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Investigation of the stability of the steepland forest soils in the coast mountains, southwest British… O'Loughlin, Colin Lockhart 1972

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(51MI AN- INVESTIGATION  OF THE  STABILITY  OF THE STEEPLAND FOREST S O I L S IN THE COAST MOUNTAINS, SOUTHWEST B R I T I S H COLUMBIA  by  COLIN LOCKHART O'LOUGHLIN M.Sc.  (Geol.), University  A THESIS SUBMITTED THE  of Canterbury  ( N Z ) , 1968  IN PARTIAL FULFILMENT OF  REQUIREMENTS  FOR THE DEGREE OF  DOCTOR OF PHILOSOPHY (Forest  in the  Hydrology)  Faculty of  Forestry  We a c c e p t t h i s  thesis  as c o n f o r m i n g  to the required  THE UNIVERSITY OF B R I T I S H O c t o b e r 1972  COLUMBIA  standard  In p r e s e n t i n g t h i s t h e s i s  i n 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 degree 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  t h a t the 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  study.  I f u r t h e r agree 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  for  s c h o l a r l y purposes may be g r a n t e d by t h e Head o f my Department o r by h i s representatives.  It  i s understood 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 .  Department o f F o r e s t r y 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 , Vancouver 8, Canada.  ABSTRACT  In B r i t i s h Columbia's Coast Mountains a large area of forested 2 steepland of probably more than 100 km , i s c l e a r f e l l e d annually by high-lead logging methods.  Although the deforestation of steep slopes  has been shown to s e r i o u s l y accelerate mass wasting i n Alaska, Oregon and elsewhere in the United States, the e f f e c t s of c l e a r f e l l i n g on slope s t a b i l i t y are l a r g e l y unknown f o r coastal B r i t i s h Columbia.  The  objective of t h i s study was to determine the extent and seriousness of l a n d s ! i d i n g on undisturbed, forested slopes and on c l e a r f e l l e d slopes and e l u c i d a t e some of the natural and human-caused f a c t o r s which are detrimental to slope s t a b i l i t y i n the Coast Range. 2 Within a study area of 640 km , 77 large landslides of the debris avalanche or debris s l i d e type were discovered.  Landslides not associated  with roads were predominantly confined to long, uniform slopes of over 30° underlain by poorly-drained podsolic s o i l s .  Slopes underlain by  shallow, regosolic s o i l s were r e l a t i v e l y r e s i s t a n t to mass wasting. Large landslides were more frequent on c l e a r f e l l e d areas than on undisturbed slopes.  Road c o n s t r u c t i o n , which was responsible f o r 14 large  landslides and more than 100 smaller f a i l u r e s , appeared to be more d e t r i mental to the s t a b i l i t y of the Coast Range slopes'  than other a c t i v i t i e s  c a r r i e d on by man. A network of simple piezometers established in steep drainage depressions revealed that the piezometric surface w i t h i n the s o i l mantle approached the ground surface during r a i n storm or snow melt periods to cause marked increases in pore water pressures.  Curvilinear relationships  between the piezometric head and the d a i l y (24 hr.) r a i n f a l l indicated that d a i l y r a i n f a l l s which exceeded approximately 120 mm caused complete  i i  saturation of most drainage depression s o i l s . 2 larger than 800 kg/m  Pore water pressures  were recorded at the base of the s o i l mantle.  Such pressures s i g n i f i c a n t l y reduce the e f f e c t i v e normal stress a c t i n g on potential f a i l u r e planes at the s o i l - b a s a l t i l l or soil-bedrock i n t e r face and decrease the s t a b i l i t y of the slope. General observations of the growth habits of tree root systems in the steepland s o i l s suggested that roots help bind the cohesionless s o i l s i n t o a coherent mantle and anchor the mantle to the substratum. Direct shear tests performed in the f i e l d indicated that the s o i l strength was l i n e a r l y r e l a t e d to the bulk weight of roots in the s o i l .  Where root  networks were dense the s o i l ' s shear strength may be increased by several . 2 hundred kg/m  compared to s o i l s with few or no roots.  Under saturated  conditions the f o r e s t s o i l s ' shear strengths are l a r g e l y derived from the apparent cohesion provided by the tree root network.  Laboratory strength  t e s t s of tree roots showed that Douglas f i r and cedar tree roots d e t e r i o r a t e r a p i d l y a f t e r death of the parent t r e e .  Within 3 to 5 years a f t e r c u t t i n g  of the parent tree, small roots may lose over h a l f of t h e i r o r i g i n a l t e n s i l e strength. Five small landslides were investigated with simple, approximate, s t a b i l i t y analyses encompassing a range of possible shear strength and s o i l water conditions.  The r e s u l t s confirmed that the s t a b i l i t y of r o a d - f i l l  slopes as well as n a t u r a l , f o r e s t s o i l slopes  is  very s e n s i t i v e to changes  in the ground water conditions and that the s t a b i l i t y of the f o r e s t s o i l mantle on steeper slopes i s very dependent on the reinforcement provided by roots.  In the l i g h t of these findings i t i s apparent that current  forest c u t t i n g and road b u i l d i n g practices on the steep slopes of B r i t i s h Columbia's Coast Mountains are not compatible with sensible mountainland management which fosters protection of the s o i l resource.  iii  TABLE OF CONTENTS Page INTRODUCTION CHAPTER 1  1 LANDSLIDES AND THEIR IMPORTANCE IN THE COAST MOUNTAINS  CHAPTER 2  3  Introduction  3  Physical s e t t i n g of the study area  3  F i e l d data c o l l e c t i o n  12  Use of a e r i a l photographs  13  Types of mass wasting  17  S i g n i f i c a n c e of landslides in the study area  24  Relationship of landslides to s i t e factors  27  E f f e c t of c l e a r c u t t i n g on l a n d s l i d e occurrence  33  Road construction and l a n d s l i d e s  38  Natural revegetation of l a n d s l i d e scars  43  Discussion and conclusions  43  PIEZOMETRY STUDIES ON STEEP SLOPES  46  Introduction  46  Piezometric instrumentation  51  Results  55  Relationship between piezometric head, r a i n f a l l  g 3  and subsurface flow  CHAPTER 3  Estimation of maximum pore water pressures  66  Pore water pressures and e f f e c t i v e stresses  68  Conclusions  68  THE IMPORTANCE OF TREE ROOTS TO SLOPE STABILITY  71  Introduction  71  Rooting habits of f o r e s t trees on the Coast Range slopes A f i e l d study of the e f f e c t of tree roots on the  72 75  s o i l ' s resistance to shear Laboratory t e s t i n g of root strength Summary and conclusions  go 104  iv Page CHAPTER 4  A DETAILED EXAMINATION OF SELECTED LANDSLIDES  106  Introduction  106  S t a b i l i t y analysis  107  Landslide 1 (road-cut slope)  110  Landslide 2 (road-cut slope)  114  Landslide 3 ( r o a d - f i l l slope)  117  Landslide 4 ( r o a d - f i l l slope)  121  Landslide 5 (natural slope)  124  Concluding remarks  127  BIBLIOGRAPHY APPENDIX 1  130 DETAILS OF MAJOR STORM RAINFALLS AT SEYMOUR FALLS AND CLEVELAND DAM, SOUTHWESTERN BRITISH  138  COLUMBIA, 1959-1970 APPENDIX 2  TECHNIQUES FOR CALCULATING VOID RATIOS AND SATUR-  ^  ATED UNIT WEIGHTS APPENDIX 3  DETAILS OF SLOPE ANALYSES  141  LIST OF TABLES  Table 1  Page Some mean physical properties of steepland s o i l s and unweathered t i l l  9  from the Coast Range,  southwestern B.C. 2  Details of high a l t i t u d e c o l o r and c o l o r i n f r a r e d  14  photography taken 16-7-71 over southwestern B.C. 3  Information obtainable from various types of aerial  4  19  photographs.  A summary of l a n d s l i d e survey data c o l l e c t e d i n  25  the B r i t i s h Columbia Coast Range, 1970. 5  Landslide number, area and quantity of debris moved 26 2  per km of the study area 6  Numbers of landslides before and a f t e r logging, Coast 34 Range, southwestern B.C.  7  Landslide d e n s i t i e s and q u a n t i t i e s of l a n d s l i d e debris produced w i t h i n selected watershed areas i n the Coast Range, southwestern B.C.  8  36  Landslide d e n s i t i e s on forested and c l e a r f e l l e d  37  mountain slopes f o r regions i n northwestern North America and northern Japan. 9  Maximum piezometric head and equivalent pore water pressure recorded at piezometer stations i n the  56  Coast Range, southwestern B.C. 10  Regression equations describing r e l a t i o n s h i p s between piezometric head (Y) and d a i l y r a i n f a l l  (X), Coast  52  Range, southwestern B.C. 11  Saturated permeability c o e f f i c i e n t s of steepland  55  s u b s o i l s , Coast Range, southwestern B.C. 12  Results of d i r e c t shear t e s t s on s o i l s containing roots.  si  Page 13  Results soils  14  of d i r e c t shear t e s t s  w i t h no  2 Diameter  (cm) a n d t e n s i l e s t r e n g t h  (kg/cm )  of  sampled from l i v i n g t r e e s  and  roots  Results  trees.  f o r Douglas  cedar root strength Total  d e f o r m a t i o n Al  red  at the rupture 2  point  a n d m o d u l u s o f e l a s t i c i t y E (kg/cm ) f o r  cedar  Means, extremes diameters of  (cm)  Factors  roots  of safety  method o f s l i c e s (BMS)  tested in  and s t a n d a r d  45 cm a t l a n d s l i d e 18  f i r and w e s t e r n  98  data.  and D o u g l a s f i r r o o t s 17  96  from  o f a n a l y s e s o f v a r i a n c e and m u l t i p l e  range t e s t s  16  87  steepland  roots.  stumps o f f o r m e r 15  on  growing  100  tension.  deviations  in subsoils  of  103  below  headscarps. (FS)  (OMS)  c a l c u l a t e d by t h e  ordinary  and by t h e B i s h o p m e t h o d  f o r a road-cut slope  at the s i t e of  113  Landslide  1. 19  Factors  of safety  method o f s l i c e s (BMS)  (FS)  c a l c u l a t e d by t h e  ordinary  (OMS)  a n d by t h e B i s h o p  method  f o r a road-cut slope at the s i t e of  116  Landslide  2. 20  21  22  Factors  o f s a f e t y c a l c u l a t e d by t h e o r d i n a r y  of s l i c e s  for a road-fill  Landslide  3.  Factors  for a road-fill  Landslide  4.  for  slope at the s i t e  o f s a f e t y and t h e a p p a r e n t c o h e s i o n  s t a b i l i t y for a natural  Landslide  5.  121  of  o f s a f e t y c a l c u l a t e d by t h e o r d i n a r y  of s l i c e s  Factors  slope at the s i t e  method  method 123  of  required  slope at the s i t e  of  126  vii LIST OF FIGURES  Figure 1  Page Map of study area showing l o c a t i o n of  4  lands!ides. 2  Oblique c o l o r a e r i a l photograph of study area.  7  3  P a r t i c l e s i z e d i s t r i b u t i o n curves f o r f o r e s t  ^  s o i l B horizons. 4  Stereogram of part of the lower Capilano catchment.  15  5  Color i n f r a r e d stereogram of part of the middle  -jg  Seymour V a l l e y . 6  Small debris s l i d e in upper Harvey Creek catchment.  7  An almost v e r t i c a l headscarp of a large debris  21  avalanche i n the middle Seymour catchment. 8  The upper part of a large debris avalanche i n the  22  Seymour catchment. 9 10  A debris avalanche in second growth f o r e s t . Top  D i s t r i b u t i o n of Landslides with respect to  slope  29  Bottom 11  Top  22  Longitudinal p r o f i l e s of selected l a n d s l i d e s .  D i s t r i b u t i o n of l a n d s l i d e s with respect to  aspect Bottom  3 0  D i s t r i b u t i o n of l a n d s l i d e s with respect to  altitude. 12  Smooth-surfaced d i o r i t e bedrock forming the s l i d i n g surface near the head of a debris avalanche in the  ^  upper Capilano catchment. 13  Road-caused debris avalanches r e s u l t i n g from the collapse of r o a d - f i l l and s i d e - c a s t materials on a  4  ^  39° slope in the Howe Sound area. 14  Flow net and neutral stresses f o r a s l o p i n g , i s o t r o p i c , saturated s o i l mantle r e s t i n g on impermeable substratum.  48  viii Page  Figure  15  Topographic  and s o i l  instrumented with 16  Details the  17  18  c h a r a c t e r i s t i c s of the s i t e s  piezometers.  o f t h e two t y p e s  of  piezometers  used  in  54  study.  P i e z o m e t r i c head v a r i a t i o n s a n d 1972  at stations  Capilano  catchment.  Cl-4  during  1970,  and C l - 5 ,  1971 57  upper  P i e z o m e t r i c head r e s p o n s e s a t t h r e e s t a t i o n s the upper C a p i l a n o catchment d u r i n g  19  52  in  a rainstorm  58  i n September,  1971.  Relationships  b e t w e e n p i e z o m e t r i c h e a d and d a i l y  gg  rainfall. 20  Relationships  between p i e z o m e t r i c head and  daily  b e t w e e n p i e z o m e t r i c h e a d and  daily  60  rainfal1. 21  Relationships  61  rainfall. 22  Hypothetical surface  23  r e l a t i o n s h i p between r a i n f a l l ,  E f f e c t i v e normal  stress  w i t h no s e e p a g e and  25  26  (b)  a moist,  sloping  slide  soil  a saturated, sloping  mantle  The e x p o s e d  root system of a r e c e n t l y overturned  western red  cedar.  R e c e n t l y exposed western  red cedar r o o t  an o p e n j o i n t i n d i o r i t e  bedrock.  of shearbox  S h e a r l o a d vs direct  shear  tests  74  c o n s t r u c t i o n and i t s  displacement curves on s t e e p l a n d s o i l s  -j^  penetrating  mode  of  field.  shear  69  soil  seepage.  operation in the 27  (a)  a c t i n g on a p o t e n t i a l  mantle with  Details  64  seepage and p i e z o m e t r i c h e a d .  plane i n the case of  24  sub-  78  for four  containing  82 roots.  Page Results of d i r e c t shear t e s t s on steepland s o i l s  88  plotted on a shear stress (T) vs normal stress (a) diagram. Instron Universal Testing Machine.  93  Details of pneumatic holding clamps.  93  Results of t e n s i l e strength t e s t s on three Douglas f i r roots i n d i f f e r e n t stages of d e t e r i o r a t i o n . Diagram showing the change i n t e n s i l e strength of small Douglas f i r and cedar roots with change i n the  95 97  time elapsed since f e l l i n g of the parent t r e e . Formation of tension cracks i n f i l l  materials  near the outer edge of an abandoned logging road. I n c i p i e n t l a n d s l i d i n g along an abandoned logging road in the Howe Sound area. Small, c i r c u l a r debris s l i d e in a road-cut slope  109  109 m  (Landslide 1). Debris s l i d e in a road-cut slope (Landslide 2 ) ,  115  upper Magnesia Creek catchment. Small, c i r c u l a r f a i l u r e i n a 34° r o a d - f i l l  slope  (Landslide 3 ) , upper Harvey Creek catchment. Results of d i r e c t shear t e s t s on r o a d - f i l l materials plotted on a shear stress  (T) vs normal stress  (a)  diagram. Shallow, c i r c u l a r f a i l u r e i n a r o a d - f i l l  slope  (Landslide 4 ) , upper Magnesia Creek catchment. Longitudinal p r o f i l e of a shallow debris s l i d e on a 30°, c l e a r f e l l e d slope (Landslide 5 ) , upper Harvey Creek catchment.  118  X  ACKNOWLEDGEMENTS  I am i n d e b t e d t o Dr. B.C. G o o d e l l f o r a d v i c e and d u r i n g most phases o f t h i s p r o j e c t and t o Dr. D.S.  assistance  L a c a t e , Dr. A.  Kozak,  Dr. R.G. Campanella and Dr. 0. Slaymaker f o r c o n s t r u c t i v e comments and a d v i c e d u r i n g the p r e p a r a t i o n o f t h i s  thesis.  I am a l s o i n d e b t e d t o the New Zealand F o r e s t S e r v i c e f o r  granting  me s t u d y l e a v e and f i n a n c i a l s u p p o r t t o undertake t h i s p r o j e c t . A d d i t i o n a l f i n a n c i a l s u p p o r t was p r o v i d e d by t h e Department o f the Environment  (NCWRR), by the G r e a t e r Vancouver Water D i s t r i c t and by the  Van Dusen Graduate F e l l o w s h i p i n  Forestry.  1 INTRODUCTION  For over 50 years the forested steeplands of B r i t i s h  Columbia's  Coast Mountains have been managed p r i m a r i l y f o r the production of timber.  T r a d i t i o n a l l y , the c l e a r c u t t i n g of large areas of mature  forests with high lead methods, often followed by broadcast burning of the cleared areas, i s applied as a general si 1 v i c u l t u r a l system. b u i l d i n g usually accompanies c l e a r f e l 1 i n g .  Road  Under t h i s type of management  large areas of steep slopes, often several square kilometers i n extent, may remain in an e s s e n t i a l l y unvegetated state f o r , perhaps, 5 years or more u n t i l natural regrowth vegetation or a r t i f i c i a l l y planted c o n i f e r seedlings gradually r e e s t a b l i s h a mantle of plant cover. Within the Coastal Forest Zone of B r i t i s h Columbia approximately 2 360 km  (90,000 acres) of f o r e s t are c l e a r f e l l e d annually (B.C.  Service, 1957).  Forest  Although the records of areas c l earfel fled annually do  not include information on land slope, i t i s c e r t a i n that a large acreage of steeplands is logged over each year.  Possibly a t h i r d or more of the  t o t a l area cut over annually w i t h i n the Coastal Forest Zone, some 120 to 140 km (30,000 to 40,000 acres) has slopes exceeding 20°. 2  This estimation  i s regarded as being not unreasonable by Dr. Smith, Faculty of Forestry, U n i v e r s i t y of B r i t i s h Columbia (personal  communication).  On steep, forested lands i n southeast Alaska, i n central Oregon and in northern Idaho, accelerated rates of mass wasting have been associated with c l e a r f e l l i n g (Bishop and Stevens 1964, Dyrness 1967 and Gonsior and Gardner 1971).  However, i n s p i t e of the importance of  c l e a r f e l l i n g on the steeplands of western B r i t i s h Columbia, the e f f e c t s  2  on s o i l s t a b i l i t y in t h i s region have not, to the present w r i t e r ' s knowledge, come under i n v e s t i g a t i o n .  In f a c t , the p o s s i b i l i t y that  c l e a r f e l l i n g on steep slopes may r e s u l t i n accelerated e r o s i o n , erosibn i n t h i s instance being used in i t s broadest sense, has not, i n general, been given more than passing a t t e n t i o n . out that  Golding (1968) points  erosion i s not generally recognised as a problem in  B.C. . . . . " although, he also indicates that erosion on logged areas i n the Province may be l o c a l l y severe.  J e f f r e y (1968) a t t r i b u t e s t h i s lack  of i n t e r e s t i n the welfare of the s o i l resource to the f a c t that land management in western Canada i s so strongly devoted to wood production that other e f f e c t s of logging, such as increased sediment y i e l d s , are not taken i n t o c o n s i d e r a t i o n . This thesis examines the natural s t a b i l i t y of the forested t e r r a i n over a small portion of the Coast Mountains i n southwestern B r i t i s h Columbia and the changes which occur in slope s t a b i l i t y when the f o r e s t vegetation i s removed from steep slopes.  The i n v e s t i g a t i o n i s  wholly concerned with mass wasting, that i s , the downward movement of s o i l and rock materials under the influence of g r a v i t y without benefit of the c o n t r i b u t i n g force of independent agencies such as flowing water or wind (Leopold et al_ 1964).  The study should help to place the  importance of mass wasting w i t h i n the Coast Range environment in correct perspective and e l u c i d a t e some of the natural and human-caused factors which are detrimental to the s t a b i l i t y of steep, forested slopes.  3 CHAPTER 1  LANDSLIDES AND THEIR IMPORTANCE IN THE COAST MOUNTAINS  Introduction During 1970 a combined f i e l d and a e r i a l photograph survey was completed of the types, numbers and volumes of l a n d s l i d e s on undisturbed forested slopes and c l e a r f e l l e d slopes over a segment of the Coast 2 Mountains.  The area investigated forms a 640 km , t r i a n g u l a r block  of steep mountain country l y i n g between Howe Sound and Indian Arm north of the c i t y of Vancouver (Figure 1).  A major question was  whether or not the large scale c l e a r f e l l i n g operations, which are employed in the Coast Mountains, s e r i o u s l y influence the s t a b i l i t y of steep slopes.  This chapter presents physiographic information of the  area and of i n d i v i d u a l l a n d s l i d e s i t e s to provide information on which to base the s p e c i f i c studies of Chapters 2, 3 and 4. Physical s e t t i n g of the study area Geology.  The area investigated forms part of the southern end of  a large complex of plutonic and metamorphic rocks that characterises the B r i t i s h Columbian coastal geology.  The bedrock i s composed of  many d i o r i t i c bodies of d i f f e r i n g age and s i z e , numerous i s o l a t e d masses of volcanic and sedimentary rocks v a r i o u s l y i n t e r p r e t e d as roof pendants or remnants of older t e r r a i n and, i n a d d i t i o n , vast q u a n t i t i e s of gneiss and migmatite (Armstrong 1965, Roddick 1965).  The most  common rock types are d i o r i t e and quartz d i o r i t e in which hornblend i s  Figure  1.  Map o f study area showing  location of  landslides.  5  more abundant than b i o t i t e .  Armstrong estimates that p l u t o n i c rocks  underlie approximately 80 percent of the area in the v i c i n i t y of Howe Sound. Natural outcrops and roadcut exposures of the d i o r i t e rock generally display smooth, s u p e r f i c i a l l y weathered surfaces r e l a t i v e l y f r e e of open j o i n t s , i n d i c a t i n g that the bedrock i s , f o r the most p a r t , impermeable. For t h i s reason deep seepage i s not considered to be an important part of the hydrology of the v a l l e y sides w i t h i n the study area.  Exceptions to  t h i s do occur, however, where deeply weathered and f r a c t u r e d b r e c c i a s , f i n e grained sedimentary rocks, and low grade s c h i s t s belonging to the Gambier Group (Roddick 1965) form the basement rock in the Howe Sound area. Pleistocene g l a c i a l i c e inundated most of the area covered by the survey except, p o s s i b l y , the highest peaks above 1,800 meters (Geological Association of Canada 1958).  At l e a s t three major g l a c i a t i o n s (Seymour,  Semiamu and Vashon) affected the southern parts of the Coast Range while the existence of a fourth g l a c i a t i o n (Sumas) of more l i m i t e d extent has been proposed by Armstrong (1956). The r e t r e a t i n g ice l e f t widespread t i l l ness f l a n k i n g the v a l l e y slopes. Seymour v a l l e y s reveal t i l l  Exposures in Howe Sound, Capilano and  thicknesses in excess of 4 m at a l t i t u d e s  below 700 m but at higher a l t i t u d e s the t i l l distributed.  deposits of v a r i a b l e t h i c k -  i s thinner and haphazardly  Except where i t i s weathered, the t i l l  i s grey in c o l o u r ,  highly compacted and impermeable to water, presumably a consequence of the great i c e pressures which exerted themselves on the v a l l e y slopes during the g l a c i a t i o n s .  The t i l l  possesses a hard consistency but tends  to rupture along planes approximately p a r a l l e l to the slope surface when h i t with a maddock.  Nevertheless, i t s high shear strength i s evidenced  by i t s obvious s t a b i l i t y .  6  The presence of t h i s unweathered t i l l s t a b i l i t y of the s o i l overburden.  substratum influences the  Not only does the t i l l  surface form  an impermeable b a r r i e r to downward water flow, thereby encouraging saturation of the s o i l mantle during storm periods, but i t also determines the  general lower l i m i t of root penetration; only o c c a s i o n a l l y do s o l i t a r y  tree roots enter cracks in the compacted t i l l .  Furthermore, the s o i l - t i l l  i n t e r f a c e i s well defined and often smooth i n l o n g i t u d i n a l p r o f i l e , causing i t to act as a s l i d e plane.  Details of the f i l l ' s  physical  properties are presented in Table 1. The topography strongly r e f l e c t s the youthfulness of the mountains and the e f f e c t s of the g l a c i a t i o n s (Figure 2 ) .  The higher peaks, of  maximum a l t i t u d e 2,025 meters, are connected by uneven ridges interrupted by deep transverse saddles and steep-walled narrow v a l l e y s .  The average  r e l i e f approximates '1,200 meters.  SoiIs.  Large and often abrupt v a r i a t i o n s occur in s o i l depths,  p r o f i l e development, textures and drainage conditions from one part of the  same slope to another.  Lesko (1961) and Brooke (1966) described  and c l a s s i f i e d the s o i l s of the southern and eastern parts of the area according to the National S o i l Survey Committee's c l a s s i f i c a t i o n system for  Canada.  More recently a s o i l survey of the area by the B r i t i s h  Columbia Department of A g r i c u l t u r e , S o i l Survey D i v i s i o n resulted in the recognition and mapping of more than twenty s o i l s e r i e s . the  However, f o r  purposes of the present l a n d s l i d e study, three broad, e a s i l y recog-  niseable categories of steepland s o i l s were distinguished on the basis of t h e i r drainage c h a r a c t e r i s t i c s , parent material and p r o f i l e development. Category 1. S o i l s developing on moderately to well drained s i t e s over t i l l , colluvium or alluvium.  These s o i l s show good horizon develop-  Figure 2.  An o b l i q u e c o l o r a e r i a l  photograph o f s t u d y a r e a .  IA,  I n d i a n Arm; SV, Seymour V a l l e y ; CV, C a p i l a n o V a l l e y ; HS, Howe Sound.  8  ment with i n c i p i e n t to s u b s t a n t i a l signs of p o d s o l i z a t i o n .  L, F and  H horizons together are usually less than 35 cm deep while the Ae horizon, i f present, i s less than 5 cm t h i c k .  The B horizon ranges in colour from  l i g h t y e l l o w i s h brown to dark brown, normally possesses weak crumb or weak blocky s t r u c t u r e , i s usually a g r a v e l l y or bouldery sandy loam i n texture and i s f r i a b l e without any noticeable accumulation of c l a y . horizons r a r e l y exceed 90 cm i n t h i c k n e s s .  B  In terms of the Canada S o i l  Survey Committee's s o i l c l a s s i f i c a t i o n f o r Canada (Canada Department of A g r i c u l t u r e , 1970) these s o i l s range from Orthic Ferro Humic Podsols to Mini Ferro Humic Podsols. Category 2. S o i l s developing on poorly drained s i t e s over unweathered till  or bedrock.  Drainage depressions, seepage hollows and f l a t midslope  benches are usually characterised by waterlogged s o i l s l a c k i n g well marked e l u v i a l and i l l u v i a l horizons.  