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Landslides at Breckenridge, Pineview Golf Club, and Rockcliffe
Mitchell, R. J.
NATIONAL RESEARCH COUNCIL O F CANADA DIVISION O F BUILDING RESEARCH
LANDSLIDES A T BRECKENRIDGE, PINEVIEW G O L F CLUB, AND k O C K C L I F F E by R. J. M i t c h e l l ANALYZED I n t e r n a l R e p o r t No. 3 7 2 of t h e Division of Building R e s e a r c h OTTAWA N o v e m b e r 19 69
LANDSLIDES A T BRECKENRIDGE, PINEVIEW GOLF CLUB, AND ROC;<CLIFFE by R. J. Mitchell P R E F A C E During t h e p a s t y e a r a r e a p p r a i s a l of the m e c h a n i c a l s t r e n g t h of Leda c l a y u n d e r low effective confining s t r e s s e s h a s r e s u l t e d in a new a p p r e c i a t i o n f o r the m e c h a n i s m s leading to s l o p e i n s t a b i l i t i e s . S m a l l m i c r o f i s s u r e s existing within t h e a p p a r e n t l y i n t a c t c l a y give r i s e , u n d e r low s t r e s s e s , to d i l a t a n t behaviour and predominantly ' f r i c tionalf s h e a r
-
ing r e s i s t a n c e a s t h e m a t e r i a l s h e a r s into s m a l l nodules o r blocks. T h i s m o d e of f a i l u r e i s c o n s i s t e n t with c e r t a i n f i e l d o b s e r v a t i o n s r e g a r d i n g t h e f a c t o r s contributing t o i n s t a b i l i t i e s . Stability a n a l y s i s b a s e d on t h e s e c o n s i d e r a t i o n s h a s r e s u l t e d in r e a s o n a b l y good c o r r e l a t i o n with s e v e r a l r e c e n t l a n d s l i d e s ; t h r e e of t h e s e a r e d e s c r i b e d and analyzed i n t h i s r e p o r t . S o m e g e n e r a l conclusions b a s e d on t h e s e s t u d i e s and o t h e r r e c e n t investigations a r e d i s c u s s e d a t the end of t h i s r e p o r t .Ottawa
November 1969
N. B. Hutcheon D i r e c t o r
LANDSLIDES AT BRECKENRIDGE, PINEVIEW GOLF CLUB, AND ROCKCLIFFE
by
R. J. Mitchell
I. THE BRECKENRIDGE LANDSLIDE
The B r e c k e n r i d g e landslide of A p r i l 19 63 i s a typical example of the r e t r o g r e s s i v e type of sliding that m a y occur in n a t u r a l slopes of Leda clay. F o r t h r e e days, during heavy rainfall, i n t e r m i t t e n t sliding r e s u l t e d in the liquification and flow of about 30, 000 cubic y a r d s of clay. The a n a l y s i s of this landslide (1) r e s u l t e d in d i s c u s s i o n s about the extent of the initial s l i d e and, consequently, the s t r e s s r a n g e in which s h e a r s t r e n g t h t e s t s should b e conducted. T r i a x i a l t e s t s c a r r i e d out under low effective s t r e s s e s
[ ( a t l f a t , ) / 2 ]
<
1. 0 k g / c m 2 indicate that the c o m p r e s s i v e s t r e n g t h of the clay i s g r e a t l y dependent on the m e a n effective s t r e s s . The i n i t i a l s l i d e and subsequent r e t r o g r e s s i v e sliding a t B r e c k e n r i d g e a r e analyzed with r e f e r e n c e to t h e s e data.INTRODUCTION
The landslide which occur r e d on the B r e c k e n r i d g e c r e e k s l o p e s i n A p r i l 1963 h a s been documented by Crawford and Eden (1). F i g u r e s 1, 2 and 3 a r e reproduced f r o m this publication and show respectively: a view of the a r e a involved; a plan and section of the slope; and the r e s u l t s of s e v e r a l groups of t r i a x i a l t e s t s . The f a i l u r e s u r f a c e shown in F i g u r e 2 was established by vane borings. The s h e a r s t r e n g t h r e s u l t s i n F i g u r e 3 w e r e obtained f r o m consolidated undrained and d r a i n e d t r i a x i a l c o m p r e s s i o n t e s t s ( s o m e with a', d e c r e a s i n g ) and a r e d i s c u s s e d in d e t a i l by Crawford and Eden. The following points s u m up, briefly, the s t a t u s of the B r e c k e n - r i d g e s l i d e a n a l y s i s to date:
(1) Maximum springtime p i e z o m e t e r r e a d i n g s indicate full s a t u r a t i o n n e a r the toe of the slope ( r U = 0. 62) reducing to r U = 0. 5 n e a r the top of the slope.* F o r conditions existing a t the t i m e of f a i l u r e i t was suggested t h a t a n a v e r a g e r u value of 0. 62 i s m o s t applicable.
(2) The stability a n a l y s i s by Crawford and Eden r e s u l t e d in a c r i t i c a l c i r c l e incorporating the e n t i r e height of the slope a s sketched in
F i g u r e 2 ( c i r c l e 1). The f a i l u r e c r i t e r i o n f r o m group 2 t e s t s ( F i g u r e 3 ) was favoured a s being c l o s e to the a v e r a g e in s i t u s t r e s s e s and r e s u l t e d in a calculated safety f a c t o r of 1. 12 f o r r u = 0. 62, and 1 . 2 6 f o r r u = 0.5.
*
hyw w h e r e hyw i s the water p r e s s u r e in f e e t of water, H i s the r u - - -
(3) Subsequent d i s c u s s i o n by T. C. Kenney(2) challenged the a d - m i s s i b i l i t y of c i r c l e 1 in F i g u r e 2. He suggested t h a t b e c a u s e the length-to-width r a t i o of the initial slide was kinematically r e s t r i c t e d the l a r g e s t possible s c a r p would be s e m i - c i r c u l a r in plan a s sketched in F i g u r e 2 ( c i r c l e 2).
(4) In t h e i r original publication, Crawford and Eden noted the d i s c r e p a n c y between the established f a i l u r e plane and their c r i t i c a l c i r c l e ( F i g u r e 2), and o b s e r v e d that this indicated a higher f r i c t i o n a l component of strength. Considered together, group 2 and group 3 t e s t s ( F i g u r e 3) suggest a pronounced r e - duction of s t r e n g t h with reduced m e a n effective s t r e s s , but no f u r t h e r study of the f a i l u r e c r i t e r i o n in the low s t r e s s region was initiated a t that time.
TRIAXIAL TESTS IN THE LOW STRESS REGION
A s e r i e s of constant p p
C
= ( o+
2 ol, ) / 3]
d r a i n e d t r i a x i a l c o m - p r e s s i o n t e s t s was c a r r i e d out with v a l u e s of p ranging f r o m 0 . 2 k g / c m 2 t o 0. 8 k g / c m 2 on tube s a m p l e s taken f r o m j u s t below the established f a i l u r e s u r f a c e a t a location approximately indicated on F i g u r e 2. One of t h e s e t e s t s was p e r f o r m e d under a constant r a t e of change of s t r e s s e s ; the r e m a i n d e r w e r e conducted by changing the confining p r e s s u r e and a x i a l s t r e s s in s m a l l i n c r e m e n t s . E a c h i n c r e m e n t a l loading t e s t was c a r r i e d out over a p e r i o d of about t h r e e days with roughly eight equal i n c r e m e n t s to f a i l u r e . The f a i l u r e points f r o m t h e s e t e s t s a r e plotted, together with s o m e of the data f r o m e a r l i e r t e s t s , i n F i g u r e 4. Typical s t r e s s - s t r a i n r e l a t i o n s h i p s a r e shown, f o r t h e low s t r e s s t e s t s , i n F i g u r e 5. F r o m t h e s e t e s t d a t a the following ob- s e r v a t i o n s a r e m a d e r e g a r d i n g the behaviour of the c l a y under low confining s t r e s s e s :(1) The f a i l u r e c r i t e r i o n f o r t e s t s conducted a t values of p l e s s than 0. 8 k g / c m 2 is defined in t e r m s of conventional p a r a m e t e r s by a low value of c ' and a high value of
a ' .
(2) A l i n e a r deviatoric s t r e s s - s h e a r s t r a i n (q, C ) relationship e x i s t s
up to a deviatoric s t r e s s approaching one-half of the deviatoric s t r e s s a t f a i l u r e
(quit).
