Structural and engineering geology of the East Gate Landslide, Purcell Mountains, British Columbia, Canada

Engineering Geology 84 (2006) 183 – 206 www.elsevier.com/locate/enggeo

Structural and engineering geology of the East Gate Landslide, Purcell Mountains, British Columbia, Canada Marc-André Brideau a,⁎, Doug Stead a , Réjean Couture b b

a Simon Fraser University, Burnaby, BC, Canada Geological Survey of Canada, Ottawa, ON, Canada

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

Abstract 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: [email protected] (M.-A. Brideau). 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 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

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

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).

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 1 m/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

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

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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).

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).

1.2. Regional 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,

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

1.3. Structural geology

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

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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 1000 discontinuities 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.

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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 N = 145 II–Joint N = 242 III–Schistose foliation N = 255 IV–Crenulation cleavage N = 157

160° ± 20°

78° ± 20°

Planar

Rough

260° ± 30°

82° ± 20°

Planar

Rough

010° ± 20°

20° ± 10°

Planar

Smooth

154° ± 10°

30° ± 20°

Stepped

Rough

<1 1–3 <1 1–3 <1 1–3 <1 1–3

60–200 200–600 200–600 600–2000 20–60 60–200 20–60 60–200

estimates of the four discontinuity sets varied between very low persistence (< 1m) and low persistence (1– 3 m). The persistence estimates are believed to represent

a lower bound due to the two-dimensional nature of the outcrops and the very blocky structure of the rock mass. The planar and rough discontinuity sets I and II are

Fig. 5. Crenulation cleavage (A) reducing the rock mass quality of micaceous phyllite and (B) bounding blocks in quartz rich phyllite (summer 2004 photographs).

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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.

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steeply dipping and trend parallel and perpendicular, respectively, to the slope. Discontinuity set I has a close (60–200 mm) to moderate (200–600 mm) spacing, while discontinuity set II has a moderate (200– 600 mm) to wide (600–2000 mm) 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 (20–60 mm) 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 400 m behind the present headscarp. Table 2 Summary of structural domains defined at the East Gate Landslide Domain

Position on landslide

Lithology

Attitude of schistose foliation (dip → dip direction)

Average geological strength index

1

Northern side scarp

14° → 068°

20–30

2

Ridge upslope from landslide Southern side scarp

Dominant quartz-rich phyllite Subordinate mica-rich phyllite Pebble conglomerate

24° → 281°

20–30

Dominant mica-rich phyllite Subordinate quartz-rich phyllite Quartz- and mica-rich phyllite

27° → 003°

10–20

3

4

Headscarp

189

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 100 m 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)

15° → 015°

20–30

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

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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).

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Fig. 9. Geological strength index (GIS) table with shaded area representing the estimates obtained for the East Gate Landslide (from Marinos and Hoek, 2000).

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

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

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

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 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 (Is50perpendicular/Is50parallel; where Is50 is the point load index corrected for the size of the sample) between 1.55 and 1.94. This anisotropy is

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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).

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-

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

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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 10 min at 20 rpm (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 10 min at 20 rpm, 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.

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

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

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).

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%) Mica-rich phyllite perpendicular Quartz-rich phyllite parallel (average quartz content 40%) Quartz-rich phyllite perpendicular Grit parallel (average quartz content 60%) Grit perpendicular

1.59

Unconfined compressive strength (MPa)

Number samples tested

Number of tests performed

38

3

7

3.14 2.45

59 54

2 4

2 5

1.55

4.77 4.09 6.36

105 90 140

3 2 2

3 2 2

1.94

Samples were tested parallel and perpendicular to the schistose fabric.

Anisotropy index

1.55

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Table 4 Results of slake-durability testing Sample

1 cycle

2 cycles

3 cycles

4 cycles

Coarse A Coarse B Phyllite 1A Phyllite 2B Phyllite 3A Phyllite 4B Phyllite 5A Phyllite 6B 04-25-09-01 04-25-12-01 04-23-09-01 04-26-05-01 04-23-06-01 04-22-06-02 04-22-06-01 04-28-01-01

Id1

Id2

Id3

Id4

0.993 0.991 0.969 0.986 0.970 0.987 0.967 0.981 0.988 0.987 0.979 0.982 0.974 0.993 0.971 0.986

0.945 0.939 0.900 0.929 0.907 0.932 0.943 0.967 0.981 0.977 0.966 0.969 0.956 0.989 0.954 0.976

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.

0.975 0.970 0.953 0.958 0.942 0.985 0.937 0.967

0.970 0.963 0.942 0.948 0.929 0.924 0.958

Sample location

Mid-Talus Mid-Talus Lower-Talus Lower-Talus Lower-Talus Lower-Talus Lower-Talus Lower-Talus Northern side scarp Northern side scarp Headscarp Headscarp Headscarp Southern side scarp Southern side scarp Southern side scarp

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 50 MPa 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.

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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/m3) Bulk modulus (GPa) Shear modulus (GPa) Cohesion (MPa) Tensile strength (MPa) Friction angle (deg)

Fig. 15. Kinematic analysis for the headscarp of the East Gate Landslide: (A) planar sliding, (B) toppling, (C) wedge failure.

Joint Normal stiffness (GPa/m) Shear stiffness (GPa/m) Joint cohesion (MPa) Joint tensile strength (MPa) Joint friction angle (deg)

2700 1.5 1.0 0.25 0 45

4 2 0 0 20–30

2700 0.66 0.40 0.1 0 34

2 1 0 0 20–30

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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

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”

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 Friction ru (kPa) angle (deg)

Factor of safety Factor of safety (limit(strengthequilibrium) reduction)

1 2 3 4 5 6 7

0.93 1.01 1.00 1.65 1.45 1.22 1.01

100 160 200 250 250 250 250

34 37 40 45 45 45 45

0 0.1 0.2 0 0.1 0.2 0.3

0.99 – – 1.67 – – –

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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).

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).

Models in which a ubiquitous joint was introduced in the elastic–plastic Mohr–Coulomb constitutive criteria were also investigated. The overall and damaged rock-

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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.

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201

Fig. 19 (continued).

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.

5.3. Distinct-element modelling UDEC is a two-dimensional distinct-element code from Itasca (2004) that models the response of a

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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 10 m 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 (∼50 m 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 400 m 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

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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

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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.

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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).

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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

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