Slope Paper 119

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Slope Stability 2011: International Symposium on Rock Slope Stability in Open Pit Mining and Civil Engineering,

Vancouver, Canada (September 18-21, 2011)

Combining Geology, Morphology and 3D Modelling to Understand the

Rock Fall Distribution Along the Railways in the Fraser River Valley,

Between Hope and Boston Bar, B.C.

R. Macciotta Civil and Environmental Engineering, University of Alberta, Edmonton, Canada

D.M. Cruden Civil and Environmental Engineering, University of Alberta, Edmonton, Canada

C.D. Martin Civil and Environmental Engineering, University of Alberta, Edmonton, Canada

N.R. Morgenstern Civil and Environmental Engineering, University of Alberta, Edmonton, Canada

Abstract

Railways across the Canadian Cordillera have long histories of losses caused by ground hazards, rock falls

being the most frequent events. Effective use of resources for rock fall risk mitigation requires understanding of

the rock block detachment and motion processes. Analysing the physical settings, event records, and modeling

tools to study dynamic behaviour are essential. Rock fall Analyst is a rock fall process modeling extension in 3D

for geographical information systems software that can be used to gain information on the rock fall dynamic

behaviour. The railways along the Fraser Rivers west bank between Hope and Boston Bar is a study area

illustrating these concepts.

1 Introduction

Rock falls are frequent hazards in transportation corridors cutting through mountainous areas (Bunce et al. 1997,

Evans & Hungr 1993, Hungr et al. 1999, Hoek 2007, Dorren 2003). In particular, railways across the Canadian

Cordilllera have long histories of losses caused by ground hazards, rock falls being the most frequent events

(Hungr et al. 1999, Lan et al. 2010). A rock fall is defined as the detachment of one or more blocks from a steep

slope and its descent mainly by falling through the air, bouncing and rolling (Cruden & Varnes 1996, Bjerrum &

Jrstad 1968). Even though these events have small volumes compared to other slope processes (Whalley 1984)

their frequency and rapidity make them threats to the railways operating in the Cordillera.

Rock fall studies, like all geotechnical-related problems, are characterized by the uncertainties. Quantitative

prediction of behaviour in such problems, even under ideal circumstances, may not be reliable (Morgenstern

2000). As such, when these problems potentially endanger society, engineers (in a conscious or unconscious

manner) use some form of risk assessment to evaluate the hazard (Fell et al. 2005, Macciotta et al. 2010).

Formal risk assessment procedures create a framework for the assessment of these hazards and focus engineering

efforts and expenditures on the highest risk areas (Pine & Roberds, 2005). Bunce et al. (1997) showed how rock

fall risks along transportation corridors could be quantitatively assessed at specific locations. However, when

entire transportation corridor sections are to be assessed qualitative risk assessments are often employed when

evaluating mitigation strategies.

Due to the difficulties associated with predicting rock fall location, timing, volume and motion (Hantz et al.

2003, Dorren 2003, Azzoni & De Freitas 1995), mitigation strategies are mostly reactive, providing protection or

avoiding the hazard after an event has occurred. To proactively mitigate the risks of rock falls, regularinspections are usually employed. These inspections are a form of qualitative risk assessment. To improve this

qualitative risk assessment and effectively use the resources available for rock fall risk mitigation, understanding

the processes leading to rock block detachment and the factors affecting their motion is essential. This requires


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the analysis of the physical settings, the records of events in the area, and the use of modeling tools to study

dynamic behaviour.

This paper presents an analysis of the geological and morphological setting of an important transportation

corridor along the Canadian Cordillera, and its relation to the observed rock fall distribution. Three dimensional

rock fall modelling is applied at a specific location to illustrate its use to further understand the rock fall

distribution and observed concentration of events.

2

Study area

The Canadian Pacific Railway Cascade subdivision is located in the Canadian Cordillera, along the Fraser River

valley, South-west British Columbia. Both the Canadian Pacific Railway (CP) and the Canadian National

Railway (CN) utilize this corridor. The section between miles 0 and 40 of the CP Cascade subdivision (along the

west riverbank) was analyzed because of its history of event records dating back to the 1940's. These records

include date, location and volume of the events, as well as the probable source height, weather conditions and

any site observations considered relevant by the inspector. Figure 1 shows the location of the study area as well

as the mileage along CP's track.