(3) S m a l l positive volume c h a n g e s ( c o m p r e s s i o n ) f o r ¶/rqult s
$
g e n e r a l l y oc-
c u r , followed by deformation a t n e a r l y constant volume. Specimens began to d i l a t e significantly a t (q/qult>
0. 8) p r i o r to f a i l u r e . The o v e r - a l l volume change f o r a l l t e s t s was no g r e a t e r than 0 . 2 p e r cent.T h e s e data indicate that the m a t e r i a l behaviour below f a i l u r e i s con- s i s tent with the behaviour expected f r o m heavily overconsolidated clay.
An a b r u p t change in the mode of f a l i u r e o c c u r s a t v a l u e s of p below 0. 8 k g / c m 2 ( F i g u r e 4 ) , and i t h a s
L
een postulated that this r e s u l t s f r o m m i c r o f i s s u r e s within the specimen.The constant p s t r e s s path was chosen b e c a u s e i t r e d u c e s ' r e c o v e r a b l e ' volume changes due to changes in m e a n n o r m a l effective s t r e s s and allows the v o l u n ~ e changes due to s h e a r to be studied in- dependently. This path a l s o intercct;~ts n o r m a l f a i l u r e c r i t e r i a a t a sufficiently l a r g e angle to allow the f a i l u r e point to b e defined r e a s o n - ably a c c u r a t e l y .
S L I P CIR C L E ANALYSIS
The stability a n a l y s i s was c a r r i e d out using Bishop's equation
( 3 ) , p r o g r a m m e d f o r the IBM 360 c o m p u t e r , by A r s e n e a u l t ( 4 ) . This
calculation a s s u m e s that the f a i l u r e condition b e attained simultaneously a t a l l points on the slip c i r c l e ' and r e p r e s e n t s a t h e o r e t i c a l upper bound. An unsafe e s t i m a t e of the safety f a c t o r m a y r e s u l t f r o m this a n a l y s i s , p a r - ticularly if the s o i l exhibits a peak s t r e n g t h followed by a d e c r e a s e in
shearing r e s i s t a n c e . In s o m e heavily overconsolidated s o i l s i t h a s been found that p r o g r e s s i v e f a i l u r e s occur in n a t u r a l slopes with a calculated
s h e a r i n g r e s i s t a n c e corresponding to a ' r e s i d u a l strength' ( 5 ) . In con- t r a s t , the c r i t i c a l s l i p c i r c l e has been found to c o r r e i a t e reasonably well with slope f a i l u r e s i n other c l a y s o i l s where, undoubtedly, the t h r e e - dimensional n a t u r e of the r e a l p r o b l e m c o m p e n s a t e s f o r the o v e r e s t i m a t e of the safety f a c t o r in the two-dimensional a n a l y s i s . In many n a t u r a l c l a y s s u c h a s Leda clays, the s l i p c i r c l e a n a l y s i s h a s not yet been adequately evaluated.
The s l i p c i r c l e a n a l y s i s i s used to d e t e r m i n e a ' c r i t i c a l c i r c l e ' f o r given s t r e n g t h p a r a m e t e r s . As mentioned e a r l i e r , i t i s n e c e s s a r y to p l a c e kinematic r e s t r i c t i o n s on s u c h calculations if the r e a l g e o m e t r y of the slope i s not a m e n a b l e to two-dimensional a n a l y s i s . Kenney(6)has suggested a method of i n t e r p r e t a t i o n whereby a ' c r i t i c a l s t r e s s curve' i s obtained f o r a given slope g e o m e t r y by analyzing n u m e r o u s kinematically acceptable c i r - c u l a r a r c s . F o r given geometry and ground water condition and a safety f a c t o r of 1. 0, a l l calculated values of c1 and
@',
when plotted in a ( T , DE ) space, will define a unique point which m a y be called a n ' a v e r a g e s t r e s s pointf f o r that p a r t i c u l a r c i r c l e . An envelope enclosing n u m e r o u s ' a v e r a g e s t r e s s pointsf f o r m s the ' c r i t i c a l s t r e s s c u r v e ' which m a y be c o m p a r e d to the a p p r o p r i a t e f a i l u r e c r i t e r i o n f r o m l a b o r a t o r y t e s t s in o r d e r to e s t i m a t e the stability of the slope. The m a i n a t t r i b u t e of this i n t e r p r e t a t i o n s e e m s to b e that the calculations e s t a b l i s h envelopes of p o s s i b l e field f a i l u r e con- ditions in s t r e s s s p a c e that c o v e r the r a n g e of n o r m a l s t r e s s e s . T h e s e en- velopes c a n b e c o m p a r e d d i r e c t l y only to l a b o r a t o r y t e s t s c a r r i e d out a t the a p p r o p r i a t e n o r m a l s t r e s s e s .C r i t i c a l s t r e s s c u r v e s w e r e calculated f o r the B r e c k e n r i d g e s l i d e using the IBM s y s t e m 360 computer to s o l v e Bishop's stability equation. I n F i g u r e 6a v a r i o u s g e o m e t r i c a l l y f e a s i b l e c i r c u l a r a r c s a r e d r a w n with c e n t r e s C1 to C, and passing through points 1 and 2 in the vicinity of the f a i l u r e s u r f a c e . Analysis of t h e s e c i r c l e s yields the c r i t i c a l s t r e s s c u r v e s AB (for r = 0. 62) and CD ( f o r r = 0 . 4 0 ) shown i n F i g u r e 6b.
u
An a v e r a g e value of r = u / y H i s used in die computer p r o g r a m with r =
u u
0. 62 corresponding to full s a t u r a t i o n f o r the B r e c k e n r i d g e clay.
Assuming that the initial f a i l u r e r e s u l t e d in a g e o m e t r y given by ACBDF in F i g u r e 6a, the f i r s t r e t r o g r e s s i v e s l i d e was analyzed. The computer was r e s t r i c t e d to s e a r c h i n g f o r c r i t i c a l c i r c l e s through point 3 ( F i g u r e 6a) on the f a i l u r e s u r f a c e and yielded the c r i t i c a l s t r e s s c u r v e s E F and GH in F i g u r e 6b. A second r e t r o g r e s s i v e s l i d e was analyzed in a s i m i l a r m a n n e r by approximating the g e o m e t r y of the slope a f t e r the m o s t c r i t i c a l c i r c l e ( c e n t r e C 5 in F i g u r e 6a) was a s s u m e d to s l i d e during the f i r s t r e t r o g r e s s i v e f a i l u r e . This a n a l y s i s yielded the c r i t i c a l s t r e s s c u r v e s KL and MN in F i g u r e 6b.
The f a i l u r e c r i t e r i o n ( c l , @ I ) f r o m F i g u r e 4 i s drawn in the ( t, s )
s p a c e of F i g u r e 6b i n t e r m s of m a x i m u m obliquity and c a n b e c o m p a r e d d i r e c t l y to the c r i t i c a l s t r e s s c u r v e s . This c o m p a r i s o n s u g g e s t s that the initial s l i d e would not occur until the a v e r a g e r was about 0. 5. Sub-
u
sequent r e t r o g r e s s i v e sliding, however, would o c c u r a t a v e r a g e r values u
of 0.4.
Finally, the existing s t a b l e back s c a r p was analyzed with r e s p e c t t o c i r c l e s tangent to the projected f a i l u r e s u r f a c e . T h e s e computations yielded the c r i t i c a l s t r e s s c u r v e QR in F i g u r e 6b and hence, p r e d i c t the stability of the s c a r p even under conditions of f u l l saturation.
It a p p e a r s t h a t stability i s attained a f t e r a s e r i e s of r e t r o g r e s s i o n s by the flattening of the back s c a r p and the i n c r e a s e in elevation of the f a i l u r e s u r f a c e (i. e. n a t u r a l slope flattening). The p e a r - s h a p e d g e o m e t r y of the c r a t e r f o r m e d by r e t r o g r e s s i v e sliding would a l s o contribute to stability. Variations in s a t u r a t i o n and m a t e r i a l p r o p e r t i e s (e. g . d e g r e e and d i r e c t i o n of f i s s u r i n g ) could contribute to changes in c i r c l e g e o m e t r y a s r e t r o g r e s s i o n p r o c e e d s . The stabilizing effect of previously flowed m a t e r i a l m a y contribute to the i n c r e a s e i n the elevation of the f a i l u r e s u r f a c e a s r e t r o g r e s s i o n p r o - c e e d s . In o r d e r to c a r r y out the type of a n a l y s i s d e s c r i b e d above, i t i s n e c e s s a r y to know the approximate location of the f a i l u r e s u r f a c e s o that r e a s o n a b l e t r i a l c i r c l e s c a n be drawn. No a p r i o r i method i s available t o p r e d i c t the m o s t probable location.
