Mechanism of landslides in leda clay with special reference to the Ottawa area

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Mechanism of landslides in leda clay with special reference to the

Ottawa area

Eden, W. J.

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Ser

TH1

N21d

no.

713

e.

2

onal Research Council of Canada

B r n

;eil national de recherches du Canada

MECHANISM OF LANDSLIDES I N LEDA CLAY WITH SPECIAL

REFERENCE

TO THE OTTAWA AREA

by W.

J. Eden

1/

Reprinted from

MASS WASTING

Proceedings of the 4th Guelph Symposium

on Geomorphology, 1975

p. 159

-

171

NATIONAL R E 4 6 W R a COUNCIL

I

DBR Paper No. 713

Division of Building Research

SOMMAIRE Une g p a i s s e c r o 6 t e a l t g r g e , d ' u n e p r o f o n d e u r a l l a n t j u s q u ' l 30 p i e d s e t p l u s , s'est formge s u r l a p l u p a r t d e s p e n t e s n a t u r e l l e s d ' a r g i l e Leda d a n s l a r g g i o n d ' o t t a w a . L e s ; t a p e s i n i t i a l e s d e s g l i s s e m e n t s d e t e r r a i n d a n s l e s p e n t e s d ' a r g i l e s e l i m i t e n t

2

l a c r o 6 t e . A u - d e s s o u s d e l a c o u c h e s u p e r f i c i e l l e , l e s a r g i l e s d e l a c r o 6 t e a l t g r g e c o n t i e n n e n t u n r g s e a u d e f i s s u r e s tres r a p p r o c h g e s . L o r s d ' u n c i s a i l l e m e n t , l ' a r g i l e a l t ; r & - ~ 4 9: mpose e n n o d u l e s p l u s p e t i t s q u i p e u v e n t se dgcomposer

2

- I ,. + - ~ r

-

,

.,

q u a s i - l i q u i d e d a n s l e c a s d

'

un r e m a n i m e n t g l o b a l . L'h?j -"L-,-* t -' c o m p o r t e m e n t a u c i s a i l l e m e n t d e c e t t e a r g i l e s u r p l a c e st aard- ie 1 r a t o i r e a p e r m i s d e m e t t r e a u p o i n t u n e m g t h o d e d ' a n a l y . d e l a s t = b ' l z t & q u i t i e n t c o m p t e d e s p r 0 ~ r i g t 6 . s d e r g s i s t a n c e d e l ' a r y ' e f l s s u r g e e t d e s v a r i a t i o n s du n i v e a u p h r g a t i q u e . C e t t e m i t . ~ e a ;ti : & i f i g e d a n s d e nombreux c a s d e r u p t u r e s d e t a l u s .

MECHANISM OF LANDSLIDES IN LEDA CLAY WITH

SPECIAL REFERENCE TO THE OTTAWA AREA

W.

J.

Eden

ANALYZED

ABSTRACT

I

A t h i c k weathered c m s t , extending t o depths of 30

feet or more, has developed on most of the natural

slopes i n Leda clays i n the Ottawa region. The i n i t i a l

stages of landslides i n the clay slopes are confined t o the c m s t . BeZp the surface layer the clays of the weathered c m s t contain a system of closely spaced

fissures. When sheared, the weathered.clay breaks d o m

i n t o small nodules which can be further broken down i n t o

a quasi-liquid when thoroughly remoulded. From the

shear behm<our of t h i s cZa!j obserued i n the fieZd and

i n the La;70rafory, a methan' of staE1:lity m a l y s i s has been

devehped which kzkee i n h account :ht? s t r q t h p r o p e r -

tie8 of the f i s s u r e d clay an2 c F i g a o ir, groundwa-csr levels. The method laae beev c7:cckad far ~ e t r s r a Z _slope fai Zurea.

T h i s c o n t r i b u t i o n i s r e s t r i c t e d t o l a n d s l i d e a c t i v i t y t h a t takes p l a c e i n t h e t o p 30 ft. ( 1 0 m) o f Leda c l a y , i n t h e weathered o r f i s s u r e d c r u s t p r e s e n t on most c l a y slopes. The concepts d e s c r i b e d have been d e r i v e d from s t u d i e s o f a number o f s p e c i f i c l a n d s l i d e s i n t h e Ottawa area o v e r t h e p a s t 15 y e a r s . The paper s h o u l d be s e t a g a i n s t t h e back- ground p r o v i d e d by s e v e r a l o t h e r c o n t r i b u t i o n s t o t h i s Symposium (Gadd 1975, Kenney 1975, LaRochelle 1975, and Torrance 1975).