Figure 1. Study area and slope stability problems along the CP's Cascade subdivision. (a) steep cut along mile

2.9, typical of the section between Boston Bar and Yale. Scar covered with shotcrete and debris

from previous events are evident. (b) typical rock fall encountered during field assessment along

mile 14.6. (c) unstable blocks along mile 5.7. (CP personal communication).


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Instabilities documented along this section include rock, soil and snow falls, where rock falls account for more

than 80% of the records. Figure 1(a) shows a typical slope cut along CP's Cascade subdivision. The steepness of

the section of the Fraser River canyon between Boston Bar and Yale required steep slope cuts through

tectonically altered rock in order to accommodate the track. Figure 1 also shows the scar and deposits of a

previous event (a), the hazards of rock blocks coming to rest along the track (b) and potential future events (c).

2.1

Geological setting

From Boston Bar to Hope, the Fraser River lies along the junction between the Coast Mountains and the Cascade

Mountains. Several orogenic episodes took place in this area which involved folding and faulting, different

levels of metamorphism and intrusion (McTaggart & Thompson 1967). This intense deformation during the mid-

Cretaceous to Early Tertiary time resulted in complexly folded rock mass cut by north-south trending faults

(Monger 1970). These faults are associated with broad zones of weak materials and differential weathering,

glacial scouring and river erosion (Piteau 1977).

The Hope and Yale Faults together with many other major cross faults to the east and to the north of the studyarea comprise the heavily faulted system known as the Fraser River fault zone (Piteau 1977) and are illustrated

in Figure 2. It appears that the latest movements of the steeply dipping Hope and Yale faults were dip-slip, and

that the Fraser River Fault zone might have been active intermittently from Early Cretaceous to relatively recent

times (McTaggart & Thompson 1967). According to Monger (1970), there was an episode, in the EarlyTertiary, of reverse and strike-slip faulting concentrated along these zones. The latest movement of the Hope

Fault was probably in the late Eocene. A study by Coleman and Parrish (1991) describes a Tertiary dextral

strike-slip fault system. The Fraser River fault zone is considered inactive (Trettin 1961, Wernicke et al. 1987).

The complex geological context is evidenced in the various lithologic units present in the study area. For

convenience, these have been grouped in rock units of similar lithological characteristics in Figure 2. The

Sedimentary unit 1 consists of chert, pelite, limestone, argillite and slates (B.C. Ministry of Energy 2005). Chert

is probably the most abundant, with a high percentage of ribbon chert layers separated by slate or argillite.

Massive and thick-bedded chert can be found in some areas. Limestone occurs as isolated beds continuously

interbedded with weakly metamorphosed volcanic rocks. Pronounced metamorphism is observed at and near the

contact with the Metamorphic unit 1, where strong shearing is observed. Rock masses are locally broken and

closely jointed in some areas (McTaggart & Thompson 1967).

Metamorphic unit 1 is also known as "Custer Gneiss". It mainly comprises granite gneiss with abundant

pegmatite dikes, pelitic schist and amphibolite (B.C. Ministry of Energy 2005). North of Hope, this unit is

closely jointed, faulted and mylonitized from pervasive shearing so that the rock is rather friable (Monger 1970).

Rock masses at and near contacts with other units are highly sheared and deformed, and folding trends are

mainly northwest (McTaggart & Thompson 1967). The Metamorphic unit 2 mainly consists of pelitic schists

local amphibolite, minor ultramafic rocks and siliceous schist (B.C. Ministry of Energy 2005). These rocks have

been subjected to repeated folding, apparently in a nearly constant direction (northwest). Foliation and bedding

are parallel and disposed in open or recumbent and overturned folds (McTaggart & Thompson 1967, Monger

1970).

Intrusive rock unit is comprised of granodiorites, diorites, quartz diorites and granites; sheared, with different

grades of foliation, pale gray to white colour. They show great variety due to variation in their original

composition or texture, and due to the various degrees of shearing and alteration. Intense shearing is observed atand near the contacts with other units and major structures (McTaggart & Thompson 1967, Monger 1970). The

Sedimentary unit 2 is the earliest within the study area (Eocene). It consists of sandstones, conglomerate and

argillite (B.C. Ministry of Energy 2005).