I t i s suggested t h a t the s l i p c i r c l e stability a n a l y s i s m a y give r e a s o n - a b l e values f o r s a f e t y f a c t o r s in Leda c l a y slopes if the r e a l f a i l u r e a r c is
a c c u r a t e l y known and can be r e a s o n a b l y approxirnated by a c i r c l e . The m o s t c r i t i c a l c i r c l e s calculated (i'cjr l a b o r a t o r y v a l u e s of c' and
Q
'
) by this a n a l y s i s may, however, not c o r r e s p o n d to the r e a l f a i l u r e a r c in many c a s e s . Thus i t would a p p e a r that the s l i p c i r c l e a n a l y s i s is l i m i t e d in application until sufficient p r e c e d e n c e i s established, o r until sufficient knowledge r e g a r d i n g the f a i l u r e m e c h a n i s m i s available, to p r e d i c t the probable location of f a i l u r e s u r f a c e s a p r i o r i .CONCLUSIONS
The following conclusions a r e tentative in lieu of f u r t h e r c o r r e l a t i o n : (1) In the region of n o r m a l s t r e s s e s applicable to existing n a t u r a l slopes in Leda c l a y a f a i l u r e c r i t e r i o n substantially different f r o m the u s u a l Hvorslev o r Mohr-Coulomb c r i t e r i o n f o r overconsolidated c l a y s i s found f r o m t r i a x i a l c o m p r e s s i o n t e s t s . The high value of 41 ( in this c a s e 35 " )
and low c' (0. 05 k g / c m 2 ) approximating the f a i l u r e condition in the low s t r e s s region a r e supported by field observations r e g a r d i n g the c r i t i c a l r o l e of
groundwater conditions on slope instabilities.
(2) The s l i p c i r c l e a n a l y s i s a p p e a r s to c o r r e l a t e reasonably well with the f a i l u r e c r i t e r i o n i n the low s t r e s s region if s l i p c i r c l e s fitted to the kinematic r e q u i r e m e n t s of the slope g e o m e t r y a r e analyzed. A safety f a c t o r of 1. 06 i s obt ained (point P in f i g u r e 6b) f o r Q = 35
"
and r varyingu
f r o m 0. 52 a t the top of the slope to r u = 0. 62 a t the toe of the slope. T h e s e a r e the p a r a m e t e r s c o n s i d e r e d m o s t applicable a t the t i m e of the Brecken- ridge slide. I t would a p p e a r t h a t 'peak strengths' a r e mobilized along the m a j o r portion of the f a i l u r e a r c simultaneously in Leda clay. The n a t u r e
of t h e s e flow s l i d e s adds confirmation to this hypothesis s i n c e a t r e m e n d o u s amount of e n e r g y m u s t be available a f t e r the f a i l u r e a r c i s f o r m e d to
completely r e w o r k the clay and t r a n s p o s e i t hundreds of y a r d s f r o m the c r a t e r over r e l a t i v e l y f l a t t e r r a i n .
(3) M i c r o f i s s u r e s a r e a p p a r e n t if undisturbed s p e c i m e n s of the Brecken- ridge Leda c l a y a r e broken. T h e s e a p p e a r s i m i l a r to f r a c t u r e planes in rock. T h e s e m i c r o f i s s u r e s a r e c o n s i d e r e d i n s t r u m e n t a l i n the dilatant behaviour
and predominantly 'frictionall s h e a r i n g r e s i s t a n c e under low confining p r e s s u r e s . GOLF CLUB LANDSLIDE
GENERAL DESCRIPTION
In November 1967 a s m a l l landslide involving about 2, 000 cubic y a r d s of Leda c l a y o c c u r r e d in the w e s t bank of Green C r e e k adjacent to the P i n e - view Golf Club. F i g u r e 7 shows the location of the slide; F i g u r e 8 is a photograph of the i m m e d i a t e s l i d e a r e a .
A e r i a l photographs ( a i r photos L7
-
16 and L7- 17 produced by A. E.Simpson Ltd., Montreal, November 1957) showed that the 1967 s l i d e oc- c u r r e d in the v e r y shallow c r a t e r of a n e a r l i e r slip. A s t a d i a s u r v e y of the s l i d e a r e a was s u p e r i m p o s e d on a 2$-ft contour m a p p r e p a r e d f r o m the 19 57 a i r photos t o produce the c e n t r e l i n e section shown in F i g u r e 9 .
Typical c r o s s s e c t i o n s and the s i d e s c a r p elevations (unaffected by the s l i d e except f o r s u r f a c e tension c r a c k s ) a r e a l s o shown on F i g u r e 9. The s l i d e o c c u r r e d i n the top half of the 60-ft-high bank and was not initiated by toe e r o s i o n . Active toe e r o s i o n i s evident in s l o p e s immediately e a s t of the slide; t h e s e s l o p e s e x i s t a t a n a v e r a g e inclination of about 40" and a r e a l m o s t 50 f t i n height. The a v e r a g e inclination on the c e n t r e l i n e of the slope which failed was about 2 8 " .
GROUND WATER CONDITIONS
It c a n b e s e e n f r o m F i g u r e 7 t h a t the upland behind the landslide i s p a r t i a l l y d r a i n e d by deep gullies. Although the ground water r e g i m e a t the t i m e of the s l i d e i s not known, it i s thought t h a t two f a c t o r s could con- t r i b u t e to high p o r e water p r e s s u r e s : the old s l i d e m i g h t have a c t e d a s a d r a i n a g e sink f o r the higher banks on each side; s u r f a c e f r e e z i n g m i g h t have p r e v e n t e d d r a i n a g e out of the s l o p e thus causing a t e m p o r a r y r i s e i n water p r e s s u r e s . P i e z o m e t e r s installed a f t e r the s l i d e (locations shown on F i g u r e 9 ) indicate slight a r t e s i a n conditions a t the toe of the s l i d e while the upland p h r e a t i c l e v e l h a s been about 8 f t below ground l e v e l during the s u m m e r .
The G r e e n C r e e k s l o p e s have s o m e n a t u r a l s p r i n g s and a complex ground water r e g i m e m a y exist. Since the d a t e of the slide, w a t e r h a s been con- tinuously percolating f r o m the ground a t the b a s e of the s c a r p f o r m i n g a pond i n the c r a t e r that d r i e s up only during extended p e r i o d s of hot d r y weather ( F i g u r e 9 ) .
It i s evident f r o m F i g u r e 7 that the s t e e p s l o p e s to the e a s t of the s l i d e a r e drained f r o m behind and to the e a s t a s well a s toward the C r e e k .
It i s suggested t h a t the b e t t e r d r a i n a g e of t h e s e s l o p e s m a y explain t h e i r existence a t s t e e p e r inclinations than the s l o p e which failed. This suggestion i s c u r r e n t l y being investigated by p i e z o m e t e r installations.
LOCATION O F S L I P SURFACE
A s e r i e s of s a m p l e s (54 m m NGI piston s a m p l e r ) w e r e taken a t two locations on the c e n t r e l i n e of the s l i d e a s shown on F i g u r e 10. Below points 1 and 2 in F i g u r e 10 the clay was apparently undisturbed while s h e a r planes and discontinuities existed in the overlying m a t e r i a l . At s e v e r a l other locations a one-in. - d i a m e t e r p e a t s a m p l e r was u s e d to locate the undisturbed m a t e r i a l . Below the toe of the s l i p the depth of spoil was m e a s u r e d ; the p r e - s l i d e ground s u r f a c e was located in a s i m i l a r m a n n e r . F u r t h e r evidence t h a t
the s l i d e was confined to the top halt of the slope was provided by the f a c t that w a t e r flowin; f r o m the toe of the s l i d e quickly c u t a n a r r o w gully out of the spoil m a t e r i a l to the depth of the p r e - s l i d e s u r f a c e , exposing r o o t s and other evidence of previous exposure.
DESCRIPTION O F SLIDE MATERIAL -
The G r e e n C r e e k s l o p e s in t ! ~ e i m m e d i a t e a r e a of the s l i d e c o n s i s t of s e v e r a l f e e t of stiff f i s s u r e d b r ~ w n (weathered) Leda c l a y overlying deep deposits of g r e y Leda clay with black mottling. The c l a y i s overconsolidated with a n a v e r a g e P value of about 3 k g / c m 2 . M o i s t u r e content i s g e n e r a l l y slightly above the ?iquid l i m i t a ~ d both of t h e s e d e c r e a s e with depth.
