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The perpetual landslide Summerland, British Columbia Riglin, Linda Diane 1977

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THE PERPETUAL LANDSLIDE SUMMERLAND, BRITISH COLUMBIA B.S., Colorado State University, 1974 THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF In THE FACULTY OF GRADUATE STUDIES (Department of Geological Sciences) We accept t h i s thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA A p r i l , 1977 Linda Diane Riglin, 1977 by LINDA DIANE RIGLIN MASTER OF SCIENCE In p r e s e n t i n g t h i s t h e s i s in p a r t i a l f u l f i l m e n t o f the r e q u i r e m e n t s f o r an advanced deg ree a t the U n i v e r s i t y o f B r i t i s h C o l u m b i a , I agree that t he L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r r e f e r e n c e and s t u d y . I f u r t h e r a g r e e t h a t p e r m i s s i o n f o r e x t e n s i v e c o p y i n g o f t h i s t h e s i s f o r s c h o l a r l y p u r p o s e s may be g r a n t e d by the Head o f my Depar tment o r by h i s r e p r e s e n t a t i v e s . I t i s u n d e r s t o o d t h a t c o p y i n g o r p u b l i c a t i o n o f t h i s t h e s i s f o r f i n a n c i a l g a i n s h a l l not be a l l o w e d w i t h o u t my w r i t t e n p e r m i s s i o n . Depar tment o f 6 £~6 C ^GJaJ ^ £ C / ^'M^ S^ The U n i v e r s i t y o f B r i t i s h C o l u m b i a 2075 Wesbrook Place Vancouver, Canada V6T 1W5 Date i ABSTRACT The purpose of thi s study i s to understand the environment for f a i l u r e of the Perpetual Landslide. To achieve t h i s objective a f i e l d i n v e s t i g a t i o n was c a r r i e d out to evaluate the movement pattern of the s l i d e and i t s geologic and hydrologic environments. This information, along with ground water flow and s t a b i l i t y models was used to define major controls on s t a b i l i t y . The following points are made: (1) The s l i d e moves by ro t a t i o n and t r a n s l a t i o n of blocks with a t r a n s i t i o n to flow movement at the toe. (2) At depth, the f a i l u r e surface l i e s within the T e r t i a r y sedimentary rocks. Exposed gouge consists primari-l y of clay (most l i k e l y remolded claystone and some c l a y - r i c h T.J I L / wizn dispersed peboxes uiiQ rock f ragiuen'cs . (3) D i s c o n t i n u i t i e s including inherent heterogene-i t y between and within geologic units, weathering, and j o i n t i n g are s i g n i f i c a n t to the unstable s i t u a t i o n . (4) Changes i n stress equilibrium, p a r t i c u l a r l y those caused by removal of overburden and l a t e r a l support with downcutting i n Trout Creek Canyon, are l i k e l y important i n the o r i g i n of the s l i d e . (5) The proposed mechanism of f a i l u r e i s : (a) The progressive reduction from peak to re-11 s i d u a l strength of the claystone. (b) In addition to the high water table, high pore-water pressures along the f a i l u r e surface. At the present time, the s l i d e ' s continuous movement i s a c t i n g to e s t a b l i s h a new s t a b i l i t y i n equilibrium with t h i s ground water flow system and lower strength. i i i CONTENTS Page CHAPTER 1 INTRODUCTION 1 1.1 L o c a t i o n 1 1.2 P h y s i o g r a p h i c s e t t i n g and s i t e d e s c r i p t i o n 3 1.3 L a n d s l i d e c l a s s i f i c a t i o n and type of movement. 8 1.4 Geomorphology 12 1.5 Previo u s i n v e s t i g a t i o n s 18 1.6 Purpose of present study 22 CHAPTER 2 GEOLOGY 2 4 2.1 C r y s t a l l i n e rocks 24 2.2 T e r t i a r y - c r y s t a l l i n e unconformity 26 2.3 T e r t i a r y v o l c a n i c rocks 28 2.4 T e r t i a r y sediments 30 2.5 S l i d e gouge 34 2.6 P o s t - d e p o s i t i o n a l h i s t o r y of T e r t i a r y sediments 35 2.7 Quaternary d e p o s i t s 36 2.8 Late g l a c i a l and recent h i s t o r y 40 CHAPTER 3 HYDROLOGY 43 3.1 Water i n f l o w and outflow 43 3.2 Water q u a l i t y 49 3.3 Ground water flow systems 58 continued. iv CONTENTS ...continued. Page CHAPTER 4 STABILITY ANALYSIS., 66 4.1 S u r f i c i a l movement 66. 4.2 D i r e c t shear t e s t i n g 77 4.3 Methods f o r s t a b i l i t y a n a l y s i s 92 4.4 Conclusions f o r slope s t a b i l i t y a n a l y s i s 97 CHAPTER 5 DISCUSSION AND CONCLUSIONS 105 5.1 D i s c u s s i o n of r e s u l t s and o r i g i n 105 5.2 Mechanism 112 5.3 Environmental problems and recommendations H 4 5.4 Summary and c o n c l u s i o n s 120 BIBLIOGRAPHY 1 2 4 APPENDIX 4-1 Movement of stakes w i t h time 129 APPENDIX 4-2 S t r e s s - s t r a i n curves f o r dr a i n e d shear t e s t s 143 V TABLES Page Table I P h y s i o g r a p h i c l o c a t i o n of the P e r p e t u a l L a n d s l i d e 5 Table II F e a t u r e s - t h a t a i d r e c o g n i t i o n of a c t i v e or r e c e n t l y a c t i v e slump f a i l u r e i n s o i l T able III Chemical d e s c r i p t i o n of c o a l Table IV H i s t o r i c a l r e c o r d of the P e r p e t u a l S l i d e 7 0 Table V H o r i z o n t a l movement r a t e s of stakes Table VI Mechanical a n a l y s i s of the d i r e c t shear t e s t 8 0 Table VII R e s u l t s of d i r e c t shear t e s t s 8 6 Table VIII L i m i t a t i o n s of lab t e s t s 3 Table IX Comparison of methods of s t a b i l i t y a n a l y s i s Table X Record of s e i s m i c a c t i v i t y i n the area of the P e r p e t u a l L a n d s l i d e 119 F i g u r e 1-• l a . F i g u r e 1-•lb. F i g u r e 1-•2a. F i g u r e -1-•2b. F i g u r e 1-•2c. F i g u r e 1-•2d. F i g u r e 1- 3. F i g u r e 1-•4a. F i g u r e 1-•4b. F i g u r e 1- 4c. F i g u r e 1- 4d. F i g u r e 1- 4e . F i g u r e 1- 4 f . F i g u r e 1- 4g. F i g u r e 1- 4h. F i g u r e 1- 4 i . vi ILLUSTRATIONS Page Physiography o f Southwestern B r i t i s h Columbia showing r e l a t i v e l o c a t i o n of the P e r p e t u a l L a n d s l i d e 2 Topography and p o i n t s of i n t e r e s t i n the v i c i n i t y of the P e r p e t u a l L a n d s l i d e 4 Catchment b a s i n of the P e r p e t u a l L a n d s l i d e pocket Topography of P e r p e t u a l S l i d e area 6 S e c t i o n s through the P e r p e t u a l L a n d s l i d e . . . 7 D e f i n i t i o n of l a n d s l i d e p r o p o r t i o n s 9 Geomorphology of the P e r p e t u a l L a n d s l i d e . . 11 Nomenclature of the p a r t s of a l a n d s l i d e . . 14 Photograph of concave headscarp 15 Photograph of t i l l - r i c h gouge f l a n k i n g the western toe of the s l i d e 15 Photograph of crack i n r e c e n t l y regraded road I 6 Photograph of u p t h r u s t e d b l o c k s exposing c o a l and t i l l 1 6 Photograph of o x i d i z e d gouge on toe of s l i d e 1 7 Photograph of t r a n s v e r s e scarps i n toe s e c t i o n 17 Photograph of l o c a l graben on e a s t e r n toe ^ l o c k 19 Photograph l o o k i n g down sand and g r a v e l flow on the western toe 19 continued. ILLUSTRATIONS .continued. Page Fi g u r e 1-4j. Photograph summarizing geomorphic f e a t u r e s r e f l e c t i n g movement i n the lower p a r t of the s l i d e 20 F i g u r e l-4k. Photograph of fans and t a l u s aggrading i n t o and being eroded by Trout Creek.... 21 F i g u r e 1-41. Photograph of recent flow i n t o creek 21 F i g u r e 2 - l a . General geology of the P e r p e t u a l L a n d s l i d e area pocket F i g u r e 2-lb. D e t a i l e d geology of the P e r p e t u a l 1 L a n d s l i d e and Trout Creek Canyon pocket F i g u r e 2 - l c . L o n g i t u d i n a l p r o f i l e of the P e r p e t u a l L a n d s l i d e along l i n e A-B pocket F i g u r e 2 - l d . Photograph of small f a u l t s , with o f f s e t shown i n pegmatite v e i n s . . . 27 F i g u r e 2-2. Photograph of headscarp of rock s l i d e on C r a t e r Mountain 27 F i g u r e 2-4a. Photograph of upthrust b l o c k of v o l c a n i c sandstone 32 F i g u r e 2-4b.. Photograph of b r e c c i a t e d c l a y s t o n e 32 F i g u r e 2-7a. Photograph of t i l l hummocks to west of s l i d e 1 37 F i g u r e 2-7b. Photograph l o o k i n g to the southeast at P a r a d i s e F l a t s , t e r r a c e s on C r a t e r Mountain, and headscarp of rock s l i d e on C r a t e r Mountain. 37 F i g u r e 2-8 Photograph of present-day Trout Creek D e l t a 4 2 F i g u r e 3 - l a . P r e c i p i t a t i o n , i r r i g a t i o n , and evapo-t r a n s p i r a t i o n over s u r f i c i a l catchment b a s i n of s l i d e 4 ^ F i g u r e 3-lb. I n f i l t r a t i o n and s p r i n g discharge 4 ^ continued... ILLUSTRATIONS . . .continued. F i g u r e 3-2a. Fi g u r e 3-2b. F i g u r e 3-2c. F i g u r e 3-2d. F i g u r e 3-2e. F i g u r e 3-3a. F i g u r e 3-3b. F i g u r e 3-4 F i g u r e 4 - l a . F i g u r e 4 - l b . F i g u r e 4 - l c . F i g u r e 4-Id. F i g u r e 4-2a, 4-2b. F i g u r e 4-2c. F i g u r e 4-2d. F i g u r e 4-2e. F i g u r e 4-2f. F i g u r e 4-2g. F i g u r e 4-3a, 4-3b, 4-3c. v i n Page NOg + NO^ c o n c e n t r a t i o n s 51 C l - c o n c e n t r a t i o n s 53 T o t a l d i s s o l v e d s o l i d s and pH 5 ^ S p r i n g water and a i r temperatures 5 ^ Water temperatures Cross s e c t i o n A'-B' pocket Schematic flownet of in t e r m e d i a t e system c r o s s s e c t i o n C-D. P l e i s t o c e n e t e r r a c e s - R e g i o n a l ground . . , j , , i - i pocket water flow model S u r f i c i a l changes of s l i d e (1938-1970) 68 P r e - s l i d e , 1938, and 1970 topography along-l o n g i t u d i n a l s e c t i o n l i n e A-B .. 69 Diagram of h o r i z o n t a l movements of sta k e s showing components normal t o survey l i n e s (upper p a r t of s l i d e ) 7 2 V e c t o r diagram of h o r i z o n t a l movement of stakes i n true d i r e c t i o n (toe of s l i d e ) . . 73 Shear s t r e n g t h vs. e f f e c t i v e normal s t r e s s . 7 ^ Photograph of the d i r e c t shear apparatus... 8 1 Forces i n d i r e c t shear t e s t i n g 8 2 S t r e s s - s t r a i n p l o t f o r remolded m a t e r i a l . . . °* Shear s t r e s s vs. e f f e c t i v e normal s t r e s s . . . Approximate r e l a t i o n s h i p between the d r a i n e d angle o f r e s i d u a l s h e a r i n g r e s i s t a n c e and the p l a s t i c i t y index S i m p l i f i e d Bishop Method of S l i c e s 93 continued... i x ILLUSTRATIONS ...continued. „ Page Figu r e 4-3d, 4-3e. T r a n s l a t i o n a l E q u i l i b r i u m A n a l y s i s 96 F i g u r e 4-4a. Case 1, f a i l u r e model f o r e n t i r e s l i d e 100 F i g u r e 4-4b. Case 2, f a i l u r e model f o r upper p a r t of s l i d e 101 F i g u r e 5-3. Photograph of l o g jam 116 Appendix 4-1. Movement of stakes with time Appendix 4-2. S t r e s s - s t r a i n curves f o r d r a i n e d shear t e s t s X ACKNOWLEDGMENTS I am indebted to the following people f o r t h e i r help and advice during t h i s study. Dr W.H. Mathews proposed the p r o j e c t and assisted, advised, and encouraged me i n a l l stages of study. Katy Madsen helped me i n many ways, especi-a l l y during the summer of fieldwork. Among other things, she made available her well-researched knowledge of the s l i d e and directed me to useful contacts in_the Summerland area, Several people i n the C i v i l Engineering Department at the University of B r i t i s h Columbia deserve acknowledge-ment. These include: Dr. R.G. Campanella, who reviewed my slope s t a b i l i t y work and gave many hours to help me with the d i r e c t shear t e s t i n g ; Dr. J.D. Anderson, who encouraged and a s s i s t e d me i n the s t a b i l i t y analysis; and Peter Demco, who made available excellent equipment for the survey. Dr. R.A..Freeze reviewed the hydrology chapter. R.A. Hodge helped me with s t a b i l i t y analysis, and L e s l i e Smith and Steve Earle reviewed parts of the thes i s . F i n a n c i a l assistance was provided by the National Research Council i n Grant A 1107 to W.H. Mathews. The 3.C. Water Investigation Branch provided topographic maps and sections and an orthophoto of the s l i d e . The Water XI Q u a l i t y Branch of Environment Canada i n Vancouver analyzed water samples f o r NO^ + N 0 2 and C l . 1 CHAPTER 1 INTRODUCTION Between 1914 and 1917 e a r l y s e t t l e r s of the area r e f e r r e d to as P a r a d i s e F l a t s (near Summerland B.C.) n o t i c e d i n c i p i e n t s l o p e f a i l u r e i n a p a r t of the f l a t s . Subsequent movement i n the area has continued f o r some s i x t y years and the f a i l u r e has become known as the P e r p e t u a l L a n d s l i d e . Because of continued movement of the s l i d e , questions have been r a i s e d r e g a r d i n g f u t u r e land-use as w e l l as s a f e t y of p r e s e n t l y d e v e l -oped lan d i n P a r a d i s e F l a t s . Hence, t h i s study was conducted to gain an understanding f o r the degree of i n s t a b i l i t y of the s l i d e , and to eva l u a t e the g e o l o g i c and h y d r o l o g i c environments conducive to f a i l u r e . I t i s hoped that remedial measures w i l l be taken to f o r e s t a l l f u t u r e damage by the s l i d e and that t h i s case h i s t o r y may demonstrate that g e o l o g i c and h y d r o l o g i c s t u d i e s should be undertaken be f o r e development to prevent s l o p e f a i l u r e i n s i m i l a r s i t u a t i o n s . 1.1 L o c a t i o n The P e r p e t u a l L a n d s l i d e i s l o c a t e d near West Summer-land i n the Okanagan V a l l e y of B r i t i s h Columbia and i s about 416 k i l o m e t e r s (260 m i l e s ) east of Vancouver ( F i g u r e 1 - l a ) . |: 900 , 800 F i g u r e 1-la.. Physiography of Southwestern B r i t i s h Columbia shov/ing r e l a t i v e l o c a t i o n o f the P e r p e t u a l L a n d s l i d e ( a f t e r H o l l a n d , 1964). 3 I t s l o c a t i o n i s 119°41' west l o n g i t u d e and 49°34' north l a t i t u d e . The s l i d e s p i l l s i n t o Trout Creek Canyon, about 5.4 km (3.4 m i l e s ) upstream from the mouth of the creek on Okanagan Lake ( F i g u r e 1 - l b ) . 1.2 P h y s i o g r a p h i c s e t t i n g and s i t e d e s c r i p t i o n The P e r p e t u a l L a n d s l i d e l i e s w i t h i n the Thompson P l a t e a u i n the Southern I n t e r i o r of B r i t i s h Columbia ( F i g u r e 1-l a , Table I ) . An i n t r o d u c t i o n to the s l i d e i s given i n the f o l l o w i n g b r i e f s i t e d e s c r i p t i o n . Topography of land i n the v i c i n i t y of the s l i d e (as shown i n F i g u r e l-2a) v a r i e s from g e n t l y u n d u l a t i n g to extremely s l o p i n g . T h i s land may be d i v i d e d i n t o three s e c t i o n s on the b a s i s of topography: Steeply s l o p i n g to very s t e e p l y s l o p i n g (30 to 40%) t e r r a i n on the slop e s of Mt. Conkle, g e n t l y u n d u l a t i n g to u n d u l a t i n g (0.5 to 5%) P a r a d i s e F l a t s , and very s t e e p l y s l o p i n g to extremely s l o p i n g (30 to 60+%) Trout Creek Canyon (B.C. Dept. of A g r i c u l t u r e , 1972). The catchment b a s i n o f the s l i d e ( F i g u r e l-2a) i s about 1.79 x 10 6 m2 (19.2 x 10 6 f t 2 or 441.1 a c r e s ) , approx-imat e l y 0.373 x 10 6 m2 (4.01 x 10 6 f t 2 or 92.2 acres) of which i s c u l t i v a t e d . The s l i d e r e c e i v e s drainage from p r e c i p i t a t i o n and i r r i g a t i o n water on P a r a d i s e F l a t s t ogether with recharge from the e a s t - f a c i n g slope of Mt. Conkle ( F i g u r e l - 2 a ) . The s l i d e dimensions are determined from a topo-g r a p h i c p l a n view and l o n g i t u d i n a l s e c t i o n s of the s l i d e (shown i n F i g u r e s l-2b and l - 2 c , r e s p e c t i v e l y ) . I t s Figure 1-lb. Topography and points of i n t e r e s t i n the v i c i n i t y of the Perpetual Landslide-(modified from Can. Dept. of Energy, Mines, SResources , 1964) . 5 Table I. P h y s i o g r a p h i c l o c a t i o n of the P e r p e t u a l L a n d s l i d e (from H o l l a n d , 1964) Region: Canadian C o r d i l l e r a System: I n t e r i o r D i v i s i o n Southern P l a t e a u and Mountain Area S u b d i v i s i o n : I n t e r i o r P l a t e a u Area: Thompson P l a t e a u FIGURE TOPOGRAPHY OF I-2b. PERPETUAL SLIDE T A D C A modified from Water Resources Service,1975) ROUT CR., SUMMERLAND S c a l e = I•'2500 Contour In terva l = 2 M e t r e s a p p r o x i m a t e b o u n d a r y o f s l i d i n g m a s s Photography Date = SeDt. 13. 1970 P h n t n N I „ = o r * A A I • 6 5 0 in 6 0 0 I— Ul 5 5 5 0 z o I-< > U J 5 0 0 PARA DISE 6 5 0 4 5 0 6 5 0 to Ul cc 6 0 0 5 5 0 < > ui _i Ul 5 0 0 4 5 0 - 2 0 0 - 1 0 0 2 0 0 3 0 0 L E N G T H IN M E T R E S SECT ION T H R U A - B ( I ) 7 0 0 <L T r o u t Or 1938 6 5 0 6 0 0 550 5 0 0 4 5 0 C A N Y O N V I E W R O A D 2 0 0 • L E N G T H S E C T I O N THRU ( 2 ) F I G U R E E —^ !EEK TROUT CF \\ 6 5 0 6 0 0 in ui cc 550 < > ui _i 5 0 0 450 L E N G T H IN M E T R E S G_ Trout Cr». 1938 SECTION THRU (3 ) - 2 c . S E C T I O N S T H R O U G H T H E P E R P E T U A L SL IDE (SEE F I G U R E l ~ 2 b ) . E - F Trout Cr. I 9 3 8 NOTE : B KEY PLAN N T S SECTIONS T A K E N FROM S E P T E M B E R , 1970 DATA. S H O W N 1 9 3 8 DATA S H O W N WARNING P H O T O G R A P H Y FOR 1 9 3 8 IS OF R E L A T I V E L Y P O O R Q U A L I T Y AND D A T A D E R I V E D F R O M IT M A Y D I S P L A Y C O N S I D E R A B L E I N A C C U R A C Y . N O T E A P P A R E N T 8 m D I F F E R E N C E IN C R E E K L E V E L S . BRITISH COLUMBIA DEPARTMENT OF LANDS, FORESTS, AND WATER RESOURCES. W A T E R R E S O U R C E S S E R V I C E WATER INVESTIGATIONS BRANCH T O A C C O M P A N Y R E P O R T O N PERPETUAL SLIDE TROUT C R E E K SUM M E R L A N D D A T E M A Y , 1975 K E N N E D Y S M Y T H E N G I N E E R H O R . i : . . 2 , 5 0 0 FILE No 0 2 5 3 7 5 6 - 7 D W G . No 8 maximum dimensions are 425 m (1394 f t ) i n upslope l e n g t h , 419 m (1375 f t ) i n width, and perhaps 80 m (262 f t ) i n depth (based a l s o on g e o l o g i c i n t e r p r e t a t i o n s ) . These measurements are made ac c o r d i n g to Skempton's (1969) d e f i n i t i o n of land-s l i d e p r o p o r t i o n s ( F i g u r e l - 2 d ) . The s l o p e of the s l i d e s u r f a c e along p r o f i l e A-B ( F i g u r e l - 2 c ) i s about 15°. I t s approximate area i s 0.14 x 10 6 m2 (1.5 x 10 6 f t 2 ) and i t s 6 3 9 3 volume i s estimated to be 5.4 x 10 m (0.19 x 10 f t ). The m a t e r i a l i n v o l v e d i n shear f a i l u r e at depth seems to be mainly a remolded T e r t i a r y claysto'ne. F a i l u r e zones are exposed at the topographic s u r f a c e as t h r e e major sc a r p s (shown i n F i g u r e l - 2 c ^ ) i n Quaternary u n c o n s o l i d a t e d d e p o s i t s d i v i d e the s l i d e i n t o s u c c e s s i v e b l o c k s . 1.3 L a n d s l i d e c l a s s i f i c a t i o n and type of movement The type of f a i l u r e i s that of a s l i d e , i n the s t r i c t sense. The Highway Research Board L a n d s l i d e Committee i n t h e i r C l a s s i f i c a t i o n of L a n d s l i d e s (1958, P l a t e 1) d e s c r i b e a s l i d e as f o l l o w s : "Movement caused by f i n i t e shear f a i l u r e along one or s e v e r a l s u r f a c e s which are v i s i b l e or whose presence may be reasonably i n f e r r e d . M a t e r i a l i n motion not g r e a t l y deformed. Moving mass c o n s i s t s of one or a few u n i t s . Maximum dimension of u n i t s i s g r e a t e r than displacements between u n i t s . Movement may be c o n t r o l l e d by s u r f a c e s of weakness such as f a u l t s , bed-ding planes or j o i n t s . " In the P e r p e t u a l S l i d e , d i f f e r e n t i a l t r a n s l a t i o n of u n i t s downslope produces e x t e n s i o n a l areas or grabens. F u r t h e r , the s l i d e has a dominant r o t a t i o n a l charac-t e r as l o c a l u p t h r u s t i n g and backward t i l t i n g of b l o c k s 9 I L = maximum length of slide up slop* | D = maximum thickness of slide ' B a maximum breadth of slide F i g u r e l - 2 d . D e f i n i t i o n o f l a n d s l i d e p r o p o r t i o n s (Skempton, 1969). 