The L, F and H horizons u s u a l l y  t o t a l less than 20 cm deep and are underlain by a gleyed B horizon up to 100 cm deep.  The B horizon i s very stoney and may contain boulders more  than 30 cm in diameter.  These s o i l s f a l l i n t o the category of Gleyed Ferro  Humic Podsols. Category 3. Shallow s o i l s developing on bedrock.  Thin dark organic  -soils frequently occur on steep rocky slopes of over 35°.  These s o i l s  consist of a f i b r o u s , decomposed organic matter horizon less than 20 cm deep, sometimes over a t h i n f r i a b l e Ae horizon but often in d i r e c t contact with the bedrock.  Occasionally the organic matter horizon rests on a  stoney dark brown B horizon less than 30 cm deep.  These s o i l s f a l l  into  the category of L i t h i c Regosols, and F o l i s o l s (Lewis and L a v k u l i c h , 1972).  9  Table 1 presents a summary of some of the B horizon's mean physical properties based on twenty four large bulk samples taken from s o i l s belonging to categories 1 and 2.  Details of the sampling, which  was c a r r i e d out in conjunction with a series of f i e l d s o i l t e s t s , are given on page 79.  strength  P a r t i c l e s i z e d i s t r i b u t i o n curves based on  sieve and hydrometer analyses of the same samples are shown in Figure 3.  Table 1 Some Mean Physical Properties of Steepland S o i l s and Unweathered Till  from the Coast Range, southwestern B.C.  Property  S o i l B horizon  Unweathered t i l l  1,110  2,170  1 ,480  2,347  1,660  2,347  Void r a t i o (e)  1.21  0.26  Percent gravel  60.10  67.30  Percent sand  30.00  25.30  Percent s i l t  9.00  7.30  Percent clay  0.90  0.10  Dry unit weight ( )  kg/m  Y d  3  Moist unit weight ( J  kg/m  Saturated unit weight (  Y  Y  3  ) kg/m  The p a r t i c l e s i z e d i s t r i b u t i o n f o r the s o i l and the unweathered till  are s i m i l a r , suggesting that the s o i l B horizons are derived from  the weathering of a b l a t i o n t i l l which i s more permeable and of lower unit weight than the basal Vegetation.  till.  Most slopes support a dense cover of coniferous  except on the most precipitous b l u f f and bedrock areas.  forest  Western red  cedar (Thuja p i i c a t a Donn), western hem!ock (Tsuga heterophyl1 a (Rafn.)  10  Gravel  Silt  Sand  Clay  100  90  \  \  \ \  80  sz cn >  X3  \ \ \  4 .  70 60-  \ \  \  \  \ ^ \ \ \ \ \  \ \ \  \ \ \  \ \  \  \  AC  D_  \  \  50  O  \  \ \  \  30  \ \  \  \  \ \ s  20  \t  10  " " " " " — " • ^ • ^ • ^ ^ j r * — ~~  0  10  100  1.0  o  Diameter  in  o.oi  mm  F i g u r e 3.  P a r t i c l e s i z e d i s t r i b u t i o n curves f o r f o r e s t s o i l B  horizons.  S o l i d curves r e p r e s e n t l i m i t curves-between which the  measured d i s t r i b u t i o n s f o r i n d i v i d u a l samples f a l l . mean curve i s shown as a broken  line.  The approximate  0001  11  Sarg.) and Douglas f i r (Pseudotsuga menziezii (Mayr.) Franco) are the dominant canopy formers on the lower slopes.  Old growth mixtures of  these species a t t a i n merchantable volumes of 30,000 board feet or more per acre on favourable s i t e s .  At approximately 900 m the increasing  importance of mountain hemlock (Tsuga mertensiana (Bong.) C a r r ) , yellow cedar (Chaemaecyparis nootkatensis (D. Don) Spach) and amabilis f i r (Abies amabilis (Dougl.) Forb.), accompanied by a reduction i n the occurrence of western red eedar and western hemlock and the e l i m i n a t i o n of Douglas f i r , mark the lower l i m i t s of the 'Mountain Hemlock Zone' (Krajina 1965). The combination of steep slopes, weak-structured s o i l s and an impermeable t i l l or bedrock substratum would presumably create ideal conditions f o r large scale mass wasting were i t not f o r the vegetation cover whose roots help strengthen the s o i l and anchor the s o i l to the slopes.  The importance of roots to the s t a b i l i t y of slopes w i l l be  discussed in Chapter 3. Commercial logging a c t i v i t i e s w i t h i n the study area were i n i t i a t e d during the l a t t e r part of the l a s t century on the lower slopes close to Burrard I n l e t .  Logging on the steeper slopes of the lower parts of  Capilano and Seymour Valleys began at about the time of World War 1. Since that time c l e a r f e l l i n g has been c a r r i e d out in most of the major watersheds t r i b u t a r y to Howe Sound, the middle and upper reaches of the Capilano and Seymour watersheds and on the higher slopes overlooking Burrard I n l e t . In the past, as at the present time, timber harvesting on the steep slopes has been c a r r i e d out with cable systems (high lead systems). Downhill yarding to an accessible landing area i s a common p r a c t i c e .  12  Individual c l e a r f e l l e d areas, which range in s i z e from less than 0.04 km 2 to more than 4 km , seldom extend above a l t i t u d e s of 1,200 m.  Areas  cleared before 1950 support pole stands of western red cedar, western hemlock and Douglas f i r while red alder (Alnus rubra Bong.) i s u s u a l l y dominant along old railway grades, abandoned roads and on old l a n d s l i d e scars. Climate.  The study area possesses a c o o l , moist, mountain climate  notable f o r i t s heavy winter s n o w f a l l s .  Seymour F a l l s in the middle of  Seymour V a l l e y (elevation 229 m) receives an average annual p r e c i p i t a t i o n of 3,700 mm, 80 percent of which f a l l s between September and March i n c l u s i v e (Department of A g r i c u l t u r e , 1963).  The maximum measured t o t a l  p r e c i p i t a t i o n f o r a 24 hour period i s 252 mm but generally r a i n f a l l i n t e n s i t i e s are not p a r t i c u l a r l y high.  At Seymour F a l l s the maximum 5 minute,  15 minute and 1 hour i n t e n s i t i e s based on 11 years of records are 61 mm, 48 mm and 31 mm per hour r e s p e c t i v e l y .  Appendix 1 shows a summary of  r a i n f a l l i n t e n s i t y data for 26 storms i n c l u d i n g a l l the storms with r a i n f a l l t o t a l s exceeding 125 mm.  F i e l d data c o l l e c t i o n The f i e l d survey did not attempt to locate and record the d e t a i l s of a l l the mass wastage features w i t h i n the study area.  Rather, most of  the c l e a r f e l l e d slopes were v i s i t e d as well as those slopes with reasonable access (within 2 hours climbing time from the nearest road) where l a n d s l i d e s were known to occur.  At each l a n d s l i d e  s i t e the mean w i d t h ,  mean depth and length of the scar, the mean slope along each 15 m section of the scar, the a l t i t u d e and aspect of the scar head and the mean s o i l  13  depth to impermeable bedrock or unweathered t i l l were measured.  Mean  depth was the most d i f f i c u l t l a n d s l i d e c h a r a c t e r i s t i c to determine and was based on the mean depth of the scarps along the l a t e r a l margins of each scar.  The mean depths of several avalanches with i r r e g u l a r or  i l l - d e f i n e d l a t e r a l scarps and uneven f a i l u r e surfaces, were simply judged from the surface morphology of the s l i d e scar.  In a d d i t i o n , the  character of the s o i l m a t e r i a l s , the surface and subsurface hydrological c o n d i t i o n s , the nature of the vegetation and the microtopography of the slope surrounding each scar head were recorded.  The survey was l i m i t e d  to those predominantly unvegetated scars with surface areas l a r g e r than 2 approximately 90 m .  Approximately 70 percent of the major landslides  within the study area were v i s i t e d and measured. Use of a e r i a l  photographs  T r a d i t i o n a l l y , the use of a e r i a l photographs has played an important role in the study of l a n d s l i d e s .  According to Liang (1952) one of the  main advantages i n using a e r i a l photographs to i n v e s t i g a t e slope f a i l u r e s i s that the e n t i r e area of movement i s usually apparent at a glance whereas, on the ground, man i s dwarfed by natural topographic features and perspection of the f a i l e d area i s d i f f i c u l t . The study was s i g n i f i c a n t l y aided by a stereoscopic examination of several hundred a i r photos supplied by the A i r D i v i s i o n , Department of Survey Services, V i c t o r i a , B.C.  Five series of approximately 1:30,000  black and white panchromatic photographs taken in 1939, 1952, 1966, 1967 and 1969 r e s p e c t i v e l y , and three series of 1:15,000 black and white photos taken i n 1957, 1963 and 1968 enabled the locations and approximate age of many of the l a r g e r landslides to be determined.  The study of  14 sequential photographs of the same slope f a i l u r e s permitted the assessment of scar s i z e , scar form, and scar vegetation changes a f t e r the initial  sliding.  Landslides were recogniseable on black and white photo-pairs by the sharp l i n e of break at the head scarp, the l i g h t coloured appearance of the l a n d s l i d e scar which, in the case of many small l a n d s l i d e s , appeared as a t h i n white l i n e broadening towards i t s lower l i m i t s , and, in the case of older revegetated scars, by the presence of uniform l i g h t grey s t r i p s representing a l d e r and young c o n i f e r growth (Figure 4 ) .  Very d u l l  tone contrasts or poor sharpness of image or presence of a snow cover r e s t r i c t e d the usefulness of several sets of o l d e r photos but generally the 1:15,000 photos provided more information than the 1:30,000 p r i n t s . Since the completion of the o r i g i n a l l a n d s l i d e survey, very high q u a l i t y colour and colour i n f r a r e d a e r i a l photographs taken at 12,000 m a l t i t u d e from a CF-100 reconnaissance a i r c r a f t have been made a v a i l a b l e . Details of t h i s photography are given i n Table 2. Table 2 Details of High A l t i t u d e Colour and Colour Infrared  Photography  taken 16-7-71 over Southwestern B r i t i s h Columbia  Film  Lens Focal Length  225 mm Aerochrome 2445  88.9 mm  70 mm Aerochrome I.R.  76.2 mm  Filter M  l  t  e  r  ^ Height F 1  i n g  12,200 m  Wratten No. 12 Yellow  12,200 m  Shutter Speed 400  s e c  Type and Scale of P r i n t *  nr>&eC .  Contact 22.5 x 22.5cm. Scale 1:140,000 Enlargement 22.5 x 22.5 cm Scale 1: 49,000  Figure 4. Stereogram of part of the lower Capilano catchment.  A,  debris avalanches on a steep, forested s l o p e ; B, l i g h t - c o l o r e d a l d e r and maple marks l o c a t i o n s of skid t r a i l s and railway grades on slope c l e a r f e l l e d 34 years p r e v i o u s l y ; C, boundary between c l e a r f e l l e d and forested slope; D, o l d , p a r t l y revegetated debris s l i d e s on a slope c l e a r f e l l e d 32 years p r e v i o u s l y . Scale 1:15,000.  16  On colour a e r i a l photos recent large l a n d s l i d e scars appear as l i g h t brown areas which contrast markedly with the dark green c o n i f e r f o l i a g e of undisturbed slopes or the l i g h t green regrowth on old c l e a r c u t areas.  However, the small scale of photography prohibited detection of 2  small f a i l u r e s less than 1,000 m i n area.  Nor could landslides be  r e a d i l y distinguished from mining excavation features in B r i t a n n i a and Furry Creek v a l l e y s .  Colour photos were superior to black and white  photos f o r studying the gross features of the landscape, the nature of the vegetation and the broad drainage c h a r a c t e r i s t i c s of s p e c i f i c slopes. The colour oblique shown in Figure 2 provides an example of the colour tones and image d e f i n i t i o n depicted on v e r t i c a l colour p r i n t s . Colour i n f r a r e d a e r i a l photography o f f e r s many advantages over colour and panchromatic black and white photography (Mintzer 1968, Poole 1969).  Colour i n f r a r e d not only enhances the contrast of the t e r r a i n and  emphasises g u l l y shapes, drainage depressions, and erosional f e a t u r e s , but i t also creates strong contrasts between the darker coloured c o n i f e r growth and deciduous vegetation and between vegetation of d i f f e r e n t ages on c l e a r f e l l e d areas. Colour i n f r a r e d f i l m u t i l i z e s three image layers s e n s i t i z e d to green, red and i n f r a r e d instead of blue, green and red i n normal colour film.  Blue i s witheld with a yellow f i l t e r .  The v i s i b l e l i g h t component  together with the i n f r a r e d component produces a modified colour r e n d i t i o n of the subject photographed.  Depending on the proportions of green, red  and i n f r a r e d r e f l e c t e d or transmitted by the s u b j e c t s , a great v a r i e t y of colours can be produced.  For instance, healthy deciduous green  f o l i a g e ( a l d e r s , birches and maples) appear red, conifers appear dark blue to dark purple while exposed bedrock may range from greyish brown to l i g h t blue.  Where vegetation i s t h i n or l a c k i n g , poorly drained slopes  17  and g u l l y seepage areas are sometimes recognisable by t h e i r dark tones which contrast with l i g h t - c o l o u r e d dry slopes. till  Recently exposed s o i l or  surfaces are coloured white in dryer s i t u a t i o n s or l i g h t greyish  blue on moist s i t e s , causing landslides to be e a s i l y i d e n t i f i e d .  Several  narrow erosion features which were p a r t i a l l y masked by the tree canopy and not d i s t i n g u i s h a b l e on panchromatic black and white photographs were recognisable on colour i n f r a r e d photos. The colour i n f r a r e d p r i n t s did present some disadvantages, the most important being that s n o w - f i l l e d hollows and snow-chutes were d i f f i c u l t to d i s t i n g u i s h from recent l a n d s l i d e scars.  The stereogram i n  Figure 5 emphasises some of the features discussed above. A summary of the l a n d s l i d e information obtainable from each of the types of a e r i a l photographs i s presented in Table 3.  Types of mass wasting During the survey, landslides were not witnessed i n a c t i o n . Therefore, the types and rates of processes responsible f o r a p a r t i c u l a r slope scar or debris accumulation could only be judged from the e x i s t i n g form of the feature.  Furthermore, i n t e r p r e t a t i o n s were sometimes  confounded because f l u v i a l erosion or continued mass movements had strongly modified the o r i g i n a l form of the mass movement scars.  Nevertheless,  it.was possible to assign most of the f a i l u r e s investigated to one of the types of landslides discussed below. Adopting the well known l a n d s l i d e c l a s s i f i c a t i o n scheme presented by Varnes (1958), most of the mass wastage features w i t h i n the study area belong in one of three categories (or combinations of these categories)  13  Figure 5. Valley.  Color i n f r a r e d stereogram of part of the middle Seymour A, large debris avalanche on a steep slope covered with  cedar-hemlock f o r e s t ;  B, discontinuous  groves of alder-maple f o r e s t  are e a s i l y d i s t i n g u i s h e d by t h e i r l i g h t pink c o l o r ; f e l l e d area.  S c a l e , 1:49,000.  C, r e c e n t l y c l e a r -  19 Table 3 Information Obtainable from Various Types of A e r i a l  Photography  Photographs  Information Obtainable  Panchromatic B & W 1:30,000  Gross features of the landscape, slope forms, drainage patterns and broad vegetation features i d e n t i f i a b l e . Lands l i d e s over 500 in area recognisable.  Panchromatic B & W 1:15,000  Most l a n d s l i d e s over 200 in area i d e n t i f i a b l e . Type of s o i l materials and morphology of large scars recognisable.  Colour (Aerochrome) 1:140,000  Gross features of landscape, slope forms, broad vegetation features and drainage patterns i d e n t i f i a b l e . Landslides over 1,000 m^ i n area recognisable. 2 Most landslides over 500 m i n area i d e n t i f i a b l e . On bare areas s o i l d r a i n age conditions and amount of bedrock assessable. Broad vegetation features identifiable.  Colour Infrared 1:49,000  Interpretations i n Table 3 based on stereoscopic examination with and Old D e l f t Scanning Stereoscope, Model 0DSS III.  which form a t r a n s i t i o n a l series of l a n d s l i d e types from r e l a t i v e l y dry debris si ides to debris avalanches to very wet, mobi1e debris flows. Despite the plea by Rapp (1963) to reserve the term 'avalanche' only f o r mass movements of snow the term has been widely used in North America to describe mass wasting of s o i l s and rock (Bishop and Stevens Swanston 1969, 1970, Dyrness 1967 and Gray 1969).  1964,  For lack of a b e t t e r  expression the term 'debris avalanche' i s adopted i n t h i s thesis to describe a very important type of slope f a i l u r e .  As the d i s t i n c t i o n s  between debris s l i d e s , debris avalanches and debris flows are vague and  20  the terminology i s often confusing, the three main l a n d s l i d e types recognisable in the study area are described i n the f o l l o w i n g paragraphs. Debris s l i d e s .  Debris s l i d e s form when part or a l l of the vegetation  cover, humus l a y e r s , weathered s o i l mantle, and o c c a s i o n a l l y the underlying  substratum are dislodged and s l i d e downslope.  The r e s u l t i n g scar  possesses an obvious s l i d i n g plane which is often formed by the upper surface of the unweathered t i l l  or bedrock (Figure 6 ) .  The l o n g i t u d i n a l  p r o f i l e s of large debris s l i d e scars were usually l i n e a r to gently concave upwards but many of the smaller s l i d e s along stream sides and on road-cut and r o a d - f i l l slopes appeared to be r o t a t i o n a l f a i l u r e s possessing spoonshaped scars s i m i l a r to the features described as 'slumps' by Sharpe (1938) and Varnes (1958).  In plan view the scars ranged in shape from  broad oval features to long narrow trenches.  Most debris s l i d e s possessed  well marked head scarps while the lower parts of several scars were covered with detached but p a r t l y i n t a c t blocks of s o i l and tree root masses, i n d i c a t i n g that movement occurred i n a s l i d i n g manner rather than by flowing.  Debris s l i d e s ranged from 15 m to 150 m i n length, averaging  approximately 80 m. Debris avalanches.  Debris avalanches, recognisable by t h e i r long  narrow V or crescent shaped scars sometimes over 600 m i n length, were the  largest mass wastage features on undisturbed and c l e a r f e l l e d slopes.  The scars c h a r a c t e r i s t i c a l l y tapered towards t h e i r heads where v e r t i c a l scarps usually marked the s i t e s of the i n i t i a l f a i l u r e s (Figure 7). Many debris avalanches began in shallow drainage depressions or other wet s i t e s (Figure 8 ) , suggesting that groundwater played an important r o l e in their release.  At the s i t e s of i n i t i a l  f a i l u r e the basal s l i d i n g plane  was often the upper surface of the unweathered t i l l  although, o c c a s i o n a l l y ,  F i g u r e 6.  Small d e b r i s s l i d e i n upper Harvey Creek c a t c h m e n t .  The s l i d e o c c u r r e d on a 34° s l o p e and exposed an unweathered, basal t i l l .  F i g u r e 7.  ;  An a l m o s t v e r t i c a l  ,  headscarp o f a l a r g e  a v a l a n c h e i n t h e m i d d l e Seymour catchment.  debris  Figure 8.  The upper part of a large debris avalanche in the  Seymour catchment.  A s i z e a b l e stream flows down the centre of  the scar during wet weather.  Figure 9.  A debris avalanche i n second growth f o r e s t .  Near  the base of the slope the scar narrows and develops a sinuous course.  23  smooth d i o r i t e bedrock surfaces formed the debris avalanche f l o o r . Most debris avalanches were less than 2 m deep except f o r those formed on slopes underlain by deep c o l l u v i a l gravels or coarse alluvium. Apparently debris m o b i l i z a t i o n begins when large i n t a c t masses of s o i l break away from the slope leaving a well defined headscarp.  These  masses transform i n t o debris avalanches when stresses cause breakdown i n the s o i l structure as explained by Swanston (1967).  Often the l a n d s l i d e  track narrowed near i t s lower l i m i t to form a sinuous channel bordered by raised levees (Figure 9 ) , demonstrating t h a t , as they pass downslope, debris avalanches may increase t h e i r water content and transform i n t o true debris flows.  Hence Swanston (1969) refers to ' d e b r i s avalanche-debris  flow combinations' while Dyrness (1967) c l a s s i f i e s s i m i l a r composit mass wasting as ' d e b r i s avalanches with e a r t h f l o w s ! .  Debris flow formation i s  discussed by Johnson and Rahn (1970) who note that the t r a n s i t i o n from l a n d s l i d i n g to channelized flow of unsorted debris r e s u l t s from the' d i l u t i o n by water of s l i d e masses. High rates of movement during f a i l u r e were indicated by the frequent occurrence of gravel and boulders lodged in the bark and wood of tree trunks growing on the margins of avalanche scars.  In contrast to debris  s l i d e m a t e r i a l s , the transported debris avalanche materials were completely disrupted as t h e i r movement i s more akin to a true flow process rather than a s l i d i n g process. The debris avalanches measured in the f i e l d ranged in length from 2 38 m to 610 m while t h e i r mean area and mean volume* were 4,300 m and 3 4,200 m r e s p e c t i v e l y . These were r e l a t i v e l y small features compared to  Mean volume was c a l c u l a t e d as the product of scar l e n g t h , mean scar width and mean scar depth.  24  the debris avalanches and r e l a t e d types of mass wasting described by Wentworth (1943) i n Hawaii, Flaccus (1958) in the Appalachian Mountains, Simonett (1967) in New Guinea, Rapp (1963) i n Norway, Dyrness (1967) in the Cascade Mountains and Sheng (1966) in Taiwan. Debris flows.  The mass wastage features p o s i t i v e l y i d e n t i f i e d as  debris flows were s i t e d along the courses of steep permanent or ephemeral t r i b u t a r y stream channels.  Their points of o r i g i n were not well defined  except in those cases where debris s l i d e s or debris avalanches entered stream channels and transformed into wet debris flows.  Presumably, many  debris flows i n i t i a t e when temporary log dams holding back rock debris and water collapse during storm periods, releasing a saturated mobile slurry.  The debris flows which r e s u l t gather a d d i t i o n a l materials as  they pass down the stream beds leaving conspicuous t r i m l i n e s on stream g u l l y s i d e s , scouring channel beds, depositing l a t e r a l levees of coarse rock materials and sometimes overtopping the stream banks to form lobes or cones of debris on the lower slopes.  S i m i l a r features in southeast  Alaska have been described by Bishop and Stevens (1964) and Swanston (1969, 1971) as 'debris t o r r e n t s . ' The present study i s p r i m a r i l y concerned with debris s l i d e s and debris avalanches while other types of mass wasting recognisable in the area, i n c l u d i n g stream channel debris f l o w s , rock f a l l s , t a l u s s l i d e s , rock and s o i l transport by snow avalanches, and s o i l creep, were not investigated.  The term ' l a n d s l i d e ' i s used in a general sense to include  debris s l i d e s and debris avalanches.  S i g n i f i c a n c e of landslides in the study area Seventy-seven l a r g e , recent, debris avalanches and debris s l i d e s were recorded i n the study area; f o r t y - n i n e of these were investigated  "ABLE 4 A SUMMARY OF LANDSLIDE SURVEY DATA COLLECTED I N THE BRITISH COLUMBIA COAST RANGE LANDSLIDE IDENTIFICATION  LANDSLIDE TYPE  SITE TYPE*  VEGETATION COVER*  SLOPE (DEGREES)  ALTITUDE (METERS)  ASPECT !  SOIL CATEGORY  SOIL SERIES*  1970  TYPE OF BEDROCK*  AREA OF LANDSLIDE (METER )  VOLUME OF LANDSLIDE (METER 3 )  2  31 •CI -C2 FC3 FC4 HSI HS2 HS3 HS*. HS5 KS6 H57 HS8 H59 KSIO KS1I HSI 2 H3I3 KSI4 HS 15 CI C2 C3 Ci C5 C6 C7  D. s l i ' d e " D. ' s l i d e  ca  C9 CIO CII CI2 CI3 CH CI5  cie CI7 Cc:8 t9 SI S2 S3 S4 SS S6 S7 S3 S9 CIO  D.  6lids  D. D. D. D. D. D. D. D. D. D. D. D. D. D. D. D. D. D. D. D. D. •D. D. D. D. D. D. D. D. D. D. D. D. D. D. D. D. D. D. D. D. D. D. D. D. D.  aval. aval. slide aval. aval. aval. slide aval. aval. aval. aval. aval. aval. aval. aval. slide slide aval. aval. aval. aval. slide slide slide slide aval. aval. aval. aval. aval. aval. aval. slide slide ava1. aval. slide aval. slide aval. aval. aval. slide slide aval. aval.  OS OS DD OS DD OS DD R? • RF OS RF RF -RF RF RF OS . RC kC DD DD DD DD DD OS OS DD 03 OS OS DD DD DD DD OS DD OS  CF15 CF12 CF15 CF15 CF20 CF12 F CF10 CF10 CF30 CF S CF10 CF10 CF 8 CF 8 "CF10 CF10 F CF 5 CF 5 FF F F F CF30 CF30 F CF20 F F CF.30 F CF 4 FF F F  35 33 35 35 35 48 45 ' 39 37 43 41 37 36 • 36 36 37 42 45 30 35 38 34 37 31 35 31 34 37 33 32 34 32 39 35 30 36  655 488 503 533 524 884 1,052 1,036 1,052 524 951 1,021 .1.036 1,030 1, 128 1.036 1,036 762 1,067 1,073 411 274 960 808 766 329 543 469 817 762 375 1,017 463 518 503 747  DD DD DD DD DD OS DD DD DD DD DD DD DD  CF30 F F F F F F F F F F F F  41 29 34 37 36 37 36 37 33 38 24 31 38  336 1,237 917 625 732 655 1,036 1,036 732 1,265 1,250 347 1,036  .  .  36  Means  CS  Open s l o p e .  DD  Drainage depression.  ? CF  Virgin forest.  RF  F i l l s l o p e o r s l o p e formed o f side-cast materials.  RC  C u t - e l o p e above roadway.  FF  Fire-killed  .  W SSE SSE SW SSE SW ENE SW SE  1 1 1 I 1 1 11 1 1 1 1 11 11 • 1 1 1 1 I 11 11 11 11 11 1 1 11 1 1 11 11 1 11 1 11 1 1  SE NNE WSW ENE SW SE SE SE SW MfE NNE SSE SE  11 11 11 11 11 111 11 11 11 11 11 11 11  ssw sa NW liNW SSE WSW wsw SE SW WSW ssw su SW SSE S-SW WNW s tsw i.SW SSE INE ESE NE SSE SSE ESE NNW  GE WH GE GE GE GE WH WH CE GE PA PA PS WH WH SN PA WH PA S SN S SN BW S SN S CE SN SN CE WH WH WH WH WH DT WH WH S CE GE WH WH  3,430 190 90 1,020 930 1,210 2,790 1,950 2,320 460 1,860 2,690 2,320 2,510 2,320 3,900 1,300 65,030 930 370 2,230 740 1,860 1,490 370 90 560 190 1,670 5,020 460 4,000 560 2,600 1,860 280  1, 130 110 60 4,670 4,390 3,540 5,950 1,780 2,830 140 2,830 3,260 2,830 3, 110 2,120 9,510 3,170 39,630 570 230 1,390 3,570 6,510 3,170 230 60 230 60 1,020 3,060 280 3,680 340 2,380 850 2S0  Dior Dior Dior Dior Congl Dior Dior Dior Dior Dior Dior Dior Dior  460 1,390 1,770 190 2,600 4,830 8,450 6,970 1,860 460 460 650 840  230 280 850 170 1,960 1,470 10,300 8,490 850 650 420 400 510  Means  794  £  Based BW  forest.  