At the toe of the s t e e p s l o p e s w h e r e a c t i v e e r o s i o n was t a k i - ~ g p l a c e l a r g e open f i s s u r e s w e r e noted and 8-in. cubes of clay could easily be r e - moved f r o m the bank. In o r d e r to d e t e r m i n e the depth of t h e s e m a j o r f i s -
s u r e s a 1 -in. d i a m e t e r horizontal c o r e was taken a t a n angle to the d i r e c t i o n of f i s -
suring. Specimens obtained beyond about 5 f t f r o m the s u r f a c e a p p e a r e d to be intact. T h e s e s p e c i m e n s and other apparently homogeneous s p e c i m e n s
taken f r o m below the f a i l u r e plane would, however, f r a c t u r e in bending along what a p p e a r e d to b e randomly orientated p r e f e r r e d f r a c t u r e planes. With continued breaking of a s p e c i m e n one could obtain a pile of gravel-sized chunks of stiff s e n s i t i v e Leda clay that a p p e a r e d to b e f r e e of m i c r o f i s s u r e s . I n engineering t e r m s the m a t e r i a l would be d e s c r i b e d a s brittle. Reaction with a c i d s indicated the p r e s e n c e of a s m a l l quantity of c a l c a r e o u s m i n e r a l s i n the c l a y and t h e s e r e a c t i o n s on a f r a c t u r e d s u r f a c e would c a u s e spalling of s m a l l p i e c e s of clay. S e v e r a l s h e l l s w e r e found in the s a m p l e s .
TRIAXIAL STRENGTH TESTS
A s e r i e s of t r i a x i a l c o m p r e s s i o n t e s t s w e r e c a r r i e d out under low confining p r e s s u r e s on s p e c i m e n s t r i m m e d f r o m the 5 4 - m m s a m p l e s ob- tained f r o m the location shown in F i g u r e 10. S m a l l i n c r e m e n t s of a x i a l load w e r e applied simultaneously with s m a l l d e c r e m e n t s of c e l l p r e s s u r e s o the m e a n n o r m a l s t r e s s p =
(oil
+
2 o', ) / 3 was held n e a r l y constant through- out a given t e s t . I n c r e m e n t durations ranged f r o m about 400 m i n u t e s to o v e r 4, 000 m i n u t e s and typical t i m e-
a x i a l deformation c u r v e s a r e plotted in F i g u r e 11 f o r the l a s t two i n c r e m e n t s of s e v e r a l t e s t s . T h e s e c u r v e s s u g g e s t that the s p e c i m e n s would r e a c h a s t a b l e equilibrium1 condition under d e - v i a t o r i c s t r e s s e s approaching 90 p e r c e n t of the f a i l u r e load in this type of t e s t .The s t r e s s paths and f a i l u r e points f o r a l l t e s t s a r e plotted in the (q, p) s p a c e of F i g u r e 12. The condition of f a i l u r e i s a p p r o x i n ~ a t e d by two i n t e r s e c t i n g s t r a i g h t l i n e s . F o r a l l t e s t s conducted with p s 0. 5 kg/cn12
f a i l u r e o c c u r r e d on the line AA1 in F i g u r e 12. T h e s e limiting s t a t e s a r e r e a c h e d only if the effective confining p r e s s u r e i s equal to z e r o in the t r i - axial t e s t and, under this condition, the influence of end r e s t r a i n t m a y r e - s u l t in t e n s i l e r a d i a l a n d / o r t e n s i l e c i r c u m f e r e n t i a l s t r e s s e s o v e r a signi- ficant p a r t of the specimen. All t e s t s p e c i m e n s which failed under the con- dition = 0 exhibited combinations of v e r t i c a l splitting and s h e a r planes a t f a i l u r e . The two t e s t s p e c i m e n s which failed with
a t 3
> 0 (giving line BB1 in F i g u r e 12 ) showed typical s h e a r f a i l u r e s with no evidence of v e r t i c a l splitting.S t r e s s - v o l u m e t r i c - s t r a i n c u r v e s f o r a l l t e s t s a r e plotted in F i g u r e 13. All s p e c i m e n s r e a c h e d f a i l u r e a t a x i a l s t r a i n s of l e s s than one p e r cent. The stiffness modulus of the m a t e r i a l i n c r e a s e d with i n c r e a s i n g
p f o r 0. 1 s p I 0 . 5 and then d e c r e a s e d again f o r p > 0 . 5 kg/cm2. The volu-
m e t r i c s t r a i n s plotted in F i g u r e 13 indicate a difference in deformation be- haviour of s p e c i m e n s f o r which f a i l u r e c o r r e s p o n d s to l i n e AAt in F i g u r e s 12 and of s p e c i m e n s f o r which f a i l u r e c o r r e s p o n d s to line BB1. The f o r m e r s p e c i m e n s deformed a t n e a r l y constant volume to q / q 2 0. 8 followed by
u l t
significant dilation ( i n c r e a s e in volume) to f a i l u r e while the l a t t e r s p e c i m e n s d e c r e a s e d in volume throughout the t e s t s . Volume expansion m a y b e due to s h e a r o r m a y r e s u l t f r o m t e n s i l e s t r e s s e s a s s o c i a t e d with v e r t i c a l splitting. Volume c o m p r e s s i o n p r i o r to f a i l u r e m u s t b e a s s o c i a t e d with s t r u c t u r a l b r e a k - down during s h e a r deformation.
STABILITY ANALYSIS
The slope was analyzed f o r stability using a m a i n l y g e o m e t r i c a p - plication of the s l i p c i r c l e analysis, i. e . , without r e f e r e n c e to m e a s u r e d s t r e n g t h p a r a m e t e r s f o r the soil. A v a r i e t y of g e o m e t r i c a l l y f e a s i b l e c i r c l e s including those d r a w n in F i g u r e 10 w e r e analyzed using Bishop's equation and a v e r a g e r u = u / y H values of 0 . 4 and 0. 62. F o r e a c h c i r c l e a n a v e r a g e s t r e s s point in
5
8 s p a c e i s obtained and enclosed by a c r i t i c a l s t r e s s c u r v e (6).n
The c r i t i c a l s t r e s s c u r v e s obtained f o r r = 0 . 4 and r = 0. 6 a r e shown in
u u
F i g u r e 14.
In view of the mode of f a i l u r e in the s t r e s s region p 0. 5 k g / c m 2 a m a x i m u m effective s t r e s s r a t i o in the f o r m q = Mp will b e used to d e s c r i b e f a i l u r e . In plane s t r a i n the m a x i m u m s h e a r s t r e s s c r i t e r i o n i s given by
( d l -
u
( 0t 0 ) = 3 M / ( 6 t M)and is equal to unity whenu13
a t f a i l u r e i s equal to z e r o (line AAt). Clearly, the M o h r - Coulomb ( m a x i m u m obliquity) c r i t e r i o n is not applicable to this type of f a i l u r e and, consequently, the c r i t i c a l s t r e s s c u r v e s cannot be c o r r e c t l y c o r r e l a t e d with the f a i l u r e line
at3
= 0 a s drawn in F i g u r e 14 without a knowledge of the d i r e c t i o n s of the principal s t r e s s e s in situ.I t i s conceivable, however, that a f a i l u r e s u r f a c e could be c o m - posed of s h e a r planes a t 45" to thr. principal s t r e s s d i r e c t i o n s
a l l
anda',
and joined by tension c r a c k s . Such a f a i l u r e plane would develop in a direction controlled p a r t i a l l y by the g e o m e t r y of the p r o b l e m (compati- bility of d i s p l a c e m e n t s ) and p a r t i a l l y by the d i r e c t i o n s of the p r i n c i p a l s t r e s s e s . On the assumption that a f a i l u r e plane of the type d e s c r i b e d above could b e approximated by a c i r c u l a r a r c , the s l i p c i r c l e a n a l y s i s could b e used to analyze the f a i l u r e . Since the magnitudes of the s t r e s s e s (being unknown) a r e approximated, in the slip c i r c l e analysis, by con-
sidering the s t a t i c equilibrium of s l i c e s , the solution m a y be in c o n s i d e r a b l e e r r o r and does not n e c e s s a r i l y yield to s a f e e s t i m a t e f o r the safety f a c t o r .