10 are observed. T h e r e f o r e , the f a i l u r e be c l a s s i f i e d as a slump: "Movement only along i n t e r n a l s l i p s u r f a c e s which are u s u a l l y concave upward. Backward t i l t i n g of u n i t s i s common." (Highway Research Board L a n d s l i d e Committee, 1958, P l a t e 1). Motion i n p a r t s of the toe zone may be l i k e n e d to that of flow: "Movement w i t h i n the d i s p l a c e d mass such that the form taken by moving m a t e r i a l , or the apparent d i s t r i b u -t i o n s of v e l o c i t i e s and displacements resemble those of v i s c o u s f l u i d s . " (Highway Research Board L a n d s l i d e Committee, 1958, P l a t e 1). R e l a t i v e l y dry sand and g r a v e l flows bound the toe s e c t i o n of the P e r p e t u a l S l i d e ( F i g u r e 1-3). They form a system of l o n g i t u d i n a l r i d g e s which f u n n e l i n t o the s p r i n g g u l -l i e s downslope. Here, as the s p r i n g s undercut loose bank m a t e r i a l , sand, g r a v e l , and s i l t flow r a p i d l y i n t o the head of the g u l l y , and by repeated s l o u g h i n g , the c a v i t y i s enlarged. Wet flows i n v o l v e w a t e r - s a t u r a t e d sand and g r a v e l masses pour-in g out from the toe and down along the s l o p e below the s l i d e . Another type of wet flow i s i n s a t u r a t e d gouge (exposed along the l i p of the canyon) as i t may flow r a t h e r than move with shear displacement. The c l a y masses which flow down the canyon w a l l f i t Skempton's (1969) d e s c r i p t i o n of mudflow m a t e r i a l , commonly c o n s i s t i n g of unsorted, c l a y - r i c h d e b r i s i n a s o f t c l a y e y matrix. In much of the toe s e c t i o n , movement may be a com-b i n a t i o n of shear on bounding s l i p s u r f a c e s and v i s c o u s move-ments w i t h i n the f l o w i n g mass. 4 LEGEND FAULT SCARPS, TICKS DOWN-SCARP LOCAL STRIKE AND DIP OF FAULT SCARP APPROXIMATE BOUNDARY OF SLIDE BLOCK GRABEN OR DEPRESS I ON GENERAL D I RECTI ON OF SURF I CIAL MOVEMENT (DASHED,TO INDICATE UPWARD MOVEMENT FLOW MOVEMENT SEEP SPRING POND RIDGE (FAULT) ALLUVIAL FAN TALUS SCALE 300 FE E T 100, METERS 600 200 F I G U R E 1 - 3 . G E O M O R P H O L O G Y O F T H E P E R P E T U A L L A N D S L I D E 12 As the type of motion w i t h i n the s l i d e v a r i e s , so a l s o does the r a t e of movement. H o r i z o n t a l movement r a t e s range from approximately 0.1 m/yr i n the slide's upper p o r t i o n s to 18 m/yr at the toe. 1.4 Geomorphology From a geomorphic study, i t i s observed that many of the landform f e a t u r e s i n the P e r p e t u a l s l i d e are t y p i c a l of a c t i v e or r e c e n t l y a c t i v e slump f a i l u r e (Table II) i n s o i l as d e s c r i b e d by R i t c h i e (1958). (Sheared and remolded bed-rock i n the T e r t i a r y s t r a t a i s c o n s i d e r e d to act as a s o i l . ) The f o l l o w i n g d i s c u s s i o n of landform f e a t u r e s used nomenclature f o r the p a r t s of a l a n d s l i d e ( F igure l-4a) as d e f i n e d by Varnes (1958). F i g u r e l-4b shows the concave main scarp area. Flank-i n g s carps are o u t l i n e d i n the geomorphic map, F i g u r e 1-3, and l a t e r a l displacement i s i n d i c a t e d by s l i c k e n s i d e s along a f a u l t bounding the s l i d e toe i n F i g u r e l - 4 c . The geomorphic map a l s o d e l i n e a t e s grabens or depressions which are evident at the head of each block, minor scarps o c c u r r i n g i n the head and body of the s l i d e ( F i g u r e l - 4 d ) , and ponded depressions forming i n the fo o t or toe areas. Scarps f a c i n g the d i r e c t i o n o p posite to gene r a l s l i d e movement are termed backscarps and are the r e s u l t of subsidence i n nearby grabens or l o c a l u p l i f t ( F i g u r e 1-3). Upthrust t i l l , c l a y s t o n e , c o a l , and sandstone are present on the p e r i p h e r y of the toe s e c t i o n s ( F i g u r e s l-4e and f ) . Water e f f l u e n t over t h i s impermeable gouge form ponds i n depressions 13 Table i i . F e a t u r e s t h a t a i d r e c o g n i t i o n o f a c t i v e o r r e c e n t -l y a c t i v e slump f a i l u r e i n s o i l (from R i t c h i e , 1958) . T y p e o f M o t i o n .' K i n d o f M a t e r i a l S t a b l e P a r t s S u r r o u n d i n g the tiltde C r o w n M a i n S c a r p F l a n k s S l u m p i S o i l N u m e r o u s c r a c k s , most o f them c u r v e d concave t o w a r d sl ide Steep, bare , concave t o w a r d s l ide, c o m m o n l y hijrh. X a y show s tr iae a n d f u r r o w s on s u r f a c e r u n n i n g f r o m c r o w n to head . U p p e r p a r t of s carp m a y be v e r t i c a l S t r i a e o n flank s c a r p s have' s t r o n g v e r t i c a l c*»»r»i>orj*-?nt n e a r head, s t r o n g h o r i z o n t a l c o m p o n e n t n e a r foot* H e i g h t of n V n k s c a r p decreases t o w a r d foot, r lar .k o f s l ide] • m a y be h i g h e r ; h a n o r i s ; i [ i J u | g r o u n d fcurfuee 1 set w e e n foo i . | a n d t<>e. En eckcion c r a c k s ! o u t l i n e s l ide i n e a r l y siaKea .J P a r t s T h a t H a v e M o v e d B o d y -Foot T o e 1U%rr.r.ant5 o f l a n d s u r f a c e flatter fh::r» c r i m i n a l ship,? o r e v e n t i l ted - in to h i l l c r e a t i n g depress ions at c f m a i n sen r p i n w h ich per imeter ponds f o r m . T r a t i s -c r a c k s , m i n o r scarps , f;ra~ H-en1*, a n d f^'Mt blocks . A t t i t u d e of bed-dine differs f r o m s u r r o u n d * in# a r e a . T r e e s lean u p h i l l . « — — f O r i g i n a l s l u m p hJocks g-«?nerai:.y broken into srr,;: IJ * r sr. c. ss e.^  ; Ion c it u c nat c r a c k s , p r ^ i - u r e r idges . occr.sic.ir.al overt h r u s t i n g . C o m m o n l y develops a s m a l l P o n M j u s t nuove 1oot T r a n s v e r s e pressure r id t ies and crackr. c o m m o n l y de-ve lop o v e r the f o o t ; zone of u p l i f t , absence o f lar^e i n d i v i d u a l blocks, trees lean d o w n h i l l O f t e n a i o n e o f earthf low, 1c-b;He f o r m , m a t e r i a l rc l iedi over a n d b u r i e d ; trees l i e fiat o r at" v a r i o u s s n j r l c s m i x e d i n t o toe m a t e r i a l 14 MfliiV SCARP- A steep surface on the undisturbed " ground"around the periphery of the slide, caused by movement of s l ide m a t e r i e l away f r o m the undisturb -e d ground. The projection of the scarp surface ur.dsr the disturbed material becomes the surface of rupture. MINOR SCARP- A steep surface on the disturb-ed matenaTproSuced by differential movements within the eliding mess. HEAD-The upper parts of the slide material clang the contact between the d i s tu rbed material o n d - ' ' the main scarp. TOP-Thre highest point of contact between the dis-turbed~rr.ateriol and the main scarp. TOOT- The line of intersection (sometimes buried) ^ between the. lower part of the surtcce of rupture and - the original ground surface. TOE-The margin of disturbed material most dis-fanfT.'cm the m a i n scarp. TIP- The point on the toe most distant from the top of the slide. FLANK- The side of the landslide. CROWN - The material that is still in place, practically undisturbed, and cdjacent to the highest parts of the main scarp. ORIGINAL GROUND SURFACE- The slope that existed before the movement which is b e i n g cons idered took place. If this is the s t i r f o c e of cn elder landslide, that fact should be stated. LEFT ANO RIGHT- Compass directions are preferable in describ-ing cTslids, but if right and left are used They refer to the s l ide as v iewed from the cro*n. F i g u r e l - 4 a . Nomenclature of the p a r t s o f a l a n d s l i d e (Varnes, 1958). Since the toe of the P e r p e t u a l S l i d e i s c u t back by the lower s p r i n g s , toe and f o o t s e c t i o n s are nol d i f f e r e n t i a t e d and the two terms are used synonymously. F i g u r e l-4b. Photograph l o o k i n g e a s t a c r o s s headscarp which i s concave towards the s l i d e a t r i g h t . Note minor f a u l t b l o c k , as marked by l a r g e t r e e i n the lower r i g h t -hand c o r n e r . F i g u r e l-4c. Photograph of mudcracked and s l i c k e n s i d e d , t i l l - r i c h gouge f l a n k i n g western toe of the s l i d e (trend of s l i c k e n s i d e s i s from l e f t to right), . F i g u r e l-4d. Photograph l o o k i n g west to crack i n r e c e n t l y regraded road i n main body of s l i d e . F i g u r e l-4e. Photo-graph of u p t h r u s t b l o c k s exposing c o a l and t i l l near headscarp o f toe b l o c k , l o o k i n g n o r t h . F i g u r e l-4g. Photograph of t r a n s v e r s e scarps i n toe s e c t i o n . Note t h a t b l o c k s are somewhat b a c k - r o t a t e d (to r i g h t ) . S l i d e movement i s towards lower l e f t - h a n d c o r n e r . 18 i n the toe s e c t i o n . A l s o , i n t h i s area break-up of coherent b l o c k s i s evident as t r a n s v e r s e c r a c k s , ( F i g u r e l - 4 g ) , l o c a l grabens ( F i g u r e l - 4 h ) , and flow motion ( F i g u r e l - 4 i ) are observed. At the southeastern extremity of the a c t u a l s l i d e , along the canyon l i p , the lower s p r i n g s and w a t e r - s a t u r a t e d c l a y gouge are exposed. F i g u r e 1-4j p r e s e n t s a summary of the f e a t u r e s i n the toe area. The f i n a l type of landform c o n s i d e r e d , the a l l u v i a l (and t a l u s ) fans, r e s u l t from d e p o s i t i o n of s l i d e d e b r i s along Trout Creek Canyon. The d e b r i s i s a product of e r o s i o n on the s l i d e toe by the lower s p r i n g s ( F i g u r e 1-3 and F i g u r e l - 4 k ) . The fans i n t u r n , are cut back by Trout Creek, as evidenced by seasonal f l u c t u a t i o n s i n the extent of fans i n t o Trout Creek. The fans, t h e r e f o r e , do not a t t a i n long-term s t a b i l i t y , the s p r i n g s continue to erode headward, and the u n s t a b l e toe i s c o n s t a n t l y undercut. F i g u r e 1-41 shows a flow extending out i n t o Trout Creek. Note the logjam and the t u r b i d water of Trout Creek below ( t o t h e _ l e f t o f ) the fans. 1.5 P r e v i o u s i n v e s t i g a t i o n s Three p r o f e s s i o n a l analyses were conducted p r i o r to t h i s study. C a r l H a l s t e a d of the Groundwater D i v i s i o n , B.C. Water I n v e s t i g a t i o n s Branch, r e c o g n i z e d the source f o r the excess sediment l o a d of lower Trout Creek and recommended c o r r e c t i v e measures to d r a i n the s l i d e . Thurber C o n s u l t a n t s noted the p o s s i b i l i t y of r a p i d s l i d e movement r e s u l t i n g i n 19 F i g u r e l-4h. Photograph of l o c a l graben on e a s t e r n toe bl o c k . S l i d e movement i s to the r i g h t . Note backscarps. F i g u r e l - 4 i . Photograph l o o k i n g down sand and g r a v e l flow on the western toe. Note r i d g e and furrow topography. The flow i s l a t e r a l l y c o n f i n e d by gouge r i d g e t o r i g h t . 20 Figure 1-4j. Photograph summarizing geomorphic features which r e f l e c t type of movement i n the lower part of the s l i d e : 1 Tranverse ridges (benches) caused by break-up of large coherent blocks near s l i d e toe. 2 Ridge and furrow topography r e s u l t i n g from movement in sand and gravel flow. 3 Depressions (shown by accumulation of trees and vegetation) i n graben areas at the back of the toe block, with upward r o t a t i o n a l movement coming out of the photograph. 4 G u l l i e s , where springs steepen and undermine the t o e . -—*—- Lower l i m i t of s l i d e . Bedrock exposed where springs appear. 21 Figure 1-41. Photograph of recent flow into creek, looking downslope. blockage and consequent f l o o d i n g downstream. They suggested that a d e t a i l e d s u r f a c e i n v e s t i g a t i o n be c a r r i e d out. A p r e l i m i n a r y assessment was a l s o made by N e i l Wade, form e r l y of Klohn Leonoff C o n s u l t a n t s , i n October, 1974. Water sources and mechanisms of the s l i d e were d e f i n e d . P o s s i b l e remedial measures were suggested and i t was recommended that a sub-s u r f a c e i n v e s t i g a t i o n be made. The Water I n v e s t i g a t i o n s Branch r e q u i s i t i o n e d l a r g e s c a l e (1:2500) topographic mapping of the s l i d e . Maps were made from both 1938 and 1970 air.photos (map shown i n F i g u r e l-2b) f o r comparison of topography. 1.6 Purpose of present study The o b j e c t i v e of t h i s study i s to understand the type of movement and i t s seasonal behaviour, the i n f l u e n c e of parameters on s l o p e i n s t a b i l i t y , and u l t i m a t e l y , the mechanism f o r f a i l u r e . Remedial measures can then be recommended. To achieve these o b j e c t i v e s : (1) A g e o l o g i c a l study was c a r r i e d out to r e l a t e g e o l o g i c a l p r o p e r t i e s to i n t e r p r e t a t i o n s of s t r e n g t h and h y d r a u l i c c o n d u c t i v i t y . (2) A h y d r o l o g i c survey q u a n t i f i e d water discharge and recharge, analyzed water q u a l i t y , and h y p o t h e s i z e d l o c a l and r e g i o n a l groundwater flow systems. (3) A s t a b i l i t y a n a l y s i s i n c l u d e d a study of both long and short-term movement of the s l i d e , d e t e r m i n a t i o n of shear s t r e n g t h of the f a i l u r e plane m a t e r i a l , and f i n a l l y , e q u i l i b r i u m modeling of the s l i d e . A l l the aforementioned i n f o r m a t i o n was i n t e g r a t e d and an o r i g i n and mechanism f o r f a i l u r e of the s l i d e was pro posed, along w i t h recommendations f o r remedial measures. 24 CHAPTER 2 GEOLOGY The o b j e c t i v e of the g e o l o g i c a l study of the P e r p e t u a l S l i d e area was to determine l i t h o l o g y , d i s t r i b u t i o n , and s t r u c t u r e of g e o l o g i c u n i t s as these f a c t o r s apply to movement, of ground water and of the s l i d e i t s e l f . Geology of the area surrounding the s l i d e has been mapped on a r e g i o n a l b a s i s by C a i r n e s (1940) and r e v i s e d by L i t t l e (1961). T h e i r work i s p u b l i s h e d as maps 538A and 15-1961, K e t t l e R i v e r (west h a l f ) , B.C. In the present study, geology i n the immediate v i c i n i t y of the s l i d e was mapped ( F i g u r e s 2 - l a , 2-lb, and 2 - l c ) and d e p o s i t s were d i v i d e d i n t o three b a s i c u n i t s : an o l d e r c r y s t a l l i n e group (mainly J u r a s s i c a c c o r d i n g to Peto and Armstrong, (1976)), a younger v o l c a n i c and sedimentary sequence ( l i k e l y Mid-Eocene ac c o r d i n g to Mathews and Rouse (pers. comm. )), and o v e r l y i n g u n c o n s o l i d a t e d d e p o s i t s (Quaternary). 2.1 C r y s t a l l i n e Rocks Exposed i n Trout Creek Canyon, u n d e r l y i n g and bound-i n g the s l i d e , i s a massive body of g r a n o d i o r i t e and a v e i n and d i k e complex ( g r a n i t i c pegmatite and r h y o l i t e p orphyry), both of which en c l o s e a competent u n i t of limestone-sandstone ( F i g u r e s 2 - l a and b ) . The body of g r a n o d i o r i t e may have been emplaced as s i l l s s urrounding t h i s limestone-sandstone u n i t 25 or a l t e r n a t i v e l y , the g r a n o d i o r i t e may be a product of gran^-i t i z a t i o n of sediments s t r a t i f i e d with the limestone-sandstone. The l a t e r e x p l a n a t i o n i s compatible with f i e l d evidence, as a great t h i c k n e s s of g r a n o d i o r i t e i s exposed i n the canyon and t h i s g r a n o d i o r i t e i s i n conformable contact with the t h i n , but continuous limestone u n i t . Because of i n t e n s e hydrothermal and perhaps r e g i o n a l metamorphism of the c r y s t a l l i n e rocks, a l o c a l c l a s s i f i c a t i o n scheme was adopted and r e g i o n a l c o r r e l a t i o n was not attempted. The limestone grades from a l i g h t (N 7) to medium (N 5) gray, c l e a n limestone i n t o a g r e e n i s h gray (6 CY 6/1) to dark g r e e n i s h gray (5 CY 4/1) impure sandstone. Thickness v a r i e s , but approximates 10 meters and dip i s about 35° to the southwest. In many exposures the u n i t i s b r e c c i a t e d and d i s -s e c t e d by v e i n s . The igneous rocks i n c l u d e a massive, medium to f i n e -g r a i n e d , pink (5 YR 8/4) g r a n o d i o r i t e , through which Trout Creek has cut i t s canyon w a l l s . I t i s commonly very f r i a b l e and hydrothermally a l t e r e d at v e i n c o n t a c t s , j o i n t e d i n ex-posure, and weathered on i t s upper s u r f a c e ( t o form a s a p r o l i t e and a weathered rock zone). P a r t s of these h e t e r o g e n e i t i e s form c o n d u i t s f o r groundwater d i s c h a r g e i n Trout Creek Canyon. The v e i n and dike complex i n c l u d e s g r a n i t i c pegmatite which i s exposed as v e i n s along the l i m e s t o n e - g r a n o d i o r i t e contact and c u t t i n g across the g r a n o d i o r i t e . It i s commonly a s s o c i a t e d w i t h q u a r t z and c a l c i t e v e i n s and i n t e n s e a l t e r a t i o n of country rock along i t s c o n t a c t s . A l s o p a r t of t h i s complex-i s (1) a r h y o l i t e porphyry s i l l which i s exposed on the south w a l l of Trout Creek canyon and i s conformable w i t h the lime-stone, and (2) a t h i n p o r p h y r i t i c mafic d i k e , observable i n the lower north w a l l of the canyon. A m u l t i t u d e of s m a l l f a u l t s o f f s e t the v e i n s and dikes to the east of the s l i d e ( F i g u r e 2 - l d ) . Observed r e l a t i v e displacements are u s u a l l y l e s s than a few meters. 2.2 T e r t i a r y - c r y s t a l l i n e unconformity An unconformity e x i s t s between the o l d e r c r y s t a l l i n e and younger v o l c a n i c and sedimentary rocks. G e n e r a l l y , t h i s s u r f a c e trends n o r t h e a s t with a shallow northwest d i p . T h i s i contact i s shown to be that of a major f a u l t zone ( a p p r o x i -mately l o c a t e d , L i t t l e , 1961). However, d e t a i l e d mapping of the P e r p e t u a l S l i d e area has f a i l e d t o demonstrate the major f a u l t zone, and the T e r t i a r y - c r y s t a l l i n e contact i s evidenced to be an unconformity. However, di s c o n t i n u o u s northeast t r e n d -i n g f a u l t s are present around t h i s contact as w e l l as e l s e -where i n Trout Creek Canyon ( F i g u r e 2 - l a ) . The unconformity i s exposed j u s t west of the toe of the s l i d e as weathered zone (developed over the g r a n o d i o r i t e ) . A p o r t i o n of t h i s zone serves f o r ground water flow as i n d i -c ated by seepage where i t i s exposed. Discharge through t h i s zone and low s t r e n g t h i n the weathered m a t e r i a l were probably r e s p o n s i b l e f o r a rock s l i d e (see F i g u r e 2-2) on the north 28 s i d e of C r a t e r Mountain, as the f a i l u r e zone there i s along the c r y s t a l l i n e - T e r t i a r y unconformity. However, the uncon-fo r m i t y does not d i r e c t l y u n d e r l i e the f a i l u r e plane of the P e r p e t u a l S l i d e except at the s l i d e toe. 2.3 T e r t i a r y v o l c a n i c rocks V o l c a n i c rocks crop out on topographic highs surrounding the s l i d e ( F i g u r e 2 - l a ) . These rocks may be c o r r e l a t i v e with the Marron Formation, of Mid-Eocene i n age ( L i t t l e , 1961; Church, 1973). Because the v o l c a n i c s are commonly i n t e n s e l y b r e c c i a t e d , t h e i r s t r e n g t h i s decreased and secondary p e r m e a b i l i t y may be developed. However, the v o l c a n i c s , themselves, are not observed to be i n v o l v e d i n the P e r p e t u a l S l i d e . A p o r p h y r i t i c , amygdaloidal flow i s present i n l i m i t e d exposures on C r a t e r Mountain. I t i s commonly gray to o l i v e gray (5Y 5/1 to 5Y 5/2) and c o n t a i n s b i o t i t e and c h l o r i t i z e d pyroxene embedded i n a matrix of g l a s s , f e l d s p a r l a t h s , and maf i c s . Amygdaloidal and v e i n f i l l i n g s are hematite-s t a i n e d and are formed from z e o l i t e s . An o l i v e (5Y 4/4) l i t h i c t u f f with b r e c c i a , flow, and minor sandstone i s conspicuous i n exposure, comprising C r a t e r Mountain, Mt. Conkle, and a bedrock r i d g e t o n o r t h e a s t of the s l i d e . Thickness v a r i e s , but i n c r e a s e s to hundreds of meters on Mt. Conkle. The rock i n t h i n s e c t i o n may be c l a s s i -* C r a t e r Mountain i s a l o c a l name f o r the mountain approximately one k i l o m e t e r south of the s l i d e . f i e d as a l i t h i c t u f f ( W i l l i a m s , et al., 1954) of in t e r m e d i a t e composition. I t c o n s i s t s of v o l c a n i c rock fragments (commonly <4mm i n diameter), b i o t i t e , and. pyroxene l y i n g i n a g l a s s , c h l o r i t i c , and ferromagnesian matrix. In p l a c e s , i t grades i n t o v o l c a n i c sandstone or conglomerate, i . e . , v o l c a n i c c l a s t s i n a sedimentary matrix ( W i l l i a m s , et a l . , 1954). C l a s t s are g e n e r a l l y subangular to rounded, v a r y i n g i n s i z e from l e s s than 1 mm to approximately 15 cm. A f a b r i c i s apparent on C r a t e r Mountain as some c l a s t s are a l i g n e d . D i a g e n e t i c e f f e c t s i n c l u d e d e v i t r i f i c a t i o n of the g l a s s matrix, and weathering and j o i n t i n g of outcrops. S e v e r a l northeast t r e n d i n g high angle f a u l t s and j o i n t s cut these v o l c a n i c s i n C r a t e r Mountain and n o r t h e a s t of P a r a d i s e F l a t s ( F i g u r e 2 - l a ) . Near the s l i d e toe and extending to the southwest ( F i g u r e 2 - l b ) , i s a more a c i d i c , v i t r i c - l i t h i c t u f f with an almost e n t i r e l y h o l o h y a l i n e matrix. Exposed i n an ap p r o x i -mately 10 m t h i c k outcrop, i t i s an o l i v e (5Y 4/3) rock composed mainly of s o r t e d v o l c a n i c rock fragments, i n a matrix of predominately d e v i t r i f i e d g l a s s . I r o n - s t a i n e d v e i n s d i s s e c t the rock and c o n t a i n z e o l i t e s . O v e r l y i n g t h i s l i t h i c t u f f , i s a l i g h t e r - c o l o r e d v i t r i c t u f f ( W i l l i a m s , et a l . ) . T h i s medium gray (N 6) rock i s exposed i n h i g h l y b r e c c i a t e d outcrops i n a t h i c k n e s s of about 10 m. The t u f f c o n s i s t s almost e n t i r e l y , both i n matrix and c l a s t s , of g l a s s , much of which i s d e v i t r i f i e d , with f e l d s p a r and b i o t i t e embedded i n the matrix. C a l c i t e v e i n s 30 d i s s e c t the rock and are i r o n - s t a i n e d , as i n some of the matrix. A l a r g e s c a l e f l e x u r e i s evident i n the v o l c a n i c s . Northwest of the mapped area, the bedding s t r i k e s north-south, with steep d i p s to the ea s t , whereas near the toe of the s l i d e , a moderate dip to the nort h e a s t i s dominant (bedding shown i n Fi g u r e 2 - l c ) . 