THE WEATHERED CRUST

A t h i c k weathered c r u s t , commonly extending t o depths o f 30 f e e t (10 m ) , has developed on most o f t h e n a t u r a l slopes i n Leda

clay

i n t h e O t t a w a r e g i o n . T h i s c r u s t i s i d e n t i f i e d by a decrease i n undrained s t r e n g t h w i t h depth t o a minimum v a l u e as measured by t h e f i e l d vane then an fncrease w i t h depth t h e r e a f t e r . The p o i n t o f minimum s t r e n g t h marks

t h e e x t e n t o f t h e weathered c r u s t (Eden and Crawford, 1957). A system of c l o s e l y spaced f i s s u r e s o r planes o f weakness i n t h e c l a y i s another c h a r a c t e r i s t i c o f t h e weathered c r u s t . When sheared t h e clay breaks

down i n t o s m a l l b l o c k s o r nodules t h a t have

well

developed p l a n a r s i d e s ( F i g . 1 ) . The upper p l a t e shows a piece o f c l a y t h a t has f r a c t u r e d c l e a n l y a l o n g a w e l l defined s u r f a c e by s i m p l e bending a c t i o n , The l o w e r p l a t e shows t h e n o d u l a r s t r u c t u r e which becomes e v i d e n t when a

lump o f t h i s c l a y i s s u b j e c t e d t o t o r s i o n a l shear. I f t h o r o u g h l y r e - moulded, t h e nodules o f c l a y can be broken down f u r t h e r i n t o a q u a s i - f l u i d i n d i c a t i n g t h a t t h e f i s s u r e d c l a y i s s e n s i t i v e and has a n a t u r a l w a t e r c o n t e n t i n excess o f i t s l i q u i d l i m i t .

SHEAR BEHAVIOUR O F FISSURED LEDA CLAY

The shear s t r e n g t h o f Leda c l a y has been i n v e s t i g a t e d i n t e n s i v e l y by a number o f l a b o r a t o r i e s o v e r t h e p a s t 20 y e a r s . One o f t h e more r e c e n t s t u d i e s ( M i t c h e l l , 1970) r e v e a l e d t h r e e d i s t i n c t types o f b e h a v i o u r f o r f i s s u r e d Leda c l a y depending on t h e l e v e l o f mean normal s t r e s s . The

Figure 1. Nodular composition o f Leda c l a y . 160

s t r k n g t h envelope shown i n f i g u r e 2 shows t h e r e l a t i o n between maximum d e v i a t o r i c s t r e s s , q, and mean normal s t r e s s e s , p, r a n g i n g from 0 t o 2.8 kg/cm2 ( 0 t o 274.6 kN/m2) on a c l a y w i t h a p r e c o n s o l i d a t i o n pres- s u r e o f a p p r o x i m a t e l y 2 kg/cm2 (196 k ~ / m ~ ) . A t s t r e s s l e v e l s above t h e p r e c o n s o l i d a t i o n pressure, i n c r e a s e s i n shear o r d e v i a t o r i c s t r e s s causes l a r g e volume changes t o o c c u r i n t h e c l a y , w i t h t h e s t r e n g t h b e i n g d i r e c t l y dependent on t h e l e v e l o f mean normal s t r e s s . The s t r u c t u r e o f t h e c l a y i s c o m p l e t e l y d e s t r o y e d and t h e term "work hard- e n i n g p l a s t i c f l o w " has been used t o d e s c r i b e t h e behaviour.