(Lf a m a x i m u m obliquity c r i t e r i o n i s applicable, the s l i p c i r c l e a n a l y s i s yields a t h e o r e t i c a l upper bound f o r the safety f a c t o r . )
A f a i l u r e m e c h a n i s m of the type d e s c r i b e d above would n e c e s s a r i l y b e p r e c e d e d by s t r e s s r e l i e f . C r e e p movements d i r e c t e d g e n e r a l l y down- slope could a l t e r the s t r e s s distribution in such a way that the m i n o r principal s t r e s s , orientated a t s o m e a c u t e angle to the horizontal, b e c a m e of the
s a m e magnitude a s the m a x i m u m p o r e water p r e s s u r e (thus giving at3 = 0). When this condition i s attained (perhaps following y e a r s of s e a s o n a l c r e e p m o v e ~ n e n t s ) a combination of s h e a r planes and i n t e r s e c t i n g t e n s i l e c r a c k s m a y f o r m a s l i p plane which could a t any given point be orientated a t any angle between 45" and 90" with the m i n o r principal s t r e s s . The t e n s i l e c r a c k s , which a r e invariably observed on the upper t e r r a c e behind the top s c a r p , indicate that the horizontal s t r e s s e s have been r e l i e v e d a s a r e s u l t of the d i s p l a c e m e n t of the s l i d e m a t e r i a l and r e p r e s e n t s a f a i l u r e m e c h a n i s m of the type proposed above.
Lf the f a i l u r e line
a!,
= 0 i s c o m p a r e d d i r e c t l y to the c r i t i c a l s t r e s s c u r v e s in F i g u r e 14 i t would suggest that a n a v e r a g e r value of about 0.5 would b e n e c e s s a r y f o r f a i l u r e along the plane establisxed ( b e s t approximated by c i r c l e No. 14 which was found c r i t i c a l f o r @ = 3 0 ° , r = 0.4, and p a s s -u ing through point 1
-
F i g u r e 10).CONCLUSIONS AND DISCUSSIONS
The stability a n a l y s i s d i s c u s s e d h e r e a p p e a r s to give a r e a s o n a b l e c o r r e l a t i o n with t r i a x i a l s t r e n g t h t e s t s although m o r e information r e g a r d i n g the ground water r e g i m e a t the t i m e of f a i l u r e would b e n e c e s s a r y to obtain a n u ~ n e r i c a l e s t i m a t e f o r the s a f e t y f a c t o r . A f a i l u r e m e c h a n i s m other than Mohr
-
Coulomb r u p t u r e i s proposed f o r the m a t e r i a l , and f u r t h e r a n a l y s i s of slopes in the s a m e m a t e r i a l would be n e c e s s a r y i n o r d e r to e s t a b l i s h a l e v e l of confidence in this approach.A r a t h e r complex ground water r e g i m e i s suggested f o r the a r e a immediately surrounding the slope f a i l u r e and this i s c u r r e n t l y being in- vestigated by u s e of piezometer installations.
C r e e p movements have been postulated a s a c a u s e of l a t e r a l s t r e s s relief in n a t u r a l slopes. This s t r e s s relief i s a n e c e s s a r y condition f o r the f a i l u r e m e c h a n i s m proposed. Inclinometer tubes have been i n s t a l l e d in
s e v e r a l n a t u r a l s l o p e s in the Ottawa a r e a to investigate this possibility. The stability a n a l y s i s was c a r r i e d out using a v e r a g e values of r f o r the slope in o r d e r to simplify the computation. The r e s u l t s a r e useyul f o r predicting the a p p r o x i m a t e ground water r e g i m e r e q u i r e d f o r f a i l u r e . I t i s c l e a r , however, that the m a x i m u m p o r e water p r e s s u r e s m u s t be a c - c u r a t e l y known and a c c u r a t e l y r e p r e s e n t e d in the computations if a r e a s o n - able e s t i m a t e of the safety f a c t o r i s r e q u i r e d . The c o m p u t e r p r o g r a m r e - f e r e d to e a r l i e r h a s been modified to i n c o r p o r a t e a v a r i a b l e r f a c t o r .
u
The possibility of ' s t r e n g t h anisotropy' h a s not been included in p r e - ceeding d i s c u s s i o n s ' : could b e a n i m p o r t a n t f a c t o r . I t i s p o s s i b l e f o r a
Mohr - Coulomb type of r u p t u r e to occur on planes corresponding to the lower portion of the in s i t u s l i p s u r f a c e a t ' s t r e n g t h s ' m u c h l e s s than those
r n e a s u r e d in the t r i a x i a l t e s t s p r e s e n t e d h e r e i n ( c a r r i e d out on v e r t i c a l l y t r i m m e d s p e c i m e n s ) . T e s t s a r e c u r r e n t l y being conducted to investigate deviations f r o m isotropy in the m e c h a n i c a l s t r e n g t h of Leda clay.
LII. C. F. S. ROCKCLIFFE LANDSLIDE DESCRIPTION
During A p r i l 1967 s e v e r a l landslides o c c u r r e d along the south bank of the Ottawa R i v e r between the Rockcliffe a r e a in Ottawa and Hiawatha P a r k , ten m i l e s to the e a s t of Ottawa. The l a r g e s t of t h e s e landslides, involving about 30, 000 cubic y a r d s of Leda clay, o c c u r r e d on 3 A p r i l a t the Rock- cliffe Air B a s e adjacent to a l a r g e hangar. E a r l y snow c o v e r in the autumn of 1966 prevented deep f r o s t penetration in the Ottawa a r e a and the m a j o r snow m e l t (about 20 in. ) during the l a s t week of M a r c h 1967 was accompanied and followed by heavy rainfall. The level clay t e r r a c e supporting the Rock- cliffe Air B a s e d r a i n s toward the Ottawa River and field o b s e r v a t i o n s a f t e r the s l i d e indicated that the slope was fully s a t u r a t e d a t the tirne of the slide. After hearing the initial s l i d e p e r s o n n e l in the Flying Club hangar went outside to investigate. They noticed that a hut located a t the top of the bank was m i s s i n g . The s l i d e r e t r o g r e s s e d to the position shown in the plan and c r o s s s e c t i o n of F i g u r e 15. The f a i l u r e s u r f a c e shown on that f i g u r e was located by f i e l d vane testing. Continuous vane t e s t s showed a s h a r p i n c r e a s e in s h e a r i n g r e s i s - tance f r o m about 0.35 k g / c m 2 to about 1 k g / c m 2 a t the inter section between
the spoil m a t e r i a l and the ~ n d i s t u r l ~ e d clay below.
The Leda clay a t the Rockcilffe s i t e exhibits a p r econsolidation r e s s u r e i n c r e a s i n g f r o m about 2 k g / c m 2 a t a depth of 10 f t to about
.
5 k g / c m 2 a t a depth of 40 ft. The n a t u r a l m o i s t u r e content i s f a i r l y uniform between t h e s e depths a t about 68 p e r c e n t d r y weight and the liquid l i m i t of the c l a y i s equal to o r slightly g r e a t e r than the n a t u r s - m o i s t u r e content. The clay i s quil e stiff but c a n be f r a c t u r e d intos m a l l blocks without evidence of e x c e s s m o i s t u r e . F u r t h e r s h e a r i n g of the s m a l l blocks without change in m o i s t u r e content p r o d u c e s a viscous liquid. The initial f r a c t u r e into s m a l l blocks i s c o n s i d e r e d to r e s u l t f r o m m i c r o f r a c t u r e s in the clay. Above the ten f t depth the weathered c r u s t h a s a lower m o i s t u r e content and higher vane
s t r e n g t h but exhibits a s i m i l a r m i c r o f r a c t u r e pattern. TRIAXIAL TEST DATA
The f a i l u r e points f o r fully drained constant m e a n n o r m a l s t r e s s (p =
a t l
+
2 a', 1 3 = constant) t r i a x i a l t e s t s a r e plotted i n F i g u r e 16. T e s t s w e r e c a r r i e d out on s p e c i m e n s t r i m m e d vertically, horizontally, and a t 45" with r e s p e c t to the in s i t u orientation (vertically r e f e r r i n g t o the n o r m a l testing orientation). S e v e r a l t e s t s w e r e c a r r i e d out under a constant r a t e of change of s t r e s s ( t e s t duration about two days); the r e - m a i n d e r w e r e c a r r i e d out using s m a l l i n c r e m e n t a l changes in s t r e s s( t e s t duration about one week). Within this r a n g e the method and t i m e of loading does not a p p e a r to influence the data.