2.4 T e r t i a r y sediments T e r t i a r y sediments form the f a i l u r e plane of the s l i d e and i n f l u e n c e slope s t a b i l i t y both h y d r o l o g i c a l l y and l i t h o l o g i c a l l y . The rocks are w i t h i n a u n i t mapped as "Eocene or O l i g o c e n e " sediments and p y r o c l a s t i c s ( L i t t l e , 1961) and are now regarded as Mid-Eocene i n age (Mathews and Rouse, per. comm.). The T e r t i a r y sediments are l o c a l i z e d i n d i s t r i b u t i o n . T h i s may be e x p l a i n e d by the f o l l o w i n g o u t l i n e of t h e i r depos-i t i o n a l and p o s t - d e p o s i t i o n a l h i s t o r y : (1) Debris shed from v o l c a n i c and c r y s t a l l i n e h i l l s s p i l l e d i n t o and i n f i l l e d a topographic d e p r e s s i o n forming fresh-water and a l l u v i a l ( l a k e and swamp) d e p o s i t s ; and (2) Subsequent deformation and e r o s i o n , r e s u l t e d i n the d i s t r i b u t i o n of v o l c a n i c sandstone, c l a y s t o n e , and c o a l shown i n F i g u r e s 2-lb and 2 - l c . These rocks l i e conformably over p a r t of the v o l c a n i c s and lap unconformably over the n o r t h w e s t e r l y s l o p i n g e r o s i o n a l s u r f a c e of the grano-d i o r i t e . At the present time, the sediments are exposed only i n the lower s l i d e area. They are commonly i n t e n s e l y deformed as they are upthrust to the s u r f a c e as a r e s u l t of s l i d e movement. Lack of s t r e n g t h i n these rocks and t h e i r , c o n t r a s t s i n c o n d u c t i v i t i e s produce 31 a s i t u a t i o n d e t r i m e n t a l to s t a b i l i t y . V o l c a n i c sandstone i s exposed i n an Upthrust fragment, where i t i s i n t e r s t r a t i f i e d with b l a c k o r g a n i c , remolded c l a y -stone ( F i g u r e 2-4a). The sandstone i s a gray (N 6) t o brownish y e l l o w (10YR 6/6), c o a r s e - g r a i n e d sediment c o m p o s i t i o n a l l y s i m i l a r to the upper t u f f s . I t c o n s i s t s mainly of g l a s s w i t h some v o l c a n i c rock fragments, f e l d s p a r , q u a r t z , and b i o t i t e c l a s t s . In t h i n s e c t i o n , the rock seems to have l i t t l e cement, although i t i s d i f f i c u l t to d i s t i n g u i s h matrix from c l a s t s . Where exposed, i t i s commonly extremely f r i a b l e and d i s i n t e -g r a t e s upon s a t u r a t i o n . The sandstone may form a g r a n u l a r , r e l a t i v e l y c onductive zone as compared to the c l a y s t o n e or gouge. I n t e r l a y e r e d w i t h the c l a y s t o n e and c l a y gouge i n the s l i d e d e b r i s , c o a l v a r i e s from t h i n l a m i n a t i o n s to beds at l e a s t 1 m t h i c k . The c o a l i s c l a s s i f i e d as high v o l a t i l e bituminous and a chemical d e s c r i p t i o n i s given i n Table I I I . Because of the g e n e r a l l y low s t r e n g t h and b r i t t l e nature of c o a l , i t may not only form p a r t of the f a i l u r e s u r f a c e , but may p r o v i d e secondary p e r m e a b i l i t y through i t s f r a c t u r e s . Where exposed, i t i s commonly i n t e n s e l y j o i n t e d and fragments are d i s p e r s e d i n the gouge. Cla y s t o n e , which outcrops only i n the lower p a r t of the s l i d e , may be l i t h i f i e d , although i t u s u a l l y takes the form of a c l a y gouge. F i g u r e 2-4b shows l i t h i f i e d , but i n t e n s e l y sheared, f i n e - g r a i n e d c l a y s t o n e . I t i s medium gray 32 Figure 2-4a. Photograph of upthrust block of volcanic sandstone. (Note seam of clay gouge cross-cutting s t r a t i -f i c a t i o n ) . 33 T a b l e I I I . C hemical d e s c r i p t i o n o f c o a l A n a l y s i s : A i r dry m o i s t u r e % 2.63 R e s i d u a l m o i s t u r e % 0.40 T o t a l m o i s t u r e % 3.03 On Dry B a s i s : Ash % 18.87 V o l a t i l e M a t t e r % 34.28 F i x e d Carbon % 46.85 S u l f u r % 0.44 C a l o r i f i c v a l u e BTU/lb 11,381 A n a l y s i s by G e n e r a l T e s t i n g L a b o r a t o r i e s , Vancouver B.C., May 21, 1975. (N 5) to black (5Y 2.5/1) in color and very f i n e l y laminated, with some metallic interlaminations, probably p y r i t e . Although j o i n t i n g may promote l o c a l secondary permeability in the b r i t t l e claystone, t h i s rock probably i s r e l a t i v e l y impermeable compared to adjacent deposits. 2.5. Slide gouge Gouge i s apparent at the toe of the s l i d e along the canyon l i p and also in an upward r o t a t i o n a l thrust of the f a i l u r e surface (Figure 2-la). It i s a heterogeneous mixture of claystone, coal, sandstone fragments, and t i l l gravels. Its g r i t t y / c l a y (generally l i g h t gray (5Y 7/1) to grayish brown (10YR 5/2)) matrix asderived from remolded claystone and clay-r i c h t i l l . As the consolidated rock, predominantly the clay-stone, i s progressively and repeatedly sheared, i t i s remolded into t h i s clayey gouge of low strength. Because of i t s clay f r a c t i o n , the gouge i s commonly mudcracked on surface. In places an abundance of cracks indicate marked volume change in the clay with change in water content. X-ray d i f f r a c t i o n established the presence of an expanding mixed-layered clay in three gouge samples tested. Expansive clays readily adsorb water (swell), with a consequent decrease in strength. However, the extent to which th i s takes place i s not known, as the percentage of expanding clay in the gouge of the Perpetual Slide i s not defined. Generally, the c l a y - r i c h gouge i s evidenced to be r e l a t i v e l y impermeable. This affects ground water flow 35 i n two ways. The gouge promotes formation of an intermediate flow system, by forcing flow through overlying unconsolidated deposits. In addition, i t confines the upward discharge from any underlying aquifers ( l i k e l y weathered granodiorite, volcanic sandstone, and perhaps jointed claystone and coal), thereby allowing high pore water pressures to be transmitted to the base of the s l i d e . 2.6 Post-depositional history of Tertiary sediments After t h e i r deposition, the Tertiary sediments were t i l t e d easterly ( L i t t l e , 1961) and upwarped to the south-east about the T e r t i a r y - c r y s t a l l i n e unconformity. (The r e s u l t i n g configuration of bedding in the sediments (Figure 2-lc) may to some extent determine the geometry of f a i l u r e at depth.) Also af t e r the Tertiary deposition-, the ground surface was eroded. This second unconformity resulted from Oligocene and Pliocene u p l i f t and erosion, which was widespread i n the Southern Interior of B.C. (Mathews, 1968), and beveling of topography with g l a c i a t i o n . The Tertiary sediments were p r e f e r e n t i a l l y eroded, leaving r e l a t i v e l y high t e r r a i n in the volcanic and c r y s t a l l i n e rocks. In the v i c i n i t y of the s l i d e , the sediments were again loaded as about 80 m of Quaternary unconsolidated material was deposited in t h i s topo-graphic low. Because the basal Tertiary sediments were subject to loads (during b u r i a l and glaciation) greater than that of the present Quaternary overburden, they are now 36 overconsolidated. The extent of overconsolidation and degree to which i t a f f e c t s slope s t a b i l i t y , i s not known. 2.7 Quaternary deposits These deposits formed during g l a c i a t i o n and retreat of the Wisconsin ice sheet in the Okanagan Valley. They are divided into g l a c i a l t i l l , g l a c i o-lacustrine s i l t and fine sands, g l a c i a l f l u v i a l d e l t a i c sands and gravels, and terrace (or raised channel) gravels (Figures 2-la, b, and c). The dark gray (5YR 4/1) to pinkish gray (5YR 6/2), c l a y - r i c h t i l l within the s l i d e grades into a less p l a s t i c , light-grayish brown, s i l t y t i l l (which i s conspicuous to the west of the s l i d e , Figure 2-7a). With an average thick-ness of 3.5 meters, the t i l l i s exposed overlying bedrock in Trout Creek Canyon, i n the northwest corner of the map area, and i n the lower section of the s l i d e (Figures 2-la and b). Within the s l i d e , t i l l i s unsorted with a c l a y - r i c h matrix and c l a s t s ranging in s i z e from smaller (.5 cm) subangular rock fragments to larger (.3m) well-rounded boulders. Coal fragments, and plutonic and volcanic gravels have been recognized, with many of the clasts being altered to clay and limonite. Because of the clay matrix, this material has low permeability r e l a t i v e to the other unconsolidated deposits. The clay's weakness i s demonstrated by f a i l u r e within the t i l l i n a right-handed s t r i k e s l i p f a u l t on the southern boundary of the s l i d e toe. This s l i p surface, l a t e r a l l y confines the s l i d e and shows mudcracks, slickensides, and 37 Figure 2-la. Photograph of t i l l hummocks to west of s l i d e . Figure 2-7b. Photograph looking to the southeast at Pa-radise F l a t s (note i r r i g a t e d area), terraces on Crater Mtn. ( l e f t side of photograph), and headscarp of rock s l i d e on Crater Mtn. (center). 38 brecciation. The g l a c i a l lake s i l t s and fine sands are t h i n -bedded, i n many places cross-bedded, and non-resistant in exposure. They were deposited as d e l t a i c bottom-set or lower fore-set beds in water ponded by ice (Nasmith, 1962, p.25). The s i l t - s a n d unit varies in thickness and i s sporadic in i t s d i s t r i b u t i o n . Approximately 3 m thick at the s l i d e toe, exposed thickness increases ;to about 30 m to the south-west along Trout Creek Canyon and to 8 m on the northeastern s l i d e boundary. The s i l t s and fine sands are l i g h t gray (5Y 7/2) to pale yellow (5Y 7/3) i n color, well-sorted, and quartz r i c h . Deformation of bedding occurs in the east bounding scarp, where deposition of channel gravels disturbed the s i l t s Because of small p a r t i c l e s i z e , the s i l t - s a n d unit i s probably of low permeability r e l a t i v e to the overlying sands and gravel Light gray (5Y 7/2) outwash sands and gravels are sorted and s t r a t i f i e d and form two major terraces (see Figure 2-7b) which are d e l t a i c remnants of meltwater ponded against the Okanagan ice to the east. A delta was b u i l t at an average elevation of 585 m (1920 f t . ) and a lower terrace was cut into i t at 550 m (1800 f t . ) . The older, higher deposit ( c l a s s i f i e d as d e l t a i c sands and gravels) forms Paradise Flats and a small bench on Crater Mountain; the lower and younger terrace (described as terrace gravels), i s conspicuous as a tableland to the northeast of Crater Mountain and as a meander scar apparent just to east of the s l i d e (Figure 2-la). 39 The 585 m t e r r a c e i s exposed i n v e r t i c a l s e c t i o n i n a sand and g r a v e l p i t l o c a t e d i n the uppermost s l i d e block. The outwash i s composed mainly of gravels (some of p l u t o n i c o r i g i n ) w i t h smaller rock fragments, quartz, f e l d s p a r , b i o t i t e , and mica i n the s i l t s and sands. Commonly well-bedded, the d e l t a i c u n i t c o n s i s t s of s t r a t i f i e d s orted l a y e r s . Foreset beds dip approximately 12 degrees N 45 degrees E, i n d i c a t i n g progradation of the d e l t a i n t h i s d i r e c t i o n . There seems to a l s o be a coarsening of gravels i n the southwest (updelta) and a s l i g h t (downdelta) topographic slope to the northeast. In another p i t area (500 m east of the s l i d e ) anomalous d i p -ping f o r e s e t beds i n d i r t y - i n t e r s t r a t i f i e d sands and g r a v e l s , may r e s u l t from i c e blocks i n t e r u p t i n g d e p o s i t i o n . The lower t e r r a c e was cut as the l e v e l of the melt-water channel decreased. This erosion i s evident on the east bounding scarp of the s l i d e , where a l e n t i c u l a r channel deposit of coarse g r a v e l o v e r l i e s d i s t u r b e d lake s i l t s (with l o c a l convolute beds). The sands and gravels are re-worked sediments derived from the high t e r r a c e . A meander scar i n d i c a t e s a c o n t i n u a t i o n of t h i s channel and t e r r a c e deposit to the east, and the p a i r e d t e r r a c e on the south side of Trout Creek Canyon shows a s l i g h t down-delta dip to the east. A high h y d r a u l i c c o n d u c t i v i t y i s i n d i c a t e d f o r the outwash because of i t s coarse-grained, uncemented, and unconsolidated nature. 40 2.8 Late g l a c i a l and recent history An understanding of late g l a c i a l and recent history i s required in order to understand the present occurrence of deposits described i n the preceding section. The following summary of this history i s based on work by Nasmith (1962) and K v i l l (1976), and f i e l d interpretations by the author. The lowermost Quaternary unit, the t i l l , was depos-i t e d as a mantle on bedrock, during the last g l a c i a t i o n in the Okanagan Valley. The deposits overlying t i l l formed subsequently during g l a c i a l retreat. At one stage i n this retreat, a melt-water channel flowed from the west along the present route of Trout Creek, into the s l i d e area. Here, ice of the main Okanagan Valley lobe extended up Trout Creek (just to the east of the s l i d e ) and diverted meltwater to the south, through a channel known as the Trout Creek-Penticton Diversion. In the v i c i n i t y of the s l i d e , meltwater was ponded against the ice and consequently, a series of deltas was b u i l t . A major terrace formed at an average elevation of 585 m (1920 f t ) . Most of th i s deposit consists of sand and gravel, but fine sands and s i l t s are found to d i r e c t l y o verlie the t i l l and are p a r t i c u l a r l y conspicuous i n exposure along the north-eastern edge of the delta. These are interpreted as deeper water deposits, corresponding to bottomset or lower foreset beds. As the l e v e l at which water entered the Trout Creek-Penticton Diversion f e l l , a lower channel was formed at an 41 average e l e v a t i o n of 550 m (1800 f t . ) . With i c e r e t r e a t and d e c l i n e i n the ponded water below the 1800 f t l e v e l , the d i v e r s i o n was abandoned and meltwater began to flow a p p r o x i -mately along the present-day route of Trout Creek, down to the i c e i n the Okanagan V a l l e y . A s e r i e s of p r o g e s s i v e l y lower a l l u v i a l fans were b u i l t i n t o the ice-ponded water as the l e v e l of t h i s water d e c l i n e d and the base l e v e l of Trout Creek was lowered. Since Okanagan Lake reached i t s present l e v e l , Trout Creek has b u i l t a major d e l t a i n t o the lake ( F i g u r e 2-8). During l a t e g l a c i a l and recent times, mechanisms operated to b r i n g about s l o p e f a i l u r e of the P e r p e t u a l S l i d e . The downcutting of Trout Creek i n f l u e n c e d s t a b i l i t y i n two ways: (1) I t unloaded the toe of the p o t e n t i a l s l i d e by eroding p a r t of the t e r r a c e d e p o s i t s on the north s i d e of Trout Creek. (2) I t removed l a t e r a l support with downcutting and steepening of the v a l l e y w a l l s . T h i s excavated the un-c o n s o l i d a t e d d e p o s i t s to a g r e a t e r depth as w e l l as exposed u n d e r l y i n g T e r t i a r y sediments which became the f a i l u r e m a t e r i a l . The two mechanisms d i s r u p t e d e q u i l i b r u m and changed s t r e s s c o n c e n t r a t i o n s . 42 Figure 2-8. Photograph of present-day Trout Creek Delta. 43 CHAPTER 3 HYDROLOGY The ground water flow system exerts c r i t i c a l influence on slope s t a b i l i t y . In the case of the Perpetual Slide, a change i n the flow system r e s u l t i n g from i n i t i a l i r r i g a t i o n of Paradise Fl a t s (1903-1906) was followediby inc i p i e n t ground water discharge and f a i l u r e in the s l i d e area (sometime during the period from 1914 to 1917). Because of the demonstrated significance of groundwater flow to f a i l u r e of the s l i d e , a study of hydrology in i t s drainage basin was carried out. The study was threefold: Water discharge and recharge were quantified and co r r e l a t i o n between them was at-tempted, water quality was analyzed, and l o c a l and regional ground water flow systems were hypothesized by approximating hydrogeologic properties of the units mapped in the f i e l d . 