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The Breckenridge landslide
NATIONAL R E S E A R C H COUNCIL CANADA
DIVISION O F BUILDING RESEARCH
T H E BRECKENRIDGE LANDSLIDE b y W. J. E d e n , E. L. M a t y a s a n d W. I r w i n ANALYZED I n t e r n a l R e p o r t No. 316 of t h e D i v i s i o n of Building R e s e a r c h OTTAWA J u n e 1965
P R E F A C E
The Breckenridge landslide in A p r i l 1963 provided another useful c a s e r e c o r d in the study of the geotechnical p r o p e r t i e s of Leda c l a y which i s being conducted by the Soil Mechanics Section. T h i s r e p o r t
p r e s e n t s t h e field observations and l a b o r a t o r y t e s t r e s u l t s obtained in the investigation. F u r t h e r observations on the p i e z o m e t r i c conditions of the a r e a a r e in p r o g r e s s and will continue f o r a number of y e a r s .
T h e r e p o r t w a s p r e p a r e d by W. J. Eden, R e s e a r c h Officer in the Soil Mechanics Section, Dr. E. L. Matyas of Carleton University, while engaged a s a S u m m e r P r o f e s s o r , and W . Irwin, a s u m m e r student.
Ottawa June 1965
N. B. Hutcheon A s s i s t a n t D i r e c t o r
T H E BRECKENRIDGE LANDSLIDE
by W. J. Eden, E . L. Matyas and W. Irwin
In April 1963, a s m a l l earthflow involving about 30, 000 cu yd of c l a y took p l a c e on t h e s i d e of a deep r a v i n e n e a r B r e c k e n r i d g e Station, approximately 15 m i l e s northwest of Ottawa. T h e Soil Mechanics Section of t h e Division of Building R e s e a r c h i s conducting an investigation of t h e
landslide, including stability a n a l y s e s in t e r m s of both total and effective s t r e s s e s . T h i s r e p o r t p r e s e n t s t h e r e s u l t s obtained thus f a r in the
investigation.
DESCRIPTION O F THE SITE
T h e exact location of the landslide i s shown in F i g u r e 1. It i s in p a s t u r e land midway between B r e c k e n r i d g e Station on the west and B e a m i s h Hill on t h e e a s t n e a r the boundary between t h e townships of E a r d l e y and West Hull. T h e g e n e r a l elevation of the clay plain i s f r o m 330 to 350. T h e s t r e a m h a s cut a number of deep r a v i n e s in the plain, and a t the landslide s i t e , t h e r a v i n e i s m o r e than 90 ft deep. T h e s t r e a m valley i s featured by the s c a r s of a number of l a n d s l i d e s , s o m e old and
s o m e quite r e c e n t .
T h e landslide o c c u r r e d a t approximately 10 p. m. on Saturday, 20 April 1963, during a heavy r a i n s t o r m accompanied by high winds. T h e initial slide destroyed a pole s t r u c t u r e of an a u x i l i a r y l i n e of the Gatineau P o w e r Co. R e t r o g r e s s i v e m o v e m e n t s continued on Sunday, Monday, and Tuesday. After T u e s d a y , only minor sloughing o c c u r r e d around the edges of the c r a t e r . Fluid c l a y f r o m the s l i d e s flowed down the s t r e a m valley blocking the c o u r s e of the water and causing a c o n s i d e r a b l e pond. T h e s t r e a m eventually cut a new c o u r s e in undisturbed ground a t the opposite s i d e of the ravine. F i g u r e 2 i s a plan of the landslide m a d e in May 1963 f r o m a stadia survey. F i g u r e s 3 and 4 a r e views of the s l i d e showing i t s extent. T h e bank f a i l u r e t h r e a t e n e d t h e safety of a s t e e l t r a n s m i s s i o n tower of the m a i n power l i n e f r o m B r y s o n to Hull and n e c e s s i t a t e d i t s relocation.
In s u m m a r y , the landslide o c c u r r e d in a n a t u r a l c l a y bank, approximately 90 ft high with a n a v e r a g e slope of about 1 7 " . It o c c u r r e d in the s p r i n g s e a s o n just a s the f r o s t w a s leaving the ground and during a heavy r a i n s t o r m . Numerous s c a r s of p a s t landslides a r e evident in adjacent slopes.
SOIL INVESTIGATIONS
An examination a t the s c a r p showed a s u r f a c e cap of about 3 t o 4 ft of sandy silt. T h e underlying clay extended to a depth of m o r e than 100 ft. The clay n e a r the s u r f a c e a p p e a r e d t o be highly stratified. Four field vane borings and one s a m p l e boring w e r e made a t the locations shown in F i g u r e 2 . The vane t e s t s r e v e a l e d a c a s e hardened l a y e r n e a r the slopes. The boring log and t e s t r e s u l t s a r e p r e s e n t e d in F i g u r e 5.