The f a i l u r e points a r e b e s t approximated, in the m e a n n o r m a l s t r e s s r a n g e f r o m z e r o to 1 . 0 k g / c m 2 , by the c u r v e d s u r f a c e shown in F i g u r e 16. In this r a n g e of n o r m a l s t r e s s the s p e c i m e n s tended to d i l a t e slightly p r i o r to f a i l u r e a s indicated in F i g u r e 17. No significant v a r i - ation of s t r e n g t h with s p e c i m e n orientation could be noted in t h e s e t e s t s . Specimens s h e a r e d a t values of p b e t w e e n 1 . 0 k g / c m 2 and 2 . 0 k g / c m 2
suffered v o l u m e t r i c c o m p r e s s i o n ( a s s o c i a t e d with yielding of the c e m e n t e d p a r t i c l e s t r u c t u r e ) during the e n t i r e t e s t . The s t r e n g t h of the horizontally orientated s p e c i m e n s was found to be only 70 p e r c e n t of the s t r e n g t h of the v e r t i c a l l y orientated s p e c i m e n s in this s t r e s s range.
The d i r e c t i o n a l variation in s t r e n g t h i s attributed to r e s i d u a l s t r e s s e s a r i s i n g b e c a u s e the clay p a r t i c l e s a r e cemented in s i t u under a n a n i s o t r o p i c s t r e s s s y s t e m . I t i s f u r t h e r c o n s i d e r e d that the s t r e n g t h of the s o i l in this s t r e s s region i s due t o the cementation bonds s o that yielding of t h e s e bonds and f a i l u r e a r e coincident. The directional variation of s t r e n g t h i s a r e a l consideration when in s i t u f a i l u r e i s c o n s i d e r e d a t t h e s e n o r m a l effective s t r e s s e s . At lower n o r m a l effective s t r e s s e s s t r e n g t h i s attributed to
dilation and f r i c t i o n a l r e s i s t a n c e between s m a l l blocks of c e m e n t e d clay. The m i c r o f i s s u r i n g d e t e r m i n e s s h e a r i n g r e s i s t a n c e indepen- dent of the s t r e n g t h of the cementation bonds within the tiny blocks of clay.
ANALYSIS
In o r d e r to apply the usual s l i p c i r c l e a n a l y s i s (upper bound collapse m e c h a n i s m ) i t i s n e c e s s a r y to e x p r e s s the s t r e n g t h of the s o i l i n t e r m s of a planar s h e a r i n g r e s i s t a n c e . This c a n be done s i m p l y by approximating the c u r v e d s u r f a c e in F i g u r e 16 by a
s t r a i g h t l i n e fitted over the region of n o r m a l s t r e s s e s applicable t o the m o s t c r i t i c a l c i r c l e f o r the p a r t i c u l a r v a l u e of 4 ' given by this
fitted line. A t r i a l and e r r o r p r o c e s s m a y be used to obtain the m o s t c r i t i c a l f a i l u r e c r i t e r i o n .
In F i g u r e 18 s e v e r a l computed c r i t i c a l c i r c l e s a r e shown f o r different values of @I
.
The c r i t i c a l s t r e s s points f o r t h e s e c i r c l e sa r e plotted in F i g u r c !CI and a f a i l u r e c r i t e r i o n (c' = 0.12 k g / c m 2 , @I = 33 " )
was chosen f r o m F i g u r e 16 a s r e p r e s e n t a t i v e of the region of n o r m a l s t r e s s e s calculated f o r the slope. The c i r c l e s n u m b e r e d C3, C4 and
C7
in F i g u r e 18 give safety f a c t o r s v e r y c l o s e to unity (i. e . , the points 3,4, 7 in F i g u r e 19 l i e on the f a i l u r e line). A c l o s e d solution calculation f o r c i r c l e CA in F i g u r e 18 gave a safety f a c t o r of 0.92 f o r the fully s a - t u r a t e d slope (point A, F i g u r e 19). T h e s e calculations indicate that the slope, if s a t u r a t e d under a hydrostatic ground water condition, was a t a condition of limiting equilibrium. I t i s quite likely that the hut
mentioned e a r l i e r ( s e e F i g u r e 15 f o r position) was c a r r i e d out on the second s l i p ( o r f i r s t r e t r o g r e s s i v e s l i d e ) . S o m e a p p r o x i m a t e calculations r e g a r d i n g the r e t r o g r e s s i v e s l i p s s u g g e s t that m u c h of the spoil which c a m e to r e s t in the c r a t e r was m a t e r i a l f r o m the f i r s t and second r e t r o - g r e s s i v e s l i d e s . This spoil m a t e r i a l undoubtedly contributed towards stabilizing the s l i d e and a safety f a c t o r in e x c e s s of unity was obtained f o r the s t a b l e back s c a r p remaining a f t e r the slide.
CONCLUSIONS
Based on a s t r e n g t h c r i t e r i o n d e t e r m i n e d by low effective s t r e s s t r i a x i a l testing, the landslide a t Rockcliffe h a s been s a t i s f a c t o r i l y a n a - lyzed with r e s p e c t to stability. T e s t s on s p e c i m e n s t r i m m e d a t v a r i o u s orientations with r e s p e c t to the i n s i t u v e r t i c a l orientation indicate that the r e o r i e n t a t i o n of the principal s t r e s s e s in s i t u will not a l t e r the
s t r e n g t h c r i t e r i o n applicable to Leda clay slopes (ranging up to 100 f t in height).
The accepted f a i l u r e c r i t e r i o n i s a s s o c i a t e d with s t r u c t u r a l defects in the clay. The origin of t h e s e defects m a y be weathering a n d / o r s t r e s s r e l e a s e following e r o s i o n of overburden and slope cutting. The dimensions of the s t r u c t u r a l defects a r e observed to be in the o r d e r of 118 in. and r e c e n t testing, d i s c u s s e d in the follow- ing g e n e r a l conclusions, suggest that the s i z e of the specimen has little effect on the f a i l u r e mode o r the maximum shearing r e s i s t a o c e .
IV. GENERAL CONCIJUSIONS
Landslides a r e common f e a t u r e s on the s1oi~c.s of s t r e a m valleys and t e r r a c e s in Leda clay. These s l i d e s appear to be r o - tational s l i p s which often r e t r o g r e s s a ~ o r ~ s i d e r a b l e distance into the slope. The l a r g e r r e t r o g r e s s i v e s l i d e s , t e r m e d flow slides, a r e c h a r a c t e r i s e d by a horseshol: shape in plan with an a p r o n of
d e b r i s which h a s flowed out into the valley ( o r into the lower t e r r a c e ) . Some s l i d e s occur in slopes steepened by toe erosion while o t h e r s do not appear to be a s s o c i a t e d with active e r o s i o n a t a l l . Among the multitude of landslides evident in a i r photos of the Ottawa Valley a r e s e v e r a l ancient s l i d e s that have individually incorporated many a c r e s of land. The two l a r g e s t landslides that have o c c u r r e d in the Ottawa a r e a during r e c e n t y e a r s (each incorporating approximately 30, 000 cubic y a r d s ) have been d i s c u s s e d in this r e p o r t .
The shearing r e s i s t a n c e of Leda clay, when subjected to con- ventional t r i a x i a l s h e a r t e s t s f r o m a n a t u r a l s t a t e (without consoli- dation in e x c e s s of the preconsolidation p r e s s u r e ) , i s considered to be dependent upon 'cementation bonds'. Specimens attain a peak s h e a r - ing r e s i s t a n c e a t s m a l l s t r a i n s (about one p e r cent). L a r g e r p r e f a i l u r e s t r a i n s occur only if the cementation bonds a r e destroyed by m e a n n o r m a l effective s t r e s s e s approaching the p r ec onsolidation p r e s s u r e .
The s t r e n g t h of the 'cementation bonds' r e s u l t s in an a l m o s t constant peak s h e a r i n g r e s i s t a n c e (in the o r d e r of 1 k g / c m 2 ) over a range of m e a n n o r m a l s t r e s s e s l e s s than the preconsolidation p r e s s u r e . ( F i g u r e 20).
In view of this behaviour i t was supposed that Leda clay s l o p e s w e r e subject to a sudden b r i t t l e type of f a i l u r e with the intact clay being a l m o s t cornpletely remoulded into a quasi-liquid s t a t e during flow slides.
This mode of f a i l u r e was not, however, supported by the many field ob- s e r v a t i o n s f r o m slope instabilities.