3.1 Water inflow and outflow The basic processes of p r e c i p i t a t i o n , evapotran-s p i r a t i o n , and i n f i l t r a t i o n determine a s i m p l i f i e d hydrologic budget equation (modified from Gray, 1970, to apply to the Perpetual Slide drainage basin): P - E = I n (3.1a) where: P = p r e c i p i t a t i o n , E = e v a p o t r a n s p i r a t i o n , and I = i n f i l t r a t i o n , n For t h i s a n a l y s i s , the input of s p r i n k l e r i r r i g a t i o n water must a l s o be i n c o r p o r a t e d i n t o the equation: P + I - E = I ( 3 . l b ) n where: I = s p r i n k l e r i r r i g a t i o n . E q u a t i o n 3.1b i s used to c a l c u l a t e a s i m p l i f i e d h y d r o l o g i c budget f o r the s u r f i c i a l catchment b a s i n of the s l i d e ( F i g u r e l - 2 a ) . The input and output parameters i n the equation are expressed as r a t e s ( l i t e r s / s e c and c u b i c f e e t / s e c ) over the catchment b a s i n . By s u b s t i t u t i o n o f known parameters (P, I, and E, as re p r e s e n t e d i n F i g u r e 3-la) i n t o the above expres-i s i o n , the i n f i l t r a t i o n r a t e i s determined ( F i g u r e 3 - l b ) . A d i s c u s s i o n of the parameters f o l l o w s . P r e c i p i t a t i o n r e c e i v e d with the catchment b a s i n was c a l c u l a t e d by u s i n g monttiy p r e c i p i t a t i o n t o t a l s ( A g r i c u l -ture-Canada, Dec. 1974 to June 1976) recorded at the Summer-la n d Research S t a t i o n , l o c a t e d about 2 mi l e s east of the s l i d e . P r e c i p i t a t i o n input i s shown as average discharge ( l i t e r s / s e c and c f s ) f o r each month (Figure 3 - l a ) . S p r i n k l e r i r r i g a t i o n i s probably the major c o n t r i b u t o r to a r t i f i c i a l recharge i n the s l i d e catchment v i b a s i n . The maximum allowed supply of water per c u l t i v a t e d acre i s 18 gallons/minute and the c u l t i v a t e d area 125 1001 o 75 ui ui O to f£ ^ < to X CE o Ul CO (-° 3 5 0 -25 _; 1 . .J M J J 1 9 7 5 F i g u r e 3 - l a . P r e c i p i t a t i o n , i r r i g a t catchment b a s i n of s l i d e . • • IRRIGATION IRRIGATION PREC l"pI TAT I ON —•• EVAPOTRANSP I RAT I ON and e v a p o t r a n s p i r a t i o n over s u r f i c i a l INFILTRATION (MEAN MONTHLY) OR SPOT DISCHARGE OF SPRINGS LITERS/SEC I I i t I I I » I i I f I 1 1 / 1 1 1 0 — — — — CO CO CO CO 0 0 — X X r~ > 3> H o 3J =0 O > DA m m H —I O CO 0 O > CO -n TJ c r* — "0 — 0 2: m £7) m to CO I I I 0 CUBIC FEET/SEC — 1 — ro ( F i g u r e l-2a) , i s approximately 92.2 acres..; Hence, 105 l i t e r s / sec (3.71 c f s ) may enter the catchment b a s i n as s p r i n k l e r i r r i g a t i o n water, d u r i n g the f i v e month i r r i g a t i o n p e r i o d . The value of 105 l i t e r s / s e c i s d i r e c t l y s u b s t i t u t e d f o r I i n equation 3.lb,even though a p o r t i o n of t h i s water i s evaporated b e f o r e r e a c h i n g the ground. The l o s s of t h i s water may be compensated by the t o t a l input of the other water sources not i n c l u d e d i n the equation. These man-made sources i n c l u d e : domestic and i r r i g a t i o n water overflows ( F i g u r e l - 2 a ), the u n l i n e d i r r i g a t i o n d i t c h ( F i g u r e l - 2 a ), s e p t i c tanks, and pos-s i b l y , cracks i n the b u r i e d water l i n e ( F i g u r e l - 2 a ). The e v a p o t r a n s p i r a t i o n r a t e , E, i s the t h i r d f a c t o r a f f e c t i n g the q u a n t i t y of water a v a i l a b l e to the s l i d e . E i s c a l c u l a t e d on the b a s i s of a consumptive use f a c t o r , FACTOR^, and p o t e n t i a l e v a p o t r a n s p i r a t i o n , PE, where: E = FACT0R c u X PE (3.1c) " P o t e n t i a l e v a p o t r a n s p i r a t i o n , PE, i s the amount of water a given p l a n t , i n a given c o n d i t i o n , w i l l use i n e v a p o r a t i o n and t r a n s p i r a t i o n i f s u f f i c i e n t water i s a v a i l a b l e i n the s o i l t o meet the demand." Gray, (1970, p. 3.44). PE, i n the study of the P e r p e t u a l S l i d e , was c a l c u l a t e d on the b a s i s of a c o n t i n -uous s t r e t c h of v e g e t a t i o n c o v e r i n g the whole ground. The long-term monthly PE t o t a l s (determined over the 30 year p e r i o d , from 1931-1960, f o r Summerland by Coligado, 1968) were u t i l i z e d to ev a l u a t e e v a p o t r a n s p i r a t i o n over the s l i d e c a t c h -ment b a s i n . Gray (1970, p. 3.44) s t a t e s that E, " A c t u a l 48 e v a p o t r a n s p i r a t i o n r e f e r s to the a c t u a l l o s s of water by the pro c e s s e s " ( e v a p o r a t i o n and t r a n s p i r a t i o n ) "as i n f l u e n c e d by the combined e f f e c t s of demand.and a v a i l a b l e water supply." T h e r e f o r e , a c t u a l e v a p o t r a n s p i r a t i o n v a r i e s with the amount of v e g e t a t i o n , among other f a c t o r s . To account f o r the i n f l u e n c e of v e g e t a t i o n , PE i s m u l t i p l i e d by a consumptive use f a c t o r of 1.00 and 0.85 f o r c u l t i v a t e d and u n c u l t i v a t e d j areas' ( F i g u r e l - 2 a ) , r e s p e c t i v e l y , i n the c a l c u l a t i o n o f t o t a l e v a p o t r a n s p i r a t i o n over the s l i d e drainage b a s i n . T h i s e v a p o t r a n s p i r a t i o n i s shown as average discharge ( l i t e r s / s e c and c f s ) i n F i g u r e 3 - l a . I , i n f i l t r a t i o n , i s a " c a t c h a l l " term, the sum of the known values i n equation 3.1b. Excess water from s u r f i c i processes w i t h i n the catchment b a s i n i n f i l t r a t e s i n the t e r -race area and o v e r l a n d flow i s n e g l i g i b l e . P a r t of the i n f i l t r a t i o n i s r e s p o n s i b l e f o r changes i n s o i l and ground water sto r a g e . The remainder i s a v a i l a b l e f o r ground water flow, which u l t i m a t e l y d i s c h a r g e s to s p r i n g s on the s l i d e and t o Trout Creek. Accor d i n g to equation 3.1b, when water i n p u t , i . e . i r r i g a t i o n and p r e c i p i t a t i o n , exceeds e v a p o t r a n s p i r a t i o n , i n f i l t r a t i o n i n c r e a s e s by the d i f f e r e n c e . As mentioned, a p o r t i o n of t h i s increment i n c r e a s e s r a t e s of ground water recharge and consequent d i s c h a r g e . T h i s c a u s a l r e l a t i o n s h i p i s observed as a l a r g e water input above the s l i d e r e s u l t s i n s e v e r a l , h i g h - d i s c h a r g e s p r i n g s ( F i g u r e l-2a) occurring 49 where gouge and bedrock are exposed below ( i n the s l i d e ) . In F i g u r e 3-lb, o v e r l a i n on the histogram of i n f i l t r a t i o n , are the spot d i s c h a r g e s f o r the upper and lower s p r i n g s . The high i n f i l t r a t i o n o r recharge i n the summer months, most of which i s due to i r r i g a t i o n , i s f o l l o w e d by maximum s p r i n g d i s c h a r g e i n November-December. In other words, i r r i g a t i o n recharge c r e a t e s a water t a b l e r i s e which i s r e s p o n s i b l e f o r i n c r e a s e d flow through and di s c h a r g e from the system. There i s a two to three month time l a g between f l u c t u a t i o n s i n t h i s input, and response of s p r i n g discharge to these changes. Sim-i l a r time l a g s (although s h o r t e r i n length) have been observed i n the Trout Creek Fan (Thurber C o n s u l t a n t s , L t d . , 1973) i n response t o the high water l e v e l i n the creek d u r i n g the s p r i n g f r e s h e t , and i n the discharge o f Ribbleworth and LaFleche creeks as i n f l u e n c e d by i r r i g a t i o n w i t h i n t h e i r drainage b a s i n s (Le-Breton and Coulson, 1975). 3.2 Water q u a l i t y The purpose of water q u a l i t y a n a l y s i s was to de t e r -mine sources f o r recharge of the s l i d e s p r i n g s , and t o attempt to i n t e r p r e t type of ground water flow from the c h a r a c t e r i s t i c s of the s p r i n g water. To determine recharge sources, NO^ + NO^ and C l ~ were used as n a t u r a l t r a c e r s i n a water sampling program. N i t r a t e compounds are s u f f i c i e n t l y s o l u b l e that n i t r a t e i s taken out of n a t u r a l water only through b i o l o g i c a l 50 a c t i v i t y or through e v a p o r a t i o n (Davis and DeWiest, 1966). NOg + NOg c o n c e n t r a t i o n s are analyzed i n s p r i n g water of the s l i d e i n o r d e r to estimate the q u a n t i t y of water coming from c u l t i v a t e d P a r a d i s e F l a t s . In t h i s area, n i t r o g e n compounds ( p r i m a r i l y NH^NO^ or (NH^gSO^) are used as f e r t i l i z e r s ; and there i s b i o l o g i c a l i n put of n i t r o g e n i n t o the system by b a c t e r i a , and perhaps, from sewage e f f l u e n t . I t has been shown t h a t i r r i g a t i o n and consequent l e a c h i n g of the s o i l i n a g r i c u l t u r a l areas may i n c r e a s e NOg l e v e l s i n nearby ground water or streams (Hem, 1970). In F i g u r e 3-2a, r e l a t i v e l y high NOg + NG"2 l e v e l s i n the s p r i n g s of the P e r p e t u a l S l i d e may be compared to the near zero background c o n c e n t r a t i o n s i n v a r i o u s s u r f i c i a l water samples (Trout Creek, the Summerland domestic water r e s e r v o i r , domestic water of P a r a d i s e F l a t s , and i r r i g a -t i o n water). The comparison suggests that water from the c u l t i v a t e d area i s a s i g n i f i c a n t c o n t r i b u t o r to water i n the s l i d e . The h i g h NO^ + NOg c o n c e n t r a t i o n s i n s p r i n g water i n -l a t e January may be caused by lower p l a n t consumption of NO~ + NOg i n the f a l l . Bourgeois and L a v k u l i c h (1972) note a s i m i l a r i n c r e a s e i n NO^ d u r i n g October i n f o r e s t s o i l s , thus, the w i n t e r peak i n F i g u r e 3-2a may imply a time l a g of a few months. T h i s and o t h e r f l u c t u a t i o n s i n the curve may be caused by a combination of both b i o l o g i c a l and d i l u t i o n a l f a c t o r s . C i r c u l a t i o n of the c h l o r i d e i o n i n the h y d r o l o g i c c y c l e i s l a r g e l y through p h y s i c a l processes. I t moves with M A M J J A S O N D J F M A M J I 9 75 I 1976 F i g u r e 3-2a. N0 o + NO ~ c o n c e n t r a t i o n s . Ln water through s o i l s , w i t h l i t t l e r e t a r d a t i o n or l o s s because of i t s i n a c t i v i t y i n n a t u r a l chemical and b i o l o g i c a l processes (Hem, 1970; Davis and DeWiest, 1966). F i g u r e 3.2b shows the C l c o n c e n t r a t i o n s i n s p r i n g water compared to background c o n c e n t r a t i o n s ( i n Trout Creek, above the s l i d e and i n i r r i g a ^ r t i o n w a t e r ) . I t should be noted that t h i s background concen-t r a t i o n s averages 0.8 ppm, w h i l e domestic water has r e l a t i v e l y h igh C l - values of 2.5 ppm (February, 1976), and 5.2 ppm (June, 1976) ( F i g u r e 3-2b). The c o n c e n t r a t i o n i n s p r i n g water, which f l u c t u a t e s about 3 ppm cannot be e x p l a i n e d by input of i r r i g a t i o n or p r e c i p i t a t i o n ( p r o v i d e d that Trout Creek, upstream from the s l i d e , can be taken to be i n d i c a t i v e of p r e c i p i t a t i o n ) . F a c t o r s which may be r e s p o n s i b l e f o r the C l c o n c e n t r a t i o n s i n the s p r i n g water are: (1) input of domestic water from Para-d i s e F l a t s , both the overflow ( d i s c h a r g e = 0.5 l i t e r s / s e c or 0.02 c f s ) and sewage e f f l u e n t ( d i s c h a r g e = 1.7 l i t e r s / s e c or 0.06 c f s ) ; (2) f l u s h i n g of atmospheric dust and s a l t o f f of p l a n t s with p r e c i p i t a t i o n or s p r i n k l e r i r r i g a t i o n ( L a v k u l i c h , p e r s o n a l communication); (3) leaks from the domestic water l i n e , r e s u l t i n g i n g r e a t e r input of t h i s water i n t o the system; and (4) c o n c e n t r a t i o n of C l by e v a p o r a t i o n upon s u r f i c i a l d i s -charge. S i g n i f i c a n t c o n t r i b u t i o n of C l by s o l u t i o n of the sedimentary rocks i s u n l i k e l y because they were d e p o s i t e d i n a f r e s h water, not a marine environment. CONCENTRATION IN LOWER SPRINGS CONCENTRATION IN UPPER SPRINGS BACKGROUND CONCENTRATION DOMESTIC WATER OVERFLOW A / \ J F M A M J J A S O N I 9 7 5 F i g u r e 3-2b. CI c o n c e n t r a t i o n s . 54 A n a l y s i s of water q u a l i t y i n the P e r p e t u a l S l i d e area i n c l u d e d d e t e r m i n a t i o n of three other c h a r a c t e r i s t i c s ; pH, t o t a l d i s s o l v e d s o l i d s , and temperature. F i g u r e 3-2c shows the pH and t o t a l d i s s o l v e d s o l i d s , TDS - ( o r the t o t a l i o n i c c o n c e n t r a t i o n ) , measured i n the upper s p r i n g s of the s l i d e and i n Trout Creek. The analyses were made i n the f i e l d u s i n g a wide range pH k i t (a c o l o r i m e t r i c t e c h n i q u e ) , and an e l e c t r i c a l c o n d u c t i v i t y meter. Conversion of u n i t s o f c o n d u c t i v i t y to TDS i n p a r t s per m i l l i o n was made u s i n g the approximation: TDS (ppm) = 0.7 X e l e c t r i c a l c o n d u c t i v i t y (umho/cm) (3.2) For the sample p o p u l a t i o n , the pH averages about 8.9 i n the upper s p r i n g water, 8.4 i n the creek water above the s l i d e , and 8.6 i n the creek water j u s t below the s l i d e . A l s o , f o r the p o p u l a t i o n sampled, the TDS averages 350 ppm i n the upper s p r i n g s ; and where Trout Creek above the s l i d e has a mean TDS l e v e l of about 60 ppm, j u s t below the s l i d e , the average c o n c e n t r a t i o n i s 100 ppm. In ground water, TDS commonly i n c r e a s e s as a f u n c t i o n of r e s i d e n c e time, which i s determined by the flow r a t e s and the length o f the flow path, and with subsequent c o n c e n t r a t i o n by e v a p o r a t i o n i n the s u r f i c i a l d i s -charge area. A p h y s i c a l q u a l i t y c h a r a c t e r i s t i c of water, temper-ature, was a l s o measured. In t h i s a n a l y s i s , s p r i n g water temperatures were compared to a i r and s u r f a c e w a t e r - ( i . e . Trout Creek) temperatures ( F i g u r e s 3-2d and 3-2e). From the compar-1000 i r9 75.0-P H TDS OR pH OF — UPPER SPRINGS — TROUT CREEK DOWNSTREAM FROM SLIDE — TROUT CREEK UPSTREAM FROM SLIDE to 2 £ ct 500-250 -TDS F i g u r e 3-2c. T o t a l D i s s o l v e d S o l i d s and pH, 55 301 20H UJ r> or o LU — 0 _ *~ toH o8- J A S 1975 TEMPERATURE OF LOWER SPRINGS AIR TEMPERATURE . -° ( a p p r o x i m a t e , ot t i m e of s p r i n g °" t e m p e r a t u r e o b s e r v a t i o n ) 0 N D J I F M A M J I S 7 6 30i UJ 20 tr t— ~ < a tr o UJ O L UJ 10 TEMPERATURE OF UPPER SPRINGS AIR TEMPERATURE ( a p p r o x i m a t e , a t t i m e of s p r i n g o t e m p e r a t u r e o b s e r v a t i o n ) J A 1975 N D " 1 ' C J F M A M J 1976 F i g u r e 3-2d. S p r i n g water and a i r temperatures. 30 — TEMPERATURE — TEMPERATURE OF LOWER OF U P P E R SPRINGS SPRINGS rr z> r- — < O rr o LU — o_ 20H _ TEMPERATURE OF TROUT CREEK - DOWNSTREAM FROM SLIDE - UPSTREAM FROM SLIDE LU r- IOH M M J J 1 9 7 5 0 N D F i g u r e 3-2e. Water temperatures 58 i s o n , i t i s i n t e r p r e t e d that s p r i n g water i s i n s u l a t e d from s u r f a c e extremes as i t shows r e l a t i v e l y l i t t l e f l u c t u a t i o n J with a i r temperatures (taken from Canada A g r i c u l t u r e Summer-land Research S t a t i o n , 1975-1976), whereas s u r f i c i a l water shows g r e a t e r seasonal v a r i a t i o n i n temperature. The measure-ments (daytime) show s p r i n g water temperatures to average 12°C ( J u l y 1975 to June 1976). The water q u a l i t y a n a l y s i s of the P e r p e t u a l S l i d e area i n d i c a t e s t h a t a s i g n i f i c a n t p o r t i o n of water i n the s l i d e o r i g i n a t e s from c u l t i v a t e d P a r a d i s e F l a t s . F u r t h e r i n t e r p r e t a t i o n s about recharge sources or ground water flow cannot be made on the b a s i s of t h i s a n a l y s i s . 3.3 Ground water flow systems Ground water flow systems are c o n t r o l l e d by s e v e r a l i n t e r r e l a t e d f a c t o r s : the water t a b l e c o n f i g u r a t i o n , the g e o l o g i c c o n f i g u r a t i o n ( s p a c i a l d i s t r i b u t i o n of h y d r a u l i c c o n d u c t i v i t y ) ; and the ground water b a s i n depth to length r a t i o . A water t a b l e can be con s i d e r e d to commonly f o l l o w topography i n a subdued manner i n upland areas and be c o i n c i -dent with the topographic s u r f a c e i n v a l l e y s . Because of the way the p o t e n t i a l f o r flow i s d i s t r i b u t e d , water moves from t o p o g r a p h i c a l l y high recharge areas towards the discharge areas i n the t o p o g r a p h i c a l l y low p a r t s of the b a s i n (Freeze and Witherspoon, 1967). The i n f l u e n c e of the g e o l o g i c c o n f i g -u r a t i o n on ground water flow i n a consequence of the geometry of formations and the h y d r a u l i c c o n d u c t i v i t y c o n t r a s t s between formations. I t a c t s to d i v i d e ground water flow i n t o a system of u n i t s that permit passage of a p p r e c i a b l e amounts of water ( a q u i f e r s ) and, u n i t s that do not ( a q u i t a r d s ) . F u r t h e r , the g e o l o g i c c o n f i g u r a t i o n can a f f e c t the i n t e r r e l a t i o n s h i p s between flow systems, the s u r f i c i a l p a t t e r n of recharge and d i s c h a r g e areas, and q u a n t i t i e s of flow d i s c h a r g e d through the system (Freeze and Witherspoon, 1967). The b a s i n depth to l e n g t h r a t i o i s r e s p o n s i b l e f o r the type, shape, and s c a l e of the flow systems. Three s c a l e s of flow systems, r e s u l t i n g from these c o n t r o l s may be o p e r a t i v e i n the ground water drainage b a s i n o f the s l i d e . (1) A r e g i o n a l system with recharge at the major topographic high, Mt. Conkle, and discharge at the major topographic low, Trout Creek, i s d e p i c t e d by the flow model i n F i g u r e 3-4 and l a r g e - s c a l e topography i s shown i n F i g u r e 3-3a. The r e g i o n a l flow of t h i s area i s i n f l u e n c e d by heterogeneous geology and may be a consequence of the a c t i o n of a r e g i o n a l a q u i f e r . The extent to which t h i s system develops i s p a r t i a l l y dependent on the depth to l e n g t h r a t i o of the b a s i n . (2) An i n t e r m e d i a t e system ( P a r a d i s e F l a t s and s l i d e areas) i s modeled w i t h gross topography and geology i n F i g u r e 3-3b. T h i s system i s c h a r a c t e r i z e d by the f o l l o w i n g : one or more topographic highs and lows are £ 2 0 0 0 . c > U l < ui to ui > o cn < z o < > ui U l 8 0 0 1600 1400 SL I DE D' I ui ui or o o IMPERMEABLE BOUNDARY K S « AQUIFER H0RIZ0NTAL = I.00 V E R T I C A L =0.10 HYDRAULIC CONDUCTIVITY CONTRASTS AQUITARD H 0 R I Z 0 N T A L = 0 . 0 l V E R T I C A L =0.01 HORIZONTAL. SCALE = V E R T I C A L SCALE Figure 3 -3b. Schematic flownet of intermediate system, cross section C'-D'enlarged from Figure l _ 2 a . tn o l o c a t e d between i t s recharge and discharge areas; i r r i g a t i o n water pro v i d e s a r t i f i c i a l recharge which r a i s e s the ground water t a b l e i n the v i c i n i t y of the i r r i g a t e d area; and h e t e r -ogeneous geology i n f l u e n c e s the p a t t e r n of flow because of h y d r a u l i c c o n d u c t i v i t y c o n t r a s t s between formations. topographic high) and a discharge area (at a topographic low) which l i e adjacent to each other. S e v e r a l of these systems are s c h e m a t i c a l l y shown i n F i g u r e 3-3b. They develop because topographic cuts ( s l i d e s c a r p s ) p r o v i d e l o c a l r e l i e f . Although not i n c l u d e d i n the model, b a r r i e r s of c l a y gouge, which are upthrust to the s u r f a c e w i t h i n the s l i d e , a l s o produce s u r -f i c i a l d i s c h a r g e . The b a s i n depth (and length) of l o c a l and i n t e r m e d i a t e flow systems may be l i m i t e d by occurrence of impermeable s t r a t a . boundaries imposed on the flow b a s i n s , determine the i n t e r -mediate flow system modeled i n F i g u r e 3-3b and the r e g i o n a l flow models presented i n F i g u r e 3-4. Before the models are examined, t h e i r l i m i t a t i o n s must be s t a t e d : (1) G e o l o g i c c o n t a c t s are approximated at depth by e x t r a p o l a t i o n of s u r f i c i a l outcrops and a t t i t u d e s , s i n c e no d r i l l h o l e i n f o r m a t i o n i s a v a i l a b l e . (2) H y d r a u l i c c o n d u c t i v i t i e s are estimated. They are assigned p r i m a r i l y on a l i t h o l o g i c b a s i s , although weather-(3) A l o c a l system c o n s i s t s of a recharge area (at a Topographic and g e o l o g i c a l c o n f i g u r a t i o n s as w e l l as 62 i n g and f r a c t u r i n g of u n i t s , as w e l l as model l i m i t a t i o n s are d e t e r m i n a t i v e f a c t o r s . (3) Assumptions w i t h i n the models themselves, are e x p l a i n e d by Hodge (1976), and as a p p l i e d to the flow systems pre s e n t e d here, are: (a) The system i s assumed to be f u l l y s a t u r a t e d with the ground water t a b l e at the topographic s u r f a c e . (b) The 2-dimensional cross s e c t i o n assumes no flow l a t e r a l l y i n or out of the system. (c) The flow i s s t e a d y - s t a t e , the d i r e c t i o n and magnitude of the flow v e l o c i t y are constant w i t h time and the water t a b l e i s time i n v a r i a n t . (d) Boundaries can be imposed on the system. (Impermeable boundaries are u t i l i z e d i n the models). (e) Other assumptions i n v o l v i n g porous media and Darcy's law are s t a t e d by Hodge (1976). The model f o r the i n t e r m e d i a t e flow system was produced by a f i n i t e d i f f e r e n c e technique and a f i n i t e element technique was u t i l i z e d f o r the r e g i o n a l flow model. Both methods are numerical s o l u t i o n s to the equation of s a t u r a t e d , s t e a d y - s t a t e flow w i t h the boundaries as i n d i c a t e d f o r each model. In t'he i n t e r m e d i a t e flow system as shown i n F i g u r e 3-3b, geology has been simplifxed::,into an- a q u i f e r ( g l a c i a l f l u v i a l and g l a c i o - l a c u s t r i n e d e p o s i t s ) and an u n d e r l y i n g a q u i t a r d ( t i l l , c l a y s t o n e , and other bedrock), with assumed h o r i z o n t a l c o n d u c t i v i t y contrasts,between the two u n i t s d i f f e r i n g by 2 orders of magnitude. The model shows l o c a l flow above the l e s s conductive s t r a t a with some flow through the consolidated deposits, down to Trout Creek. In the f i e l d , springs, g u l l i e s , or t h i c k l y vegetated areas are located approximately i n the discharge areas indicated by the flownet. Because of assump-tions made to produce t h i s model, i t gives only a q u a l i t