C l e a r f e l l e d a r e a . (Numbers i n d i c a t e a p p r o x i m a t e y c a r a str.ee c l e a r f e l l i n g } .  F sed Dior Dior Dior Dior F sed Dior Dior F sed Congl F sed Schl F sed F sed F sed Dior Dior Dior Dior Dior Dior Dior Dior Dior Dior Dior Dior Dior Dior Dior Dior Dior Dior Dior Dior Dior  time I n |  3,113  APPROXIMATE AGE (YEARS) 3 - 5 2 - 3 3 - 5 12 -18 12 -18 3-5 3 -10 3 -10 3 -10 2 - 3 3 -10 3 -10 3 -10 2-3 3 -10 3 -10 3 -10 18 -30 3-5 3-5 1 - 2 12 -18  - -  12 -18 18 -30 2-5  2-5 12 - i a 3-5 12 -18 k - 1 2-3 3 -12 7  -12  4-7 1 - 2 \ - I 3 -12 3 - 7 3 -12  3-5  2,972  on s o i l s e r i e s nap (1:50,000) produced by B.C. Dept. of  Burwell Series.  GE  Colden Ears  PN  Paton S e r i e s .  WH  Whonnock  DT  Dennet S e r i e s .  SN  Strachen S e r i e s .  CE  Cannel S e r i e s .  S  F sed  Fine grained sedimentary rock.  Dior  Diorite or related plutonic  Schi  Schist.  Congl  Conglomerate.  Sayres  rock.  Agri.  Series.  Series.  Series.  ro cn  26  in the f i e l d while the remaining twenty-eight were i d e n t i f i e d on 1968 and 1969 a e r i a l photographs.  Their d i s t r i b u t i o n i s shown in Figure 1 and  a summary of each l a n d s l i d e ' s measured c h a r a c t e r i s t i c s appears in Table 4. The importance of l a n d s l i d i n g in terms of l a n d s l i d e numbers, area of landslides and weight of l a n d s l i d e debris transported per square kilometer can be gauged from Table 5.  As the mean values of measured debris  avalanche areas and volumes were assigned to the twenty-eight debris avalanches not measured in the f i e l d , while a moist unit weight of 3 1,480 kg/m  was used for converting volumes to weights, the two righthand  e n t r i e s in Table 5 are no b e t t e r than rough approximations. Table 5 Landslide number, area and quantity of debris moved per square kilometer of the study area  Mean number of l a n d s l i d e s per km? 0.12  Mean area of l a n d s l i d e s , m? per km2 427  ^ L ^ ^ f 5 1  °  l^t V e  612  Compared to other forested mountain regions f o r which data are a v a i l a b l e (Katsumi 1965, Sheng 1966, Simonett 1967, and Dyrness 1967), landslides in the Coast Range study area are less frequent and influence a smaller percentage of the t o t a l area, i n d i c a t i n g that the s o i l mantle is r e l a t i v e l y stable.  Nevertheless, landslides are an important feature  of the environment of some steep slopes.  27 T h i r t y - s i x debris avalanches and debris s l i d e s , representing 47 percent of a l l landslides in the study area, ran out into t r i b u t a r y stream beds or main stream channels.  Generally, t r i b u t a r y catchments  with large landslides descending i n t o t h e i r stream channels possessed recently aggraded stream beds i n t h e i r lower reaches.  For instance, 3 3  large debris avalanches had deposited approximately 10,400 m of debris i n t o the main stream channel of Strachen Creek which i s a steep bedrock gorge to w i t h i n a few hundred meters of i t s confluence with S i s t e r s  Creek.  Most of the l a n d s l i d e materials had passed down the gorge to the lower reaches of S i s t e r s Creek, and possibly as f a r as the Cleveland Resevoir. The Coast Range landslides have an a d d i t i o n a l hydrological icance because they form part of the surface drainage network.  signifMost large  l a n d s l i d e scars possessed ephemeral streams down the length of t h e i r l o n g i t u d i n a l axes and, in many cases, the streams o r i g i n a t e d at the l a n d s l i d e headscarp as a subsurface seepage.  In a l l cases i n v e s t i g a t e d ,  however, a e r i a l photographs showed that discernable surface drainage features did not e x i s t on these s i t e s p r i o r to the time of f a i l u r e . Furthermore, f i e l d inspections showed that surface drainage features were often absent (four notable exceptions e x i s t ) upslope from the headscarps of l a n d s l i d e s .  Presumably, the p r e - l a n d s l i d e downslope drainage was  f a c i l i t a t e d p r i m a r i l y by subsurface flow and the occurrence of a l a n d s l i d e , t h e r e f o r e , represents an a d d i t i o n to the surface drainage network. Relationship of landslides to s i t e factors Microtopoqraphy.  Thirty-one of the f o r t y - n i n e landslides investigated  in the f i e l d i n i t i a t e d in drainage depressions, small seepage hollows or on poorly drained parts of open slopes.  Many of these s i t e s were underlain  by s o i l s belonging to Category 2 which exhibited some degree of mottling  28  in the B horizon, a condition which normally, but not always, r e f l e c t s poor drainage conditions and high water tables f o r long periods during the year (Canada Department of A g r i c u l t u r e , 1970).  Hack and Goodlett  (1960) noted that debris avalanches on the steep slopes of the Central Appalachian Mountains commonly o r i g i n a t e in hollows although they do not mention the drainage conditions at the s i t e s of l a n d s l i d e release. Debris avalanches frequently occur w i t h i n l i n e a r seepage depressions on steep slopes in southeastern Alaska (Swanston 1969) which, combined with the f a c t that depression s o i l s often displayed evidence of former i n s t a b i l i t y , led Swanston to conclude that depressions enlarge themselves by l a n d s l i d i n g .  Large l i n e a r drainage depressions occur on uniform  slopes which are e a s i l y recognised on 1:15,000 a e r i a l photographs.  On  the other hand many slopes have 'stepped' l o n g i t u d i n a l p r o f i l e s c o n s i s t i n g of short slope facets separated by bedrock outcrops and v e r t i c a l rock walls.  The discontinuous s o i l mantle on these 'broken' slopes i s not  susceptable to l a n d s l i d i n g . Slope.  Most large landslides originated on s i t e s which f a l l w i t h i n  a r e l a t i v e l y narrow range of slope steepness.  Figure 10 (top) shows that  over 75 percent of the measured landslides occurred on slopes from 31° to 39° i n c l u s i v e .  ranging  Sites with surface slopes greater than 40°  are usually extremely rocky and underlain by t h i n regosolic s o i l s (Category 3 s o i l s ) not prone to large scale f a i l u r e . Aspect.  Approximately 75 percent of the landslides investigated  possessed aspects with a southerly component, SSE being the most frequent p  aspect, accounting f o r 20 percent (Figure 11 ( t o p ) ) .  A chi squared (  x  )  t e s t was used to determine whether or not the d i s t r i b u t i o n of landslides with respect to aspect- d i f f e r e d s i g n i f i c a n t l y from the d i s t r i b u t i o n  29  8|  Figure 10. Bottom.  Top.  D i s t r i b u t i o n of l a n d s l i d e s with respect to s l o p e .  Longitudinal p r o f i l e s of selected l a n d s l i d e s .  Figure 11. Top. Bottom.  D i s t r i b u t i o n of l a n d s l i d e s with respect to aspect.  D i s t r i b u t i o n of l a n d s l i d e s with respect to a l t i t u d e .  31  which would be expected i f aspect exercised no influence on l a n d s l i d e locations.  2 2 The t e s t y i e l d e d a x equal to 45.3 (tabulated x at 0.01  l e v e l of s i g n i f i c a n c e and 15 degrees of freedom i s equal to 30.6) i n d i c a t i n g that aspect s i g n i f i c a n t l y influences the l o c a t i o n of slope failures.  However, t h i s s i g n i f i c a n c e i s based on the premise that slopes  of a l l aspects are equally represented.  There i s a preferred o r i e n t a t i o n  of slopes towards the south i n the Capilano and Seymour catchments (Slaymaker, personal communication) but a sample of 134 points obtained by l a y i n g a dot g r i d over a 1:50,000 topographic map of the study area, showed that t h i s southerly bias was very weak f o r the t o t a l study area. S i x t y - f o u r points f e l l on slopes with a southerly component, f i f t y - n i n e points f e l l on slopes with a northerly component and eleven, points could not be c l a s s i f i e d . There i s one probable reason why l a n d s l i d e o r i e n t a t i o n i s biased towards a southerly d i r e c t i o n .  In Furry Creek, Strachen Creek and Harvey  Creek, in most of the large t r i b u t a r y catchments in the upper Capilano V a l l e y , and in two t r i b u t a r y catchments draining the western slopes of the middle Seymour V a l l e y , the north-facing slopes are rocky and broken, a condition which discourages l a n d s l i d e formation, while south-facing slopes are r e l a t i v e l y uniform and underlain by an extensive unweathered till  substratum.  These physiographic differences presumably r e s u l t from  the general North to South flow d i r e c t i o n of the Pleistocene i c e sheets which inundated the Coast Mountains (Armstrong and Brown 1954).  If the  landslides associated with roads were neglected,the percentage of f a i l u r e s with a southerly component in t h e i r o r i e n t a t i o n remained at approximately 75 percent.  32  Altitude.  The d i s t r i b u t i o n of l a n d s l i d e s with respect to a l t i t u d e  i s depicted in Figure 11 (bottom).  The f a c t that landslides were most  numerous between a l t i t u d e s of 1,000 and 1,100 m i s seen to coincide with the f a c t of numerous road associated f a i l u r e s w i t h i n t h i s range of altitude.  If road-caused landslides are discounted, then the apparent 2  influence of a l t i t u d e becomes questionable.  A x  t e s t showed that the  d i s t r i b u t i o n of l a n d s l i d e s , except f o r road-caused f a i l u r e s , was not s i g n i f i c a n t l y d i f f e r e n t at the 0.05 level of s i g n i f i c a n c e from the uniform d i s t r i b u t i o n formed by assuming a l l a l t i t u d e classes contained 2 equal numbers of l a n d s l i d e s . The c a l c u l a t e d x with 10 degrees of freedom 2 was equal to 11.20 compared to 18.31, the tabulated x  for 10 degrees of  freedom at the 0.05 l e v e l of s i g n i f i c a n c e . Bedrock geology.  The o v e r a l l uniformity of the d i o r i t e bedrock over  a large portion of the study area would tend to r u l e against bedrock type as a cause of s p a t i a l v a r i a b i l i t y in l a n d s l i d e numbers.  Moreover, the  widespread covering of compacted t i l l , the upper surface of which often forms the basal f a i l u r e plane, reduces the influence of the bedrock on landsliding.  Nevertheless, the f i e l d survey data showed that bedrock  differences may exercise some control over large slope f a i l u r e s r e l a t e d to road c o n s t r u c t i o n .  On slopes underlain by sedimentary and metamorphic  rocks belonging to the Gambier Series (Armstrong 1965), road construction had caused s i x major debris avalanches but on slopes underlain by d i o r i t e and r e l a t e d igneous rocks, the s i t u a t i o n on approximately 90 percent of the logged-over slopes v i s i t e d , there were only eight major f a i l u r e s related to roads.  However, the e f f e c t of bedrock type on road-caused  debris avalanches may be more apparent than r e a l . on the Gambier Series originated i n road f i l l  The s i x landslides  or sidecast materials and  33  steep slopes ( a l l f a i l u r e s o r i g i n a t e d on slopes steeper than 35 ) combined with poor drainage conditions seemed to be the main f a c t o r s c o n t r i b u t i n g to the slope f a i l u r e s . Of greater importance than the bedrock composition i s the nature of the bedrock's upper surface.  I f the s o i l mantle rests d i r e c t l y on bedrock,  then smooth uniform bedrock surfaces p a r a l l e l to the slope surface favour s l i d i n g as in the cases of debris s l i d e s C17 (Figure 12) and S3. Soils.  The poorly drained s o i l s , belonging to Category 2 were  associated with 25 l a n d s l i d e s , while the s o i l s at Category 1 subject to only moderately good drainage but d i s t i n g u i s h a b l e from Category 2 s o i l s by the lack of well defined mottling in t h e i r B horizon, accounted f o r f i f t e e n slope f a i l u r e s .  Only one debris s l i d e , S3, had formed in the  shallow Category 3 s o i l s .  The close a s s o c i a t i o n between landslides and  p o t e n t i a l l y very wet s o i l s , supports a concept that pore water pressures from high water tables i s a p r i n c i p a l f a c t o r leading to slope f a i l u r e . An attempt was made to place the f a i l e d s o i l s in the s o i l series categories mapped by the S o i l Survey D i v i s i o n , B.C. Department of A g r i c u l t u r e (see Table 4 ) .  Although t h i s placement should be regarded as  t e n t a t i v e i t does appear that the Golden Ears, Whonnock, P a l i s a d e , and Strachen s o i l series are most subject to mass wasting.  E f f e c t of c l e a r c u t t i n g on l a n d s l i d e occurrence Croft and Adams (1950), Bishop and Stevens (1964), Dyrness (1967), Gray (1969) and Fujiwara (1970) have f i r m l y established that the removal of f o r e s t s may s e r i o u s l y increase the incidence of l a n d s l i d i n g on steep slopes.  The protective r o l e of a f o r e s t cover i s p a r t l y manifested  through the mechanical strengthening of the s o i l by tree root systems  34  (Turmanina 1965, Endo and Tsurata 1969, Gray 1969, and Swanston 1969, 1970) and p a r t l y through the modification of s o i l moisture d i s t r i b u t i o n s and possibly pore water pressures  (Bethlamy 1962, P a t r i c et al 1965,  Hall in 1967 and Gray 1969). Although the Coast Range l a n d s l i d e survey revealed an increase i n the density of l a n d s l i d e s on c l e a r f e l l e d areas, the extremely e r r a t i c d i s t r i b u t i o n of slope f a i l u r e s on c l e a r f e l l e d and undisturbed  slopes,  combined with the lack of information s u i t a b l e f o r accurately dating most l a n d s l i d e events, complicated the assessment of s t a b i l i t y changes f o l l o w i n g logging.  Table 6 i n d i c a t e s the broad changes in l a n d s l i d e  frequencies a f t e r logging on areas c l e a r f e l l e d between 1957 and 1968. The pre-logging information was obtained from 1957, 1963 and 1966 a e r i a l photographs. Table 6 Numbers of landslides Before and A f t e r Logging,  Coast  Range, Southwestern B r i t i s h Columbia Approx. area c l e a r f e l l e d between 1957 and 1968  Howe Sound Drainages  13.7 km  Number of landslides 1957  1970  3  11  Capilano Catchment  0  3  Seymour Catchment  0  0*  2  * Two small s l i d e s occur on a c l e a r f e l l e d slope in the middle Seymour catchment.  35 Eight of the 14 landslides recorded in 1970 were p r i m a r i l y caused by road c o n s t r u c t i o n . In order to i n v e s t i g a t e the e f f e c t s of f o r e s t removal on l a n d s l i d e occurrences in more d e t a i l , the l a n d s l i d e d e n s i t i e s and the q u a n t i t i e s of l a n d s l i d e debris transported per square kilometer on c l e a r f e l l e d slopes and undisturbed slopes were determined in ten small watershed areas.  Estimates of l a n d s l i d e volumes, and consequently weights of debris  moved, were mainly based on the data presented i n Table 4 but, in a d d i t i o n , rough f i e l d estimates of the volumes of small landslides (not included in Table 4) on forested slopes in Furry Creek catchment (2), B r i t a n n i a Creek catchment (1), Lynn Creek catchment (4), Magnesia Creek catchment (1) and Harvey Creek catchment (1) and on c l e a r f e l l e d slopes in the middle Seymour catchment (2) were included i n the c a l c u l a t i o n s .  Within each  subcatchment the broad physical form of the forested slopes used in the analysis was s i m i l a r to that of the c l e a r f e l l e d slopes except i n Magnesia Creek and the middle Seymour catchment where the forested slopes were generally steeper and at higher elevations than the logged slopes.  Areas  were measured from enlargements of 1:50,000 topographic maps and from 1:15,000 a e r i a l photographs with a polar planimeter.  The r e s u l t s are  presented in Table 7. A t t e s t was employed to determine i f the mean l a n d s l i d e density and the mean quantity of l a n d s l i d e debris produced per square kilometer f o r c l e a r f e l l e d slopes were s i g n i f i c a n t l y d i f f e r e n t from the equivalent means f o r undisturbed slopes.  The data were f i r s t tested by assuming  that population variances were homogeneous, an assumption which F t e s t s showed to be i n v a l i d .  This necessitated the use of a t  1  1968, p. 230) designed to take i n t o account heterogeneous  t e s t (Walpole variances.  In the f i r s t set of t e s t s a l l l a n d s l i d e s were considered, in the second  36 set only those f a i l u r e s not caused by road construction were included.  Table 7 Landslide Densities and Quantities of Landslide Debris Produced Within Selected  Watershed Areas in the Coast Range, Southwestern B r i t i s h Columbia  Catchment  No. of landslides per km2 Clearfelled  Tons of l a n d s l i d e debris per knr  Forest  Clearfelled  Forest  A  B  C  D  E  F  Britannia  0.32  0.32  0.05  539  539  293  Furry  0.75  0.30  0.05  2,227  212  180  Brunswick  6.25  2.50  0.19  20,573  6,808  1,639  Magnesia  1.43  0.48  0.30  6,484  56  1,933  Harvey  1.11  0.56  0.19  3,075  469  1 ,227  Strachen  2.50  2.50  0.56  6,290  6,290  2,716  Sisters  0.64  0.64  0.16  126  126  81  U Capilano  0.95  0.95  0.09  1,917  1,917  412  M Seymour  0.59  0.59  0.22  252  252  823  Lynn  0.42  0.42  0.04  1,890  1,890  240  Means  1.50  0.93  0.19  4,337  1 ,856  954  A and D  A l l landslides considered  B and E  Road-caused l a n d s l i d e s not considered  The t e s t r e s u l t s are tabulated below: A  vs  C  t'  =  2.31* with  9 degrees of freedom. (Tabulated t.05 = 2.26)  B  vs  C  t'  =  2.71* with 10 degrees of freedom. (Tabulated  "  = 2.23)  D  vs  F  t'  =  1.73  with  9 degrees of freedom. (Tabulated  "  = 2.26)  E  vs  F  t'  =  1.05  with 11 degrees of freedom. (Tabulated  "  =2.20)  37  The r e s u l t s of the analysis i n d i c a t e that the density of l a n d s l i d e s was s i g n i f i c a n t l y higher, at the 5 percent level of s i g n i f i c a n c e , on c l e a r f e l l e d slopes compared to forested slopes.  However, t h i s s t a t i s t i c a l  s i g n i f i c a n c e may have l i t t l e real meaning, as evidence that the c l e a r f e l l e d slopes and the  forested slopes were comparable was l i m i t e d to s i m i l a r i t i e s  in gross slope morphology. The mean l a n d s l i d e d e n s i t i e s f o r logged and forested slopes presented in Table 7 are s i m i l a r to those recorded by Dyrness (1967) in the H.J. Andrews Experimental Forest, Oregon but are much smaller than the d e n s i t i e s presented by Fujiwara (1970) f o r two unstable mountain areas in northern Japan (Table 8 ) . Table 8 Landslide Densities on Forested and C l e a r f e l l e d Mountain Slopes f o r Regions in Northwestern North America and Northern Japan  Source of information  Region  Number of landslides per km Forest Logged A B  Ratio of B:A  O'Loughlin 1972  Coast Range B.C. .  0.2  1.5  7.5  Dyrness 1967  Cascades, Oregon  0.1  1.0  10.0  Fujiwara 1970  Nth.Japan a) B i r a t o r i b) Urahoro  9.0 3.0  80.0 25.0  8.9 8.3  Although the natural resistance to mass wasting in the undisturbed conditon varies widely from region to region, the number of landslides per unit area increases by a s i m i l a r f a c t o r a f t e r logging, implying that the r e l a t i v e e f f e c t s of deforestation on the s o i l ' s resistance to f a i l u r e do not vary  38  greatly over a wide range of g e o l o g i c a l , s o i l , topographical and c l i m a t i c conditions. The importance of l a n d s l i d i n g in terms of numbers of events i s not as meaningful as the volumes or weights of l a n d s l i d e debris transported. The t ' t e s t s showed that the q u a n t i t i e s of l a n d s l i d e materials on c l e a r f e l l e d slopes were not s i g n i f i c a n t l y d i f f e r e n t from the q u a n t i t i e s moved on undisturbed slopes, a consequence of the large v a r i a t i o n in debris weights from catchment area to catchment area on the same class of slope and the l a r g e r mean s i z e of landslides on forested slopes.  Nevertheless,  the mean weight of debris moved per unit area on logged slopes was approximately 4.5 times l a r g e r than the equivalent mean weight on forested slopes.  More than two t h i r d s of t h i s debris increase on cleared areas i s  a t t r i b u t a b l e to road-caused f a i l u r e s (Table 7).  Road construction and landslides Road construction has been i d e n t i f i e d as a major cause of mass wasting on steep slopes i n many d i f f e r e n t environments (Fredriksen 1963, 1970, Dyrness 1964, 1967, Sheng 1966, J e f f r e y 1968, Gonsior and Gardner 1971, Lantz 1971 and Nobel and Lundeen 1971).  Road construction i s more  detrimental to the s t a b i l i t y of the mountain slopes w i t h i n the study area than any other a c t i v i t y c a r r i e d on by man.  Approximately 260 km of  access roads and logging roads, i n c l u d i n g old disused roads that are now i n various states of d e t e r i o r a t i o n and r e c o l o n i z a t i o n by vegetation, e x i s t in the study area.  Road lengths were measured on 1:15,000 a e r i a l  photographs and enlargements of 1:50,000 topographic maps.  Of these  about 180 km traverse moderately steep to steep slopes, and thereby create favourable s i t u a t i o n s f o r the development of f a i l u r e s on the disturbed areas adjacent to the road r i g h t of way.  39 Fourteen landslides of the seventy-seven recorded were d i r e c t l y r e l a t e d to road construction while possibly three a d d i t i o n a l f a i l u r e s (HS1, HS14 and HS15) were associated with road construction less than 200 m upslope from t h e i r points of o r i g i n .  Of the landslides d i r e c t l y  related to road formation, seven o r i g i n a t e d in sidecast or road f i l l materials at or below the level of the road surface, f i v e i n i t i a t e d in natural s o i l s and colluvium immediately below the outer edge of the road, and two resulted from the collapse of s o i l and c o l l u v i a l materials on steep, cut slopes. However, the constraints on the survey resulted in strong underestimation of the importance of road construction as a primary cause of mass wasting.  If the minimum l a n d s l i d e s i z e f o r i n c l u s i o n i n the survey  2 2 had been set at 10 m instead of 90 m , then more than 100 additional debris avalanches and debris s l i d e s would have been included and most of these would have been road-caused events.  For example, along an approx-  imately 4 km section of an abandoned logging road in Furry Creek v a l l e y at 590 m above s e a - l e v e l , twelve small landslides occurred on cut and f i l l slopes.  Although the q u a n t i t i e s of materials displaced by such f a i l u r e s  are small compared to those moved by many of the large debris avalanches, secondary f l u v i a l erosion r e s u l t i n g from blockage of roadside ditches i s common. Four f a c t o r s appear to be most s i g n i f i c a n t in the formation of l a n d s l i d e s r e l a t e d to logging roads; (1) the natural steepness of the t e r r a i n , (2) the amount of sidecasting of s o i l and rock materials onto the slopes below the r o a d - l i n e during road c o n s t r u c t i o n , (3) the drainage c o n d i t i o n s , (4) the rate at which disused roads become recolonized by vegetation. These factors are b r i e f l y discussed below.  Figure 12.  Smooth-surfaced d i o r i t e bedrock forming the s l i d i n g  surface near the head of a debris avalanche i n the upper Capilano catchment.  Figure 13.  Road-caused debris avalanches r e s u l t i n g from the  collapse of r o a d - f i l l and s i d e - c a s t materials on a 39° slope i n the Howe Sound area.  41 Natural slope steepness.  p  Large l a n d s l i d e s (over 90 m in area)  r e l a t e d to roads did not occur on v a l l e y sides with slopes less than 3 0 ° , while ten debris avalanches had formed on areas sloping at 36° or more. Road-caused l a n d s l i d e s have not occurred in the Capilano and Seymour c a t c h ments where most of the access roads and logging roads are located on the  j  v a l l e y f l o o r s or on the lower v a l l e y sides with slopes less than 35°. Steep slopes generally require l a r g e r road cuts and the placement of l a r g e r q u a n t i t i e s of f i l l Sidecasting.  and sidecast materials than gentle slopes.  Failures in the sidecast and f i l l  materials most frequent-  ly o r i g i n a t e d at or near the outer road surface edge, producing long narrow scars up to 3 m in depth (Figure 13).  I n s t a b i l i t y p r i n c i p a l l y derives  from the loose, uncompacted condition of the sidecast mantle combined with saturated conditions r e s u l t i n g from the concentration of road surface runoff onto the s i d e c a s t i n g s , p a r t i c u l a r l y along the incurved sections of o roads.  Failures associated with s i m i l a r conditions in the Idaho B a t h o l i t h  region received d e t a i l e d study by Gonsior and Gardner (1971).  Swanston  (1971) indicates that sidecasting and road f i l l s may encourage slope f a i l u r e s by overloading the slope below the roadcut and by obstructing upslope s o i l  drainage.  Drainage c o n d i t i o n s .  At l e a s t eight road-caused debris avalanches  were d i r e c t l y associated with excessive seepage c o n d i t i o n s , a consequence of inadequate road drainage f a c i l i t i e s .  I n s u f f i c i e n t roadside d i t c h  c a p a c i t i e s , often r e s u l t i n g from cut-slope f a i l u r e s , and a lack of functionable c u l v e r t s , caused road surface ponding along f l a t s e c t i o n s , or uncontrolled surface runoff and g u l l y i n g on steep s e c t i o n s , of abandoned logging roads i n Furry Creek, Brunswick Creek, Magnesia Creek and Harvey Creek catchments.  Such conditions lead to saturated slope mantles and  a weakened resistance to mass wasting.  The plugging of roadside drains  42  by compacted snow during the spring thaw may also be a primary cause of road surface g u l l y i n g and, i n d i r e c t l y , a cause of mass wasting at higher a l t i t u d e s (Goodell, personal communication). Reyegetation of disused roads.  It was found t h a t , at a l t i t u d e s  below 1,000 m, abandoned logging roads are usually invaded by red alder seedlings w i t h i n 2 to 3 years a f t e r the cessation of logging.  