OCCURRENCE AND GENESIS OF ALPINE LINEARS DUE TO GRAVITATIONAL DEFORMATION IN SOUTH WESTERN, BRITISH COLUMBIA

Derek Kinakin BSc, Simon Fraser University 2002

THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF

MASTER OF SCIENCE

In the Department of Earth Sciences

O Kinakin 2004

SIMON FRASER UNIVERSITY

Fa11 2004

All rights reserved. This work may not be reproduced in whole or in part, by photocopy or other means, without permission of the author. APPROVAL

Name: Derek Kinakin Degree: MSc Title of Thesis: Occurrence and genesis of alpine hears due to gravitational deformation in South Western, British Columbia

Examining Committee: Chair: Dr. Peter Mustard Associate Professor Department of Earth Sciences, SFU

Dr. Doug Stead Senior Supervisor Professor Department of Earth Sciences, SFU

Dr. Brent Ward Supervisor Associate Professor Department of Earth Sciences, SFU

Bruce Thomson, MSc., PGeo. Supervisor Ministry of Water, Land & Air Protection (Ret.)

Tom Stewart, MSc., PEng. External Examiner Civil Engineer B.C. Hydro

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Simon Fraser University Library Burnaby, BC, Canada Alpine linears are found on many slopes in south western, British Columbia. The genesis of these features is commonly related to gravitational deformation of rock slopes. A preliminary stress analysis of selected ridge morphologies indicates that the resulting stress fields are different for each basic ridge type analysed, indicating that various deformation mechanisms may be active in producing alpine linears. An integrated system of GIs and numerical modelling is applied to a study of Mount Mercer, British Columbia. Detailed geomorphic and engineering geological mapping indicates that linears observed along the ridgeline of Mount Mercer are due to rock slope deformations. Potential failure mechanisms are evaluated for kinematic feasibility and resulting failure morphology. The results of the study indicate that toppling does not appear to be a suitable rock mass deformation mode for the failures at Mount Mercer; bi-planar failure and rock slumping are demonstrated to be suitable for the rock slope failures. DEDICATION

For my parents, Peggy and Mike.

And

For Krista. ACKNOWLEDGEMENTS

The support of my supervisory committee, Dr. Brent Ward and Mr. Bruce Thornson, has been instrumental in the completion of this work. The comments and suggestion from my external examiner, Mr. Tom Stewart, have been greatly appreciated. The assistance of my fellow students, Marc-Andre Brideau and Nichole Boultbe, has been a key factor in the completion of the research presented in this document. The teaching, advice, and encouragement provided by my senior supervisor, Dr. Doug Stead, has resulted in my significant personal growth as a research scientist and applied geomorphologist. TABLE OF CONTENTS .. Approval ...... II ... Abstract ...... III Dedication...... iv Acknowledgements ...... v Table of Contents ...... vi List of Figures ...... ix List of Tables ...... xiv Chapter 1 Introduction ...... 1 1. 1 Alpine linears...... 2 1. 1.1 Geomorphology of alpine linears...... 4 1.1.2 Processes forming alpine linears ...... 6 1.2 Rock slope interaction matrix ...... 9 1.2.1 Interaction matrix details ...... 10 1.3 Summary ...... 12 Chapter 2 Analysis of the distribution of stress in natural ridge forms ...... 13 2.1 Introduction ...... 13 2.2 Stress modelling and analysis ...... 13 2.3 Past methods and examples ...... 14 2.4 Methods ...... 15 2.4.1 Model properties...... 15 2.4.2 Simulation of rock mass properties ...... 15 2.4.3 Geometry creation ...... 16 2.4.4 Procedures ...... 18 2.5 Results ...... : ...... 19 2.5.1 Model validation ...... 19 2.5.2 Stress distributions with elastoplastic materials ...... 20 2.5.3 Gravity loading (K = 0.3) ...... 20 2.5.4 Tectonic and gravity loading (K=0.5. 1. and 2) ...... 23 2.6 Discussion...... 27 2.7 Conclusions...... 30 Chapter 3 Tools for landslide analysis ...... 32 3.1 Introduction ...... 32 3.1.1 Previous GIs applications in engineering geology / geotechnics ...... 32 3.1.2 GIs applications for site investigation...... 34 3.2 Tool integration: The landslide research toolbox ...... 35 3.2.1 Digital data collection and storage...... 36 3.2.2 Data manipulation and visualization ...... 40 3.2.3 Modelling and analysis ...... 44 3.2.4 Continuing work on the "Landslide Research Toolbox" ...... 45 Summary ...... 46 Chapter 4 A field investigation of sackung at Mount Mercer. British Columbia ...... 47 Study location...... 47 Local and regional physiography ...... 48 Climate ...... 48 Bedrock geology ...... 50 The Chilliwack Group ...... 50 The Cultus Formation ...... 51 Surficial geology and geomorphology ...... 51 Landsliding in the Chilliwack Valley ...... 52 Geomorphic and engineering geological mapping and data collection ...... 54 Ridge morphology ...... 54 Structural geology ...... 54 Engineering geological mapping ...... 58 Geomorphology mapping ...... 69 Geomorphological domains ...... 79 Geomorphic processes responsible for sackungen features at Mount Mercer ...... 81 Ridge-top...... 81 "Slab failure" ...... 85 The main scarp complex ...... 90 Discussion ...... 99 Failure initiation and timing ...... 99 Slope instability mechanisms: Seismic accelerations...... 100 Slope instability mechanisms: Pore water pressure ...... 100 Slope instability mechanisms: Erosion ...... 102 Temporal pattern of failure ...... 102 Current activity ...... 104 Summary ...... 104

Chapter 5 Numerical modelling. of sackung . forming . rock slope deformation mechanisms...... 106 Introduction ...... 106 Models in geomorphology ...... 107 Geomechanics and geomorphology ...... 107 Model verification and validation ...... 107 Failure mechanisms resulting in sackung ...... 109 Toppling ...... 110 Rock slumping ...... 112 Active / passive failure (two wedge failure or bi-planar failure) ...... 114 Model setup ...... 118 UDEC ...... 120 Model properties and geometry ...... 121 Simulation results ...... 129 "Slab failure" ...... 129 MSC failure simulations...... 136 Discussion...... 147 Summary ...... 153

vii Chapter 6 Conclusions and Summary ...... 154 6.1 Conclusions ...... 154 6.2 Summary ...... 155 6.3 Recommendations for future work ...... 157 References...... 158 Appendices ...... 166 Appendix 1 : Derivation of rock material properties for geomechanical modelling ..... 166 Introduction ...... 166 Hoek-Brown failure criterion...... 167 GS I ...... 169 Derivation of properties ...... 170 Conversion from RocLab outputs to Mohr-Coulomb failure criterion ...... 170 Appendix 2: SFU-Geotech data model ...... 173 Appendix 3: Field and GIs data ...... 173 Appendix 4: Simulation files ...... 174

viii LIST OF FIGURES

Figure 1: The locations of selected alpine linears identified in South Western, B.C ...... 2 Figure 2: Bedrock linears commonly identified in alpine areas. Vertical and horizontal displacements are typically on the order of meters. Certain features have lateral extents of hundreds of meters to kilometers...... 4 Figure 3: Ridge-top trench (dopplegrate) highlighted by snow, Handcar Peak, B.C ...... 5 Figure 4: Antiscarp, Handcar Peak, B.C...... 5 Figure 5: Formation of ice-marginal melt water channels during deglaciation...... 7 Figure 6: Suggested interaction matrix for mass movements in bedrock ...... 10 Figure 7: Sample DEM (with draped hillshade image) for one set of B.C. ridges used in this study. 1: Devastation Creek; 2: Pika Ridge; 3: Upper Ryan River; 4: Handcar Peak. Hillshade generated from B.C. TRIM I1 data...... 17 Figure 8: Mean ridge forms derived from 13 cross-sections. Classified according to terminology from Cruden and Hu (1999). The main valley slope (MS) and secondary valley slope (SS) for each ridge form is identified. Ridge profiles are normalized...... 18 Figure 9: Contours of s, (horizontal stress) for an elastic model of a hogback ridge. These results closely match those from previously published analytical solutions ...... 20 Figure 10: Contours of s, and sxxfor gravity loading (K = 0.3) and high GSI (90) for a hogback ridge...... 21 Figure 11: Locations of tensile stresses from FLAC modelling for a hogback ridge...... 22 Figure 12: Shear stress (sxy) concentrations from FLAC modelling for a dogtooth ridge (GSI = 90). The shallow stress concentration on the main slope is coincident with major failures in ridges used to derive the dogtooth ridge form in this study ...... 22 Figure 13: Increase in magnitude and change of location of displacements in slope toe of dogtooth ridge with a reduction of GSI from 90 to 30 ...... 23 Figure 14: Horizontal (s,) and vertical (syy)stress contours from FLAC modelling for castellate ridges. GSI = 70 and K = 0.3 ...... 23 Figure 15: Contours of s, and s, for hogback ridge with tectonic loading (GSI = 50, K = 2)...... 25 Figure 16: Contours of sxyfor hogback ridge under tectonic loading (K = 0.5 and K = 2, GSI = 70) ...... 26 Figure 17: Additional displacements in slope toe of dogtooth ridge at high tectonic stress (K = 0.5 and 2, GSI = 70) ...... 26 Figure 18: Contours of s, for castellate ridge at K = 0.5 and K = 2 (GSI = 50) from FLAC modelling...... 26 Figure 19: The formation of an asymmetric ridge occurs from differential erosion by a tributary / cirque glacier and a main valley glacier. This topography is typical of glacially modified terrain in the Cordillera of North America...... 29 Figure 20: Cartoon showing the comparison of stress distribution (under gravity and tectonic stress, K > 0.3) in normalized ridge profiles with sackung overlaid for reference...... 30 Figure 21 : Work flow (outer ring) and data flow (inner ring) of the "landslide research toolbox." ...... 36 Figure 22: Layout of the SFU-Geotech data model. PK = Primary key for data table. FK1 and FK2 = Feature links between tables...... 39 Figure 23: An example of a map rapidly produced using GIs and several data inputs...... 42 Figure 24: Perspective view of TRIM II DEM of Garibaldi Peak and Brohm Ridge (British Columbia) with simulated shadows and a colour ramp representing elevation...... 42 Figure 25: Hill shade image of same area in Figure 24...... 43 Figure 26: Scanned and rectified aerial photo of Highway 99 and part of Brohm Ridge "draped" over a DEM...... 43 Figure 27: Data from the data base (rock mass properties) and the GIs (profiles from the DEM) can be exported for use with numerical modelling packages...... 44 Figure 28: DXF output from UDEC is imported in the GIs for additional processing and analysis...... 45 Figure 29: Mount Mercer is located in the Chilliwack River Valley, south western British Columbia...... 47 Figure 30: Hillshade image illustrating the general physiography of the Chilliwack River Valley. Mount Mercer is approximately centre in the image...... 49 Figure 31 : Precipitation normals (1971 - 2000) from the Chilliwack River Hatchery at the base of Mount Mercer (49' 4' N, 121' 42' W, Elev: 213 m) Data source: Environment Canada, 2004 ...... 49 Figure 32: General bedrock geology, surficial deposits and landslides in the Chilliwack River Valley. Data sources: Evans and Savigny, 1994; Massey et al., 2003; Saunders, 1985; Thomson, 1998...... 53 Figure 33: Cross section through Mount Mercer...... 54 Figure 34: Generalized structural geology of Mount Mercer, adapted based on a basin wide cross section by Monger (1967)...... 56 Figure 35: Recumbent fold in Unit 1 observed in outcrop at the ridge line ...... 57 Figure 36: Unit 1 (Cultus Fm.) in outcrop. Scale in photo is 1 meter square...... 59 Figure 37: Discontinuity types, distribution and mean planes mapped in Unit 1...... 60 Figure 38: Unit 2 (foliated basalt of the Chilliwack Grp.) in outcrop below Unit 3. Scale in photograph is 0.5 meters square...... 62 Figure 39: Discontinuity types, distribution and mean planes mapped in Unit 2...... 63 Figure 40: Unit 3 in outcrop. Note slickensided surface dipping toward the SE ...... 65 Figure 41 : Discontinuity types. distribution and mean planes mapped in Unit 3 ...... 66 Figure 42: Subtle trench features on the western part of the Mount Mercer ridge...... 70 Figure 43: Ridge-top trench occurring above normal scarps on eastern part of the Mount Mercer ridge...... 71 Figure 44: Centre line azimuths of all ridge-top trenches / depressions mapped at Mount Mercer...... 71 Figure 45: Trench feature occurring below normal scarp in the central part of the ridge ...... 72 Figure 46: Fissures below normal scarp in central part of the ridge ...... 74 Figure 47: Tension crack occurring perpendicular to the ridge line in the central part of the ridge . The crack passes through the ridge-top ...... 74 Figure 48: Normal scarp in Unit 1 at central part of ridge...... 76 Figure 49: Normal scarps below eastern part of the ridge . These scarps occur near the top of the south east aspect slope of Mount Mercer...... 76 Figure 50: Strikes of scarp segments on the south east aspect slope of Mount Mercer ...... 77 Figure 51: Benched topography and observed geology on the south-east aspect slope of Mount Mercer...... 78 Figure 52: Slope map derived from DEM data for the south-east aspect slope of Mount Mercer ...... 78 Figure 53: Engineering geomorphology map of Mount Mercer with geomorphic domains: Ridge.top. "Slab Failurev.and Main Scarp Complex highlighted...... 80 Figure 54: Trends of ridge-top trenches compared to the strikes of steeply dipping joints and ridge line orientations...... 82 Figure 55: Unit 1 outcropping in a vertical wall at the head of a north facing cirque ...... 84 Figure 56: Overview of the morphological features of the "slab failure." ...... 86 Figure 57: "Slab failure" and mean discontinuities mapped in Unit 1 ...... 87 Figure 58: Basic geometry of an active / passive ("2 wedge" failure) ...... 89 Figure 59: Force vectors expected in failure mass. based on landform type ...... 90 Figure 60: Overview the Main Scarp Complex and associated slope zones based on landforms and overall slope ...... 92 Figure 61: Normal scarp segments compared with discontinuity strikes indicate that the normal scarps follow existing joints ...... 93 Figure 62: The failure is moving toward the south-east. The upper part of the failure is defined by compound head-scarps and lateral and rear release zones indicated by trenches...... 93 Figure 63: Simplified kinematic analysis indicates that neither sliding nor toppling is a likely failure mode in the main scarp complex of Mount Mercer ...... 95 Figure 64: Apparent displacements in the upper part of the MSC illustrated by both landform development and possibly displaced bedrock units...... 98 Figure 65: Probable location of basal zone / plane required for rock slumping to occur based on bedrock geology and surface morphology ...... 98 Figure 66: Suspected limit of the Lake Chelan earthquake of 1842. The Richter scale magnitude in the central zone is estimated at 7.4. The Chilliwack Valley falls within this central, high magnitude area...... 100 Figure 67: Conceptual Time I Displacement curves for a failure dominated by rock slumping ...... 104 Figure 68: Simplified representation of an infinite slope model for dry conditions. The factor of safety (F) is the ratio of the resisting forces (r, shear strength) of the material to the driving forces (d) due to gravity acting on the failure plane...... 109 Figure 69: Toppling of columns within a slope resulting in obsequent scarps and tension cracks. The interaction of toppling columns and non-toppling sections of the rock mass are highlighted...... 1 1 1 Figure 70: Rock slumping is due to the instability of individual blocks or columns and results in compound head-scarps, a mid slope bench, and a steep toe. Figure based on physical and numerical models by (Muller and Hofmann, 1WO), (Gaziev and Rechitshi, 1974a), (Gaziev and Rechitshi, 1974b). and (Kieffer, 1998)...... 114 Figure 71: An active block driving a passive block on a bi-planar failure surface results in the development of a well defined head-scarp, possible grabens and upper slope deformation, anti-scarps, slope bulging and dilation of the slope material. See text for complete symbol explanation...... 118 Figure 72: The location of cross sections for models of the slab failure (S-S') and the MSC (M-Me)...... 123 Figure 73: Block A based on slope profile S - S' and discontinuity measurements from Unit 1 and model block B based on slope profile M - M' and discontinuity measurements from units 2 and 3 for UDEC modelling. Note displacement constraints on the model boundaries used for each set of simulations...... 124 Figure 74: Complete UDEC block used for "slab failure" simulations. Area of interest is the ridge-top and upper slope...... 129 Figure 75: Initial configuration of the "slab failuren simulation 1...... 131 Figure 76: Toppling occurrs only when the friction angle on the base plane is reduced to the unrealistic value of < 5' resulting in the toe blocks sliding. The resulting surface morphology is dominated by obsequent scarps, no head-scarp is formed...... 132 Figure 77: Displacement vectors for the toppling model (simulation 1) of the "slab failure." ...... 132 Figure 78: Initial configuration of simulation 2 for the "slab failure." ...... 133 Figure 79: Magnified view of the head-scarp developed in simulation 2. Also note the development of minor obsequent scarps and minor rotations of block defined by the toppling joints ...... 134 Figure 80: Displacement vectors for model 2 illustrating a zone where displacement vectors are parallel to JS 3 (I), a zone where displacement vectors are parallel to JS 1 (Ill) and a transition zone between the two (11)...... 135

xni Figure 81: Zoomed view of the head-scarp developed in model 3. The observed landforms are similar to those in simulation 2 ...... 136 Figure 82: Rock slumping model in UDEC based on a physical model of Gaziev and Rechitshi 1974a ...... 138 Figure 83: Various examples of initial elastic simulation of rock slumping based on the geometry of the MSC...... 139 Figure 84: Model geometry and boundary condition overview for the MSC simulations...... 140 Figure 85: Overview of the final slope geometry of simulation 1b ...... 141 Figure 86: Magnified view of the ridge line of simulation 1b based on the MSC. A compound head-scarp is produced...... 142 Figure 87: Base of columns in model MSC 1. The columns deformed due to the gravitational stress. 'A'-frame voids develop as blocks slip and back rotate ...... 143 Figure 88: Magnified view of compound head-scarp development in simulation 2b ...... 144 Figure 89: 'A'-frame voids develop between blocks in simulation 2b. The displacements along the cross joints are also visible...... 145 Figure 90: Magnified view of the development of the compound head-scarp in simulation 3b ...... 146 Figure 91: 'A'-frame voids and horizontal voids opening along the cross joints near the basal surface of simulation 3b...... 147 Figure 92: Simulated scarp heights increase from the ridge-top to a maximum at the mid-slope and decease toward the slope toe...... 151 Figure 93: Cross section (FC 1 and FC 2) created during the field study of Mount Mercer, British Columbia illustrating normal scarp heights at the ridge- top and upper slope of the MSC...... 151 Figure 94: Equivalent step-path structures formed by a combination of joints, required for the application of toppling, rock slumping, and bi-planar failure mechanisms to large, complex slopes...... 153 LIST OF TABLES

Table 1: Mohr-Coulomb material properties used for FLAC modelling. Properties were derived from published experimental data and scaled values from RocLab. Density was assumed to be 2600 kg/m3 for all model runs...... 16 Table 2: Ridges from B.C. and Colorado with known sackung used to develop the mean ridge forms for modelling...... 18 Table 3: GIs applications for investigation methods suggested by Eberhardt et al., 2002...... 34 Table 4: Summary of selected data for mapped engineering geology units...... 67 Table 5: Summary of selected discontinuities properties collected from mapped engineering geology units...... 68 Table 6: The progression of simulations used in the current study to investigate the development of sackung by various rock slope failure mechanisms...... 120 Table 7: Summary of basic discontinuity properties used for the UDEC simulations in the current study...... 126 Table 8: Rock mass properties used in the current study compared to previously published material properties used for FD (finite difference) and DE (distinct element) models...... 128 Table 9: Discontinuity properties used in this study compared to previously published discontinuity properties. Tensile strength = 0 MPa or NA for compared studies. Dilation angle = 0' or NA for compared studies...... 129 Table 10: Overview of "slab failure" simulation results...... 130 Table 11: Overview of initial simulations of rock slumping and rock slumping simulations based on MSC data...... 137 Table 12: A summary of the geometric and geomorphic characteristics of rock slumping, toppling, and bi-planar failure mechanisms...... 152 CHAPTER 1 INTRODUCTION

Linear geomorphic features such as scarps, anti-scarps, trenches, and tension cracks have been identified in alpine areas all over the world. Suggestions for the formation of these landforms have included melt-water erosion, earthquakes, and gravitational deformations within mountain slopes. Published evidence has shown that some sets of these features can be linked to mass movements, while others have a more ambiguous genesis.

In British Columbia, numerous ridges and mountains have been identified as locations of alpine linears (Fig. 1). Greater understanding of the processes responsible for the formation of these landforms is necessary before evaluating their potential hazards. At the time of writing, a minimum of 10 sets of alpine linears in south-westem British Columbia have been described in previous publications (Bovis, 1982; Bovis and Evans, 1995; Clague and Evans, 1994; Evans and Savigny, 1994; Stewart, 1997). As the expansion of tourism and development into the mountainous regions of B.C. continues, the evaluation of the hazards associated with alpine linears will become increasingly important. Although the features are not themselves generally hazardous, the processes which they reflect may be. Many questions regarding the genesis of these features remain.

Throughout this thesis two broad goals are pursued:

1. Provide researchers with an overview of techniques that are useful in studying alpine linear features.

2. By using an integrated approach of geomorphology and rock mechanics, provide additional insights into the processes that may result in alpine linears. It Alpine Linear Locations

Figure 1: The locations of selected alpine linears identified in South Western, B.C.

1.1 Alpine linears

Bedrock linears (Fig. 2) have been observed in numerous locations around the world. Such features are commonly considered a surface expression of some deep-seated failure mode in rock slopes. First identified in Europe, sets of scarps, anti-scarps (up-hill facing scarps, obsequent scarps), and ridge-top trenches were associated with gravitational slope deformations. Zischinsky (1969) used the term sackungen (German: sagging) for the process resulting in these landforms. Frequently, the extent of these bedrock linears is on the order of kilometers. Displacements (horizontal in the case of fissures or tension cracks, vertical/horizontal in the case of antislope scarps, and vertical/horizontal for trenches) can range from less than a meter to tens of meters. These features are reported in a wide array of bedrock types. Detailed reviews of the literature of bedrock linears have been completed by previous authors (McCalpin and Irvine, 1995; Stewart, 1997).

Zischinsky (1969) first introduced the term sackungen to describe a process of deep- seated gravitational deformation of schists, phyllites and gneisses that did not require a continuous slip surface. The linears or sackung were assumed to be the surface expression of this gravitational creep. Beck (1 967) reported linear scarps ("gravity faults") in the massive sedimentary rocks of the New Zealand Alps. These features were thought to be caused by gravitational displacements in a rock mass weakened or damaged by earthquakes. Scarps (L'~b~eq~io~~scarps" and "scarplets") have been observed in Scotland by de Freitas and Watters (1 973) and Holmes and Jarvis (1985). In both cases, toppling of blocks within the slope was suggested as the mode of deformation and therefore the mechanism of scarp formation. Bovis (1990) has suggested toppling within volcanic and igneous rocks as the mechanism for the large antislope scarp observed at Affliction Creek, British Columbia. More complex kinematic mechanisms resulting in sackung have been recently suggested (Alfonsi et al., 2004; Kieffer, 1998). Explanations for the formation of tension cracks, ridge-top trenches and antislope scarps based on stress distributions in slopes have been presented by several authors (Radbruch-Hall et al., 1976; Savage and Vames, 1 987).

Recently, "sackungen" has been used to refer to the landforms and not the process (McCalpin and Irvine, 1995). Other authors have suggested terminology such as mass rock creep (Chigira, 1992) or deep-seated slope deformations (Agliardi et al., 200 1). Alternative explanations (melt water erosion, earthquakes) suggested for the formation of these landforms have created further confusion in the terminology used when discussing the landforms and associated causative mechanisms.

To ensure a clear and productive discussion on the current topic, based on existing literature, the following definitions are presented:

Linears (Alpine Linears): linear geomorphic features, occurring in bedrock in alpine and sub-alpine areas, including:

Scarps Anti-scarps (reverse scarps, obsequent scarps, uphill facing scarps) Tension cracks (fissures) Slope benches Ridge-top trenches (dopplegrate) These features may or may not be formed in relation to the gravitational deformation of rock slopes.

Sackungen: the process of gravitational deformation or mass rock creep. Sometimes used to refer to the entire mountain where the process is occurring.

Sackung : surficial features formed by sackungen. Double Ridges Trenches f Fissures

Antiscarps

Figure 2: Bedrock linears commonly identified in alpine areas. Vertical and horizontal displacements are typically on the order of meters. Certain features have lateral extents of hundreds of meters to kilometers.

1.I .I Geomorphology of alpine linears Jahn (1964) identified gaping joints and wide fissures, ridge-top trenches, and slope trenches as features potentially caused by slow moving failures occurring on the mountain scale. Radbruch-Hall et al. (1976) observed horizontal linear trenches and up-hill facing scarps (anti- slope scarps) on steep slopes and ridge crests as products of sackungen. Bovis and Evans (1996) and McCalpin and Irvine (1995), summarizing previous literature, have identified the previously mentioned landforms as "linears".