a t i v e idea of ground water flow from Paradise F l a t s to the s l i d e . The regional flow model i s shown i n a northwest-southeast oriented section, along l i n e A -B i n Figure l-2a. The section extends across the topographic divide of Mt. Conkle so that a v e r t i c a l impermeable boundary was not a r b i t r a r i l y assigned beneath t h i s high. However, near Mt. Conkle, the section l i n e i s neither perpendicular to topographic nor water table contours, and some ground water flow i s out of the 2-dimensional system. A more r e a l i s t i c representation of ground water flow may have been obtained by using a curved section. Given topographic and approximate geologic data as interpreted by the author, R.A. Hodge, (1976) modeled regional flow for the system, which i s shown i n Figure 3-4, taken d i r e c t l y from Hodge's Figure 5-10. Figure 3-4a shows a homogeneous i s o t r o p i c regional system. In Figure 3-4b, heterogeneity has been introduced i n the form of u n i t s : (A) T e r t i a r y volcanics, (B) Older g r a n i t i c complex, (C) T e r t i a r y sedimentary rocks, (D) G l a c i o - l a c u s t r i n e s i l t and g l a c i a l t i l l , and (E) G l a c i a l outwash sand and gravel. In Figure 3-4c, layered anisotropy has been incorporated i n t o 64 the v o l c a n i c s , w i t h h i g h e r c o n d u c t i v i t e s p a r a l l e l to a 31 degree down-section d i p of bedding pl a n e s . In an attempt to b e t t e r s i m u l a t e f i e l d c o n d i t i o n s , i n F i g u r e 3-4d, the water t a b l e has been lowered, the T e r t i a r y sediments and g l a c i o -l a c u s t r i n e s i l t a s s i g n e d a n i s o t r o p i c c o n d u c t i v i t e s , and a l e s s conductive s a p r o l i t e zone i n t r o d u c e d over the g r a n i t i c rocks. In F i g u r e 3-4e, i r r i g a t i o n i s modeled by r a i s i n g the water t a b l e i n p r o x i m i t y to the i r r i g a t e d area. A more conductive t r a n s i t i o n - w e a t h e r e d rock zone o v e r l i e s the g r a n i t i c rocks i n F i g u r e 3-4f, and a s e c t i o n of the T e r t i a r y sediments i s assigned a h i g h e r c o n d u c t i v i t y . Hodge (1976) conducted a slope s t a b i l i t y a n a l y s i s on a p a r t i c u l a r f a i l u r e s u r f a c e f o r models 3-4a, 3-4c, 3-4d, 3-4f, and found a 55% range i n f a c t o r s of s a f e t y f o r the d i f f e r e n t models. He concluded t h a t i t i s p o s s i b l e t h a t changes i n ground water flow such as those i l l u s t r a t e d c o u l d r e s u l t i n s i g n i f i c a n t changes i n s t a b i l i t y and consequent f a i l u r e . I f , i n f a c t , p a r t s of the t e r r a c e l i e w i t h i n areas of r e g i o n a l ground water flow discharge as i n d i c a t e d by the more complex r e g i o n a l models, and the water t a b l e i s high, an u n s t a b l e s i t u a t i o n i s c r e a t e d , e s p e c i a l l y when compounded by flow from P a r a d i s e F l a t s . However, s i n c e the g e o l o g i c c o n t a c t s were i n f e r r e d , vand h y d r a u l i c c o n d u c t i v i t e s are only estimated, i n both models no f i n a l c o n c l u s i o n s about the e f f e c t s of the d i f f e r e n t g e o l o g i c u n i t s on ground water flow s h o u l d be drawn. More i n f o r m a t i o n , such as d r i l l h o l e , piezometer, and w e l l data would be necessary. 66 CHAPTER 4 STABILITY ANALYSIS Two p r o j e c t s were undertaken i n order to understand the mechanics of the P e r p e t u a l S l i d e . The f i r s t i n v o l v e d a study of long-term movement based on h i s t o r i c a l records (post 1900) and an a i r photograph comparison (1938 and 1970). In a d d i t i o n , s h o r t - t e r m movement was e v a l u a t e d by s u r v e y i n g the s l i d e d u r i n g a p e r i o d from the summer of 1975 to the summer of 1976. The second p r o j e c t , a sl o p e s t a b i l i t y a n a l y s i s , was undertaken to gain i n s i g h t i n t o the c o n d i t i o n s r e s p o n s i b l e f o r the observed movement or f a i l u r e . I n i t i a l l y , the shear s t r e n g t h of the s l i d e gouge ( d e s c r i b e d i n S e c t i o n 2.5) was determined. Then, t h i s s t r e n g t h , as a constant, and assumed environmental parameters, as v a r i a b l e s , were i n c o r p o r a t e d i n t o e q u i l i b r i u m models. In t h i s way, an attempt was made to f i n d the most c r i t i c a l s i t u a t i o n f o r f a i l u r e . 4.1 S u r f i c i a l movement A study of long-term movement was c a r r i e d out u s i n g topographic maps and s e c t i o n s p r o v i d e d by the Water Invest-i g a t i o n s Branch of the P r o v i n c i a l Government. The maps and s e c t i o n s which showed topography as determined from 1938 and 1970 a i r photographs served t o d e f i n e s l i d e movement between these y e a r s . F i g u r e 4 - l a shows the s u r f i c i a l changes, two of which are of major importance: (1) subsidence i n depressions or e x t e n s i o n a l areas near the 560, 550, and 530 meter contours, and i (2) upward movement near the 540 meter contour of the middle b l o c k . An important c o n c l u s i o n that can be drawn by com-p a r i n g the topography as i n f e r r e d p r i o r to the s l i d e , as i t was i n 1938, and again i n 1970 ( F i g u r e 4 - l b ) , i s that most of the s l i d e movement seems to have taken p l a c e p r i o r to 1938. T h i s c o n c l u s i o n i s supported by the h i s t o r i c a l r e c o r d of the s l i d e . The r e c o r d i s compiled l a r g e l y from accounts given by r e s i d e n t s of P a r a d i s e F l a t s and i s presented i n Table IV. A c c o r d i n g to t h i s h i s t o r y i r r i g a t i o n and domestic water was brought i n from o u t s i d e the s l i d e drainage b a s i n f o r many years p r i o r to 1938. F a i l u r e , with a head s c a r p near i t s present-day l o c a t i o n , had a l s o taken p l a c e b e f o r e 1938, probably by 1917. These events are compatible with the i n t e r -p r e t a t i o n that a l a r g e amount of movement oc c u r r e d p r i o r to 1938. A r e c o r d of s h o r t - t e r m movement was o b t a i n e d from a survey of the s l i d e d u r i n g a p e r i o d from the summer of 1975 to the summer of 1976. S e v e r a l l i n e s of stakes were s e t approximately p e r p e n d i c u l a r to the d i r e c -t i o n s of s l i d i n g i n June and J u l y of 1975 ( F i g u r e s 4 - l c and 4 - l d ) . These were surveyed at r e g u l a r i n -t e r v a l s throughout the year i n an attempt to d e f i n e A LEGEND -550- 1938 CONTOUR LINE (C . I . = 10 METERS) -4 CHANGE IN ELEVATION (METERS, 1938-1970) CENTRAL ZONE OF UPLIFT o CONTROL POINT A a CROSS SECTION LINE Z Z ROAD SCALE F-» »-» 50 0 50 (METERS) 100 F i g u r e 4~I a . Sur f i c i a I changes of s l i d e U 9 3 8 to 1 9 7 0 ) . 68 ,r650 400 L E N G T H IN M E T E R S 600 P R E - S U O E TOPOGRAPHY ( I N F E R R E D ) 1938 TOPOGRAPHY , 1970 TOPOGRAPHY Trout Creek 1938 FIGURE 4-lb. PRE-SLIDE, 1938,8.1970 TOPOGRAPHY ALONG LONGITUDINAL SECTION LINE A-B (SEE FIGURE l~2c). LD 70 Table IV . H i s t o r i c a l r e c o r d of the P e r p e t u a l S l i d e ( A l l i n f o r m a t i o n i s by p e r s o n a l communication unless otherwise i n d i c a t e d . ) Approximate Date 1903 - 1906 1907 1912-1913 1914-1917 1920-1925 1927 1930 Event I r r i g a t i o n water was f i r s t brought i n to P a r a d i s e F l a t s . (Summerland Development Company L i m i t e d , sub-d i v i s i o n map, 1904). The present-day s l i d e area was f l a t as i n d i c a t e d by the road l a i d a c r o s s what i s now the top b l o c k of the s l i d e (Summerland Development Company L i m i t e d , s u b d i v i s i o n map, 1907). P r i o r to t h i s date no subsidence had taken p l a c e . P a r t s of the f l a t s were c u l t i v a t e d as orchards ( J . M a r s h a l l , L. Monfort, M. Fenwich). Two n a t u r a l s p r i n g s were present along Trout Creek Canyon below P a r a d i s e F l a t s , one s e v e r a l hundred f e e t south ( J . M a r s h a l l ) , and the other on the e a s t e r n f l a n k of the present-day s l i d e (H. M i t c h e l l ) . The s l i d e was i n i t i a t e d (H. Fenwich, J . M a r s h a l l , D. Dunham), with sub-sidence i n the western s e c t i o n f i r s t (H. M i t c h e l l ) . The backscarp was present near the road which bounds the s l i d e (L. Gould, D. Dunham), but no other scarps were apparent (L. Gould). Trout Creek water was n o t i c e a b l y s i l t y (S. Fenwich). P r e v i o u s to t h i s time s e v e r a l s p r i n g s were present on the e a s t e r n f l a n k of Mt. Conkle (W. Davis, J . S t r a c h e n ) . continued 71 Table IV . ... continued. Approximate Date 1929-1933 1930's 1950 to present Event Dates and accounts vary, but a s o - c a l l e d " s p r i n g " was b l a s t e d j u s t t o the northwest of P a r a d i s e F l a t s and any water was l o s t underground ( J . Howard, L. Gould, M. Fenwich). E a r t h d i t c h e s , then wooden, and f i n a l l y metal flumes were used to t r a n s p o r t i r r i g a t i o n water (L. Gould, K. Blagbourne). More water was brought i n f o r i r r i g a t i o n . Concrete flumes r e p l a c e d metal ones (K. Blagbourne), except f o r a 1500' le n g t h of overflow l i n e which remains an e a r t h d i t c h . ( D i s t r i c t of Summerland E n g i n e e r i n g Study P l a n of E x i s t i n g I r r i g a t i o n Systems, June 1972). i g u r e 4 - l c . am of h o r i z o n t a l ents of sta k e s showing components normal to survey l i n e s (upper p a r t of s l i d e ) . SCALE (METERS) 50 MOVEMENT COMPONENT SCALE r H I—I 1 0 2 (METERS) LEGEND 1970 CONTOUR LINE ( C . I . = 1 0 METERS) — * MOVEMENT OF STAKE (6/1975 TO 6/1976) •~* MOVEMENT OF STAKE (7/1975 TO 5/1 976) A INSTRUMENT STATION A CONTROL POINT FOR SURVEY LINE ^ to — SURVEY L I N E i n t r u e d i r e c t i o n (toe of s l i d e ) . — SURVEY LINE 74 seasonal and s p a c i a l f l u c t u a t i o n s i n movement. v, Two methods were used: (1) A s t a d i a survey was made of the upper s e c t i o n of the s l i d e . Readings were taken from i n t e r v i s i b l e instrument s t a t i o n s l o c a t e d on s t a b l e ground at the ends of s t r a i g h t survey l i n e s ( F i g u r e 4 - l c ) . Small departures of stakes from the o r i g i n a l s t r a i g h t l i n e s were taped. Large departures were c a l c u l a t e d u s i n g h o r i z o n t a l angles and s t a d i a d i s t a n c e s . Changes i n e l e v a t i o n s of s t a k e s were measured us i n g v e r t i c a l angles and - s t a d i a d i s t a n c e s . (2) A t r i a n g u l a t i o n survey made use of a b a s e l i n e l o c a t e d on the t e r r a c e on the south s i d e of Trout Creek Canyon. Th i s enabled the p o s i t i o n of movement stakes on the s l i d e toe to be l o c a t e d more e x a c t l y and, as a r e s u l t , o r i g i n a l d i s t a n c e s , d e f l e c t i o n s , and e l e v a t i o n s c a l c u l a t e d from s t a d i a were c o r r e c t e d ( F i g u r e 4 - l d ) . E l e v a t i o n determinations u s i n g method (1) may be i n e r r o r by 0.2 to 0.3 m. In c o n t r a s t , e l e v a t i o n s c a l c u l a t e d by t r i a n g u l a t i o n are estimated to be accurate w i t h i n - 0.02m. For e i t h e r method, h o r i z o n t a l departure of stakes from l i n e s may be i n e r r o r by 0.01 m. I t i s concluded that e l e v a t i o n s determined by s t a d i a alone, may be i n a c c u r a t e . Consequently, no i n t e r p r e t a t i o n s are made from s m a l l , s t a d i a - c a l c u l a t e d changes i n e l e v a t i o n s . R e l i a b l e survey measurements demonstrate that s l i d e movement i s a f u n c t i o n of l o c a t i o n w i t h i n the s l i d e and time of year. Movement of s e l e c t e d stakes i s given i n Appendix 4-1 and Table V l i s t s departures (per year) of stakes from the survey l i n e s . The general trend of increased rate of movement downslope may be attributed to conditions at the toe: (1) the mudflow type movement, (2) the maintenance of steep slopes by headward erosion of -gullies, (3) the addition of flow & s u r f i c -i a l movement on steep toe slopes (sloughing) to movement along the f a i l u r e plane at depth, and (4) the large d r i v i n g force behind, and lack of f r i c t i o n a l resistance at the toe. Thompson and Hayley (1975) found a movement pattern, s i m i l a r to that of the Perpetual Slide, i n the L i t t l e Smoky Landslide, Alberta, where: "Vectors representing movement are, in general, shortest near the scarp and increase to a maximum at the toe." They use thi s phenomenon to confirm operation of retrogressive f a i l u r e , that i s , i n s t a b i l i t y i n i t i a t e d at the toe, with progressive f a i l u r e upslope. The toe moves at a r e l a t i v e l y fast rate and provides l i t t l e support for blocks upslope. This mechanism i s not required to explain increase in rates of movement downslope i n ,the Perpetual S l i d e , but i t remains a possible mode of f a i l u r e . Movement rates also increase toward the i n t e r i o r of the s l i d e , as i l l u s t r a t e d by Figures 4-lc and 4-ld, and d i r e c -tions of movement vary within the s l i d e . Modification of the general southeasterly trend of movement i s observed (Figure 4-ld and Appendix 4-1) at the toe. Lower li n e s are found to converge toward g u l l i e s created by the springs. Short-term 76 Table V . H o r i z o n t a l movement r a t e s of stakes ( g i v e n as averages f o r each l i n e of stakes) L i n e Date Upper p a r t o f s l i d e NO LM RS JK WX 6/75 tc> 6/76 7/75 to 5/76 Apparent downslope movement r a t e ( p e r p e n d i c u l a r to survey l i n e ) 2i .08 m/yr s i . 18 m/yr .13 m/yr .51 m/yr 1.49 m/yr A c t u a l downslope movement r a t e ( t r u e d i r e c t i o n ) Toe of s l i d e : TI» UV. TE 7/75 to 5/76 7/75 to 2/76 8.12 m/yr 13.09 m/yr 15.69 m/yr 9.01 m/yr 15.90 m/yr 18.30 m/yr 77 upward movement i s apparent on the e a s t e r n s e c t i o n of the toe. T h i s may be caused by backward t i l t i n g of coherent b l o c k s as shown by T g ( F i g u r e 4 - l d , Appendix 4-1) durin g October, 1975. Seasonal f l u c t u a t i o n s are p a r t i c u l a r l y e v ident i n toe s t a k e s . T h e i r average monthly movement i s at maximum i n November (Appendix 4-1). T h i s c o r r e l a t e s c l o s e l y to the time of maximum discharge of the s p r i n g s d u r i n g November and Dece-mber. The b a c k - l i n e s , WX and JK, d i s p l a y l a r g e r r a t e s of move-ment f o r the p e r i o d from October-November 1975 to May-June 1976, than f o r the summer of 1975. T h i s i s a r e s u l t of prog-r e s s i v e i n c r e a s e i n movement r a t e s or seasonal a c c e l e r a t i o n during the f a l l and wi n t e r , only. To conclude, a rough c a l c u l a t i o n of mass d i s p l a c e d (at the toe) i s made, u s i n g v e r t i c a l and h o r i z o n t a l movements 3 of toe s t a k e s . I t i s estimated that 11,500 m of m a t e r i a l i s removed per year, assuming the shear s u r f a c e extends h o r i z o n t -a l l y beneath the s l i d e from the p o i n t s at which s p r i n g s occur i n the canyon. 4.2 D i r e c t shear t e s t i n g A s e r i e s of d i r e c t shear t e s t s were conducted to determine the s t r e n g t h of the gouge. The best value obtained has been used as a constant i n a s t a b i l i t y a n a l y s i s , together w i t h an i n f e r r e d water t a b l e , to d e f i n e c r i t i c a l p i e z o m e t r i c and f a i l u r e s u r f a c e s . 78 A review of s t r e n g t h theory concerning s o i l i s a p p r o p r i a t e p r i o r to d i s c u s s i o n of d i r e c t shear t e s t i n g . The theory i s a p p l i e d here because the gouge i s co n s i d e r e d to behave as a s o i l . A c c o r d i n g to the Coulomb-Terzaghi f a i l u r e c r i t e r i o n , the shear s t r e n g t h of a s o i l at a p o i n t on a p a r t i c u l a r plane i s a l i n e a r f u n c t i o n of the e f f e c t i v e normal s t r e s s on the plane at the time of f a i l u r e . T h i s l i n e , the f a i l u r e e n v e l o p e ( p l o t t e d i n F i g u r e 4.2a), i s d e f i n e d by the e f f e c t i v e s t r e s s equation: T, = a' tan + c" (4.2a) t f where: x^ = shear s t r e s s at f a i l u r e , which i s the s o i l ' s shear s t r e n g t h , and i s r e s i s t e d only by the s o i l s k e l e t o n ; "cr'f = e f f e c t i v e normal s t r e s s on the f a i l u r e plane at the time of f a i l u r e , which i s c a r r i e d by the s o i l i t s e l f ; <j>' = e f f e c t i v e angle of s h e a r i n g r e s i s t a n c e , assumed to be a m a t e r i a l constant; and c' = e f f e c t i v e cohesion i n t e r c e p t , equal to zero f o r the type of t e s t s ( r e s i d u a l s t r e n g t h ) used i n t h i s study ( F i g u r e 4-2b). Because = f u n c t i o n ( a ' ^ ) , f a i l u r e occurs at any p o i n t where a c r i t i c a l combination of shear s t r e s s and e f f e c t i v e normal s t r e s s i s o p e r a t i v e . A p r a c t i c a l demonstration of t h i s s t r e n g t h theory i s g iven by mechanical a n a l y s i s of the d i r e c t shear t e s t (Table VI, F i g u r e s 4-2c and 4-2d). B a s i c a l l y , the d i r e c t F i g u r e 4-2 a & b. Shear s t r e n g t h v s . e f f e c t i v e normal s t r e s s . 80 Table VI . Mechanical a n a l y s i s of the d i r e c t shear t e s t (see F i g u r e 4-2c) The shear plane i s allowed to develop i n the s o i l by m a i n t a i n i n g a s m a l l gap between the upper and lower h a l f of the sample box. The normal s t r e s s i s c r e a t e d by a p p l y i n g weights to a l o a d i n g hanger, which r e s t s on top of the box. The shear s t r e s s i s imparted along the plane by moving the upper h a l f of the box and s o i l r e l a t i v e to the lower h a l f w i t h a h o r i z o n t a l yoke. The shear f o r c e i s measured with a p r o v i n g r i n g -l o a d i n g d i a l a t t a c h e d to the h o r i z o n t a l yoke. The normal and h o r i z o n t a l displacements of the s o i l are determined by extensometers. 81 F i g u r e 4-2c. Photograph of the d i r e c t shear apparatus. 1 sample box 2 l o a d i n g hanger 3 h o r i z o n t a l yoke 4 p r o v i n g r i n g - l o a d i n g d i a l 5 extensometers 82 NORMAL FORCE l , 1 "< o • / , SHEAR i L j I I F i g u r e 4-2d. Forces i n d i r e c t shear t e s t i n g ( modified a f t e r Lambe, 1951). SHEAR DISPLACEMENT F i g u r e 4-2e.. S t r e s s - s t r a i n p l o t f o r remolded m a t e r i a l . 