Fault breccia is considered in this study as a distinct lithology from an engineering perspective. It consists of

intensively sheared material associated with the faults in the area and could have a marked influence in the rock

fall events frequency where the railway alignment approaches the fault traces.


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Figure 2. Rock units and faults in the study area, (with information from B.C. Ministry of Energy 2005).

2.2 Geomorphologic setting

The study area has been glaciated with the highest peaks staying above the ice. Several of these peaks are horns,

and cirques are common. Lower mountains stood below the Pleistocene ice sheet and are rounded. Valleys were

glaciated and are generally characterized by U-shaped cross sections and hanging side valleys and most drainage

is structurally controlled (Monger 1970).

The first 27 miles of the study area (roughly between Boston Bar and Yale), has a very constricted, steeply U-shaped section; with a river gradient approaching 1.5 m per km (0.15%), and with steep side slopes where rock

benches are common. The bottom of the valley has been notched to a depth of about 30 m by stream action,

resulting in a small V-shaped cut in the base of the glacial scoured surface. This has lead to a succession of

constrictions within a larger canyon where the river velocity increases significantly. The slopes in the area are

generally sparsely forested, steep and irregular. Common features are steep scarp faces, bare rock buttresses and

spurs, as well as numerous sharp bends and curves. These, together with well-developed alluvial fans, have led

to significant lateral erosion by the river, resulting in toe unloading and steepening of the valley slopes. Evidence


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of major post glacial or inter glacial slides is observed between miles 6 and 9, and between miles 23 and 26

which probably caused blockage of the river (Piteau 1977).

In the southerly 13 miles between Yale and Hope, the valley widens and the river gradient averages about 0.7 m

per km (0.07%). In this section the valley is broader and with densely forested slopes. A reason for the valley

section change at Yale is the change in the rock mass at this location (Figure 2). Rocks between Yale and Hope

(Metamorphic unit 1) would have been more easily scoured and excavated by glacial action than the slope

forming material immediately upstream, thus resulting in a wider section. Alluvial fans in the 40 mile section

can be regarded as static or dormant, thus the rate of lateral river erosion is not likely to increase with time,

however, the lateral river erosion opposite to alluvial fans will continue to take place (Piteau 1977).

A topographic break consisting of north-trending hills with relatively low elevations when compared to the

surrounding mountains is present on the west side of the river valley. These antiforms clearly follow the

alignments of both the Hope and Yale Faults, between miles 16.5 and 40 and are a consequence of their

presence. Analyzing the elevation of the drainage divide adjacent to the CP track, immediately to the west

(Figure 3), shows that the presence of these hills limit the height of the potentially unstable slopes. The

occurrence of the Hope and Yale faults (and associated orogenies) not only provides the context of sheared and

altered rock units, but also influences the geometry of the slopes adjacent to the track. Typical sections along the

40 miles show average slope angles between 27 and 32 degrees, with most of them between 24 and 34 degrees

(except in areas where tributary creeks discharge to the Fraser River from the west). From about mile 27 at Yale,the slopes change their constantly steep configuration, to become gentler as they approach the track (see Figures

3 and 4). This is consistent with the description of this section of the Canyon provided by Piteau (1977). The

approximate location of this slope change coincides with a change in the rock unit comprising the slope, from a

stronger, less sheared, Intrusive-rock unit; to the highly altered Metamorphic unit 1.

Given the river gradients and typical cross sections, it is believed that the slopes in the first 27 miles are at an

active state of river erosion. The slopes in the southern 13 miles are believed to be in an abandoned state of

erosion, and also show a slower rate of mass wasting, with less frequent events, as they approach their ultimate

(stable) state.

3 Spatial distribution of events

CP rock fall records in this section include the event volume and location. Recorded events include blocks

landing on the tracks and blocks caught within the ditches or behind protective walls. The event recording

standards have evolved, where smaller events or events with no consequences were not consistently recorded in

the early years. So, the entire set of records is not statistically uniform for the full time range they were kept.

However, this is not considered to greatly influence their spatial distribution. The authors assume for this study

that recording standards were consistent along the entire section for any given year.

Figure 3. Drainage divide elevation immediately adjacent to CP track and typical cross sections of the canyon


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Figure 4. Typical sections along the study area (west side).