The geomorphic characteristics of each individual type of linear provide only limited suggestions to the mode of formation. Ridge-top depressions (Fig. 3) have been noted up to 2 kilometers in length (Clague and Evans, 1994). Some occurrences of these landforms causes a division of the ridge line, resulting in a "double ridge" or dopplegrate (Jahn, 1964). The depths and widths of these features are on the order of 1-10 meters. Scarps (normal scarps, down-hill facing scarps) can be laterally extensive (- 1000 meters) and have significant vertical relief (10's of meters) (Thompson et al., 1997). Anti-scarps (or reverse scarps, obsequent scarps, uphill facing scarps) (Fig. 4) commonly have heights less than 10 meters, but can be laterally extensive (100's of meters) (de Freitas and Watters, 1973). Tension cracks can occur at a range of lengths and widths. t Figure 3: Ridge-top trench (dopplegrate) highlighted by snow, Handcar Peak, B.C.

Figure 4: Antiscarp, Handcar Peak, B.C. 1.I .2 Processes forming alpine linears The landforms cited as alpine linears are common geomorphic features. The key to determining the genesis of these landforms is examination in context. The relationships of these landforms to each other are important when suggesting possible processes of formation of the landform group. Several processes resulting in alpine linears have been previously suggested.

1.I .2.1 Melt water erosion

The formation of ice-marginal channels by melt water erosion of bedrock is well documented (Benn and Evans, 1998; Bennett and Glasser, 1996). These melt water channels form as water erodes both into the ice and adjacent bedrock along the lateral margins of the ice mass (Fig. 5). Typically associated with other ice marginal features, these channels can have either a complete channel cross section or only one wall and a channel floor; the other wall being previously formed by the ice mass. The channels can be expected to start and stop abruptly (Bennett and Glasser, 1996).

The general morphology of the ice marginal melt water channels could represent features that would be termed slope parallel ridge-top trenches and slope benches / anti-scarps during modem investigations. Most of the alpine linears reported occur on slopes which were formerly glaciated. However, many alpine linears are laterally extensive scarps which show no evidence of fluvial erosion, such as: a sinuous planform, smooth walls and floor, or remnant bedload material. Holmes and Jarvis (1 986) indicate that rock slope linears which strike at oblique angles to the slope are not likely related to glacial erosion processes. Melt water channel

Melt water channel 1

Figure 5: Formation of ice-marginal melt water channels during deglaciation.

1.1.2.2 Faulting and seismicity

Linears associated with active faults have been suggested by Agliardi et al. (2001) and Bell and Eisbacher (1996). The recent Denali earthquake (Alaska, 2002 M 7.9) was observed to produce linear scarps on mountain slopes and ridge-tops. The linkage of some of these landforms to tectonic creep along faults or displacements during earthquakes is strengthened by their distribution through zones of surface rupture (Stewart, 1997). However, the definite determination of a "tectonic scarp" from a sackung is difficult as gravitational deformation is commonly concentrated around zones of intense tectonic damage (faulting and shearing) (Beck, 1967; Radbruch-Hall and Varnes, 1976). Detailed studies of individual, extensive hears are required to establish whether active fault processes may be responsible for the creation of the landforms (Clague and Evans, 1994; Thompson et al., 1997). 1.1.2.3 Gravitation deformation

Most of the literature regarding alpine linears has focused on the gravitational deformation of rock slopes as the mechanism of linear formation. Jahn (1 964) ,referring to dopplegrate or double-ridges, noted that they are formed by the "the loosening of the rock mass by gravitational and tectonic forces (stresses)". As previously mentioned, Zischinsky (1 969) used the term sackung to refer to surface manifestations of deep seated sagging of rock masses due to gravity. Radbruch-Hall et al. (1976) noted that linears in the Colorado Rockies form due to rock mass deformations in three settings (spreading of rigid blocks overlying soft rocks, sagging and bending of foliated rocks, differential displacements in hard and fractured crystalline rocks). Savage and Vames (1 987) observed that "gravity driven plastic yielding of over steepened and closely jointed high mountain ridges" would result in linears. More current authors (Agliardi et al., 2001 ; Bovis and Evans, 1996; Bovis and Stewart, 1998; Kieffer, 1998) have examined the kinematic mechanisms of failure occumng during the gravitational deformation of the rock slopes.

It has been suggested that the mechanisms of sackungen and linear formation should be treated separately from triggers for the mechanisms (McCalpin and Irvine, 1995). Cited precursors for the formation of sackung include:

The over steepening of valley walls by glaciers resulting in increased gravity / tectonic stress differences (Savage and Varnes, 1987).

Weakening of the rock mass through seismic shaking (Beck, 1967)

Neo tectonic uplift / differential isostatic rebound or subsidence since the end of the last glaciation (Agliardi et al., 2001; Clague and Evans, 1994; Radbruch-Hall et al., 1976).

Rapid stream erosion (Radbruch-Hall et al., 1976)

Rock slope debuttressing by receding glacial ice (Bovis, 1990; Thompson et al., 1997).

As a starting point for the analysis of any geomorphic feature, one must examine the endogenic and exogenic processes which may be involved in the formation of the feature(s). Endogenic processes are those which occur within the rock or soil mass under investigation. Driven by far field events, these processes may "set the stage" for the exogenic processes. Tectonic modifications of geologic units through folding and faulting are common examples of endogenic processes. Regional groundwater flow or hydrothermal fluid flow can also be considered endogenic processes. Exogenic processes are best described as those which affect surficial or shallow material on a local scale. Weathering (chemical and mechanical) and erosion are exogenic processes.

1.2 Rock slope interaction matrix

The development of sackung is, by definition, caused by mass movements. Mass movements are products of the interactions of various factors. Jahn (1964) proposed that the initial morphology of slopes and ridges, the rock structure, and the climatic processes on the mountain are important in the development of sackungen. Radbruch-Hall et al. (1 976) notes importance of rock strength, rock mass relationships, rock mass geometry. Savage and Varnes, (1 987) show the importance of slope geometry and rock mass properties in producing concentrations of stress. In their review of sackungen formation, McCalpin and Irvine (1 995) point out the importance of interactions between the stress field, rock mass geometry, and rock mass condition. In addition, they note that the formation of sackungen landforms is independent from the triggering event. Previous authors (Bovis and Evans, 1996; Giardino et al., 2004) recognized that rock mass structure is important in determining failure mode.

From these (and other) previous studies, linkages between the following have been noted:

(1) Rock Mass (quality and intact properties)

(2) Stress (gravitational, tectonic, pore water)

(3) Geometry (slope and discontinuities)

(4) Erosion (fluvial, glacial, colluvial)

(5) Weathering (physical and chemical)

Utilizing an interaction matrix (Hudson, 1992), the results of the coupling of these factors can be explored (Fig. 6). The core factors are arranged along the leading diagonal with processes linking the factors derived from the previous overview of suggested means of sackungen formation from the literature. Figure 6: Suggested interaction matrix for mass movements in bedrock.

1.2.1 Interaction matrix details The interactions between the main factors included in the matrix are well established in rock mechanics and geomorphology literature. Although examined separately, the results of all of these interactions affects the development of mass movements in bedrock.

Implicit in many of these interactions is the passage of time. The results of some interactions ('weathering and rock mass') can be observed annually, while others ("erosion and geometry") require 100,000's of years before the results are apparent. Some of the interactions are only valid for specific time periods ("erosion and rock mass") and others occur irrespective of any time scale ("rock mass and stress"). The results of many of the interactions are similar, yet occur over different scales of time and space ("weathering", "erosion" and "stress" can all reduce rock mass quality).

Tectonic and local stress concentrations within the rock mass can result in the creation of rock mass structures during active plate tectonics, closure of open discontinuities, or micro- fractures in intact rock during an earthquake. All of these deformations of the rock mass are caused by differential stresses over various temporal and spatial scales (Goodman, 1989). New discontinuities and structures form in orientations determined by the stress regime of the rock mass; rock mass properties (stiffness, strength) affect how stress is distributed through the entire mass in question. Strong and stiff units can carry more stress, reducing stress in units below them (Goodman, 1989).

At the earth's surface, erosion is more pronounced in zones of low rock mass quality. Valleys form through fluvial, glacial, and colluvial processes that commonly follow damage zones related to regional discontinuities (Goodman, 1989; Scheidegger, 198 1). Valley and ridge morphometry is determined by the active erosion processes. Fluvial erosion results in "v"-shaped valleys, whereas glacial erosion results in "U"-shaped valleys. The resulting slope geometry affects where shallow compressive and tensile stress concentrations occur (Goodman, 1989; Pan et al., 1994). Although these stresses are most commonly examined in valley cross section, stress is a tensor and should be considered in three dimensions. Stress concentrations in three dimensions can be created by various topographic features such as convex "noses" on slopes (tensile and shear stresses) and cirques (possibly high compressive stresses at the cirque floor).

The removal of large volumes of overburden by erosion results in the unloading of underlying bedrock. Bedrock displacements occur in response to stress relief. Low compressive stresses allow existing discontinuities to open and control the displacement behaviour of the rock mass. Additional damage to the rock mass can occur indirectly as the rock mass accommodates the stress relief or directly through the stress concentrations and pore water pressures associated with glacial erosion.

Zones of tectonics damage are more susceptible to physical and chemical weathering due to pre-existing discontinues and fractures which focus water and increase the surface area available for chemical weathering. These weathering processes result in the further reduction of rock mass quality through the breakdown of constituent rock minerals and the expansion of existing rock fractures. The quality of the rock mass has been shown to affect the final slope geometry, as high quality rock masses deform less and can maintain steeper slopes than low quality rock masses (Augustinus, 1992; Augustinus, 1995a; Selby, 1980).

The geometry of the slope and discontinuities in the rock mass control which kinematic modes of failure can occur within the rock slope (Hoek and Bray, 198 1). As the strength along existing discontinuities is reduced through weathering, discrete displacements along the discontinuities can occur. The depth and extent of weathering affects the volume involved in landsliding.

The general landscape evolution described by the interaction matrix creates slopes, reduces rock mass quality and creates discontinuities within the (forming or existing) rock slopes. The overall result of the above interactions is the increased probability of landsliding, and in turn the production of sackung.

1.3 Summary

Several geomorphic processes can form linears; however, the remaining chapters of the current thesis will emphasise the exploration of sackungen and alpine linears related to mass movements. The previously introduced interaction matrix provides a framework by which important factors related to mass movements and the subsequent linears can be explored.

In Chapter 2, the interactions of the rock mass properties, slope geometry and stresses will be further examined. Chapter 3 introduces a set of tools and methodologies for the study of large landslides. Chapter 4 demonstrates the application of these tools in investigating the field relationships of mass movements and sackung with reference to Mount Mercer, B.C. Potential failure modes for mass movements in high mountain slope, such as those suggested by the field study at Mount Mercer, B.C., are explored by simulating the interactions of discontinuities, slope geometries, and rock mass properties through n~merical'modelin~in Chapter 5. Finally, Chapter 6 will provide a summary for the current work and a critical discussion of the current research for future studies of extensive natural slope deformations and sackung. CHAPTER 2 ANALYSIS OF THE DISTRIBUTION OF STRESS IN NATURAL RIDGE FORMS

2.1 Introduction

Stress distributions related to topography have been previously examined in two dimensions using symmetrical (Savage and Swolfs, 1986) and asymmetric (Pan et al., 1994; 1995) ridge profiles with assumed elastic material properties. Links between stresses in ridges and the formation of ridge-top trenches, antiscarps, and tension cracks (collectively termed: sackung) in alpine areas have been postulated by several previous authors (Ferguson, 1967; Radbruch-Hall et al., 1976; Savage and Varnes, 1987). Several British Columbia and Colorado ridges associated with sackung, or bedrock linears, do not reflect the general shapes previously analysed. The effects of the natural topography on the stress distributions in a ridge may have important implications for sackung development. Two-dimensional elastic analysis may prove too simplistic to analyse the stress distributions in an alpine ridge as elastic material properties fail to realistically simulate the response of the fractured, weathered, and discontinuous natural rock mass. Previous work is extended by exploring the effects of representative topographic profiles and elastoplastic constitutive models on the distributions of stress in ridges. These distributions of stress are then examined with respect to the occurrence of sackung (bedrock linears).

2.2 Stress modelling and analysis

The concept of stress is used to describe the intensity of-internal forces in a body under the influence of a set of applied surface forces (Brady and Brown, 1985). These internal forces can cause strain in a body that may eventually lead to plastic (unrecoverable) deformation or brittle fracture. In the current study, the body is defined by the ridge form. The applied surface forces include the gravitational stress (near-field stress) provided by the weight of the ridge and tectonic stress (far-field stress) from plate interactions beyond the boundary of the model. The strains or deformations of the rock mass result in the production of specific landforms.

The geomechanics convention for describing stresses is used throughout this thesis ("compressive stresses are positive"). Geomechanics and engineering geology techniques provide useful tools for the geomorphologist. Scheidegger (1963) relied on geomechanical principles when reviewing the development of valley patterns in the Alps. Selby (1980), following work by Bienawski (1976) on rock mass quality, developed the concept of Rock Mass Strength (RMS). Augustinus (1995a; 1995b; and Selby, 1990) applied RMS and rock engineering principles to slope stability issues in New Zealand.

2.3 Past methods and examples

Within the literature, two methodologies have been applied to the two- dimensional stress analysis of ridge forms: closed form mathematical solutions and numerical continuum analysis. The distributions of stress for symmetric ridges have been analysed using a closed form solution for an elastic half plane (Savage et a]., 1985; Savage and Swolfs, 1986). The use of the conformal mapping technique allowed the analysis of both gravitational stress (ridge body weight) and constant tectonic stress (far field compression) applied to the model. This method was later applied to the problem of sackung formation by Savage and Varnes (1987) by comparing stress values fiom the solution with values of material yield strength to determine which parts of the ridge could be expected to fail and deform, resulting in sackung formation. The velocities of deformation in these failure regions were used to divide the slope into regions of extending (near ridge-top), plug (mid-slope) and compressive (slope toe) plastic flow. Tension cracks were linked to the zones of extension; antislope scarps were linked to zones of extending flow.

Pan et al. (1995) and Hasebe and Wang (2003) presented methods of conformal mapping that were used in the analysis of irregular ridge and slope boundaries. Pan et al. (1995) addressed the issue of the distributions of stress based on the mechanical properties and structure associated with rock masses in addition to the ridge shape. The solution provided by Pan et al. (1995) was elegant as it reduced complex asymmetric ridges into the constituent symmetrical ridge.forms. All of these analytical solutions showed that both the mechanical properties of the rock and the geometrical configuration of the ridge were important in the predicted final stress distributions.

Radbruch-Hall et al. (1 976) used a finite element model (FEM) analysis to show the stress distributions in slopes within two rock types where sackung were observed. A simple conceptual slope, assuming elastoplastic constitutive criterion, was analysed. Tensile stresses, developed near the ridge crest, provide a promising explanation for the formation of sackung. Kohlbeck et al. (1979) presented an elastic FEM analysis for a valley bounded by two irregular ridge forms. Their main interest was in the relationship between principal stress orientations and observed jointing patterns. 2.4 Methods

To further investigate the links between the distributions of stress in natural slopes and sackung formation, a two-dimensional continuum analysis was undertaken assuming elastoplastic response. The complete ridge form was modelled to assess the effects of the overall ridge geometry on the distributions of stress in the ridge slopes. The modelling of stress distributions was completed using a finite difference-based geomechanical code. Three core models were built representing each ridge form of interest. Each ridge was modelled assuming progressively lower elastoplastic material properties.

FLAC 4 (HCItasca, 2002), a finite difference modelling program, was chosen for the stress analysis. Available for the PC platform, the FLAC software has been used extensively for modelling rock and soil in slopes and rock in underground settings. It is a recognized, robust tool for this type of analysis. In addition to its efficient handling of the computations required for this analysis, FLAC provides very reasonable visualizations of the model outputs.

2.4.1 Model properties

Geomorphic researchers that have considered bedrock/landform systems within the literature have correlated process to bedrock lithology. The variations in geomechanical properties within a single lithology are great enough to make such correlations difficult. Some success has been achieved through the relation of rock mass classification systems to geomorphic features (Selby, 1980; Agustinus, 1992, 1995). The adoption of a rock mass classification system allows the estimation of geomechanical properties for use in modelling. As sackung are products of the stress 1 strain interaction within the rock mass and the total strain is limited by the quality of the rock mass (assuming no geological structural controls), representative rock mass properties are required for the analysis.

2.4.2 Simulation of rock mass properties

The model input data (Table 1) were generated for each core model using a combination of published laboratory data and scaled parameters derived from the methods described in

Appendix 1. Initial material properties were created for intact and unweathered rock (GSI = 100) with properties, UCS (unconfined compressive strength) and mi (a material constant), reflecting a strong rock (such as a granite). The values for the deformation modulus, produced by RocLab (Rocscience, 2003) for a GSI of 100, are significantly higher than the expected range of values for granite. To reduce the modulus, the "disturbance value" was set at one (D = 1) for all property sets developed. The "disturbance value" (D) compensates for the upper bound values produced by the Hoek-Brown criterion when applied to intact rock (Hoek et al., 2002). Varying from 0 - 1, the "disturbance value" represents damage to the rock mass by processes equivalent to rock blasting and removal of overburden in a mining operation.

Table 1: Mohr-Coulomb material properties used for FLAC modelling. Properties were derived from published e~perim~ntaldata and scaled values from ~oc~ab.Density was assumed to be 2600 kglm3 for all model runs. Initial Stage 1 Stage 2 Stage 3 Stage 4 (intact rock) (GSI 90) (GSI 70) (GSI 50) (GSI 30) Bulk modulus (K), GPa 58.6 27.1 7.5 2.2 0.7 Shear modulus (G), GPa 35.7 21 .O 7.0 2.2 0.7 Cohesion, MPa 16.5 9.9 5.0 2.8 1.4 Friction angle 53.0 48.7 37.8 26.4 16 Tensile strength, MPa 4.0 1.5 0.23 0.034 0.005 Modulus of deformation (4,GPa 89.0 50.0 16.0 5.0 1.6 Poisson's Ratio (v) UCS, MPa GSI 100 90 70 50 30

2.4.3 Geometry creation

Ridge forms representative of natural ridges were created by analyzing a population of ridge cross-sections with known sackung. Thirteen cross-sections (nine from SW British Columbia and four from western Colorado) were created from British Columbia TRIM (25 m) and USGS (30 m) digital elevation models (DEMs) of each source area (Fig. 7) in ArcGIS (ESRI, 2002). All of the ridges have been sampled from formerly glaciated terrain and represent current alpine and subalpine environments. Maximum local relief for the ridges ranged from 1000 - 1800 meters.

The 13 cross-sections were set to a common base level ("0")by subtracting the lowest elevation of each cross-section from all of the elevation values in that cross-section. In addition, the cross-sections were normalized by dividing the "x" (horizontal) and "y" (vertical) values by the maximum elevation of each cross-section. This method of normalization ensured that slope angles and general morphology were maintained.

From the 13 normalized cross-sections, three groups were readily defined based on the morphology of the ridge (Fig. 8). These groups can be categorized using the tenninology of Cruden and Hu (1999) (based on morphology, ignoring the bedding orientation of the material in the peaks) for the shape of mountain peaks in the B.C. and Alberta Rocky Mountains. The categories and associated mountain peaks used in this study are listed in Table 2.

Figure 7: Sample DEM (with draped hillshade image) for one set of B.C. ridges used in this study. 1: Devastation Creek; 2: Pika Ridge; 3: Upper Ryan River; 4: Handcar Peak. Hillshade generated from B.C. TRIM II data. Mean Ridge Profiles

Figure 8: Mean ridge forms derived from 13 cross-sections. Classified according to terminology from Cruden and Hu (1999). The main valley slope (MS) and secondary valley slope (SS) for each ridge form is identified. Ridge profiles are normalized.

Table 2: Ridges from B.C. and Colorado with known sackung used to develop the mean ridge forms for modelling.

Name Location Ridge Morphology Class Mount Breakenridge B.C. Hog back Walheach B.C. Hog back Devastation Creek B.C. Hog back Pika Ridge B.C. Hogback Surprise Ridge Colorado Hogback Mount Nast Colorado Hogback Cheam Peak B.C. Dogtooth Johnson Peak B.C. Dogtooth Katz B.C. Dogtooth Bald Eagle Colorado Castellate Upper Ryan River B.C. Castellate Mount Massive Colorado Castellate Handcar Peak B.C. Castellate

2.4.4 Procedures

The mean cross-sections were used for the modelling procedure. These three representative ridge forms were used for the two dimensional elastic and elastoplastic stress analyses for two stress states: gravity stress only

combined gravity and tectonic stress (K = 0.5, K = 1, K = 2)

Increases in tectonic stress are simulated by increasing the initial horizontal stress in the model. The relationship between the horizontal and vertical stresses can be expressed as K, where

Under only gravity loading, the K value can be approximated to 0.3.

2.5 Results

The distributions of three stress orientations are considered for each ridge form. Stresses in the horizontal plane (s,) result from the Poisson's effect and any simulated tectonic stress applied to the model. Vertical stresses (s,) are created by the weight of the ridge under the modeled gravitational acceleration. Shear stress (s,,) occurs tangential to the plane being considered. Displacement patterns for each ridge are also reported.

2.5.1 Model validation

Preliminary models were undertaken assuming an elastic constitutive criterion. All of the models of elastic materials illustrated distributions of stress closely matching those results produced by previously mentioned analytical solutions. Compressive stress dominated the ridge- tops and slopes. Some minor tensile stresses were observed in the parts of the models that coincide with valleys. Concentrations of stress near the inflection points and toes of slopes were observed (Fig. 9). These concentrations of compressive stresses can also be observed in Pan et al., (1 995).

The addition of a constant tectonic stress to the elastic models, following the methodology of Savage and Swolfs (1 986), produced little change in the overall pattern of stress distribution. Increases in the values of compressive stress in the model were observed. These results also agree closely with published analytical solutions for elastic materials. Results confirm that FLAC is an appropriate tool for modelling stress in these ridges. S, S, contours (MPa) for elastic model (K = 0.3)

Figure 9: Contours of s, (horizontal stress) for an elastic model of a hogback ridge. These results closely match those from previously published analytical solutions.

2.5.2 Stress distributions with elastoplastic materials

Further modelling, undertaken to illustrate the changes in stresses for each ridge form, assumed elastoplastic material properties. Simple gravity loading and combined gravity and tectonic loading were considered.

2.5.3 Gravity loading (K = 0.3)

Hogback: Under gravity loading only, stress patterns change little as the material properties are reduced. Initially, s,, concentrations are located at the ridge-top and toward the crest of the main slope (Fig. 10). Some spatially localized -s,, (tensile) stresses are observed in the valley bottoms. Tensile stresses are observed in the s, distributions. Localized tensile stresses in the vertical plane develop in the ridge-tops near the ground surface (Fig. 1 I). Otherwise, the s, stresses are compressive and follow the general topography of the ridge. On the steeper secondary slope of the ridge, a minors,, concentration is observed. At high GSI values (70-90), only minor displacements in the ridge occur. At low GSI values (< 30), more significant displacements (1x10-' m) occur, with maximums at the ridgetop. Under gravity loading only, all significant displacements occur as compression of the ridge. Dogtooth: Very low (approaching zero) s, arid s,, stresses are produced in the peak at all GSI values. No tensile stresses are observed for the GSI values modeled. Concentrations of shear stress (s,,,) occur in the steep main slope of the ridge (Fig. 12). This area is coincident with zones of rapid failure initiation observed in the ridges used to develop the dogtooth ridge form. At very low GSI (-30), a horizontal shear zone develops at the base of the main slope of the ridge. Displacement patterns are similar to the hogback ridge under gravity loading. The shear zone developed, at the base of the lnain slope at low GSI values, is reflected by additional displacements at the slope toe (Fig. 13).

Castellate: At high GSI values, s,, stresses approach zero near the ridgetop (Fig. 14). A r on cent ration of compressive s, stress is located at the inflection point of the main slope. As the GSI values are reduced, an s, concentration develops at the crest of the main slope and s,, values approach zero at the &ansition point from the ridge-top to the shallow dipping secondary ridge slope. The s,, patterns are consistent over the various assumptions of material propmies with stress contours following the topography (Fig. 14). No tensile stresses develop at the ridge-top. The displacenlenrs of the ridge material are similar to the hogback and dogtooth ridges. At very low GSI values (- 30), some additional movement out of the main slope is observed.

Figure 10: Contours of s, and sx, for gravity loading (K 0.3) and high GSI (90) for a hogback ridge. Figure 11: Locations of tensile stresses from FLAC modelling for a hogback ridge.