83 shear apparatus s u b j e c t s the s o i l to a c e r t a i n normal l o a d and g r a d u a l l y a p p l i e s a shear f o r c e along a h o r i z o n t a l plane i n the s o i l . Shear s t r e s s i s taken by the s o i l along t h i s predetermined plane. The s o i l r e s i s t s i n c r e a s e s i n shear s t r e s s as i t i s loaded h o r i z o n t a l l y . In theory, some p o i n t i s reached, where the maximum f r i c t i o n a l r e s i s t a n c e i s m o b i l i z e d over the e n t i r e plane. Slippage occurs across t h i s s u r f a c e , because the s o i l w i l l not take any f u r t h e r i n c r e a s e i n shear s t r e s s . The shear s t r e s s at f a i l u r e , T ^ , i s d e f i n e d as shear s t r e n g t h , and according to equation 4.2a, i s a f u n c t i o n of the e f f e c t i v e normal s t r e s s on the f a i l u r e plane. T e s t i n g c o n d i t i o n s are determined from knowledge of the parameters measured i n the lab as w e l l as understanding the s t a t e of the f a i l u r e m a t e r i a l i n i t s n a t u r a l environment. P e r t i n e n t t e s t i n g c o n d i t i o n s are c o n s o l i d a t i o n and f r e e -drainage of the s o i l . Reason f o r t h i s treatment i s made c l e a r by a n a l y z i n g the s t r e s s system of the t e s t . As d i s c u s s e d i n the pre v i o u s paragraph, shear s t r e n g t h i s a f u n c t i o n of e f f e c t i v e normal s t r e s s . T h i s e f f e c t i v e s t r e s s i s d e f i n e d by the f o l l o w i n g : a ' = a - u (4.2b) where: a' = e f f e c t i v e s t r e s s , that c a r r i e d by the s o i l ; a = t o t a l s t r e s s , equal t o the t o t a l overburden load; and u = pore water pr e s s u r e , that p a r t of the t o t a l s t r e s s r e s i s t e d by an i n c r e a s e i n f l u i d p r e s s u r e . Since pore water pr e s s u r e cannot be monitored i n a d i r e c t shear 84 t e s t , i t i s necessary to s t a b i l i z e i t at zero. T h i s i s done by: e n c l o s i n g the sample i n a water bath, i n s u r i n g f r e e access to water; c o n s o l i d a t i n g the sample p r i o r to shear, a l l o w i n g adequate time f o r drainage; and s h e a r i n g the s o i l at a slow r a t e so t h a t u remains equal to zero, even with volume changes d u r i n g shear. In t h i s way, a f r e e d r a i n i n g system i s guaranteed. Since u = 0, e f f e c t i v e s t r e s s i s known and i s equal to the overburden s t r e s s , i . e . , a' = a' (from Equation 4.2b). Lambe (1951) has d e s c r i b e d the general procedure of t h i s c o n s o l i d a t e d - d r a i n e d type of t e s t . Another p a r t i c u l a r of the t e s t i n g i s that the r e s i d - . u a l , remolded s t r e n g t h of the gouge i s determined. Bishop e t . a l . (1971) s t a t e s that remolding may be c o n s i d e r e d to destroy cohesive ( o r cementation) bonds and e l i m i n a t e any d i l a t a n c y accompanying f a i l u r e . F i g u r e 4.2e shows the g e n e r a l s t r e s s -s t r a i n path f o r remolded m a t e r i a l . The r e s i d u a l s t r e n g t h i s d e f i n e d as "the u l t i m a t e s h e a r i n g r e s i s t a n c e a f t e r very l a r g e displacements under f u l l y d r a i n e d c o n d i t i o n s " ( T e r z a g h i and Peck, 1967, p. 121). Note that s t r e n g t h s t a b i l i z e s at a constant ( r e s i d u a l ) value at l a r g e displacements i n F i g u r e 4-2e. The d i f f e r e n c e between any remolded "peak" and r e s i d u a l s t r e n g t h may be caused by r e - o r i e n t a t i o n of c l a y p a r t i c l e s adjacent to the s l i p s u r f a c e (Bishop, 1971). By p r e - s h e a r i n g and repeated t e s t i n g of the sample, i n t h i s study, i t i s thou-ght that the constant, r e s i d u a l value f o r <j> ' i s measured. T h i s parameter i s a p p l i c a b l e to the most c r i t i c a l f a i l u r e s i t -85 u a t i o n . The r e s i d u a l , remolded s t a t e of gouge i s a l s o comp-a t i b l e w i t h f i e l d o b s e r v a t i o n s : (1) The gouge i s a remolded m a t e r i a l produced by s h e a r i n g of the o r i g i n a l d e p o s i t s , p a r t i c u l a r l y , the c l a y s t o n e ; (2) S l i c k e n s i d e s are a predominant f e a t u r e i n the f i e l d . The range of e f f e c t i v e normal loads used i n the t e s t s i s the l a s t d e t a i l t o be d i s c u s s e d . Skempton (1969) s t a t e s that i t i s d e s i r a b l e to t e s t c l a y over the range of expected normal e f f e c t i v e s t r e s s to be encountered i n the f i e l d . T h i s procedure i s recommended because <f>' i s not n e c e s s a r i l y constant, that i s , may not be a l i n e a r f u n c t i o n of a '^. The range 2 of normal loads used i n t h i s study i s from 2 to 6 kg/cm . T h i s range i s thought to cover most s t r e s s c o n d i t i o n s on the a c t u a l f a i l u r e s u r f a c e . However, the s t a b i l i t y a n a l y s i s 2 2 ( F i g u r e 4-4a), shows extremes of 1 kg/cm and 7 kg/cm a c t i n g on the base of s l i d e s l i c e s . R e s u l t s f o r each of three l o a d i n g c o n d i t i o n s are given i n the form of s t r e s s - s t r a i n curves i n Appendix 4-2 and are summarized i n Table V I I . Some d e v i a t i o n of p o i n t s from curves (Appendix 4-2) can be e x p l a i n e d by l a c k of homo-geniety over the f a i l u r e s u r f a c e and l o c a l b u i l d - u p of pore water p r e s s u r e . Test r e s u l t s are c o n s i s t e n t i n the r e l a t i o n -s h i p between shear s t r e n g t h and p r e s s u r e . T h i s r e l a t i o n s h i p i s shown by p l o t t i n g , f o r each of the three l o a d i n g c o n d i t i o n s , the average values of shear s t r e s s at f a i l u r e a g a i n s t the 86 Table VII . R e s u l t s of d i r e c t shear t e s t s Normal l o a d Shear s t r e n g t h R e s i d u a l angle of s h e a r i n g r e s i s t a n c e 4150 psf 9 (2.03 kg/cm ) 1440 psf 0 (0.71 kg/cm ) 19. 2 o 8150 p s f „ (3.98 kg/crn ) 2710 p s f 9 (1.33 kg/cm ) 18.4 o 12,470 p s f 9 (6.09 kg/cm ) 4100 p s f 9 (2.00 kg/cm ) 18.2 o A l l parameters given i n terms of e f f e c t i v e s t r e s s . 87 e f f e c t i v e normal s t r e s s ( F i g u r e 4 - 2 f ) . The shear s t r e n g t h parameters, c' and <(>' are d e f i n e d by the best f i t t i n g l i n e of these f a i l u r e p o i n t s . F i g u r e 4-2f shows that $' equals 18.5 degrees, and because o r i g i n a l s o i l s t r u c t u r e s and bonds have been destroyed by "remolding", cohesion i s not a c t i v e and i s equal to zero. Although the r e s u l t s are c o n s i s t e n t , l i m i t a t i o n s of l a b t e s t s must be recognized. These are l i s t e d i n Table V I I I . D i s c r e p a n c i e s between f i e l d and l a b s t r e n g t h s are d i s c u s s e d by Skempton (1969). The reader i s r e f e r r e d t o Lambe (1951), Sowers and Sowers (1970), and Bishop (1971) f o r examination of the disadvantages of d i r e c t shear t e s t i n g . D e s p i t e l i m i t a t i o n s of the t e s t s , the aspects of the f a i l u r e envelope ( F i g u r e 4-2f), i t s l i n e a r i t y and l a c k of a cohesion i n t e r c e p t , are those p r e d i c t e d by s o i l mechanics theory.- A l s o , the $' value o b t a i n e d from the t e s t s i s a reasonable one. F i g u r e 4-2g shows an approximate r e l a t i o n s h i p , which i s a p p l i c a b l e to n a t u r a l gouge, between <j> ' and p l a s t i c i t y index. Gouge from the P e r p e t u a l S l i d e has a p l a s t i c i t y index of 18. The corresponding, d r a i n e d <t>' of 18.5 degrees p l o t s s l i g h t l y above that expected, but i s a reasonable value. 89 Table V I I I . L i m i t a t i o n s of l a b t e s t s Sampling: 1. Number of samples t e s t e d : Only one gouge sample was t e s t e d because of time l i m i t a t i o n i n the l a b . 2. S i z e of sample: Samples should be of adequate volume to give a r e p r e s e n t a t i v e n a t u r a l s i z e d i s t -r i b u t i o n of p a r t i c l e s (Skempton, 1969). Sand-size - g r i t i s present i n the gouge sampled from the Pe r p e t u a l S l i d e , however, i n the f i e l d , t h i s gouge co n t a i n s l a r g e r rock fragments, the e f f e c t s of which are not c o n s i d e r e d i n lab shear s t r e n g t h t e s t s . T e s t i n g : 1. R e v e r s a l of displacement d i r e c t i o n : S t r i a e , i n d i c a t i n g l a r g e displacements i n one d i r e c t i o n , are observed on n a t u r a l gouge. These l a r g e displacements are necessary to reduce the m a t e r i a l to i t s r e s i d u a l s t a t e and are a t t a i n e d i n the l a b by r e v e r s i n g the d i r e c t i o n of the d i r e c t shear box. Accor d i n g t o Bishop (1971, p. 322) because the lab t e s t f a i l s "to s i m u l a t e the f i e l d c o n d i t i o n of a l a r g e r e l a t i v e displacement u n i n t e r u p t e d by changes i n d i r e c t i o n , " the u l t i m a t e , r e s i d u a l shear s t r e n g t h may not be measured. 2. P r e d e t e r m i n a t i o n of the f a i l u r e s u r f a c e : T h i s can be j u s t i f i e d because the predetermined f a i l u r e s u r f a c e i s s l i c k n e s i d e d before t e s t i n g and t h e r e f o r e , i s the weakest and most c r i t i c a l s u r f a c e . Non-uniform s t r e s s and s t r a i n d i s t r i b u t i o n s on the f a i l u r e plane: The s o i l f a i l s p r o g e s s i v e l y , f i r s t at the box edges. Non-uniform s t r e s s e s and s t r a i n s r e s u l t , as the s t r e n g t h of the specimen over the e n t i r e f a i l u r e plane i s not m o b i l i z e d at the same i n s t a n t to r e s i s t shear. 4. Indeterminate nature of s t r e s s e s : The complete s t a t e of s t r e s s e s w i t h i n the s o i l can be determined only at f a i l u r e . c ontinued 90 Table V I I I . . . . continued. 5. Unknown changes i n area of shear: The c a l c u l a t e d s t r e n g t h i s i n a c c u r a t e when c l a y i s smeared o u t s i d e the presumed area of s o i l - s o i l c o n t a ct. 6. Rate of s h e a r i n g : There i s no c o r r e l a t i o n between speed and s t r e n g t h , f o r the slow r a t e s of displacements used i n t h i s study. Skempton (1969) demonstrated that there was l i t t l e v a r i a t i o n i n r e s i d u a l shear s t r e n g t h ( i n t e s t s on brown London Clay and weathered Edale Shale) with changes i n shear r a t e s . </-> UJ '•'•! u J t:.j 50 i i — on U J . f/7 U J u_" U J <3" 20 10 \ \ \ GOUGE: FROM THE .PERPETUAL SLIDE A F T E R K A N J I (.1970) 20 40 • 'PLASTICITY INDEX, % 60 80 FIGURE 4-29, APPROXIMATE RELATIONSHIP BETWEEN THE DRAINED ANGLE OF RESIDUAL SHEARING RESISTANCE AMD THE PLASTICITY U 3 4.3 Methods f o r s t a b i l i t y a n a l y s i s L i m i t e q u i l i b r i u m methods, wherein, a l i m i t i n g c o n d i t i o n i s reached everywhere, at the same time, along an assumed f a i l u r e s u r f a c e , were used i n s t a b i l i t y a n a l y s i s of the P e r p e t u a l S l i d e . The s l o p e f a i l e d as the f a c t o r of s a f e t y , the r a t i o between a v a i l a b l e shear s t r e n g t h of the f a i l u r e m a t e r i a l and shear s t r e s s causing f a i l u r e , approached one. Only s t a t i c e q u i l i b r i u m was considered, the f o r c e s i n the system being constant with time. T h i s e q u i l i b r i u m was analyzed i n a v e r t i c a l l o n g i t u d i n a l s e c t i o n of u n i t t h i c k n e s s . The S i m p l i f i e d Bishop Method of S l i c e s as w e l l as t r a n s l a t i o n a l e q u i l i b r i u m a n a l y s i s were a p p l i e d to the s t a b i l i t y e v a l u a t i o n . The Bishop method (as d i s c u s s e d by C r a i g , 1974) assumes a c i r c u l a r - a r c f a i l u r e and i n v o l v e s d i v i s i o n of the s l i d e i n t o l o n g i t u d i n a l v e r t i c a l s l i c e s f o r c o n s i d e r a t i o n of moment e q u i l i b r i u m . The f a i l u r e arc may be as shown i n F i g u r e 4-3c, w i t h overburden above t h i s s u r f a c e d i v i d e d by v e r t i c a l planes i n t o s l i c e s of width b ( F i g u r e 4-3a). The base of each s l i c e i s assumed to be p l a n a r . The f o r c e s ( F i g u r e 4-3a), per u n i t dimension normal to the s e c t i o n , are d e f i n e d by C r a i g (1974): (1) The t o t a l weight of the s l i c e , W, i s equal to Y^hb, where y^ = the t o t a l u n i t weight; (2) The t o t a l normal f o r c e on the base of the s l i c e , N, can be broken down i n t o two p a r t s : (a) the boundary water f o r c e , U, which equals the pore water pressure at the c e n t e r of the base, u , times the 93 Figure' 4~3a Figure 4-3b N' found by summing forces in this d i r e c t i o n resuitant of o 11 side forces assumed to act in this direction FORCES C O N S I D E R E D F i g u r e 4~3c MOMENT EQUILIBRIUM .r sin 6 7\ R e e n t e r of r o t a t ion r a d i u s of c i r c u l a r a r c h = h e i g h t m e a s u r e d o n c e n t e r l i n e of s l i c e & = i n c l i n a t i o n of b a s e to h o r i z o n t a l f o r e a c h s l i c e b = w i d t h of e a c h s l i c e Si = l e n g t h of t h e b a s e o f t h e s l i c e F o r c e s o n t h e u p s l o p e a n d d o w n s l o p e e d g e s o f s l i c e s a r e d e n o t e d by l a n d 2 , r e s p e c t i v e l y . F i g u r e s 4-3 a, b, & c. S i m p l i f i e d Bishop Method o f S l i c e s ( m o d i f i e d a f t e r C r a i g , 1974 and Lambe, 1969) . length of the base, a , and (b) the e f f e c t i v e normal force, N' = a '£ , which equals the e f f e c t i v e stress over the length of the base of the s l i c e . N' i s found by resolving forces on the s l i c e v e r t i c a l l y ; (3) The shear force on the base, S, i s equal to s £ , where s i s the shear strength required to maintain e q u i l -ibrium; (4) The i n t e r s l i c e forces consist of: (a) the v e r t i c a l shear forces on the si d e s i o f the s l i c e , X^ and X^, and (b) , the resultant of the t o t a l horizontal forces on the sides of the s l i c e , E^ and . This s i m p l i f i e d method i s an approximate one because: (1) The resultant forces on the sides of the s l i c e s are assumed to be horizontal, -and : .., X^ - = 0 (Figures 4-3a and 4-3b); (2) V e r t i c a l shear force and horizontal i n t e r s l i c e force equilibrium are not s a t i s f i e d . In t h i s analysis, the moment of the shear strength along the entire f a i l u r e arc (the r e s i s t i n g moment) i s compared to the moment of weight of the f a i l i n g mass (the driving moment) for evaluation of the factor of safety. The second method used to determine s t a b i l i t y of the Perpetual Slide i s one which has been developed by the C i v i l Engineering Department at the University of B r i t i s h Columbia. In t h i s method, i n t e r s l i c e forces are considered and trans-l a t i o n a l equilibrium of a l l forces shown i n Figure 4-3d i s s a t i s f i e d by evaluating components normal and tangential to the f a i l u r e surface (Figure 4-3e). The potential f a i l u r e mass can be divided into the s l i c e s l i k e those used i n the Bishop analysis and the forces on these s l i c e s are: (1) W and U, as defined i n Bishop's Method; (2) N', the e f f e c t i v e force normal to the f a i l u r e surface, derived by summing, for a l l forces shown in Figure 4-3d, the components normal to the f a i l u r e surface; (3) and Ug, the water forces acting normal to the s l i c e edges; (4) R, the shearing resistance of the " s o i l " along the f a i l u r e plane; and (5) F-^  and F 2, the i n t e r s l i c e forces between s l i c e s , which are resultants of Bishop's E and X forces. The t r a n s l a -t i o n a l program defines F^ and Fg by angles and Pg, the i n c l i n a t i o n s between i n t e r s l i c e forces and normals to the s l i c e faces. Translational equilibrium i s calculated progressively downslope. The factor of safety, F.S., i s found by an i t e r a -t i v e procedure whereby the strength of the f a i l u r e material i s altered u n t i l the s l i d e i s i n t r a n s l a t i o n a l equilibrium. While the t r a n s l a t i o n a l analysis affords a more complete force examination, Bishop's Method may better apply to the f a i l u r e mode of the Perpetual S l i d e . Because upthrust 96 Figure 4~3e / I : / f o r c e s r e s o l v e d / p e r p e n d i c u l a r a n d j p a r a l l e l t o f a i l u r e s u r f a c e F i g u r e s 4-3 d & e. T r a n s l a t i o n a l E q u i l i b r i u m A n a l y s i s (modified a f t e r Anderson, 1975). 97 and r o t a t e d b l o c k s are common f e a t u r e s , a n a l y s i s o f r o t a t i o n a l (moment) e q u i l i b r i u m may be the most r e a l i s t i c approach. A comparison of t h e two ,methods i s g i v e n i n Table IX. 4.4 C o n c l u s i o n s f o r s l o p e s t a b i l i t y a n a l y s i s The purpose of the s l o p e s t a b i l i t y a n a l y s i s was t o d e f i n e a r e a s o n a b l e environment f o r s l o p e f a i l u r e . To d e t e r -mine t h i s environment, c o n f i g u r a t i o n s o f the f a i l u r e s u r f a c e , w a t e r t a b l e , and p i e z o m e t r i c s u r f a c e were v a r i e d w i t h i n l i m i t s e s t a b l i s h e d by f i e l d i n v e s t i g a t i o n . The most c r i t i c a l of t r i a l f a i l u r e s u r f a c e s are shown i n F i g u r e s 4-4a and 4-4b. These s u r f a c e s are dependent on t h e geology and topography of t h e s l i d e . The t r i a l s u r f a c e encom-p a s s i n g t h e e n t i r e s l i d e , i n case 1 ( F i g u r e 4-4a), i s de-l i n e a t e d by t h e headscarp and the c l a y gouge zone at t h e t o e . In case 2 ( F i g u r e 4-4b), the t r i a l s u r f a c e f o r the upper p a r t o f t h e s l i d e i s d e t e r m i n e d by the headscarp and u p t h r u s t c l a y gouge above the t o e b l o c k . A c c o r d i n g l y , the lower u n i t ( t h e c l a y s t o n e , t i l l , c o a l , and sandstone) i s shown r o t a t e d up. T h i s u p t h r u s t can be r e a s o n a b l y n e g l e c t e d i n case 1. A g a i n , t h e c r i t i c a l t r i a l f a i l u r e s u r f a c e s are t o a g r e a t e x t e n t , d e t e r m i n e d by t h e i n f e r r e d s u b s u r f a c e g e o l o g y . C r i t i c a l c o n f i g u r a t i o n s of both the w a t e r t a b l e and p i e z o m e t r i c s u r f a c e are a l s o r e p r e s e n t e d i n the a f o r e m e n t i o n e d f i g u r e s . The p h r e a t i c s u r f a c e i s i n f e r -r e d from s u r f i c i a l f i e l d e v i d e n c e , i n c l u d i n g p r o x i m i t y 98 Table IX . Comparison of methods of s t a b i l i t y a n a l y s i s S i m p l i f i e d Bishop Method of S l i c e s T r a n s l a t i o n E q u i l i b r i u m Method E q u i l i b r i u m Moment e q u i l i b r i u m : The moment about 0, the center of r o t a t i o n , of the weight of the s o i l mass i s compared t o the moment of r e s i s t i n g f o r c e s a c t i n g on the shear s u r f a c e ( F i g u r e 4-3c). T r a n s l a t i o n a l e q u i l i b r i u m : The e q u i l i b r i u m of f o r c e s perpend-i c u l a r and p a r a l l e l to the f a i l u r e s u r f a c e i s c o n s i d e r e d ( F i g u r e 4-3e). Forces I n t e r s l i c e f o r c e s have no v e r t i c a l component ( F i g u r e 4-3b). Forces are r e s o l v e d v e r t i c a l l y and N', the normal e f f e c t i v e s t r e s s on the f a i l u r e s u r f a c e , i s found by summing v e r t i c a l components of f o r c e s . R e s u l t a n t of a l l s i d e f o r c e s i s assumed to be h o r i z o n t a l . U l t i m a t e l y , the f o r c e s N' and S ( F i g u r e 4-3a) are c o n s i d -ered i n the e q u i l i b r i u m used to e v a l u a t e F.S. I n t e r s l i c e f o r c e s are con s i d e r e d , and may have both v e r t i c a l and h o r i z o n t a l components. N' i s found by r e s o l v i n g f o r c e s p e r p e n d i c u l a r to the f a i l u r e s u r f a c e . The f o r c e s ( F i g u r e 4-3d) F 1,F2,Ui,U2,U, arid W, as w e l l as t h e i r r e s u l t a n t s , N' and R, are e v a l u a t e d p r o g r e s s -i v e l y downslope and are c o n s i d -e r e d i n the e v a l u a t i o n of F.S. by an i t e r a t i v e procedure. F a c t o r of S a f e t y , F.S F.S. i s d e f i n e d as the r a t i o o f a v a i l a b l e shear s t r e n g t h to that r e q u i r e d t o m a i n t a i n e q u i l i b r i u m . F.S. i s d e f i n e d as the f a c t o r by, which " s o i l " r e s i s t a n c e had to be a l t e r e d to produce e q u i l -i b r i u m . F.S..= A v a i l a b l e shear s t r e n g t h  Shear s t r e n g t h r e q u i r e d f o r moment e q u i l i b -rium. F.S. .= A v a i l a b l e shear s t r e n g t h Shear s t r e n g t h r e q u i r e d f o r t r a n s l a t i o n a l e q u i l -i b r i u m . continued Table IX . ... continued. Method Difference between the methods of analysis l i e s e s s e n t i a l l y i n the equilibrium considered, the way in which forces are resolved (N' determined), treatment of i n t e r s l i c e forces, and manner i n which F,S. i s calculated. / 1 „ CENTER OF 0 ROTATION TRANSLATIONAL EQUILIBRIUM F . S . = I . 10 MOMENT EQUILIBRIUM F . S . = 1 . 1 2 OVERBURDEN IS DEFINED AS ALL MATERIAL ABOVE THE FAILURE PLANE. ASSUMED 100 ASSUMED OVERBURDEN UNIT WEIGHT (d = dry, s= saturated) y d = I 5 80 KG/M3 X = 19 10 KG/M3 2 0 0 300 400 LENGTH IN METERS O O Figure 4 * 4 o . Cose I, f a i l u r e model • for ent i re s l i d e (cross s e c t i o n olonQ l ine A " B of F i g u r e l _ 2 b ) . OVERBURDEN IS DEFINED AS ALL MATERIAL ABOVE THE FAILURE PLANE. ASSUMED OVERBURDEN UNIT WEIGHT <d=dry, s = „ oturotad ) _CENTER OF y.O ROTATION T R A N S L A T I O N A L E Q U I L I B R I U M . F . S . = 1 . 