Structural and engineering geology of the East Gate Landslide ...

Structural and engineering geology of the East Gate Landslide, 

Purcell Mountains, British Columbia, Canada 

Abstract 

Marc-André Brideau a,⁎ , Doug Stead a , Réjean Couture b 

a Simon Fraser University, Burnaby, BC, Canada 

b Geological Survey of Canada, Ottawa, ON, Canada 

Received 19 July 2005; received in revised form 27 January 2006; accepted 31 January 2006 

The East Gate Landslide is a prehistoric landslide that was reactivated in January 1997. The slope failure took place in the lower 

greenschist metasedimentary units of the Precambrian Horsethief Creek Group. The Grizzly Creek Thrust is a regional overturned 

fault that coincides with the location of the headscarp of the East Gate Landslide. Four discontinuity sets were recognised from 

detailed engineering geological mapping of the headscarp and surrounding area. The main scarp of the section reactivated in 1997 

was sub-divided into three structural domains based on its position within the landslide, lithology, and orientation of the 

discontinuity sets. Limit-equilibrium techniques, finite-difference (FLAC) and distinct-element (UDEC) codes were used to 

investigate the failure mechanism of the 1997 event. The results of the field observations and numerical models suggest that the 

1997 failure involved a complex mechanism incorporating components of rock-slumping, bi-planar, and pseudo-circular failure 

that was controlled by both the orientation of the discontinuity sets and reduced rock-mass quality due to tectonic deformation. 

© 2006 Elsevier B.V. All rights reserved. 

Keywords: GSI; Limit-equilibrium; Finite-difference; Distinct-element 

1. Introduction 

The East Gate Landslide is located on the eastern side 

of the Beaver River Valley in Glacier National Park, 

British Columbia (Fig. 1). The Beaver River Valley is 

bounded to the east by the Dogtooth Range of the 

Purcell Mountains and to the west by the Hermit and Sir 

Donald ranges of the Selkirk Mountains. In January 

1997, an important retrogressive failure took place 

within the rock mass above the oversteepened head 

scarp (Fig. 2). During the following days and weeks, the 

large intact block slumped down to a few hundred 

meters below the head scarp. The rock mass disin- 

⁎ Corresponding author. 

E-mail address: mbrideau@sfu.ca (M.-A. Brideau). 

Engineering Geology 84 (2006) 183–206 

0013-7952/$ - see front matter © 2006 Elsevier B.V. All rights reserved. 

doi:10.1016/j.enggeo.2006.01.004 

tegrated completely after less than a few hundred meters 

of slumping, due to the high degree of fracturing and 

low rock-mass quality, and transformed from a debris 

pile into both debris and mud flows. In both 1999 and 

2003, mudflows from the upper slope debris impacted 

the Trans-Canada Highway (Highway 1), which is 

situated at the base of the East Gate Landslide (EBA 

Engineering Consultants Ltd., 2004). 

1.1. Previous work 

www.elsevier.com/locate/enggeo 

A kinematic analysis performed by Couture and 

Evans (2000), using joint sets recognised in the headscarp, 

suggested toppling as a feasible failure mechanism. 

A pseudo-rotational or rock slumping mechanism 

was subsequently proposed as complementary to the

184 M.-A. Brideau et al. / Engineering Geology 84 (2006) 183–206 

toppling (Couture and Evans, 2000; EBA Engineering 

Consultants Ltd., 2004). Debris is now accumulating at 

varied elevations on flatter, bench-like sections of the 

slope, forming unstable piles of disintegrated rock, in 

which large ripples and open fissures perpendicular to 

the flow direction indicate complex movements and 

down-slope displacement of debris (Couture and Evans, 

2002). The benches are assumed to be bedrockcontrolled 

because their continuation is observed outside 

of the failure area (Couture and Evans, 2000). 

Fig. 1. Location map of the East Gate Landslide in southwestern Canada. 

Fig. 2. Overview of the East Gate Landslide (fall 2003 photograph). 

Ground-based monitoring and analysis of highresolution 

digital elevation models (DEM) of the debris 

indicate significant transfer of materials from the upper 

sections of the debris mass towards the lowermost part. 

In addition, the lowermost part of the debris mass 

exhibits high rates of movement, averaging 1m/month 

(Couture et al., 2004). This section of the debris remains 

the primary source of material that, once combined with 

runoff from snowmelt and heavy rainfalls, triggers 

seasonal debris-flow events that may impact the

highway. In addition, areas of the main escarpment 

show a large concentration of cracks and opened fissures 

that have opening rates varying from 7 to 603mm/year 

(Couture et al., 2004). Hence, the debris mass has the 

potential to be continuously fed by down-slope 

movement of material from the upper parts of the 

landslide. Bedrock lineaments were identified in aerial 

photographs and in field investigations by previous 

workers (Couture and Evans, 2000; EBA Engineering 

Consultants Ltd., 2004). A hazard assessment of the 

current conditions and a review of the mitigative options 

were prepared by EBA Engineering Consultants Ltd. 

(2004). 