The spatial distribution of events along the 40 miles is presented in Figure 5. Events are concentrated in the first

15 miles with the exception of miles 19 to 20, 24 to 26 and, 38 to 39. Also, the events are not evenly distributed,

but concentrated at certain locations (peaks). It is believed the spatial distribution of events corresponds to

differences in general settings (i.e. lithology and geomorphologic process) and each location's particular

characteristics (i.e. slope steepness, vegetation, previous protective works).

Previous studies of this section by Piteau (1977), focused on CN's track located on east river bank. This work

was updated by C.H. Lim in 2007 (personal communication) who focused on CP's track on the west river bank.

These studies suggested that river erosion opposite to alluvial fans, river bends and postglacial landslides, were

important factors influencing the spatial distribution of the rock fall events. The data also showed that more than

80% of the recorded events along the west river bank originated from slope heights of less than 50m. Figure 5

shows that the distribution of events is generally associated with average slope angles of the initial 50m from the

track exceeding a threshold between 20 and 30 degrees.

Figure 5. Rock fall spatial distribution and average slope angle of the initial 50m from the track, west river

bank.


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Figure 6 presents the relation between the event distribution and the geologic-geomorphologic setting. The

presence of hard intrusive rocks between Boston Bar and Yale (miles 2 through 27) led to a steep canyon after

glacial scouring. River erosion, mostly opposite to valley constrictions caused by alluvial fans, further steepened

some of these slopes. The river gradients and typical steep cross sections in this area indicate the slopes are

likely to be at an active state of river erosion, thus transitions to flatter slopes before the track alignment are

expected to be rare and steep cuts to be necessary to accommodate the rail alignment. However, the presence of

the Hope and Yale faults led to the formation of antiforms about half the height of nearby mountain peaks, whereglacial erosion would have reshaped them to rounder hills. This would explain the presence of softer reliefs

between miles 15 and 27, limiting the potential source areas when compared to miles 2 through 15.

Between miles 27 and 40, the valley widens when compared to the previous section. This corresponds to a

change in lithology to a metamorphosed, sheared unit, where glacial erosion and the presence of the antiforms

promoted a softer relief. It is worth noting that between miles 38 to 40 the relief is soft relative to the canyon

topography with the exception of a ridge, where the track is located. Here a steep cut is needed to accommodate

the railway alignment, being the likely cause for the concentration of events.

Events have been recorded along mileage 28 and 33, where the average slope angle of the initial 50m from the

track does not exceed 20 degrees. The railway alignment at mile 28 was accommodated by a steep cut on altered

rock. This cut slope, about 15 m high, sits below a gentler slope. Our analysis is based on the slope angle of the

initial 50m from the track and conditions at mile 28 gave an average slope less than that observed. Mile 33,however, does not show any apparent conditioning for the occurrence of rock falls. It has to be noted that only

one event was recorded in 1957 at mile 33, where a rock block was found lying on the track.

Figure 6. Event distribution and geologic-geomorphologic setting. Preventive and protective work completed

is also shown.


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4 Three dimensional modelling of rock falls

Modelling of rock fall trajectories, and in particular three dimensional modelling, can aid in the design of

protective measures (Giani et al. 2004, Dorren 2003), and increase our knowledge on the factors affecting the

spatial distribution of the events (Agliardi & Crosta 2003, Monnet et al. 2010). A section of the study area

between miles 6.7 and 8 was selected to illustrate the influence of topography on the rock fall trajectories when

modelled in three dimensions and the importance of the slope profile near the track in the spatial distribution ofevents. Modelling was done using Rockfall Analyst (RA), a three dimensional rock fall trajectory modelling

extension for ArcGIS software (Lan et al. 2007, 2010). The software output includes rock fall path, velocity,

height and energy for given sources and slope materials based on the particle kinematics (assuming parabolic

trajectory while flying and an equivalent frictional model while rolling). Energy loss is estimated by energy

restitution coefficients after each impact for rolling or sliding, depending on the slope materials.