3 0 WATER ASSUMED: • T A B L £  P ,= 32° 9 r o v | l , n 6. - 32° s i l t 300 4 0 0 LENGTH IN METERS Figure, 4 - 4 b . Cose 2 , f a i l u r e model for upper part of s l ide (cross s e c t i o n a l o n g l i n e A - B of F i g u r e I- 2 b). O 102 of the i r r i g a t e d area to the s l i d e , and the l o c a t i o n of dense vegetation and seeps w i t h i n the s l i d e . Factors which determine the p i e z o m e t r i c heads ( a c t i v e along the f a i l u r e surface) are the p o s i t i o n of t h i s water t a b l e , probable groundwater flow, assumed c o n d u c t i v i t y c o n t r a s t s between formations, and geo-metry of the f a i l u r e s u r f a c e . The most c r i t i c a l surfaces are shown i n Figures 4-4a and 4-4b, and are the highest that are reasonably p o s s i b l e . In a n a l y s i s of the Perpetual S l i d e , v e r t i c a l s l i c e s were considered f o r moment and t r a n s l a t i o n a l e q u i l i b r i u m using the methods described i n the preceding s e c t i o n . Although the surfaces and assumed geotechnical parameters (Figures 4-4a and 4-4b) were; reasonable, the a n a l y s i s was s e n s i t i v e to changes i n these v a r i a b l e s . This dependancy must be kept i n mind when con s i d e r i n g the f o l l o w i n g conclusions: (1) A high piezometric surface i s e s s e n t i a l f o r i n s t a b i l i t y . (2) The water t a b l e must be high enough to unload the overburden s t r e s s and to be compatible w i t h t h i s high p i e z -ometric surface. (3) The f a c t o r of s a f e t y i s minimized along the c r i t i c a l t r i a l surfaces presented. Shallow surfaces do not d i s s e c t the weak m a t e r i a l and do not have the mass to generate s i g n i f i c a n t g r a v i t a t i o n a l d r i v i n g f o r c e . Deeper surfaces are s t a b l e because of the large amount of ^©verburden above the / 103 middle p a r t and the upturned toe of the t r i a l s e c t i o n s . S t a b i l i t y i n these areas r e s u l t s from zero or negative slope of the f a i l u r e s u r f a c e . (4) F a i l u r e of the e n t i r e s l i d e (case 1) i s more c r i t i c a l than separate f a i l u r e of the upper p a r t of the s l i d e (case 2); ^ Case 1 Case 2 F.S. = 1.12 F.S. = 1.30 Moment a n a l y s i s F.S. = 1.10 F.S. = 1.30 T r a n s l a t i o n a l a n a l y s i s . There i s agreement between the two e q u i l i b r i a c o n s i d e r e d f o r each case. C e r t a i n l y , a f a c t o r of s a f e t y near one, o b t a i n e d from the f i r s t case, i s r e a l i s t i c . A lower F.S. i m p l i e s s t a b -i l i t y too low to m a i n tain the continuous-type f a i l u r e of the P e r p e t u a l S l i d e . The r e s u l t s of the a n a l y s i s may be questioned on the f o l l o w i n g b a s i s : Would the e q u i l i b r i u m of each of the t h r e e b l o c k s ( c r o s s s e c t i o n A-B, F i g u r e 2 - l c ) c o n s i d e r e d as i n d i v i d u a l u n i t s , be more c r i t i c a l ? In a study of the L i t t l e Smoky L a n d s l i d e , Thompson and Hayley (1975) found that a n a l y s i s of r e t r o g r e s s i v e f a i l u r e of separate b l o c k s gave more reasonable f a c t o r s of s a f e t y (1.30 to 1.03) than e q u i l -i b r i u m c o n s i d e r a t i o n of the e n t i r e s l i d e mass (F.S. = 1.7). Haug et a l . (1976) a l s o found t h a t the r e t r o g r e s s i v e mechanism was v a l i d i n e x p l a i n i n g f a i l u r e at Beavercreek, Saskatchewan. However, i t i s not necessary to r e s o r t to a r e t r o g r e s s i v e mechanism f o r the P e r p e t u a l S l i d e , because a reasonable F.S. 104 r e s u l t e d from a n a l y s i s of the e n t i r e s l i d e ; and the upper p a r t of the s l i d e , e v a l u a t e d as a separate u n i t , was found to be more s t a b l e than the s l i d e as a whole. That the most c r i t i c a l t r i a l f a i l u r e s u r f a c e appears to be c i r c u l a r i s a l s o s u b j e c t to q u e s t i o n . I t must be recog-n i z e d that t h i s t r i a l s u r f a c e i s an approximation of some a c t u a l f a i l u r e s u r f a c e which most l i k e l y i s not c i r c u l a r . F i e l d o b s e r v a t i o n s show d i s t o r t i o n , breakup: of s l i d e b l o c k s , and graben formation, a l l of which, a c c o r d i n g to R i t c h i e (1958), i n d i c a t e a n o n - c i r c u l a r s l i p s u r f a c e . However, these d i s -r u p t i o n s may a l s o be e x p l a i n e d by separate movement of b l o c k s i n t e r n a l to an e n c l o s i n g (perhaps near c i r c u l a r ) f a i l u r e s u r -face . Hodge (1976) concluded, as others have, that s t a b -i l i t y a n a l y s i s are r e l i a b l e only when the imput data i s accur-a t e l y determined. Lack of data p l a c e s c o n s t r a i n t s on any i n t e r p r e t a t i o n s made from t h i s e q u i l i b r i u m e v a l u a t i o n . F i g u r e 4-4a does i l l u s t r a t e a probable s i t u a t i o n f o r f a i l u r e and i s the most c r i t i c a l of the p o s s i b i l i t i e s i n v e s t i g a t e d . Given approximations i n h e r e n t i n the a n a l y t i c a l methods and i n e v a l u a t i o n of i n t e r a c t i v e v a r i a b l e s , the q u e s t i o n as to how c l o s e l y the most c r i t i c a l s i t u a t i o n was approached would best be answered by a more comprehensive a n a l y s i s u s i n g d r i l l h o l e , p i e z o m e t r i c , and groundwater e x p l o r a t i o n data. 105 CHAPTER 5 DISCUSSION AND CONCLUSIONS 5.1 D i s c u s s i o n o f r e s u l t s and o r i g i n C o n s i d e r a t i o n of the f i e l d study and the ground water and s t a b i l i t y analyses suggest the f o l l o w i n g c l a s s i f i c a t i o n of f a c t o r s r e s p o n s i b l e f o r the P e r p e t u a l S l i d e : (1) L i t h o l o g y and s t r a t i g r a p h y as r e l a t e d to s t r e n g t h and (2) S t r e s s h i s t o r y h y d r a u l i c c o n d u c t i v i t y of (3) S t r u c t u r e g e o l o g i c u n i t s (4) Ground water (5) Climate The s l i d e seems to be adequately e x p l a i n e d by these c o n t r o l s . However an understanding of f a i l u r e i s l i m i t e d by incomplete d e f i n i t i o n of these f a c t o r s because of l a c k of subsurface i n f o r m a t i o n and l a c k of time f o r s t r e n g t h t e s t s on more than one gouge sample. Consequently, the f a i l u r e model of the s l i d e u t i l i z e d the a v a i l a b l e f i e l d and lab i n f o r m a t i o n to e s t imate some of -the slope p r o p e r t i e s which a f f e c t s t a b i l i t y , such as subsurface g e o l o g i c boundaries and overburden u n i t weight. I t i s hoped that the r e s u l t i n g e v a l u a t i o n of f a i l u r e may give any more i n t e n s i v e a n a l y s i s some d i r e c t i o n . The c l a s s i f i c a t i o n system of f a c t o r s r e s p o n s i b l e f o r f a i l u r e i s s i m i l a r to that adopted by Scott-and Brooker (1968). 106 The f a c t o r s are d i s c u s s e d , as they apply to the P e r p e t u a l S l i d e , i n the f o l l o w i n g paragraphs. (1) L i t h o l p g y and s t r a t i g r a p h y : Varnes (1958) s t a t e s that the f a c t o r s c o n t r i b u t i n g to low shear s t r e n g t h may be d e r i v e d from the i n i t i a l s t a t e of the m a t e r i a l or changes i n t h i s s t a t e . Reference i s made to " I n h e r e n t l y weak m a t e r i a l s " or to those that may become weak. These i n c l u d e : Sedimentary c l a y s and s h a l e s , o r g a n i c m a t e r i a l , decomposed rocks, and v o l c a n i c t u f f , which may a l t e r to a clayey m a t e r i a l . Patton and Hendron (1974, p. 9) s t a t e that "the presence of t h i c k , low, p e r m e a b i l i t y rocks" or t h i c k f a u l t zones, "would tend to be a s s o c i a t e d with zones of excess pore-water pressures w i t h i n and at the base of s l o p e s of major r i v e r v a l l e y s . " The P e r p e t u a l S l i d e , w i t h l o w - p e r m e a b i l i t y u n i t s l o c a t e d proximal to Trout Creek Canyon, i s s u s c e p t i b l e to the development of high pore-water p r e s s u r e s . These u n i t s are a l s o types which are commonly weak. A c c o r d i n g l y , an attempt was made to r e l a t e the s p a t i a l d i s t r i b u t i o n of l i t h o l o g i e s & associated:propert±es to a q u a l i t a t i v e e v a l u a t i o n of both s t r e n g t h and r e l a t i v e h y d r a u l i c c o n d u c t i v i t y . F a c t o r s which are p e c u l i a r to the P e r p e t u a l S l i d e and probably c r i t i c a l to i t s i n s t a b i l i t y are d i s c u s s e d i n t h i s context. The weathering zone developed on c r y s t a l l i n e rocks may not only decrease support at the s l i d e toe, but may a l s o a f f e c t r e g i o n a l ground water flow. The unconformable contact which marks t h i s p r o f i l e c o n t a i n s a flow zone and d i r e c t l j ^ 107 u n d e r l i e s the s l i d e only at i t s toe. Hodge (1976, p. 88) concluded t h a t : "Weathering p r o f i l e s can commonly r e s u l t i n a l e s s conductive zone c o n f i n i n g another w i t h the r e s u l t i n g flow system extremely d e t r i m e n t a l to s t a b i l i t y . " To determine i f t h i s type of c o n f i n e d s i t u a t i o n i n f l u e n c e s the P e r p e t u a l S l i d e , d r i l l hole data would be needed to d e f i n e the l o c a t i o n of the weathered zone i n the subsurface and p i e z o m e t r i c l e v e l s be measured to determine i t s i n f l u e n c e on water pres-sures . From the study of exposed geology, i t i s concluded t h a t the f a i l u r e s u r f a c e at depth l i e s w i t h i n the T e r t i a r y sediments, p r i m a r i l y the c l a y s t o n e . T h i s f a i l u r e i n p a r t r e s u l t s from the l i t h o l o g i c d i s t i n c t i o n between the sediments, which i n t u r n , causes d i f f e r e n c e s i n r e l a t i v e s t r e n g t h and c o n d u c t i v i t y . Bedding and j o i n t i n g no doubt a l s o a f f e c t the geometry of f a i l u r e and ground water flow. The c l a y s t o n e , which i s sheared and remolded i n t o a c l a y e y gouge of r e s i d u a l s t r e n g t h , i s the key to the proposed mechanism of continuous f a i l u r e , although l o c a l shear un-doubtably takes p l a c e through f r i a b l e sandstone and b r i t t l e c o a l . Besides p o s s e s s i n g low s t r e n g t h i n i t s d i s p l a c e d and remolded s t a t e , the c l a y s t o n e may act as an impermeable cap over u n d e r l y i n g d e p o s i t s (weathered c r y s t a l l i n e rocks, v o l c a n i c sandstone, and f r a c t u r e d c l a y s t o n e and c o a l ) . The s i t u a t i o n of impermeable rock or gouge co n f i n i n g the upward di s c h a r g e from any u n d e r l y i n g a q u i f e r s and a l l o w i n g high pore-water 108 p r e s s u r e s to b u i l d u p w i t h i n the slope i s not uncommon and has been d e s c r i b e d by Patton and Hendron (1974). Hodge (1976 p. 83) concluded from s t u d i e s of h i s ground water flow models that "A f a u l t gouge or g e o l o g i c u n i t need not be impermeable or even of exceedingly l e s s c o n d u c t i v i t y than the surrounding m a t e r i a l to adversely a f f e c t the flow regime i n such a way as to cause i n s t a b i l i t y . " The e f f e c t of Quaternary d e p o s i t s on s t a b i l i t y r e s u l t s from i n t e r b e d d i n g and l a t e r a l v a r i a t i o n of the d e p o s i t s . The o l d e s t Quaternary u n i t , the t i l l , grades from a c l a y - r i c h m a t e r i a l w i t h i n the s l i d e , to a more s i l t y , l e s s p l a s t i c m a t e r i a l i n exposures o u t s i d e of the s l i d e . Weakness of the c l a y - r i c h t i l l i s demonstrated by a major s l i p s u r f a c e formed w i t h i n i t on the southern s l i d e toe. F u r t h e r , mudcracks on t h i s s u r f a c e i n d i c a t e presence of expansive c o n s t i t u e n t s . Because of i t s r e l a t i v e l y low c o n d u c t i v i t y , the t i l l (and a l s o the c l a y gouge) promotes flow through o v e r l y i n g g r a n u l a r d e p o s i t s ; t h i s flow subsequently undercuts the s l i d e toe. In a d d i t i o n , c o n d u c t i v i t y c o n t r a s t s e x i s t w i t h i n the g r a n u l a r d e p o s i t s : The g l a c i o - l a c u s t r i n e s i l t and f i n e sand i s not as f r e e d r a i n i n g as the g l a c i o - f l u v i a l sand and g r a v e l , and as a consequence, appears more s a t u r a t e d i n the f i e l d . I n t e r -bedding between the two u n i t s may cause h i g h e r pore-water pressures i n l o c a l l y c o n f i n e d systems. Perched water t a b l e s may be a l s o c r e a t e d . (2) S t r e s s h i s t o r y : Because the T e r t i a r y s t r a t a once were s u b j e c t to loads ( b u r i a l and i c e ) g r e a t e r than t h e i r 1 0 9 present overburden load, they are o v e r c o n s o l i d a t e d . The e f f e c t on rock s t r e n g t h of o v e r c o n s o l i d a t i o n and consequent rebound wi t h reduced s t r e s s caused by e r o s i o n , i s d i s c u s s e d by S c o t t and Brooker (1968) and Bjerrum (1966). The extent to which any rebound took p l a c e i n the s t r a t a i n response to T e r t i a r y or P l e i s t o c e n e ( g l a c i a l ) unloading i s not known. The s t r e s s h i s t o r y r e s u l t i n g from f l u v i a l processes i s b e t t e r d e f i n e d . Downcutting i n t o t e r r a c e d e p o s i t s d u r i n g l a t e g l a c i a l and Recent time, had the e f f e c t of unloading over-burden on the toe of the p o t e n t i a l s l i d e , removing l a t e r a l support f o r the s l o p e , and exposing a s m a l l s e c t i o n of the c l a y s t o n e where i n i t i a l f a i l u r e may have occu r r e d . P r e s e n t l y , the toe of the s l i d e i s s u b j e c t to s t r e s s from upslope m a t e r i a l , and t h i s i s p a r t i c u l a r l y d e t r i m e n t a l to s t a b i l i t y with continued u n d e r c u t t i n g of the toe by s p r i n g e r o s i o n . (3) S t r u c t u r e : F r a c t u r e s i n the T e r t i a r y s t r a t a r e s u l t from deformation a f t e r d e p o s i t i o n and d i s t u r b a n c e of s t r e s s e q u i l i b r i u m with e r o s i o n . The f r a c t u r e s may act as planes of weakness with consequent l o s s i n mass shear s t r e n g t h . Secondary p e r m e a b i l i t y may have been c r e a t e d through j o i n t i n g by the above mechanisms or by a c t u a l shear displacement d u r i n g s l i d i n g . Mathews (pers. comm.) found that water l o c a l l y flowed f r e e l y through c l a y s t o n e i n t o a hand-augered h o l e . (4) Ground water flow: The importance of ground water to slope f a i l u r e i s confirmed by: (a) The h i s t o r i c a l r e c o r d of the s l i d e : I r r i g a t i o n water was f i r s t brought i n t o P a r a d i s e F l a t s sometime i n the 110 p e r i o d 1903-1906. In f o l l o w i n g years (1914-1917), sub-sidence was noted by s e v e r a l r e s i d e n t s . New s p r i n g s appeared at the base of the s l i d e d u r i n g or a f t e r the i n i t i -a t i o n of f a i l u r e . (b) The r a t e o f movement of the s l i d e measured from the summer of 1975 to the summer of 1976: Seasonal v a r i a t i o n i n s u r f i c i a l movement ( i . e . maximum movement i n November), c o r r e l a t e s c l o s e l y with seasonal v a r i a t i o n of s p r i n g d i s c h a r g e (maximum flow i n November-December). From the topographic and p o s t u l a t e d h y d r o l o g i c and g e o l o g i c environments, i n f e r e n c e s can be made as t o how ground water flow a f f e c t s s t a b i l i t y : (a) High pore-water pre s s u r e s are probably generated along the f a i l u r e zone of the. s l i d e , thereby d e c r e a s i n g e f f e c t i v e s t r e s s and shear s t r e n g t h i n t h i s zone. ( i ) Both r e g i o n a l and in t e r m e d i a t e ground-water discharge systems (as i l l u s t r a t e d i n S e c t i o n 3.3), with flow from Mt. Conkle and P a r a d i s e F l a t s r e s p e c t i v e l y , are probably o p e r a t i v e . L o c a l flow systems are a l s o probably a c t i v e w i t h i n the s l i d e i t s e l f . These d i s c h a r g e systems i n d i c a t e pore-water p r e s s u r e s i n excess of h y d r o s t a t i c p r e s s u r e s . ( i i ) I t i s i n f e r r e d from g e o l o g i c and hydro-l o g i c i n v e s t i g a t i o n s t h a t there are s i g n i f i c a n t c o n d u c t i v i t y c o n t r a s t s between u n i t s i n the s l i d e area. As p r e v i o u s l y mentioned, these g e o l o g i c c o n d i t i o n s are conducive to high I l l pore-water pressures developing below the base of the s l i d e . ( i i i ) In c o n f i r m a t i o n of the above, s t a b i l i t y a n a l y s i s i n d i c a t e s that a high p i e z o m e t r i c s u r f a c e (and water t a b l e ) are necessary f o r f a i l u r e . (b) Regional flow models suggest that changes i n ground water flow c o u l d r e s u l t i n s i g n i f i c a n t changes i n s t a b i l i t y . (c) A high water t a b l e can have the f o l l o w i n g e f f e c t s : ( i ) I n c r e a s i n g the t o t a l u n i t weight of the over-burden, while d e c r e a s i n g the e f f e c t i v e l o a d on the f a i l u r e zone. For example, a h i g h water t a b l e c o u l d be r e s p o n s i b l e f o r f a i l u r e by t h i s mechanism i f the s l o p e of the f a i l u r e s u r f a c e were g r e a t e r . ( i i ) E l i m i n a t i n g a i r - w a t e r s u r f a c e t e n s i o n i n v o i d s between p a r t i c l e s . ( i i i ) H y d r a t i n g c l a y m i n e r a l s i n gouge so as to cause s w e l l i n g and decrease shear s t r e n g t h . The extent of t h i s i s not known. However, d e s s i c a t e d gouge i s o f t e n mud-cracked and x-ray d i f f r a c t i o n e s t a b l i s h e d the presence of an expanding c l a y m i n e r a l i n the three gouge samples t e s t e d . From the h y d r o g e o l o g i c a l f a c t o r s c o n s i d e r e d above, i t i s concluded that ground water i s the most s i g n i f i c a n t s i n g l e f a c t o r i n i t i a t i n g slope i n s t a b i l i t y . ( 5 ) Climate: The l a s t of the parameters a f f e c t i n g f a i l u r e are c l i m a t i c i n f l u e n c e s , p r e c i p i t a t i o n and evapor-a t i o n . These a l t e r s a t u r a t i o n of m a t e r i a l and the q u a n t i t y 112 of water a v a i l a b l e f o r ground water flow. They must be c o n s i d e r e d , along with the input of i r r i g a t i o n water, as they a f f e c t the discharge of water from the s l i d e & consequently i t s s t a b i l i t y . 5.2 Mechanism A mechanism which i s commonly a c t i v e i n c l a y s and c l a y s h a l e s i s p r o g r e s s i v e f a i l u r e . T h i s has been e x t e n s i v e l y s t u d i e d by Skempton (1964, 1969) and Bjerrum (1966). Progres-s i v e f a i l u r e i s the gradual r e d u c t i o n from peak to r e s i d u a l s t r e n g t h of a c l a y , with p r o g r e s s i v e development of the f a i l u r e s u r f a c e upslope. A s i m i l a r concept, that of r e t r o -g r e s s i v e f a i l u r e , has been adopted to e x p l a i n l a n d s l i d e s i n Cretaceous c l a y s h a l e s i n A l b e r t a and Saskatchewan. Thomson and Hayley (1975) d e s c r i b e t h i s , as f a i l u r e " c o n s i s t i n g of a s e r i e s of b l o c k s , each a c t i n g as a separate e n t i t y but a l l merging to a common lower shear zone." L i k e p r o g r e s s i v e f a i l u r e , the i n d i v i d u a l b l o c k s f a i l p r o g r e s s i v e l y from the toe to the s c a r p . Where exposed i n the P e r p e t u a l S l i d e , the f a i l u r e m a t e r i a l i s a gouge c o n s i s t i n g of rock fragments i n a c l a y matrix, much of t h i s c l a y i s predominantly the remolded product of the c l a y s t o n e . The d i r e c t shear t e s t s determined the e f f e c t i v e s t r e n g t h of t h i s gouge to have the r e s i d u a l angle of s h e a r i n g r e s i s t a n c e = 18.5°, with the cohesion i n t e r -cept = 0, under most expected l o a d i n g c o n d i t i o n s . A pro-113 g r e s s i v e mechanism seems to be o p e r a t i v e , i n the sense that the peak s t r e n g t h of the c o n s o l i d a t e d rock ( c l a y s t o n e ) has been reduced by s h e a r i n g and consequent remolding to t h i s r e l a t i v e l y ; low r e s i d u a l s t r e n g t h . N e i t h e r f a i l u r e of the s l o p e as an i n t a c t mass nor as i n d i v i d u a l b l o c k s has been e s t a b l i s h e d as the f a i l u r e mechanism. Moment and t r a n s l a t i o n a l s t a b i l i t y a n a l y s i s d i d i n d i c a t e t hat f a i l u r e of the e n t i r e s l o p e i s more c r i t i c a l than separate f a i l u r e of the upper p a r t of the s l i d e . The u p t h r u s t gouge zones w i t h i n the s l i d e can be e x p l a i n e d by i n t e r n a l r o t a t i o n of separate b l o c k s w i t h i n an e x t e r n a l e n c l o s i n g f a i l u r e s u r f a c e . A more i n t e n s i v e a n a l y s i s i n v o l v i n g the three i n d i v i d u a l b l o c k s (as shown i n F i g u r e 2-2c), with adequately d e f i n e d m a t e r i a l p r o p e r t i e s and subsurface informa-t i o n , c o u l d determine i f r e t r o g r e s s i o n i s a c t i v e . The g e o l o g i c , h y d r o l o g i c , a n d s t a b i l i t y s t u d i e s i n d i c a t e that ground water has a s i g n i f i c a n t i n f l u e n c e on s t a b i l i t y . Both a h i g h water t a b l e , r e s u l t i n g from a r t i f i c i a l water sources i n P a r a d i s e F l a t s , and a high p i e z o m e t r i c s u r f a c e , caused t y the h i g h groundwater t a b l e , topography, and heterogeneous geology, were necessary f o r f a i l u r e i n the s t a b i l i t y a n a l y s i s conducted. The r e s u l t s of t h i s study support p r o p o s a l of the mechanism of f a i l u r e to be: (1) The p r o g r e s s i v e r e d u c t i o n from peak to r e s i d u a l s t r e n g t h of the c l a y s t o n e . 