Boring BS -3 indicated the clay to be reasonably uniform throughout i t s depth with an a v e r a g e n a t u r a l water content of 7 9 p e r cent, liquidity index of 1. 36, and 82 p e r cent c l a y - s i z e p a r t i c l e s . The p o r e
-
water s a l t concentration was low with s o m e indication of a slight i n c r e a s e with depth. The clay was only slightly overconsolidated by about 1500 psf and a drying c r u s t extended t o a depth of about 20 ft. T h e field vane t e s t s for boring BV-2 indicated a minimum undrained s h e a r s t r e n g t h of 840 psf a t 31 ft and an a l m o s t l i n e a r i n c r e a s e in strength with depth below the drying c r u s t .LABORATORY SHEAR STRENGTH RESULTS
T o determine the effective s t r e s s s h e a r s t r e n g t h p a r a m e t e r s , consolidated undrained and drained t r i a x i a l t e s t s w e r e conducted on
undisturbed s a m p l e s over the full depth r a n g e of boring BS -3. Thirty-four consolidated undrained and 8 drained t e s t s w e r e conducted and the r e s u l t s a r e given in F i g u r e
6.
The r e s u l t s a r e plotted a s 1/2(a1 -d ) against3
1 / 2 ( d + b ) s o that an average line could be obtained by tfie method of l e a s t
1 3
s q u a r e s . The actual f a i l u r e envelope f o r m e d by the Mohr c i r c l e s i s r e l a t e d to this line by simple e x p r e s s i o n s (Bishop et a1 1960) and yielded s h e a r s t r e n g t h p a r a m e t e r s of c t = 344 p s f and
6'
=
20. 2".F o r e a c h t e s t maximum deviator s t r e s s was considered a s the f a i l u r e c r i t e r i a . Con olidation p r e s s u r e s used in the CIU t e s t s
5
ranged f r o m 1 to 6 kg/cm
.
The drained t e s t s w e r e conducted byconsolidating the sample t o a p r e s s u r e slightly below the preconsolidation p r e s s u r e , then failure was induced by d e c r e a s i n g the c e l l p r e s s u r e .
F a i l u r e s o c c u r r e d a t s t r a i n s f r o m 1. 0 to 8. 5 p e r cent i n the drained t e s t s . T h e r e s u l t s of the two types of t e s t s w e r e reasonably consistent. F i g u r e 6
shows that a s a t i s f a c t o r y c o r r e l a t i o n was achieved in determining the b e s t straight line.
P O R E PRESSURE RATIO, r
u
P i e z o m e t e r s w e r e not i n s t a l l e d at t h i s s i t e and consequently t h e p o r e w a t e r p r e s s u r e s had t o be a s s u m e d . Initially, a value of
r
=
0. 2 was used, a s this value had been used in t h e a n a l y s i s of the Green C r e e k ulandslide w h e r e p i e z o m e t e r s had r e v e a l e d a downward gradient in t h e ground w a t e r . It i s believed that t h e a s s u m e d value of r i s too low in this c a s e . P i e z o r n e t e r s a r e to be installed to obtain a n a c t u a value.
STABILITY ANALYSIS
A stability a n a l y s i s using Bishop's (1955) method of s l i c e s was u s e d t o d e t e r m i n e the factor of safety of the slope shown in F i g u r e 7. T h i s slope c o n s i s t s of s t r a i g h t - l i n e approximations to the a c t u a l slope which was determined by stadia s u r v e y and was used to facilitate
computation by computer a s d e s c r i b e d by Irwin (19641. The a n a l y s i s indicated that the m o s t c r i t i c a l portion of the slope ( c i r c l e 48) w a s t h e portion having t h e s t e e p e s t gradient. Using t h e r e s u l t s of CIU t e s t s ,
c i =- 250 psf a r d
4
'
-
20. 8 ' , the computed factor of s a f e t y was 0. 88. F o r the over - a l l stability of the slope ( c i r c l e 32) a value of F=
1. 0 4 was calculated.B y including t h e r e s u l t s sf the drained t r i a x i a l t e s t s , c '
i n c r e a s e d f r o m 250 psf t o 350 psf and
#'
d e c r e a s e d f r o m 20.8" t o 20. 2 " , and the factor of safety for the two c i r c l e s changed f r o m 0. 88 to 1. 01 and f r o m l . 0 4 tr, 1. 13.F i g u r e 8 ( a ) d e m o n s t r a t e s t h e sensitivity of the f a c t o r of safety t o v a r i a t i c n in t h e cohesion i n t e r c e p t , c ' . B e c a u s e t h e r e w e r e no field piezometric l e v e l s available, various values of r w e r e used in t h e
u
a n a l y s i s and over a limited r a n g e the influence of variation in r i s shown
u in F i g u r e 8 (b).