1.2. Regional geology 

The eastern side of the Beaver River Valley is 

composed of rocks from the late Pre-Cambrian Horsethief 

Creek Group (Wheeler, 1963) (Fig. 3). The 

Horsethief Creek Group represents a shallowing upward 

megacycle that was deposited during an intracratonic 

rifting event of the Late Proterozoic (Kubli, 1990). The 

metamorphic grade of the Horsethief Creek Group on 

the eastern side of the Beaver River Valley corresponds 

M.-A. Brideau et al. / Engineering Geology 84 (2006) 183–206 

to the chlorite zone of the lower greenschist facies 

(Kubli, 1990). The Horsethief Creek Group is subdivided 

in a series of slate, grit, and carbonate divisions 

(Poulton, 1970; Poulton and Simony, 1980; Kubli, 

1990). The slate divisions are predominantly composed 

of pelites metamorphosed to slate or phyllite, while the 

grit divisions are comprised of weakly metamorphosed 

granule or pebble conglomerates. The grit divisions also 

contain a subordinate amount of interbedded laminated 

slate and sandstone with rare carbonate horizons (Kubli, 

1990). 

1.3. Structural geology 

The Dogtooth Range is composed of a series of 

southwest-dipping thrust sheets, which form part of an 

imbricate thrust system (Kubli, 1990). In a regional 

geological context, the Dogtooth Range is located on the 

eastern limb of the northern extension of the Purcell 

Anticlinorium (Wind, 1967). The rocks on the eastern 

side of the Beaver River Valley have been complexly 

folded, with the bedding (S0) striking north–northwest 

and dipping to the east. Older thrust faults have been 

folded into a vertical or overturned position (Poulton, 

Fig. 3. Geologic map of the East Gate Landslide (geology modified from Kubli, 1990; Poulton and Simony, 1980). 

185


1970). The Grizzly Creek Thrust is a regional overturned 

thrust fault mapped by previous geologists 

(Poulton and Simony, 1980; Kubli, 1990) (Fig. 3). The 

Grizzly Creek Thrust fault was first suggested by 

Couture and Evans (2000) to coincide with the headscarp 

of the East Gate Landslide. A pervasive schistose 

fabric (S1) is present; it was subsequently deformed by a 

crenulation cleavage (S2) striking northwest to northeast 

and dipping to the east. The Beaver River Valley follows 

the Beaver River Fault, a normal fault that created the 

Purcell Trench (Wheeler, 1963). 

2. Discontinuity sets and structural domains 

The attitudes and characteristics of approximately 

1000discontinuities were recorded at 66stations along 

rock exposures in the failure scar and on the ridge 

upslope from the East Gate Landslide (Fig. 4). 

Discontinuity terminology for spacing and persistence 

follows the suggested method from the International 

Society for Rock Mechanics (ISRM, 1978). Four 

dominant discontinuity sets were recognised within the 

study area (Table 1). The dip- and strike-persistence 

Fig. 4. Attitudes of approximately 1000 discontinuities in the failure scar and ridge upslope from the East Gate Landslide. (A) Contoured plot of the 

poles to discontinuities and (B) symbolic pole plot of the discontinuity types recognized at the East Gate Landslide. Both stereonets are lower 

hemisphere projection, Schmidt nets.

Table 1 

Summary of discontinuity-set characteristics, East Gate Landslide 

Discontinuity set Dip direction Dip Large-scale roughness Small-scale roughness Persistence (m) Spacing (mm) 

I–Joint 160°±20° 78°±20° Planar Rough


Fig. 6. Structural domains at the head and upslope from the East Gate Landslide. The field stations are represented as dots on the map. Stations outside 

of domain boundary are associated with the location of tension cracks and do not include structural measurements.

steeply dipping and trend parallel and perpendicular, 

respectively, to the slope. Discontinuity set I has a close 

(60–200mm) to moderate (200–600mm) spacing, 

while discontinuity set II has a moderate (200– 

600mm) to wide (600–2000mm) spacing. The planar 

and smooth discontinuity set III represents a foliation 

related to the schistose fabric of the phyllite and granule 

conglomerate present at the study site. The predominantly 

stepped and rough discontinuity set IV is related 

to the crenulation cleavage. Fig. 5A illustrates how the 

crenulation cleavage significantly reduces the rock-mass 

quality in the micaceous phyllite, whereas Fig. 5B 

shows the crenulation cleavage bounding larger blocks 

within the quartz-rich phyllite. Both discontinuity sets 

III and IV strike obliquely relative to the slope, dipping 

into the slope at between 10° and 40°, and are 

characterised by a very close (20–60mm) to close 

(60–200mm) spacing. 

Four structural domains were recognised at the East 

Gate Landslide (Fig. 6). The structural domains were 

divided based on their locations on the landslide 

(headscarp vs. sidescarp), the variation in lithology, 

and the attitude of the discontinuity sets (Table 2). 

Domain 1 encompasses the northern section of the field 

site, which includes the sidescarp of the recently reactivated 

area. Domain 1 is composed of quartz-rich 

phyllite with subordinate interbeds of mica-rich phyllite. 

The second domain is based on measurements acquired 

on the ridge 400m behind the present headscarp. 

