a s s i s t a n c e i n obtaining

s u c h p e r m i s s i o n .

P u b l i c a t i o n s of the Division m a y b e obtained by m a i l - ing the a p p r o p r i a t e r e m i t t a n c e ( a Bank, E x p r e s s , o r P o s t Office Money O r d e r , o r a cheque, m a d e p a y a b l e t o t h e R e c e i v e r G e n e r a l of Canada, c r e d i t NRC) t o t h e National R e s e a r c h Council of Canada, Ottawa. K1A OR6. S t a m p s a r e n o t a c c e p t a b l e .

A l i s t of a l l p u b l i c a t i o n s of the D i v i s i o n i s a v a i l a b l e and m a y b e obtained f r o m the P u b l i c a t i o n s Section, Division of Building R e s e a r c h , National R e s e a r c h Council of Canada, Ottawa. KIA OR6.