During the p a s t y e a r a r e a p p r a i s a l of the mechanical s t r e n g t h of Leda c l a y under low effective confining s t r e s s e s h a s r e s u l t e d in a new appreciation of the m e c h a n i s m s leading to slope instabilities. S m a l l
m i c r o f i s s u r e s existing within the apparently i n t a c t clay give r i s e , under low effective s t r e s s e s , to dilatant behaviour and predominantly 'frictional' s h e a r i n g r e s i s t a n c e a s the m a t e r i a l s h e a r s into s m a l l nodules o r blocks.
The f a i l u r e c r i t e r i o n obtained f r o m d r a i n e d t r i a x i a l t e s t s on
1
1,-in. d i a m e t e r s p e c i m e n s of Leda clay f r o m s e v e r a l s i t e s i s typified, in t e r m s of conventional planar r e s i s t a n c e p a r a m e t e r s , by a c' of a - bout 0 . 1 k g / c m 2 and a (Pr of about 35 " ( F i g u r e 20). The dilatant be- haviour during s h e a r h a s been examined by both i n c r e m e n t a l and con- s t a n t r a t e of loading t e s t s c a r r i e d out under constant values of m e a n
-
n o r m a l s t r e s s (i. e. p =(all
t 2ot, ) / 3 = constant). Other effectives t r e s s paths have been employed showing that the f a i l u r e i s reasonably independent of the s t r e s s path providing the preconsolidation p r e s s u r e i s not exceeded. The m i c r o f i s s u r e d s t r u c t u r e c a n be s e e n when a s l i c e of Leda clay i s f r a c t u r e d in two ( F i g u r e 2 l a ) and then c r u m b l e d into s m a l l nodules ( F i g u r e s 21b) by a twisting action. The nodules a r e quite s e n s i t i v e a n , ' .n b e easily remoulded into a quasi-liquid s t a t e .
The above mode of f a i l u r e r e s u l t s (due to the high v a l u e s of @ I )
in c r i t i c a l c i r c l e s that a r e c o n s i s tent with field observations r e g a r d i n g the locations of f a i l u r e s u r f a c e s and of kinematically acceptable s l i p s u r f a c e s . The dependence of the shearing r e s i s t a n c e on the n o r m a l effective s t r e s s i s supported by the f a c t that s l o p e f a i l u r e s in Leda clay a l m o s t invariably occur during periods of high ground water l e v e l s
(and usually during a r a i n s t o r m ) . The conventional s l i p s u r f a c e a n a l y s i s h a s been used in conjunction with the low s t r e s s f a i l u r e c r i t e r i o n to achieve r e a s o n a b l y good n u m e r i c a l c o r r e l a t i o n with s e v e r a l r e c e n t land- s l i d e s in the Ottawa a r e a .
S e v e r a l s t r a i n controlled d r a i n e d t r i a x i a l t e s t s on l$-in. d i a m e t e r s p e c i m e n s w e r e c a r r i e d out and only a slight reduction of the s h e a r i n g r e s i s t a n c e was noted a t l a r g e values of s t r a i n . A s p e c i a l t e s t was con- duc t e d w h e r e the t r i a x i a l s p e c i m e n was loaded in i n c r e m e n t s (drained) t o about 90 p e r c e n t of i t s f a i l u r e load. I t was found that the deformation of the s p e c i m e n could b e controlled, a t this s t r e s s level, by the p o r e water p r e s s u r e . A slight i n c r e a s e in the back p r e s s u r e (approximately 0.05 k g / c m 2 ) would r e s u l t in a r a t e of s t r a i n r e a d i l y o b s e r v a b l e on the d i a l gauge m e a s u r i n g axial deformation. The deformation could be a r
-
r e s t e d by lowering the back p r e s s u r e to i t s p r i o r value. This r e s u l t s u g g e s t s t h a t high s e a s o n a l ground water m a y c a u s e deformations in Leda clay s l o p e s without instability. Complete f a i l u r e may, in fact, de- pend on the availability of s u r f a c e water (via tension c r a c k s ) to s a t i s f ythe dilatant tendency i n the zones 01 s h e a r .
Not a l l i,:da c l a y s exhibit the mode of f a i l u r e d e s c r i b e d h e r e . Clay f r o m the Green C r e e k valley ( P i n e l - i t T v Golf Club Landslide) i s a n example of a Leda clay which does not show a s h ~ a r f a i l u r e in t r i a x i a l c o m p r e s s i o n a t deviatoric s t r e s s e s l e s s than that attributed to the 'cementation bonds'. (The applied deviatoric s t r e s s in the t r i a x i a l a p - p a r a t u s i s r e s t r i c t e d in the low str,-.ss region by the unconfined c o m - p r e s s i o n line given by on3 = 0. )
This clay, when broken, shows a typical b:-,ttle f r a c t u r e s u r f a c e without the f i s s u r e s o r f r a c tur ca planes that a r e a p p a r e n t in F i g u r e s 2 l a
and 21b. A twisting action will not r e s u l t in the f o r m a t i o n of nodules a s noted f o r the Leda c l a y f r o m B r e c k e n r i d g s and Kockcliffe. Thtt :: ;If
club landslide i s one of a limitec; number of landslips whichhave o c c u r r e d in this valley in r e c e n t y e a r s and i t m a y b e a s s o c i a t e d with a local a r t e s i a n
ground water condition. Compared with other a r e a s w h e r e the c l a y i s m i c r o f i s s u r e d , the G r e e n C r e e k valley i s c o n s i d e r e d f a i r l y inactive with r e g a r d to slope instabilities and s e v e r a l 40" slopes i n this clay a r e apparently s t a b l e to heights of about fifty ft. The e x i s t e n c e of nonfis s u r ed c l a y s (excepting l a r g e s u r f i c i a l f i s s u r e s ) m a y b e helpful in establishing the c a u s e of m i c r o f i s s u r i n g in other clays.
In addition to the t r i a x i a l t e s t s c a r r i e d out on 1s-in. d i a m e t e r
1
s p e c i m e n s , r e c e n t t e s t s on 2,-in.
-
and 4-in.-
d i a m e t e r s p e c i m e n s of s i m i l a r clay indicate that t h e r e i s little, if any, s i z e effect i n testing t h e s e m i c r o f i s s u r e d specimens.Detailed testing suggests that the a c t u a l f a i l u r e c r i t e r i o n in the low s t r e s s region i s a c u r v e d envelope a s sketched in F i g u r e 20. The c u r v e d s h a p e i s a s s o c i a t e d with the v a r i a t i o n in dilatant tendencies, de- pendent p r i m a r i l y on the v a l u e of the m e a n n o r m a l s t r e s s . This c u r v e d f a i l u r e c r i t e r i o n i s s i m i l a r to the s h a p e of the " c r i t i c a l s t r e s s c u r v e t 1
(6), obtained f o r the instable slope by calculating v a r i o u s combinations of c' and iP t (with corresponding c r i t i c a l c i r c l e s ) that yield a safety
f a c t o r of unity. This c o m p a r i s o n indicates that, although one s l i p c i r c l e m a y be theoretically c r i t i c a l , many s l i p s u r f a c e s m a y e x i s t simultaneously a t a n e a r l y c r i t i c a l equilibrium. In other words a n extensive zone of s o i l m a y b e deforming a t the t i m e of f a i l u r e .
An extensive zone of deformation would a p p e a r to explain the m u l t i - plicity of tension cracking observed a t the back s c a r p of r e c e n t s l i d e s .
Often the ground s u r f a c e h a s slipped down in a s e r i e s of s t e p s (in c.onjunction with the tension c r a c k s ) indicating a number of s l i p s u r f a c e s . Extensive deformation (and dilation) together with multiple slippage would r e a s o n a b l y
b e a s s o c i a t e d with a flow slide. Much of the 'flow' m a t e r i a l would b e t r a n s p o r t e d a s a flexible nodular blanket riding on a s l i p p e r y l a y e r of remoulded c l a y nodules. This nodular s t r u c t u r e was a p p a r e n t in t h e d e b r i s of s e v e r a l landslides which w e r e investigated during the s p r i n g of 1969.