Table 2 

Summary of structural domains defined at the East Gate Landslide 

Domain Position on 

landslide 

1 Northern 

side scarp 

2 Ridge 

upslope from 

landslide 

3 Southern 

side scarp 

Lithology Attitude of 

schistose 

foliation 

(dip→dip 

direction) 

Dominant 

quartz-rich 

phyllite 

Subordinate 

mica-rich 

phyllite 

Pebble 

conglomerate 

Dominant 

mica-rich 

phyllite 

Subordinate 

quartz-rich 

phyllite 

4 Headscarp Quartz- and 

mica-rich 

phyllite 


Average 

geological 

strength 

index 

14°→068° 20–30 

24°→281° 20–30 

27°→003° 10–20 

15°→015° 20–30 

Domain 2 is comprised of granule to pebble conglomerates 

that are characteristic of the grit divisions of the 

Horsethief Creek Group. The relative attitude of 

discontinuity sets I and II is different in domain 2 as 

compared to the other domains. Crenulation cleavage 

was not obvious in domain 2 outcrops, being replaced 

by a discontinuity set. Domain 3 encompasses the 

southern sidescarp of the landslide and is characterised 

by two very well defined discontinuity sets III and IV. 

Domain 3 is composed of mica-rich phyllite with 

subordinate interbeds of quartz-rich phyllite. The central 

section of the landslide, which includes all of the 1997 

failure headscarp area, is designated domain 4 and is 

composed of interbeds of quartz-rich and mica-rich 

phyllite. 

3. Tension cracks, anti-slope scarps and bedrock 

lineaments 

The locations, orientations and relative lengths of 

tension cracks, anti-slope (uphill-facing) scarps, and 

bedrock lineaments recognised at the study area are 

shown in Fig. 7. A typical tension crack present behind 

the southern sidescarp is shown in Fig. 8. A series of 

tension cracks immediately behind the main escarpment 

has been monitored by the Geological Survey of Canada 

and Parks Canada since 2000. The monitored features 

were visited and augmented by the first author with new 

features recognised during fieldwork performed in the 

summer of 2004. EBA Engineering Consultants Ltd. 

(2004) first reported the presence of anti-slope scarps 

100m upslope from the headscarp. Some of the antislope 

scarps cut across contour lines. Three ground 

traverses from the headscarp to the anti-slope scarp 

position were conducted in order to evaluate the 

presence of tension cracks or anti-slope scarps. Only a 

few subdued features were observed along these 

traverses and were included in Fig. 7. The bedrock 

lineaments were identified from aerial photographs (30 

BCB 96083 194–196) and have a similar trend to the 

lithological contacts and regional faulting, and are hence 

assumed to be the surface expression of these features. 

4. Engineering Geology 

4.1. Geological strength index (GSI) 

189 

The geological strength index (GSI) was developed 

by Hoek and Brown (1997) to provide a quantitative 

evaluation of rock-mass quality for engineering purposes. 

The GSI considers the structure and surface 

conditions of the rock mass (Fig. 9). The spatial


Fig. 7. Bedrock lineaments and tension cracks at the East Gate Landslide. Tension cracks were recognised during fieldwork, while bedrock lineaments 

were identified from aerial photographs. 

Fig. 8. Example of a tension crack located behind the southern sidescarp (summer 2004 photograph).

distribution of the GSI estimates obtained at the East 

Gate Landslide is shown in Fig. 10. As illustrated in 

Figs. 11 and 12, the rock-mass quality at the study site is 

poor. Fig. 11 illustrates the subtle field expression of the 

Grizzly Creek Thrust Fault. The only two stations with a 

GSI as high as 40–50 were located on the northern 

sidescarp of the 1997 failure (Fig. 10). The majority of 

the headscarp area corresponds to GSI estimates 

between 20 and 40. Fig. 13 illustrates that there was 

no clear correlation between the GSI estimates and the 

identified structural domains. However, from field 


Fig. 9. Geological strength index (GIS) table with shaded area representing the estimates obtained for the East Gate Landslide (from Marinos and 

Hoek, 2000). 

191 

observation there was a correlation between the 

lithology of the outcrop and the GSI value and the 

quartz rich phyllite consistently having a higher GSI 

than the adjacent micaceous phyllite. 

The distribution of the 61 GSI estimates obtained in 

the headscarp area was transformed into a 3D surface 

using the “Surfer” code (Golden, 2002) to further 

investigate potential tectonic controls on the rock-slope 

instability (Fig. 14). The locations of the photographs of 

field outcrops (Figs. 5, 11, and 12) are indicated on the 

GSI surface in Fig. 14. A rose diagram of the orientation


of the discontinuity and tension cracks is provided with 

the 3D GSI surface in order to allow comparison 

between the measured structures and the GSI surface. 

4.2. Point-load testing 

Fig. 10. Spatial distribution of the geological strength index (GSI) estimates for the head for the East Gate Landslide. 

Point-load tests were undertaken in order to characterise 

the intact strength properties of the different 

materials present at the East Gate Landslide. The tests 

were conducted according to the International Society 

for Rock Mechanics (ISRM, 1985) guidelines for 

irregular blocks. The results of the point load tests are 

summarised in Table 3. The unconfined compressive 

strength of the different lithologies increases with quartz 

content. These results correspond to the field estimates 

that the mica-rich unit could be easily excavated with a 

rock hammer (12.5–50MPa; “R3-medium strong rock” 

according to Brown (1981) and Hoek and Brown 

(1997)) and the quartz-rich units could only be broken 

with a single blow by a rock hammer (50–100MPa; 

“R4-strong rock”). All the lithologies revealed a 

strength anisotropy index (I s50perpendicular/I s50parallel; 

where I s50 is the point load index corrected for the size 

of the sample) between 1.55 and 1.94. This anisotropy is

attributed to the planes of weakness provided by the 

schistose foliation of the rocks. According to Fig. 4 

these planes of weakness dip obliquely into the slope. 