An undrained o r total s t r e s s a n a l y s i s w a s conducted on c i r c l e 48 using the r e s u l t s of field vane s t r e n g t h s . F o r a uniform s t r e n g t h of 900 psf the calculated factor of safety i s 1. 5. T o obtain a factor of
safety of 1. 0, a n undrained strength of 600 psf m u s t be u s e d , which i s not consistent with t h e o b s e r v e d field vane s t r e n g t h s . T h u s , it m u s t b e
concluded that the undrained a n a l y s i s in t h i s i n s t a n c e gives a n unsafe a s s e s s m e n t s f slope stability.
DISCUSSION
T h e a n a l y s i s of the B r e c k e n r i d g e landslide h a s indicated that the calculated factor of safety for the original slope c l o s e l y
approximated t h e t h e o r e t i c a l value of unity. It h a s a l s o been d e m o n s t r a t e d that t h e stability a n a l y s i s i s r a t h e r s e n s i t i v e to changes in the value of c 1
and t o changes in t h e p o r e p r e s s u r e r a t i o r
.
uIn the a n a l y s i s of t h e G r e e n C r e e k landslide (Matyas 1964) it was shown that a s i m i l a r i n t e r p r e t a t i o n of t e s t r e s u l t s and p o r e
p r e s s u r e r a t i o led t o a r e l a t i v e l y high calculated factor of safety. If the actual f a c t o r of safety of the G r e e n C r e e k slope did approach a value of unity, i t might b e a r g u e d that t h e r e i s s o m e d i s a g r e e m e n t in the
i n t e r p r e t a t i o n of the t e s t r e s u l t s f r o m the two s i t e s . T h e r e does not yet a p p e a r to be any explanation to c l a r i f y t h i s anomaly. It might be a r g u e d , however, that the two s i t e s have been subjected t o different
s t r e s s h i s t o r i e s ( B r e c k e n r i d g e i s about 50 ft higher than t h e G r e e n C r e e k s i t e ) and consequently t h e r e might b e s o m e fundamental differences in the behaviour of the soil with r e s p e c t to s h e a r strength.
REFERENCES
(1) Bishop, A. W. 91955). T h e u s e of t h e slip c i r c l e in the stabi1,ity a n a l y s i s of slopes. G6otechnique, Vol. 5, No. 1 , p. 7.
$ 2 ) Bishop, A. W . , I. Alpan, G. E. Blight and I. B. Donald (1960). F a c t o r s controlling the s t r e n g t h of p a r t l y s a t u r a t e d cohesive soils. R e s e a r c h Conference on Shear Strength Cohesive S o i l s , P r a c s . A m e r . Soc. Civ. E n g r s . p. 503-532.
( 3 ) Irwin, W. $1964). T h e u s e of a digital computer for solving slope stability p r o b l e m s . National R e s e a r c h Council, Division of Building R e s e a r c h , Computer P r o g r a m No. 19,
November 1964.
( 4 ) Matyas, E. L. (1964). T h e G r e e n C r e e k Landslide. National R e s e a r c h Council, Division of Building R e s e a r c h , Internal R e p o r t 309. J a n u a r y 1965.
S c a l e
1:50000
F I G U R E 1
Base of dismantled
O
transmission tower
/ - - -
Approxi mate location of
50
-
0
50
100
S c a l e i n F e e t
0B V - 2 ( V a n e b o r i n g )
B V - 3 ( S a m p l e b o r i n g )
F I G U R E 2
P L A N O F B R E C K E N R I D G E L A N D S L I DE A N D L O C A T I O N O F B O R I N G S
A R 9 3 6 8 - 2FIGURE 3
AERIAL V I E W O F BRECKENRIDGE LANDSLIDE
F I G U R E 4
:
loo
F I G U R E 7 S L O P E S T A B I L I T Y A N A L Y S I S B R E C K E N R I D G E L A N D S L I D E0
I
- L E G E N D-
Circles T h r o u g h P o i n t #1: Centre Contours o f Factor o f Safety- Circles T h r o u g h P o i n t #2: Centre 0
-
-
Contours o f Factor o f Safety