Figure 12: Shear stress (sxy) concentrations from FLAC modelling for a dogtooth ridge (GSI = 90). The shallow stress concentration on the main slope is coincident with major failures in ridges used to derive the dogtooth ridge form in this study. Figure 13: Increase in magnitude and change of location of displacements in slope toe of dogtooth ridge with a reduction of GSI from 90 to 30.

Figure 14: Horizontal (s,,) and vertical (s,) stress contours from FLAC modelling for castellate ridges. GSI = 70 and K = 0.3

2.5.4 Tectonic and gravity loading (K=0.5, 1, and 2)

Hogback: At high GSI (70-90) values and low K (K = 0.5) values, s, concentrations at the ridge-top and crest are observed. As the tectonic stresses are increased, s, stresses at the ridge-top increase (Fig. 15). When the GSI values are significantly lower, low stresses dominate the ridge-top at all values of tectonic stress. At high values of tectonic stress, zones of failure begin to develop at the toe of the main slope. The s, stress distributions show tensile stress at the ridge-top at low tectonic stress values (K= 0.5) and high GSI (70-90). As the tectonic stress increases, the concentrations of tensile stress remain the same or increase slightly. No tensile stresses develop at any value of tectonic stress at low GSI values (30).

No significant concentrations of shear stress occur at low tectonic stresses and high GSI (70-90) values. As the tectonic stress increases, concentrations of shear stress develop at the toes of the main and secondary slopes (Fig. 16). Lower GSI values result in a similar distribution of shear stress. The depth of the shear stress concentration below the main slope toe increases (Fig. 16) at higher tectonic stress (K=2).

For materials with high GSI values (90), subdued rebound displacements are observed for all values of tectonic stresses with displacement values remaining constant over the range of tectonic stress. Reduced GSI values result in an increase in vertical rebound with the increase in

horizontal stress. At the highest horizontal stresses (K = 2), the maximum displacements are located in the valleys and at the main slope toe.

Dogtooth: Dogtooth ridge forms showed little difference between s, distributions in high and low GSI rock masses. Very low stresses (< 5 MPa) are observed in the ridge peak, and approach zero toward the ground surface across the range of modelled K values. Vertical stress is

similarly very low (approaching zero) in the ridge peak over the range of GSI and K values (K =

0.5, l,2) modelled. At low GSI (30-50) and high Kvalues (K = 2), distortions in the stress contours develop in response to shearing and failure. Assuming high GSI values, the main distribution of shear stress moves from the upper slope to the toe of the main slope with an increase in K. With low GSI rock masses and a high K, the main shear stress concentration occurs deep below the growid surface of the main slope. For the high GSI (70-90) rock masses, displacements are dominated by rebound at all K values. The models with low GSI values show only minor initial displacements toward the main valley. An increase in the K value (from 0.5 to 1 to 2) results in additional displacement of the ridge towards the main valley at the toe of the main slope (Fig. 17). Significant rebound (on the order of meters) is simulated in the valley bottoms.

Castellate: At high GSI (70-90) values of the s, stresses predicted are low at the ridge- top, although they do not become tensile. An increase in the tectonic stress (K > 0.5) results in an increase in the compressive stress at the ridge-top. A reduced GSI at low K values results in s, concentrations at the toe and inflection point of the main slope. As K increases (from 0.5 to 1 to 2) stresses in the main slope crest increase and the main slope begins to indicate the potential for yielding (Fig. 18).

The top of the ridge shows isolated areas of tensile vertical stresses for the high GSI material. An increase in the K value results in a reduction in the tensile stress and compressive concentrations developing in the toe and inflection point of the main slope. Models assuming a low GSI (- 30) rock mass indicate no tensile zones in the ridge-top; an increase in the value of K results in a very limited change in simulated stress distribution.

In high GSI (70-90) rock masses, concentrations of shear stress occur at the inflection point of the main slope for all values of K. Low GSI materials and low values of K produce concentrations of shear stress at the inflection point and toe of the main slope. As the K value increases, the concentration of shear stress at the toe of the slope increases in magnitude and depth.

Modelled displacements in the high GSI material show a constant minor rebound over the range of assumed values for K. For low GSI materials very minor rebounds are associated with low values of K. The maximum displacements occur at the toe of the main slope as material moves toward the valley. Increases in the values of K result in significant increases in rebound of the material, especially in the valleys. The toe of the main slope moves significant amounts in response to the yielding of the main slope.

Figure 15: Contours of s, and s, for hogback ridge with tectonic loading (GSI = 50, K = 2). Figure 16: Contours of s, for hogback ridge under tectonic loading (K = 0.5 and K = 2, GSI = 70).

Figure 17: Additional displacements in slope toe of dogtooth ridge at high tectonic stress (K = 0.5 and 2, GSI = 70).

Figure 18: Contours of s, for castellate ridge at K = 0.5 and K = 2 (GSI = 50) from FtAC modelling. 2.6 Discussion

The choice of symmetrical ridge forms in some previous studies has not been representative of mountains affected by glacial erosion processes. A simple survey of a DEM from an area with a glacial history shows that few ridges are symmetric. Instead, many are formed by the differential erosion of smaller alpine and cirque glaciers on one side of the ridge combined with the effects of larger valley glaciers on the other (Fig. 19). Slopes produced by these erosion processes can be very steep. Many slopes from study areas for this paper range from 28" to 38'. Kawabata et al., (2001) showed mean slopes for locations above the slope toe for glaciated terrain in the Japanese Alps to be in the range of 25" to 35'. The current author suggests that analyses in the literature do not always reflect realistic geometries of the ridges.

The use of an elastoplastic constitutive model is important in representing the global response of a natural rock mass to stresses. The example of the Kirsch solution (Goodman, 1989) from underground mining illustrates the effect of elastic versus elastoplastic materials on the distributions of stress. The solution for stresses around an underground opening (tunnel) in an elastic material can be simply determined from the analytical solution developed by Kirsch (Goodman, 1989). Damage and fracturing immediately around the tunnel, however, results in a distribution of stresses different than the ideal solution. Bray's solution (1967) for a tunnel in elastoplastic conditions illustrates how the generation of fractures around the tunnel results in a reduction in stresses in the fractured zone. Outside of this fractured and damaged zone the distributions of stress match the general pattern of stress predicted by the Kirsch solution. Although the previous example has different in situ conditions of stress than considered in this study, it illustrates that the response of an intact material to stress is significantly different that the response of a fractured material.

The accumulation of damage and fractures in natural rock masses exposed at the surface is caused by a combination of exogenous and endogenous forces (Gerber, 1980; Scheidegger, 1980). Exogenic forces refer to weathering and erosion processes. Endogenic forces refer to internal (tectonic) stresses. The accumulation of damage in the rock mass from the combination of these endogenic and exogenic processes alters the response of the final material (Zolotarev, 1973). Kohlbeck et al. (1979) suggested the importance of modelling mountains and valleys with an elastoplastic substrate. Considering the weathering and erosion processes acting on a rock mass, the zone of elastoplasticity should definitely encompass the near surface zones of the model.

The typical field locations of sackung are compared to concentrations of stress produced in this study in Fig. 20. The results indicate that different stress fields are responsible for the development of sackungen in each type of ridge. The simulated tensile stresses and very low compressive stresses observed in the tops of the hogback ridges would allow the activation and opening of existing structures in the rock mass.

The distributions of stress for dogtooth ridges do not produce regions of tensile stress in zones where sackung are typically found. A significant s, stress concentration on the main slope of the ridge may provide a clue to the mode of deformation responsible for the development of sackung. The ridges used to derive the dogtooth ridge form all have major historic failures that were initiated on the main slope. These failures coincide with the location of the concentrations of shear stress. The development of sackung may be linked to the general shallow instability in these zones of the dogtooth ridges. Links between the sackung and the catastrophic failure of some of the ridges used in this study have previously been suggested (von Sacken et al., 1992). The large rock slides and sackung may be caused by the same instability mechanism within the rock mass.

The stress distributions for the castellate ridge forms agree poorly with the known locations of sackung in the ridges used to develop the mean ridge form for this study. This may indicate that the formation of sackung in castellate ridge forms is independent of stresses in the ridge. In these slopes, activation of pre-existing discontinuities may be accomplished through tectonic or seismic activity. Further exploration of faults near the castellate ridges may be justified.

The conceptual separation of the process of gravitational deformation and the mode of deformation is important. Gravitational deformation indicates that strains in the rock mass are dominated by gravitation stresses, the interactions of the blocks within the mass can occur through various modes (toppling, sliding, slumping, etc.). The unique stress fields for each ridge type indicates that different modes may be more likely under one stress field than another. These modes may be simple sliding or toppling; however, more complex combinations are also possible. The mean stress distributions for the mean profiles of ridges should be reflective for many ridges in formerly glaciated regions. In addition, sackungen are known to form in many bedrock types. Sackung, however, are not found on every ridge. The controlling factor in the formation of these features is a combination of the discontinuities in the rock mass and the stress conditions. The importance of existing discontinuities in the rock mass for the formation of sackungen has been suggested by previous authors (Bovis and Evans, 1996). Many of these suggestions are based on the coincidence of discontinuity orientations with the orientation of the sackung. Possible simple modes of failure (toppling, sliding) based on kinematic analysis of existing discontinuities have been used in attempts to explain some sackungen in British Columbia (Bovis and Evans, 1996). To better understand the development of sackung, further exploration into the interactions of blocks within natural rock slopes is required.

Ridges in @ladatedTernin I - I

Figure 19: The formation of an asymmetric ridge occurs from differential erosion by a tributary I cirque glacier and a main valley glacier. This topography is typical of glacially modified terrain in the Cordillera of North America. ,s, ,s, tensrte stress zone and 0 s,, stressme 10 v

0 5 L 2 0 43 Features

Figure 20: Cartoon showing the comparison of stress distribution (under gravity and tectonic stress, K > 0.3) in normalized ridge profiles with sackung overlaid for reference.

2.7 Conclusions

In formerly glaciated regions of British Columbia and Colorado, asymmetric ridge profiles are common. From the limited sample used in this study, three ridge types were identified based on overall ridge morphology. Each of these ridge types will undoubtedly be characterised by different distributions of horizontal, vertical and shear stress. The mode of deformation in the rock mass, resulting in sackung, will be different due to these varied stress fields.

Sackung are products of gravitation deformation of the rock mass of a mountain slope. This deformation is the result of the interactions of the stress field and the rock mass. To understand the mode and location of formation of sackung, details of the distribution of stress for a ridge must be known. To ensure that the stress analysis produces representative stress fields, appropriate rock mass properties and ridge geometries are required. The rock mass properties chosen must represent the fractured, weathered and inhomogeneous nature of a natural rock mass; elastoplastic properties are required. The generation of rock mass properties for numerical modeling is an inherently difficult procedure. The author demonstrates the significant potential offered by a combined use of the Geological Strength Index (GSI) and the Hoek-Brown criterion in estimating the rock mass strength properties necessary to characterize rock slope geomorphic processes using numerical techniques. By adopting rigorous engineering geological mapping techniques, the confidence in the data used for a geomorphic study can be significantly increased.

To ensure that numerical rock slope stress analyses produce representative stress fields, appropriate rock mass properties and ridge geometries are required. Previous studies have frequently assumed either idealised elastic properties andlor conceptualised slope geometries in the derivation of rock slope stress distributions. Elastic properties, however, fail to realistically represent the fractured, weathered and inhomogeneous deforming nature of a natural rock slope. In this paper the authors adopt an elasto-plastic constitutive criterion. Results of elasto-plastic finite difference models of the three identified ridge types (assuming varied in situ stress ratios) provide hrther insight into the operative slope deformation mechanisms and their relation to natural topographical stress distributions.

This chapter has demonstrated the potential importance of topography induced stresses on high mountain slope deformation in glaciated regions of British Columbia and Colorado. A recognized hrther control on specific modes of deformation in individual rock slopes are the existing rock mass discontinuities (e.g. joints) and the kinematics of block interactions within deforming rock masses, with respect to the simulated in situ stress conditions. Future chapters examining sackung development will focus on these processes incorporating discontinuum rock mechanics concepts constrained by geomorphic observations. Investigations emphasizing the interaction of geomorphic processes and rock mechanics principles are a hndamental prerequisite for hrthering the understanding of complex natural rock slope deformations. CHAPTER 3 TOOLS FOR LANDSLIDE ANALYSIS

3.1 Introduction

Integrating data management and manipulation techniques provided by modem geographical information systems (GIs) with standardized engineering geological site investigation techniques results in a powerful research methodology. GIs can serve as the hub of a suite of integrated mapping and analysis tools. The spatially enabled relational database, at the core of the GIs, provides data storage and management capabilities. Digital data collection allows rapid updating of the GIs as well as increased quality of collected data. Rapid extraction of data from the database for engineering analysis is easily accomplished by the querying and manipulation tools available to the desktop GIs user. Finally, the increasing computational speed of personal computers has allowed the development of powerful 2D and 3D visualization tools for spatial data. Data limited problems benefit from this integrated approach as all of the available data can be leveraged to their full potential.

Detailed reviews of the history and development of GIs technology are available in the literature (Mark et al., 2004). The key components of a GIs are:

A spatially enabled relational database.

A two-dimensional map display, through which map objects 1 data layers are connected to the relational database. Both vector and raster data formats should be supported.

Not a required component, but available in most currently available GIs software packages:

A module for three-dimensional visualization of supported data sets.

3.1.1 Previous GIs applications in engineering geology 1 geotechnics

Previous uses of GIs for geotechnical purposes have focused on the regional or multi-site scale. Giardino et al. (2004) describe GIs techniques useful in the analysis of large landslides in the Italian Alps through a combination of mapping, data management, and visualization. Dermentzopoulos and Katsaridis (1 997) describe data management and GIs for urban planning projects in Greece. A GIs developed to assist the construction of MRT lines in Singapore is reviewed by Kimmance et al. (1 999). GIs as a data source for slope stability analysis, through limit equilibrium methods, in forested terrain of the Pacific Northwest has been demonstrated by Miller (1 995). Integrating geotechnical mapping and analysis with digital field data collection and GIs applications in general has been reviewed by Rengers et al. (2002). Aste and Badji (1 996) describe a tightly integrated system of software programs for slope engineering analysis. Their Slope Engineering Toolbox (SET) combines an input module (for new and existing data), an enhancement module (for handling and modelling), and a communications module for cartographic expression and 3D visualization. Currently, this tightly integrated set of tools allows for the analysis of block stability, rock fall and circular failure analysis based on digital elevation data. Loosely integrated approaches have also been suggested (Giardino et al., 2004); however, these systems do not rely on existing engineering geological terminology or established classification systems.

More advanced GIs applications including spatial modeling and analysis techniques have been presented by several authors. Aldridge (1 999) combined fuzzy set methods with thematic map data in an attempt to develop a model describing the spatial occurrence of landslides. Dai and Lee (2001) pursued a similar goal through multi correspondence analysis (MCA), a common spatial analysis technique. By overlaying thematic data layers and comparing the combinations of data to known landslide locations, inferences about future landslide locations are made. Rollerson (2003) used similar multi-correspondence analysis combining data from terrain attribute studies and known landslide locations within the GIs to produce "semi-quantative" hazard maps. Remote instrument monitoring has been combined with software agents through GIs to develop decision support systems for reservoir slope stability by Hutchinson et al. (2004).

Slope stability analysis methods have been previously integrated with GIs. Pack et al. (1 998) combined the infinite slope concept with a simple hydrological model to predict unstable terrain on a digital elevation model. The resulting SINMAP (Stability Index Mapping) module is available for ArcView GIs (ESRI, 2000). Tetsuro et al. (2001) combined custom Microsoft Visual Basic programs with the GIs to analyse slopes for 3D circular failures. Guzzetti et al. (2002), as part of the European Damocles project, leveraged GIs techniques and custom code to produce 3D rock fall simulations on a regional basis. Gunther et al. (2002) demonstrate how slope geometry combined with regional geological structure information can be combined through kinematic analysis concepts to predict slope instabilities according to specific failure modes. 3.1.2 GIs applications for site investigation

GIs provides a number of useful tools for engineering geology and geotechnics research at the field site scale, specifically for slope stability analysis. Building from the suggested integration of slope instability analysis techniques presented by Eberhardt et al. (2002), suggestions of how the GIs can be applied at each stage are provided in Table 3.

Table 3: GIS applications for investigation methods suggested by Eberhardt et al., 2002. Investigation Parameters investigated GIS applications method View existing digital maps, DEM data with draped Previous investigations, literature digital photography. Enhanced initial Desk Study review, available data. interpretation through the use of hillshade images and other GIs visualization techniques.

Field mapping, scanline surveys, Data collection with a field computer can reduce Site investigation observations of instability, data errors and missed observations. Allows hydrogeologic observations. rapid data updates to main database.

Determination of rock mass Samples are indexed by their location of Laboratory strength and material behaviour collection. Data is included in the main database. testing including discontinuity shear Spatial distribution of lab properties assists in the strength evaluation. classification of rock mass domains.

Discontinuity measurements, material properties, Kinematic feasibility, deterministic and slope topography is extracted from the GIs. Conventional limit equilibrium (i.e. Factor of Thematic map layers representing index values stability analysis Safety), probabilistic sensitivity (GSI, RMR, Q) can be derived from other data analysis. layers.

Simulation of slope deformation Rock properties and slope geometry are exported Numerical and stability, analysis of from the GIs. modeling progressive failure and shear Numerical model output can be imported to the surface development. GIs for additional processing or visualization.

Instruments are linked to the database allowing Monitoring of 3-D deformations, Field monitoring remote monitoring and updates of the office ground water and microseismicity. database. 3.2 Tool integration: The landslide research toolbox

The goals of the "Landslide Research Toolbox" (LRT) developed in this study are the collection, management, and analysis of data at the local site scale. These goals are accomplished by the integration of digital data collection / mapping systems, GIs, and geomechanical modelling / analysis packages. Currently, the components of the LRT are at the "first level integration" ("separate but equal") described by Ehlers et al. (1989). Each component functions separately, but data is easily moved from one application to another. This loose integration provides a flexibility that allows the user to use their preferred software packages for each component of the LRT. As well, flexibility in hardware choices can be realized.

The following "first level integration avenues", in the style of Ehlers et al. (1 989), are available for GIs and geomechanics modeling through the LRT:

The ability to move data based on simple queries from the GIs to the geomechanics model. Data should include topography (cross sections and DEMs), material properties and discontinuity measurements. Data queries should be defined visually by selecting stations / locations or programmatically using simple natural language.

The ability to import the outputs of the geomechanics modeling into the GIs for additional visualization or processing. Model outputs should maintain a spatial registration to increase the ease of reintegration with the GIs.

Within the "Landslide Research Toolbox" framework, GIs is a central hub for the basic research work flow (Fig. 21). The initial reconnaissance of the study area is completed with available maps and aerial photographs. These data may require conversion to a digital format to be imported into the GIs. Digital elevation maps and 3D visualization may also be useful at this stage. Field mapping and site investigation utilize the digital data collection system to a significant degree. Once all of the available data has been aggregated in the GIs, conceptual models of failure modes or processes can be constructed with the aid of data visualization routines from the GIs. The conceptual models are tested with geomechanical modelling software using topography and rock properties representative of the field data extracted from the GIs. These tests may lead to improvements or modifications of the conceptual model; they may also lead to information on where to focus additional field work. Figure 21: Work flow (outer ring) and data flow (inner ring) of the "landslide research toolbox."

3.2.7 Digital data collection and storage

Following the examples of the Geological Survey of Canada (GSC) and the Yukon Geological Survey (YGS) (Lipovsky et al., 2003) a digital field data collection system has been created by combining consumer level handheld computers with mobile database software. Hardware requirements for the data collection system are minimal (replaceable battery, removable media support, and moderate screen resolution). Currently available, inexpensive ($100 - $200) handheld computers are more than adequate. For the present study, the Sony Clie PEG-SLlO/U (grey scale screen, 33MHz processor, 8mb RAM, and Memory stick support) and the Sony Clie PEG-T665C (colour screen, 66MHz processor, 16mb RAM, and Memory stick support) were used successfully as data collection tools. Several mobile database software packages exist each with its own positive and negative features. The ability to "synch" with a desktop or GIs database is the main requirement for choosing a mobile database product. In addition, each data base product allows various amounts of control over data input interface design. Pendragon Forms (Pendragon Software Corporation, 2003) was used in this study, as well as previous work (Lipovsky et al., 2003).

Previous attempts at data collection with field computers have yielded mixed results. Struik et al. (1991) describe several issues encountered testing a technologically limited data collection system. Concerns with the system included:

0 Speed: data entry and verification was slow at the outcrop and in the field camp.

Geologically degrading: field researchers are forced to fit all of their observations within a limited and highly structured frame work.

Intimidating: complex codes and new computer skills are required for to use the system. This results in a steep learning curve which makes the system difficult for new users.

Some of these concerns have been addressed by updated technology and careful data model design. Other concerns, however, are subjective and cannot be addressed by any technological or system design improvements. Applications of this type of data collection system are most appropriate where the data required are specific and objective.

A custom data model for geotechnical and terrain data is required for the basis of the

mobile database and the GIs database. The SFU-- Geotech data model (Fig. 22) combines elements of the B.C. Terrain Classification system (Howes and Kenk, 1997) with standard geotechnical site investigation observations (discontinuity survey, rock mass classification, field photos, testing results). Designed as a series of relational tables, the data model can be implemented in any common relational database. The general structure of the data model breaks up the constituent classification systems (B.C. Terrain Classification, NGI Q, GSI, RMR) into

, data tables based on subjects under observation. The data required by each classification system can be simply extracted from the database using standard queries. The database on the handheld computer is mirrored by the database on the desktop PC. Each table contains a "primary key", or one piece of data that serves as a unique identifier for each feature in the data set. Links (or relations) between the tables occur through a hierarchy of association. For example, all Schmidt hammer test results are associated with a specific rock mass (geotechnical unit). Each rock mass observation is associated with a station, which in turn is associated with a traverse. Finally, each traverse is associated with a particular project. "One to many" relationships are used throughout the data model (e.g. one station for many rock mass observations, one rock mass observation for many Schmidt hammer tests, etc.). The relatively small community of engineering geologists and applied geoscientists is a benefit when designing portable data models. Semantic interoperability of datasets should be possible within the geospatial information community of engineering geologists and applied geoscientists. Geospatial Information Communities (GICs) are groups who share similar data models, feature definitions, and data applications (Pundt, 2002). To benefit from this feature of the community, data model designers should focus on using existing data definitions, classifications, and standards within the community instead of defining new concepts for their database projects. Database / data model designers must work closely with domain experts (engineering geologists, geotechnical engineers, and applied geoscientists).

To ensure portability among numerous researchers, care has been taken to base the data and data definitions on established engineering geological concepts. The use of standard rock mass classification systems (RMR, Q, GSI) provide the main source of data included in the SFUGeotech data model. These data have been supplemented by data required for structural geology, joint surveys, and other common field tasks. Engineering geological conventions for describing data (e.g. joint spacing, weathering grade) have been adopted from established sources (Anon, 1977; Anon, 1980). The design of the data model is facilitated by the previous standardization of engineering geological observations. Figure 22: Layout of the SFU-Geotech data model. PK = Primary key for data table. FKI and FK2 = Feature links between tables. 3.2.2 Data manipulation and visualization

The most powefil and overlooked feature of the GIs is the ability to create new data by combining and manipulating existing data sets (Rengers et al., 2002). In a field of study where data limited problems are common, this ability alone makes GIs a useful tool.

Another useful feature of the GIs is the ability to combine data of various scales for display and analysis (Halounova, 2002). In addition, outputs (e.g. maps) can be printed on demand based on the most up-to-date data for the various scales (Fig. 23). Additional visual outputs (based on Digital Elevation Models) useful for site scale investigations are available including:

Hill shade images

Slope maps

Photographic overlays

Hillshade images (Figs. 24 and 25) are derived from digital elevation (DEMs) data by calculating the shadow intensity expected for each DEM grid cell based on the sun position. The highlights and shadows produced increase the visibility of terrain features in the study area. Slope maps are derived by an algorithm which compares elevation differences across cells in the DEM. The slope map provides a colour coded output for rapid illustration of slope changes over the site. A GIs with digital elevation models or digital terrain models (DEMs or DTMs) can be used to determine slope angle, slope shape, slope azimuth, and slope height. These geometric characteristics are most efficiently determined through the GIs (Halounova, 2002). Also, "before" and "after" digital elevation surfaces for landslides can be subtracted from each other to rapidly calculate the volumes of material lost and gained on different parts of the study area. Vector data can also be manipulated efficiently in the GIs. The trends and lengths of linear features can be rapidly calculated using freely available scripts (Tchoukanski, 2004). The same tools can also be used to calculate various geometrical features of polygons (area, perimeter, centroid, etc.).

Rectified scanned aerial photographs or satellite images can be "draped" over the DEM to produce photo realistic terrain models (Fig. 26). Viewing angles not possible from a helicopter can be used to review the site and may lead to significant improvements in the interpretation of landforms. The importance of data visualization techniques cannot be over emphasized. Hoek (1999), in a discussion of 3-D models of geology, notes that the "advantages of these three- dimensional computer generated models are enormous. The model can be rotated or viewed from any direction, enlarged, sectioned and components can be removed or added at will." These same advantages are available through the GIs when reviewing 3-D geological and terrain models.