4.3. Slake-durability 

A series of slake-durability tests was conducted to 

investigate the physical weathering properties due to a 

series of wetting and drying cycles of the different 

phyllites present at the study area. The rapid break- 


Fig. 11. Northern sidescarp of the East Gate Landslide. Note the subtle change in GSI values interpreted to represent the field expression of the 

overturned Grizzly Creek Thrust (summer 2004 photograph). 

193 

down of the failed mass reported by Couture and 

Evans (2000, 2002) could have been due to the 

material properties of the phyllite or to the closely 

spaced discontinuities within the rock mass. The 

samples for the slake-durability tests were collected 

from the headscarp, talus, mid-section, and deposition 

areas of the landslide. An attempt was made to collect 

a wide range of lithological variation. An initial series 

of tests (Coarse A to Phyllite 6B; Table 4) was 

conducted following guidelines from the American


Fig. 12. Headscarp of the 1997 slope failure with structural domains represented (summer 2004 photograph). 

Society for Testing and Materials (ASTM, 1987). 

None of the samples of the initial series of tests 

exhibited more than 10% disintegration after two 

cycles of 10min at 20rpm (Id2>0.90) (Table 4). 

Similar Id2 values were obtained by Ramamurthy et al. 

(1993) for phyllites from the Himalayan region. A 

second series of tests was performed using four cycles 

of 10min at 20rpm, as suggested in an alternate 

procedure by Richardson and Long (1987). Samples 

from this second series of tests again failed to exhibit 

a disintegration of more than 10% (Id4>0.90) (Table 

4). No relation between the slake durability and either 

the location on the slope or the lithology of the sample 

was observed in either series of tests. 

Fig. 13. Distribution of the GSI estimates as a function of the identified structural domains.

4.4. Kinematic analysis 

A kinematic analysis of sliding, toppling, and 

wedge failure mechanisms was performed on the 

mean of the discontinuity sets identified in domain 4 

because this domain encompassed the area involved 

in the 1997 failure (Fig. 15). A slope attitude of 

45°→270° (dip →dip direction) and a 30° friction 

angle was used in the analysis (Fig. 15A). A very low 

friction angle of 20° was also considered along the 

Fig. 14. Three-dimensional GSI surface related to the structural data. 

Table 3 

Point-load test results for different lithologies recognised at the East Gate Landslide 

Lithology Point load index 

(MPa) 

Mica-rich phyllite parallel 

(average quartz content 25%) 


schistose fabric to assess the sensitivity of the friction 

angle because it was described in the field as smooth 

and planar. Such a low effective frictional strength 

could also be considered to simulate the effect of high 

pore-water pressures. Toppling along some discontinuities 

of set II is feasible (Fig. 15B). Planar failure is 

not feasible with a 30° friction angle and is 

marginally possible using a 20° friction angle (Fig. 

15A). Wedge failures do not appear kinematically 

feasible (Fig. 15C). 

Unconfined compressive 

strength (MPa) 

Number samples 

tested 

1.59 38 3 7 

Number of 

tests performed 

Mica-rich phyllite perpendicular 3.14 59 2 2 1.55 

Quartz-rich phyllite parallel 

(average quartz content 40%) 

2.45 54 4 5 

Quartz-rich phyllite perpendicular 4.77 105 3 3 1.94 

Grit parallel (average quartz content 60%) 4.09 90 2 2 

Grit perpendicular 6.36 140 2 2 1.55 

Samples were tested parallel and perpendicular to the schistose fabric. 

195 

Anisotropy 

index


Table 4 

Results of slake-durability testing 

Sample 1 cycle 2 cycles 3 cycles 4 cycles Sample location 

Id1 Id2 Id3 Id4 

Coarse A 0.993 0.945 Mid-Talus 

Coarse B 0.991 0.939 Mid-Talus 

Phyllite 1A 0.969 0.900 Lower-Talus 

Phyllite 2B 0.986 0.929 Lower-Talus 





04-25-09-01 0.988 0.981 0.975 0.970 Northern side scarp 

04-25-12-01 0.987 0.977 0.970 0.963 Northern side scarp 

04-23-09-01 0.979 0.966 0.953 0.942 Headscarp 

04-26-05-01 0.982 0.969 0.958 0.948 Headscarp 

04-23-06-01 0.974 0.956 0.942 0.929 Headscarp 

04-22-06-02 0.993 0.989 0.985 Southern side scarp 

04-22-06-01 0.971 0.954 0.937 0.924 Southern side scarp 

04-28-01-01 0.986 0.976 0.967 0.958 Southern side scarp 

Several difficulties are encountered, however, when 

considering toppling as the dominant failure mechanism 

for the East Gate Landslide. Firstly, the prominent basal 

surfaces of the blocks are dipping into the slope with 

only a subordinate number of planes dipping downslope. 

Secondly, the spacing of the discontinuity sets 

(Table 1) creates tabular blocks which do not favour a 

block toppling mechanism (Wyllie and Mah, 2004). 