Figure 7 presents some of the results obtained from the RA model for the section between miles 6.7 and 8. The

model considered values of restitution coefficients and friction angles used in previous studies in the area and

calibrated against historical records (Lan et al. 2007). These were taken for vegetated slopes with normal and

tangential restitution coefficients of 0.2 and 0.5 respectively and equivalent rolling friction angle of 30 degrees.The sources were estimated by analysing source locations containing sharp topographic contrast, slope steepness

and considering that most source areas recorded in CP rock fall event data base are located within the first 50 m

of elevation from the track. A plan view with the modelled rock fall distribution is shown in Figure 7 togetherwith a typical cross section of the rock fall path and the variation of the block velocity.

A comparison between the recorded and predicted event frequency is presented in Figure 7c. The model results

are in general agreement with the observations except for mileage 6.7, 7.3 and 8. At mileage 8 only one event

was recorded and the source for that event is uncertain. Mitigation work-records shows that at mileage 7.2 and

7.3 rock bolts and shotcrete were used extensively to stabilize the slope when compared to other locations.

Mileage 6.7 recorded substantially more rock fall events than predicted. This section is located at the discharge

of a small creek which could act as a chute for blocks detached from sources located at higher elevations than the

50 m source location used for these analyses.

In these analyses we assumed the same number of blocks are detached from each source location. Given the

same frequency of block detachment and the same restitution coefficients one would expect a uniform spatial

event distribution along the track. The observed agreement between the model and measured events obtainedusing a single model for a 2 km long section suggests that accounting for three-dimensional changes in

topography is likely the factor controlling the spatial distribution of events. Variations in the material covering

the slope and the associated energy restitution coefficients appear to play a minor role. The analyses were carried

out within the Intrusive rock unit, where cut slopes were used throughout the entire section to accommodate the

track alignment. These findings may not be applicable to the metamorphic or sedimentary units in the area.

Models are currently being developed for other track sections to check on the validity of these observations

within other rock units and slope-track configurations.

5 Summary and discussion

Methodologies for managing risks associated with rock falls along transportation corridors through mountainous

terrain are moving towards more proactive approaches rather than reacting after the event occurrence. Theauthors believe that, with the increasing capabilities in data storage and processing, this trend will lead to a wide

application of quantitative risk analysis for decision-making regarding mitigation measures. Routine inspections

and on site assessment of the slopes currently constitute a qualitative risk assessment, which is the basis for

resource allocation for mitigation works along transportation corridors. Understanding the factors affecting the

spatial distribution of events is considered essential to better allocate the resources available for risk mitigation

and to estimate the event spatial probabilities within a quantitative risk analysis framework.


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Figure 7. Rockfall Analyst model output for mileage 6.7 through 8. Plan view of the predicted rock fall paths

(a). Section view of a predicted rock fall trajectory and its velocity at mileage 7.1 (b). Comparison

between the predicted rock fall distribution and the distribution from CP recorded events.

The analysis of this section of the CP Cascade subdivision shows that the geological and morphological setting

determines the 40 mile scale distribution of events, while the site specific distribution (peaks) is influenced by

local three-dimensional topography immediately up-slope of the railway. The event distribution appears to

correlate to slopes angles above a threshold value between 20 and 30 degrees for the initial 50 m from the track.

This was observed regardless the geologic or morphologic context. However, geology and morphology appears

to determine the frequency of these steep slopes, within the 40 mile section. It is worth noting that a threshold

value between 20 and 30 degrees is consistent with the minimum rock fall shadow angle estimated at about

27.5o. This shadow angle is defined as the angle between the apex of the rock fall talus slope and the farthest

run-out boulder. Evans & Hungr (1993) reported that typical shadow angles range between 22oand 30o. They

argued that this shadow angle appears to be the lower limit of the rolling friction angle of large boulders overtalus slopes. This same concept could explain the absence of events at flatter slopes near the track. For these

slopes, the shadow angle is more likely to intercept the slope before reaching the track.

The model results are consistent with observations. It seems that three dimensional changes in topography islikely the factor controlling the spatial distribution of events for the geologic/geomorphologic context of the

modelled section. These results are encouraging for the use of three dimensional modelling to better allocate

resources for risk mitigation along transportation corridors. The influence the material cover has on the event

distribution is expected to increase with increasing distance to the source. Models are being developed in other


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sections of the study area within other rock units and slope-track configurations to understand the rock fall

behaviour in other lithologic and topographic context.

Acknowledgements

This work was supported by the Canadian Railway Ground Hazard Research Program, and the Natural Sciences

and Engineering Research Council of Canada

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