This  growth i s obviously helping to protect road surfaces against g u l l y i n g and the root systems are presumably adding strength to the road f i l l s and sidecastings.  However, above a l t i t u d e s of 1,000 m natural revegetation of  the roads is a slow process. and f i l l  At these higher a l t i t u d e s obsolete roadbeds  slopes may remain devoid of vegetation f o r many years.  Disused  roads in upper Magnesia Creek, f o r instance, remain e s s e n t i a l l y unvegetated (alder seedlings occur o c c a s i o n a l l y i n wetter roadside ditches) although logging terminated here approximately 5 years ago. Kochenderfer (1970) emphasised the importance of road care a f t e r logging in order to condition abandoned roads against e r o s i o n . Practices such as the removal of wooden c u l v e r t s , construction of waterbars, smoothing and outsloping of road surfaces and the a r t i f i c i a l revegetation of steep roadbeds and unstable f i l l  slopes a f t e r the  completion of logging have been l a r g e l y ignored in the study area. Employment of these techniques, better provision f o r drainage and, most important of a l l , much improved road planning and route s e l e c t i o n (Larse, 1971), are urgent requirements to ensure that timber harvesting on the steep Coast Range slopes a t t a i n s some degree of c o m p a t i b i l i t y with sensible watershed management.  43  Natural revegetation of l a n d s l i d e scars Red alder usually colonizes l a n d s l i d e scars w i t h i n 3 to 4 years a f t e r the time of f a i l u r e .  On exposed, compacted t i l l  s i t e s alder may  remain the only c o l o n i z i n g vegetation f o r 10 years or more but on uncons o l i d a t e d c o l l u v i a l deposits or rocky debris accumulations, cedar and hemlock seedlings often e s t a b l i s h themselves amongst young alder growth before the scars are 4 years o l d .  Two l a n d s l i d e s in the lower Capilano  watershed and one in the lower Lynn Creek catchment, a l l of which appeared to be fresh and unvegetated in 1939, are completely revegetated with young c o n i f e r forest but are s t i l l graphs.  recognisable on 1968 1:15,000 a e r i a l photo-  On the other hand, several scars which o r i g i n a t e d between 1952  and 1957 remained e s s e n t i a l l y unvegetated in 1970 (landslides FC4, HS1, C10 and C12).  T h i s , combined with the observation that the vegetation  on several ancient l a n d s l i d e scars has f a i l e d to develop beyond a low shrub-stunted c o n i f e r growth stage, suggests that snow avalanches, which recur on many debris avalanche_sites each w i n t e r , retard the natural revege t a t i o n process.  Discussion and conclusions The combination of steep slopes, shallow, weak-structured s o i l mantles, heavy seasonal r a i n f a l l s , and the presence of an impermeable till  substratum or impermeable smooth-surfaced igneous bedrock i n d i r e c t  contact with the s o i l , predisposes c e r t a i n undisturbed slopes i n the Coast Range to catastrophic f a i l u r e s .  However, on the densely forested  v a l l e y s i d e s , s o i l mass wasting occurs at a subdued rate compared with other forested mountain regions f o r which data are a v a i l a b l e .  Consequently  i t would be misleading to claim that l a n d s l i d i n g , under undisturbed  44  c o n d i t i o n s , i s of great economic importance in the study area.  On the  other hand, man's a c t i v i t i e s can s u b s t a n t i a l l y increase the importance of mass wasting to the point where landslides may be a serious threat to the permanence of the s o i l resource of c e r t a i n steep slopes.  Measure-  ments i n d i c a t e that the average q u a n t i t i e s of s o i l and rock debris d i s lodged by landslides on the more unstable c l e a r f e l l e d slopes with roads are more than 10 times l a r g e r than the average q u a n t i t i e s transported on comparable undisturbed slopes (Table 7).  As f l u v i a l surface e r o s i o n ,  small scale mass movements and f l u v i a l g u l l y i n g are not considered i n these estimates the increases i n t o t a l erosion r e s u l t i n g from logging and road construction i s , presumably, much greater.  Gonsior and Gardner  (1971), f o r instance, c i t e sediment production increases of more than a thousandfold a f t e r roadbuilding and c l e a r f e l l i n g in steep forested watersheds in Idaho.  Fredriksen (1970) found that t o t a l stream-carried  sediment y i e l d from a small watershed in western Oregon a f t e r the completion of roadbuilding and patchwork logging-, was approximately 100 times greater than the sediment y i e l d from a nearby undisturbed watershed. Although rather dramatic increases i n mass wasting f o l l o w i n g timber harvesting on some areas o f the steep Coast Range slopes are indicated by Table 7, i t i s apparent that a^great d i v e r s i t y e x i s t s in the natural s t a b i l i t y of steep areas.  The survey established several general points  d i r e c t l y r e l a t e d to t h i s v a r i a b l e s t a b i l i t y . (1) Long uniform slopes are more susceptible to l a n d s l i d i n g than s h o r t , broken, stepped, slopes with large amounts of exposed bedrock. (2) Poorly drained hollows, l i n e a r drainage depressions, and wet seepage zones are more l i k e l y to be l a n d s l i d e i n i t i a t i o n points than well drained convex i n t e r f l u v e s separating drainage  depressions.  45  (3) Mountainsides with slopes greater than 30° but less than 40° are most l i k e l y to s u f f e r l a n d s l i d e s . (4) C l e a r f e l l i n g and road construction increase v u l n e r a b i l i t y to the physiographic f a c t o r s of points (1), (2) and (3). Although general information of the sort obtained in the survey provides guidelines f o r f i e l d project planning, c r i t e r i a to be employed in making (a) r e l i a b l e assessments of the l a n d s l i d e potential of natural and a r t i f i c i a l slopes and (b) meaningful predictions of slope s t a b i l i t y changes a f t e r logging or roadbuilding should, i d e a l l y , be based on understanding of the basic processes and mechanisms involved in l a n d s l i d e i n i t i a t i o n .  To gain such understanding, three d e t a i l e d  studies were made of the hydrological and strength c h a r a c t e r i s t i c s of representative slope materials and of some of the processes which render them susceptible to f a i l u r e .  46 CHAPTER 2 PIEZOMETRY STUDIES ON STEEP SLOPES  Introduction On the steep, s o i l - c o v e r e d slopes of the Coast Mountains, downslope drainage i s predominantly by subsurface flow (Chamberlin, 1972).  The  p r o l i f e r a t i o n of springs and seepages from the s o i l mantle, p a r t i c u l a r l y at the soil-impermeable t i l l  i n t e r f a c e as exposed along road cutslopes  and at the headscarps of l a n d s l i d e s , suggests that the basal s o i l horizons are susceptible to saturation and that t h i s condition may p e r s i s t f o r long periods of the year in drainage depressions and in other preferred subsurface drainageway zones.  It i s well known that saturated s o i l mantles  favour mass wasting, a f a c t borne out by the common a s s o c i a t i o n of l a n d s l i d e s with wet slope s i t e s . According to Cedergren (1967), s o i l water lowers s t a b i l i t y and contributes to slope f a i l u r e s by:(a) causing s o i l p a r t i c l e s to migrate to escape e x i t s , r e s u l t i n g in piping and erosional f a i l u r e s . (b) reducing or e l i m i n a t i n g cohesive strength. (c) increasing neutral pore water pressures and thereby reducing e f f e c t i v e stresses and shear strength. (d) producing h o r i z o n t a l l y i n c l i n e d seepage forces which increase downslope tangential forces on s o i l masses and the p o s s i b i l i t y of f a i l u r e . (e) l u b r i c a t i n g f a i l u r e planes a f t e r small i n i t i a l movements occur. (f) supplying an excess of f l u i d that becomes trapped i n s o i l pores during earthquake or other severe shocks and promotes l i q u i faction failures.  47  During heavy rainstorms the free water surface (phreatic  surface)  in the shallow, steepland s o i l s approaches the s o i l surface and accentuates some or a l l of the mechanisms described by Cedergren.  Possibly  a l l of these mechanisms influence the i n i t i a t i o n of f a i l u r e s in the study area but i t i s l i k e l y that the e f f e c t of s o i l water on the e f f e c t i v e stresses i n the s o i l mantle and the influence of seepage pressures are the factors of most importance. The pore water pressure in the s o i l i s r e l a t e d to the piezometric head (the height to which water r i s e s in a piezometer) by the expression  2 u  =  hp = v, = w  pore water pressure, kg/m  2 (lbs/ft )  piezometric head, m ( f t . ) 3 3 unit weight of water, kg/m ( l b s / f t )  The development of p o s i t i v e pore water pressures a f f e c t s the stabi l i t y of a sloping s o i l mass by reducing the e f f e c t i v e stress acting i n a d i r e c t i o n perpendicular to the p o t e n t i a l f a i l u r e surface.  The empirical  Coulomb's Law which can be w r i t t e n as s  =  c ' + a* tan 0' 2  s  =  c' =  — -  (2)  2  s o i l shear strength, kg/m  (lbs/ft ) 2 2 e f f e c t i v e s o i l cohesion, kg/m ( l b s / f t ) 2 2  a' =  e f f e c t i v e normal s t r e s s , kg/m  (lbs/ft )  0' =  e f f e c t i v e i n t e r n a l f r i c t i o n angle,  degrees  indicates that the s o i l shear strength i s d i r e c t l y dependent on the magnitude of the e f f e c t i v e normal stress a  =  a' = a - u t o t a l normal s t r e s s , kg/m  — — (lbs/ft )  —  (3)  48  In order to develop t h i s reasoning f u r t h e r , consider the diagram in Figure 14 showing part of a s l o p i n g , saturated, cohesionless through which water i s seeping downslope. meable t i l l  soil  The s o i l rests upon an imper-  layer and thus represents a s i t u a t i o n which i s not uncommon  in the study area.  Figure 14.  The free water surface coincides with the s o i l  surface  Flow net and neutral stresses f o r a s l o p i n g ,  i s o t r o p i c , saturated s o i l mantle r e s t i n g on impermeable substratum.  which i s p a r a l l e l to the potential s l i d e plane at the t i l l  surface.  It  is assumed that the s o i l i s i s o t r o p i c or that the maximum s o i l permeability i s in the d i r e c t i o n p a r a l l e l to the slope, permitting construction of a simple flow net.  The flow l i n e s  (the phreatic surface)  p a r a l l e l to the s o i l surface while the equipotential l i n e s 0 are normal to i t .  and ^ n  and  are  0 ] n+  This highly i d e a l i s e d flow s i t u a t i o n i s e x a c t l y the  49  same as that assumed by Taylor (1948) i n his s t a b i l i t y a n a l y s i s of an i n f i n i t e , cohesionless  slope with seepage o c c u r r i n g .  The pore water  pressures operating on the s o i l ' s basal plane, delineated by a  n  and  a ^ i , are shown as u = u ,, and are equivalent to the v e r t i c a l Ii = n+l n n+l n h -| m u l t i p l i e d by the u n i t weight of water.  The t o t a l u p l i f t force  n+  u a c t i n g on a^ - a ^ n +  is U  =  u  n >  Al  (4  -  )  assuming unit length i n the t h i r d dimension. Taylor shows that the e f f e c t i v e normal stress a' at depth z i s a' = (Y " Y ) . z - c o s a  (5a)  2  w  S  = Yk« « os z  c  (5b)  a  3  Y  S  = saturated s o i l unit weight, kg/m 3  Y^ = buoyant s o i l unit weight, kg/m and that the tangential stress T, a c t i n g p a r a l l e l to the slope on the potential f a i l u r e plane is represented by T = Y - z . s i n a . cosa  (6)  s  The e f f e c t of seepage forces applied by the moving water to the s o i l skeleton through f r i c t i o n a l drag are included i n the computation of the tangential stress T.  The seepage forces cause the tangential  stress to be proportional to the saturated unit weight of the s o i l whereas the e f f e c t i v e normal stress a ' i s proportional to the buoyant unit weight. It follows from consideration of the Mohr-Coulomb c r i t e r i a f o r s o i l strength (Lambe and Whitman, 1969) t h a t , f o r cohesionless m a t e r i a l s , -i = tan 0' (7a)  Y  = — tana b  -  Y  and that the steepest possible stable slope a  c r  is  (7b)  50  a As  = tan  c r  i s t y p i c a l l y about h a l f Y  tan 0 ' )  (8)  f o r many cohesionless m a t e r i a l s , the  s  saturation of a s o i l mantle accompanied by downslope seepage reduces the maximum stable slope ( d e c l i v i t y ) to approximately one h a l f that f o r an unsaturated cohesionless s o i l mantle.  Furthermore, the safety f a c t o r FS  f o r a saturated cohesionless slope with seepage i s defined as F S  =  — Y S  tan 0' tana  (9)  which y i e l d s a value approximately one h a l f that for a slope with no saturation and downslope subsurface flow. Two aspects of the groundwater hydrology of the steep slopes in the Coast Range appeared to warrant i n v e s t i g a t i o n : (a) establishment of the r e l a t i o n s h i p ( i f i t e x i s t s ) between the amount of r a i n f a l l received and the piezometric head at the base of the s o i l mantle. (b) ascertainment of the maximum pore water pressures which develop in the steepland s o i l s on varying topographies, p a r t i c u l a r l y i n l a n d s l i d e susceptible drainage  depressions.  Both aspects are i n t i m a t e l y t i e d into the estimation of in s i t u e f f e c t i v e stresses.  I f a defineable r e l a t i o n s h i p e x i s t s between r a i n f a l l  and piezometric head r i s e , then i t should be possible to predict the groundwater conditions i f given measured r a i n f a l l q u a n t i t i e s .  Moreover,  i f the maximum pore water pressures that are l i k e l y to develop during extreme storm periods could be p r e d i c t e d , then t h i s information could be used to q u a n t i t a t i v e l y define the most c r i t i c a l s t a b i l i t y conditions of p o t e n t i a l l y unstable t e r r a i n .  To i n v e s t i g a t e the rainfall-groundwater  conditions on steep slopes, r a i n f a l l and piezometric measurements were  51  undertaken on representative s i t e s .  Piezometric instrumentation Four s i t e s , three of which are considered to be p o t e n t i a l l y unstable, were selected f o r the piezometry study.  At each s i t e a l i n e of piezometers,  each recording the maximum piezometric head or maximum pressure head at the soi1-impermeable t i l l  or soil-bedrock i n t e r f a c e , was e s t a b l i s h e d .  S i t e SI i s a very shallow, l i n e a r depression located on an uneven poorly-drained slope in the middle Seymour catchment.  The s o i l s in the  depression are shallow, stoney and rest on an unweathered t i l l  mantle.  S i t e S2 consists of a narrow, steep depression which o r i g i n a t e s in a gentle, forested hollow and extends downslope onto a steep c l e a r f e l l e d slope.  The upper part of the depression i s underlain by shallow s o i l s  over bedrock but at lower elevations the s o i l s are deeper and r e s t on compacted t i l l .  A small debris s l i d e has r e c e n t l y formed i n the upper part of  the depression while a dry rocky channel extends along the bottom of the depression's  lower section but at no time was surface runoff observed at  this s i t e . S i t e S3 i s located on a very well drained, convex slope which forms an i n t e r f l u v e between two s m a l l , t r i b u t a r y stream v a l l e y s .  The r e l a t i v e l y  deep s o i l s on t h i s slope are underlain by coarse colluviurn and compacted till;  The s i t e was c l e a r f e l l e d and burnt approximately 5 years ago. S i t e Cl i s a steep, poorly drained depression underlain by stoney  s o i l s which show evidence of former i n s t a b i l i t y .  The depression  is  morphologically s i m i l a r to an adjacent depression in which l a n d s l i d e Cl3 occurred in 1969.  S i t e Cl i s , t h e r e f o r e , regarded by the present w r i t e r  as a l i k e l y future l a n d s l i d e l o c a t i o n .  The lower parts of the s i t e are  drained by a s m a l l , ephemeral stream channel.  Figure 15.  Topographic and s o i l c h a r a c t e r i s t i c s of the s i t e s  instrumented with piezometers.  53 Details of the instrumented s i t e s and the locations of the piezometers are shown in Figure 15. Two types of piezometers were i n s t a l l e d .  Both types of instruments  are considered to be simple piezometers rather than observation w e l l s , i n that they respond to water pressures at s p e c i f i c locations w i t h i n the s o i l mantle.  Figure 16 i l l u s t r a t e s the d e t a i l s of these instruments.  The  l a r g e r diameter piezometer tubes employ powdered cork which adheres to a steel rod to i n d i c a t e the maximum piezometric head r i s e .  A foam p l a s t i c  r i s e r i n s i d e a transparent, a c r y l i c tube performs a s i m i l a r function i n the smaller diameter piezometers.  Preliminary t e s t s indicated that both  types of instruments recorded r e l i a b l y in saturated sand. The tubes were inserted in augered holes which penetrated the s o i l mantle to the upper l i m i t s of the unweathered t i l l or the bedrock.  Coarse  sand was poured i n t o each hole so that the basal portion of each tube, when forced i n t o the sand, was surrounded by a permeable medium.  Each  tube was sealed i n place with a r e l a t i v e l y impermeable layer of c l a y .  Ten  cork type piezometers were established i n September 1970 and an a d d i t i o n a l 12 double-tube piezometers were i n s t a l l e d in August 1971.  The double-tube  piezometers were e a s i e r to i n s e r t and read i n the f i e l d than the l a r g e r tubes but the use of the t h i n - w a l l e d , aluminium conduit allowed bending by snowcreep at the ground surface. R a i n f a l l was measured with 2 automatic, 12 inch capacity gauges located i n the upper Capilano catchment at s i t e Cl and in the upper Seymour Valley approximately 2 km WNW of s i t e SI r e s p e c t i v e l y . were v i s i t e d as soon as possible a f t e r each major storm.  The piezometers  54  rubber  -soil  stopper  surface  8 inch steel pipe 2 inches i.d.  '  32 inch aluminium _  w  o  conduit  5 inch i.d. 5 inch diameter steel  rod  16 inch acrylic recording tube i inch i.d.clay  layer  foam plastic  --jr^-ltf*powdered  cork nylon water  uniform —  Figure 16.  riser  gauze  cover  intake  holes  sand  layer  unweathered  till  Details of the two types of piezometers used i n the study,  (a) As water level r i s e s i n tube powdered cork i s c a r r i e d upwards. A f t e r reaching point of maximum r i s e water descends leaving cork adhered t6 steel rod which can. be withdrawn from tube f o r measurement of piezometric head,  (b) Rising water l e v e l i n ijiner tube c a r r i e s  buoyant, foam-plastic r i s e r upwards. a c r y l i c tube  The water f i l m between f l o a t and  permits r i s e r to remain at point of maximum water r i s e  which i s measured by removing inner tube.  55  Results Heavy r a i n f a l l was found to cause sharp increases in the piezometric head at most piezometer s t a t i o n s .  Table 9 l i s t s the maximum recorded  r i s e at each s t a t i o n and the equivalent pore water pressures.  The  average maximum piezometric head f o r a l l stations w i t h i n drainage depressions was 585 mm whereas that f o r the s t a t i o n s on the well drained, convex slope was only 103 mm. Figure 17 i l l u s t r a t e s the seasonal trends in piezometric head at two stations during 1970, 1971 and 1972.  Groundwater l e v e l s in drainage  depressions remained low during the winter months (January to A p r i l i n c l u s i v e ) but with the onset of snowmelt in A p r i l , l e v e l s rose and t h e r e a f t e r f l u c t u a t e d about a r e l a t i v e l y high average level u n t i l  late  November when colder weather and a change from r a i n to snow caused decline.  However, on the well drained, convex slope detectable r i s e s  in the pressure head only occurred during the l a r g e s t storms. The groundwater l e v e l in drainage depressions responds in a very s e n s i t i v e manner to r a i n f a l l as shown in Figure 18.  Within one hour  a f t e r the commencement of heavy r a i n of the intense but b r i e f September storm, the l e v e l continued to r i s e at a rate of approximately 60 mm per hour.  Unfortunately, the time of maximum r i s e was not determined but  was presumed to coincide with the cessation of heavy r a i n f a l l at 8 p.m. on the 10th September. The r e l a t i o n s h i p s between piezometric head and accumulated d a i l y rainfall  (24 hour t o t a l r a i n f a l l ) at 19 piezometer s i t e s are shown in  Figures 19, 20 and 21.  The c a l c u l a t e d curves which best describe these  data (Table 10) are expressions of the polynomial Y = b  Q  + b  1  1+ b  2  X  -  (10)  56 Table 9 Maximum Piezometric Head and Equivalent Pore Water Pressure Recorded at Piezometer Stations in the Coast Range, Southwestern B r i t i s h Columbia  Piezometer  Maximum piezometric head mm  k  SI -1 SI-2 SI-3 SI-4 SI-5 Sl-6  737 457 886 432 711 732  S2-1 S2-2 S2-3 S2-4 S2-5  356 737 406 Did 853  S3-1 S3-2 S3-3  132 81 97  Cl-1 Cl -2 Cl-3 Cl-4 Cl-5 Cl-6* Cl-7* Cl-8*  Did 292 597 404 615 813 508 409  not  inches  kg/m  29.0 18.0 34.9 17.0 28.0 28.8  737 457 886 432 711 732  150.8 93.6 181.5 88.4 145.6 149.8  356 737 406  72.8 150.8 83.2  853  174.7  132 81 97  27.0 16.6 19.8  292 597 404 615 813 508 409  59.8 122.2 82.7 125.8 166.4 104.0 83.7  14.0 29.0 16.0 function 33.6  correctly  5.2 3.2 3.8 not  Equivalent pore water pressure  function 11.5 23.5 15.9 24.2 32.0 20.0 16.1  correctly  2  lbs/ft  2  Maximum piezometric heads and pore water pressures a f t e r the rapid ablation and disappearance of a 1.7 meter deep snowpack are l i s t e d be.l ow. 851 Cl-6 851 33.5 174.2 Cl-7 889 35.0 889 182.0 Cl-8 1,029 40.5 1,029 210.6  Figure 17.  Piezometric head v a r i a t i o n s during 1970, 1971 and 1972 at  s t a t i o n s Cl-4 and C l - 5 , upper Capilano catchment.  58  Figure 19.  Relationships between piezometric head and d a i l y r a i n f a l l  Figure 20.  Relationships between piezometric head and  daily r a i n f a l l .  61  Figure 21.  Relationships between piezometric head and d a i l y r a i n f a l l .  62  Y  =  piezometric head in m i l l i m e t e r s  X  =  accumulated d a i l y r a i n f a l l in m i l l i m e t e r s  Table 10 Regression Equations Describing Relationships Between Piezometric Head (Y) and Daily R a i n f a l l  (X), Coast Range, Southwestern B r i t i s h  Columbia  2 R *  Piezometer  Equation of best f i t  SI-1  Y = 432.5 - 2 , 1 7 2 . l l + 4.3X  .91  51.7  13  Sl-2  Y = 388.7 - 2,017.9j + 0.7X  .79  44.5  13  SI-3  Y = 537.5 *** 3 3 37 5 • 3 ^+ 5.8X  .89  80.8  15  SI-4  Y =  590.7j + 4.3X  .86  46.7  15  SI-5  Y = 641.6 - 2,608.0^ + 1.5X  .92  34.8  14  Sl-6  Y = 753.7  3) 557 • 3"^"+ 0.2X  .94  33.9  13  S2-1  Y = 242.0  + 2.IX  .85  36.2  10  S2-2  Y = 189.9  + 7.3X  .93  53.0  12  S2-3  Y = 333.6  + 7.2X  .96  45.9  12  Cl-2  Y = 342.3 - 1,591.3^ + 0.5X  .98  11.0  10  Cl-3  Y = 289.4 - 1,225.81 + 3.9X  .96  35.0  10  Cl-4  Y = 220.0 -  705.2^- + 2.OX  .87  32.0  21  Cl-5  Y = 377.8 - 1,506.91 + 3.OX  .92  41.1  21  Cl-6  Y = 342.8 -  1,006.6^ + 4.4X  .88  59.4  21  Cl-7  Y = 274.6 - 1 ,225.2^-+ 2.7X  .97  27.6  9  Cl-8  Y =  .97  28.7  6  81.6 -  —  33.6 -  48.2^ + 4.IX  2 *R  =  m u l t i p l e c o e f f i c i e n t of determination  **SE^  =  standard e r r o r of estimate  SE^**  The curves vary considerably from piezometer to piezometer, presumably a r e f l e c t i o n of s p a t i a l differences i n s o i l physical  prop-  e r t i e s , source area of subsurface flow and configuration of the t i l l  or  bedrock-soil i n t e r f a c e .  Relationship between piezometric head, r a i n f a l l and subsurface flow A r i s e in the free water surface and, consequently, in the piezometric head w i l l only occur i f the supply of water from the s o i l surface and from i n t e r f l o w and excessive to any t r a n s p i r a t i o n al l o s s , exceeds the seepage a b i l i t y of the s o i l . Chamberlin  The piezometer study, combined with the findings of  (1972), indicated t h a t , during heavy r a i n f a l l , water passes  r e a d i l y through the s o i l mantle and moves over the impermeable t i l l  or  bedrock in a shallow, saturated l a y e r at the base of the B horizon. Flow i s directed towards the drainage depressions  from adjacent slopes.  The concentration of subsurface water in the depressions causes a rapid increase in the thickness of the saturated zone and a concomitant increase in the s o i l ' s a b i l i t y to conduct water downslope. Swanston (1967a) proposed that the r e l a t i o n s h i p s between increments of r a i n f a l l  ( A P ) , the downslope subsurface flow rate ( A Q ) and changes in  piezometric head (Ah ) could be described by the simple model depicted in Figure 22.  During a rainstorm, small i n i t i a l values of Q and a large  volume of water moving downslope cause hp to increase r a p i d l y .  As hp  increases, Q also becomes l a r g e r which, u l t i m a t e l y , leads to a condition of e q u i l i b r i u m between r a i n f a l l , seepage and piezometric head. The nature of t h i s r e l a t i o n s h i p depends on the s o i l a b i l i t y to conduct water downslope.  mantle's  Spatial v a r i a b i l i t y in s o i l permeab-  i l i t y may p a r t i a l l y account for the variable response of the piezometric head between piezometer stations during the same storm.  The saturated  64  AP  Figure 22.  Hypothetical r e l a t i o n s h i p between r a i n f a l l ,  seepage and piezometric head.  subsurface  (After Swanston 1967a).  permeabilities of eight large s o i l samples taken from the lower B horizon near piezometers SI-2  (samples 1 and 7 ) , SI-3  (samples 2, 3 and  8) and Cl-5 (samples 4, 5 and 6) were determined in the laboratory by a constant head method s i m i l a r to the method described by Klute (1966). C i r c u l a r disks of s o i l , approximately 14 cm in diameter and 10 cm long, were c a r e f u l l y trimmed from l a r g e , i n t a c t s o i l blocks and placed in 16 cm long, a c r y l i c c y l i n d e r s f i t t e d with a perforated plate at one end. The sides of the disks were sealed to the a c r y l i c c y l i n d e r w a l l s with a sealing compound.  