Thirdly, the tension cracks surveyed behind the headscarp 

opened (with only two exceptions) on the 

slumping (cataclinal) discontinuity and not on the 

toppling (anaclinal) discontinuity. Fourthly, the observed 

failures since 1997 all have exhibited pseudocircular 

slumping topography. Finally, the present-day 

headscarp morphology is controlled by the southstriking 

and steeply west-dipping (cataclinal) discontinuity 

set II. 

Using the discontinuity sets recognised at the East 

Gate Landslide, and considering both kinematic analysis 

and field observations, a conceptual 3D block diagram is 

proposed (Fig. 16). In this model, discontinuity set I 

would facilitate the development of lateral release 

surfaces. The interaction of discontinuity sets II, III 

and/or IV suggests that a rock-slumping (Kieffer, 1998, 

2003) or active-passive wedge (Coulthard, 1979; Stead, 

1984) mechanism might be appropriate. Such mechanisms 

are further complicated by the presence of the 

Grizzly Creek Thrust fault which has degraded the rockmass 

quality at the base of the unstable mass. In the 

proposed conceptual model, the toppling (anaclinal) 

discontinuities are attributed to fault damage associated 

with the overturned Grizzly Creek Thrust. 

5. Numerical modelling 

The failure mechanism of the 1997 event at the East 

Gate Landslide was investigated using limit-equilibrium, 

finite-difference and distinct-element models. The 

limit-equilibrium model was used as a preliminary 

assessment of the dependence of the critical failure 

geometry on the strength of the rock mass. The rock 

mass was assumed of sufficiently low-rock mass quality 

(as opposed to a weak intact rock mass) to be considered 

as an equivalent continuum material. The finite 

difference code was used to model the stress–strain 

relations within the rock slope. The distinct-element 

code allowed the control exerted by both discrete 

structures and rock-mass strength to be investigated. 

The cross section used in all of the models was derived 

from a pre-1997 detailed topographic map of the East 

Gate Landslide provided by the Geological Survey of 

Canada. The Mohr–Coulomb parameters used for the 

rock mass in the various models were estimated using 

RocLab software (RocScience, 2002). Two sets of 

properties were derived. The first set of properties 

related to the overall quality of the rock mass observed 

at the East Gate Landslide (Table 5). The uniaxial 

compressive strength (UCS) measured perpendicular to 

the foliation for the mica-rich phyllite of 50MPa and a 

GSI value of 30 were used as input in RocLab. The 

second set of properties described the intensely 

deformed material associated with the Grizzly Creek 

Thrust Fault (Table 5). A reduced GSI value of 15 

(based on field observation) was used as input in 

RocLab.

Fig. 15. Kinematic analysis for the headscarp of the East Gate 

Landslide: (A) planar sliding, (B) toppling, (C) wedge failure. 


Fig. 16. Conceptual block model of the East Gate Landslide showing 

the discontinuity sets and the tectonic structures influencing slope 

stability. 

5.1. Limit-equilibrium modelling 

SLIDE by RocScience (2004) is a slope-stability 

program that evaluates the stability of circular and noncircular 

slip surfaces in soil or rock slopes using verticalslice 

limit-equilibrium methods. The Spencer (1967) 

and Morgenstern and Price (1965) analysis methods 

were used in the models investigated. These methods are 

rigorous limit-equilibrium techniques that satisfy both 

force and moment equilibrium. The first series of 

models investigated circular and non-circular surfaces 

for a Mohr–Coulomb material with the cohesion and 

friction-angle values for the overall rock mass (Table 5). 

These models suggested a factor of safety (FOS) of ∼1.6 

Table 5 

Material and discontinuity properties used in the numerical models of 

the East Gate Landslide 

Overall rock mass Damaged rock mass 

Material 

Density (kg/m 3 ) 2700 2700 

Bulk modulus (GPa) 1.5 0.66 

Shear modulus (GPa) 1.0 0.40 

Cohesion (MPa) 0.25 0.1 

Tensile strength (MPa) 0 0 

Friction angle (deg) 45 34 

Joint 

Normal stiffness (GPa/m) 4 2 

Shear stiffness (GPa/m) 2 1 

Joint cohesion (MPa) 0 0 

Joint tensile strength (MPa) 0 0 

Joint friction angle (deg) 20–30 20–30 

197


Fig. 17. Global minimum slip surface of the East Gate Landslide 

obtained from Spencer's limit-equilibrium method for (A) the overall 

rock-mass material properties and (B) the damaged-rock-mass 

properties. The critical slip surface obtained in (B) resembles the 

actual 1997 slope failure. 

for both the Spencer and Morgenstern–Price methods 

(Fig. 17A). The minimum FOS was obtained for a deepseated 

movement with a top of the surface 

corresponding with the location of the anti-slope scarps. 

The damaged rock-mass properties were then investigated 

with respect to their effects on the stability of the 

slope and on the shape of the minimum circular and noncircular 

surfaces. For a cohesion value of 0.1MPa and a 

friction angle of 34°, a circular failure surface of similar 

shape and cross sectional area to the 1997 failure event 

developed for the Spencer and Morgenstern–Price 

methods (Fig. 17B). The models investigated for a 

non-circular slip surface with the reduced material 

properties had larger volumes than the circular failure 

and the 1997 event. 