Beyond the a s sociation of i n s t a b i l i t i e s with abundant ground and s u r f a c e water, the detailed climatological and geological conditions contributing to instabilities a r e not yet fully understood. Some hypoth- e s e s r e g a r d i n g p o s s i b l e geological f a c t o r s have r e c e n t l y been communi- cated t o the a u t h o r s and a r e the subjects of c u r r e n t r e s e a r c h . The
spring s e a s o n s of 1967 and 1969 have both been accompanied by n u m e r o u s i n s t a b i l i t i e s along the south bank of the Ottawa River. Both t h e s e p e r i o d s w e r e p r e c e d e d by winter s e a s o n s during which e a r l y and continuous snow cover prevented deep f r o s t penetration. Low f r o s t penetration m a y lead t o e a r l y infiltration of melting snow c o v e r and s p r i n g r a i n s with l e s s
s u r f a c e r u n off.
P e r h a p s the I ~ ~ U S L i m p o r t a n t a s p e c t yet unexplained i s the develop-
m e n t of the m i c r o f i s s u r i n g . What f a c t o r s contribute to the f o r m a t i o n of t h e s e c l o s e l y spaced h a i r l i n e f r a c t u r e s ? What conditions d e t e r m i n e the extent and r a t e of f o r m a t i o n of t h e s e f i s s u r e s ? L a r g e f i s s u r e s , a p p a r e n t in the oxidized c r u s t , a r e attributed to shrinkage of the c l a y due to l o s s of m o i s t u r e content. S i m i l a r f i s s u r e s m a y a p p e a r in clay exposed to the a t m o s p h e r e by erosion. L a r g e s c a l e f i s s u r i n g was o b s e r v e d t o g r e a t depths i n the Ottawa s e w e r tunnel i n t e r c e p t o r shaft but the blocks of clay which w e r e r e m o v e d w e r e apparently not m i c r o f i s s u r e d . T h e s e l a r g e f i s s u r e s could have developed a s a r e s u l t of l o c a l s t r e s s relief. M i c r o f i s s u r i n g m a y be a r e s u l t of s t r e s s relief due to overburden r e m o v a l and s l o p e cutting. The f o r m a t i o n of m i c r o f i s s u r e s by this m e c h a n i s m would depend p r i m a r i l y on the relationship between the swelling p r e s s u r e s and the s t r e n g t h of the 'cementation bonds' in the clay. A second p o s s i b l e c a u s e of m i c r o f i s s u r i n g m a y be the fatiguing effect of s e a s o n a l changes in t e m - p e r a t u r e a n d / o r ground water p r e s s u r e s on the 'cemented.' clay. T h e s e and o t h e r plausible hypotheses might b e investigated by detailed mapping of the depths and locations of the m i c r o f i s s u r e d m a t e r i a l .
Recognition of the m i c r o f i s s u r e d n a t u r e of s o m e Leda c l a y s h a s been i n s t r u m e n t a l i n under standing the m e c h a n i c s of landslides. An under
-
standing of the c a u s e s and distribution of m i c r o f i s s u r e s m a y lead to aACKNOWLEDGEMENTS
The studies r e p o r t e d h e r e i n developed in conjunction with s i m i l a r work c a r r i e d out by W. J . Eden on a landslide a t O r l e a n s , Ontario. The author i s grateful f o r the d i s c u s s i o n s held with M r . Eden throughout t h e s e studies. Some of the f a c t o r s d i s c u s s e d i n the g e n e r a l conclusions originate f r o m t h e s e discussions.
Appreciation i s extended to P.M. J a r r e t t f o r the availability sf
the original field r e c o r d s f r o m Rockcliffe and f o r his continued i n t e r e s t in this landslide.
F o r t h e i r excellent fieid and l a b o r a t o r y work, appreciation i s ex- tended to D. C. MacMillan, K. T i m m i n s and A. L a b e r g e of the te.:hnical staff, Geotechnical Section, Division of Building R e s e a r c h , N.R. Z. REFERENCES
1. Crawford, C.B. and W. J. Eden, (1967). Stability of n a t u r a l slopes in s e n s i t i v e clay. A. S. C. E. Journ. Soil Mechanics and Foundation Engineering, Vol. 93, No. SM 4, p. 419-436.
2. Kenney,T. C., (1968). Discussion to Stability of n a t u r a l s l o p e s in s e n s i t i v e c l a y by C. B. Crawford and W. J. Eden. A.S. C. E. Journ. Soil Mechanics and Foundation Engineering, Vol. 94, No. SM 5,
p. 1185-1190.
3. Bishop, A. W. (1955). The u s e of the slip c i r c l e in the stability a n a l y s i s of s l o p e s Geotechnique 5 : 1 : 7-17.
4. Arseneault, J. G., (1967). A p r o g r a m f o r solving slope stability p r o b l e m s DBR c o m p u t e r p r o g r a m No. 27.
5. B j e r r u m , L . , (1966). Mechanism of p r o g r e s s i v e f a i l u r e in s l o p e s of overconsolidated p l a s t i c c l a y s and c l a y s h a l e s . 3 r d T e r z a g h i L e c t u r e , A. S. C. E . S t r u c t u r a l Eng. Conf., F l o r i d a .
6. Kenney,T.C., (1967). Slide behaviour and s h e a r r e s i s t a n c e of a quick clay d e t e r m i n e d f r o m a study of the landslide a t Selnes, Norway. P r o c . Geotechnical Conf., Oslo, Vol. 1, p. 57-64.
S C A L E : I# METERS
3 0
-
ORIGINAL SLOPE
---
~ P R E S E N T
SURFACE
SECTION A - A
F I G U R E
2
P L A N OF BRECKENRIDGE S L I D E AREA
F I G U R E
4
T R l A X 1 A L S H E A R I N G S T R E N G T H D A T A
0
0
0.4
0. 8
1 . 2
1. 6
S H E A R S T R A I N ,
%
m
-I
ncremental Loacling
S h o w n A b o v e
D I L A T I O N
C O M P R E S S I O N
V O L U M E T R I C S T R A I N ,
%
F I G U R E
5
S T R E S S - S T R A I N R E L A T I O N S H I P S F O R
C O N S T A N T
p T E S T S
B R 4 4 0 7 - 4A p p r o x P o s i t i o n
0
X i l
?
0
30
40
50
60
70
H O R l Z O N T A L D l S T A N C E , M E T E R S
0.
5
0.4
(Un i t i a l C r i t i c a l
3
E c)-
- 0 70.
3
YC r i t e r i o n ,
@'-35"
( f r o m t r i a x i a l t e s t s
0.
2
i n
l o w s t r e s s r e g i o n )
0.1
F I G U R E
6
A N A L Y S I S O F T H E B R E C K E N R I D G E L A N D S L I DE
( a ) T R I A L C I R C L E S
(b)
C R I T I C A L S T R E S S C U R V E S
B R 4 4 0 7 - 5FIGURE
7
0
2 0
40
6 0
8 0
100
120
140
160
H O R I Z O N T A L D l S T A N C E , FEET
F I G U R E
10
F I G U R E 11
T I M E - D E F O R M A T I O N C U R V E S N E A R F A I L U R E
2 . 4 t
1
1
3-2 10.3I
Vert Split14
3-4 0.4 Vert SplitI 1
3-51
0.51
Vert Split/
)
3-7 0.6 Shear 65" 3-8 0.7 Shear 45"&65" 3-3 0.8 Shear 65"1
Test No. 3-1 0 F a i l u r e P o i n t s A l l T e s t s a t C o n s t a n t N o r m a l S t r e s s , p 0 0 . 4 0. 8 1 . 2 - 2 . 0 - 1 . 2 - 0 . 4 0 0 . 4 1. 2 2. 0 p = ( U / + ~ U ; 13, kglcrn2 D I L A T I O Nb
V % C O M P R E S S I O N Value of 0.2 kglcJ2 F I G U F E 1 2 F I G U R E 13 T R I A X I A L F A I L L I R E C R I T E R I O N V O L U M E T R I C S T R A I N S I N C O N S T A N T p ' T R I A X I A L T E S T S 80 FailureMode PlaneI
F I G U R E 1 4
S T A B I L I T Y C O M P A R I S O N S
B R 4 4 0 7 - / /1
I
M i n i m u m
-S t r e n g t h
-M o s t P r o b a b l e
F a i l u r e C r i t e r i o n
L i m e
/
t
-f o r A p p l i c a b l e
/
-S t r e s s R e g i o n
/ C r i t i c a l S t r e s s
/
C u r v e s
R,=
0.40
- - - -/
/
/
/
1
1180
I- LLGROUND SURFACE
-
160
z 0-
I----
3
140
W J W120
0
5 0
100
150
2 0 0
2 5 0
HORIZONTAL D I S T A N C E , F T
FIGURE
15
PLAN AND PROFILE OF ROCKCLIFFE LANDSLIDE
0