The samples were selected so that 6 t e s t s d e t e r -  mined the permeability p a r a l l e l to the natural slope i n the f i e l d and 2 t e s t s provided estimates of the v e r t i c a l permeability.  Decayed root  channels as well as undecayed roots were interspersed w i t h i n the s o i l disks which were soaked in water f o r several hours p r i o r to t e s t i n g . The t e s t r e s u l t s are presented in Table 11. Table 11 Saturated Permeability C o e f f i c i e n t s of Steepland Subsoils, Coast Range, Southwestern B r i t i s h Columbia  Sample  Orientation  Permeability C o e f f i c i e n t cm/sec  1  p a r a l l e l to slope  0.0058  2  0.0056  3  "  "  "  0.0051  4  "  "  "  0.0055  5 6 7  0.0007 "  "  "  vertical  0.0010 0.0005  8  "  0.0030  9*  "  0.00002  * Sample of unweathered, compacted t i l l .  The mean downslope permeability was 0.0039 cm/sec (standard deviation = 0.0024 cm/sec) equivalent to 337 cm/day which corresponds to a rapid class of permeability according to O'Neal 1952 (in Klute 1965). t i l l , by comparison, has a very slow permeability.  The  These r e s u l t s imply  that the B horizon i s capable of t r a n s m i t t i n g large volumes of water downslope and that substantial changes in permeability occur from one part of a slope to another. In a recent study, Aubertin (1971) demonstrated that macropores, including those r e s u l t i n g from the decay of tree roots, g r e a t l y increase  66  the permeability of f o r e s t s o i l s by permitting rapid transmittance of water along paths of low r e s i s t a n c e .  Variations in the c o n t i n u i t y and  density of root channels w i t h i n the B horizon of drainage depression s o i l s probably contribute to v a r i a b l e permeability and to uneven p i e z o metric surfaces.  Estimation of maximim pore water pressures During the period of observation, the maximum recorded d a i l y r a i n f a l l s in the upper Seymour catchment and the upper Capilano catchment were 74 mm and 86 mm r e s p e c t i v e l y .  Many of the curves in Figures 19,  20 and 21 suggest that d a i l y r a i n f a l l s of 130 to 150 mm would cause the piezometric head to r i s e to the s o i l surface.  According to the r a i n f a l l  records at Seymour F a l l s , r a i n f a l l events of these magnitudes have an expected recurrence i n t e r v a l of approximately 2 years.  As the pressure  head shown in Figure 19, 20 and 21 i s probably less than the height of the phreatic surface above the impermeable substratum under conditions of downslope seepage, i t can s a f e l y be assumed that most drainage depression s o i l s saturate completely during very heavy rainstorms of t h i s magnitude.  The presence of ephemeral surface runoff channels and  i n c i p i e n t channels in many depressions substantiates t h i s assumption. A b r i e f reconsideration of Figure 14 permits a comparison of the calculated pore pressures, assuming complete saturation and seepage p a r a l l e l to the slope, with the pressures measured in the f i e l d .  If  values of 90 cm and 35° are assigned to z and a r e s p e c t i v e l y , then the pore water pressure u at the base of the s o i l mantle i n Figure 14 i s 2 U  =  Z . y  W  . C O S  = 604 kg/m  A  2  "  (11  )  67 which corresponds to a piezometric head of 604mm.  However, t h i s  was exceeded at eight piezometer s t a t i o n s with comparable slopes  pressure (Sl-1,  S l - 3 , S l - 5 , S l - 6 , S2-2, S2-5, Cl-5 and Cl-6) even though the s o i l mantle at four of these s i t e s was less than 90 cm deep, i n d i c a t i n g that pore pressures are l a r g e r than the pressure given by equation 11 and may approach the hydrostatic pressure. 2 = 900 kg/m  f o r z = 90 cm.  In Figure 14 the hydrostatic pressure  The curves i n Figures 19, 20 and 21 i n d i c a t e  that pore pressures of t h i s magnitude can be expected in some drainage depression s i t e s when d a i l y r a i n f a l l s exceed 120 mm. The maximum recorded piezometric heads at three stations occurred during the 1972 snowmelt period (Table 9).  At the beginning of March  1972 the snowpack at s i t e C-l was approximately 1.5 m deep but when the s i t e was r e v i s i t e d on the 3rd May the snowpack had almost disappeared. During A p r i l , r a i n on r a p i d l y melting snow caused the piezometric head l e v e l s at stations C l - 6 , . C l - 7 and Cl-8 to r i s e to heights above the ground surface.  P o s s i b l y , at some stage during the snow a b l a t i o n period,  the s o i l mantle and the lower part of the snow pack saturated to produce large piezometric heads.  Klock (1972) discovered that the water retention  c a p a b i l i t y of a sandy s o i l was increased and the permeability was decreased at temperatures close to 0°C.  Low temperature snowmelt water may cause  a decrease in the c o n d u c t i v i t y of the Coast Range steepland s o i l s which promotes a buildup of pressure head in the s o i l mantle. The piezometric data provides no i n d i c a t i o n that c l e a r f e l l i n g promotes increased s o i l pore water pressures although the small numbers of piezometers involved in the study precluded s t a t i s t i c a l t e s t i n g of this.  The average maximum pore water pressure recorded at the four  stations established in forested drainage depressions ( C l - 7 , C l - 8 , Sl-6 2 2 and S2-5) was 626 kg/m compared with 573 kg/m f o r the t h i r t e e n  68 staions i n s t a l l e d in c l e a r f e l l e d drainage  depressions.  Pore pressures and e f f e c t i v e stresses The piezometer study established that pore water pressures of 800 2 kg/m  or more may develop in the basal regions of the s o i l mantle in  drainage depressions.  The magnitude of the e f f e c t i v e stress  changes  which accompany such pressure r i s e s can be gauged from Figure 23 which compares a moist, sloping s o i l mantle with a saturated s o i l mantle with seepage.  Typical f i e l d values of y » Y » <*> u, and z are used f o r m  s  computing the e f f e c t i v e normal stresses acting on the potential f a i l u r e 2 plane. A pore water pressure increase of 800 kg/m at the base of the s o i l mantle causes a 76 percent decrease i n the e f f e c t i v e normal stress a ' which, as shown by equation 2, reduces the s o i l ' s resistance to shear.  Under moist s o i l conditions Chamberlin  (1972) measured pore water 2  tensions (negative pore water pressures) of approximately ( 20 cm water) near the base of the f o r e s t s o i l mantle. pressures r a i s e the normal e f f e c t i v e s t r e s s .  200 kg/m Negative  Consequently, the value  f o r the normal e f f e c t i v e stress f o r the moist condition shown in Figure 23 i s a conservative estimate as pore water tensions were not taken i n t o consideration.  The e f f e c t s of pore pressures on slope s t a b i l i t y w i l l be  examined in more d e t a i l in Chapter 4. Conclusions (1) Heavy r a i n f a l l causes a rapid r i s e in the piezometric head measured at the base of the s o i l mantle in drainage depression s i t e s . However, on well drained slopes measureable piezometric heads only develop during very large storms.  69  Figure 23.  E f f e c t i v e normal stress a c t i n g on a potential s l i d e plane  in the case of (a) a moist, sloping s o i l mantle with no seepage and (b) a saturated, sloping s o i l mantle with seepage.  70  (2) At most piezometric stations a well defined, c u r v i l i n e a r r e l a t i o n s h i p e x i s t s between the d a i l y r a i n f a l l received and the piezometric head.  Such a r e l a t i o n s h i p permits the estimation of pore water  pressures i f r a i n f a l l i s measured. 2 (3) In many drainage depressions,  pore water pressures of 800 kg/m  or more can be expected in the lower B horizon during intense  rainstorms.  Pressures of t h i s magnitude r e s u l t in substantial reductions in the e f f e c t i v e normal stress acting on potential f a i l u r e planes at the base of the B horizon which, in t u r n , decrease the slope s t a b i l i t y .  71  CHAPTER 3 THE IMPORTANCE OF TREE ROOTS TO SLOPE STABILITY  Introduction Although the r o l e played by tree roots in the s t a b i l i z a t i o n of steep slopes has been conjectured by several i n v e s t i g a t o r s  (Wentworth  1943, White 1949, Penck 1953, Bishop and Stevens 1964, Swanston 1969, Gray 1969, Gonsior and Gardner 1971, Fujiwara 1970, and Rice and Foggin 1971) very l i t t l e q u a n t i t a t i v e work has been accomplished which i n d i c a t e s to what degree and by what mechanisms the tree root network enhances the s o i l ' s resistance to f a i l u r e .  Less i s known about the r a p i d i t y at which  the added mechanical strength provided by roots, disappears a f t e r a forest stand i s c l e a r f e l l e d .  There i s l i t t l e doubt that the s t a b i l i z i n g  functions of root systems are very important, p a r t i c u l a r l y where s o i l mantles are shallow and mass wasting occurs w i t h i n the rooting zone as i s the case in the Coast Mountains study area. the s o i l down to the compacted t i l l  Where the tree roots penetrate  or substratum, root systems may not  only add mechanical strength to the s o i l mantle but also modify the stress d i s t r i b u t i o n w i t h i n the s o i l mass by t r a n s f e r r i n g surcharge loads to the substratum.  Furthermore, i f roots penetrate the basal t i l l  or cracks i n  the bedrock then the root network helps to attach the s o i l mantle to the underlying substratum. On the other hand, where trees are blown over or overturned by g r a v i t y , tree roots are responsible f o r l o c a l disturbances of the s o i l mantle (Lutz and Gfiswold 1939, Stephens 1956 and Stone 1969) which may contribute to l a r g e r , damaging slope f a i l u r e s (Wright and M i l l e r 1952, Schweinfurth 1967, Jackson 1966, and Swanston 1969).  However, these  72  adverse e f f e c t s are generally considered to be i n s i g n i f i c a n t , at l e a s t in temperate regions, compared to the s t a b i l i z i n g functions performed by roots (Gray 1969). Coast Range.  This c l e a r l y i s the case in the B r i t i s h Columbia  In f a c t , in a region where c l e a r f e l l i n g i s associated with  an increased frequency of l a n d s l i d e s , i t could be argued that the strength reinforcement provided by roots, or lack of i t , i s the p r i n c i p a l f a c t o r controlling soil s t a b i l i t y . In t h i s chapter an attempt i s made to assess the importance of tree root systems to the shearing resistance of the s o i l and the r a p i d i t y at which the additional strength provided by roots decreases a f t e r c l e a r felling.  Rooting habits of f o r e s t trees on the Coast Range slopes The root development of f o r e s t trees i s a most important consideration i n an evaluation of the e f f e c t s of f o r e s t vegetation on slope stability.  Deep rooted species are more l i k e l y to o f f e r greater protection  against s o i l f a i l u r e than shallow rooted species and widely-spreading root systems which intermingle with adjacent root systems serve a greater protective function than r e s t r i c t e d root systems which form a dense, i s o l a t e d b a l l at the base of the tree stem (Kostler et_ al_, 1968). Although a s p e c i f i c study of root development was not conducted in the present study, general information on rooting habits was provided by exposed root systems at the margins of l a n d s l i d e s , along fresh road cuts and at the s i t e s of wind thrown t r e e s . The rooting habits of the major tree species in the study area are strongly influenced by the physical c h a r a c t e r i s t i c s of the steepland s o i l s , p a r t i c u l a r l y t h e i r depth.  On shallow s o i l s i t e s where basal  till  73  or bedrock occurs w i t h i n 50 centimeters of the ground surface, western red cedar, western hemlock and mountain hemlock develop p l a t e l i k e root systems which display vigorous l a t e r a l root development (Figure 24). The western cedar root system shown in Figure 24 was exposed when the parent tree (98 centimeters dbh) was blown over a few months before the photograph was taken.  The oval root p l a t e , which measured 8.5 meters  by 5.0 meters, consisted of a dense r a d i a l system of large l a t e r a l roots up to 25 centimeters in diameter which gave r i s e to smaller secondary, t e r t i a r y and quarternary roots.  The deepest l a t e r a l roots forming  the basal surface of the root plate had extended to a depth of 30 c e n t i meters where they had been t i g h t l y pressed against the smooth surface of the d i o r i t e bedrock.  Roots up to 4 centimeters t h i c k had penetrated a  l a r g e , open j o i n t in the d i o r i t e (Figure 25).  S i m i l a r p l a t e - l i k e root  systems were observed on overturned western and mountain hemlock trees growing on shallow s o i l s over bedrock.  Large Douglas f i r trees growing  on shallow, rocky s o i l s tend to develop massive l a t e r a l roots which may extend more than 5 meters from the root stock.  Lloyd e_t al_ (1956) point  out that p l a t e - l i k e root systems are j u s t as c h a r a c t e r i s t i c of Douglas f i r as they are of other species where s o i l s are shallow. On deeper s o i l s the root systems of Douglas f i r and cedar extend into the lower B horizon.  A p a r t l y exposed Douglas f i r root system  attached to a stump 140 centimeters in diameter, extended 96 centimeters in depth to the upper l i m i t s of a compacted t i l l  substratum on a steep,  c l e a r f e l l e d slope i n the middle Seymour catchment.  An excavation under  the stump showed that the root system consisted of at l e a s t three l a r g e , c e n t r a l l y - s i t u a t e d , v e r t i c a l roots up to 30 centimeters in diameter and many large l a t e r a l s with numerous branches mainly concentrated in the  74  Figure 24.  The exposed root system of a r e c e n t l y overturned  western red cedar.  The lower surface of the p l a t e - l i k e root system  had developed in d i r e c t contact with smooth bedrock.  Figure 25.  Recently exposed western red cedar root penetrating  an open j o i n t i n d i o r i t e bedrock (a).  75  upper 50 centimeters of s o i l .  Smaller secondary and t e r t i a r y r o o t s ,  many derived from the central v e r t i c a l s i n k e r s , were common in the lower B horizon but did not enter the t i l l  substratum.  Although Douglas f i r i s  c h a r a c t e r i s t i c a l l y a deep rooting species (McMinn 1963) t h i s a t t r i b u t e i s not manifested in the steepland s o i l s where basal t i l l  or bedrock  forms an impedence to roots. Freshly-exposed, unweathered t i l l  surfaces i n the bottoms of l a n d -  s l i d e scars often e x h i b i t imprints of roots which followed tenuous courses across the t i l l ' s surface, but, except f o r small roots embedded in the top few centimeters of t i l l , very few larger roots were observed to penetrate the tough, unweathered t i l l  matrix.  Schlots et al_ (1956)  noted that Douglas f i r r o o t s - f a i l e d to penetrate compacted Vashon t i l l in western Washington but P a t r i c and Swanston (1968) discovered that cedar roots f r e e l y entered s i m i l a r compacted t i l l  in Alaska.  Small cedar  or mountain hemlock roots did enter cracks i n fractured compacted t i l l in the upper regions of l a n d s l i d e s CIO and C12. In summary, the roots of mature Douglas f i r , cedar and western hemlock u t i l i z e most of the s o i l volume on the steep slopes underlain by shallow s o i l s and, although they do not generally penetrate the compacted till  matrix, roots do extend through cracks and j o i n t s i n the t i l l  bedrock substratum.  or the  Presumably, root systems contribute s i g n i f i c a n t l y  to the s t a b i l i t y of the forested slopes by binding the s o i l mass together and by helping to anchor the s o i l mantle to the substratum.  A f i e l d study of the e f f e c t of tree roots on the s o i l ' s resistance to shear The influence of tree, roots on the shearing strength of s o i l s  has,  to the present w r i t e r ' s knowledge, only been measured d i r e c t l y in Japan.  76  Endo and Tsuruta (1968) conducted a number of f i e l d shear t e s t s on a uniform nursery s o i l containing varying q u a n t i t i e s of l i v i n g alder r o o t s . Using a large shear-box device equipped with a gauge or recorder to measure deformation, they found that s o i l shear strength increased with the bulk weight of roots in the s o i l and that the r e l a t i o n s h i p could be adequately described by a l i n e a r equation with the form S  =  r  a(R + b)  -  (12)  2 = increased s o i l shear strength due to roots, kg/m 3 R  = bulk weight of fresh roots, g/m  a and b = empirical constants A s i m i l a r study by Takahasi (1968) indicated that the apparent cohesion of a f i e l d s o i l depended on the quantity of white b i r c h roots i n the s o i l according to the formula C  =  a - bD + cW 2  C  = apparent s o i l cohesion, kg/m  D  = depth of shear plane, cm  —  (13)  2 W a, b, c  = weight of b i r c h r o o t s , kg/m  (weight of roots per volume of  s o i l , D x 1m x lm) = empirical constants  The d i f f i c u l t i e s involved in measuring the component of s o i l  strength  provided by the tree root network are compounded under conditions of s o i l heterogeneity.  In f a c t , the determination of meaningful shear strength  parameters of extremely v a r i a b l e bouldery and g r a v e l l y s o i l s i s a d i f f i c u l t i f not impossible task.  The problems were minimized by the  Japanese workers by studying r e l a t i v e l y homogeneous s o i l s on f l a t , uniform sites.  However, e x t r a p o l a t i o n of t h e i r r e s u l t s to heterogeneous mountain  s o i l s would be of dubious value.  Accordingly, the mountain s o i l s of the  77  Coast Range were investigated d i r e c t l y in an attempt to determine the r e l a t i o n s h i p between the tree root network and the steepland s o i l s ' resistance to shear. Methods.  Direct shear t e s t s were conducted on the subsoils of two  f r e s h l y c l e a r f e l l e d slopes formerly occupied by mixtures of old growth Douglas f i r , cedar and hemlock and on the subsoils of a forested slope supporting ah old-growth cedar-western hemlock stand.  The s o i l s at the  t e s t s i t e s were representative of s o i l Categories 1 and 2, a l l possessing yellowish-brown, g r a v e l l y sandy-loam B horizons generally less than 100 cm deep and r e s t i n g on compacted basal t i l l or d i o r i t e bedrock. A portable, steel shear-box, 30.5 cm x 30.5 cm x 15.3 cm (1 f t . x 1 f t . x 0.5 f t . ) in s i z e was employed (Figure 26).  The shearing force  was provided by a small hand winch while the magnitude of the shearing force was recorded with a set of high q u a l i t y scales of capacity 90.7 kg (200 lbs) and s e n s i t i v i t y of 0.22 kg (0.5 l b s ) .  At each t e s t s i t e an  excavation was dug to i s o l a t e a r e l a t i v e l y undisturbed block or column of subsoil approximately the same s i z e as the shear-box.  Each column  was c a r e f u l l y trimmed with a k n i f e so that the shear-box f i t t e d snugly over the column which supported the f u l l weight of the box and any a d d i t ional load applied to the top of the box.  The blocks were sheared at  t h e i r bases and the shear planes normally sloped towards the v a l l e y bottom at angles between 5° and 10°.  The shearing force was applied in the  downslope d i r e c t i o n . Attempts to conduct the t e s t s at constant displacement rates were unsuccessful.  Rather, the shearing force was increased by 4.5 kg (10  lbs) increments each 60 seconds up to the time when displacement rates increased to approximately 0.5 cm per minute.  Thereafter, 2.3 kg (5 l b s )  78  -30-5 cmE o in  6  rO  Shear box constructed of 1/8" steel plate-  •30-5 cm-  fK Anchor  3—•f^t*  Scales  Winch  Figure 26.  Shear box  D e t a i l s of shearbox construction .and i t s mode of operation  i n the f i e l d .  79  shear load increments were applied every 60 seconds u n t i l sudden or complete f a i l u r e occurred.  The maximum shear strength of the s o i l was  taken to be the maximum shear force recorded on the scales divided by the area of the s o i l column.  During each t e s t the shear displacement was  measured with a displacement gauge.  Tests in which the s o i l  columns crumbled instead of f a i l i n g along a reasonably w e l l - d e f i n e d shear plane were regarded as unsatisfactory and were not taken i n t o account. Approximately 40 s o i l columns were cut but only 24 successful tests were completed. At the completion of each t e s t a 2,500 c c , r e l a t i v e l y undisturbed s o i l sample was r e t r i e v e d from the s o i l immediately adjacent to the f a i l u r e plane and taken to the laboratory f o r p a r t i c l e s i z e analysis and determinations of moisture content, unit weight and root weight content. The technique adopted f o r c o l l e c t i n g the s o i l samples was rather laborious but e f f e c t i v e .  A roughly c i r c u l a r s o i l column, approximately  20 cm in diameter and 15 cm t a l l , was i s o l a t e d at each t e s t s i t e and a l a r g e , c i r c u l a r , metal container, 15 cm in diameter and 14.5 cm deep, was placed, open end down, on top of the column.  The sides of the column  were c a r e f u l l y trimmed with a knife and protruding roots were cut with a secateur so that the container could be worked down over the column without s e r i o u s l y d i s t u r b i n g the s o i l .  A f t e r the container was f u l l y  occupied the column was cut flush with the open end of the container which was then sealed with a l i d . Samples were weighed in the undisturbed s t a t e , then broken up and the roots separated by hand and weighed and, f i n a l l y , s o i l and roots were ovendried at 100° and reweighed.  P a r t i c l e s i z e d i s t r i b u t i o n s were  80  measured by dry s i e v i n g and, f o r p a r t i c l e s passing a No. 200 s i e v e , by hydrometer analysis according to the methods described by Lambe(1951). The mean diameter and weight of p a r t i c l e s retained on 5 cm (2 in) sieve were measured separately. Five a d d i t i o n a l samples of steepland subsoils from the 40 to 60 cm depth zone were c o l l e c t e d in the same manner as described above f o r determination of the saturated unit weight (y ) and the void r a t i o (e).  Two  s m a l l , undisturbed samples were also taken from the unweathered t i l l  sub-  stratum f o r s i m i l a r determinations.  Methods and r e s u l t s are presented  in Appendix 2. A l l the s o i l columns tested contained r o o t s , mostly less than 2 cm i n diameter, and, as the shear plane depths were s i t u a t e d between 40 and 60 cm below the o r i g i n a l ground surface, most of the roots were presumed to be tree roots.  Results and d i s c u s s i o n .  The d e t a i l s of the s o i l s tested and the  t e s t r e s u l t s are summarized in Table 12.  At the t e s t loads employed  (normal loads on the shear plane), shear strengths ranged from 208 kg/m to 415 kg/m . 2  S o i l columns which did not contain large boulders or large roots over 4 cm in diameter generally f a i l e d along a well defined plane.  As  the columns d i s p l a c e d , small roots crossing the shear plane were usually sheared o f f d i r e c t l y but larger roots over 0.5 cm in diameter tended to p u l l out of the s o i l block or f a i l in t e n s i o n . Typical shearing force-displacement curves are shown in Figure 27. S o i l s with many roots (curves 4 and 15) attained t h e i r peak shear force at a displacement of 2.6 cm and 2.1 cm r e s p e c t i v e l y , but the curves f o r  T a b l e 12.  f,  1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24  R e s u l t s o f D i r e c t Shear T e s t s on S o i l s C o n t a i n i n g Roots  Normal load kg-  Normal stress kg/m  Shear load kg  Shear stress kg/m  Water content g/g  Dry u n i t weight kg/m  % Gravel  29.4 29.1 29.4 29.1 29.4 29.1 29.4 29.2 29.4 29.1 29.1 29.2 29.1 29.1 29.1 29.2 29.2 29.4 29.4 29.2 29.4 29.4 29.2 29.2  316.5 313.2 316.5 313.2 316.5 313.2 316.5 314.3 316.5 313.2 313.2 314.3 313.2 313.2 313.2 314.3 314.3 316.5 316.5 314.3 316.5 316.5 314.3 314.3  31.3 31.5 20.4 37.2 19.5 22.5 19.5 29.9 29.0 26.8 22.2 22.0 19.3 20.9 36.3 27.2 20.0 21.8 20.9 38.6 34.0 34.0 20.4 21.8  336.9 339.3 219.7 400.3 209.9 241.7 209.9 322.2 312.4 288.0 239.2 236.8 207.5 224.6 390.6 292.9 214.8 234.3 224.6 415.0 366.2 366.2 219.7 234.3  44.9 35.7 36.6 38.1 39.6 48.6 33.7 21.3 20.5 33.4 20.6 25.4 23.6 18.2 30.7 48.6 36.1 25.3 33.5 49.8 38.8 31.0 34.6 36.1  1,090 1,037 1,207 965 878 910 917 1,377 1,391 1,066 1,112 1,192 1,341 1,388 1,256 936 1,074 1,066 1,066 918 1,045 1,163 1,149 1,098  51.8 61.6 67.4 41.1 51.2 60.0 61.5 61.9 61.9 59.8 64.2 60.4 63.1 69.4 69.5 50.1 59.1 60.9 60.9 70.2 49.4 65.8 66.5 56.5  27.6 24.2 16.2 47.2 40.7 34.2 31.1 27.4 27.4 31.4 27.6 29.9 30.3 26.0 20.1 35.1 30.8 29.4 29.4 22.0 41.2 27.4 26.0 34.9  19.0 13.9 16.3 11.3 7.5 4.9 6.7 9.2 9.2 7.1 6.2 8.1 5.3 3.3 10.2 12.2 9.1 9.7 9.7 6.4 9.4 6.7 7.0 8.6  1.6 0.3 0.1 0.4 0.6 0.9 0.7 1.5 1.5 1.7 2.0 1.6 1.3 1.3 0.2 2.6 1.0 0.1 0.1 1.4 0.1 0.1 0.5 0.1  2; 000 1,300 2,700 9,300 400 1,300 400 1,100 700 3,700 3,300 1,300 2,000 400 7,100 2,700 700 400 400 8,000 5,300 2,700 2,700 400  26.1  281.1  33.5  1,107  60.1  30.0  9.0  0.9  2,513  2  2  % Sand  % Silt  % Clay  3  Root content g/m 3  " I n c l u d e s w e i g h t o f s o i l column above s h e a r p l a n e p l u s w e i g h t o f s h e a r box a d j u s t e d f o r s l o p e a c c o r d i n g t o t h e r e l a t i o n N = W cosa.  50  Curves 4 and 15 for soils with many roots Curves 3 and 7 for soils with few roots  40 E o £  30  O O _l  o 20 x: in  15 10  0 00  10  20  ±__L  3-0  Shear  Figure 27.  Displacement,centimeters  Shear load vs shear displacement curves f o r four  d i r e c t shear t e s t s on steepland s o i l s - containing roots.  83  s o i l s with few roots (curves 3 and 7) show that the peak shear forces coincide with r e l a t i v e l y small displacements.  