Since no constraints were available on the groundwater 

conditions, its effect on the slope stability was 

investigated by considering various values for the “ru” 

coefficient, which models the pore-water pressure as a 

fraction of the vertical earth pressure for each slice along 

the critical slip surface (“ru”=0 for dry condition and 

“ru”=1 for artesian pore-water pressure rising above 

ground the height of the soil column modeled). For “ru” 

of 0.3, the factor of safety is 1.0 for overall rock mass 

strength properties, but the critical slip surface is similar 

to Fig 17A, which is larger than the observed failure 

surface for the 1997 event (Table 6). 

5.2. Finite-difference modelling 

FLAC is a 2D finite-difference modelling code 

from Itasca (2002a), which models the stress–strain 

response of a continuum material (e.g., soil or rock) to 

loading (static or dynamic). The advantages of the 

finite-difference code over the limit-equilibrium technique 

are that no failure path needs to be specified and 

the elastic and plastic behaviour of the material can be 

included in the analysis. A factor of safety can be 

computed using the strength-reduction technique 

(Dawson et al., 1999) in the FLAC/Slope module 

(Itasca, 2002b). The factor of safety values obtained in 

FLAC/Slope for the overall and damage rock 

properties (Table 5) of the East Gate Landslide 

correlated with the factors of safety obtained using 

the limit equilibrium method. 

The first model investigating the stress–strain 

behaviour in the East Gate Landslide using FLAC, 

modelled the overall rock-mass properties using an 

elastic–plastic Mohr–Coulomb constitutive criterion 

(Table 5). The maximum shear strain increment contour 

plot illustrates a small shear-strain concentration at the 

base of the material that failed in 1997 (Fig. 18A). In the 

second model, the elastic–plastic Mohr–Coulomb 

properties were reduced to reflect the properties of the 

damaged rock mass. The maximum shear-strain increment 

concentration observed in this model was four 

Table 6 

Factor of safety obtained for limit-equilibrium analyses using different 

combinations of material strength properties and “ru” pore-pressure 

coefficients 

Model Cohesion 

(kPa) 

Friction 

angle 

(deg) 

ru Factor of safety 

(limitequilibrium) 

1 100 34 0 0.93 0.99 

2 160 37 0.1 1.01 – 

3 200 40 0.2 1.00 – 

4 250 45 0 1.65 1.67 

5 250 45 0.1 1.45 – 

6 250 45 0.2 1.22 – 

7 250 45 0.3 1.01 – 

Factor of safety 

(strengthreduction)

orders of magnitude greater than in the overall rock 

mass and it encompassed a zone only slightly smaller 

than the material that failed in 1997 (Fig. 18B). 


Fig. 18. FLAC numerical model of the East Gate Landslide. Maximum shear strain increment contour plots for (A) the overall rock-mass quality 

observed at the site and (B) the damaged-rock-mass quality. Note that the contour intervals are four orders of magnitude larger in (B) than in (A). 

199 

Models in which a ubiquitous joint was introduced in 

the elastic–plastic Mohr–Coulomb constitutive criteria 

were also investigated. The overall and damaged rock-


mass properties listed in Table 5 were investigated with 

the addition of a ubiquitous discontinuity set dipping 

10° into the slope (anaclinal). This discontinuity set 

represents the intersection of discontinuity sets III 

(schistose foliation) and IV (crenulation cleavage). 

The behaviour of the models with and without the 

Fig. 19. UDEC numerical models of the East Gate Landslide. Velocity vectors for model with fault as (A) single discontinuity, (B) a set of parallel 

discontinuities. Plasticity state of the nodes from the model with fault as (C) single discontinuity and (D) a set of parallel discontinuities.

ubiquitous joint criterion was similar, with the exception 

that the ubiquitous models developed a wider zone of 

elements failing in tension behind the headscarp of the 

East Gate Landslide. 


Fig. 19 (continued). 

5.3. Distinct-element modelling 

201 

UDEC is a two-dimensional distinct-element code 

from Itasca (2004) that models the response of a


discontinuous medium, such as a jointed rock mass, to 

loading (static or dynamic). The material and discontinuity 

properties used in a series of models are listed in 

Table 5. The models investigated assumed that the 

blocks making up the rock mass behaved as an elastic– 

plastic Mohr–Coulomb material. The schistose foliation 

was simulated using the ubiquitous Mohr–Coulomb 

criteria with an assumed direction of weakness dipping 

10° into the slope. 

Several discontinuity geometries were represented in 

the UDEC models. First, the discontinuity set II anaclinal, 

discontinuity set II cataclinal, and discontinuity 

III (schistose foliation) (Fig. 6) were represented 

individually in the models to investigate their influence 

on slope movement. Discontinuity set I was not 

considered in the models presented here because it is 

parallel to the cross section investigated. Discontinuity 

set I is important, however, because it provides lateral 

release to the blocks. This first series of models found 

that the anaclinal discontinuities (toppling) create an 

extensive zone of opening and shearing upslope from 

the headscarp of the failure while the model with 

cataclinal discontinuities (slumping) creates a more 

localised opening and shearing along discontinuities 

upslope from the headscarp. A second series of models 

compared the representation of the Grizzly Creek Fault 

as a single discontinuity and as a set of parallel 

discontinuities (similar to representation in Fig. 16). 