A t mean normal s t r e s s l e v e l s below t h e preconsol i d a t i o n p r e s s u r e ( f r o m 1.0 t o 2.0 kg/un2 (98.07 t o 196.14 kN/m2) i n f i g u r e 2 ) , t h e shear s t r e n g t h i s n e a r l y c o n s t a n t . I n t h i s s t r e s s range, i t i s t h o u g h t t h a t t h e s t r e n g t h depends l a r g e l y on t h e bonds t h a t e x i s t between c l a y p a r - t i c l e s . M i t c h e l l (1970) showed t h a t t h e s t r e n g t h shows g r e a t e r a n i s o t r o p i c e f f e c t s i n t h i s range than i n h i g h e r o r l o w e r s t r e s s I ranges. Shear occurs w i t h l i t t l e change i n volume a l o n g w e l l d e f i n e d

!

f a i l u r e planes. Shear behaviour i n t h i s range i s c h a r a c t e r i s t i c o f

I

b r i t t l e m a t e r i a l .

A t low l e v e l s of mean normal s t r e s s ( f r o m 0 t o 1 .O kg/cm2 ( 0 t o 98.07 kN/m2) i n f i g u r e 2 ) , a t h i r d t y p e o f shear behaviour occurs due t o t h e presence of t h e c l o s e l y spaced f i s s u r e s . The c l a y mass behaves as densely packed g r a n u l a r m a t e r i a l

,

w i t h one nodule moving r e 1 a t i v e t o another under shear s t r a i n . T h i s r e l a t i v e movement causes an i n c r e a s e i n volume and a t t r a c t s w a t e r t o t h e shear zone. T h i s f e a t u r e i s con- s i d e r e d i m p o r t a n t f o r s l o p e s t a b i l i t y , s i n c e a l l l a n d s l i d e s thus f a r * s t u d i e d i n d e t a i l have o c c u r r e d when a s u p p l y o f f r e e w a t e r was a v a i l - a b l e a t t h e ground s u r f a c e , e i t h e r from r a i n f a l l o r from m e l t i n g snow. I n t h i s range, t h e s t r e s s i s s t r o n g l y dependent on t h e l e v e l o f mean normal s t r e s s and t h e s t r e n g t h envelope ( F i g . 2) i s p r o b a b l y curved i n t h e l o w s t r e s s range. For purposes o f a n a l y s i s , however, i t can be r e p r e s e n t e d b y a s t r a i g h t l i n e .

Thus c l a y s from t h e weathered c r u s t show t h r e e d i s t i n c t modes o f shear behaviour, depending on s t r e s s l e v e l . The low s t r e s s range i s

a p p r b p r i a t e f o r s l o p e s t a b i l i t y c o n s i d e r a t i o n s ; an a p p r e c i a t i o n o f t h e shear b e h a v i o u r e x p l a i n s many o f t h e f e a t u r e s observed i n l a n d s l i d e s , such as t e n s i o n cracks, evidence o f creep, and r e l a t i o n of l a n d s l i d e a c t i v i t y w i t h s u p p l y o f water.

I

EXAMPLES OF LANDSLIDES

Several l a n d s l i d e s t h a t have been s t u d i e d i n some d e t a i l w i l l b e des- c r i b e d b r i e f l y t o , i l l u s t r a t e t h e e f f e c t s of shear behaviour on s l o p e s i n Leda c l a y . The f i r s t example i s o f a f a i l u r e i n a r o a d - c u t near Orlezns, O n t a r i o ( F i g . 3), d e t a i l s o f which have been r e p o r t e d by Eden and J a r r e t t (1971). T h i s l a n d s l i d e o c c u r r e d i n a 3 5 - f t - h i g h (10.5-111) r o a d c u t i n October 1965 a f t e r a p e r i o d o f heavy r a i n f a l l . The c u t had been made 5 y e a r s p r e v i o u s t o t h e f a i l u r e . F o l l o w i n g t h e f a i l u r e , i t was p o s s i b l e t o l o c a t e t h e f a i l u r e s u r f a c e . With t h i s i n f o r m a t i o n , t h e l a n d s l i d e c o u l d b e used t o back c a l c u l a t e t h e shear s t r e n g t h r e q u i r e d f o r s t a b i l i t y , w i t h i n t h e 1 im i t s p e r m i t t e d , by know- l e d g e of t h e groundwater c o n d i t i o n s . Piezometers were i n s t a l l e d i n t h e s l o p e t o a s c e r t a i n t h e groundwater c o n d i t i o n s a f t e r t h e f a i l u r e occur- red. Samples were a l s o o b t a i n e d and s u b j e c t e d t o shear s t r e n g t h determi n a t i o n s i n t h e 1 a b o r a t o r y .