The desktop database provides data storage and manipulation to the user or group. Rock mass classifications can be completed and displayed rapidly using SFU-Geotech. Once the field observations of discontinuities (spacing, roughness, weathering, etc.) and bedrock (UCS estimate, weathering, structure, etc.) have been made, simple queries can be used to extract the appropriate observations for the Q or RMR classifications from the database. Automated calculations for the final index value of each classification method are completed for every station. As each station in SFU-Geotech has a spatial location attached to it (typically UTM from GPS) the classification results can be displayed graphically in the GIs. If sampling density is high enough, gridded / contoured representations of field data values are possible. Figure 23: An example of a map rapidly produced using GIs and several data inputs.

Figure 24: Perspective view of TRIM II DEM of Garibaldi Peak and Brohm Ridge (British Columbia) with simulated shadows and a colour ramp representing elevation. Figure 25: Hill shade image of same area in Figure 24.

Figure 26: Scanned and rectified aerial photo of Highway 99 and part of Brohm Ridge "draped" over a DEM. 3.2.3 Modelling and analysis

The diverse problems encountered in researching large landslides (difficult terrain, lack of sub-surface data) require a range of tools. Any geotechnical analysis or modelling package can be included in the LRT due to the nature of the component integration. The researcher is free to use their engineering judgement in regards to the best analysis tool for any problem. Cross sections, discontinuity data and rock mass properties can be rapidly extracted from the GIs using visual tools for specific areas of interest on the slope (Fig. 27). In this study, cross sections are created from DEM data in ArcGIS using EZ Profiler (Huang, 2003).

The flow of information is bidirectional; outputs from the models can be imported into the GIs for further analysis. As an example, void space development in a UDEC (Universal Distinct Element Code) model can be quantified using the GIs. Outputs of model cross sections in DXF format from UDEC can be imported directly into the GIs (Fig. 28). The vertical cross section is then treated as a map and void spaces can be delineated using polygons. The location and area of the polygons (voids) can be rapidly determined. The potential for a relationship of void area 1 failure plane location can be examined. The integrated nature of the LRT provides the possibilities for using GIs in a non-standard way.

Rock Properties

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Within the "Landslide Research Toolbox" framework, GIs is a central hub for the basic research work flow. The combination of digital data collection, GIs, and geomechanics modelling provides a system that is greater than the sum of its parts. By using digital data collection systems data errors can be reduced, data transfer from field notebook to digital format is avoided, and database maintenance is simplified. Error checking of data inputs is possible through data input masks. GIs allows new and existing data to be leveraged to its maximum potential. Rapid production of maps is possible, even in a field camp setting, through the LRT. Rapid calculations and powerfd visualizations can be realized using tools available in the GIs. Geomechanical models benefit from the data available from the GIs. As well, results from the geomechanical models can be further analysed in the GIs. An integrated set of tools such as the "Landslide Research Toolbox" is a prerequisite when examining complex, natural slope instabilities. CHAPTER 4 A FIELD INVESTIGATION OF SACKUNG AT MOUNT MERCER, BRITISH COLUMBIA

Over the summer of 2003, geomorphic and geotechnical mapping of previously identified sackung features and bedrock failures (Savigny 1990b; Thomson, 1998) at Mount Mercer, British Columbia was undertaken. A combination of aerial photographic interpretation, GIs techniques, stereographic kinematic analysis and field mapping was used to determine the extent, possible mechanisms, and morphology of the sackung features and related landsliding.

4. I Study location

Mount Mercer (with Mount Thurston) forms a generally east-west trending ridge near the center of the Chilliwack River Valley. The Chilliwack River valley is located in southwest British Columbia approximately 90km east of Vancouver, British Columbia. The drainage boundary of the east-west trending valley straddles the Canada 1 U.S.A border (Fig. 29). Situated within the north-western flank of the Cascade Mountains, the main valley is completely inside Canada; however, the Chilliwack River's headwaters are in the U.S.

Figure 29: Mount Mercer is located in the Chilliwack River Valley, south western British Columbia 4.2 Local and regional physiography

The summit of Mount Mercer is approximately 1680 meters above sea level. Total relief above the floor of the Chilliwack Valley is approximately 1420 meters. Mount Mercer is defined by four cirques and the main Chilliwack River Valley. Three cirques form the northern side of the ridge and terminate in hanging valleys in the Chipmunk Creek valley. The fourth cirque is on the south side of the ridge and empties into the Chilliwack Valley. The major south aspect slope of Mount Mercer is part of the wall of the main valley. The Mount Mercer ridge is 4.5 to 5 kilometers long. A "bend" in the ridge near its center point results in a division of the main south slope into a south-west aspect section and a south-east aspect section (Fig. 30).

Elevations in the remaining parts of the valley range from 2473 meters in the area near the international border to 25 meters at the basin mouth (Thomson, 1998). Influenced by the tectonic history of the area and heavily modified by glacial processes, the physiography of the region is characterized by rugged peaks at high elevations and terraces composed of thick glacial drift at lower elevations. Cirques, hanging valleys, U-shaped valleys, and truncated spurs are common throughout the Chilliwack River basin.

Climate

The climatic regime of the Chilliwack Valley is typical of South West British Columbia. Two annual hydrograph peaks can be expected (Thomson, 1998). The first occurs during late spring1 early summer in response to snow melt. The second occurs during the late fall I early winter in response to heavy rains or rain on snow events. Snow can exist almost year round on the highest peaks in the basin. Even ridges at moderate elevations (1 500 meters - 1600 meters) can be expected to have some snow until late August. Rainfall normals for the Chilliwack River Hatchery, near the base of Mount Mercer, are given in Fig. 3 1. Figure 30: Hillshade image illustrating the general physiography of the Chilliwack River Valley. Mount Mercer is approximately centre in the image.

------Precipitat~onNormals (1971 - 2000) from the Chilliwack River Hatchery

Rainfall Snowfall

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec I - - ---

Figure 31: Precipitation normals (1971 - 2000) from the Chilliwack River Hatchery at the base of Mount Mercer (49' 4' N, 121' 42' W, Elev: 213 m) Data source: Environment Canada, 2004 4.4 Bedrock geology

The Cascade Mountains are composed of a north-north west trending granitic and gneissic core flanked by belts of volcanic and sedimentary rock. Regional low-grade metamorphism has locally altered some of the flanking rocks (Monger, 1967). The extreme eastern end of the Chilliwack Valley is dominated by Cenozoic igneous intrusions. The central and western parts of the valley are made up of sedimentary and volcanic rocks ranging fiom the Devonian to the Lower Cretaceous (Massey et al., 2003).

The sedimentary and volcanic rocks in the Chilliwack Valley have undergone two phases of tectonic folding and faulting (Monger, 1967). The initial folding was to the north-west and resulted in recumbent isoclinal folds with hinge surfaces trending north-west. Accompanying the folds are low angle thrust faults, also trending to the north-west (Monger, 1967; Thomson, 1998). The second phase of folding and faulting caused the existing fold axes and beddings to plungeldip toward the north-east. The predominant geological units in the valley are the Chilliwack Group (Upper Carboniferous to Early Triassic) and the Cultus Formation (Upper Triassic to Jurassic).

4.4.1 The Chilliwack Group

The Chilliwack Group has been subdivided into seven main sequences 1 sections (Monger, 1967). The observed base of the group is dominated by amphibolites of an uncertain age. Above these rocks are undifferentiated quartz rich argillites, also of unknown age. Pennsylvanian in age (Monger, 1967), the lower clastic sequence of the Chilliwack group is approximately 750 m thick and composed of fine grained volcanic arenites and argillites. An argillaceous limestone (Red Mountain Limestone, approximately 60rn thick) lies between the lower clastic sequence and the upper clastic sequence. The upper clastic sequence (Upper Pennsylvanian or Lower Permian) is 135 m - 240 m thick and composed of coarse to medium grained volcanic arenites and argillites with some local conglomerates and tuffs. Overlying the upper clastic sequence is a typically cherty (90 m thick) Permian aged limestone. The upper most sequence in the Chilliwack Group is made up of Permian volcanics. This volcanic sequence (200 m - 600 m thick) is characterized by three main units: altered basic to intermediate flow rocks (I), tuffs (2), cherts and argillites (3). 4.4.2 The Cultus Formation

Ranging in age from Upper Triassic to Upper Jurassic, the Cultus Formation is separated from the Chilliwack Group by a disconfomity. The unit is composed of fine and medium grained volcanic arenites, argillites and slates within the main Chilliwack Valley. The total apparent thickness of the Cultus formation is 1200m (Monger, 1967).

4.5 Surficial geology and geomorphology

The Chilliwack Valley has been subject to glacial ice from both valley and alpine sources. Two main sources of valley ice are responsible for much of the deposited material in the trunk valley: The Fraser lobe and the Chilliwack Valley glacier. The Fraser lobe, part of the Cordilleran Ice Sheet, was located at the western margin of the valley and at times completely blocked the mouth. Some evidence suggests that the Fraser lobe penetrated into the Chilliwack Valley (Saunders et al., 1987). The valley head (eastern end) was the source of the Chilliwack Valley glacier. This ice mass retreated from the valley mouth during the later half of the Late Wisconsinan. The retreating glacial ice deposited thick sandurs at various points in the valley.

Of significant interest is the consequence of the presence of these two ice masses in close spatial proximity. With the mouth of the valley blocked by Fraser Valley ice and the Chilliwack Valley glacier retreating up-valley, a large lake formed in the ice free section of the valley. From approximately 11,700 years B.P. (Saunders et al., 1987) the central portion of the valley was free of ice. Two periods of lake formation separated by a period of forested landscape apparently existed from 11,700 - 11,100 years B.P. (Saunders et al., 1987). After 11,100 years B.P., the main valley and mouth of the Chilliwack Valley were essentially ice free. The contributions of alpine glaciers in the drainage are unclear; however, it is assumed that effects would be locally

. confined.

Deposits in the main valley resulting from this complex glacial history are summarized in surficial mapping completed by Saunders (1985) (Fig. 32). The lower elevations of the valley are dominated by glacio-lacustrine and outwash deposits. Terraces at moderate elevations are composed of glacial outwash, outwash / lacustrine deposits / till, and landslide deposits. At higher elevations veneers of till are observed. Local alpine moraines and erratics can be found on ridge- tops. Current fluvial and colluvial processes in the valley bottom continue to rework the older glacial deposits. 4.6 Landsliding in the Chilliwack Valley

General surveys of landslides in and near the Chilliwack Valley have been published by Evans and Savigny (1 994) and Thomson (1 998). Mass movements in both the Quaternary deposits and the bedrock of the Chilliwack Valley are common (Fig. 32). A significant failure in glacio-lacustrine and outwash deposits (Slesse Park Slump) has been reviewed by Fletcher (et a]., 2002). Debris flow studies in the Chilliwack Valley and surrounding region have been undertaken by Jakob et al. (1997). Large bedrock mass movement events on the northern edge of the Chilliwack Valley drainage have been reviewed by Naumann (1 991) (Cheam Slide) and Stewart (1997) (Walheach Sackung). It is interesting to note that approximately 70% of the known bedrock landslides occur in either the Chilliwack Group or Cultus Formation. Of these, over half appear to be rock slumps or sackungen (occumng with tension cracks, anti-scarps, etc.). The Chilliwack Valley drainage provides landslide researchers with a wealth of potential study sites (Thomson, 1998). S a tB U,u-2 5s "IILI05g [1: .zv,5<,"oYFEw: - 22 20% 2 ,,, ,,, c .=3'-3a-ogg - "gi?(r++- 3 5 S3 Z~$'20%Z$dI ILI_I ycdIYO0> I IIIC.1. I 4.7 Geomorphic and engineering geological mapping and data collection

4.7.1 Ridge morphology

The basic morphology of Mount Mercer was established using approximately northwest - southeast cross sections though the digital elevation data for the ridge (Fig. 33). These cross sections are normal to the main set of sackung features observed. The basic ridge form is similar to others found in formerly glaciated regions. A steep (30' - 40') main slope forms part of the wall of the Chilliwack Valley. A definite peak exists at the ridge line. The back slope of the ridge is formed by a series of cirques. This ridge form is "dogtooth", loosely applying the terminology of Cruden and Hu (1999).

Chiirnunk Greek huntMiarcer Ridge tine Chilliwack River Valley I f I

Figure 33: Cross section through Mount Mercer.

4.7.2 Structural geology

Subsurface data is not available for Mount Mercer. However, a structural interpretation of the bedrock units of Mount Mercer is provided by Monger (1967). This interpretation provides some constraint on the geometrical relationships between the geological units of interest at Mount Mercer. From a cross section and description from the above thesis, major structural features at Mount Mercer can be considered in two classes: folds and faults. The general folding of rocks at Mount Mercer (Fig. 34) follow the regional trend as mapped by Monger (1 967). Recumbent, isoclinal folds with axial planes verging toward the north east are the regional structure. However, tight overturned folds are not uncommon in the rocks of Mount Mercer (Fig. 35). The result of these recumbent folds is the near horizontal bedding and foliation of many of the Mount Mercer rocks. In addition, the stratigraphic sequence of rocks in the upper ridge matches the sequence of original deposition. The secondary folding (resulting in a hinge line trending northwest) described by Monger (1 967) is not apparent from the current field work at Mount Mercer.

Two main sets of faults have been mapped at and around Mount Mercer (Monger, 1967). Thrust faulting after folding of the rocks in the Chilliwack Valley appeared to result in little noticeable displacement between the rock units in this study. These faults have been mapped at angles ranging from 5' - 35'. The fault planes dip toward south - southeast. At least two thrust fault planes have been proposed to cut Mount Mercer at depth.

In addition to thrust faults, one reverse fault has been mapped in the col between Mount Mercer and Mount Thurston (Monger, 1967). This high angle plane dips toward the northeast. Formed during the second stage of regional deformation, this fault has elevated the units of Mount Mercer above the corresponding units in Mount Thurston. The trace of this fault can be seen from aerial photograph coverage of the south aspect slope on the ridge to the north of the col. The previously mapped faults appear to be the minimum possible planes which intersect Mount Mercer. From the aerial photograph interpretation other possible fault traces are apparent. Mount Mercer Chipmunk Creek Ch~ll~wackR~ver I ,~. I 1 Zone of sackung +. / I

. . ---.. ---<------.--.-. .. -.. ----... . -... ___--. .-- -

I Cultus Fm. (Jurassic - Triassic) -I I I volcanic Sequence I L-- I (Permian ) ..-...... _.Disconformable Contact Limestone (Permian) Chilliwack Grp. Conformable Contact Upper Clastic ,.-.- Thrust Fault Sequence J

Figure 34: Generalized structural geology of Mount Mercer, adapted based on a basin wide cross section by Monger (1967). 111

Figure 35: Recumbent fold in Unit 1 observed in outcrop at the ridge line 4.7.3 Engineering geological mapping

Four geotechnical units were identified along the ridge and upper slope of Mount Mercer. The spatial distribution and apparent thicknesses of each unit was mapped. Geotechnical data including rock descriptions (rock type, texture, and colour), weathering state, basic structure, discontinuity orientations, discontinuity spacing, block size, block shape, and unconfined compressive strength estimates were collected. Indexes describing the expected geomechanical behaviour of the rock mass (GSI and Q) were used to summarize the field data.

4.7.3.1 Unit 1: Cultus Formation

The top unit stratigraphically in the study area, unit 1 is confined to the western part of the ridge. Fissures and scarps occur in Unit 1 along the ridgeline. Outcrops in undisturbed and mostly undisturbed parts of the ridge line were used for data collection. The thickness of Unit 1 is approximately 100 m, estimated from mapped outcrops.

The rock is mainly a bedded arenite composed of volcanic fragments (Fig. 36). The texture of the unit varies from fine (silt sized) to medium (coarse sand); typically each individual bed has a consistent grain size. The unit has slight (w 1) surface weathering which gives it an orange and tan colour. Fresh surfaces are green to grey. The rocks are heavily jointed with discontinuity spacing range from moderately narrow to wide (20 mm - 200 mm). Blocks are typically tabular or equant shaped with sizes ranging from 64 mm - 256 mm. Four distinct joint sets (one coincident with bedding) have been observed (Fig. 37). Planar (as opposed to undulating or stepped) surfaces are dominant. Joint surfaces are rough.

Intact UCS (unconfined compressive strength) estimated from point load testing lab samples is 100 MPa. This unit can be considered a strong rock (R3).Based on the weathering / alteration conditions and the extent ofjointing, a GSI (Geological Strength Index) estimate for this unit is in the range of 30 - 50. The Q value determined for this unit from the available field data is in the range of 1 - 10. Figure 36: Unit 1 (Cultus Fm.) in outcrop. Scale in photo is 1 meter squal Ecliral Angle Lnwer Hendsphrre i95 Poles 195 Errtries

F15her Concenlraf~ons % of total per 1 0 % area

No Blas Correction Max. Con:. = 6.5276%

quai Angle Lmrnemisghere ID5 Poles 1'35 Ern

Figure 37: Discontinuity types, distribution and mean planes mapped in Unit 1. 4.7.3.2 Unit 2: Foliated basalt of the Chilliwack Group

Potentially 600 m thick, Unit 2 is a horizontally foliated basaltic rock. Mapped in the central and eastern part of the ridge, the unit dominates the ridge and upper slope area of Mount Mercer. The horizontally foliated rock is dark grey and red on surfaces with sight weathering (wl). Fresh surfaces are dark green to black (Fig. 38). Foliation spacing is not constant and decreases from close (6 mm - 20 mm) to very close (< 6 mm) toward contacts with other units. Quartz veining perpendicular to the foliation is common near the contacts between units 2 and 3.

Blocks defined by joints in observed outcrops range in size from 64 mm - 256 mm and are tabular. In addition to the foliation, 3 joint sets have been identified within Unit 2 (Fig. 39). Joint spacing ranges from moderately narrow to moderately wide. Most joint surfaces are either planar and rough or planar and smooth.

Estimates of UCS from point load testing gave a range of strengths from 130 - 180 MPa. The samples used for point load testing were observed to separate along the foliation during testing, irregardless of the testing orientation. A mean anisotropy index of 2.5 was calculated from all unit samples testing with the point load apparatus. GSI estimates are very low for this unit due to its foliated nature. A range from 25 - 45 may be appropriate. Q estimates for Unit 2 are on the order of 0.1 - 1 .O. Figure 38: Unit 2 (foliated basalt of the Chilliwack Grp.) in outcrop below Unit 3. Scale in photograph is 0.5 meters square. TYPE

Folialinn 1301 Joml (931 Others

Equal Angle Cower Hernwphere 152 Poles 152 Entries

Fisher Concenlrations %of tolal per 1.0 % area

No Bias Correction Man. Conc. = 9.8798%

Equal Angle Lower Hemisphere 16Poles 152 Enlries

Figure 39: Discontinuity types, distribution and mean planes mapped in Unit 2. 4.7.3.3 Unit 3: Tuff and agglomerate of the Chilliwack Group

A distinctive unit in the study area, Unit 3 is a massive volcanoclastic sedimentary rock within the upper part of the Chilliwack Group (Fig. 40). Unit 3 can be found in the same spatial distribution as Unit 2. In fact, Unit 3 appears as discrete, discontinuous beds within Unit 2. Typically 1 - 1.5 meters thick in outcrop this unit is resistant to erosion, as illustrated by its prominence in outcrop.

Composed of ash and volcanic fragments, grain size varies from coarse (2 mm - 60 mm) to fine (0.002 mm - 0.06 mm). Most surfaces are slightly weathered (wl) and are grey, fresh surfaces are green. Some fractures in the unit are quartz filled near the contacts of the units above and below (Unit 2). Four regular discontinuity sets occur in this unit (Fig. 41). Most discontinuities are planar and rough, a few show slickensides.

UCS estimated from point load testing of lab samples is approximately 200 MPa. The GSI estimates for this unit range from 60 to 70. Q estimates are in the range of 1 - 10. Figure 40: Unit 3 in outcrop. Note slickensided surface dipping toward the SE. TYPE

Fault (21 Joint [451

[no data] [II

Equal Angle Lower Hemisphere 52 Pdes F;2 Entries

Fisher Concentraltons $6 of tolal per 1 0 YO arm

No Bias Correction Max. Conc. = 7.9J999"o

Equal Angle Lower Hemisphere 52 Poles 52 Entries

Figure 41: Discontinuity types, distribution and mean planes mapped in Unit 3. 4.7.3.4 Unit 4: Upper limestone of the Chilliwack Group

Unit 4 is located at mid slope on the eastern extent of the ridge. This unit is a massive limestone or cherty-limestone. The limestone weathers dark grey and chert nodules are sometimes prominent. From previous bedrock mapping (Monger, 1967), this unit lies stratigraphically below Unit 2. Only limited data was collected from this unit.

Three stepped and rough discontinuities sets were observed within one outcrop of Unit 4. The spacing of these discontinuities was typically wide (200 mm - 600 mm). UCS estimates for the limestone from point load testing give a range of 90 - 100 MPa. GSI estimates from field observation are from 50 - 70. Q is estimated to be in the range of 1 - 10.

Selected data for the mapped geotechnical units in summarized in Tables 4 and 5.

Table 4: Summary of selected data for mapped engineering geology units. Point Unit Geotechnical Composition Weathering Load UCS Weight GSI Q Unit /Alteration Estimate (KNlm3, (MPa) Slight I Fine sand very b~ocky 109 21.9 30-50 1-10 stone to shale (~1) Laminated I Slight 2 Basalt sheared (wl) 155 26.6 25-45 0.1-1 Ash and Slight 3 Volcanic Blocky 209 30.4 60-70 1-10 fragments (wl) Slight 4 Limestone Blocky 93.4 25.4 50-70 1-10 (wl) Table 5: Summary of selected discontinuities properties collected from mapped engineering geology units.

Geotechnical Joint Set Mean Spacing Curvature Roughness Comments Unit Weathering Orientation

wide Planar Smooth wl (slight) I1 to bedding (60 - 200mm) moderately wide Stepped wl (slight) I1 to strike of JS-1 (200 - 600mm) Rough - moderately wide Stepped wl (slight) (200 - 600mm) Rough -- 1- to JS2 moderately wide Planar wl (slight) (200 - 600mm) Rough moderately narrow to moderately wide Planar Smooth wl (slight) I1 to foliation (20 - 200mm) moderately narrow to I1 to strike of JS-1, Quart moderately wide Planar Rough wl (slight) - fili common (20 - 200mm) moderately narrow to moderately wide Stepped Rough wl (slight) (20 - 200mm) moderately wide Planar Smooth wl (slight) (200 - 600mm) wide Planar Rough wl (slight) I1 to Unit 2 foliation (60 - 200mm) - wide 2 Planar Rough wl (slight) I1 to strike of JS-1 (60 - 200mm) - wide 3 Planar Rough wl (slight) - to JS-2 (60 - 200mm) - 1- wide Rough1 4 Planar wl (slight) (60 - 200mm) Slickensided 4 ALL wide (60 - 200mm) Stepped Rough wl (slight) 4.7.4 Geomorphology mapping

From colour aerial stereo photographs of Mount Mercer a number of geomorphological features (scarps, tension cracks, slope benches, and ridge-top depressions) are immediately apparent. Smaller discrete landforms on the middle and lower slopes of Mount Mercer are obscured by trees; however, general slope trends can be seen using a combination of aerial photographs and digital elevation models.

4.7.4.1 Ridge-top trenches I depressions

Depressions parallel to the ridge line were mapped along the length of Mount Mercer. These depressions seem typical of both "ridge-top trenches7'and "double ridges" (or dopplegrate) identified in literature dealing with sackungen and massive scale slope movements (Bovis 1982; Jahn 1964; Radbruch-Hall 1976). The depressions are readily visible from recent aerial photographs (August 2002) due to snow protected f'rom melting by the depressions. The features range f'rom subtle, ambiguous flat bottom depressions (Fig. 42) to large, steep sided trenches (Fig. 43). Most of the trenches mapped have similar trends (Fig. 44).

Ridge-top depressions occurring away (mainly on the western end of the ridge) from other sackungen features are typically shallow, subtle landforms. These flat bottomed depressions do not form along the ridge centre line, but off towards one side. The sides of the depressions slope gently and the transition from depression side to bottom is very gradual. Depths of these shallow depressions is never greater that 1.5 meters; widths range fiom 4 - 8 meters. The total lengths of these shallow depressions range fiom 30 - 40 meters. From the aerial photographs the depressions appear linear, however in the field the features appear to curve slightly in plan.