Possibly roots impart an  additional e l a s t i c i t y to the s o i l which enables i t to undergo considerable s t r a i n without f a i l i n g completely. Because of the inherent c r u d i t y of the t e s t i n g procedure (the s h o r t comings of d i r e c t shear t e s t s discussed by Sowers and Sowers 1970, Lambe (1951) and Sowers (1963) were added to by the poor experimental control that existed during t e s t i n g i n the f i e l d ) , the r e s u l t s of the experiment provide no more than a broad i n d i c a t i o n of the e f f e c t s of the root network and the physical properties of the s o i l on i t s resistance to shear.  Table 12 indicates that the s o i l columns varied considerably  in t e x t u r e , moisture content and dry unit weight as well as in root content.  In order to ascertain the r e l a t i v e influence that these  variables exercise on the s o i l ' s shear resistance the data were subjected to a m u l t i p l e regression a n a l y s i s . Y  =  Y  =  X-| = X  2  =  b  Q  + bX 1  ]  The model adopted had the form  + b X 2  2  +•  b X y  dry unit weight, kg/m percent gravel by weight  X^ =  percent sand by weight  X  =  percent s i l t by weight  Xg =  percent clay by weight  Xy =  (14)  2 s o i l shear r e s i s t a n c e , kg/m 3 3 s o i l moisture content, cm /cm 3  X^ =  5  y  fresh root weight, g/m  3  of s o i l  To what extent the data v i o l a t e the basic assumptions of  regression  and j u s t how s e r i o u s l y the e x i s t i n g v i o l a t i o n s a f f e c t the analysis are not known by the present w r i t e r .  The most serious inherent weakness of  84  the analysis may derive from a lack of independency among the p r e d i c t o r v a r i a b l e s , p a r t i c u l a r l y i n the cases of Xg and  which, according to  the c o r r e l a t i o n matrix f o r these data, are negatively c o r r e l a t e d (r^ 4 0.8).  =  Beginning with the t o t a l model (equation 3) a stepwise e l i m i n a t i o n procedure was adopted in which the independent variables were eliminated one by one, e l i m i n a t i o n at each stage depending on that v a r i a b l e d i s p l a y ing the l e a s t s i g n i f i c a n t p a r t i a l F value.  The t o t a l regression model p  accounts f o r 70 percent of the v a r i a t i o n i n s o i l shear strength (R n = 24).  = 0.70,  Successive e l i m i n a t i o n of X^, X^, Xg, Xg and X^ reduced the  regression model to Y  = b  Q  + b X 5  5  + b X 7  (15)  -r  ?  2 which explains 60 percent of the v a r i a t i o n i n strength (R n = 24) and i s s i g n i f i c a n t at the 0.01 l e v e l ( F  2  20  =  = 0.60,  16.0).  Throughout  the e l i m i n a t i o n procedure, X-, remained the most s i g n i f i c a n t v a r i a b l e , exemplifying the importance of roots to the steepland s o i l s '  strength.  The simple regression of s o i l shear resistance on the s o i l root content (X ) accounted f o r 56 percent of the t d t a l shear resistance v a r i a t i o n 2 (r = 0.56, n = 24). The equation i s 7  Y  =  229.4 + 0.02 X  -----  ?  and is s i g n i f i c a n t at the 0.01 level (F^  2  2  =  28  -r  «1)«  (16)  This r e l a t i o n s h i p  applied s p e c i f i c a l l y to the conditions of normal loading during the 2 tests (approximately 315 kg/m ).  The l a s t term in equationl6 provides  a f i r s t approximation of the s o i l ' s apparent cohesion due to the binding e f f e c t s of the roots. C a  =  The r e l a t i o n s h i p can be represented by 0.02 R  -  -  (17)  85  C a  =  apparent cohesion, kg/m  R  =  root weight, g/m  3  However, as the shear t e s t s are influenced only by the v e r t i c a l roots crossing the p o t e n t i a l shear plane at the base of the columns, t h i s empirical estimate of the s o i l ' s apparent cohesion i s probably very conservative.  In the undisturbed s o i l mantle, l a t e r a l roots as well as  v e r t i c a l roots intermingle and may sometimes g r a f t together (Eis to help bind the s o i l i n t o a coherent l a y e r .  1972)  Furthermore, columns could  not be s u c c e s s f u l l y cut in s o i l s containing roots with diameters l a r g e r than 3 to 4 centimeters.  Therefore the shear t e s t s measured only the  strengthening e f f e c t of the smaller tree roots. A second series of d i r e c t shear tests were made in the f i e l d to e s t a b l i s h an estimate of the angle of i n t e r n a l f r i c t i o n of the steepland s u b s o i l s .  Nine d i r e c t shear t e s t s were conducted on a g r a v e l l y -  sandy-loam B horizon developed over compacted t i l l on a 20 degree slope in the middle Seymour catchment. previously.  The slope had been c l e a r f e l l e d 6 years  The subsoils selected f o r the t e s t s contained no l i v i n g  roots except f o r occasional fireweed (Epilobium angustifolium)  roots  l e s s than 1 m i l l i m e t e r in diameter and very few s m a l l , decayed roots. At three s i t e s l a r g e , undisturbed blocks of subsoils were i s o l a t e d by careful excavation and then each block was divided i n t o several s o i l columns separated by narrow trenches.  The methods used remained the  same as those used to t e s t s o i l s with roots but the loading conditions normal to the potential shear plane were v a r i e d .  Three successful  tests,  each with a d i f f e r e n t loading c o n d i t i o n , were completed at each s i t e . The depths of the shearing planes ranged between 40 and 60 cm below the o r i g i n a l ground surface.  At the time of t e s t i n g the s o i l s were  86  extremely moist but not saturated.  The r e s u l t s of the tests are pres-  ented i n Table 13 and shown g r a p h i c a l l y i n Figure 28. Because only the normal stresses  {a) and shear stresses  (T) on a  s i n g l e plane are known i n a d i r e c t shear t e s t , i t i s necessary to assume that the stresses at f a i l u r e are i n the r a t i o — = tan 0 (Lambe and a Whitman 1969) i n the case of cohesionless s o i l s .  The low c l a y and s i l t  content of the steepland subsoils suggests that these materials are e s s e n t i a l l y cohesionless.  The curves of T vs a f i t t e d to the three sets  of test r e s u l t s (Figure 28) produce f r i c t i o n angles of 40, 41 and 34 degrees r e s p e c t i v e l y . If values of the apparent cohesion estimated from equation 17, an i n t e r n a l f r i c t i o n angle of 40 degrees, and e f f e c t i v e normal stress  values  s i m i l a r to those which can be expected at the basal regions of a t y p i c a l sloping s o i l mantle i n the study area, are inserted i n t o Coulomb's shear strength equation, then i t i s possible to assess the importance of roots to the t o t a l shear resistance of the steepland s o i l s i n q u a n t i t a t i v e terms.. For a moderately dense tree root network (R = 8,000 g/m ) and an unsaturated, 90 centimeter deep s o i l sloping at 35 degrees ( a = 900 1  kg/m , see Figure 23) the shear resistance at the basal plane i s s = 160 + 900 . 0.839 =915-kg/m  2  and the root network provides approximately 18 percent of the t o t a l shear strength.  However, under saturated s o i l conditions the e f f e c t i v e 2  normal stress reduces to 200 kg/m (Figure 23) and the shear resistance at the basal plane i s s = 160 +200 . 0.839 =328 kg/m  2  In t h i s case the root network provides 49 percent of the t o t a l strength.  shear  Table 13  Results of Direct Shear Tests on Steepland S o i l s with no roots  S i t e One Normal load kg  Normal stress kg/m2  Maximum shear load kg  S i t e Two  Maximum shear stress kg/m 2  Maximum shear load kg  Maximum shear stress kg/m2  S i t e Three Maximum shear load kg  Maximum shear stress kg/m2  29.5  318  21.8  235  25.3  272  30.7  330  48.0  517  33.6  362  32.3  348  41.3  445  81.5  877  63.2  680  58.1  625  72.3  778  Dry unit weight kg/m  1,180  1,055  1,094  Moisture content cm /cm  38.6  35.0  27.8  % gravel  62.0  53.8  49.9  % sand  27.3  36.4  39.1  % silt  8.5  8.0  11.2  % clay  2.2  1.8  0.8  3  3  3  With the aid of an i n f i n i t e slope analysis (Taylor 1948, Lambeand Whitman 1969) the safety factors f o r the two conditions of slope draina can be computed and used to check whether or not the estimates of shear strength are reasonable.  The safety f a c t o r FS f o r a slope i s defined  by Lambeand Whitman as Fs =  a v a i l a b l e shear strength shear stress required f o r e q u i l i b r i u m  1,000  Figure 28.  Results of d i r e c t shear t e s t s on .steepland s o i l s  plotted on a shear stress (T) vs normal s t r e s s (a) diagram.  89  and, in the case of the hypothetical slopes shown i n Figure 23, Chapter 2, i s evaluated at the soil-unweathered t i l l i n t e r f a c e . For the unsaturated condition FS  =  C + a tan 0 a Y . z . sina . cosa 1  =  m  915 626  1.46  Q 1 C  and f o r the saturated condition FS  =  C + a ' tan 0  =  a  Y  s  . z . sina . cosa  ^  =  Q  A  7  702  T h e o r e t i c a l l y , a slope with a safety f a c t o r less than 1.0 cannot remain in a s t a b l e condition and must f a i l .  As poorly drained s o i l s  subject to p e r i o d i c saturation e x i s t on slopes of 35 degrees or steeper in the study area, the very low FS c a l c u l a t e d f o r the saturated condition presumably derives p r i n c i p a l l y from an inadequate estimate of the apparent cohesion C due to the tree roots. a  To r a i s e the safety f a c t o r to a 2  value of 1.0 the apparent cohesion must be increased to 540 kg/m  which,  judging by the estimates of cohesion due to tree roots made by Endo and Tsuruta (1968) and Takahasi (1968), i s a more r e a l i s t i c value than the estimate provided by equation 17. Then, at s a t u r a t i o n , the tree network accounts f o r 71 percent of the s o i l ' s shear strength. Although the estimates of the apparent cohesion and the s o i l ' s f r i c t i o n angle are extremely crude, these c a l c u l a t i o n s serve to show that the s t a b i l i t y of s a t u r a t i o n - s u s c e p t i b l e s o i l s of drainage  depressions  and of other poorly-drained, sloping s i t e s may be l a r g e l y dependent on the cohesion derived from the root network.  Loss of t h i s root strength-  ening e f f e c t through c l e a r f e l l i n g and subsequent decay of the root network w i l l , according to the analyses presented above, predispose such s o i l s  90  to imminent f a i l u r e during heavy rainstorm periods when the s o i l mantle a t t a i n s a condition of p a r t i a l or complete s a t u r a t i o n .  Laboratory t e s t i n g of root strength The d e t e r i o r a t i o n of tree roots accompanied by a decrease i n t h e i r strength i s believed to be the most important means by which c l e a r f e l l i n g reduces the s t a b i l i t y of steep, forested slopes (Croft and Adams 1950, Bishop and Stevens 1964, and Fujiwara 1970).  However, very few attempts  have been made to measure the strength of roots d i r e c t l y and less i s known about the rate at which t h i s strength i s l o s t a f t e r the death of the parent t r e e .  Strength measurements of root wood of various hardwood  and c o n i f e r t r e e species reported by Fengel i n Brown et_ al_ (1952) i n d i c a t e that root wood i s mechanically weaker than stem wood.  A  d e t a i l e d study of the strength of l i v i n g tree roots was conducted by Turmanina (1965).  He tested the roots, sampled from several species of  hardwood and c o n i f e r s , i n tension in order to d i s t i n g u i s h those species with the strongest root systems and, t h e r e f o r e , with the most d e s i r a b l e q u a l i t i e s f o r r e i n f o r c i n g s o i l s against l a n d s l i d e s . The condition of partly-exposed roots at the headscarps and l a t e r a l scarps of many l a n d s l i d e scars in the Coast Range study area suggests that a high percentage of the l a r g e r , s t r u c t u a l roots,as well as the smaller roots at the margins of l a n d s l i d e s , f a i l  in tension.  roots often extend some distance out from the head and l a t e r a l  Broken scarps  (Figure 7 ) , i n d i c a t i n g that they were subjected to considerable p u l l i n g forces before they f i n a l l y ruptured.  Shear f a i l u r e s as well as tension  f a i l u r e s were discovered at most l a n d s l i d e s i t e s but, g e n e r a l l y , a l a r g e r number of roots appear to f a i l  in tension.  During the shear box  91  experiment i t was also noted that many roots f a i l e d i n tension. Therefore, the t e n s i l e strength of tree roots appears to be a very important mechanical property c o n t r i b u t i n g to the s t a b i l i t y of slopes. The purpose of the research reported i n t h i s section was to determine the t e n s i l e strength of tree roots sampled from l i v i n g trees and i d e n t i f y the rate at which the root strength deteriorates a f t e r the parent trees are f e l l e d .  Methods.  Roots were taken from mature, l i v i n g , Douglas f i r , western  red cedar and western hemlock trees and from Douglas f i r and cedar stumps on slopes c l e a r f e l l e d at various times in the past (Table 14). growing in the B horizon of poorly drained s o i l s in drainage and on other wet s i t e s were c o l l e c t e d .  Only roots depressions  At each s i t e , roots were taken  from two or three trees or stumps of the same species.  In order to ensure  that i n t e r - t r e e root grafts did not influence the r e s u l t s of the strength t e s t s on roots from stumps, only those stumps located well away from l i v i n g trees at the margins of c l e a r c u t areas, were sampled. . Eis (1972) has shown that i n t e r - t r e e root g r a f t s have permitted Douglas f i r stumps to remain a l i v e and to continue to grow f o r many years a f t e r the parent trees were f e l l e d .  The root samples, which ranged from 1 m i l l i m e t e r to  2 centimeters i n diameter, were excavated with a hand shovel, traced to t h e i r respective parent tree or stump, l a b e l l e d , sealed in p l a s t i c bags and taken to the laboratory f o r t e s t i n g .  Uniform, s t r a i g h t  sections  were selected from the root samples, trimmed to a length of 25 centimeters and debarked.  Approximately 5 to 10 samples were prepared i n t h i s manner  at one time and then promptly tested i n tension to avoid excessive moisture loss from the root t i s s u e s .  Wangaard (1950) pointed out t h a t ,  92  above the f i b r e saturation point (25 to 30 percent water content by weight f o r most woods), the strength properties of wood are l i t t l e affected by changes i n moisture content.  However, i f the wood water  content i s permitted to f a l l below the f i b r e saturation p o i n t , then marked increases in strength may occur. Root t e s t i n g was performed with a Floor Model TT-CML Instron U n i versal Testing Machine equipped with an Instron 5,000 kilogram c a p a c i t y , r e v e r s i b l e l o a d c e l l (No. 2511.304).  Type 3D pneumatic-hydraulic clamps  (Figures 29 and 30) with f l a t , non-serrated jaw faces 5 centimeters square were employed to g r i p the root ends.  The load speed and recorder chart  speed were set at 2 cm per minute f o r a l l tension t e s t s . The determination of a s a t i s f a c t o r y technique f o r gripping the root sample ends without permitting slippage or damage to the wood tissues posed the most serious experimental problem.  Preliminary exper-  iments showed that tension tests could be conducted s a t i s f a c t o r i l y without damage to the root ends i f short segments of p l a s t i c tubing about 4 cm long were f i t t e d over the root ends and attached with epoxy-resin glue. A clamping force of approximately 300 kilograms at the jaw faces was s u f f i c i e n t to prevent slippage up to tensions of 100 kilograms.  Further  experimentation showed that gluing was not necessary i f a clamping force of 550 to 600 kilograms was applied to the root ends protected by p l a s t i c sheathing.  Under these conditions t e n s i l e forces of 100 kilograms  or more could be applied to the roots without slippage and with only minor crushing of the root ends. At the beginning of each t e s t the upper and lower sets of jaws were set at 18 centimeters apart; t h i s represented the length of the root section strained in tension.  Each t e s t was continued u n t i l the  Figure 29.  Instron Universal Testing Machine showing recorder  housing ( f a r l e f t ) , holding clamps and control panel ( f a r r i g h t ) .  Figure 30.  Details of the pneumatic holding clamps (Type 3D).  94  root, which was strained at a constant rate of 2 centimeters per minute, ruptured.  If the root fractured at the jaw edges, then the r e s u l t s of  that t e s t were not considered.  A f t e r the completion of each t e s t , the  mean diameter of the root was measured with a d i a l gauge near the point of breakage and the equivalent root c r o s s - s e c t i o n a l area was computed. Test r e s u l t s were automatically recorded as a p l o t of t e n s i l e load in kilograms against time in minutes (Figure 31) and from these data the ultimate t e n s i l e load in kilograms and the t o t a l l o n g i t u d i n a l deformation ( A l ) i n centimeters were determined.  Ultimate t e n s i l e loads were conver-  ted into maximum t e n s i l e strengths i n kilograms per square centimeter. Approximately 200 tests were completed, i n c l u d i n g a l i m i t e d number oh l i v i n g roots of red alder and fireweed (Epilobium a n g u s t i f o l i u m ) , a vigorous c o l o n i z e r of c l e a r c u t areas in the Coast Range.  Results and d i s c u s s i o n . results.  Table 14 presents a summary of the t e s t  Regressions of t e n s i l e strength on mean root diameter were  c a l c u l a t e d f o r the 12 groups of cedar and Douglas f i r roots.  A l l the  r e l a t i o n s h i p s were n o n s i g n i f i c a n t , i n d i c a t i n g t h a t , w i t h i n the rather narrow range of root diameters sampled, diameter does not s i g n i f i c a n t l y influence the t e n s i l e strength. The t e n s i l e strength of cedar and Douglas f i r roots declined r a p i d l y with increasing time a f t e r f e l l i n g of the parent t r e e s .  Douglas f i r roots  sampled from stumps which had been cut.3 years previously, possessed less than h a l f the mean strength of roots taken from l i v i n g t r e e s .  Cedar  roots displayed a slower decline in strength up to 5 years a f t e r c u t t i n g of the parent tree at which time approximately h a l f t h e i r o r i g i n a l mean t e n s i l e strength had been l o s t .  These trends are shown i n Figure 32.  Only l i v i n g roots of western hemlock were t e s t e d .  L i v i n g roots of red  Root from living Douglas fir 4-0 CD  RP 30  CD  C S area =0-059 c m Load at RP = 35-2 kg Maximum tensile strength =593-0 kg , Deformation at RP = 2-5cm ' E = 1317 k g / c m 2  C  e  2-0  E  Tangent line  1-0  / c m  2  00  10  15  20  25  30  35  L o a d , kilograms Root from Douglas fir stump cut 3 years before test CO CD  CS area = 0-116 c m L o a d at RP = 22-5 kg Tangent line yf Maximum tensile strength = 194-8 k g / c m Deformation at RP = 0-8 cm E = 6840 k g / c m 2  3 C  CD  E  2  5  10  ±  15 20 .. 25 L o a d , kilograms  Root from Douglas fir stump cut 10 years before test CO CD C  CD  £  30 C S area = 0-245 c m Load at RP = 9-0 kg Maximum tensile strength = 36-7 k g / c m Deformation at RP =0-2 cm 2  2-0 1-0  o-o*=»=»——J'  ±  2  5 10 15 Load,kilograms  Figure 31.  Results of t e n s i l e strength t e s t s on three Douglas f i r  roots in d i f f e r e n t states of d e t e r i o r a t i o n . PL = proportional  limit.  RP = rupture p o i n t ;  2  96 T a b l e 14.  Diameter- (cm) and t e n s i l e s t r e n g t h  2 (kg/cm ) o f r o o t s  sampled  f r o m l i v i n g t r e e s and f r o m stumps o f f o r m e r t r e e s  (1)  Western r e d c e d a r  LivingI t r e e  Period elapsed 2  3  NB  B  s i n c e stump was c u t ( y e a r s ) 5  B  6  B  8  B  Diam  TS  Diam  TS  Diam  TS  Diam  TS  Diam  TS  Diam  TS  .190 .104 .246 .157 .145 .155 .157 .152 .175 .185 .135 .104 .086 .132 .079 .261  485.4 337.2 393.3 510.9 854.0 812.1 677.2 454.4 389.5 502.2 562.4 834.5 396.1 450.2 870.0 585.5  .175 .122 .099 .310 .274 .079 .368 .478 .188 .201 .185 .145 .254 .368 .272 .310  398.0 473.8 451.1 507.6 522.3 496.0 142.8 351.2 562.3 480.8 442.8 435.2 217.7 356.9 551.1 450.4  .173 .249 .213 .231 .318 .335 .284 .277 .300 .518 .490 .523 .503 .513 .511 .254  47,3.5 4d3.0 372.0 362.4 364.2 306.8 325.6 336.6 268.8 411.4 185.2 235.5 259.1 198.1 220.9 349.2  .521 .368 .406 .490 .267 .112 .140 .203 .445 .419 .337 .368 .411 .277 .298 .165  351.3 341.0 216.9 317.5 207.2 237.6 515.5 198.4 269.3 136.6 266.0 429.2 363.9 169.4 131.2 287.5  .597 .919 .552 .752 .528 .411 .488 .429 .267 .305 .272 .272 .348 .333 .374 .137  130.4 35.4 173.9 71.4 113.5 26.3 41.7 75.3 97.8 18.1  .368 .485 .358 .721 .663 .638 .453 .482 .669 .485 .238 .372 .364 .745 .521 .330  49.0 30.9 49.6 70.5 37.3 25.7 69.5 74.5 38.7 56.2 81.8 57.7 28.7 35.9 24.5 43.5  569.7 (2)  427.5  317.0  277.4  .89.8  64.0 94.7 87.5 76.4 189.5  48.4  86.6  Douglas f i r  Living tree  P e r i o d e l a p s e d s i n c e stump was c u t ( y e a r s ) 2  NB  3  B  5  B  8  B  10  B  Diam  TS  Diam  TS  Diam  TS  Diam  TS  Diam  TS  Diam  TS  .163 .206 .203 .533 .274 .208 .147 .231 .384 .345 .140 .127 .152 .229 .114  687.7 432.1 511.4 519.5 59 3.0 453.2 590.2 443.5 457.2 552.6 728.2 966.6 961.1 652.5 889.3  .292 .191 .564 .572 .325 .312 .231 .259 .501 .372 .249 .357 .441 .208 .419  402.3 602.3 416.5 412.8 550.2 412.9 441.1 398.7 439.4 468.8 293.4 482.7 518.9 442.2 592.1  .140 .384 .279 .157 .277 .351 .203 .165 .288 .129 .180 .341 .258 .435 .608  180.8 194.8 228.4 253.2 181.7 124.0 238.7 234.8 204.7 191.2 220.8 103.8 267.8 288.9 119.4  .495 .246 .376 .470 .500 .538 .620 .478 .455 .531 .511 .762 .447 .447 .221  124.9 207.1 118.9 123.9 76.7 57.9 115.2 139.4 130.4 150.0 134.5 126.5 169.0 156.3 183.8  .510 .393 .402 .510 .584 .367 .421 .693 .450 .718 .936 .580 .386 .647 .671  20.3 52.1 7.2 13.9 33.3 44.1 37.8 20.8 26.6 46.0 50.8 30.8 46.5 57.9 40.8  1.242 .724 .533 .965 .559 .605 .457 .737 .582 .990 .731 1.140 .647 .850 .858  43.2 29.1 20.6 6.4 36.7 19.2 21.7 30.7 25.2 26.4 15.7 30.6 35.2 16.8 17.9  629.2  458.3  202.2  134.3  35.3  25.2  Period separating time of cutting of parent tree and time of root strength test (Years )  Figure 32.  Diagram showing the change i n t e n s i l e strength of small  Douglas f i r and cedar roots with change i n the time elapsed since f e l l i n g of the parent t r e e .  98  alder and fireweed were considerably weaker than l i v i n g c o n i f e r r o o t s . The root strength data f o r Douglas f i r and western red cedar were investigated with analyses of variance and Duncan's m u l t i p l e range t e s t s in order to determine whether or not the differences i n mean root strength at various times a f t e r c l e a r f e l l i n g were s i g n i f i c a n t .  The t e s t r e s u l t s  are summarized in Table 15. Table 15 Results of Analyses of Variance and M u l t i p l e Range Tests f o r Douglas f i r and Western Red Cedar Root Strength Data  (1)  Douglas f i r  One way analysis of variance y i e l d e d an  =  119.48**  A Duncan's m u l t i p l e range t e s t on the root strength means of the s i x root classes produced the f o l l o w i n g r e s u l t s at the 0.05 level Living tree  (2)  Period elapsed since f e l l i n g of parent tree 2 3 5 8 10  (yrs)  Western red cedar  One way analysis of variance y i e l d e d an F^  5  g Q  j  =  56.41**  A Duncan's m u l t i p l e range t e s t on the root strength means of the s i x root classes produced the f o l l o w i n g r e s u l t s at the 0.05 l e v e l Living tree  Period elapsed since f e l l i n g of parent tree 2 3 5 6 8  (yrs)  99  Both ANOVA tests were highly s i g n i f i c a n t .  The M u l t i p l e Range tests  indicated t h a t , w i t h i n the same species, the mean t e n s i l e strength decreased s i g n i f i c a n t l y from one class of roots to the next class  (pro-  gressing from l e f t to r i g h t ) except in those cases where adjacent classes are underlined. From the traces of t e n s i l e load against time i t was possible to i n v e s t i g a t e the e l a s t i c behaviour of roots i n tension.  The load-time  curves f o r roots sampled from l i v i n g trees (Figure 31) e s s e n t i a l l y consist of an i n i t i a l s t r a i g h t portion w i t h i n the e l a s t i c range and a curved portion which l i e s in the p l a s t i c range. sometimes apparent but was disregarded.  An i n i t i a l curvature i s  That part of a curve separating  the e l a s t i c range from the p l a s t i c range is c a l l e d the proportional  limit  and i s delineated where a l i n e , tangent to the curve in the e l a s t i c range, deviates from the curve (Brown et_ al_ 1952). The modulus of e l a s t i c i t y represented by r  _  T  E  "  S. A  . 1 _  — -  (18)  Al  2 E  =  modulus of e l a s t i c i t y , kg/cm  Tp  =  t e n s i l e load at proportional l i m i t , kg 2  A  =  cross sectional area of root specimen, cm  1  =  gage length of specimen = 18 cm  =  deformation at proportional l i m i t , cm  Al  was determined f o r cedar and Douglas f i r roots taken from l i v i n g trees and from stumps cut 2 years and 3 years p r i o r to t e s t i n g .  Roots sampled  from older stumps generally produced load-time curves which did not possess defineable proportional l i m i t s .  The t o t a l deformation at the rupture point  Table 16 Total deformation A l (cm) at the Rupture Point and Modulus of p E l a s t i c i t y (E) kg/cm f o r Cedar and Douglas F i r Roots Tested i n Tension  (1) L i v i n g tree Al  Cedar  Time elapsed since stump was cut (years) 2 3 5 6 8 Al  Al  Al  Al  Al  2.0 2.0 1.5 1.3 1.3 2.6 1.3 2.2 2.8 2.8 3.4 3.7 1.7 2.8 2.2 2.9  33,300 5,010 5,105 12,630 17,640 7,756 9,376 6,428 7,515 3,228 8,550 8,460 15,030 6,000 2,000 5,618  2.4 2.1 1.7 0.9 2.4 2.2 0.9 1.1 2.4 2.3 1.0 1.5 2.0 1.5 2.0 2.4  5,348 7,515 11,655 15,840 12,600 18,000 4,260 9,270 10,680 12,690 13,320 42,300 1,959 846 1,089 798  3.2 2.2 3.0 1.1 0.9 2.9 3.6 3.0 3.1 1.6 2.0 1.1 2.8 1.4 1.6 1.1  6,264 6,680 3,420 8,550 7,515 6,840 4,110 6,345 4,980 11,610 7,020 3,394 7,020 31,750 5,220 7,853  1.0 1.3 0.6 1.5 0.5 1.2 1.1 0.4 0.7 0.2 0.9 1.2 0.4 0.3 0.8 1.2  0.5 0.6 1.0 0.4 0.6 0.1 0.2 0.5 0.5 0.2 0.3 0.2 0.4 1.4 0.3 0.4  0.5 0.3 0.9 0.4 0.1 0.2 0.1 1.0 0.4 0.2 0.6 0.8 0.1 0.2 0.5 0.2  2.3  9,605  1.8  10,511  2.2  8,036  0.8  0.5  0.4  (2)  Douglas f i r  L i v i n g tree Al  3.0 1.5 2.7 2.5 3.1 1.7 2.7 2.0 1.3 2.5 1.5 2.5 1.4 1.7 1.8  E 2,004 16,596 25,020 1,317 9,818 9,652 4,635 3,750 4,860 11,430 7,950 7,290 6,840 8,460 11,475  Time elapsed since stump was cut (years) 2 3 5 8 10 Al  1.0 2.4 0.6 1.1 2.5 1.7 1.6 1.8 1.7 1.3 2.2 2.0 1.8 0.6 1.5  E 12,060 3,870 7,920 16,110 10,305 5,683 11,400 11,880 6,250 8,742 6,810 6,645 10,130 4,200 5.810  Al  E  1.1 4,500 0.8 7,380 0.9 11,790 0.8 6,624 0.6 5,265 0.8 3,690 0.6 9,360 0.8 6,840 1.7 8,860 0.9 10,420 0.8 5,204 1.9 3,418 0.5 8,610 0,8 9,730 0,9 7,110  Al  0.9 0.9 0.8 0.8 0.6 0.7 0.8 0.9 0.8 1.2 0.7 0.7 '1.1 1.1 09 f  Al  Al  0.5 0.8 0.4 0.2 0.2 0.5 0.9 1.2 0.4 1.3 1.2 0.6 0.8 0.4 0.6  0.5 0.8 0.2 0.1 0.2 0.2 0.1 0.4 0.8 0.2 0.9 0.1 0.3 0.7 I.T  101  was also estimated f o r a l l t e s t s .  