Fig. 19A and B show the velocity vectors for the model 

with the fault as a single discontinuity and a set of 

parallel discontinuities respectively. Fig. 19B outlines 

more clearly a semi-circular zone of material that is 

slightly larger than the 1997 failure outline. Fig. 19C 

and D represent the plasticity state of the nodes 

composing the two models. Fig. 19D outlines a zone 

at the toe of the East Gate Landslide where a 

concentration of nodes has failed by slip along the 

ubiquitous joints. Also occurring in both models are 

tensile failures in the nodes behind the headscarp. 

6. Discussion 

The results of the slake-durability tests suggest that 

the observed rapid breakdown of the failed material 

reported by Couture and Evans (2000, 2002) is not a 

material property. It is possible that the apparent lack of 

breakdown indicated by the slake-durability test is 

related to the inability of the testing method to readily 

simulate the material's physical weathering due, for 

example, to freeze–thaw cycles. Alternatively, the rapid 

material breakdown at the headscarp of the 1997 failure 

of the East Gate Landslide may be highly localized and 

controlled by tectonic damage due to major structures 

such as the Grizzly Creek Thrust. 

The headscarp of the 1997 event is covered by a 

thin film of silt- and clay-size material while the cliff 

face just 10m away from the headscarp did not have 

such a thin film. This condition was also observed 

during fieldwork by the third author in 1999. Silt- and 

clay-size material appears to have been moved 

predominantly by surface runoff on the headscarp. A 

localized source of the fine material could not be 

observed directly in the field; however, the groundwater 

conditions and the influence of groundwater on 

the stability of the slope at the East Gate Landslide are 

not well known. Field mapping by the first author in 

August 2004 indicated one seepage zone located at the 

base of the central section of the headscarp and a 

second one at the base of the northern sidescarp. Both 

occurred in micaceous phyllite with very low GSI 

values (0–10), indicating possible structural control. 

EBA Engineering Consultants Ltd. (2004) also noted 

groundwater seepage at the base of the first bench in 

the debris material (∼50m from the headscarp). A low 

temperature (approx. −10°C) period preceded warm 

temperatures (approx. 0°C) in the days before the 

1997 landslide. It was suggested that such low 

temperatures would have reduced the permeability 

due to freezing of the natural conduit. This would 

have led to high pore-water pressure when the 

temperature increased, thereby further reducing the 

stability of the slope by reducing the effective friction 

along the discontinuity surfaces (EBA Engineering 

Consultants Ltd., 2004). 

The delineated structural domains suggest that the 

southern sidescarp of the East Gate Landslide has 

subsided vertically and rotated counter-clockwise relative 

to the central and northern sections. This confirms 

the preliminary observation by Couture and Evans 

(2000) that the southern sidescarp appeared to be 

displaced. Domain 3 is a culmination of a progressive 

counter-clockwise rotation of the strike of the discontinuity 

sets from domains 1, 3, and 4. This is further 

supported by field observations that the southern portion 

of the landslide is bounded by lineaments. Domain 2 is 

not considered further here because it is 400m from the 

other domains and possibly is affected by another fault 

system. 

Circular rock-mass and rock-slumping failure in 

weak highly jointed rock masses has been recognised 

in the past in rock cuts and open-pit mines (Sjoberg, 

2000; Wyllie and Mah, 2004). Couture and Evans 

(2000) noted that, because of the highly fractured 

nature of the rock mass at the East Gate Landslide, a

pseudo-circular retrogressive failure may have been 

possible. The proposed conceptual model and the 

results from preliminary numerical models appear to 

support a complex rock slumping/bi-planar mechanism 

that approaches a circular failure due to the poor rockmass 

quality. The geometry used in the numerical 

analysis was based on the conceptual model presented 

in Fig. 16. The numerical models can be constrained 

by the geological structures observed on site. Shear 

displacement along discontinuities in models that 

include steep slumping (cataclinal) discontinuity 

matches agree closely with the location of the tension 

cracks immediately behind the main escarpment and 

of the anti-slope scarps observed in the field. This is 

in contrast to the discontinuum models that simulated 

only toppling (anaclinal) discontinuities and which 

developed extensive zones of extension and shear that 

did not match field observations. The inclusion of the 

Grizzly Creek Thrust in the numerical models was 

shown to have an important effect on the failure 

outline, its representation as a set of parallel discontinuities 

led to stress concentration in the toe of the 

landslide and facilitated slip along the ubiquitous 

(foliation) discontinuities. The heavily fractured nature 

of the rock mass introduced two complications in the 

numerical models. First, the material properties 

observed in the field and estimated using RocLab 

were at the boundary between a weak rock mass and 


soil behaviour. Secondly, it was impossible to 

represent a discontinuity spacing in the distinctelement 

model that would be of the same order of 

magnitude as observed in the field. Another limitation 

of the models presented here is that the 3D nature of 

the failure cannot be accurately represented in the 2D 

models. Schistose foliation and crenulation cleavage 

were represented as one plane dipping 10° into the 

slope, while in practice they are two distinct planes 

striking obliquely with respect to the slope (Fig. 16). 