114 (2) In a d d i t i o n to a high water t a b l e , high pore-water p r e s s u r e s along the f a i l u r e s u r f a c e . 5.3 Environmental problems and recommendations Environmental hazards are determined by the be-havio u r of the s l i d e . T h i s behaviour can be c o n s i d e r e d to f o l l o w three p a t t e r n s , movement c o n t i n u i n g at the same r a t e , movement i n c r e a s i n g , or movement dec r e a s i n g . If remedial measures are not taken, movement may continue much the same as i t has i n the past year, 1975-1976, and i n 1974-1975 (Madsen, pers. comm.). C r i t i c a l s i t u a t i o n s may then develop i n the fans below the s l i d e and i n r e l a t i v e l y u n d i s t u r b e d ground behind scarps which now bound the s l i d e . The r i s k of damming Trout Creek by the fans i n c r e a s e s as they extend f a r t h e r i n t o the creek. T h i s danger i s p a r t i c u l a r l y imminent duri n g the time of minimum discharge of Trout Creek ( g e n e r a l l y l a t e summer through w i n t e r ) and maximum discharge of s p r i n g s from the s l i d e (November-December). At t h i s time, maximum d e p o s i t i o n by the fans i s produced and s l i d e d e b r i s commonly o b s t r u c t s Trout Creek. Fan d e p o s i t i o n has been observed to s i g n i f i c a n t l y a l t e r the flow path of the creek and ponding (upstream from the fans) does occur ( F i g u r e s l-4k & 1). Sudden br e a c h i n g of a major b l o c k -age of the creek c o u l d cause f l o o d i n g downstream. The extent of damage would depend upon the q u a n t i t y of ponded water r e l e a s e d and d e b r i s washed downstream. Compounding the 115 hazardous s i t u a t i o n i n the canyon i s a l o g jam ( F i g u r e 5-3), which l i e s i n between the fans of the two western-most s p r i n g s . P r e s e n t l y , i t i s at l e a s t 25 f e e t across and extends out from the south w a l l of the canyon to .deflect and p a r t i a l l y b l o c k Trout Creek. The l o g d e b r i s adds to the s l i d e m a t e r i a l a v a i l -able to o b s t r u c t Trout Creek. Moreover, log-choked f l o o d d e b r i s c o u l d b l o c k c u l v e r t s through which Trout Creek flows i n p a r t of i t s d e l t a . I f movement continues at i t s present r a t e , the p o s s i b i l i t y of r e t r o g r e s s i v e f a i l u r e i n back of the present s l i d e s hould a l s o be considered. In f a c t , some scarp development and minor block subsidence behind the present headscarp was l a r g e enough to be c a s u a l l y observed d u r i n g the study. T h i s o c c u r r e d near A N ( F i g u r e 4 - l c ) , where a minor b l o c k dropped a>bout one foot between January 1975 and June 1976. Movement was a l s o noted by the survey t o occur behind a f l a n k i n g scarp (at £± W, F i g u r e 4 - l c ) and i n another p a r t of the crown area (at N ?, F i g u r e 4 - l c ) . Although the survey d e t e c t e d no movement i n areas more than a few f e e t back of the major scarps or i n areas approaching the bounding roads, i t i s p o s s i b l e that some movement may take p l a c e over tens of years. The p o s s i b i l i t y of r e t r o g r e s s i o n deserves c o n s i d e r a t i o n because the present domestic water l i n e runs along the road which bounds the s l i d e and the new p r e s s u r i z e d , domestic-i r r i g a t i o n water l i n e i s b u r i e d along the south s i d e of t h i s road ( F i g u r e l - 2 a ) . Although rupture i n e i t h e r l i n e may not occur f o r years, i t i s l i k e l y t h a t d i f f e r e n t i a l subsidence 116 117 w i l l eventually stress the systems. Any leaking water released within the s l i d e drainage basin would adversely a f f e c t s t a b i l i t y . Accurate predictions of retrogression, however, require a better d e f i n i t i o n of the f a i l u r e surface and the sedimentary rock contacts, and a s t a b i l i t y analysis which extends behind the present headscarp. Another problem, degradation of water i n Trout Creek, i s an ongoing consequence of s l i d e springs emptying into the creek. The Trout Creek Ecological Overview prepared by Smyth (1975, p. 10) states that between mile 0 to 2 from the mouth of the creek, "excessive temperature and t u r b i d i t y strongly l i m i t f i s h e r i e s p o t e n t i a l . " In addition, fine sediments have f i l l e d i n the voids i n the gravel bed to the point that spawning sucess i s very low. The deterioration of Trout Creek (and subsequently, Okanagan Lake) as well as the p o t e n t i a l hazards mentioned above, warrant concern i f s l i d e movement i s allowed to continue as i t has i n the past. Catastrophic movement may be i n i t i a t e d by rupture in the water l i n e , t o r r e n t i a l r a i n , or an earthquake. The f i r s t two occurrences increase ground water flow, the role of which has been summarized in Sections 5.1 and 5.2. The t h i r d event, seismic action i s commonly known to trigger massive f a i l u r e . The detrimental action may be i n the form of additional accelerations that can modify the state of stress within the slope (Varnes, 1958). Although the area i s i n seismic zone 1 (Canada National Research Council, 1975), a 118 zone which does not commonly experience high a c c e l e r a t i o n s from earthquakes, the s e i s m i c r e c o r d of the area should be examined (Table X). I t i s p o s s i b l e that a' major earthquake, such as the one o c c u r r i n g i n Southwestern B.C. i n 1946, may be f e l t with s u f f i c i e n t i n t e n s i t y i n the s l i d e to d i s t u r b i t s present s t a t e of q u a s i - e q u i l i b r i u m . The consequences of c a t a s t r o p h i c movement, are the same as those of continued movement, only t h e i r hazard i s much g r e a t e r . These consequences are the blockage of Trout Creek f o l l o w e d by breaching and f l o o d i n g downstream, and the d e s t r u c t i o n of p r o p e r t y i n P a r a d i s e F l a t s caused by r e t r o -g r e s s i v e f a i l u r e . In a d d i t i o n , l i v e s are threatened by un-a n t i c i p a t e d c a t a s t r o p h i c f a i l u r e . The l a s t of the three e v e n t u a l i t i e s i s a decrease i n movement. The most d i r e c t remedial measure to b r i n g t h i s about i s to improve s u r f a c e drainage. Recommendations f o r i n t e r - 1 c e p t i n g s u r f a c e water before i t d r a i n s i n t o the slope f o l l o w : (1) E l i m i n a t e overflow and use of the s e c t i o n of u n l i n e d d i t c h by o p e r a t i o n of the new, s e l f - c o n t a i n e d , p r e s s u r i z e d system (expected to be i n use i n 1977). I f t h i s system i s delayed i n o p e r a t i o n , a l t e r n a t e measures are recommended: (a) Pipe or flume i r r i g a t i o n and domestic water overflow to bypass the s l i d e and empty i n t o Trout Creek. (b) Completely l i n e a l l s e c t i o n s of the overflow d i t c h , e l i m i n a t i n g approximately 1500 f e e t of u n l i n e d d i t c h 119 Table X. Record of seismic a c t i v i t y i n the area of the Perpetual Landslide (1900 - 1951), derived from Milne, (1956). 1915 August 18. 6:05 A.M.,$ = 48°32' N; A = 121°26' W. The tremor was f e l t at Summerland. "An earthquake was f e l t from Seattle to Enderby, and from V i c t r o i a to the Okanagan Valley." 1918 December 6. 00:41 A.M. <j> = 49 3/4°N; A = 126 h 0 W;, M = 7.G. "The earthquake was f e l t at V i c t o r i a , Vancouver, Kelowna, and Seattle, but not at Vernon, Penticton, Ghilliwack, or Armstrong i n the i n t e r i o r . " , 1936 March 28. 1:15 A.M. V. 119.5 W. Penticton and Kelowna 1946 June 23. 9:13 A.M. cj) M = 7.3. The "B.C. Earthquake." epicenter $ = 50.5°N ;A = residents generally awakened. = 49° 52' N; A = 124°55' W: 1949 Feb. 4. An earthquake was f e l t at Summerland and Peachland. 1949 Aug. 20. 8:03 P.M. y = 54.2°N; A = 133..5°W; M = 8.0. F e l t as far east as Jasper and as far south as Portland. 120 (northwest corner of P a r a d i s e F l a t s , F i g u r e l - 2 a ) . (2) Zone the areas proximal to s l i d e scarps a g a i n s t any f u r t h e r development. The area already developed w i t h i n the drainage b a s i n ( F i g u r e l-2a) should be managed i n such a manner as to admit no i n c r e a s e d discharge of water i n t o the s l i d e . (3) R e s t r i c t i o n of the q u a n t i t y of domestic'and i r r i g a t i o n water used i n P a r a d i s e F l a t s i s a matter f o r c o n s i d e r a t i o n by the m u n i c i p a l i t y of Summerland. More d e t a i l e d recommendations cannot be made on the b a s i s of t h i s study. 5.4 Summary and c o n c l u s i o n s From geomorphic c o n s i d e r a t i o n s , i t i s concluded t h a t the P e r p e t u a l S l i d e i s a composite l a n d s l i d e , c o n s i s t i n g of: ( 1 ) Slump movement, r o t a t i o n a l f a i l u r e along a concave upward s u r f a c e which produces a backward t i l t to the f a i l i n g mass, as w e l l as d i f f e r e n t i a l t r a n s l a t i o n of b l o c k s downslope, which r e s u l t s i n graben formation. (2) A t r a n s i t i o n from competent movement to s o i l flow, i n the toe s e c t i o n . The g e o l o g i c study shows t h a t : (1) At depth, the f a i l u r e s u r f a c e l i e s w i t h i n T e r t i a r y sedimentary rocks. Exposed gouge c o n s i s t s p r i m a r i l y of c l a y (most l i k e l y remolded c l a y s t o n e and some c l a y - r i c h 121 t i l l ) w ith d i s p e r s e d pebbles and fragments of c o a l , c l a y s t o n e , and sandstone. (2) D i s c o n t i n u i t i e s i n the form of inhe r e n t h e t e r o -geneity between and w i t h i n l i t h o l o g i e s , weathering, and j o i n t i n g are a l l l i k e l y to i n f l u e n c e m a t e r i a l s t r e n g t h and the ground water flow system, and hence are s i g n i f i c a n t to the u n s t a b l e s i t u a t i o n . (3) Changes i n s t r e s s e q u i l i b r i u m caused by r e d u c t i o n of former l o a d a f t e r b u r i a l and g l a c i a t i o n are probably important i n the o r i g i n of the P e r p e t u a l S l i d e . Of p a r t i c u l a r s i g n i f i c a n c e i s removal of overburden and l a t e r a l support by l a t e and p o s t - g l a c i a l downcutting i n Trout Creek Canyon. From the g e o l o g i c , h y d r o l o g i c , and survey p r o j e c t s i t i s concluded that ground water seems to be the c o n t r o l l i n g f a c t o r i n i t i a t i n g s l o p e i n s t a b i l i t y . (1) F l u c t u a t i o n s i n the ground water t a b l e g r e a t l y i n f l u e n c e s t a b i l i t y . T h i s i s concluded from the h i s t o r i c a l r e c o r d , the a n a l y s i s of the r e g i o n a l ground water flow model, and the survey, which demonstrates a c o r r e l a t i o n between r a t e of movement and discharge of s l i d e s p r i n g s . F u r t h e r , water q u a l i t y data shows high NOg + NO^ c o n c e n t r a t i o n s , which suggest a l a r g e a r t i f i c i a l water input from the c u l t i v a t e d area. (2) A system with high pore-wafer p r e s s u r e i s probable f o r the topographic and p o s t u l a t e d g e o l o g i c c o n f i g u r a t i o n s . T h i s p r e s s u r e , which i s due to c o n d u c t i v i t y c o n t r a s t s and 122 ground water d i s c h a r g e , where a c t i n g w i t h i n and below the f a i l i n g mass, decreases s t a b i l i t y . (3) A high ground water t a b l e with accompanying high pore-water p r e s s u r e s along the f a i l u r e s u r f a c e i s necessary to cause f a i l u r e i n the most c r i t i c a l s t a b i l i t y models. T h i s c o n c l u s i o n r e s u l t s from the s t a b i l i t y a n a l y s i s which u t i l i z e d ^ r = 18.5, the value o b t a i n e d from d i r e c t shear t e s t i n g , and other assumed parameters. The proposed mechanism of f a i l u r e i s : (1) The p r o g r e s s i v e r e d u c t i o n from peak to r e s i d u a l s t r e n g t h of the c l a y s t o n e . (2) In a d d i t i o n t o a h i g h water t a b l e , h i g h pore-water p r e s s u r e s along the f a i l u r e s u r f a c e . Remedial measures i n c l u d e improving s u r f a c e drainage to reduce s u r f a c e input of water and zoning to prevent d e v e l -opment which c o u l d i n c r e a s e the input of water to the s l i d e . The P e r p e t u a l L a n d s l i d e i s a phenomenon o c c u r r i n g i n response to a conducive g e o l o g i c environment and to i n i t i -a t i o n by an a r t i f i c i a l i n c r e a s e i n ground water flow. Since t h i s i n i t i a t i o n , the s t r e n g t h of the f a i l u r e m a t e r i a l has decreased to a r e s i d u a l value. The s l i d e ' s continuous move-ment now a c t s to e s t a b l i s h a s t a b i l i t y i n e q u i l i b r i u m with the ground water flow system and t h i s lower s t r e n g t h . The study of the s l i d e has demonstrated how a s u r f i c i a l g e o l o g i c a l and h y d r o l o g i c a l a n a l y s i s can h e l p e v a l u a t e s l o p e s t a b i l i t y and guide any f u r t h e r subsurface i n v e s t i g a t i o n s . I t has a l s o 123 r e a f f i r m e d the need to understand the g e o l o g i c and h y d r o l o g i c environment before development i n order t o avoid subsequent slope f a i l u r e . 124 BIBLIOGRAPHY A g r i c u l t u r e Canada, 1974-1976, Weather o b s e r v a t i o n s : Research S t a t i o n , Summerland, B.C. Anderson, J . D., 1975, Slope s t a b i l i t y by n o n - c i r c u l a r a n a l y s i s : Dept. of C i v i l E n g i n e e r i n g , Univ. of B r i t i s h Columbia, Vancouver, B. C. B.C. 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L., 1976, Strontium i s o t o p e study of the composite b a t h o l i t h between P r i n c e t o n and Okanagan Lake: Canadian J o u r n a l of E a r t h Sciences v. 13, no. 11, p. 1577-1583. R i t c h i e , A. M., 1958, R e c o g n i t i o n and i d e n t i f i c a t i o n of l a n d s l i d e s : L a n d s l i d e s and E n g i n e e r i n g P r a c t i c e , (ed. E. B. E c k e l ) , Highway Research Board S p e c i a l Report 2,9, pp. 48-68. S c o t t , J . S. and Brooker, E. W., 1968, G e o l o g i c a l and e n g i n e e r i n g aspects of Upper Cretaceous s h a l e s i n Western Canada: Geol. Surv. Can., Paper 66-37, 75 pp. Skempton, A. W., 1964, Long-term s t a b i l i t y of c l a y s l o p e s Geotechnique, v. 14, no. 2, pp. 77-102. Skempton, A. W., and Hutchinson, J . N., 1969, S t a b i l i t y of n a t u r a l s l o p e s and embankment foundations: Proc. of the 7th I n t e r n a t . Conf. on S o i l Mech. and Found. 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Freehan and Company, pp. 149-15 7. P e r s o n a l communication: L. M. L a v k u l i c h , 1976, Department of S o i l S cience, Univ. of B r i t i s h Columbia, Vancouver, B.C. P e r s o n a l communication: K. Madsen, 1975, Summerland, B.C. P e r s o n a l communication: W. H. Mathews, 1977, Dept. of Geol. S c i e n c e s , U n i v e r s i t y of B r i t i s h Columbia, Vancouver, B . C . P e r s o n a l communication: R. E. Rouse, 1977, Dept. of Geol. Sciences, U n i v e r s i t y of B r i t i s h Columbia, Vancouver, B. C. Appendix 4-1 Movement of Stakes With Time • K E Y -C U M U L A T I V E H O R f Z O N T A L D E P A R T U R E O F S T A K E F R O M S T R A I G H T S U R V E Y L I N E (measured perpendicular to l ine) C U M U L A T I V E C H A N G E I N E L E V A T I O N O F S T A K E M O N T H L Y A V E R A G E H O R I Z O N T A L D E P A R T U R E O F S T A K E F R O M S T R A I G H T S U R V E Y L I N E (measured perpendicular to line) M O N T H L Y A V E R A G E C H A N G E I N E L E V A T I O N O F S T A K E 12.5 i 10.0 -\ 75 J CO or UJ UJ s ACTUAL H O R I Z O N T A L MOVEMENT OF S T A K E = 3.15m (JU L Y 1 9 7 5 t o MAY 1976) TRUE D I R E C T I O N OF MOVEMENT N I63°I0'E 5.0 H 2.5 H O R I Z O N T A L M O V E M E N T D O W N f S L f D E D T M 1 A 1 M 1 J 1 J 1 9 7 5 "L'JlJ N » 1 H O R I Z O N T A L M O V E M E N T UP S L I D E L O S S IN E L E V A T I O N ' - " " * T i * - * —* ' J GAIN IN 1 9 7 6 ELEVAT ION l—' 1,2.5 >i I 1975 I 1976 1 2 . 5 1 1 9 7 5 1 1976 12.5-1 !975 I 1976 141 (S3 cn 2 lu. 0 METERS :z 2tt Appendix 4-2 S t r e s s - S t r a i n Curves f o r Drained Shear T e s t s t—• 0 . 4 0 . 3 1 2 , 4 7 0 p.s.f, { 6 . 0 9 .-kg/cm2) -++-o 0 . 2 0.1 T E S T + F O R W A R D | o F O R W A R D ^ 0 . 1 2 X 1 0 " 3 . 6 X ! 0 o F O R W A R D j 0 . 9 2 X I 0 " 5 2 7 . 9 X I 0 " 5 A V E R A G E R A T E O F S T R A I N ( f e e t / m i n ) ( c m / m i n ) 0 . 4 1 X 1 0 * 5 12.5 X I 0 " 5 -5 ( p . s . f . ) T ( k g / c m 2 ) fir 3 9 9 0 1 . 9 5 1 7 . 7 ° 4 1 0 0 2 . 0 0 1 8 . 2 ° 4 2 0 0 2 . 0 5 1 8 . 6 ° 4 ! 0 0 2 . 0 0 1 8 . 2 ° 0 0.010 _L ° - . ° a 0 (feet) ° - 0 3 0 _L J. 0.040 I 0 . 2 0 0 . 4 0 0 . 6 0 ( C m ) 0 . 8 0 1 . 0 0 1 . 2 0 HORIZONTAL DISPLACEMENT j. 0.050 1 . 4 0 M -P-0.4 0.3 L o in UJ cm w 0.1 L 0.2 8150 p.s.f. (3.98kg/cmr) + TEST RATE OF STRAIN ( f e e t / m i n ) ( c m / m i n ) (p.s . f + FORWARD | 0.13 X I 0 " 4 3.8 X 10" 4 2 5 3 0 F0RWARDj a - 4 4 o 0.10 X 1 0 3 . 0 X 10" 2 7 8 0 o F0RWARD 2 0 .05X I 0 " 4 I .5X 10" 4 2 7 8 0 0 FORWARD 3 0 . 0 8 X I 0 " 4 2 . 3 X 10" •4 2 7 7 0 AVERAGE 2710 ( k g / c m ^ ) 1.24 I . 3 6 1.36 1.35 1.33 17.2° 18.8° 18.8° 18.8° 18.4° 0 0 0-010 0.20 a 0 2 ° ( f e e t ) 0 0 3 0 I 0 .040 I 0.40 0 . 6 0 t c m j 0.80 1.00 1.20 HORIZONTAL DISPLACEMENT .40 i 0.050 0.4 03 /<<*-o \— < Q2 rr CO co LU or h-</> o.i = 4l50p.S.f (2.03 kg/cm2) - if TEST RATE OF STRAIN 0r' { f e e t /min) (cm /min) (p. s . f . ) ( k g / c m 2 ) 4- FORWARD4 0.14 X I 0 " 4 4.3 X 1 0 " 4 1 450 0.71 19.3° ?/ 0 FORWARD* 0.15 X ! 0 " 4 4.6 X 1 0~ 4 1460 0.71 19.4* r 0 F0RWARD 6 0.I8X ! 0 ' 4 5.3 X 1 0 " 4 1 4 0 0 0.68 18.6° o FORWARD7 0.30X I 0 " 4 9.1 X I0" 4 . 147 0 0.72 19.5° 1 1 AVERAGE l I 14 4 0 | 0.71 19.2° 0 .010 1 I I 1 1 .030 1 .040 1 ! .050 0.20 0.40 0.60 {cm) ° - 8 0 1.00 1.20 1.40 HORIZONTAL DISPLACEMENT t—• CTl F i g u r e I~2Q. C a t c h m e n t b a s i n o f t h e P e r p e t u a l L a n d s l i d e LEGEND CONTOUR LINE (C . I . = 100 FT. ) » SURFACE DRAINAGE —> (EPHEMERAL) SLIDE BOUNDARY ~ CATCHMENT BASIN BOUNDARY ) IRRIGATION FLUME IRRIGATION DITCH (UNLINED) DOMESTIC WATER LINE NEW PRESSURIZED WATER LINE — ROAD POND A' B' C R O S S S E C T I O N ' ' L I N E CULTIVATED (IR-RIGATED) AREA WITHIN CATCH-MENT BASIN ° ( M E T E R S ) 1 5 0 MT. C O N K L E PARADISE F L A T S S L I D E F i g u r e 3 - 3 a . C r o s s s e c t i o n A ' - B 1 . B' 70 O O m m n 3coo i 9 0 0 2500 750 UJ Ui Ul Y-Ul S 2000 600 I5C0 450 * > ROAD CONTOUR (INTERVAL, iOO F I G U R E 2 - l a GENERAL GEOLOGY OF THE PERPETUAL LANDSLIDE AREA SCALE 300 600 FEET 100 200 METERS APPROXIMATE MAGNETIC DECLINATION, 23 e EAST 600 900 1 I 1200 L E N G T H IN F E E T 500 1800 J 2! 00 2400 F I G U R E 2 - l c LONGITUDINAL PROFILE OF THE PERPETUAL LANDSLIDE ALONG LINE A - B (SEE FIGURE 2-lb) Figure 3-4 Pleistocene Terraces oooos fn oil c o s r i . o r iter ropy It parallel re bedding QUATERNARY f~n RECENT FLOODPLAIN [~g~| TERRACE GRAVEL |~3~[ DELTA IC SANDS AND GRAVELS f l - ] GLACIO LACUSTRINE FINE SANDS 1 1 AND S ILTS H3 T ILL TERTIARY COAL / CLAYSTONE / VOLCANIC X SANDSTONE ACIDIC TUFFS ' VITRlC LITHIC CLASTS CLASTS TERTIARY OR OLDER [8~1 RHYOLITE PORPHYRY ["in GRANITE PEGMATITE [To] PORPHYRITIC MAFIC PRE-TERTIARY T f l GRANODIORITE 1 2 12 0 b L IMESTONE/ IMPURE SANDSTONE GEOLOGICAL BOUNDARY (DEFIN E D, A PPROXI MATE) BEDDING (INCLINED, HORIZONTAL) CROSS-BEDDlNG GNEISSOSITY FAULT (DEFINED, APPROXIMATE, ASSUMED) JOINT SLICKENSIDE IRON ALTERATION CROSS SECTION LINE R E M O L D E D T O G O U G E W I T H I N S L I D E SCALE 600 100 METERS 200 APPROXIMATE MAGNETIC DECLINATION, 23° EAST DETAILED GEOLOGY OF THE PERPETUAL LANDSLIDE AND TROUT CREEK CANYON 

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