More definite, greater relief depressions or trenches are found on the central and eastern parts of Mount Mercer. These trenches form near other sackung features discussed below. Depths for these trenches are up to 5 meters; widths range fiom 5 to 15 meters. The sides of these features are steep and well defined. In some cases, rock units outcrop in the sides of these features. Some of these trenches are clearly related to other features along the ridge. One large trench feature along the ridge line in the central portion of the ridge is formed below a head-scarp of an obvious failure (Fig. 45). Other ridge-top trenches form along the ridge line behind a number of normal scarps on the upper ridge slope of Mount Mercer. Figure 42: Subtle trench features on the western part of the Mount Mercer ridge. &pareill Slr %e ? rnax plar~er;: arc at outer c~rcle

Trend l Piunye ol Face Norrnal = 0.90 (d~rectedaway horn mwerj

G ,?lane- Plotied Wnh~!)45 and 90 Degrees of Umng Face

Figure 44: Centre line azimuths of all ridge-top trenches I depressions mapped at Mount Mercer. Figure 45: Trench feature occurring below normal scarp in the central part of the ridge. 4.7.4.2 Tension cracks 1 fissures

Tension cracks can be found in the rocks of the central and eastern parts of the Mount Mercer ridge. Always found near other alpine hears, the tension cracks 1 fissures vary in orientation and magnitude (length I width). Most fissures occur on the upper slopes of Mount Mercer oriented perpendicular to the fall line of the slope. These features occur in obviously disturbed I deformed rocks below normal scarps. Some of these features may be better grouped with anti-scarps than tension cracks. The depths of the fissures are generally less than a meter. Aperture of the fissures at Mount Mercer can reach two meters. In the central part of the ridge, fissures occur in groups and can reach 20 meters in length (Fig. 46).

Two tension cracks of interest occur in the central part of the ridge. This part of the ridge hosts a failure mass which may still be active. Aligned with the eastern side scarp of the feature is a large tension crack which cuts through the ridge-top perpendicular to the ridge line (Fig. 47). Approximately five meters long and three meters deep, this tension crack occurs in Unit 1 (Cultus Fm.). The orientation, size and situation of the feature make it unique among sackung features at Mount Mercer. A tension crack of similar size occurs roughly parallel to the slope at the apparent toe of the failure at the central part of the ridge. a"'

Figure 46: Fissures below normal scarp in central part of the ridge.

Figure 47: Tension crack occurring perpendicular to the ridge line in the central part of the ridge. The crack passes through the ridge-top. 4.7.4.3 Normal scarps

Normal (or "down-hill facing") scarps are found in both the central and eastern parts of the Mount Mercer ridge. Related to the failure in the central part of the ridge is a normal scarp (actually the back scarp of the failure) approximately 2 10 meters long (Fig. 48). The height of the scarp ranges from 2 - 3 meters. Units 1,2 and 3 are visible in different sections of the scarp.

The eastern part of the ridge contains four well defined normal scarps in close proximity (Fig. 49), as well as other more ambiguous features associated with severe slope changes which may be scarps. Starting from the ridge line, this zone of scarps extends 500 meters down slope. The spacing of the scarps ranges from 50 - 100 meters. Most of the scarps are 400 - 500 meters long; however, some can be traced for up to 1000 meters. Scarp height increases away from the ridge line. Scarp near the ridge line are approximately 1 - 2 meters in height. Scarps further down slope can be as high as 10 meters.

Many of the scarps are arcuate in plan. Some appear to define "bowl-like" concavities in the main eastern slope of Mount Mercer. The general appearance of these concavities is similar to failure scars left behind by rotational mass movements. The orientations of scarp segments are shown in Figure 50. Figure 48: Normal scarp in Unit 1 at central part of ridge.

Figure 49: Normal scarps below eastern part of the ridge. These scarps occur near the top of the south east aspect slope of Mount Mercer. Figure 50: Strikes of scarp segments on the south east aspect slope of Mount Mercer.

4.7.4.4Anti-scarps and stepped I benched topography

Anti-scarps (or "uphill facing scarps") as previously described in the published literature are rare at Mount Mercer. Those that do exist are more likely sections of other landforms (trenches). More prevalent on the upper slopes of Mount Mercer is stepped 1 benched topography (Fig. 5 1).

Starting within the zone of normal scarps and extending to mid-slope, flat benches occur over the south-east aspect slope of Mount Mercer. These benches occur laterally adjacent to some normal scarps and below others in the upper parts of the slope. In the mid slope of Mount Mercer, benches occur above sharp slope breaks. The benches are easily visible, both in the field and on slope maps generated from the DEM data (Fig. 52). All of the benches trend perpendicular to the fall line of the slope. The lateral extent of these benches ranges from 300 - 500 meters. The benches can be up to I00 meters wide, but 50 meters is most common. Legend

1-4 1-4 Foliated basalt (01) 1 I Unseen geology

16 - 42 Dip - Dip Direction of Foliations

L I 50 Metres

Figure 51: Benched topography and observed geology on the south-east aspect slope of Mount Mercer.

Figure 52: Slope map derived from DEM data for the south-east aspect slope of Mount Mercer. 4.7.5 Geomorphological domains

Mapping completed using aerial photographs, DEMs, and field checking identified a significant number of sackungen features. From the mapping (Fig. 53) it was clear that certain landforms could be grouped together as they were related by process as well as location. Three geomorphic domains were identified:

Ridge-top

Slab Failure

Scarp Complex

Processes responsible for landforms in each domain are treated in turn.

4.8 Geomorphic processes responsible for sackungen features at Mount Mercer

The sackungen type landforms mapped at Mount Mercer are a product of both endogenetic and exogenetic processes. Endogenetic processes (e.g. tectonics) are responsible for the geological structure of Mount Mercer; where the exogenetic processes (erosion and weathering) have taken advantage of these structures to produce the ridges, valleys and slope forms (Scheidegger, 1981). The general bedrock structure in the geomorphic domains of interest is known to a limited extent. However, the erosional or weathering processes responsible for the scarps, anti-scarps and trenches remain to be explored.

Ridge-top depressions on the western part of the Mount Mercer ridge are encapsulated in this geomorphic domain. As well, other trenches and depressions likely not associated with activity in the remaining two domains are considered to be part of the ridge-top domain. Possible mechanisms for ridge-top trenches associated with the "main scarp complex" and the "slab failure" will be discussed in the appropriate sections.

As previously mentioned, most of the trench 1 depression features occur parallel to the local ridge line of Mount Mercer. The azimuths of the centre lines (Fig. 54) of mapped trenches compared with high angle joint sets mapped on Mount Mercer show that most trenches and depressions are aligned with the strike of mapped discontinuities. However, the trenches do not necessarily match the most common discontinuities mapped.

The coincident orientation of the ridge-top trenches with structural features of the rock mass and the ridge line does not indicate the actual mode of formation. Based on the morphology of these features, two processes previously mentioned (Chapter 1) are plausible for creating these depressions: melt water erosion and gravitational deformation of the rock mass. Western ridge line - - = Figure 54: Trends of ridge-top trenches compared to the strikes of steeply dipping joints and ridge line orientations.

4.8.1.1 Melt water erosion

Reviewed in Chapter 1, the general morphology of the ice marginal melt water channel fits well with the subtle trench features observed on the western most part of the Mount Mercer ridge. The ridge parallel orientation of the subtle trenches would also fit with formation by ice marginal drainage. No depositional ice marginal features are observed along the ridge line of Mount Mercer. However, from the morphology of the ridge in general and the erratics found during mapping, glacial ice overtopped the current summit of Mount Mercer.

4.8.1.2 Gravitational deformation

Obvious deformation features are not visible down slope of the subtle ridge-top depressions. However, at least one of the depressions is situated directly above a vertical wall of rock (Fig. 55) which forms the back of a north facing cirque. Examination of the rock outcropping at the cirque headwall reveals extensive vertical and sub-vertical joints which create tall slabs of rock which tend to topple from the rock face into the cirque creating aprons of talus. If these observed vertical and sub-vertical joints continue in the rock mass beneath the ridge-top depression toppling may produce the depressions observed at the surface. Goodman (1 989) notes the development of obsequent scarps at the top of a large topple in North Wales. These scarps form two sided depressions (one steep side, one gentle side) which occur with tension cracks. No tension cracks are found associated with the subtle depressions at Mount Mercer.

Without detailed subsurface data, these limited observations cannot provide a definite answer to the formation of these subtle ridge depressions. Based on the current rock mass data for this part of the ridge, either mechanism of formation is possible. The morphology of the features is more in agreement with formation by melt water processes. Alternatively, these features may be the result of modem nivation processes on the ridge-top. Figure 55: Unit 1 outcropping in a vertical wall at the head of a north facing cirque. 4.8.2 "Slab failure"

Starting from the ridge-top on the south-westem aspect slope is a moderately size (area = 53000 m2) bedrock failure. The general morphology of the failure is a well defined head-scarp, a bench and fissured mid-slope mass and a steep toe (Fig. 56). Unit 1 is the only geological unit visible at the failure.

The head-scarp of the failure is coincident with the ridge crest. Parts of the former ridge line have dropped below the current head-scarp due to the failure. Vertical displacements at the head-scarp range from 4 - 8 meters. The horizontal displacements at the surface near the head- scarp are on the order of 4 - 10 meters. From the shape of the head-scarp and displacements observed in the field, the failure does not seem to be moving in a direction perpendicular to the ridge line. Instead, the main mass is moving obliquely out of the slope (toward the south east).

The failure mass has several cracks and fissures on its surface and immediately behind parts of the head-scarp. These fissures trend parallel to the head-scarp and are of highest density near the western part of the failure. Likely related to the deformation of the "slab" during movement, the fissures found both behind the head-scarp and within the failure mass show minor or very minor infill of material. The lengths of the fissures are on the order of 1 - 10 meters, with apertures rarely greater than one meter.

The lateral scars of the failure mass are very well defined. The eastern limit of failure is marked by a steep, vegetated slope. The western limit of failure is marked by a tension crack which cuts perpendicular through the ridge. This fissure has been previously reviewed (see above: Tension Cracks /Fissures).

An additional tension crack occurs parallel to the ridge line, down slope of the fissured failure mass. Approximately 2 meters deep, 3 meters long and 1 meter wide this tension crack is laterally adjacent to a "berm" like feature which continues along the mid to lowe; part of the failure, terminating at the eastern side scarp. Below these features the slope becomes very steep (<35"). Not accessible in the field, but easily seen from aerial photos, is a disturbed area further down slope of the berm and lower tension crack (Fig. 57).

Slab Failure

W E

Orientations S

/ - / Failure Mass SFR Slope face r~ghtl left /Tansion Crack

Figure 57: "Slab failure" and mean discontinuities mapped in Unit 1.

4.8.2.1 Slab failure kinematics

Following the suggested technique of Bovis and Evans (1996), kinematic analysis of discontinuities mapped in relatively undisturbed zones was carried out. Discontinuities in units 1 and 2 were used to explore potential failure modes for the "slab failure." For previously studied slopes with sackungen features two failure modes have been suggested: sliding and toppling.

To produce a sliding failure a plane within the rock mass must daylight at the surface of the mountain slope at the failure toe. In addition, this daylightkg plane must dip steeper than the friction angle of the material making up the plane. No mean discontinuity plane measured at the "slab failure" daylights at the slope surface near the failure toe. Some of the bedding planes measured at the "slab failure" approach horizontal. These may daylight at the slope toe. However, they do not dip steeply enough to cause a sliding failure.

Toppling failures can occur when a steeply dipping anaclinal (dipping counter to the slope dip) joint set is present. Columns formed by this joint set can topple out of the slope face, typically resulting in steep vertical faces dominated by rock fall process or a distributed slope deformation without a defined head-scarp. Joint set 4 mapped at the "slab failure" dips at a steep angle counter to the slope. The lateral and observed vertical persistence of these joints are on the order of several meters. Toppling of columns defined by this joint set may be possible. However, the surface morphology of the slide is not consistent with toppling columns. In addition, the tabular blocks have a low base / height ratio (0.1 - 0.3) making toppling of individual blocks unlikely. From these simple observations it seems clear that neither of the previously suggested failure modes is the dominant mode at the "slab failure."

The morphological characteristics of the failure are similar to the general morphology of a bi-planar active /passive (or "two wedge") failures first identified in clay cored dams and later in mine spoil piles (Coulthard, 1979). A well defined head-scarp with a benched and deformed upper and mid-slope occurs in the upper block. The lower block is relatively undeformed, as it is driven along the basal failure surface.

To produce an active /passive failure in rock certain geometrical requirements must be satisfied by discontinuities. A bi-planar failure surface must exist consisting of an upper (or back) steeply dipping plane and a lower (or basal) shallow dipping failure surface (MacLaughlin et al., 2001). Finally, a plane separating the upper block from the lower block should be oriented dipping into the slope to provide the maximum transfer of force from the upper block to the lower block (Fig. 58). It should be noted that although the problem is conceptualized as two blocks, in the rock mass thousands of blocks actually exist. The conceptual "blocks" are really zones in the failure where either an active or passive mode dominates.

Bedding in Unit 1 has a mean orientation of 10" / 042 " (dip / dip direction). However, the range of values includes planes which are horizontal or dip very gentle out of the face below the "slab failure". These planes may form the lower (or basal) failure surface. High angle cataclinal discontinuities were measured in Unit 1 (68" / 191 "). These discontinuities could provide the upper (or back) failure surface. Finally, visible in data from Unit 1 is a set of anaclinal discontinuities (the previously mentioned joint set 4) which could form the cross plane between the upper and lower blocks of the active /passive failure.

The simplistic analysis of mean discontinuities provides only a basic insight into the actual the kinematics of this complex 3D failure. A detailed review of the surficial landforms provides additional support to the active /passive failure mode. A simplified force map is shown in Figure 59. This conceptual diagram shows the likely orientations of forces required to produce specific landforms observed at the slab failure surface. At the head-scarp of the failure, material is dropping in elevation and moving away from the ridge line. The tension crack cutting perpendicular to the ridge line is opening is response to this movement. Larger fissures on the upper surface of the failure mass give the clear impression of being formed by compressive forces causing the material to thrust and buckle. The berm feature lower on the failure is also fomed in a compressive environment. The general pattern of these forces indicate an upper block moving against a lower block providing some resistance. The likely separation of the two blocks is expressed at the ground surface by the berm feature. The actual toe of the failure occurs at the disturbed area visible from the aerial photos down slope of the bem.

Active (driving) block

Figure 58: Basic geometry of an active I passive ("2 wedge" failure) A - Head scarp

Cross section Plan view Figure 59: Force vectors expected in failure mass, based on landform type.

4.8.2.2 Slab failure and sackungen

Sackung features found in relation to the slab failure include tension cracks, scarps and benched topography. All of these features are clearly related to the failure which is occurring. Whether these scarp features would be classified as true sackung may be debatable. The tension crack which defines north-eastern side scarp of the "slab" failure is of some interest in terms of sackungen, as the crack's continuation through the ridge from one slope face to the other may indicate some connection to the more regional pattern of trench and depression features on Mount Mercer. The strike of this crack seems to agree with the orientation of some ridge-top trenches and mapped faults from other locations on the ridge.

4.8.3 The main scarp complex

Alpine linear landforms (fissures, scarps and anti-scarps) are present throughout the south east aspect slope of Mount Mercer. The "main scarp complex" (MSC) is composed of three zones (Fig. 60) based on overall slope angle and morphological features. The steep upper slope (1) contains compound head-scarps which are controlled by discontinuities (Fig. 61) and associated anti-scarps. Benched topography is found in the low angle mid slope area (2). The lower slope of Mount Mercer is steep with topography obscured by forest (3).

The combination of landforms in the "main scarp complex" (MSC) indicates the occurrence of a significant mass movement (surface area - 1000000m2). This failure has been previously noted by researchers in the Chilliwack River Valley (Savigny, 1990b; (Thomson 1998). No associated debris pile has been located by any previous researchers. Thomson (1 998) suggested that the failure occurred during the previous glacial period, with the debris being deposited on the valley glacier and subsequently moved down valley. Sackung features occurring in the MSC are related to the development of this mass movement. As a sackung forming mass movement, no debris pile is expected as features are formed by gravitational mass rock creep rather than catastrophic failure.

The displacement trend of the creeping failure is at an oblique angle to the ridge line (approximately south-east) (Fig. 62). The lateral extent of this failure mass appears to be defined by the "bend" in the Mount Mercer ridge to the west and the end of the ridge to the east. The down slope extent of this zone is less clear.

Investigation of this forested area found significant instability indicators such as jack- strawed trees and hummocky terrain. The depth of surficial material (mainly till with some colluvium) is much greater than at higher ridge elevations, yet still a veneer (< lm). Near the middle of zone 2 a creek head can be found. This creek exists well through the summer months and is likely fed by snow melt water stored in the rock mass of the upper ridge. The creek continues through zone 2 and zone 3 to the valley bottom where it empties into the Chilliwack River. Zone 2

/ - Anti - Scarp i Bench ' . Fadure Mass /& Trench 1 Depression /' \ . -..------.. Possible Fault Trace Approximate Scarp Limestone Contact

Figure 60: Overview the Main Scarp Complex and associated slope zones based on landforms and overall slope. Figure 61: Normal scarp segments compared with discor~tinuitystrikes indicate that the normal scarps follow existing joints.

Main Scarp Complex

Figure 62: The failure is moving toward the south-east. The upper part of the failure is defined by compound head-scarps and lateral and rear release zones indicated by trenches. 4.8.3.1 Main scarp complex kinematic analysis

Suggested kinematic modes for large failures in British Columbia have been limited to toppling and sliding in previous studies (Bovis, 1996 ; Nichols et al., 2002). Although these simple failure modes may not be completely representative of slope failures in irregular terrain on a large scale, the kinematic testing methods provide some guidance in understanding the failure. Following the suggested technique of Bovis and Evans (1 996), kinematic analysis of discontinuities mapped in relatively undisturbed zones was carried out.

The mean planes of four discontinuity sets mapped in units 2 and 3 above the head-scarps of the "Main Scarp Complex" (or MSC) have been analysed. From the mean joint sets, neither toppling nor sliding is kinematically admissible (Fig. 63). Within each joint set, it is likely a few planes may allow either mode to occur. However, these outliers from the mean will not have a significant effect on the overall failure mode of the mass. Because the persistence and condition of the discontinuities is relatively uniform, the effects of only a few joints with slightly different orientations would likely be outweighed by the mean plane orientations. Based on this analysis, the simple kinematic failure modes used to construct conceptual models for other large sackungen forming failures in British Columbia are not applicable for Mount Mercer. o Bedding [29] v Fault [2] Foliation [30] + Joint [I381

x [no data] [1]

0 Others [4] W E

Equal Angle Lower Hemisphere 204 Poles 204 Entries

,fl--\ \ I Daylight envelope I '.-A' f14-\ / / I ' / S ' I/,' Toppling envelope I./ /

Figure 63: Simplified kinematic analysis indicates that neither sliding nor toppling is a likely failure mode in the main scarp complex of Mount Mercer. 4.8.3.2 Alternative failure modes: rock slumping

Rock slumping is a compound failure mode requiring both inter layer slip and back rotation of rock blocks (Kieffer, 1998). As demonstrated by several authors using physical models (Gaziev and Rechitski, 1974a; Aydan et al., 1992; Kieffer, 1998), rock slumping provides a possible mode of failure for large rock slopes which develop sackung features.

The term rock slumping has been used by past researchers to described a landslide in bedrock that results in surficial features similar to a soil slump. Kieffer (1998) details both the kinematic requirements and resultant morphology of rock slumping. For rock slumping to occur two joint sets are required. The first is closely spaced set dipping steeply with the slope, but not daylighting. The second required joint set must be shallow dipping and daylighting. Pure sliding should not be admissible on these shallow dipping planes. These daylighting planes may occur throughout the rock mass under analysis, or a single base plane may also provide the necessary kinematic freedom for rock slumping to develop.

Rock slumping observed in physical models produced well defined morphological features in the slope. Kieffer (1 998) describes four characteristic landform sets associated with rock slumping:

A steep, well defined head-scarp (possibly compound) (1)

A series of minor stepped scarps in the upper slope (2)

Mid-slope topographic benches (3)

A steep toe (4)

4.8.3.3 Main scarp complex and rock slumping

Considerable evidence exist which indicates that rock slumping is the dominant failure mode responsible for sackung features in the "main scarp complex" (MSC). The geomorphology of the MSC fits the general landforms described by Kieffer (1 998). The compound and well developed head-scarp of the MSC is indicative of rock slumping. An apparent pattern of displacement may also be illustrated by mapped geotechnical units near the head-scarp (Fig. 64). Foliations measured in these units appear to be back rotated compared to foliations measured at the ridge-top. Combined with the anti-scarps, the mid-slope benches and overall slope changes over the length of the failure, the MSC area of Mount Mercer closely resembles rock slumping morphology.

Some of the kinematic requirements for rock slumping can also be satisfied by the joint data collected from the MSC. A steeply dipping back joint set exists in the MSC. Two possible faults mapped in Unit 3 in the upper slope also follow this cataclinal (dipping with slope) orientation. The foliation of the rock mass is near horizontal, but likely does not provide the basal plane required for rock slumping. Two other options from the structural geology exist for a basal plane or zone (Fig. 65) which would allow block slumping to develop.

Two thrust faults have been mapped at the base of Mount Mercer (Monger, 1967). The location of these two faults may be too low in elevation to be involved in the failure as the failure morphology suggests that the limit of failure is in the limestone unit of Mount Mercer. An additional thrust fault may exist above these faults.

In addition to a continuous plane, a zone of damaged rock would form a basal zone allowing rock slumping to develop. Muller and Hofmann (1970) used physical models to demonstrate a combined toppling / rock slumping failure occurring in a slope with a weak layer near the slope base. At Mount Mercer, damage associated with the previously described thrust faults may produce a weak layer within the slope. In addition to thrust faults, folding of the rocks of the Chilliwack Group is common. Monger (1967) inferred a set of folds near the current down slope limit of failure. These tight, isoclinal folds result in extensive jointing around the fold zone. Accommodation thrusts commonly form through the hinges of these tight folds. The axial planes of these folds dip gently to the south east. Either the thrust faults or folds may form the basal plane or zone required for rock slumping to occur. Legend

-- - - - Foliated basalt (Unlt 2) 'a ,-. . ,> .., :;. LS Jir$-,;] Massive tuff (Unit 3) r. .+. -,

I]Unseen geology

16 - 42 Dip - Dip D~rectionof Foliat~ons ' 50 Metres '. A'

Figure 64: Apparent displacements in the upper part of the MSC illustrated by both landform development and possibly displaced bedrock units.

Mount Mercar Ridge Line I

Figure 65: Probable location of basal zone I plane required for rock slumping to occur based on bedrock geology and surface morphology. 4.8.3.4 Main scarp complex and sackungen

Strong evidence indicates that a rock slumping mode of rock mass creep is occurring at Mount Mercer. The normal scarps, trenches and benches observed on the south-east aspect slope of Mount Mercer are truly sackung features, as they are produced by the slow deformation of the rock mass. The occurrence of these features in the foliated sedimentary bedrock of the Chilliwack Group corresponds well with the features occurring in the foliated rocks of the Alps termed sackung by Zischinsky (1 966; 1969).

The compound normal scarps observed in zone 1 are the head-scarps for the massive failure. The general orientations of these head-scarps follow the orientations of discontinuities observed in the field. Units 2 and 3 can be found in most normal scarps in the MSC. Overall displacement of the failure mass is difficult to judge. However, estimates fiom combined displacement measurements from the compound head-scarps indicate vertical displacements on the order of 10 to 20 meters.

A large trench occumng in zone 1 on the ridge line, behind the scarps indicates a lateral 1 rear release of the failure mass. This trench has two definite steep sides. The maximum depth of the ridge is approximately 1.5 meters; maximum width is 8 meters. Units 2 and 3 were mapped in outcrops in this trench. The anti-scarps in zone 1 and benched topography in zone 2 may be the result of deformations in the rock mass. These features occur roughly parallel to the slope contour. Bench features become lost in the trees on the lower slope.

4.9 Discussion

4.9.1 Failure initiation and timing

General comments regarding the triggering mechanisms and the progress of the failures resulting in sackung features at Mount Mercer can be made based on basic slope stability mechanics and models of the proposed failure modes. Mechanisms typically related to slope instability (seismic accelerations, pore water pressure, erosion, time) are discussed with respect to Mount Mercer. Results of previously published physical and numerical models illustrate the temporal pattern of displacements which may occur through each of the proposed failure modes. Based on potential failure triggers and the expected rates of failure, inferences about the timing of formation of the sackung features can be made. 4.9.2 Slope instability mechanisms: Seismic accelerations

Failure initiation through seismic activity for landslides in South Western, British Columbia has been reviewed by Naumann (1 991). Timing of major landslides (Hope Slide I + 11, Katz Slide, Cheam Slide, Lake of the Woods Slide) compared with the timing of large subduction style earthquakes showed little agreement. One of the reasons for this lack of correlation is that smaller earthquakes may actually have provided the trigger for these slides. Smaller earthquakes, such as the Lake Chelan event of 1842 (Fig. 66), may have provided enough acceleration (7.4 M) (Keefer, 1984) to destabilize the slopes at Mount Mercer.