The r e s u l t s appear i n Table 16.  The mean deformation at the rupture point decreased from 2.3 and 2.1 centimeters f o r roots taken from l i v i n g cedar and Douglas f i r , r e s p e c t i v e l y , to 0.8 and 0.9 centimeters f o r roots sampled from cedar and Douglas f i r stumps cut 5 years previously.  The modulus of e l a s -  t i c i t y , however, did not show a s i m i l a r , well-marked trend as the roots d e t e r i o r a t e d , suggesting that the s t i f f n e s s or r i g i d i t y of the wood does not a l t e r during the e a r l y stages of decay.  According to Kennedy (1958)  e l a s t i c i t y i s one of the mechanical properties of wood l e a s t a f f e c t e d by decay, at l e a s t in i t s e a r l y stages. The a b i l i t y of l i v i n g roots to elongate without rupturing in response to t e n s i l e stresses may permit the s o i l mantle on steep slopes to undergo small scale d i f f e r e n t i a l movements or creep without serious loss of strength. The rapid strength d e t e r i o r a t i o n of the smaller tree roots, such as tested in t h i s study, i s presumably the consequence of fungal decay, although d e t a i l e d microscopic examinations of root wood were not made. According to Wallis and Ginns (1968), the stumps of most coniferous species i n Coastal B r i t i s h Columbia are susceptible to Fomes spore i n f e c t i o n .  annosus  Following i n f e c t i o n , the stumps and roots of Douglas  f i r and of western hemlock,in p a r t i c u l a r , are r a p i d l y invaded by the fungus and often s u f f e r advanced root decay w i t h i n 2 to 3 years. However, the rapid root d e t e r i o r a t i o n rates measured i n the experiment reported above are u n l i k e l y to be representative of the whole tree root system because the l a r g e r s t r u c t u a l roots of Douglas f i r and cedar may remain reasonably sound and i n t a c t f o r many years a f t e r the parent trees are cut down.  For instance, old cedar stumps in the lower Capilano v a l l e y  102  on areas c l e a r f e l l e d in the 1930's, have retained t h e i r roots of diameters greater than approximately 15 cm.  Although these roots are in  an advanced state of decay, some r e t a i n central cores of r e l a t i v e l y sound wood.  The smaller members of these old root systems have decayed and  disappeared, probably many years ago.  McMinn (1963) also noted that  many of the massive roots of a mature Douglas f i r tree f e l l e d more than 25 years previously were only p a r t i a l l y decayed. The d u r a b i l i t y of large roots raises the question as to whether or not the smaller roots less than, say, 3 centimeters in diameter, are very important to slope s t a b i l i t y .  The evidence provided by measurements of  root sizes at the l a n d s l i d e s i t e s suggests that they are.  At the head-  scarps of three l a n d s l i d e s (S4, CIO and C12 i n Table 4) the diameters of 150 exposed, broken roots growing in s o i l or t i l l  below a depth of  approximately 45 centimeters were measured with a p a i r of c a l i p e r s . The mean diameters were 1.4 cm, 0.7 cm and 1.1 cm (Table 17), emphasising that small roots c o n s t i t u t e a major part of the root network i n the lower B horizon.  This confirms the findings of McMinn (1963) who  discovered that large Douglas f i r roots tend to p r o l i f e r a t e when they extend i n t o stoney or bouldery subsoils and that large roots are mainly confined to the upper s o i l l a y e r s .  In very shallow s o i l s , however, large  roots extend to the base of the B horizon and may be one of the main reasons why s o i l s less than 40 centimeters deep are very r e s i s t a n t to large scale mass wasting. The changes which occur in root strength and, consequently, i n the s o i l ' s resistance to f a i l u r e a f t e r c l e a r f e l l i n g have several management implications.  I f the decaying root network i n the s o i l of a steep,  c l e a r f e l l e d slope i s q u i c k l y reinforced with a vigorously expanding, replacement network from natural f o r e s t regrowth or a r t i f i c i a l l y planted  103  Table 17 Means, extremes and standard deviations of diameters of roots growing in subsoils below 45 centimeters at l a n d s l i d e headscarps  Landslide No.  No.of roots measured  Mean diameter cm  Max diameter cm  Min diameter cm  Standard deviation cm  S4  50  1.4  8.8  0.1  1.9  CIO  50  0.7  3.2  0.1  0.8  C12  50  1.1  5.2  0.1  1.1  c o n i f e r s , then i t i s conceivable that the reduction in slope s t a b i l i t y brought about by c l e a r f e l l i n g w i l l be minimal.  The root study reported  above i n d i c a t e s that a s u b s t a n t i a l loss of root strength occurs w i t h i n 3 to 5 years a f t e r c l e a r f e l l i n g .  Such a period provides very l i t t l e time  for the regrowth vegetation to develop an e f f e c t i v e root network.  On  the g r a v e l l y , sandy-loam, steepland s o i l s of the Coast Range, root d e v e l opment of Douglas f i r and hemlock seedlings i s slow.  On c l e a r f e l l e d  areas i n the Seymour catchment, Douglas f i r s a p l i n g s , planted approximately 5 years previously, possess root systems which are predominantly confined to the upper 40 centimeters of s o i l .  Pruning of taproots in the nursery  may account f o r these r e l a t i v e l y shallow root systems.  A l l Douglas f i r  planting stock used in the study area receives two and usually three separate root prunings while being raised in the nursery (Mr. Armit, B.C. Forest Service nursery, Surrey; personal communication).  McMinn  (1963) points out that an aggressive taproot i s l i k e l y to be a d e s i r a b l e s u r v i v a l c h a r a c t e r i s t i c , p a r t i c u l a r l y on low f e r t i l i t y s i t e s or on dry  104  sites.  An aggressive taproot which imparts strength to the subsoil  would also be a d e s i r a b l e c h a r a c t e r i s t i c on steep, unstable s i t e s . Therefore, the use of unwrenched, c o n t a i n e r i s e d , Douglas f i r seedlings rather than open-grown, root-pruned seedlings f o r revegetating steep, p o t e n t i a l l y unstable slopes would provide some advantages. I f the decay rate of the root systems of c l e a r f e l l e d trees could be retarded u n t i l the young replacement root network was well established throughout the s o i l mantle, then the impact of the decay e f f e c t on the s t a b i l i t y of sloping s o i l s might be reduced considerably.  The a p p l i c a t i o n  of borax to stumps (Wallis and Ginns 1968) immediately a f t e r f e l l i n g prevents fungal i n f e c t i o n and i s , perhaps, the most obvious and most p r a c t i c a l means of slowing the process of root d e t e r i o r a t i o n .  For  p r a c t i c a l purposes t h i s treatment could be r e s t r i c t e d to steep drainage depression  s i t e s and to areas over 35° in slope.  The exceptional a b i l i t y of red alder to colonize and grow r a p i d l y on wet, cutover slopes and exposed surfaces of t i l l  and s u b s o i l s , at  the same time producing a widely spreading root system of slender l a t e r a l s , makes t h i s species p o t e n t i a l l y valuable f o r s t a b i l i z i n g steepland s o i l s . The establishment of a l d e r plants from nursery raised seedlings or from cuttings on p o t e n t i a l l y unstable, c l e a r f e l l e d slopes, a p r a c t i c e which i s common in parts of Europe and in New Zealand, could prove to be an e f f e c t i v e counter-landslide measure i n the Coast Range of B r i t i s h Columbia.  Summary and conclusions (1) General observations of the growth habits of tree roots i n the Coast Range steepland s o i l s suggest that roots help bind the noncohesive  105  s o i l s into a coherent mantle and anchor the s o i l mantle to the substratum. (2) Direct shear tests conducted i n the f i e l d provide a d d i t i o n a l evidence that the tree roots do, indeed, provide a cohesive e f f e c t which may, in s o i l s containing high root d e n s i t i e s , increase the s o i l shear strength by several hundred kilograms per square meter.  Under saturated  s o i l c o n d i t i o n s , the s o i l ' s shear strength may be l a r g e l y derived from the apparent cohesion provided by the tree root network. (3) Laboratory strength t e s t s of tree roots from l i v i n g trees and from stumps in various stages of decay show that a f t e r c l e a r f e l l i n g , the smaller roots less than 3 centimeters i n diameter, which c o n s t i t u t e a large proportion of the root network in the lower s u b s o i l s , r a p i d l y lose t h e i r strength.  Within 3 to 5 years a f t e r c l e a r f e l l i n g the small roots  of Douglas f i r and cedar stumps may lose more than h a l f t h e i r o r i g i n a l t e n s i l e strength. (4) Several simple management techniques could be employed to help reduce the impact of c l e a r f e l l i n g on the s t a b i l i t y of steep slopes.  These  include the p l a n t i n g of p o t e n t i a l l y deep-rooting, unwrenched, c o n t a i n e r i z e d Douglas f i r seedlings on steep, c l e a r f e l l e d areas, treatment of f r e s h l y - c u t stumps with borax, and the a r t i f i c i a l establishment of red alder seedlings or cuttings on p o t e n t i a l l y unstable, wet areas.  106 CHAPTER 4  A DETAILED EXAMINATION OF SELECTED LANDSLIDES  Introduction The evidence presented in the e a r l i e r chapters indicates that the s o i l water conditons and the density and condition of the tree root network are two very important f a c t o r s i n f l u e n c i n g the s t a b i l i t y of forest s o i l s .  However, the large v a r i a t i o n in the s i z e , shape, topog-  raphy and subsurface s i t e conditions among d i f f e r e n t l a n d s l i d e s , suggests that each f a i l u r e r e s u l t s from a unique combination of movement-promoting and movement-resisting f o r c e s .  Moreover, several large debris avalanches  and debris s l i d e s have formed on well drained forested s i t e s where excessive pore water pressures and lack of a strong, well developed root network are u n l i k e l y to have been major causes of f a i l u r e .  On the other  hand, there e x i s t steep, poorly-drained seepage depressions on c l e a r f e l l e d slopes w i t h i n the study area which appear to be l i k e l y l a n d s l i d e areas but, nevertheless, show no evidence of recent mass wasting.  Such anomalies  i n d i c a t e that general rules explaining the cause of l a n d s l i d e s may lead to many m i s i n t e r p r e t a t i o n s in l a n d s l i d e s t u d i e s .  Probably most landslides  r e s u l t from a combination of many causes with one f a c t o r being f i n a l l y dominant as suggested by Baker (1953).  This i s why attempts to assess  the r e l a t i v e s t a b i l i t y of slopes and to predict the l o c a t i o n of future landslides are often unsuccessful. It follows that there are many advantages in undertaking a d e t a i l e d i n v e s t i g a t i o n of a sample of c a r e f u l l y selected i n d i v i d u a l landslides which are, at l e a s t p a r t l y , amenable to q u a n t i t a t i v e study.  Although i t  i s normally impossible to accurately define the stress f i e l d and the subsurface conditons at f a i l u r e , careful examination of the f a i l e d s i t e s  107  combined with an analysis of the estimated forces i n f l u e n c i n g the slope at time of f a i l u r e often permit an understanding of the p r i n c i p a l causal f a c t o r s .  This chapter describes the nature and r e s u l t s of the  q u a n t i t a t i v e analyses of f i v e selected l a n d s l i d e s .  Stability  analysis  Simple, approximate, s t a b i l i t y analyses based on the theory of p l a s t i c e q u i l i b r i u m were used to i n v e s t i g a t e the selected l a n d s l i d e s . Most, i f not a l l , of the l a r g e r landslides in the study area are not amenable to q u a n t i t a t i v e analyses because t h e i r topography has been modified by erosion since f a i l u r e and because the shearing strength chara c t e r i s t i c s of the materials involved cannot be determined with any degree of accuracy.  Even in the cases of the selected l a n d s l i d e s , where s o i l s  were r e l a t i v e l y homogeneous and reasonably accurate estimations of the dimensions, volumes, and weights of the displaced s o i l s could be made, the strength of the s o i l s i s probably so v a r i a b l e and the number of d i f f e r e n t forces a c t i n g i s so large that the t h e o r e t i c a l studies provide l i t t l e more than a broad i n d i c a t i o n of what i s l i k e l y to have occurred. The ordinary method of s l i c e s and the Bishop method of s l i c e s , assuming c i r c u l a r surfaces of f a i l u r e , were applied to four f a i l u r e s associated with roads.  In a d d i t i o n , an i n f i n i t e slope analysis was used  to i n v e s t i g a t e a f i f t h l a n d s l i d e on a steep, c l e a r f e l l e d slope. 3 b r i e f l y describes each analysis method.  Appendix  Detailed d e s c r i p t i o n s , a p p l i c -  ations and l i m i t a t i o n s of the method of s l i c e s are presented by Bishop (1955), Lambeand Whitman (1969), Whitman and Bailey (1967), Wu (1966) and Zaruba and Mencl (1969).  Lambeand Whitman (1969) and Taylor (1948)  o u t l i n e the theory and a p p l i c a t i o n s of i n f i n i t e slope analyses.  108  Before a r e l i a b l e s t a b i l i t y analysis can be conducted several fundamental p r e - r e q u i s i t e s must be met: (1) The geometry of the l a n d s l i d e i n c l u d i n g the scar topography must be accurately described.  Only r e l a t i v e l y fresh landslides which  showed no evidence of p o s t - f a i l u r e erosion were selected f o r a n a l y s i s . In a d d i t i o n , the selected l a n d s l i d e s possessed s m a l l , well defined scars which permitted reasonably accurate ' l e v e l and s t a f f surveys to be made of t h e i r dimensions and shape. (2) The s o i l p r o p e r t i e s , p a r t i c u l a r l y the shearing strength and the unit weight, must be known.  Unit weights were estimated from undisturbed  bulk samples and, in the case of road f i l l method.  m a t e r i a l s , by a sand replacement  The strength parameters were based on f i e l d shear t e s t s .  To  compensate f o r the inaccuracies in the strength t e s t r e s u l t s , several analyses were made of each f a i l u r e using d i f f e r e n t shear strength parameter values which embraced the range of measured values.  Although there was  no way of gauging which values, i f any, were appropriate, r e p e t i t i v e analyses provided information on the s e n s i t i v i t y of the slope s t a b i l i t y to changes in the s o i l shear strength. (3) The external loads, i f any, should be c o r r e c t l y defined.  The  weight of the vegetation was not considered to be important in any of the  failures investigated.  However, the weight of the snow on the slopes  may have contributed to f a i l u r e , e s p e c i a l l y in the case of the debris avalanches in road f i l l m a t e r i a l s .  A heavy snowpack on the f l a t crown  portions of the landslides would greatly add to the downslope shearing f o r c e s , but, because these loads were impossible to d e f i n e , they were not considered in the analyses. (4) The groundwater or seepage conditions at the time of f a i l u r e must  109  Figure 33.  Formation of tension cracks in f i l l  materials near  the outer edge of an abandoned logging road.  Figure 34.  I n c i p i e n t l a n d s l i d i n g along an abandoned logging  road in the Howe Sound area.  110  be c o r r e c t l y i n t e r p r e t e d . The most c r i t i c a l c o n d i t i o n , that of complete saturation of the s o i l mantle with the top, flow l i n e coincident with the s o i l surface, was assumed f o r 3 landslides on poorly drained slopes. Piezometer measurements on poorly drained f o r e s t s o i l s i t e s showed that the assumption of seepage p a r a l l e l to the slope may cause serious estimates of the true pore water pressures  under-  (Chapter 2) but on slopes  underlain by r e l a t i v e l y uniform road f i l l materials t h i s assumption i s probably v a l i d .  P a r t i a l l y saturated conditions were assumed for 2 l a n d -  s l i d e s on well drained slopes. (5) The s o i l materials at f a i l u r e should behave in a r i g i d - p l a s t i c manner.  In other words, the i n i t i a l l y displaced s o i l mass should move  as one or more d i s c r e t e , s o l i d u n i t s .  Within the study area the formation  of tension cracks (Figure 33) and slump steps (Figure 34) along road r i g h t of-ways and o c c a s i o n a l l y on steep, unstable slopes i n d i c a t e i n c i p i e n t l a n d s l i d i n g of d i s c r e t e , coherent blocks of s o i l or road f i l l  materials.  The greatest u n c e r t a i n t i e s in the s t a b i l i t y analyses a r i s e from the s e l e c t i o n of the strength parameters and pore water pressures. Landslide 1 (road-cut slope in upper Magnesia Creek catchment). The collapse or slumping of road-cut slopes represents one of the commonest types of mass wasting features in the study area, although 2 the i n d i v i d u a l f a i l u r e s are r a r e l y more than 100 m in area. Landslide 3 1 was a small debris s l i d e i n v o l v i n g the f a i l u r e of approximately 6 m of forest s o i l and colluvium from a steep, cut slope of 63 degrees onto an abandoned logging road in upper Magnesia Creek.  The slope had been  cut about the same time that the general area had been c l e a r f e l l e d , approximately 5 to 7 years p r i o r to the time of f a i l u r e . At the time of the l a n d s l i d e survey (July 1970) the scar surface  lli  Scale  0  Figure 35.  1  meters  2  Small, c i r c u l a r debris s l i d e i n a road-cut slope  (Landslide 1 ) .  Broken l i n e shows the most c r i t i c a l a n t i c i p a t e d  ground water t a b l e c o n d i t i o n s . surveyed surface.  The heavy l i n e indicates the  Area (a) represents the accumulation of f a i l e d  debris on the road surface.  112  was fresh and had, presumably, been exposed no more than 2 to 3 months. The scar formed a toe f a i l u r e with a concave l o n g i t u d i n a l p r o f i l e approximately c i r c u l a r in shape (Figure 35). The s o i l consisted of a dark-brown, gravelly-sandy-loam 130 cm deep which gradually passed i n t o a 50 cm t h i c k layer of grey-brown, g r a v e l l y colluvium containing boulders up to 20 cm in diameter.  Dead  tree roots f r e e l y penetrated the s o i l down to the c o l l u v i a l gravels. Ruptured roots extended out from the upper part of the f a i l u r e surface and the absence of gleying or mottling in the subsoils suggested that saturation and excessive seepage were not a problem.  Two r e l a t i v e l y  undisturbed s o i l samples taken from a rdepth of approximately 100 cm 3 produced a mean moist unit weight of 1,450 kg/m and a mean saturated 3 unit weight of 1,680 kg/m . Shear strength t e s t s were not made of the s o i l s in the v i c i n i t y of Landslide 1 or Landslide 2.  Rather, a number of s t a b i l i t y analyses  were conducted using d i f f e r e n t 0 values ranging in the v i c i n i t y of the 0 values, f o r s i m i l a r f o r e s t s o i l s determined by d i r e c t shear t e s t s in the middle Seymour catchment (Figure 28). On a s i m i l a r dry s i t e in the middle Seymour catchment, piezometers S3-1, S3-2 and S3-3 (Figure 15, Chapter 2) showed t h a t , during wet periods, the s o i l saturated in a t h i n layer immediately above the impermeable t i l l or bedrock substratum.  The most c r i t i c a l seepage condition assumed in the  analyses of Landslides 1 and 2 was a 30 cm saturated zone immediately above the bedrock with a drawdown e f f e c t near the l a n d s l i d e toe (Figure 35 and Figure 36).  Pore water pressures were estimated with the  relationship 1  where  =  n  ,.Yw W'W  C0S  2  <*  u. = pore water pressure at the f a i l u r e surface at the mid  point of s i ice i . h,, = w  v e r t i c a l distance between the f a i l u r e plane and the r  water table at the mid point of s l i c e i . S t a b i l i t y analyses were made with the .ordinary method of s l i c e s and with the Bishop method f o r the p a r t i a l l y saturated condition and f o r an unsaturated condition with no seepage.  Results are shown in  Table 18. Table 18 Factors of Safety (FS) Calculated by the Ordinary Method of S l i c e s (QMS) and by the Bishop Method (BMS) f o r a road-cut slope at the s i t e of Landslide 1  Seepage condition  Method  Factor of Safety  0 35°  36°  37°  38°  39°  40°  Moist, no seepage  OMS  0.47  0.49  0.51  0.52  0.54  0.56  P a r t i a l l y saturated  OMS  0.37  0.39  0.41  0.42  0.44  0.45  P a r t i a l l y saturated  BMS  0.38  0.41  0.4.3  0.45  0.46  0.47  The c a l c u l a t e d factors of safety f a l l well below u n i t y , i n d i c a t i n g that the s o i l possessed a d d i t i o n a l strength derived from the tree root network and, p o s s i b l y , from some other f a c t o r not recognised in the o  analysis.  An apparent cohesion of 250 kg/m  n  9  and 200 kg/m  f o r 0 = 35  and 4 0 ° , r e s p e c t i v e l y , i s s u f f i c i e n t to r a i s e the f a c t o r of safety to a value of 1.0 f o r the moist condition with no seepage.  Under p a r t i a l l y 2  saturated conditions with seepage, apparent cohesions of 300 kg/m 260 kg/m  2  (0 = 35° and 40° r e s p e c t i v e l y ) are required to r a i s e the  and  114  f a c t o r of safety to 1.0. As the only roots in the v i c i n i t y of the shear plane were dead tree roots, i t i s probable that root d e t e r i o r a t i o n a f t e r c l e a r f e l l i n g was an important f a c t o r promoting the debris s l i d e .  However, the most  obvious cause of f a i l u r e was the steepness of the road cut.  For cuts  in cohesionless sands Smith and Cedergren (1963) recommend that the slope should be no steeper than 1%:1  or approximately 33°.  For reasons  of economy, logging road .cuts in the study area usually slope at angles greater than 45° and, consequently, are subject to numerous f a i l u r e s . Landslide 2 (road-cut slope in upper Magnesia Creek catchment) Landslide 2 was a debris s l i d e with a shallow, oval scar on a roadcut slope of 49° about 20 meters north of Landslide 1.  The s o i l and  drainage conditions were very s i m i l a r to those at the Landslide 1 s i t e although the c o l l u v i a l gravel l a y e r at the base of the s o i l B horizon contained more large boulders than the.same l a y e r at the former s i t e . The f a i l u r e surface had cut through the rooting zone and dead tree roots extended from the upper and middle parts of the scar surface. presents the l o n g i t u d i n a l section of the scar. 3  Figure 36  The displaced d e b r i s ,  c o n s i s t i n g of 7.5 m of p a r t l y i n t a c t blocks of s o i l , d i s i n t e g r a t e d s o i l and colluvium and two large tree stumps, had accumulated on the road surface and plugged a road side drainage d i t c h . S t a b i l i t y analyses were made under assumptions of a moist condition with no downslope seepage and a saturated s o i l layer 30 cm t h i c k immediately above the impermeable bedrock.  The r e s u l t s appear in Table 19.  As was the case with the f i r s t l a n d s l i d e examined, the low factors of safety i n d i c a t e that the s t a b i l i t y of the cut slope was l a r g e l y dependent on the strengthening e f f e c t of the tree root systems.  For the  115  Figure 36.  Debris s l i d e in a road-cut slope (Landslide 2 ) , upper  Magnesia Creek catchment.  The most c r i t i c a l a n t i c i p a t e d ground water  table condition i s .depicted by the broken l i n e .  116  Table 19  Factors of Safety (FS) Calculated by the Ordinary Method of S l i c e s (OMS) and by the Bishop Method (BMS) f o r a Road-cut slope at the s i t e of Landslide 2  Seepage condition  Method  Factor of Safety 0  35°  36°  37°  38°  39°  40°  Moist, no seepage  OMS  0.73  0.76  0.79  0.81  0.84  0.87  P a r t i a l l y saturated  OMS  0.59  0.61  0.64  0.66  0.68  0.71  P a r t i a l l y saturated  BMS  0.60  0.62  0.64  0.67  0.69  0.72  p a r t l y saturated c o n d i t i o n , apparent cohesions of 240 kg/m 35 ) and 160 kg/m  (0 =  (0 = 40 ) are required to r a i s e the f a c t o r s of  safety to 1.0. Between J u l y 1970 and J u l y 1972, f i v e a d d i t i o n a l small debris s l i d e s formed on cut slopes near Landslides 1 and 2.  P r i o r to 1970 only  two events had occurred on these slopes over a 5 to 7 year period since the cut slopes were formed.  This increased slumping a c t i v i t y a f t e r  1970 suggests that root strength d e t e r i o r a t i o n reached a c r i t i c a l approximately 5 to 7 years a f t e r c l e a r c u t t i n g .  stage  However, the 1970-1971  and 1971-1972 winters produced above average snowpacks w i t h i n the study area and possibly caused very wet and unstable s o i l conditions during the snowmelt period. Landslide 3 ( r o a d - f i l l slope in upper Harvey Creek catchment) Some of the most damaging landslides in the study area o r i g i n a t e when r o a d - f i l l materials collapse to form debris avalanches.  The  l a r g e s t and most damaging examples are located along abandoned logging roads which have f a l l e n i n t o d i s r e p a i r (Figure 13). Landslide 3 was a debris avalanche which o r i g i n a t e d when a portion of a 34° f i l l  slope collapsed at the outer edge of an abandoned  road in upper Harvey Creek catchment.  logging  The upper part of the f a i l u r e scar  formed a spoon-shaped hollow in poorly compacted, gravelly-sand,  fill  materials but downslope the scar narrowed to form a shallow trench, i n d i c a t i n g that the debris avalanche had become a debris flow below the i n i t i a l f a i l u r e zone.  