S t a b i l i t y analyses a r e based on t h e assumption t h a t t h e s u r f a c e o f f a i l u r e can be r e p r e s e n t e d b y a c i r c u l a r a r c . The r e s u l t s o f t h e a n a l y s i s f o r t h e Orleans c u t a r e presented i n f i g u r e 4. The upper drawing shows s e v e r a l t r i a l a r c s o r c i r c l e s i n c l u d i n g one which approx- imates t h e f a i l u r e s u r f a c e . Each a r c on t h e f i g u r e i s t h e most c r i t i -

01

U n c o n f i n e

C o m p r e s s i

S p e c i m e n s

d

o n

I

Q

1

S t r e s s l n t e r m e d i a t e

.I'

R e g i o n S t r e s s R e g i o n H i g h S t r e s s

o d

F a i l u r e

P e a k S t r e n g t h R e g i o n W o r k

D u e t o

A t t r i b u t e d t o

H a r d e n i n g

'

_{P l a n e s o f}

_{C e m e n t a t i o n}

W e a k n e s s

B o n d s

"

. 4

. 8

1. 2

1.

6

2.

0

2 . 4

2.

8

p,

k g l c m 2

Figure 2 . Mechanical s t r e n g t h of Leda c l a y .

Figure 3. View of c r a t e r o f f i r s t s l i d e a t Orleans, Ontario, October, 1965.

H O R I Z O N T A L

D l

STANCE, FEET

Figure

4. Analysis

o f

the Or1 eans lands1 ide.

(a) cri ti'cal toe circles;

(b)

critical stress curves.

c a l one found f o r a s e r i e s o f t r i a l a r c s passing through t h e t o e o f t h e s l o p e u s i n g a c o n s t a n t a n g l e o f s h e a r i n g r e s i s t a n c e f o r t h e c l a y ( + I ) . The l o w e r drawing i s a c r i t i c a l s t r e s s p l o t (Kenney, 1967) i n which t h e average mean normal s t r e s s f o r a c r i t i c a l t r i a l c i r c u l a r a r c a t one s e t of shear s t r e n g t h parameters i s p l o t t e d a g a i n s t t h e shear s t r e n g t h r e - q u i r e d f o r s t a b i l i t y f o r t h e c r i t i c a l a r c . The p l o t t h u s o b t a i n e d can be repeated f o r d i f f e r e n t assumptions o f t h e l e v e l of groundwater t a b l e . F i n a l l y , t h e s t r a i g h t l i n e r e p r e s e n t a t i o n , i n terms of apparent cohesion C ' and a n g l e o f s h e a r i n g r e s i s t a n c e

6'

,

o f t h e s t r e n g t h envelope o b t a i n e d from l a b o r a t o r y t e s t i n g can be compared w i t h t h e c r i t i c a l s t r e s s p l o t s (Eden and M i t c h e l l , 1970). F i g u r e 4 i l l u s t r a t e s t h a t t h e h i g h a n g l e o f s h e a r i n g r e s i s t a n c e ( @ I ) found i n 'the l a b o r a - t o r y t e s t s a r e r e a l i s t i c i n terms o f t h e a n a l y s i s conducted on t h e a c t u a l f a i l u r e .

With t h e use o f computers, i t i s p o s s i b l e t o t a k e i n t o account v a r i a t i o n s i n t h e groundwater l e v e l s and a curved f a i l u r e envelope. Lo and Lee (1974) i l l u s t r a t e such methods.