Figure 66: Suspected limit of the Lake Chelan earthquake of 1842. The Richter scale magnitude in the central zone is estimated at 7.4. The Chilliwack Valley falls within this central, high magnitude area.

4.9.3 Slope instability mechanisms: Pore water pressure

The effects of pore water pressure on the stability a rock mass are well established. For discontinuity controlled failures, like those proposed for Mount Mercer, the increase in pore water pressure along discontinuities reduces the shear strength along the planes by decreasing the effective normal stress on a discontinuity plane. This can result in the total destabilizing forces acting on a natural slope being greater than the stabilizing forces within a natural slope. The mechanism of shear strength reduction on a plane due to pore water pressure is demonstrated by:

Shear strength along a discontinuity (Mohr-Coulomb):

~=c+o'tan@~ (Eq. 4.1)

Effective normal stress: (Eq. 4.2)

Where: z= shear strength, c = cohesion, o' = effective normal stress, o=normal stress, u = pore water pressure, 4 = friction angle

Then, as u increases, o ' decreases resulting in a reduction of total shear strength available on a discontinuity.

Hoek ( 198 1) points out:

"...it is water pressure rather than moisture content which is important in defining the strength characteristics of hard rocks, sands and gravels. In terms of the stability of slopes in these materials, the presence of a small volume of water at high pressure, trapped within the rock mass, is more important than a large volume of water discharging from a free draining aquifer."

With this in mind, a review of hydrogeologic regimes at Mount Mercer is required. Two time periods are of interest: deglaciation and the present.

During deglaciation the slopes of Mount Mercer would have been subject to large volumes of melt-water. Much of this water would flow through englacial plumbing or along the ice /bedrock interfaces. Some of this water would be expected to infiltrate the bedrock through existing open joints and fractures. The Mount Mercer ridge would have had melt water inputs from both the valley glacier along the south side of the ridge and the cirque glaciers on the north side of the ridge.

During deglaciation in the Cordillera, the upper ridge areas become ice free first, followed by the valley bottoms (Clague, 2002). This results in melt water inputs at the to ridge- tops with the only outlets being conduits back into / onto the glacier or into sub-glacial cavities. The inputs at the ridge-tops, combined with inputs along the remaining ice I bedrock interface can be expected to be greater than the out flow at the ice I bedrock interface. The result of this would be an increasing head in the rock mass, leading to high pore water pressures.

The current hydrology of Mount Mercer is completely dominated by precipitation. Most of the precipitation occurs as rain, however small amount of snow can be expected to last well into the summer months. Three creeks were observed to flow throughout the summer, originating from the mid - slopes of Mount Mercer. These creeks must be fed by ground water stored in the rock mass of the upper ridge. Estimates of the current head at Mount Mercer are not available. If high pore water pressures do currently exist at Mount Mercer, they would be concentrated in the zone above the creek heads. Bovis (1990) postulated that variations in monitored displacements of a sackung forming rock slope movement at Affliction Creek, British Columbia could be linked to variations in groundwater recharge related to snow pack volumes.

4.9.4 Slope instability mechanisms: Erosion

"Glacial debuttressing" is a commonly cited cause of mountain slope failure (Bovis, 1990; Savage and Varnes, 1987; Thompson et al., 1997). The concept assumes that either the slope was unstable before the ice came into contact with the slope, or some process(es) reduced the strength of the slope material during the time that the ice was in contact with the slope. Already mentioned is reduction of shear strength along discontinuities by high pore water pressures, which could lead to slope failure once the mountain side was ice free.

As outlined by the interaction matrix in Chapter 1, slopes which have been modified by glacial or melt water erosion processes may become unstable. Key blocks removed from the slope could create instability by providing additional kinematic freedom to the remaining blocks in the slope. Erosion of material at the toe of the slope reduces the weight of blocks at the toe of the failure which may resist movement. Finally, although abrasion of the rocks in the slope by rock transported in ice remove only small amounts of material, new fractures may be formed in the rock mass during the process. These fractures may combine to form new discontinuities, which may allow failure to occur.

4.9.5 Temporal pattern of failure

A definite trigger cannot currently be linked to the failures at Mount Mercer. However, each trigger appears to provide different limiting ages for the mass movements. Acquiring age control on some of the sackung features associated with the mass movements may provide evidence which points to a specific trigger or combination of triggering events.

If the deformations at Mount Mercer began in response to glacial debuttressing, the ages of failures are relatively easy to estimate, as the deglaciation of the Chilliwack River Valley has been well established (Saunders et al., 1987). The main southern slope of Mount Mercer was ice fiee by the middle of the Fraser Glaciation. The evidence for this is the existence of a lake at the base of the mountain which extended to approximately 500m (a.s.1.) elevation. If the MSC failure initiated due to debuttressing, the age of the movement could be near 12000 years B.P. The "slab failure" would likely be much younger if related to glacial debuttressing as it is within a hanging cirque that may not have been ice fiee until well after the end of the Fraser Glaciation (the actual timing is unknown). It is unlikely that these features pre-date the last glaciation, as there is clear evidence for ice over topping the ridge (granitic erratics and striations on exposed bedrock) but no evidence of ice or water erosion on the angular, steep outcrops found on the ridge-top and upper slope.

If the failures are related to seismicity, they could be relatively young. The previously mentioned Lake Chelan Earthquake would be the most recent earthquake which could have triggered failures on the scale observed at Mount Mercer.

Based on the conceptual failure models for the "slab failure" and the MSC, some inferences regarding the development of each failure over time can be made. Surveys of two - wedge failures by Coulthard (1979) indicate that the initial failure occurs rapidly, then stabilizes for some period. Long term stability cannot be assumed. The rock slumping mechanism that may dominate the MSC slope has a well established temporal pattern of displacements determined fiom physical models. The initial movement of the failure mass is abrupt and rapid. Next, the failure mass creeps as internal deformations of the failing rock mass dominate the displacements. Two possible end points for rock slumping have been observed. Some rock slumps stabilize after some period of deformation; others fail catastrophically (Kieffer, 1998) (Fig. 67). Initial Creeping I Possible ock slump failure I end points I mass I I Catastrophic I failure I I I I I

1 i I I I I Time

Figure 67: Conceptual Time I Displacement curves for a failure dominated by rock slumping.

4.9.6 Current activity

No monitoring currently exists any where on Mount Mercer. From the field investigation, both failures appear to be still active. Apparently fresh fissures opening behind the head-scarp of the "slab failure" indicate continued instability. Hummocky terrain on the mid and lower mid- slope of the MSC area, as well as jack-strawed trees, indicate that this area of thin till cover is also unstable. The apparent instability in the mid to lower slope may be linked to bedrock instability.

4.10 Summary

Geomorphic and geotechnical mapping of Mount Mercer has provided several clues to the origin of sackung features on the ridge. Most trenches, scarps, anti-scarps and benches can be linked to the occurrence of significant mass movement processes. Some other trench features may be produced by glacial melt water or be products of fault displacement. The formation of sackung features on the SE aspect slope of Mount Mercer could be explained by a failure occumng by rock slumping. A poorly (if at all) recognized failure mode in British Columbia, the rock slumping conceptual model fits both the geomorphic features and discontinuity orientations observed in the field. Further application of the rock slumping model to other sackungen slopes in British Columbia may produce further insights into those slopes.

The timing and activity of the sackung development at Mount Mercer is currently unknown. Trenching in the depressions between scarps may result in sedimentalogical evidence (McCalpin and Irvine, 1995; Thompson et al., 1997) to tie the scarp formation to a specific time period. No monitoring currently exists; however , the accessibility of the field area would make setting up a monitoring program relatively easy. Conceptual models proposed for both the "slab failure" and the 'main scarp complex" will be evaluated with geomechanical numerical modes in Chapter 5. CHAPTER 5 NUMERICAL MODELLING OF SACKUNG FORMING ROCK SLOPE DEFORMATION MECHANISMS

5.1 Introduction

Geomorphic systems are complex, multi-scale, and open. In general, the quantity and quality of data required to perform a detailed analysis of these systems is not available. Insights into these data limited systems can still be gained by combining the available data with modeling techniques. Models provide a laboratory where experiments can be carried out on simplifications of natural systems. The information provided by models commonly result in new interpretations of existing data or further suggestions on how to collect more data (Starfield and Cundall, 1988).

The goal of modeling is to simulate real world systems, not to duplicate them. In the geosciences modeling can take many forms. Statistical models are commonly used to describe systems where significant data is available, but little is known about the operative processes. Physical models provide researchers with scaled analogues of natural process. Closed form solutions, or mathematical relationships, derived from statistical or physical models are useful for simple applications. Numerical models leverage the power of computers to simulate systems through equations describing the interactions of the system constituents. Geological materials are typically represented as continuous bodies (continuum models) or as numerous bodies separated by discontinuities which can interact (discontinuum models). Various numerical methods exist for geomechanical models including (Hoek et al., 199 1):

rn Boundary element

Finite element

rn Finite difference

rn Distinct element

In geomechanics and applied geomorphology, numerical models have become common tools. Advanced codes combined with modem computing power have made modeling a useful approach for many problems (Starfield and Cundall, 1988). In this study modeling provides a means to test hypotheses regarding the mode of failure and kinematics of a potentially deep- seated, gravitational mass movement. In the absence of significant subsurface data, the simple models presented in this chapter provide additional insight into the mountain slope 1 rock mass interactions that result in slope failures and the associated landforms as they relate to sackung formation.

5.1.1 Models in geornorphology

Modeling has been used to examine several facets of geomorphology. Thomas and Nicholas (2002) simulated the development of braided river flow using cellular automata models. Harbor (1992; et al., 1988) has used time stepping numerical models to simulate the development of valley cross sections by glacial erosion processes. Bovis and Stewart (1998) used geomechanical numerical models to try to explain the development of slope morphological features similar to those in this study.

5.1.2 Geornechanics and geornorphology

Geomechanical and engineering geological techniques have a long history of use in geomorphological research. Scheidegger (1963) relied on geomechanical principles when reviewing the development of valley patterns in the Alps. Selby (1980) built on the work of Bieniawski (1976) on rock mass quality, when developing the concept of Rock Mass Strength (RMS). Augustinus (1995a; 1995b; and Selby, 1990) used RMS as well as other engineering principles when approaching slope stability issues in New Zealand. Modern geomorphic analysis of mass movements must leverage existing engineering analysis tools, just as fluvial geomorphology studies leverage existing hydraulic engineering tools.

5.1.3 Model verification and validation

Oreskes et al. (1994) emphasizes that the verification ("establish the truth of') models in the earth sciences is impossible as they are not closed systems. The input parameters required for a model cannot be completely known, the collection of data required for models occurs at multiple scales, and all input data are affected by inferences and assumptions regarding the system under study.

The term "validation" is often incorrectly used by both modellers and those who rely on the results of modeling. Validation denotes legitimacy (Oreskes et al., 1994). A valid model may be one which has no logical errors; however the models (or the outputs) are not necessarily true. Verification and validation are not synonymous and a valid model may not be an appropriate representation of the system of interest. The infinite slope method (Fig. 68) that is commonly used for simple slope stability analysis, fails to represent key characteristics of landslides. Material heterogeneity, rear and lateral release surfaces, the commonly non-planar nature of failure surfaces, and many other factors are not included within the infinite slope formalism. The model abstracts so many parts of the natural system that the "factor of safety" values calculated by the infinite slope method should rarely yield reasonable results, yet many researchers (Pack et al., 1998) consider the method valid.

Counter to the above philosophical arguments, Rykiel(1996) attempts to provide a more practical framework for verification and validation of models. Verification is a "demonstration that the modeling formalism is correct." To verify a model, the logic used to construct the model and the mechanics (mathematics, computer code) must be shown to be error free. A verified model does not guarantee a model which is valid or produces valid outputs. Validation is a multi- step process through which the model is demonstrated it is acceptable for its attended use; the result of this definition is that for any model the performance standards and range of applicability of each model must be established during the initial model design phase. Within this frame work, the infinite slope model could be correctly considered to be a valid model if it is limited to applications that match the model assumptions (relatively homogeneous material, planar failure surface, large slope compared to failure initiation area, etc.).

This advice can also be found in Starfield and Cundall's (1988) suggestions for a rock mechanics modeling methodology. Among the many suggestions they present are:

"It (a model) is an intellectual tool that has to be designed or chosen for a specific task"

"Instead of trying to validate ("prove to be true") a model, one should aim to gain confidence in it"

Using the terminology of Rykiel (1 996), the second suggestion is actually part of the validation process.

Before building the models for this study, the goals of the models and validation standards should be explicitly defined. Models of mass movement mechanisms created for the current study are used to explore the potential failure modes active during hypothetical deep seated gravitational deformation and are evaluated in three ways (Rykiel, 1996):

Data validity: Are the rock mass properties used in the models reasonable when compared to other published values and for the processes that may be operative at Mount Mercer, British Columbia? Conceptual validity: Are the mechanisms proposed and simulated reasonable or justifiable compared to other mechanisms proposed for large slope failures?

Operational (whole model) validity: Are the morphologies and displacement patterns produced by the models representative of those observed at Mount Mercer, British Columbia.

F=.t/d if F < 1, failure will occur

forc=O T = c + Q (tan 4) F=(tan$)/(tanQ) d=Wsin8 a=W(ms8) W = Hy 4 = internal friction angle of material c = material cohesion y = unit weight of material f3 = slope angle

Figure 68: Simplified representation of an infinite slope model for dry conditions. The factor of safety (F) is the ratio of the resisting forces (T, shear strength) of the material to the driving forces (d) due to gravity acting on the failure plane.

5.2 Failure mechanisms resulting in sackung

Potential failure mechanisms responsible for the deformations and related sackung on mountain slopes are reviewed in detail before considering the results of the model simulations. These failure mechanisms have been suggested based on the kinematic analysis of recorded discontinuity measurements and mapped morphological features at Mount Mercer. 5.2.1 Toppling

The toppling of rock columns as a mode of slope instability was first recognized in open pit mines, coastal rock slopes, and road cuts (Goodman, 1989; Hoek and Bray, 198 1). Based on the distinctive landforms and the rotation of geologic structures expected from toppling blocks (Fig. 69), many authors have suggested toppling as a mode of failure for the sagging and deformation of natural slopes (Bovis, 1982; Bovis and Evans, 1996; de Freitas and Watters, 1973; Muller, 1968; Pritchard and Savigny, 1991).

Mechanics: Several subtypes of toppling exist; however, all toppling failures have similar basic mechanical principles. Rock columns defined by steeply dipping anaclinal joints rotate forward. Inter-column slip must occur for the columns to rotate. Typically, toppling begins at the slope face and retrogresses back into the slope (Hoek and Bray, 198 1). Primary toppling requires the instability of individual columns. Secondary toppling occurs as blocks rotate forward due to applied forces from other primary failure modes. In natural slopes, toppling has been observed to drive sliding blocks near the slope toe (de Freitas and Watters, 1973).

For primary toppling to be kinematically admissible, two geometrical relationships must be satisfied:

1. The planes defined by the anaclinal discontinuities should strike in a similar direction as the slope face (originally +/- 15 O, later increased to +/- 30') (Goodman, 1989).

2. For inter-columns slip to occur, a normal to the anaclinal planes must dip less steeply than a line dipping at the slope angle plus the friction angle of the discontinuities defining the columns (Goodman and Bray, 1977): $, +(90-S)

Where +r is the friction angle on the toppling planes, 6 is the dip of the planes, and a is the slope angle.

Occurrence: From studies of engineered slopes, three types of primary toppling have been identified (Goodman, 1989; Hoek and Bray, 1981). Flexural toppling occurs with tall thin columns. Thinly bedded / foliated sedimentary and metamorphic rocks can be subject to flexural toppling. Block toppling occurs in columns made up of individual blocks. The base of a block toppling failure is typically well defined (Hoek and Bray, 1981). Block-flexure toppling occurs in columns divided by many cross joints. Small displacements on the cross joints result in a flexural like failure. Morphologv: The size and intensity of morphological features produced by a toppling failure are dependent on the type of toppling failure. In all cases, two main landforms are expected:

1. Obsequent scarps (or anti-scarps, uphill facing scarps)

2. Tension cracks

Observed above engineered slopes, these landforms are produced at the intersection of the tops of the columns and ground surface. Tension cracks open as individual columns topple away from each other. Obsequent scarps form as the sub-vertical block faces are exposed as the block rotate forward. Tension cracks are most prevalent in flexural toppling. Obsequent scarps are most noticeable as products of block toppling (Hoek and Bray, 1981). Normal scarps are not expected associated with primary toppling failures.

Figure 69: Toppling of columns within a slope resulting in obsequent scarps and tension cracks. The interaction of toppling columns and non-toppling sections of the rock mass are highlighted. 5.2.2 Rock slumping

A frequently overlooked failure mode in field studies (Kieffer, 1998), rock slumping has been clearly demonstrated in both physical models (Muller and Hofmann, 1970; Gaziev and Rechitshi, 1974a; Gaziev and Rechitshi, 1974b; Kieffer, 1998) and numerical models (Kieffer, 1998; Starfield and Cundall, 1988). Through these modeling exercises, the distribution and types of landforms associated with rock slumping has been established (Fig. 70).

The term rock slumping has been used by past researchers to described a landslide in bedrock which results in surficial features similar to a soil slump. As the surficial features were similar, some past researchers assumed that the failure mode was also similar, envisioning a rotational failure plane developing in a highly fractured rock mass. However, Kieffer (1 998) successfully demonstrates that rock slumping morphology can be produced by failures which develop with a bi-planar failure surface. In these cases rock slumping is caused by the back rotation of individual blocks 1 columns distributed throughout the rock mass which is failing (Fig. 70). The failure mode is controlled by discontinuities and does not require a rock mass to behave like a soil.

Mechanics: Two sets of discontinuities are required for rock slumping to occur. A closely spaced, steeply dipping cataclinal (dipping with the slope) set that does not daylighting should intersect a shallow dipping, daylighting discontinuity set (or single plane) (Fig. 70). Pure sliding should be not be admissible on these shallow dipping planes as the friction angle of the interface should be greater than the dip (4 > 6%). Sliding should be possible on the steeply dipping cataclinal discontinuities (4 < &). During failure, individual columns back rotate passively loading, or surcharging, (P) the columns behind and actively driving (A) columns ahead. As the column (block) rotates, face-to-face contacts between blocks are lost and replaced by edge-to- face contacts. The loading of block comers may result in rounding of the blocks as the comers fail and fiacture in tension. For displacements to occur, the component of the column weight transferred to the shallow discontinuities as a shear force must overcome the frictional resistance (as well as any cohesion which may exist due to rock bridges). Tall, thin columns with a centre of gravity outside of the block base are more likely to slump as more of the column weight is mobilized as a shear force along the steep cataclinal discontinuities and transferred to the shallow basal discontinuities. Occurrence: Expected for slopes composed of rock masses with discontinuities dipping steeply with the slope (cataclinal joints). Considerable deformation within the failure mass and a characteristic surface morphology can be expected. Characteristic "A"-frame voids develop near the bases of the columns.

As with toppling, sub-types of rock slumping can be identified including (Kieffer, 1998):

Flexural slumping: continuous thin rock columns flex and fracture.

Blockflexural slumping: continuous thin columns are cut by many cross joints.

Blockslumping: wide columns are cut by cross joints. Significant internal deformation is expected as most of the displacements occur through block rotation.

Morphology: Physical and numerical models of rock slumping produced well defined morphological slope features (Fig. 70). Kieffer (1 998) describes four characteristic landform sets associated with rock slumping:

A steep, well defined crown (head-scarp)

A series of minor stepped scarps in the upper portion of the failure mass

Mid-slope topographic benches

A steep toe

These features are similar to those observed as a result of an active I passive failure. Several similarities appear to exist between the two failure modes (e.g. resulting morphology, geometric requirements, "zones" dominated by active or passive behaviour); however, there is a significant difference. An active I passive failure develops by the cumulative interaction of two or more blocks (or "zones" composed of blocks, soil, etc.). Rock slumping occurs due to the forces acting on a single block. Each block in a rock slump fails due to its own geoinetrical characteristics. Similar to a toppling failure, each block fails because the centre of gravity of the block falls outside the block's base, creating a turning moment about its centre. Rock slumping and toppling can be considered "mirror images" of each other in terms of mechanics (Kieffer, 1998). Flexural slumping Block - flexural slumping

Compound normal scarps Compound normal scarps {head scarp) Wad scw~)

'A' - frame voids 4 > 8s

Figure 70: Rock slumping is due to the instability of individual blocks or columns and results in compound headscarps, a mid slope bench, and a steep toe. Figure based on physical and numerical models by (Muller and Hofmann, 1970), (Gaziev and Rechitshi, 1974a), (Gaziev and Rechitshi, 1974b), and (Kieffer, 1998).

5.2.3 Active / passive failure (two wedge failure or bi-planar failure)

First used for the analysis of earth cored dam stability (Sultan and Seed, 1967; Seed and Sultan, 1976), the active / passive concept has also been used in the analysis of rock slopes (Goodman, 1989; Soe Moe et a]., 2003; Stead, 1984; Stead and Scoble, 1983). The basic concept of the active /passive analysis divides the failure mass in question into two (or more) zones or "wedges" defined by the forces within each zone (Fig. 7 1). The upper zone or wedge is "active" as the some component of the wedge weight is transferred to the lower "passive" zone(s). The addition of these forces to the lower zone results in failure of the lower zone. In soil slopes these zones are defined by discontinuities that develop within the mass. In rock slopes these zones are can be defined by existing discontinuities or discontinuities which develop through intact rock fracture.

Mechanics: Standard two-wedge analyses techniques from (Coulthard, 1979) are based on back analysis of mine spoil pile failures.

Consider the case of two blocks, under dry conditions, at limiting equilibrium (where the mean factor of safety along the interfaces of both blocks is 1). The force transferred from the active block to the passive block is PI,which is equal to P2. P can be calculated by:

Resolving the forces of the upper block parallel ( TI) and perpendicular (N,) to the upper failure surface:

Where eI is the angle of the upper failure plane, 6is the angle between P and a perpendicular to the interface of the wedges, and pis the angle from horizontal to the perpendicular of the interface of the wedges.

Then, expressing the shear force (8)in terms of the normal force (Nl):

At limiting equilibrium of the upper block (F1= 1):

Where SI is the shear strength on the upper failure plane, Dl is the length of the upper failure plane, and cl, 4I ate the cohesion and friction, respectively, on the upper failure plane.

Finally, combining and rearranging the previous equations to results in an expression for the value of P: 4(W,sine, - W, cos8, tan~,)+c,D, P= 4 (cos(6 + ,O - 8,)) - (sin(6+ ,O - 8,) tan 4,

Knowing the value of the force transferred from the upper block to the lower block allows for the factor of safety (F2), and the stability (F2 5 1 results in slope failure) of the lower block. To derive the equation for the factor of safety of the lower block, the block force components are resolved using equations 5.1 and 5.2 (replacing the necessary variables with those representing the lower block, e.g. W, + W2) and expressed in terms of the normal force using equation 5.3. F2can then be calculated by:

c,D, + (W, cos 8, tan 4, - P cos(6 + ,O - 8, )) tan 4, F2 = (5.5) W, sin 8, + Psin(6 + ,O - 8, )

In addition to the block weights and discontinuity geometries, the properties on the interfaces (c, 4) are required for the analysis. The cohesion and friction values can be estimated from laboratory testing or engineering judgment or back calculated assuming a limiting equilibrium at the time of failure. Finally, to complete the analysis some estimate of 6, the angle between P and a perpendicular to the interface which P acts on. Sultan and Seed, (1 967) suggested that 6 could be estimated by:

6 = tan tan 4,

In reference to rock slopes, some authors (MacLaughlin et al., 2001) have suggested that these failures can be very stable once the lowest comer of the upper block comes into contact with the basal failure plane. This conclusion, although theoretically plausible, may not allow for the influence of tensile rock fracture and block disintegration during failure. Occuwence: In soil masses, active /passive failures can be expected when the slope (Coulthard, 1979):

contains a weak discontinuity plane

consists of highly stratified material

rests on a strong base

In rock slopes the geometric requirements for a bi-planar failure are (Nathanail, 1996):

A shallow dipping, daylighting discontinuity or discontinuity set forming the lower failure plane

A steeply dipping, non-daylighting discontinuity or discontinuity set forming the upper failure plane

and

A cross joint separating a block on the upper failure plane from a block on the lower failure plane.

A zone of plastic deformation or tensile fracture between the active and passive blocks (Soe Moe et al., 2003)

Morphology: An active / passive failure produces similar landforms in both soil and rock (Fig. 7 1). Well developed head-scarps can be expected, exposing the upper failure plane. Deformation of the upper zone is common. Grabens may form above the head-scarp in multi- block systems (MacLaughlin et al., 2001) or in response to block disintegration. Displacements near the head-scarp are dominantly vertical; at the failure toe they are horizontal. Bulging of the lower slope is common and significant internal dilation of the dope materials can be expected. The overall failure morphology in both soil and rock is similar to a rotational soil slump; however a circular failure plane does not exist. Additional morphological features may be expected in rock slopes as displacements on existing joints are expressed at the ground surface. active wedae

head scarp slope deformation

void space / dilation

Figure 71: An active block driving a passive block on a bi-planar failure surface results in the development of a well defined head-scarp, possible grabens and upper slope deformation, anti-scarps, slope bulging and dilation of the slope material. See text for complete symbol explanation.