The condition of uprooted vegetation near the  foot of the scar indicated that the debris avalanche occurred no more than 3 to 4 months p r i o r to i t s examination. approximately 180 cm of gravelly-sand f i l l  Near the headscarp,  rested on d i o r i t e bedrock but  118  0  Figure 37.  1 2 meters  Small, c i r c u l a r f a i l u r e i n a 34  3 ) , upper Harvey Creek catchment.  3  j j  r o a d - f i l l slope (Landslide  The f i l l consisted of gravels  sands (28%), s i l t s (4%) and clay (-) and was l o o s e l y compacted (e = 0.48).  (68%),  downslope the f i l l  overburden thinned to approximately 100 cm.  up to 1 m i n diameter were embedded in the f i l l . roots in these m a t e r i a l s .  Boulders  There were no tree  The curved, f a i l u r e surface was roughly  c i r c u l a r in l o n g i t u d i n a l section with an estimated radius of 17.7 m (Figure 37). The unit weight of the f i l l  materials was determined by the sand  replacement method (Capper and Cassie 1963). 20 cm x 20 cm x 10 cm, was dug in the f i l l  A hole, approximately  and the excavated materials  were c o l l e c t e d in p l a s t i c bags f o r water content and p a r t i c l e s i z e d i s t r i b u t i o n determinations.  The volume of the hole was measured by  f i l l i n g i t with dry, uniform sand of known dry, unit weight.  Measure-  ments made at two locations near the scar head produced a mean dry unit weight of 1 ,730 kg/m  3  3 and a mean saturated unit, weight of 2,130 kg/m .  Direct shear tests by the same shear box equipment and methods described in Chapter 3, were made on excavated columns of f i l l near the head of Landslide 3.  materials  Two sets of t e s t s produced f r i c t i o n angles  of 38° and 41° r e s p e c t i v e l y (Figure 38).  However, these r e s u l t s may be  very misleading because the cohesionless gravels and sands tended to crumble w i t h i n the shear box a f t e r small t e s t displacements rather than f a i l along a d i s t i n c t i v e shear plane. The s i t e was very poorly drained.  Surface water ponded on the  uneven road surface at the l a n d s l i d e head during wet periods and i t was assumed i n the s t a b i l i t y analyses that the f i l l  materials attained  complete saturation and that seepage occurred p a r a l l e l to the slope. The ordinary method of s l i c e s was used to c a l c u l a t e the factors of safety f o r the slope under dry and saturated conditons (Table 20).  Figure 38.  Results of d i r e c t shear t e s t s on r o a d - f i l l materials  p l o t t e d on a shear stress (T) vs normal stress (a) diagram.  Table 20  Factors of Safety Calculated by the Ordinary Method of S l i c e s f o r a R o a d - f i l l slope at the s i t e of Landslide 3  Seepage condition  Factor of Safety 38 o  39°  40°  41°  Dry  1.46  1.52  1.57  1.63  1.69  Saturated  0.84  0.88  0.91  0.94  0.97  0  The s t a b i l i t y of the dry slope appears to be adequate.  42°  However,  at s a t u r a t i o n , the s t a b i l i t y of the slope i s reduced to a c r i t i c a l state as indicated by the factors of safety which are less than 1.0. Landslide 4 ( r o a d - f i l l slope i n upper Magnesia Creek catchment) Landslide 4 o r i g i n a t e d when part of a loose, cohesionless f i l l  slope  collapsed at the outer edge of an abandoned logging road i n upper Magnesia Creek.  The r e s u l t i n g scar formed a shallow,curved hollow with an approx-  imately c i r c u l a r l o n g i t u d i n a l p r o f i l e (Figure 39). The gravelly-sand 3 f i l l had a dry unit weight of 1,670 kg/m and a saturated unit weight of 3 2,030 kg/m .  At the road edge the f i l l was about 150 cm t h i c k and  rested on weathered greywacke and low-grade s c h i s t bedrock. A small seepage discharged from the cut slope face immediately above the  l a n d s l i d e s i t e and, as there was no roadside d r a i n , the upslope  drainage passed through or across the road bed gravels onto or through  122  Figure 39.  Shallow, c i r c u l a r f a i l u r e in a r o a d - f i l l slope (Landslide  4 ) , upper Magnesia Creek catchment.  The f i l l was loosely compacted  (e = 0.62) and was composed of gravels (53%), sands (38%), s i l t s (8%) and clays  (1%).  the  fill  slope below the road.  The road surface was deeply rutted  by s m a l l , surface runoff channels. The ordinary method of s l i c e s was used to analyse t h i s l a n d s l i d e and i t was assumed that the slope would be subject to complete s a t u r ation during heavy storms.  The shear strength of the f i l l  materials  were not measured at t h i s s i t e (the l a n d s l i d e was not accessible by vehicle) but i t was assumed that the e f f e c t i v e 0 value or values f o r these materials l a y w i t h i n the range of 0 used i n analyses of Landslide 3. Table 21 Factors of Safety Calculated by the Ordinary Method of S l i c e s f o r a R o a d - f i l l slope at the s i t e of Landslide 4  Seepage condition  Factor of Safety  0  38°  39°  40°  41°  42°  Dry  1.02  1.06  1.10  1.14  1.18  Saturated  0.54  0.56  0.58  0.60  0.62  The r e s u l t s of these analyses show that the f i l l  slope, even i n  a dry c o n d i t i o n , did not possess a wide margin of safety against failing.  Therefore, i t seems u n l i k e l y that the slope attained a state  of complete saturation before s l i d i n g .  In f a c t , the c a l c u l a t e d f a c t o r s  of safety suggest that a very small r i s e in the pore water pressures, r e s u l t i n g from the saturation of the basal regions of the f i l l , would have been s u f f i c i e n t to jeopardize the s t a b i l i t y of the slope.  124  Although the poor drainage condition at the head of t h i s l a n d s l i d e was a p r i n c i p a l cause of f a i l u r e , the steepness of the f i l l was also instrumental in bringing about movement.  slope (41°)  In the Howe Sound area  l a n d s l i d i n g of the r o a d - f i l l s i s mainly confined to slopes over 33 to 36 degrees. Downslope from the i n i t i a l f a i l u r e zone many debris avalanches, including those o r i g i n a t i n g in r o a d - f i l l s , appear to revert to debris flows which continue to the v a l l e y bottoms.  Gonsior and Gardner (1971)  point out that l i q u i f a c t i o n f a i l u r e s commonly occur in poorly compacted, saturated f i l l the  slopes in northern Idaho.  loose, saturated f i l l  P o s s i b l y , shear f a i l u r e s in  slopes w i t h i n the study area, are accompanied  by a volume decrease which t r i g g e r s 1 i q u i f a c t i o n .  I f the f i l l s had  received some compaction during road c o n s t r u c t i o n , then the frequency of l a n d s l i d i n g might have been reduced. Landslide 5 (natural slope in upper Harvey Creek catchment) Landslide 5 was a small debris s l i d e (HS 14 i n Table 4, Chapter 1) which occurred in a shallow drainage hollow on a 30° c l e a r f e l l e d slope in upper Harvey Creek catchment.  Approximately 570 m of s o i l had s l i d  downslope i n t o a small stream g u l l y to expose a f l a t , even, unweathered till  surface (Figure 40).  A s l i g h t l y overhanging head wall and v e r t i c a l  l a t e r a l scarps o u t l i n e d the f a i l u r e scar which broadened towards i t s toe  to form a narrow, fan-shaped feature.  Water seeped from the base  of the headwal1. The s o i l s in the v i c i n i t y of the l a n d s l i d e ranged between 50 and 80 cm in depth, were poorly drained and possessed stoney, mottled B horizons.  Cedar and hemlock roots f r e e l y penetrated the subsoils down  to the s o i l - t i l l  i n t e r f a c e and small roots extended i n t o the t i l l .  The  Scale 0  Figure 40.  ~~5™"~ 10 meters  Longitudinal p r o f i l e of a shallow  debris- s l i d e on a 3 0 ° , c l e a r f e l l e d slope i n upper Harvey Creek catchment (Landslide 5).  The r e c t i l i n e a r  f a i l u r e surface coincided with the s o i l - t i l l i n t e r f a c e .  s o i l B horizon contained scattered boulders of weathered d i o r i t e up to approximately 30 cm in diameter.  Two undisturbed samples of the  B horizon gave a mean moist unit weight of 1 ,550 kg/m  3  and a saturated  3  unit weight of 1,790 kg/m .  S o i l shear t e s t s were not conducted at  this s i t e . Factors of safety and the apparent cohesion required f o r s t a b i l i t y were c a l c u l a t e d f o r d i f f e r e n t s o i l strength and s o i l water conditions by an i n f i n i t e slope analysis (Table 22). Table 22 Factors of Safety and the Apparent Cohesion Required f o r S t a b i l i t y f o r a Natural Slope at the s i t e of Landslide 5  Seepage condition  Factor of Safety  0  35°  36°  37°  38°  39°  40°  Moist, no seepage  1.21  1.26  1.31  1.35  1.40  1.45  Saturated, seepage p a r a l l e l to slope  0.53  0.56  0.58  0.60  0.62  0.64  Apparent Cohesion Required f o r S t a b i l i t y kg/m Saturated, seepage p a r a l l e l to slope  216  206  197  2  187  177  167  The r e s u l t s of the analysis i n d i c a t e t h a t , even without the benefit of a s t a b i l i z i n g root system, the slope would not have f a i l e d as long as the s o i l remained in an unsaturated c o n d i t i o n .  However, the topography  and the s o i l drainage condition at t h i s s i t e leaves l i t t l e doubt that  127  the slope did saturate during wet periods.  Under these circumstances  i t i s obvious from the very low c a l c u l a t e d f a c t o r s of s a f e t y , that the s t a b i l i t y of the slope depended heavily on the a d d i t i o n a l strength imparted by the tree root network.  The d e t e r i o r a t i o n of the root net-  work f o l l o w i n g c l e a r f e l l i n g appears to be the dominant causal f a c t o r in the release of t h i s debris s l i d e .  Concluding remarks Although the analyses described i n the preceding section are fraught with u n c e r t a i n t i e s they, nevertheless, r a t i f y many of the conclusions o u t l i n e d i n the e a r l i e r chapters.  In p a r t i c u l a r , the analyses of  f a i l u r e s in the n a t u r a l , steepland s o i l s (Landslide 1, 2 and 5) a f f i r m t h a t , under the most adverse s o i l water conditions l i k e l y to have occurred at these s i t e s , the slopes depended to a large degree on the tree root network f o r t h e i r s t a b i l i t y .  There i s , however, a strong p o s s i b i l i t y  that s o i l s with a high content of rocks may derive a d d i t i o n a l resistance to shear from the i n t e r l o c k i n g or keying e f f e c t s of large boulders i n close contact with each other.  As the support provided by the tree roots  i s gradually l o s t , roadcuts in the natural s o i l s with slopes greater than 35° become very susceptible to shallow f a i l u r e s .  The a r t i f i c i a l  estab-  lishment of c o n i f e r seedlings on the steep, cut faces and on the natural slopes at the crown of the c u t s , soon a f t e r road c o n s t r u c t i o n , would possibly prevent a great number of these s m a l l , shallow, debris s l i d e s . The highly unstable road f i l l s on steep slopes c o n s t i t u t e the most serious mass wasting problem in the study area.  The loose structure of  the f i l l s examined and t h e i r high void r a t i o s suggest that these materials received very l i t t l e or no compaction during road c o n s t r u c t i o n , presumably  ,128  causing the f i l l s to have lower f r i c t i o n angles than e q u i v a l e n t , compacted materials and possibly rendering very loose f i l l s and s i d e castings susceptible to 1 i q u i f i c a t i o n f a i l u r e s . Adverse road drainage conditions are, however, the most obvious cause of the large mass movements associated with roads.  The analyses  of Landslides 3 and 4 show that the s t a b i l i t i e s of the steeper f i l l slopes are extremely s e n s i t i v e to water table r i s e s .  Although post-  logging treatments such as water bar construction and o u t s l o p i n g , smoothing, and a r t i f i c i a l l y revegetating road surfaces might have prevented some l a n d s l i d e s , there are logging roads above 800 m a l t i t u d e w i t h i n the study area which traverse slopes so steep that such treatments would have probably been l a r g e l y i n e f f e c t i v e .  On these high, steep slopes  mass wasting must be regarded as an i n e v i t a b l e consequence of road construction. There i s no doubt in the w r i t e r ' s mind that current f o r e s t removal and road b u i l d i n g practices in many parts of the study area are at complete variance with sensible mountain-land management.  I f road  b u i l d i n g and timber removal threaten to destroy the s o i l resource, increase the sediment load of streams and r i v e r s and generally degrade the mountain environment, then, regardless of the conditions of land tenure, the standing timber values, and the magnitude of the potential p r o f i t s to be gained, road b u i l d i n g and logging cannot possibly be comp a t i b l e with long term environmental goals.  There i s a real and urgent  need to define those mountain f o r e s t areas which can be managed f o r timber production without degrading the s o i l resource and hydrologic functions of those areas and those f o r e s t s which should be l e f t i n t a c t to f u l f i l a protective f u n c t i o n .  By way of example, i t i s possible to recognise  and designate c e r t a i n areas of mountain f o r e s t s in the study area, as  being unsuitable f o r management by t r a d i t i o n a l , c l e a r f e l l i n g methods. C r i t e r i a of such areas are as f o l l o w s : (1) Forested slopes steeper than 35°.  Mass wasting, p a r t i c u l a r l y  debris avalanches associated with roads, becomes very troublesome on slopes steeper than 35°.  In the past, slopes as steep as 50° have been  clearfelled. (2) Forests above a l t i t u d e s of 1,000 meters.  Although high level  benches and other areas with stable t e r r a i n do e x i s t at a l t i t u d e s above 1,000 meters, construction of 'switch-back' road systems to provide access to these areas creates a serious mass wasting problem. (3) Forests adjacent to mountain t o r r e n t s . (Riparian s t r i p s of f o r e s t , perhaps 200 to 300 feet wide, would provide protection against the accumulation of logging debris in the stream channels and would help maintain the s t a b i l i t y of the stream banks.) (4) Forests growing on poorly drained slopes and in large d r a i n age depressions subject to p e r i o d i c s a t u r a t i o n . Forest areas included in (1), (2) and (3) can be recognised and delineated on 1:30,000 and possibly smaller scale a e r i a l  photographs  but shallow drainage depressions and slopes with poorly drained s o i l s are d i f f i c u l t to i n t e r p r e t on photographs and t h e i r i d e n t i f i c a t i o n normally requires a ground i n s p e c t i o n .  Approximately 30-40 percent of the  t o t a l area c l e a r f e l l e d w i t h i n the study area over the l a s t 10 years would probably be c l a s s i f i e d as unsuitable f o r timber production i f t h i s forest c l a s s i f i c a t i o n scheme was adopted.  BIBLIOGRAPHY  Armstrong, J . E . , 1956. S u r f i c i a l geology of the Vancouver area, B r i t i s h Columbia. Geol. Surv. Can. Paper 55-40, 16 pp. , 1965. Geology, Vancouver North, B r i t i s h Columbia. Geol. Surv. Can. Map 1152A. Armstrong, J . E . and Brown, W.L., 1954. Late Wisconsin marine d r i f t and associated sediments of the Lower Fraser V a l l e y , B r i t i s h Columbia, Canada. B u l l . Geol. Soc. Amer., 65, p. 349-364. Aubertin, G.M., 1971. Nature and extent of macropores in f o r e s t s o i l s and t h e i r influence on subsurface water movement, USDA Forest Serv. Res. Paper NE-192, 33 pp. Baker, R.F., 1953. Analysis of c o r r e c t i v e actions f o r highway lands l i d e s . Proc. Amer. Soc. Civ. Eng., 79, 25 pp. Baker, R.F. and Gray, H., 1960. Design of foundations, embankments and cut slopes. In 'Highway engineering handbook'. McGraw H i l l  Book  Co., p. 11-1-11-80. Barr, D.J. and Swanston, D.N., 1970. Measurement of creep in a shallow, slide-prone t i l l  s o i l . Amer. J . S c i . , 269, p. 467-480.  Bethlamy, N., 1962. F i r s t year e f f e c t s of timber removal on s o i l moisture. B u l l . Int. Assoc. S c i , H y d r o l . , 7, p. 34-38. Bishop, A.W., 1955. The use of the s l i p c i r c l e in the s t a b i l i t y analysis of slopes. Geotechnique, 5, p. 7-17. Bishop, D.M. and Stevens, M.E., 1964. Landslides on logged areas in southeast Alaska. USDA Forest Serv. Res. Paper N0R-1, 18 pp. B r i t i s h Columbia Forest Service, 1957. Continuous f o r e s t inventory of B r i t i s h Columbia. Dept. of Lands and Forests. 223 pp. Brooke, R.C. 1966. Vegetation-environment r e l a t i o n s h i p s of subalpine mountain hemlock zone ecosystems. Ph.D. Thesis, U n i v e r s i t y of B r i t i s h Jolumbia, 225 pp.  131  Brown, H.P., Pashin, A.S. and F o r s a i t h , C . C , 1952. Textbook of wood technology, Volume 2. McMillan and Sons Inc.* 783pp. Canada Department of A g r i c u l t u r e , 1970. The system of s o i l c l a s s i f i c a t i o n f o r Canada. Queen's P r i n t e r f o r Canada, Ottawa. 249 pp. Capper, P.L. and Cassie, W.F., 1963. The mechanics of engineering s o i l s . Spon Publishing Co., London, p. 250-252. 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I n t e n s i t i e s (mm/hour) 5 minutes  28-4-59 19-11-60 14,15-1-61 30-8-61 2-1-62 19-11-62 30-12-62 13-9-63 21-10-63 25-11-63 22-12-63 24-12-63 30-11-64 5-10-65 15-12-66 17-12-66 1-10-67 10-10-67 10-12-67 22-12-67 18-1-68 17-9-68 22-9-69 5-4-70  15 minutes  24 hour t o t a l  1 hour  10.7 30.5 33.5 18.3 13.2 18.3 21.3 61.0 25.9 21.3 27.4 21.3 18.3 51.8 15.2 15.2 27.4 30.5 30.5 18.3 16.8  10.2 24.4 28.5 17.3 11.2 16.8 14.2 47.8 19.3 17.3 24.4 19.3 13.2 43.7 12.2 11.7 20.3 25.4 24.4 14.2 14.2  8.9 15.0 25.4 9.4 10.7 13.2 10.9 15.2 15.2 11.7 20.6 17.8 11.4 31.0 10.2 9.7 16.5 21.6 18.8 12.2 10.4  30.5 15.2  20.3 13.2  9.9 9.9  (mm) 127.3 101.6 365.8 178.8 118.1 146.1 105.7 37.6 148.3 155.7 116.8 244.1 163.8 169.4 127.8 145.8 140.0 110.7 134.6 121.9 119.9 106.2 112.5 114.1  (b) Capilano Dam 30-11-64 15-12-66 1-10-67 7-10-67 10-10-67 10-12-67 22-12-67 18-1-68 17-9-68 22-9-69  124.2 93.7 51.3 116.1 48.0 41.7 71.9 157.5 124.5 75.9  Appendix 2  (a)  Determination of void r a t i o s f o r steepland subsoils Each undisturbed, 2,500 cm  C and weighed.  sample of s o i l was oven-dried at 105°  An approximately 150 g sub-sample of the dried subsoil  was used f o r determination of the s p e c i f i c g r a v i t y of the s o i l by the water immersion method described by Lambe (1951).  solids  The void r a t i o  i s given as  e  =  void r a t i o  g  =  w  =  s p e c i f i c g r a v i t y of the s o l i d s 3 unit weight of water g/cm 3  G Y V  =  W = g  t o t a l volume of sample cm weight of the s o i l s o l i d s g  The d e r i v a t i o n of t h i s formula i s given i n Lambe (1951) p. 154.  (b)  Determination of void r a t i o s f o r compacted t i l l 3 I r r e g u l a r , approximately 400 cm  i n t a c t samples of cement-like t i l l  were oven-dried, weighed, coated with a t h i n l a y e r of parafin wax and immersed i n water to obtain t h e i r volume.  The s p e c i f i c g r a v i t y of the  s o l i d s and the void r a t i o were obtained in the same manner as f o r the subsoils.  140  (c)  Determination of the saturated unit weight f o r subsoil and t i l l  The saturated unit weight Y  s  f o r both types of material was  obtained from Y  (d)  s  =  r^-r^ w  {  Y  Results Subsoils. Sample  551 552 553 554 555  Specific gravity of s o l i d s  Saturated unit weight kg/m  Void r a t i o  2.4627 2.4473 2.4584 2.4910 2.4631  1,650 1,690 1,750 1,620 1,610  1.25 1.09 0.93 1.41 1.38  2.4613  1,660  1.21  2.6794 2.6799  2,370 2,300  0.23 0.29  2.6797  2,340  0.26  3  Till. TI T2  141  Appendix 3  (1)  Slope analysis by the ordinary method of s l i c e s  The free body cut from a slope by a c i r c u l a r f a i l u r e surface ((a) in above figure) i s divided up i n t o several s l i c e s or segments and the e q u i l i b r i u m of each of these s l i c e s i s considered.  The unknown  forces a c t i n g on each sli<?e include the weight of the s l i c e W^, the normal force N.j and the shear force T. a c t i n g on the f a i l u r e plane, the normal forces F.. and F^  +n  on the f a i l u r e a r c .  and the shear forces t^ and t 1  + n  pressures IK a c t i n g  In the ordinary method of s l i c e s i t i s assumed  that the forces a c t i n g on the sides of a l l s l i c e s have zero r e s u l t a n t  i n the d i r e c t i o n normal to the f a i l u r e arc f o r that s l i c e . The f a c t o r of safety f o r the free body i s defined in terms of moments about the centre of the f a i l u r e a r c . FS = moment of shear strength along f a i l u r e arc moment of weight of free body i=n  _ R (sci. "  i=l  RF"^. Where  1  i=n + tan 0 z N.) jsQ  sin  ,  1  u  n ;  .  a  R = radius of the f a i l u r e arc n = number of si ices  In the ordinary method of s l i c e s N  i  = W. cosa . - U.  — — - —  = W. cosa . - u l i  (2a) -  i  Combining equations (1) and (2b) and c a n c e l l i n g the  FS  i=n • z [cl,- + (W. cosa. - u . l . ) tan 0] = i=l i=n z W. s i n a . i=l 1  1  1  1  1  1  (2b) R's  (3)  1  This approximate s o l u t i o n , which neglects the forces on the sides of the s l i c e s , y i e l d s a safety f a c t o r which i s generally 5 to 15 percent but may be 40 to 50 percent below the values obtained by more rigorous methods, p a r t i c u l a r l y i f the pore pressures are large and the curvature of the f a i l u r e arc i s high. (2)  Bishop method of s l i c e s Bishop (1955) refined the computation of the f a c t o r of safety by  considering the forces a c t i n g on the s l i c e sides and assuming they have  143  zero r e s u l t a n t in the v e r t i c a l d i r e c t i o n .  Under these circumstances  Bishop showed that W  1  N, =  - u l i  i  coso  -  1  C  l  i  S  i  FS  n  a  i  — — — — -  () 4  cosa^ + (tan 0 sina^)  1  FS Combining equations (1) and (4) i n j [ c l , cos., • ( H , -  1  =  r  FS  =  i  —  U 1  1 , cosc.)tan H ]  C O S a  .  t  ( t a n 0  F S  i = n . s , i = l  W. s i n a . 1  1  As FS appears on both sides of equation (5), an i t e r a t i v e procedure must be employed to assess FS but convergence of t r i a l s i s very r a p i d .  Often  the FS obtained by the ordinary method of s l i c e s i s used as the i n i t i a l jtrial  value.  To aid the c a l c u l a t i o n s , charts can be used to evaluate  the function cosa^ + (tan 0 sina^) FS Generally, the f a c t o r of safety produced by the Bishop method i s 1 to 10 percent higher than the f a c t o r of safety given by the ordinary method of s l i c e s .  D i f f i c u l t i e s in using equation (5) can be encountered  i f the pore water pressures involved i n the problem are large and the FS i s less than u n i t y .  (3) I n f i n i t e slope analysis According to Taylor (1948) an i n f i n i t e slope designates  " — a  constant slope of unlimited extent which has constant conditions and constant s o i l properties at any given distance below the surface of the s l o p e . "  Such ideal conditions are r a r e l y shown by natural slopes  but, i f the thickness of the s o i l mantle or mantle of p o t e n t i a l l y unstable slope materials i s small compared to the height of the slope, then the slope may be c a l l e d i n f i n i t e .  Under these circumstances  the f a i l u r e plane i s p a r a l l e l to the slope. Although the mineral portions of the steepland f o r e s t s o i l s are predominently cohesionless, f o r p r a c t i c a l purposes these s o i l s can be considered cohesive because of the e f f e c t s imparted by the tree root network.  The analysis of cohesive, i n f i n i t e slopes i s o u t l i n e d below.  ( i ) Analysis of unsaturated i n f i n i t e slopes with no seepage  In the diagram above showing part of a dry, i n f i n i t e slope the stress components on the base plane at depth z are T = y^Z S i n a COSa a = y,z  COS a 2  S u b s t i t u t i o n of these expressions i n t o the f a i l u r e law T  c + tan 0 CT  y i e l d s the equation c Y  2 = cos a (tana - tan 0)  d  (6)  and rearrangement of the terms gives the cohesion needed f o r stability p  c = Y  D  cos a (tana - tan 0)  (7)  ( i i ) Analysis of saturated i n f i n i t e slopes with seepage  If the s o i l i s saturated with the free water surface at the ground surface and seepage occurs p a r a l l e l to the slope, then the stresses acting on the base plane are T  = Y Z cosa sina S  ,  2  = Y^Z  0"  COS  a  S u b s t i t u t i n g these expressions i n t o the f a i l u r e law T = c + atan 0 gi ves —=• Y„Z  =  c o s a (tana - — 2  X  Y  tan 0)  's s and the cohesion required for s t a b i l i t y i s r  (8)  c  = Y5  Z  C O S  (tana  a  C  - — tan 0) :  Y  (9)  S  The f a c t o r of safety (FS) f o r an i n f i n i t e , cohesive slope with seepage p a r a l l e l to the slope i s FS =  a v a i l a b l e shear strength shear stress required f o r e q u i l i b r i u m 2 C + y,Z cos a tan 0  -  lb  YZ g  (  1  0  )  S i n a COSa  For cohesionless materials the FS reduces to FS =  Y  tan 0  b  Y tan S  a  (11)  Sample C a l c u l a t i o n s o f F a c t o r s o f S a f e t y (FS) f o r a Road-cut Slope a t t h e s i t e o f L a n d s l i d e 1 O r d i n a r y Method o f S l i c e s Slice  Area  1. i  a. l  u. x  u.l. i i  W. i  Bishop Method o f S l i c e s (W\ cosaj_u.l.)tan0 l l  W. s i n a • i l  (W -  1 u.l.cosa.) cosa^ + i l l (tan0.sina /FS) tan 0 FS=.38 FS=.40 kg ±  a x b  a x b  FS=.38  FS= .40  i  2 m  o  m  kg/m  kg  m  kg  kg  1  0.121  42  0.549  122.1  67.0  190.7  51.9  127.6  98.6  0.506  0.523  49.9  51 .6  2  0.204  46  0.457  182.8  83.5  319.3  96.9  229.6  182.9  0.495  0.512  90.5  93 .6  3  0.251  50  0.457  243.7 111.4  387.0  96.2  296.4  220.8  0.487  0.504  107.5  111 .3  4  0.307  54  0.488  122.1  59.6  456.0  146.0  368.9  294.7  0.481  0.499  141.8  147 .1  5  0.372  58  0.610  -  -  539.4  200.1  457.4  377.6  0.478  0.497  180.5  187 .7  6  0.279  66  0.762  -  -  404.6  115.3  369.8  283.2  0.478  0.498  135.4  141 .0  7  0.102  72  0.945  -  -  147.9  32.0  140.7  103.5  0.485  0.507  50.2  52 .5  738.4  1,990.4  755.8  784 .8  FS Y  kg  TR  =  1,450 kg/m 1,680 kg/m  3  0 = 35°  738 .4 1990 .4  0.371  1 s t t r y : FS  784.8 = 0.394 1990.4  2nd t r y : FS  755.8 = 0.380 1990.4  

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