T h i s a n a l y s i s has s e v e r a l imp1 i c i t assumptions which a r e r a r e l y r e a l - i z e d i n a c t u a l slopes. Even where s o i l c o n d i t i o n s a r e v e r y u n i f o r m , t h e a c t u a l f o m t h a t a l a n d s l i d e may t a k e i s l a r g e l y d i c t a t e d by geo- l o g i c a l o r s t r u c t u r a l d e t a i l s a t t h e s i t e . F i g u r e s 5 and 6 i l l u s t r a t e l a n d s l i d e a c t i v i t y a l o n g B r e c k e n r i d g e Creek. F i g u r e 5 i s a view o f an e a r t h f l o w t y p e o f l a n d s l i d e which o c c u r r e d i n A p r i l , 1963. T h i s l a n d - s l i d e has been s t u d i e d i n d e t a i 1 (Crawford and Eden, 1967). One o f t h e f e a t u r e s o f t h e s i t e i s v e r y u n i f o r m s o i l c o n d i t i o n s f o r t h e f u l l h e i g h t o f t h e 9 0 - f t . (27-m) r a v i n e . As t h e s o i l c o n d i t i o n s were u n i - form, i t i s s u r p r i s i n g t o n o t e t h e t y p e o f l a n d s l i d e a c t i v i t y t h a t o c c u r r e d on t h e o p p o s i t e s i d e o f t h e stream i n 1969 ( i l l u s t r a t e d

i n

F i g . 6 ) . Here t w o d i f f e r e n t t y p e s o f l a n d s l i d e a c t i v i t y developed a t t h e same t i m e . The s m a l l e r one on t h e l e f t o c c u r r e d a l o n g a c u r v e d f a i l u r e s u r f a c e , s i m i l a r t o t h e l a n d s l i d e a t Orleans. The l a r g e r s l i d e on t h e r i g h t o c c u r r e d on a p l a n a r s u r f a c e p a r a l l e l t o and 10 ft. below t h e s u r f a c e ( M i t c h e l l and Eden, 1972). F i g u r e s 7, 8 and 9 I l l u s t r a t e f u r t h e r t h e v a r i e t y o f f o r m o f l a n d s l i d e a c t i v i t y . The t h r e e photo- graphs were taken i n t h e Ottawa v i c i n i t y i n April-May, 1971. F i g u r e 7 i s a view o f t h e South N a t i o n R j v e r l a n d s l i d e which extended 70 acres ( 2 8 h a ) i n area (Eden, F l e t c h e r , M i t c h e l l , 1971). F i g u r e 8 i s a view o f a much s m a l l e r l a n d s l i d e a l o n g Mud Creek which shows some i n d i c a t i o n o f r e t r o g r e s s i o n . F i g u r e 9 shows l a n d s l i d e a c t i v i t y i n a 9 0 - f t . (27-m) h i g h bank of Green Creek j u s t e a s t o f Ottawa.

I n A p r i 1, 1971, 1 andsl i d e a c t i v i t y developed

In

a TOO-ft. (30-m) high t e r r a c e s l o p e i n t h e Town o f Gatineau. A housing c o n t r a c t o r was con- s t r u c t i n g a number o f row house u n i t s , each u n i t b e i n g s e t

on

i t s own graded t e r r a c e up t h e slope. Oversteepening of t h e s l o p e caused land- s l i d e a c t i v i t y t o develop d u r i n g t h e snow m e l t p e r i o d . F i v e s e p a r a t e 1 andsl i _{de events were recorded o v e r} a 7-day p e r i o d (Eden, 1972)

.

F i g u r e 10 i s a view o f t h e l a n d s l i d e a c t i v i t y a t t h e e a s t e r n end of t h e p r o j e c t and i s t h e s i t e o f 4 s e p a r a t e d i s t i n c t movements. F i g u r e 11 shows t h e wreckage o f a p a r t i a l l y completed u n i t which s t o o d i n t h e p a t h o f a s m a l l l a n d s l i d e which developed i n t h e weathered c r u s t o f t h e c l a y . F i n a l l y f i g u r e 12 shows t h e b u l g e t h a t developed a t t h e t o e o f a s t e e p c u t s l o p e which has been made t o p r o v i d e a l e v e l y a r d area f o r t h e housing u n i t s .

CREEP OF SLOPES

A t s i t e s o f l a n d s l i d e a c t i v i t y , movements o f t h e c l a y s l o p e n o t l e a d i n g t o f a i l u r e a r e i n d i c a t e d by t h e appearance o f t e n s i o n c r a c k s a t t h e c r e s t o f a s l o p e and by b u l g i n g a t t h e t o e ( F i g . 1 2 ) . Indeed, many accounts by witnesses o f l a n d s l i d e s mention t h e presence o f a c r a c k a t

Figure 5. A e r i a l view o f e a r t h f l o w i n v o l v i n g 30,000 cubic yards of c l a y

-

Breckenridge Creek, Quebec, A p r i l 1963.

Figure 6. View o f two small l a n d s l i d e s t h a t occurred i n s p r i n g of

1969

-

Breckenridge Creek, Quebec.

Figure 7 . South N a t i o n R i v e r Lands1 i d e . May 1971. Lasselman, Ontario.

Figure 8. L a n d s l i d e along Mud Creek. May 1971. Navan, O n t a r i o . 166

Figure 9. L a n d s l i d e a l o n g Green Creek. A p r i l 1971. Near OttaNa, O n t a r i o .

Figure 10.

LandsliQs. Gattnew,

Quebec.

A p r i l 1971. 167

Figure 11. Small l a n d s l i d e and wreckage o f house. Gatineau, Quebec. A p r i l 1971.

Figure 12. Bulge a t t o e of slope. Gatineau, Quebec. A p r i l 1971.

Figure 13. Crack i n pavement as r e s u l t of creep movement. Cumberland. O n t a r i o .

t h e c r e s t o f t h e slope.

To s t u d y movements of slopes, i n c l i n o m e t e r tubes were i n s ta 7 l e d i n

1968 and 1969 i n s i x steep s l o p e s which were near, b u t n o t a f f e c t e d by, r e c e n t l a n d s l i d e a c t i v i t y . The p l a s t i c tubes were i n s t a l l e d t o a

depth o f 30 f t (9-m). Readings o f i n c l i n a t i o n were made a t 1-rn

i n t e r v a l s s t a r t i n g a t t h e b o t t m and proceeding t o the s u r f a c e .

Re-

s u l t s o f t h e program a r e r e p o r t e d i n d e t a i l i n M i t c h e l l and Eden (1972). A1 7 s i x i n s t a l l a t i o n s showed some annual movement, o c c u r r i n g m a i n l y i n t h e spring when groundwater c o n d i t i o n s were high. Over a 3-year p e r i o d

movements were measured r a n g i n g

frm 0.5 cm t o f a i l u r e i n one case.

The movements l n d i c a t e d t h a t r e l a t i v e l y deep-seated movements d i d o c c u r on steep slopes w i t h movements o c c u r r i n g d u r i n g p e r i o d s o f h i g h g r o u n k a t e r l e v e l . S u r f a c e i n d i c a t i o n s of such creep movements

are

i l l u s t r a t e d i n f i g u r e s 13 and 14. F i g u r e 73 i s a view o f a c r a c k which opened i n t h e s p r i n g o f 1973 i n t h e s u r f a c e o f Highway

No.

77 n e a r Cumberland. O n t a r i o . Figure 1 4 i n d i c a t e s t h e condi ti on o f an a1 ignrnent monument e r e c t e d a p p r o x i m a t e l y 20 y e a r s e a r l i e r a l o n g t h e same h i g h - way. Other evidence o f creep i s being accumulated f r o m

measurements

of b r i d g e s spanning s t e e p s i d e d r a v i n e s i n Leda

c l a y .

CONCLUSIONS

L a n d s l i d e a c t i v i t y i n t h e weathered c r u s t o f L e d a ' c l a y occurs i n a v a r i e t y o f forms l a r g e l y determined by g e o l o g i c a l d e t a i l a t a g i v e n s i t e . Such a c t i v i t y under c e r t a i n c o n d i t i o n s can by a process o f r e t r o g r e s s i o n l e a d t o v e r y l a r g e l a n d s l i d e s such as t h e South N a t i o n R i v e r l a n d s l i d e .

Landslides t a k e p l a c e d u r i n g p e r i o d s o f h i g h groundwater l e v e l s when t h e r e i s a s u p p l y o f s u r f a c e w a t e r a v a i l a b l e . T h i s o b s e r v a t i o n meets w i t h t h e requirement f o r w a t e r t o s a t i s f y t h e d i l a t a n c y r e s u l t i n g from shear s t r a i n i n t h e f i s s u r e d c l a y .

The concept o f a curved f a i l u r e envelope e x p l a i n s f e a t u r e s f r e q u e n t l y observed a t l a n d s l i d e s such as t e n s i o n cracks a t t h e c r u s t , evidence o f m u l t i p l e f a i l u r e s u r f a c e s , creep i n s t e e p slopes and v a r i a t i o n i n t h e geometric form o f t h e lands1 i d e s u r f a c e .

There i s evidence t h a t slow deep-seated movements, which c o u l d be c a l l e d creep, o c c u r i n slopes s t e e p e r t h a n 3 : l . The movements o c c u r a t times o f h i g h groundwater l e v e l .

ACKNOWLEDGEMENTS

The w r i t e r

wishes

t o acknowledge the

assistance o f h i s colleagues i n

t h e Geotechnical Section p a s t and p r e s e n t f o r t h e i r a s s i s t a n c e and c o n t r i b u t i o n t o t h e s t u d i e s on s l o p e s t a b i 1 i ty. P a r t i c u l a r acknow- ledgement i s due t o Dr. R.J. M i t c h e l l o f Queen's University who c o l l a b o r a t e d w i t h t h e w r i t e r on many o f

the

s t u d i e s . T h i s paper i s a

c o n t r i b u t i o n f r o m

the

D 7 v i s i

on

o f Bui t d i n g Research, N a t i o n a l Research Council o f Canada, and i s p u b l i s h e d w i t h the approval o f the Director

of t h e D i v j s i o n .

REFERENCES

Crawford, C.B. and Eden W.J., 1967: S t a b i l i t y o f n a t u r a l slopes i n s e n s i t i v e c l a y . Proc. A.S.C.E., 93, SM4, 419-439.

Eden, W.J., 1972: Some o b s e r v a t i o n s o f Le Coteau L a n d s l i d e , Gatineau, Quebec. Can. Geotech. J . , 9 : 508-514

Eden, W.J., and Crawford, C.B., 1957: Geotechnical p r o p e r t i e s o f Leda c l a y i n t h e Ottawa area. Proc. 4 t h I n t . Conf. S o i l Mech. Found. ' ~ n g . , London, 1 : 22-27

Eden, W.J., F l e t c h e r , E.B., and M i t c h e l l , R.J., 1971: South N a t i o n R i v e r Landslide, 16 May, 1971. Can. Geotech. J., 8: 446-451. Eden, W.J., and J a r r e t t , P.M., 1971: L a n d s l i d e a t Orleans, O n t a r i o

N a t i o n a l Research Council, D i v i s i o n o f B u i l d i n g Research, Tech. Paper 321

,

Ottawa, Canada. (NRCC 11856).

Eden, W.J., and M i t c h e l l , R.J., 1970: The mechanics o f l a n d s l i d e s i n Leda c l a y s . Can. Geotech. J . , 7: 285-296.

Gadd, N.R., 1975: Geology o f Leda c l a y . Proc. 4 t h GueZph Symposium on GeomorphoZogy.

Kenney, T.C., 1967: S l i d e behaviour and shear r e s i s t a n c e o f q u i c k c l a y determined from a study o f t h e l a n d s l i d e a t Selnes, Norway. Proc. GeotechnicaZ Conference, Oslo, 1 : 57-64.

,

1975: Weathering and changes i n s t r e n g t h s i n c l a y s . Proc. 4th GueZph Symposium on GeomorphoZogy.

LaRochelle, P.A., 1975: Causes and mechanism o f l a n d s l i d e s i n s e n s i - t i v e c l a y s w i t h s p e c i a l r e f e r e n c e t o t h e Quebec Province Area.

Proc. 4 t h GueZph Symposium on GeomorphoZogy.

Lo, K.Y., and Lee, C.F., 1974: An e v a l u a t i o n o f t h e s t a b i l i t y of n a t u r a l slopes i n p l a s t i c Champlain c l a y s . Can. Geotech. J .

11: 165-181.

M i t c h e l l , R.J., 1970: On t h e y i e l d i n g and mechanical s t r e n g t h o f Leda c l a y s . Can. Geotech. J . , 7: 297-325.

M i t c h e l l , R.J., and Eden, W.J., 1972: Measured movements o f c l a y slopes i n t h e Ottawa area. Can. Jour. Earth S c i . , 9: 1001-1013.

Torrance, J.K., 1975: Leaching processes i n Leda c l a y . Proc. 4th GueZph Symposium on GeomorphoZogy.

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