5.3 Model setup

To demonstrate the influence of discontinuities on sackung development, and test conceptual failure mechanisms proposed for sackung formation a series of geomechanical models have been developed based on the geomorphic mapping and engineering geological analysis of Mount Mercer, British Columbia presented in the previous chapter. UDEC 3.1 (HCItasca, 2001) is used to construct and evaluate the models. The failure mechanisms suggested by the kinematic analysis and geomorphological investigations from Chapter 4 determine which discontinuity sets and bedrock material properties are used in the simulations.

Two sets of simulations will be undertaken using UDEC 3.1 (Table 6). The first set of simulations is based on the "slab failure" area identified during the field study. Starting from the requirements for a toppling failure (which was identified as a potential failure mechanism from the stereonet analysis), additional discontinuity sets will be added to evaluate the effects on the influence on the failure mechanism(s) resulting in sackung; similar to those observed at the "slab failure." The second set of simulations is based on the geometry and material properties associated with the "main scarp complex" identified at Mount Mercer, British Columbia These data are used to evaluate the potential for rock slumping to form sackung features observed at the MSC and explore the effects of simulating blocks of various base to height ratios, instead of continuous columns. All of the simulations undertaken in this study are used to principalIy explore potential linkages between sackung formation and failure mechanisms, not to provide detailed back analyses of the failures observed at Mount Mercer, British Columbia. This forward modelling approach has been adopted by many previous authors (Bovis and Stewart, 1998; Pritchard and Savigny, 1990) where detailed sub-surface data were not available. The progression of models in the simulations follows the example of Hencher et al. (1996), where changes in failure mode are observed as more discontinuities are added to the models. Table 6: The progression of simulations used in the current study to investigate the development of sackung by various rock slope failure mechanisms.

Simulation state Model # Purpose

1 toppling discontinuity set (JS 4) forming Evaluate the potential for a la continuous columns. toppling dominated failure. the potential for a Model Ia + 1 cataclinal discontinuity (JS 3) valuate "Slab 2a multi-block active-passive failure" resulting in a bi-planar failure surface. failure. Model 2a + reduced discontinuity spacing for the Evaluate the effects of 3a cataclinal discontinuity and bedding parallel decreased block size within discontinuities (JS 1). the slope. 1 cataclinal discontinuity set (JS 3) forming Evaluate the potential for rock columns resting on the basal plane (possible fold slumping using columns and I axis 1 damage zone). The maximum base to the resulting slope height ratio of the columns is 0.1 morphology. Model Ib + a foliation parallel discontinuity (JS 1) Evaluate the effect of shorter MSC 2b set cutting the columns, resulting in blocks with a columns on the resulting maximum base to height ratio of 0.5 surface landforms. Model 2b + reduced spacing of the foliation Evaluate the effect of square 3b parallel discontinuities resulting in a maximum blocks on the development of base to height ratio of 1 surficial landforms.

5.3.1 UDEC

UDEC 3.1 (Universal Distinct Element Code) provides a method of simulating the interactions of discrete blocks in two dimensions. In these systems the intact material properties of the blocks, as well as the mechanical properties along the discontinuities are important. Displacements of blocks due to the applied stress field are easily determined through UDEC. As displacements are updated through the simulation process, the blocks' position and orientation at any calculation time step can be determined.

UDEC is ideal for slope stability simulations as rock masses in these low stress conditions are controlled by discontinuities (joint, faults, etc.). The data requirements of UDEC can be difficult to fulfill; however, published studies of similar problems can provide some guidance. As the software is capable of showing animations of the kinematic development of failures and tracking several variables (displacements / time, block velocities / time, etc.) it is an ideal tool for geoscientists interested in mass movements and the resulting morphological features. 5.3.2 Model properties and geometry

5.3.2.1 Stress conditions and boundary conditions

Natural slopes are normally considered low stress environments for rock mechanics research. The two-dimensional modelling used in this study assumes vertical and horizontal stress acting on the model. The vertical stresses are due to the gravitational self weight of the model material. The vertical stress at any point in the model can be estimated by:

Where D is the thickness of overburden above the point of interest and y is the unit weight of the material.

Horizontal stresses are the result of tectonics or neotectonics and the Poisson's effect. As models in this study do not consider tectonic stresses, the horizontal stress (oh)is due to the Poisson's effect and can be estimated (assuming elastic materials) for any point within the model using equation 5.8:

Where K can be calculated by:

Where v is Poisson's ratio

The horizontal / vertical stress ratio of approximately 0.3 (K = 0.3) would be expected for a material with a Poisson's ratio of 0.23. Results from the general stress analysis completed for dogtooth ridges in Chapter 2 shows that only minor changes in stress distributions would be expected when horizontal stresses are increased (K = 0.5) through the addition of tectonic stresses.

The boundaries of each model block have been chosen to ensure that the area of interest in each model is unaffected by effects of the zero displacement boundaries applied to the model boundaries. Only the area of interest (failure area) of each model is composed of an elasto-plastic material model. The outer sections of each model block are assumed to behave elastically. 5.3.2.2 Groundwater conditions

Although previously discussed as important consideration for rock slope stability, groundwater is not explicitly included in the following simulations. Little or no data is available on the position of a phreatic surface at Mt. Mercer. Other than the existence of late lying snow (a potential groundwater source) and creeks mention in Chapter 4 (outlets for groundwater), data characterizing the nature of flow through the fracture rock mass along the ridge line of Mt. Mercer is unavailable. Instead of including groundwater as an additional unknown quantity in the following simulations, the results of the dry simulations are interpreted keeping the general effects of groundwater in mind.

5.3.2.3 Slope geometry

Cross section geometry for the areas of the "slab failure" and the "main scarp complex" at Mount Mercer were derived from DEMs (digital elevation models) for use in the conceptual failure mechanism models (Fig. 72). These DEMs are based on TRIM I1 (Terrain Resource Inventory Management) datasets provided by the government of British Columbia. Elevation data derived from 1: 40000 orthometrically rectified aerial photographs is used to create the TRIM I1 DEMs which have a horizontal and vertical resolution of 20 meters. This resolution provides an appropriate level of detail for the numerical models of the mountain slopes. Cross sections (Fig. 73) were drawn through the DEM parallel to the apparent centre line of the each proposed failure mass at Mount Mercer using the EZ Profiler extension (Huang, 2003) for ArcGIS 8.2 (ESRI, 2002). Figure 72: The location of cross sections for models of the slab failure (S-S') and the MSC (M-M'). s Ridge line t

metres

I hwizontal and vertimI A mnstra~nt I

M' Ridge line

Figure 73: Block A based on slope profile S - S' and discontinuity measurements from Unit 1 and model block B based on slope profile M - M' and discontinuity measurements from units 2 and 3 for UDEC modelling. Note displacement constraints on the model boundaries used for each set of simulations. 5.3.2.4 Discontinuity sets and properties

The discontinuity sets identified through the stereonet analysis as related to possible sackung forming failure mechanisms at Mount Mercer are used to define discrete blocks for the UDEC simulations (Table 7). The orientations of the discontinuities are based on the mean orientations of the discontinuity measurements collected from the undisturbed zones near the "slab failure" (Unit 1) and "main scarp complex" (units 2 and 3).

All of the discontinuity sets observed in units 1,2 and 3 at Mount Mercer have sub meter spacing. Many have strike and dip persistence greater than two meters. The spacing and persistence values observed in the field cannot be directly reproduced in the numerical model, without resulting in the computation time of the simulation becoming impractical. For the simulations in this study, the spacing of discontinuity sets are assumed to result in blocks of specific dimensions of interest (tall columns) or based on the spacing of observed landforms associated with the failure masses from the field study.

One type of discontinuity used in both model simulations requires clarification: the basal plane. For the "slab failure" simulations, the basal surface is parallel to JS 1; however, it is an artificial construct required by the simulation to limit the depth of failure. This basal surface may represent the contact between Unit 1 and Unit 2; however no direct field evidence is available. The location (depth) of the "slab" basal plane is based on the observed limit of failure and the dip of JS 1. The properties of this plane are the same as all other JS 1 discontinuities. A basal plane is also used in the MSC simulations. This plane also limits the depth of failure, but it is based on the structural geology interpretation of Monger (1967). From the geological interpretation of the mountain, the basal plane may represent part of a fault or a zone of damage related to the recumbent folding of the Chilliwack Group rocks. Mechanical properties for the assumed basal plane are based on the above tectonic origin.

The Mohr-Coulomb failure criterion is used to simulate the properties of the joints in both sets of models. All joints are assumed to have zero tensile strength and cohesion. The resistance along all of the joints in the models is provided only by the friction along the joints. To ensure displacements within the rock masses such that the kinematics and resulting surface morphologies could be evaluated, the frictional resistance along the discontinuities for the final simulations was established through a number of preliminary simulations. Table 7: Summary of basic discontinuity properties used for the UDEC simulations in the current study.

UDEC Friction angle Dip Dip direction Comments Model set spacing (degrees) (degrees) (degrees) (meters)

1 20 20 25 12 045 1 "Slab - I1 to Unit bedding failure" 3 20 20 - 30 86 220 Cataclinal joints 4 15 15 - 20 79 344 Anaclinal joints 1 50 25 - 35 17-21 010 - 046 11 to Unit 2 foliation 3 50 20 - 30 60 - 75 110 - 129 Cataclinal joints MSC Inferred from Base Single plane 10-20 5-20 - 135 geology of Monger plane (1 967)

5.3.2.5 Rock mass properties

From the field study it was clear that some of the rocks visible in outcrop at Mount Mercer have a low rock mass quality (Geological Strength Index ranging from 25 - 50). Without subsurface data, the condition of the rocks at depth cannot be ascertained. The rocks at the surface should have a similar structural history to those at depth based on the interpretations of Monger (1967). However, at the surface rocks, are subject to greater weathering and erosion. It is likely that the rocks at depth have a slightly higher rock mass quality. Therefore, material properties derived based on surficial exposures can be expected to be the lower bounds of the expected range of rock properties for a specific unit in a slope, ignoring any affects of discrete geological structures at depth.

Material properties for the "slab" failure and MSC simulations are derived from:

GSI

Unit Weight

UCS

Rock Type

These observations combined with the Hoek-Brown failure criterion and the freely available software RocLab (Rocscience, 2003), are used to estimate the material properties required for use in the UDEC simulations. The method of derivation of the final properties is detailed in Appendix 1. The material and related discontinuity properties used for the UDEC simulations are summarized in Tables 8 and 9.

5.3.2.6 Comparison to other published values

Developing appropriate model properties from field observations requires experience in geomechanics and engineering geology. To ensure that the properties used for this study are reasonable, the derived properties are compared to rock properties used in other studies of large landslides and deep seated slope deformations (Agliardi et al., 2001; Bovis and Stewart, 1998; Pritchard and Savigny, 1990; Pritchard and Savigny, 1991). The properties for this study are well within the range of previous rock slope deformation studies using similar techniques and involving similar rock types. Table 8: Rock mass properties used in the current study compared to previously published material properties used for FD (finite difference) and DE (distinct element) models.

Friction Tensile Bulk Shear Source angle Cohesion strength modulus, modulus, Rocktype Notes (degrees) (MPa) (MPa) K (GPa) G (GPa) type This Foliated UDEC See text 38 5 0.1 study - 53 3'5 - 35 basalt (DE) Agilardi Meta- FLAC = 4,85 et al., 32 0.2 0.1 10 sediments (FD) 2001

Pritchard 46 0.6 0.3 10.7 Pelites and UDEC Heather Savigny, and (DE) hill toppling schists 1991 40 0.3 0.15 9.5 8.7

Pritchard Brenda and Quartz UDEC mine 35 0.15 0.21 33 20 Savigny, diorites (DE) toppling 1990

Bovis 54 - 62 2.4 - 4.5 0.5 - 1 12-25 8.4 -17.5 Columnar Fresh and basalts UDEC Stewart, overlying (DE) 1998 48 - 54 0.03.53 - 0.0 - 0.2 0.7 -7.5 0.5 -5.1 diorite Weathered Table 9: Discontinuity properties used in this study compared to previously published discontinuity properties. Tensile strength = 0 MPa or NA for compared studies. Dilation angle = 0' or NA for compared studies.

- -- -. Friction Kn Ks Cohesion Source t angle Notes (GPa Im) (GPa Im) (ivira, (degrees) - This study 2 -20 0.8- 8.3 0 15 - 35 See text Pritchard and 1.2-10.8 0.6-5.4 Savigny, 1991 Petites and schists Pritchard and 40 20 0 25 Quartz diorites, friction angles Savigny, 1990 were reduced to 18 - 20 ~ovisand Stewart, 0.5 -1.2 25 - 38 1998

5.4 Simulation results

5.4.1 "Slab failure"

The results of the simulations based on conceptual models proposed in chapter 4 are summarized in Table 10. Possible explanations for the formation of sackung (based on data fiom the "slab failure") (Fig. 74) are detailed in the following discussion.

Figure 74: Complete UDEC block used for "slab failure" simulations. Area of interest is the ridge-top and upper slope. Table 10: Overview of "slab failure" simulation results Simulation Initial state Results # 1 toppling discontinuity set (JS 4) forming continuous No toppling due to buttressing by

la columns.JS4b=15•‹s=15m.JS1b=200 non-toppling.. - toe blocks. Model Ia + 1 cataclinal discontinuity (JS 3) resulting Developed active I passive failure resulting in slope morphological 2a in a bi-planar failure surface. JS 4 I$ = 15" s = 15 m, features similar to those observed in JS1 I$=20•‹s=50m,JS31$=200s=50m the field. Model 2a + reduced discontinuity spacing for the cataclinal discontinuity and bedding parallel Results similar to simulation 2, with 3a lower magnitudes of displacement on discontinuities (JS 1). JS 4 I$ = 15's = 15 m, JS 1 I$ = individual discontinuities. 20•‹s=20m,~S3~=200s=20m

5.4.1.1 Simulation la

The initial simulation (Fig. 75) was designed to evaluate the potential for toppling to produce sackung features similar to those observed at the "slab failure." The discontinuity sets JS 4, which was identified as a potential toppling joint set in the preliminary stereographic kinematic analysis, and JS 1 (as the basal plane) were used in this simulation. The JS 1 plane was placed at base of the rock columns to provide a basal release and limit the depth of the failure. The location of this basal plane was the down slope limit of the "slab" failure mapped in the field. To ensure toppling of JS 4, the spacing of the discontinuities was set to 15 meters to produce columns with a base to height ratio of approximately 1 : 10. The friction angle on JS 4 was initially set to 15"; which is the largest possible friction angle that would still allow toppling according to the geometrical requirements for toppling outlined above (Eq. 5.0). The friction angle on the basal plane was set to 20 degrees.

Although toppling of the columns defined by joint set 4 should be expected based on the stereographic analysis and properties, displacements did not develop. Reducing the friction angle on the JS 4 discontinuities did not result in displacements either. The toppling columns were buttressed by a group of blocks at the toe of the failure. These blocks would not topple due to their shape, as defined by JS 4 and the gradual slope; nor would they slide, driven by the toppling columns, as the basal friction was too large. To produce a toppling failure with the "slab failure" slope configuration, the friction angle on the basal plane had to be reduced to less than 5" to allow the toe blocks to be pushed by the toppling blocks, providing the required kinematic freedom (Fig. 76). Increasing the dip on the basal plane in the simulation would result in sliding at a greater basal friction angle, however, the mean trend of JS 1 is an anaclinal ("dip counter to the slope") plane dipping 5" - 10". The horizontal plane used in the simulation is already at the limit of possible orientations measured in the field study. The resulting failure can be divided into three zones based on block behaviour. The toe blocks slide as they are driven by toppling occumng further up slope. The toppling blocks in the middle of the failure do not deform as they rotate off the base plane. The upper most columns do not separate from the basal plane (little or no displacement), instead they bend and flex as the tops of the columns topple (Fig. 77). The morphology of the failure matches that expected from a toppling failure. No head-scarp develops and the ground surface is dominated by obsequent scarps.

UDEC (Version 3.10)

Figure 75: Initial configuration of the "slab failure" simulation 1. Figure 76: Toppling occurrs only when the friction angle on the base plane is reduced to the unrealistic value of < 5' resulting in the toe blocks sliding. The resulting surface morphology is dominated by obsequent scarps, no head-scarp is formed.

t'"

Figure 77: Displacement vectors for the toppling model (simulation 1) of the "slab failure." 5.4.1.2 Simulation 2a

Joint set 3 was added to the previous toppling model to explore some of the more complex kinematic possibilities for the "slab" failure. The initial configuration of the toppling model (joint set 4: 15m spacing, 15" friction angle) was modified by adding joint set 3 (50m spacing, 20" friction angle) and additional bedding planes (50m spacing, 20" friction angle). The location of one of the joint set 3 discontinuities was based on the location of the head-scarp of the "slab" failure observed in the field (Fig. 78).

The new configuration did not require any changes to the frictional resistance on the joint planes for displacements to develop. Sliding blocks along joint set 3 in a zone near the ridge-top transferred force to the lower failure mass, driving it along the basal plane. The addition of JS 3 changed the failure mechanism from simple toppling to an active /passive bi-planar failure. The surface morphology matches that expected from bi-planar failure, with the development of a head-scarp (Fig. 79) and sliding displacements at the toe of the failure (Fig. 80). The ground surface develops minor obsequent scarps as displacements occur along the planes of JS 4, in response to the internal deformations of the rock mass. The limit of failure at depth, determined by displacement vectors (Fig. 80), appears to be well above the basal plane at the intersection of the upper failure plane (JS 3 through head-scarp) and the basal plane (lowest JS 1 plane).

Figure 78: Initial configuration of simulation 2 for the "slab failure." Figure 79: Magnified view of the head-scarp developed in simulation 2. Also note the development of minor obsequent scarps and minor rotations of block defined by the toppling joints. Figure 80: Displacement vectors for model 2 illustrating a zone where displacement vectors are parallel to JS 3 (I), a zone where displacement vectors are parallel to JS 1 (111) and a transition zone between the two (11).

5.4.1.3 Simulation 3a

The spacing of JS 1 and JS 3 were reduced to 20 meters for the final simulation in the "slab failure" set (Fig. 8 1). The overall displacement pattern and surface landform development was similar to the previous simulation (2). The development of surface landforms was more subdued with the smaller joint spacing as the displacements in the model were distributed over more joints. This resulted in deformation of the failure mass, but less displacement along the basal plane. Figure 81: Zoomed view of the head-scarp developed in model 3. The observed landforms are similar to those in simulation 2.

5.4.2 MSC failure simulations

Initial simulations of rock slumping reproduced previously published experiments (Gaziev and Rechitshi, 1974b; Kieffer, 1998; Muller and Hofinam, 1970) based on physical and numerical models in UDEC 3.1. Simulations using geometry and properties for the "main scarp complex" are used to explore the morphologies produced by rock slumping in natural slopes. Initial simulations are based on elastic constitutive models to represent the rock slope materials. Several conceptual slopes and geometry combinations were tested. Models based on the "main scarp complex" have been built using elastoplastic constitutive material properties. For the MSC a series of simulations exploring the effects of block shape were run (Table 11). Table 11: Overview of initial simulations of rock slumping and rock slumping simulations based on MSC data. Simulation Initial state Results #

General morphologies and kinematics of rock slumping Establish expected morphologies are explored through simulations based on published of rock slumping in UDEC. Initial physical models. lnitial simulations based on the MSC Explored possible combinations of simulations are used to develop reasonable geometries for the basal plane orientation and friction basal plane as well as friction values. angle.

1 cataclinal discontinuity set (JS 3) forming columns resting on the basal plane (possible axial plane I damage zone). The maximum base to height ratio of Simulation resulted in flexural Ib the columns is 0.1. slumping. JS 3: I$ = 20 s = 50 m JS1 @=25,s=500m BP @ = I5 Model Ib + a foliation parallel discontinuity (JS I)set cutting the columns, resulting in blocks with a maximum base to height ratio of 0.5 Simulation resulted in block - 2b JS3:$=20s=50m flexural slumping. JS 1 I$ =25, s= 100 m BP I$ = I5 Model 2b + reduced spacing of the foliation parallel discontinuities resulting in a maximum base to height ratio of 1 Simulation resulted in block 3b JS 3: I$ = 20 s = 50 m slumping. JS 1 4 = 25, s = 50 m Bp 4

5.4.2.1 lnitial simulations

General kinematics and morphologies of failures from previously published work were successfully simulated in UDEC (Fig. 82). Simulations of blocks with elastic properties behaved similar to previously published results. Slope geometries and block distributions were estimated from published photographs of physical models constructed by Gaziev and Rechitshi (1974b) and Muller and ~ofnkinn(1970). Material and joint properties for these numerical models were chosen to ensure that failure, as observed by the above authors, along the defined discontinuities would develop.

Initial conceptual models confirmed generalizations made by Kieffer (1998) as to the friction angles required on the base planes of the models to allow rock slumping to occur (5'-10' greater than the basal plane angle). The general pattern of rock slumping development, where failure begins at the slope toe and retrogresses back toward the head-scarp was observed in all of the initial conceptual simulations (Fig. 83). The slope morphological features identified by previous authors, including compound head-scarps, mid-slope benches, and a steep slope toe, were reproduced in the UDEC simulations. As well, previous observations of sub-surface displacements (the development of 'A' - frame voids) could also be reproduced.

I I t I I ti 0.25 0.75 3.25 1.75 2.25 2.75 (metres x 1W)

Figure 82: Rock slumping model in UDEC based on a physical model of Gaziev and Rechitshi 1974a Figure 83: Various examples of initial elastic simulation of rock slumping based on the geometry of the MSC. 5.4.2.2 MSC simulations

For the final simulation of rock slumping, the location and 10" dip of the basal surface used is assumed based on the folding and unit contacts observed in the cross section of Monger (1 967). The existence of the basal surface cannot be confirmed, without borehole data. The basal surface is estimated to be at the contact between units 3 and 4 in the Chilliwack Group. The MSC simulations (Fig. 84) are run with elastoplastic material properties to allow individual blocks to deform. Three sets of simulations are run to explore the effect of varying base / height ratio on the failure progression and final simulated slope morphology.

UDEC [Varsin 3.10)

A Ftxed x and y Fixed x

Figure 84: Model geometry and boundary condition overview for the MSC simulations.

5.4.2.3 Simulation 1b

Simulation lb (Fig. 85) consists of columns with a maximum base to height ratio of 0.1 defined by JS 1 and JS 3 of the mapped geotechnical units 2 and 3. Compound normal scarps are produced near the ridge-top (Fig. 86). Similar to the elastic conceptual models displacement along the basal surface occurs as blocks back rotate and slide, forming 'A'-fiame voids (Fig. 87). There is little variation in the area of these voids along the base plane; the maximum void area (- 450 m2) is located below the ridge-top and the mean void area is - 130 m2. Flexing and bending of the columns is observed in the simulation. The elastoplastic blocks deform as more of a cohesive mass than blocks in the elastic material simulations, this due to the slight column bending observed in this simulation. The type of rock slumping observed is similar to theflexural slumping described by Kieffer (1 998).

Figure 85: Overview of the final slope geometry of simulation Ib. Figure 86: Magnified view of the ridge line of simulation lb based on the MSC. A compound head-scarp is produced. Figure 87: Base of columns in model MSC 1. The columns deformed due to the gravitational stress. 'A'-frame voids develop as blocks slip and back rotate.

5.4.2.4 Simulation 2b

The spacing of JS 1 was decreased in this simulation from 500 meters to 100 meters. The resulting blocks have a maximum base to height ratio of 0.5. The expected surface morphology of normal scarps and benches is formed (Fig. 88). Little or no column deformation is observed in simulation 2b; however, displacements are observed on the additional cross joints. The pattern of surface landforms, basal plane displacements, and void development are similar to simulation lb (Fig. 89); yet lower in magnitude. The maximum void area (located below the ridge-top) is - 150 m2; the mean void area is - 50 m2. This simulation results in an example of block-flexural rock slumping. To produce scarps of the same magnitude as those in simulation I b, the simulation

Figure 88: Magnified view of compound head-scarp development in simulation 2b. (metres x 10A2f

Figure 89: 'A'-frame voids develop between blocks in simulation 2b. The displacements along the cross joints are also visible.