Taking into account the effect of groundwater only as 

a “ru” coefficient in the limit-equilibrium models is an 

oversimplification of the potential role played by porewater 

pressure on rock slope-stability and failure 

mechanisms. These limitations of the model reduce its 

capability to investigate the influence of the 3D 

geometry of discontinuity sets and groundwater on the 

stability of the slope. However, the good correlation 

between the deformation structures observed in the 

field and those simulated in the numerical models 

suggests that the dominant mechanisms operative at 

the East Gate Landslide have been realistically 

captured. 

Slope instability along the Beaver River Valley is not 

restricted to the East Gate Landslide (Pritchard et al., 

1989). Pritchard (1989) suggested that the location of 

landslides in the Beaver River Valley was partially 

controlled by the lithology as they appeared to occur 

Fig. 20. Hillshade obtained from a digital elevation model (DEM) of the Beaver River Valley showing East Gate Landslide in relation to other 

instabilities and the trace of the Grizzly Creek Thrust. 

203


preferentially in the slate divisions. Fig. 20 is a hillshade 

of the digital elevation model (DEM) of the 

Beaver River Valley, in which at least four other large 

slope failures can be clearly recognised. The Heather 

Hill Landslide has been the subject of a previous field 

investigation and numerical modelling (Pritchard, 1989; 

Pritchard and Savigny, 1991). Two previously unstudied 

landslides south of the main study site in the Beaver 

River Valley, located at a similar elevation to the East 

Gate Landslide occur on the eastern side of the Beaver 

River Valley and within the Grit Unit of the Horsethief 

Creek Group (Fig. 21). From a reconnaissance helicopter 

flight over the headscarp of Landslide 1, it appears to 

have similar discontinuity sets as the East Gate 

Landslide (Fig. 21A). The headscarp also corresponds 

to the mapped location of the Grizzly Creek Thrust 

which is the same fault that controlled the location of the 

East Gate Landslide. The headscarp of Landslide 2 has a 

different morphology from that of the East Gate 

Landslide (Fig. 21B). Mapping undertaken by Kubli 

(1990) suggests that the Grizzly Creek Thrust does not 

follow the Beaver River Valley at this location. A debris 

flow originating from Landslide 2 reached the trail at the 

bottom of the valley in 1999. This study suggests that 

the presence of regional tectonic structures also has a 

significant influence on the location of the landslides in 

the Beaver River Valley. 

A toppling failure mechanism was proposed by 

Pritchard and Savigny (1991) for the Heather Hill 

Landslide, while a complex block-slumping/bi-planar 

Fig. 21. Two other landslides on the eastern side of the Beaver River Valley. (A) Landslide 1 has a similar morphology and is the closest to the East 

Gate Landslide. Its location corresponds to the mapped position of the Grizzly Creek Thrust. (B) Landslide 2 has different morphology in comparison 

to the East Gate Landslide (summer 2004 photographs).

pseudo-rotational mechanism is suggested from the 

present work for the East Gate Landslide. The 

discontinuity sets identified by Pritchard and Savigny 

(1991) in the headscarp of the Heather Hill Landslide 

correspond closely to the discontinuity sets presented in 

this study for the headscarp of the East Gate Landslide. 

Further work is planned to ascertain if a similar complex 

failure mechanism could also explain the features 

observed at the Heather Hill Landslide. A major 

difference between the East Gate and Heather Hill 

landslides is that the toppling joint set recognised by 

Pritchard (1989) and Pritchard and Savigny (1991) north 

and south of the Heather Hill Landslide corresponds to 

the foliation and crenulation cleavage while the toppling 

joints observed at the East Gate Landslide can be 

attributed to a sub-parallel discontinuity set associated 

with the fault damage zone of the overturned Grizzly 

Creek Thrust. 

7. Conclusions 

Four discontinuity sets and three structural domains 

were recognised in the headscarp area of the East Gate 

Landslide. The southern portion of the landslide appears 

to have subsided vertically and rotated counter-clockwise 

relative to the northern portion. Point-load tests 

revealed an anisotropy index between 1.55 and 1.96 and 

correlation between the point-load index and the quartz 

content of the samples tested. Tension cracks and 

trenches appear to be restricted to the immediate vicinity 

of the headscarp. Although kinematic analysis suggested 

that toppling was a feasible failure mechanism, field 

observations of block shape and rock-mass quality make 

simple block toppling an unlikely dominant failure 

mechanism. A 3D conceptual block diagram suggested 

that a complex rock slumping/bi-planar pseudo-rotational 

failure mechanism may be involved. Twodimensional 

limit-equilibrium, finite-difference, and 

distinct-element modelling indicates that a pseudocircular 

failure influenced by a steeply dipping cataclinal 

discontinuity set would result from the very low rockmass 

quality and the discontinuity sets recognised at the 

East Gate Landslide. 

Acknowledgements 

The authors would like to thank K. Fecova for her 

capable assistance in the field, A. Polster (Mount 

Revelstoke and Glacier National Park) for his insightful 

discussions and ongoing monitoring efforts of the East 

Gate Landslide and T.E. Kubli for discussions on the 

regional geology during a CPGS fieldtrip. Logistical 


support was provided by Parks Canada and the 

Geological Survey of Canada. The Geological Survey 

of Canada also provided the detail topographic information 

of the East Gate Landslide used in this paper. 

Funding for this project was provided from NSREC 

Discovery grant to D. Stead, Geological Survey of 

Canada Contribution 2005847. 

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