5.4.2.5 Simulation 3b

For simulation 3b, the spacing of JS I was decreased fhrther (from 100 meters to 50 meters) resulting in blocks with a base to height ratio of approximately one. The friction angles on the simulated discontinuities remain the same as in the previous simulations. The pattern of displacement in the rock mass is unchanged from the simulations lb and 2b. The magnitudes of the simulated displacements at the slope surface are less (max 5 m) than those observed in the simulations 1b (max 13 m) and 2b (max 7 m) for the same amount of computation time (Fig. 90). The displacements in simulation 3b are distributed throughout the rock mass along the higher number of discontinuity surfaces compared to the previous models (Fig. 91). Mean 'A'-frame void area is similar to simulation 2b, with a mean void area of approximately 30 m2. No block deformation is observed in these simulations. The displacements of the rock mass are controlled by the basal plane and JS 3 in this example of block slumping. 1 Compound head scarp

Figure 90: Magnified view of the development of the compound head-scarp in simulation 3b. Figure 91: 'A'-frame voids and horizontal voids opening along the cross joints near the basal surface of simulation 3b.

5.5 Discussion

Data inputs for the current simulations included derived rock mass properties and geometries from TRIM 11 data. The properties used for these simulations have been developed through a reproducible set of techniques which are common to engineering geology. When compared to properties used in previously published modeling studies of mountain slope deformation, the values used in the current study are within the lower range of those of the previous studies. For models of the complexity presented here, based on the available field data, these properties can be considered appropriate. British Columbia TRIM data is often derided based on perceived and real inaccuracies in previous iterations of the data set (TRIM I). The TRIM I1 data set used in the current study overcomes many of the issues of previous TRIM data by being based on larger scale aerial photography (1 : 40 000 versus 1 : 63 000) for data capture. Both TRIM I and TRIM I1 data products are designed for application at a scale of 1 : 20 000. This scale can be considered appropriate for slope profiles on the scale used for the simulations in the current study.

Neither of the failure mechanisms investigated for the formation of sackung similar to those observed at Mount Mercer have been recognized in studies of other large rock slope failures in British Columbia. Most sackungen or mountain scale failures in British Columbia have been attributed to toppling. These include:

Affliction Creek (Bovis and Stewart, 1998)

Mount Cume (Evans, 1987)

Heather Hill (Nichol et al., 2002)

Pika Ridge, Handcar Peak, and others (Bovis and Evans, 1996)

In some of these cases, data matching the geometric requirements and geomorphic characteristics of alternative failure modes (rock slumping) exists (Bovis and Evans, 1996). The similarities between rock slumping and toppling mechanics make it likely that rock slumping may well be as probable a failure mode for natural slopes as toppling. Evidence of this has been demonstrated by Kieffer (1 998) through case studies. As well, Agliardi et al. (2001) recognize a combination translational - rotational failure mode that produces similar landforms to those expected from rock slumping. That failure may be another example of rock slumping in a natural slope.

Through the numerical simulations presented in this chapter, failure modes reproducing many of the observed geomorphic features (well defined head-scarp, compound head-scarp, anti- scarps, slope benches, etc.) during the field study presented in Chapter 4 were demonstrated. Based on the results of the toppling models, it is clear that toppling (eitherflexural or block or block-flexural) alone is not likely to produce sackung similar to those associated with the "slab failure." The gradual change of the modelled slope resulted in the formation of blocks that would not topple. An issue for toppling in many natural slopes, it has been suggested that toppling blocks up slope of the toe blocks will drive the non-toppling blocks along available planes (de Freitas and Watters, 1973). To achieve this effect in the current simulations, an unreasonable low friction angle was required on the basal plane. The simulations that included IS 4 and resulted in an active /passive type failure reproduced many of the landforms mapped in the field. In addition, friction values for all of the discontinuities used in the simulations of active / passive behaviour were geologically reasonable. From the results of this limited set of simulations, a bi- planar (active / passive) failure controlled by IS 1 and IS 4 is a plausible mode of failure for the "slab failure".

Geomorphic and geologic evidence, from Chapter 4, had previously indicated that rock slumping was a potential mode of deformation for the MSC. The simulations presented indicate that a shallow dipping basal surface combined with discontinuities based on field observations are capable of producing normal scarps, benches and steep toe similar to those observed at the MSC of Mount Mercer. Scarps heights resulting from the simulations based on the MSC showed a general increase from the ridge-top to the mid-slope, followed by a decrease to the slope toe (Fig. 92). This general trend can be observed on the main slope of Mount Mercer through aerial photography and in the field (Fig. 93). The agreement between the field study data and modeling results in terms of geomorphic features is additional evidence supporting rock slumping as the failure mode at the MSC. Subsurface exploration is the only way to establish the failure mode with complete confidence; however, this study has illustrated that numerical modelling can provide some useful insights into these data limited situations.

Of the three failure mechanisms discussed in this chapter, toppling is the most commonly applied to natural slopes in British Columbia. Based on the current numerical modelling and previous studies mentioned above, the geometric and geomorphic of characteristics of these failure mechanisms have be described in significant detail. A summary comparison of these failure mechanisms is presented in Table 12. If an analysis of a slope is attempted relying only on simple stereographic methods and surface observations, any of these failure mechanisms could seem plausible; the choice is only limited by the workers' familiarity with common rock slope failure mechanisms.

When examining natural slopes on the scale of those considered here, stereonet kinematic analysis techniques are insufficient alone to determine the possible complex nature of block interactions within the slope. Failure modes originally developed for the analysis of engineered slopes may not be appropriate for direct application to large scale, complex natural slopes due to the assumption of discontinuity persistence required by each failure mechanism. As each of these three failure mechanisms was first defined for engineering slopes of linear slope profiles and limited height, care must be taken when attributing certain of these mechanisms to natural slopes, which can be 1000 meters high and have parabolic profiles reflecting a history of glacial erosion. The significant discontinuity persistence assumed by these failure mechanisms (and demonstrated in the models of the current and previous studies) requires special attention. Requiring discontinuities to be hundreds of meters in length may be reasonable for certain types (faults, shear zones); however, joints are commonly much less persistent. Joints may "link-up" through a step-path mechanism (Jennings, 1970) (Fig. 94) resulting in equivalent structures (Karzulovic, 2004). In some of the more simple numerical models presented in this study represent these equivalent structures act as a single discontinuity. The abstraction can be reasonable, however, more detailed investigation is required to assess whether the expectation of the formation of "persistent" equivalent structures required to facilitate certain failure modes is plausible.

The importance of groundwater conditions has not been explored in this modelling study; however, some general comments can be made. In cases where the simulations have required friction angles lower than typically expected for unaltered discontinuity surfaces groundwater may be an important factor in allowing displacements to occur. By reducing the strength 1 resistance of the discontinuities involved in the development of a particular failure mechanism, the water pressures may play a controlling role in the rate and location of failure. Further characterization of fracture fluid flow in rock slopes and its relationship to particular rock slope failure modes is clearly required. Ridge line downslope lirn~tof failure I I A Simulation lb 0 Simulation 2b

@ SiuWon 3b

slope pmfik distance (metres)

Figure 92: Simulated scarp heights increase from the ridge-top to a maximum at the mid- slope and decease toward the slope toe.

Figure 93: Cross section (FC 1 and FC 2) created during the field study of Mount Mercer, British Columbia illustrating normal scarp heights at the ridge-top and upper slope of the MSC. Table 12: A summary of the geometric and geomorphic characteristics of rock slumping, toppling, and bi-planar failure mechanisms. Mechanism Kinematic requirements Morphology Assumptions Steeply dipping cataclinal 1 dipping cataclinal Well defined, discontinuities must be discontinuity set that possibly persistent, resulting in defines columns. Dip compound, columns with a low base: angle should be greater head-scarp. height ratio. than the friction angle. rn Grabens and A (semi)continuous basal tension cracks failure plane is required. 1 cataclinal diswntinuity at Ibehind Friction angles within 5" - set that defines a basal head-scarp. 10" of the basal plane dip sliding surface. The dip Anti-scarps on are required. angle should be less than the upper slope. All columns are capable of the friction angle. Benched mid slumping individually due to slope. Steep their unstable geometry. All discontinuities sets toe. Failure proceeds upslope should strike roughly rn Internal dilation, from the slope toe. parallel to the slope face. development of Maximum horizontal "A"-frame voids. displacement occurs at the base of the columns.

1 steeply dipping Steeply dipping cataclinal anaclinal discontinuity set discontinuities must be that defines columns. Obsequent persistent, resulting in scarps columns with a low base: All discontinuities sets throughout height ratio. should strike roughly failure. All blocks near the slope parallel to the slope face. rn Head-scarp is toe must be able to topple, NOT expected or slide when driven by The slope angle should for toppling toppling block above. be greater than the sum failures. A failure plane must exist of the friction angle on the Tension or develop at depth for anaclinal discontinuities complete detachment of the and the normal to the cracking at failure mass. discontinuities (slope upslope failure limit. In practice, steep and angle > friction angle (90 regular slopes are required. - discontinuity angle)) Maximum horizontal displacement occurs at the tops of the columns. rn Well defined 1 cataclinal diswntinuity head-scarp; anti-scarps, set dipping steeper than Failure occurs as the upper tension cracks its friction angle. zone transfers force to the and deformation lower zone, driving it along features on 1 cataclinal discontinuity the base plane. upper slope. set that defines a basal The steeply dipping 0 Bulging of mid sliding surface and dips diswntinuity and shallow at an angle less than its and lower slope; dipping discontinuity must be friction angle. grabens and continuous and intersect. tension cracks Force is transferred behind head- All discontinuity sets efficiently to the lower zone. should strike roughly scarp. parallel to the slope face. rn Internal dilation Rock slumping

Active I Passive (BCplanar)

Figure 94: Equivalent step-path structures formed by a combination of joints, required for the application of toppling, rock slumping, and bi-planar failure mechanisms to large, complex slopes.

5.6 Summary

Through numerical modeling, it has been demonstrated that the failure modes proposed in Chapter 4 based on data from the "slab failure" and the "main scarp complex" (MSC) can result in sackung landforms similar to those observed at Mount Mercer, British Columbia. In addition, the importance of modeling the interactions of several joint sets when attempting to determine the failure mode has been highlighted. The analysis of extensive natural slope deformations requires a combination of techniques in order to arrive at a reasonable explanation of observed displacements. Finally, alternative failure modes which can arguably better explain a range of the landforms associated with natural slope failures than toppling alone have been described in detail and demonstrated though numerical modelling. CHAPTER 6 CONCLUSIONS AND SUMMARY

6.1 Conclusions

An integrated approach is necessary when attempting to analyze complex, natural rock slopes. Tools and techniques from geomorphology, engineering geology, and rock mechanics are required to deal with these commonly data limited problems. Data deficiencies for one analysis method may be reduced by the application of other techniques. A lack of subsurface data may severely hinder the establishment of the failure mechanism of a specific landslide; however, detailed analysis of the landslide geomorphology and conceptual modelling may still produce useful insights.

The large scale of natural slopes associated with the formation of sackung makes the application of routine engineering geological techniques challenging. Kinematic analysis, through stereonet methods, of slope blocks defined by discontinuities observed in the field can lead to implausible results if detailed information regarding the persistence of discontinuities and the size 1 shape of the blocks within the slope are not considered. Three simple mechanisms of failure are well defined for stereonet analysis: sliding, wedge, toppling. None of these mechanisms are based on large, natural rock slopes; they are instead derived from the analysis of rock slopes in road cuts and open pit mine slopes. Neither rock slumping or active / passive failures are considered in conventional stereographic kinematic analysis and hence have not been adequately investigated in sackung deformation in British Columbia.

As the ratio of block size to the slope height can have a significant affect on the mechanism of failure (Hencher et a]., 1996), filtering techniques are required to allow realistic kinematic mechanisms to be suggested. Kinematic analysis may suggest a simple failure mechanism (ex. "wedge failure"); however, the discontinuity persistence would determine the probability of a specific failure mechanism responsible for instability of the entire slope.

Only in the last few years have open pit depths approached the height of natural slopes (Karzulovic, 2004) considered in the current study and other studies of sackungen. These simple kinematic mechanisms may not readily scale to the number of block interactions suggested by the sizes of the natural slopes associated with sackungen. The stress conditions within these natural slopes, as shown in Chapter 2, must be considered as a significant control on deformation. Deformations in natural slopes are more likely to require complex failure modes.

Sackung landforms can provide a significant amount of information about possible failure modes. However, the landforms classified as sackung are generic and can be produced by several geomorphic processes. Possible sackung landforms should be investigated as a collection and the relationships between processes responsible for each type of landform should be considered. For example, sets of repeating obsequent scarps may indicate a different failure mechanism than a well defined normal scarp at the top of a slope adjacent to obsequent scarps and slope benches down slope. Even in cases where gravitational deformation can be linked to the development of alpine linears, the deformation mechanism maybe be unclear. The current work has demonstrated two failure mechanisms (active - passive and toppling) that can result in obsequent scarps distributed throughout the upper part of a failure. Certain landforms are not expected for all failure mechanisms. For example this study, as well as several others (Hencher et al., 1996; Nichol et al., 2002; Pritchard and Savigny, 1991), has demonstrated that active failures dominated by toppling do not produce well defined head-scarps. This work, extending the work of Kieffer (1 998), indicates that toppling, rock slumping, and active 1 passive should all be considered as candidate mechanisms in future studies. To advance the science of stability analysis of natural slopes, current researchers must familiarize themselves with a broader range of failure mechanisms (including the geometric requirements and geomorphic results) and increase the quality and quantity of data collected during research programs.

6.2 Summary

The current work has provided:

1. An overview of useful techniques (both new and existing) and their application in the study of alpine linear features.

2. An integrated approach involving geomorphology, engineering geology, and rock mechanics through a regional analysis and a case study. Additional insights into the geomorphic processes and tectonic predesign factors that may result in alpine linears through gravitational deformation have been provided.

To facilitate work on landslides occumng on complex, natural slopes a set of integrated tools and techniques has been established through the "landslide research toolbox." The combination of digital data collection, GIs and numerical modelling provides a powefil tool set that can be applied to geomorphic and engineering geoIogical field studies. By applying the system presented here, the quality of collected data can be improved and additional data can be created by combining existing data sets. These techniques result in improved conceptual models for landform development by maximizing the utility of all the available data.

The process of gravitation deformation or sackungen ("sagging") has been suggested for several slopes in South Western British Columbia, based on the recognition of landforms such as scarps, anti-scarps, trenches, and tension cracks. Through examining the interactions of factors important in rock mass deformation (rock mass quality, stress, geometry) resulting in particular suites of the above landforms, inferences about the mode of gravitational deformation can be made. Knowledge of the mode of deformation then allows for further analysis regarding the hazards associated with the deforming slope.

As a precursor to a field study, the interactions between stress, rock mass quality and ridge geometry were examined through a finite difference analysis. The results of the analysis illustrate that the different ridge types encountered in the northern Cordillera each have unique stress distributions, based on overall ridge morphology. These stress distributions affect the possible deformation modes occurring in the rock mass or the ridges. This type of preliminary stress analysis is useful as it provides initial information on the location of possible zones of failure. The potential benefits of such preliminary stress analyses in future surface and sub- surface rock slope investigations are significant. The geometry of a slope can have a critical influence on the potential failure mechanism and should not be over looked in designing site investigations.

A field study of sackung at Mt. Mercer, British Columbia demonstrated the usefulness of the "landslide research toolbox" and the complexity of processes possibly forming hears along the upper slope and ridgeline. Linears formed by gravitational deformation were linked to two separate landslides. The "slab failure" appears to be an active / passive failure in the poor quality bedrock of the Cultus formation. The "main scarp complex" is possibly forming through rock slumping or a related failure mode occurring on the main slope of Mt. Mercer. The conceptual models suggested for the failures are supported by geomorphic and geological evidence. Uncertainty in the exact failure mechanisms must remain as this study, like many similar sackung slopes, lacks borehole data.

To expIore the viability of the conceptual models suggested in this current study for sackung formation at Mt. Mercer, British Columbia, distinct element modelling was undertaken. Models of the interactions of blocks defined by discontinuities measured in the field study illustrated that the failure modes ("active / passive two wedge failure" and "rock slumping") suggested by the geomorphic and geological field evidence were plausible. The general simulation results indicate that the two conceptual models may be applicable to other slopes where the geometric and geomorphic requirements of rock slumping and active-passive failure are met.

6.3 Recommendations for future work

Several avenues for continuing work have been established from this study:

1. Examine additional large natural rock slopes to evaluate the potential relevance of the rock slumping conceptual model. Several possibilities candidates are available within British Columbia (Bovis and Evans, 1996).

2. Combine results from the analysis of stress distributions in ridges (Chapter 2) with discrete models of sackung forming failure mechanisms (Chapter 5) to investigate the effects of ridge morphology on failure mechanism.

3. Characterize the flow of groundwater through fracture rocks in mountain environments for glaciated, post-glaciated and present conditions and investigate how these flows interact with particular rock slope failure modes.

4. Further research into failure mechanisms appropriate for large slopes through a combined geomorphology / engineering geology approach. What is the relationship between slope size / block size / failure mode? An online survey to collect observations from other professionals and researcher should be undertaken to quantify geometric relationships that predetermine failure mechanisms. Existing landslide databases should also be reviewed.

5. Test conceptual failure mechanism models presented in the current against sub-surface data from borehole drilling or geophysical methods.

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Appendix I:Derivation of rock material properties for geomechanical modelling

Introduction

Numerical simulations of rock slopes require material properties that are representative of the rock mass being modelled. Intact rock properties, from lab testing, need to be scaled according to observations of weathering /alteration and jointing / damage of the rock mass being studied. GSI (geological strength index) provides a method of scaling intact material properties described by the Hoek-Brown failure criterion. The derivation of these values (Fig. A) is automated through the use of RocLab (Rocscience, 2003).

The geomechanical modelling software packages used in this study do not directly support the use of the Hoek-Brown failure criterion in describing rock mass properties. A conversion from the properties produced by RocLab to those used in both FLAC 4.0 and UDEC 3.1 (HCItasca, 2002) is required. The method for deriving rock mass properties is applied to numerical simulations in Chapters 2 and 5. Convert E to Kend G

Figure A. Derivation of numerical modelling properties workflow.

Hoek-Brown failure criterion

A detailed description of the history and development is provided by Hoek (2002). Originally developed from lab testing for underground purposes, the practical application of the Hoek-Brown failure criterion was realized by combining it with RMR (Bienawski, 1974). This combination allowed parameters to be estimated from field observations. The original criterion was biased toward hard rock and that failure was controlled by translation and rotation of rock blocks defined by a chaotic jointing pattern. The original equation for the Hoek-Brown criterion is adapted from an equation used to determine concrete failure.

Additional developments in the application of the Hoek-Brown failure criterion included the ability to translate the m and s parameters to the Mohr-Coulomb parameters c and 4. Of significant importance in recent years has been the replacement of RMR with GSI (Geological strength index) for estimating Hoek-Brown failure criterion parameters from geological observations.

Input parameters

Eq. 1 a: General Hoek - Brown Failure Criterion (Hoek et al., 2002)

Where:

o;= Major principal effective stress

o;= Minor principal effective stress

oCi= Uniaxial compressive strength of the intact rock

mb= Frictional properties of the rock mass

s = Coefficient reflecting jointing in the rock mass

a = Coefficient of curvature of the Hoek - Brown failure envelope

GSI = Geological Strength Index

Eq. 2: Jointing coefficient

GSI - 100 s = exp( 9-30 )

Eq. 3: Friction properties of the rock mass mb= miexp (Z-Z')

Eq. 4: Coefficient of curvature of the Hoek - Brown failure envelope

Where: mi= Frictional properties of the intact rock, D = Disturbance value of rock mass, a = Coefficient of curvature of the Hoek - Brown failure envelope, GSI = Geological Strength Index GSI

The Geological Strength Index (GSI) provides a method of translating qualitative observations into quantities for use in engineering analysis and design projects (Figure B). Working from a table that has been modified over several iterations, field workers are able to estimate a GSI value (ranging from 1 to 100) by observing the structure 1jointing intensity of the rock mass and the condition (weathering, alteration) of those joint surfaces. Practioners are encouraged to not give exact GSI values; rather a range of GSI for a rock mass is considered most appropriate.

1 SURFACE CONDITIONS

Figure 6.Chart for determining GSI value from field observation of rock masses. Source: Hoek, 2003, by permission. Derivation of properties

From the combination of the Hoek-Brown criterion, GSI, intact rock properties (UCS, mi), and problem geometry a set of rock mass properties can be generated for analysis and modelling. Outputs describing the rock mass include:

Hoek-Brown criterion parameters: Mohr-Coulomb parameters: .mb cohesion S friction angle (angle of internal a friction)

Rock Mass parameters: tensile strength uniaxial compressive strength (to determine fracture propagation) global strength (for the entire rock mass) modulus of deformation

Conversion from RocLab outputs to Mohr-Coulomb failure criterion

RocLab can be used to rapidly generate the above outputs. However, for use in the geotechnical modelling packages used in this thesis further modifications were required. Models in both FLAC and UDEC were run using either the Mohr-Coulomb criterion or the Elastic criterion to describe the rock mass properties. In addition to the Mohr-Coulomb outputs provided by rock lab, the bulk modulus and shear modulus are required. To generate the appropriate bulk and shear modulus values for the generated deformation modulus produced by RocLab, some assumptions are required.

Standard equations exist for converting Young's modulus (or the elastic modulus) with Poisson's ratio to values of bulk modulus and shear modulus:

Eq. 4: Eq. 5:

where K is the bulk modulus, G is the shear modulus, E is Young's modulus and vis Poisson's ratio. The first assumption required to use these equations is that Young's modulus can be replaced with the deformation modulus calculated from the Hoek-Brown criterion. Young's modulus describes the relationship between stress and strain for a material. It is typically determined from loading tests on intact laboratory samples of rock. The modulus of deformation describes the stress 1 strain relationship for a rock mass. As these two concepts are analogous, the suggested replacement seems reasonable. The resulting bulk and shear moduli should represent the rock mass.

The second assumption required is a value of Poisson's ratio for the rock mass. Usually determined from uniaxial or triaxial testing of laboratory samples wired with strain gauge rosettes, Poisson' ratio describes the relationship of axial strain to lateral strain under some applied stress field.

Eq. 6: Eq. 7: Eq. 8:

where:

v = Poisson's ratio, raria,= strain parallel to long axis of the sample, r,,,,, = strain perpendicular to the long axis of the sample, A I = change in length of the sample, Ad = change in diameter of the sample

Guidance in appropriate Poisson's ratio values to use in the conversion is available. A review of published combinations of Young's modulus and Poisson's ratio illustrates that a significant range of Poisson's ratio values can exist for a given value of Young's modulus. It is not clear if which of these values can be used in conjunction with the deformation modulus. Additional suggestions can be found in Hoek (2000). Examples of typical rock mass properties for very good quality, average, and poor quaIity (based on GSI value) rock masses are provided. An increase in the Poisson's ratio from 0.2 to 0.25 to 0.3 is estimated for deformation moduli of 42 GPa, 9 GPa, and 1.4 GPa respectively. The trend illustrates that a reduction in the deformation modulus results in an increase in the Poisson's ratio. These examples are from the analysis of rock masses around underground openings. These values do fall within the range of published combinations of Young's modulus and Poisson's ratio. Engineering judgement combined with this information should allow reasonable estimates of Poisson's ratio based on the deformation modulus to be produced.

Once the shear and bulk moduli values have been calculated, the properties required representing the rock mass using the Mohr-Coulomb model in FLAC and UDEC can be satisfied. Properties used for individual models are presented in the associated thesis chapters. The above methodology should produce rock mass properties with reasonable values. A simple check using the Mohr-Coulomb properties derived from RocLab 1.0 can be used to check if the combination of derived cohesion, tensile strength and internal friction angle are possible:

Based on the Mohr-Coulomb failure criterion: If:

T 2 Lax

Where T is the tensile strength of the material and:

C T =- max tan 4 Then the properties are admissible

Where T,,,, is the maximum tensile strength defined by the slope of the Mohr-Coulomb failure envelop, c is cohesion or the 'y7-intercept of the Mohr-Coulomb failure envelope, and 4 is the internal angle of friction or the angle between the Mohr-Coulomb failure envelope and the horizontal. Appendix 2: SFU-Geotech data model

An example implementation of the SFU-Geotech data model is provided in Microsoft Access format (.MDB) on the accompanying CD-Rom at:

In addition to the desktop database, pre-made data forms and lookup lists for use with Pendragon Forms (versions 3.x and 4.x) are available on the accompanying CD-Rom at:

These implementations fall under the same copyrights as this thesis document. Users of these implementations should cite this thesis as the source.

Appendix 3: Field and GIs data

Collected field data has been included in Microsoft Access format (.MDB) and can be access on the accompanying CD-Rom at:

GIs data layers created from field data and paper maps (as well as associated meta-data) is provided in ESRI shape (SHP) format and can be access on the accompanying CD-Rom at: Appendix 4: Simulation files

Input files (.DAT), results (.SAV) and images for numerical models built and run in FLAC and UDEC have been included for future reference.

FLAC

FLAC model data files and save file for the stress analysis of ridges presented in chapter 2 can be found on the accompanying CD-Rom at:

UDEC

UDEC model data files and save file for block slumping and active / passive failure presented in chapter 5 can be found on the accompanying CD-Rom at: