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Improvements to field and remote sensing methods for mapping discontinuity persistence and intact rock

bridges in rock slopes Zack Tuckey and Doug Stead

This paper was first published in Engineering Geology by Elsevier on 11 May 2016. Abstract Discontinuity persistence and intact rock bridges are critical factors that influence the stability of rock slopes. Persistence influences the extent of pre-existing potential failure surfaces, and intact rock bridges can add cohesion and tensile strength to an incipient failure surface, which must be destroyed before slope failure can occur. Despite continued research and improvements in mapping technology, computing power, and numerical modelling codes, there is no standard recommended methodology for field characterisation of intact rock bridges, or their incorporation into stability analysis. We use observations from remote sensing and field mapping investigations at three open pit mines and one natural rock slope to recommend improvements to field and remote sensing methods for mapping discontinuity persistence and intact rock bridges in rock slopes. We apply a fracture network engineering approach to discontinuity trace mapping, using new intensity factors to describe intact rock bridge trace intensity R21, and blast-induced fracture intensity B21.Weemphasize the importance of multi-scale characterisation of discontinuity persistence and rock bridges in rock slopes, and we distinguish between different classes of intact rock bridges based on scale and geometry, with particular attention given to the distinction between laboratory sample scale intact rock bridges on incipient discontinuities, and larger rock mass bridges, which represent metre or larger scale intervals of fractured rock mass that may exist between very high persistence joints and faults. 1. Introduction Slope failures in fractured rock masses frequently involve a component of shearing and dilation of pre-existing discontinuities, combined with brittle fracture of intact rock (Stead and Wolter, 2015). Persistence describes the size of discontinuities in a rock mass, and intact rock bridges represent intervals of intact rock between the tips or edges of adjacent discontinuities (ISRM, 1978). The shear strength of intact rock is typically two or more orders of magnitude greater than the shear strength of pre-existing discontinuity interfaces (Kemeny, 2005). Consequently, a small content of rock bridges (in the range of 1% to 3%), favourably distributed within the rock mass, can significantly increase the stability of a slope. Although the importance of intact rock bridges in slope stability has been qualitatively understood for decades, there are still no standard accepted methods for estimating the intact rock bridge content of a rock slope and incorporating intact rock bridges into slope stability analysis. Advances in computing power and numerical modelling codes have led to increasingly sophisticated research into complex slope failures, including explicit simulation of brittle fracture initiation, propagation, and coalescence using Discrete Fracture Networks (DFNs) and the Synthetic Rock Mass (SRM) methods (Mas Ivars et al., 2011). However, to date most numerical modelling studies of fractured rock slope failures have been limited to forensic investigations of past events or sensitivity studies, with little applicability for predictive modelling (Stead and Coggan, 2012). Technological advances have been unable to compensate for inability to reliably characterise the persistence of discontinuities and the configuration of the rock bridges that are hidden within the interior of a rock mass. We present field observations from three open pit mines (referred to as Mines A, B, and C, for anonymity) and one natural rock slope. We carried out discontinuity mapping at bench, inter-ramp and overall slope scales, using digital photogrammetry, LiDAR, and modified 2D window mapping. We used a fracture network engineering approach (after Pettitt et al., 2012) to characterise areal trace intensity of blasting induced damage and inferred intact rock bridges in bench-scale window maps. Based on our observations, we recommend improvements to field and remote sensing methods for mapping discontinuity persistence and intact rock bridges in rock slopes.

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2. Discontinuity persistence in slope stability Discontinuities in rock range in size from sub-millimetre microcracks to kilometre scale faults. Even laboratory scale specimens of intact rock with no visible discontinuities, contain a microfabric of grain scale flaws, cleavages, or cracks induced by geological processes such as cooling, tectonic deformation, and glaciation (Gudmundsson, 2011). If discontinuity persistence is large relative to slope height, then slope failure may be characterised by simple translation of massive blocks of rock mass along one or several adversely oriented discontinuities (Hoek and Diederichs, 2006). In contrast, if discontinuity persistence is small relative to overall slope height, then slope failure mechanisms are more likely to involve a complex combination of shearing and dilation of many low persistence discontinuities, and some brittle failure of intact rock (Sjöberg, 1999). Conceptual models of rock joints usually quantify persistence as the diameter or edge length of idealized circular or square discontinuities (Jennings, 1970). Discrete Fracture Network (DFN) research has explored alternative models for joint geometry and spatial distribution (Dershowitz, 1985), and for decades researchers have emphasized the need to validate statistical DFNsimulations against field mapping observations (Kulatilake et al., 1993). However, in practice most field mapping investigations simply approximate discontinuity persistence using the trace length of discontinuities intersected in scanline or window maps. Field studies have shown that trace length distributions tend to conform approximately to negative exponential, power law, or lognormal functions: kilometre scale discontinuities such as regional faults are exponentially less frequent than metre scale joints, which in turn are less frequent than microcracks (Ortega et al., 2006; Ortega and Marrett, 2000; Crosta, 1997; Priest, 1993; Baecher et al., 1977). The negative exponential distribution has also been shown to apply to discontinuity spacing (Priest and Hudson, 1976, 1981). Guidelines by the ISRM (1978) suggested five categories for discontinuity persistence, ranging from “very low” persistence for structures b1 m to “very high” persistence for discontinuities with trace length N20 m. For the sites investigated in this study, we estimated persistence using the length of discontinuity traces measured in field window maps, and also the diameter of best-fit circles, which were manually mapped to digital terrain models (DTMs) constructed from LiDAR and photogrammetry surveys. Field measurements of discontinuity persistence are subject to several important sources of bias and error, which can result in under- or over-estimation of persistence. Major sources of error are illustrated in Fig. 1 and summarised below:

Fig. 1. Major sources of bias and error in discontinuity trace length surveys.

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• Truncation occurs when discontinuities are too small or too short to measure (Baecher and Lanney,

1978; Zhang and Einstein, 1998; Sturzenegger et al., 2011). • Censoring occurs when discontinuity length extends past the sampling region. (Baecher and Lanney,

1978; Priest, 1993). • Orientation Bias occurs when boreholes, scanlines or windows are sub-parallel to discontinuities

(Terzaghi, 1965; Sturzenegger et al., 2007; Lato et al., 2009). • Length bias occurs because longer discontinuity traces have increased likelihood of intersecting a

scanline (Baecher and Lanney, 1978; Zhang and Einstein, 2000; Sturzenegger et al., 2011). • Scale bias occurs in remote sensing surveys, and arises from limitations in image resolution. Higher-

resolution imagery allows for smaller discontinuities to be distinguished, whereas lower-resolution imagery may render smaller features indistinguishable (Ortega et al., 2006; Sturzenegger and Stead, 2009).

• F-bias describes how discontinuity traces can be approximated as chords formed by the intersection of an approximately circular discontinuity plane with a sampling window. Unless a discontinuity intersects an outcrop precisely along the diameter, the chord length will always be less than the maximum dimension (Priest, 2004).

Since the 1978 publication of the ISRM suggested methods for discontinuity characterisation, open pit mines have been developed to increasingly greater slope heights. In open pits with slope heights of 1 km or more, the 20 m threshold for very high persistence is insufficient to capture the influence of large scale structures that can form major sections of potential inter-ramp and overall pit scale failure surfaces. In a study of the Palisser rockslide in Canada, Sturzenegger (2010) used photogrammetry mapping to show that apparent “extremely persistent” discontinuities can comprise step-path combinations of different joint sets and intact rock fracture, emphasizing the need for multiscale discontinuity characterisation. 3. Intact rock bridges in slope stability 3.1 Limit equilibrium methods Rock bridges can be most simply defined as the intervals of intact rock that exist between the tips of adjacent discontinuities. Jennings (1970) was the first to introduce a mathematical method for incorporating intact rock bridges into limit equilibrium slope stability assessment, by considering a theoretical slope failure surface comprising discontinuous joints interrupted by patches of intact rock, which must undergo shear failure in order for slope instability to occur (Fig. 2).

Fig. 2. The “patch” model for intact rock bridges introduced by Jennings (1970)

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Jennings proposed that intact rock bridge content could be represented with a continuity coefficient that describes the ratio of area, kA (or length, kL in two dimensions) of intact rock bridge patches to total area of the complete, idealized failure surface:

The continuity coefficients are used to derive weighted shear strength parameters expressing equivalent cohesion c and friction angle φ of the failure surface:

Using the Mohr-Coulomb shear strength criterion, a small content of intact rock bridges can add significant apparent cohesion to an idealized continuous discontinuity or failure surface (Elmo et al., 2011; Gischig et al., 2011; Karami et al., 2007). Key limitations of the limit equilibrium approach include the implicit assumptions that (1) failure occurs in shear, and (2) frictional strength and cohesion are mobilised simultaneously; progressive brittle fracturing of rock bridges is not considered. Later research showed that a fracture mechanics based approach is better able to represent the brittle fracture process by using stress intensity factors to account for concentrations at the tips of pre-existing discontinuities, and the influence of both tensile and shear fracture initiation and propagation during the brittle slope failure. Scavia (1990) showed that displacement discontinuity numerical method can be used to simulate the role of stress concentration in crack initiation and propagation, prior to the mobilisation of frictional strength, which occurs with later shear displacement. More recently researchers have applied fracture mechanics principles to a modified discontinuous deformation analysis method to simulate complex toppling failure (Zhang et al., 2010) and progressive step-path failure of rock slopes (Wong and Wu, 2014). 3.2 3D rock bridge geometry In reality, intact rock bridges do not exist only along a basal sliding surface. Instead, they occur throughout the entire volume of a rock mass. In addition to increasing the shear strength of a potential failure surface, the rock bridges dispersed throughout a rock slope also increase the stiffness and shear strength of a potential failure mass. Jennings' apparent shear strength-limit equilibrium approach is suited to simple slope stability analyses involving planar sliding. However, more complex slope failure mechanisms, involving lateral and rear release surfaces, as well as rotation and rock mass dilation, require a more complete understanding of the 3D geometry of rock bridges inside\ the slope (Elmoet al., 2007). Unfortunately, rock bridges can be difficult to rigorously characterise in the field, and exhaustive delineation of discrete centimetre to metre scale rock bridges for an entire open pit slope is not feasible. Although many laboratory scale experiments on rock bridge failure have been undertaken (Lajtai, 1969a, 1969b, 1969c; Bobet and Einstein, 1998; Wong et al., 2001; Sagong and Bobet, 2002; Gehle and Kutter, 2003), studies of rock bridge failure in field scale rock masses are limited to forensic investigations of 23 June 2016

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previous failures, where failed intact rock bridges have been identified by the presence of fresh rock or brittle fracture morphology along exposed landslide scars or rockfall detachment niches (Table 1). 3.3. Coplanar 2D rock bridges on incipient joints Hencher et al. (2011) investigated the genesis of joints in granite, describing how progressive extension and coalescence of tensile microcracks can eventually result in the formation of persistent mechanical discontinuities. They proposed that during intermediate stages of joint formation, planar structures comprising partially-coalesced microcracks can form incipient “proto joints” that are interrupted by intact rock bridges, contributing cohesion and tensile strength to the discontinuity. These proto joints closely resemble the conceptual patch model for rock bridges proposed by Jennings (1970). We suggest that intact rock bridges in this configuration may be referred to as “coplanar” intact rock bridges, because they occur in the plane of an incipient discontinuity, to which they contribute cohesion and tensile strength. Table 1 Some published estimates of rock bridge content in rock slopes

Location Measurement method Estimated rock bridge content (%)

Authors

Mont Blanc rockfalls, Drus Peak, France

Failure mapping; area % 3 Matasci et al. (2015)

Palliser slide, Alberta Canada Failure mapping; length % 2 to 3 Sturzenegger and Stead (2012)

Åknes slide, Norway Failure mapping; length % 1 to 3 Grøneng et al. (2009)

Highway roadcuts, Idaho USA Failure mapping; length % 0 to 3 Ristau (1994)

Gypsum mine, Arizona USA Failure mapping; length % 16 to 36 >5

LeBaron (2011) LeBaron (2011) Karami et al. (2007)

Diavik A154 Pit, Northwest Territory Canada

FN modelling study; length % 25 3 to 45

Moffitt et al. (2007) Elmo et al. (2011)

Rockfall scars, French Sub-Alps Failure mapping; length % 0.2 to 5.0 Frayssines and Hantz (2006)

Failed overhang road half-tunnel, Italy

Failure mapping; length % 26 Paronuzzi and Serafini (2009)

Natural slope failure, Western Alps, France

Failure mapping; area % +numerical simulation

7.3 Lévy et al. (2010)

Randa rockslide, Switzerland Length % for specific discontinuity sets, +numerical simulation

8 to 45 Gischig et al. (2011)

3.4. Non-coplanar 3D rock bridges We suggest the term “non-coplanar” to describe rock bridges between the tips of discontinuities with different orientations, belonging to different structural sets. In contrast to coplanar rock bridges, which represent in-plane patches along discrete proto joints, non-coplanar rock bridges may have irregular three dimensional

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geometry, and contribute cohesion and tensile strength to the overall volume of rock mass rather than to a specific discontinuity plane. Window mapping surveys of intact rock bridges are subject to the same orientation bias that occurs in a conventional discontinuity survey (Terzaghi, 1965). Fig. 3 illustrates the difficulty of distinguishing between co-planar and non-coplanar (3D) intact rock bridges in a 2Dwindow map. Window location and orientation has a profound impact on the 2D interpretation: three parallel mapping windows offset by a small distance result in markedly different interpretations of the rock bridge configuration. Stead et al. (2007) concluded that 3D characterisation of rock bridge geometry is critical for non-conservative brittle fracture simulation in open pit slopes. 3.5. Role of rock mass damage Natural slopes and engineered open pit mine slopes are both subject to brittle rock mass damage, resulting from progressive fracturing which degrades rock mass strength. Damage can occur slowly from endogenous natural processes within the rock mass (Gerber and Scheidegger, 1969) such as tectonic action (Brideau et al., 2005), which can interact with topography to alter in situ stress and promote rock fracturing (Miller and Dunne, 1996; Pan et al., 1994). Exogenous processes such as glacial loading and subsequent debuttressing after glacial retreat can also promote fracturing (Ziegler, 2014; Leith, 2012). Open pit mining introduces accelerated rock mass damage accumulation from production blasting and rapid removal of confinement as the slope is excavated (Hagan et al., 1978; Hoek and Karzulovic, 2000). Blasting can destroy rock bridges between pre-existing discontinuities, and also increase their persistence and aperture (Hagan, 1982; Hagan et al., 1978). Blast damage is generally confined to a shallow zone behind the pit walls, extending in the order of 0.5 to 2.5 times the bench height behind the face, depending on blasting practices (Hoek, 2012). Fig. 4 shows an example realization of a three dimensional discrete fracture network using the code Frac_Rock (Gibson, 2015) illustrating the concept of increased fracture intensity and decreased rock bridge content in the near-surface blast damaged zone. Although the blast damage zonemay be confined to the near-surface rock mass, we propose that careful examination of blasting induced fractures can provide insights into rock bridge failure: understanding the evolution of near surface blast fracturing may help to better understand mechanisms of rock bridge failure deeper inside the slope. The open pit trace maps created in this study give special attention to interpreted blast induced fractures.

Fig. 3. Impact of 2D window location on trace map interpretation

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Fig. 4. Increased fracture intensity in the near-surface blast damage zone.

4. Field investigation methods Our field investigations included modified window mapping, ground-based photogrammetry and LiDAR survey. Remote sensing methods allow survey of dangerous or inaccessible slopes, and produce a permanent electronic three dimensional record of slope conditions (Sturzenegger et al., 2011). Photogrammetry-based discontinuity mapping builds upon earlier research into discontinuity trace mapping using 2D photographs (Blin-Lacroix and Thomas, 1990; Crosta, 1997). 4.1. Digital photogrammetry Photogrammetry software applies the principles of stereoscopy to construct 3D models from overlapping photographs taken from offset camera stations. We used a specialist photogrammetry software package (Adam Technology, 2010) that applies an automated image matching process to identify matching pixels and compute their 3D locations, in addition to the location of the camera stations (Birch, 2006). Once the location of each pixel is computed, the software constructs a 3D point cloud and triangulated mesh, with accompanying colour photographic overlays. We carried out long-range surveys from distances of 500 m to 1 km using a long focal length telephoto lens (f = 100 mm to f = 400 mm), configured in an image fan layout, as described by Birch (2006), and previously applied to open pit mines and natural rock slopes by Sturzenegger (2010) and Lee (2011). For the close-range surveys for digital trace mapping, we used an f = 20mm fixed focal length lens. We registered the final digital terrain models in local mine coordinates by identifying geodetic survey prisms with known coordinates as control points. 23 June 2016

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4.2. LiDAR LiDAR devices emit a laser beam towards a target, and generate a 3D point cloud of the target by measuring the properties of the reflected light. Recent research into LiDAR for rock slope characterisation has shown that the technology can be used for conventional discontinuity mapping and may also be able to distinguish between pre-existing discontinuities and brittle intact rock fractures in forensic investigations of slope failure (Jaboyedoff et al., 2012; Sturzenegger and Stead, 2012). We used a time-of-flight scanner, which measures the distance to the rock slope based on the return time of the incident beam (Optech Incorporated, 2008). Intensity of the reflected beam is also recorded, giving an indication of the reflectivity of the rock surface, which is influenced by factors including vegetation, water seepage, weathering and mineralogy. 4.3. Modified window mapping Conventional window mapping techniques involve the measurement of discontinuity traces in a rectangular or circular window on a rock outcrop. Discontinuity surveys focus on trace length, orientation, termination style (Dershowitz and Einstein, 1988; Hodgson, 1961), and on discontinuity surface roughness, weathering, and alteration. Lithology, in situ block size, estimated intact rock strength, weathering and alteration are also recorded. The combined assessment of discontinuities and intact rock properties can then be used to assign rock mass quality measurements using rock mass classification systems such the GSI (Hoek, 1994), (M)RMR (Bieniawski, 1976, 1989; Laubscher, 1977,1990), or Q system (Barton et al., 1974). In this study, we used the GSI classification system. Trace length measurements from window mapping are used as aproxy measure of persistence (e.g. Kulatilake and Wu, 1984a, 1984b; Zhang and Einstein, 1998; Mauldon, 1998) and are used to derive estimates of 2D fracture intensity P21 (Dershowitz et al., 2000; Zhang and Einstein, 2000). Fracture intensity is one of the key input parameters needed to create 3D computer simulations of fracture networks using DFN software such as FracMan (Dershowitz et al., 1998), JointStats (JKMRC, 2000), or 3FLO (Billaux et al., 2006; Itasca, 2006). Although automated algorithms for detection of rock block edges and discontinuity traces have been increasingly investigated (for example, with 2D photographs by Hadjigeorgiou et al., 2003; and with 3D LiDAR and photogrammetry models by Lato and Vöge, 2012), we carried out manual mapping, which avoids some limitations of automated discontinuity mapping, principally (1) the need to validate automatically detected features to check for accuracy; and (2) the tendency for automated methods to underestimate the degree of fracturing in the rock mass (Lemy and Hadjigeorgiou, 2003). For each field mapping location we produced a digital trace map by manually tracing discontinuities and inferred rock bridges on perspective- corrected 2D photographs using Adobe Illustrator (Adobe, 2010). The mapping windows are approximately rectangular. We used raster image processing software ImageJ (Rasband, 2008), which has built-in functionality to compute the length statistics for each set of traces. The trace mapping procedures for this studywere broadly based on the Digital RockMass Rating (DRMR)method proposed byMonte (2004), with extensions to consider new factors for intact rock bridge intensity R21 and blast-induced fracture intensity B21. We traced inferred rock bridges between the tips of the nearest adjacent discontinuities terminating in intact rock, connecting discontinuities belonging to any structural set. Consequently, some inferred rock bridge traces represent co-planar bridges between incipient joint segments, and some traces are irregular non-coplanar bridges. Traces were drawn in an effort to completely dissect the window into an assembly of inferred blocks with complete perimeters, partially bounded by pre-existing fractures, and partially by rock bridges, keeping inferred intact rock bridge traces shorter than the length of the pre-existing discontinuities that they connect. It is important to qualify the rock bridge traces as inferred only, because actual rock bridge failure geometry can be variable, and may involve irregular fracture coalescence patterns that do not simply propagate directly from tip to tip of adjacent discontinuities (Yan, 2007).

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Blast damage traces included sub-horizontal pre-splitting fractures, and smaller irregular brittle fractures. Fig. 5 presents an example of the separate raster image layers constructed for pre-existing discontinuities, blasting damage, and inferred rock bridges. Being confined to the outcrop plane, the 2D rock bridge and blasting damage trace maps are subject to the same orientation bias as conventional discontinuity survey; however, we qualitatively compared each trace map against 3D remote sensing models in an effort to minimise unrealistic trace geometry. The sums of trace lengths are used to calculate areal intensity of pre-existing discontinuities, intact rock bridges, and blast-induced damage:

Fig.5. Digital trace mapping stages from Window E at the Mine A study site

The trace intensity statistics P21, B21, and R21 are derived from 2D mapping, and thus do not directly capture 3D geometry. However, we suggest that the trace mapping intensity measurements may be useful as input and validation data for 3D DFN simulation, where in a typical workflow, 3D DFNs are iteratively refined by comparing equivalent trace intensity values from simulated synthetic window maps against the measured trace intensity from field investigation (Elmo et al., 2014).

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The digital window maps captured more detail than conventional field mapping by applying a smaller truncation length than the field window maps (about 5 cm), and by including traces of irregular non- planar intact rock fractures that are difficult to record using conventional field scanline or window mapping techniques. We compared the results from trace mapping of inferred rock bridges with the rock mass classification and discontinuity persistence data from photogrammetry and LiDAR models, to investigate trends in inferred rock bridge size and intensity, pre-existing discontinuity persistence, blasting damage, and rock mass quality. We traced blast induced fracturing on irregular fractures with fresh surfaces, with surface morphology showing evidence of rapid crack propagation by dynamic loading. Blast induced fractures were typically irregular or undulating, and rough to very rough (JRC N 10; Barton and Choubey, 1977). Fig. 6 shows close range photographs of interpreted blasting fractures with clean surfaces and fractography features including rough hackle fringes, en echelon cracks, arrest marks, and undulations.

Fig. 6. A) Blast induced fractures with rough hackle fringe (h), and en echelon cracks (e.e.); (B) fringe undulations (u) and arrest marks (a).

5. Overview of study sites The rock mass at each study site is different: in particular the existing fracture networks at each site differ in terms of discontinuity persistence, presence of incipient joints with coplanar rock bridges, rock mass damage from blasting and natural processes, and geometry and spatial distribution of rock bridges. The four study sites are described in the following sections, and representative rock mass conditions are illustrated in Fig. 7.

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The Chief is a massive, high strength granite monolith about 45 km north of Vancouver, Canada. The mountain was previously covered by up to 1.3 km of ice at the last glacial maximum (Turner et al., 2010). Very high persistence exfoliation joints are ubiquitous; other discontinuities comprise mostly brittle intact rock fractures, forming the release surfaces for rockfalls. The rock mass is sparsely fractured and very strong (UCS N 100 MPa), supporting sub-vertical cliff faces up to 400 m high. We carried out a LiDAR survey of the west wall covering a region of interest about 150 m by 300 m.

Fig. 7. Representative rock mass conditions at the four study sites.

Mine A is an open pit excavated in strong, mostly massive granite (GSI N 70), with common occurrence of very high persistence joints (frequently N 50 m), and evidence of several sets of incipient joints with frequent termination in intact rock, retaining cohesion and tensile strength from coplanar intact rock bridges. The high intact rock strength and massive condition of the rock mass have enabled sub-vertical bench construction, with 30 m high double benches. Half barrel marks from pre-split blasting holes are frequently preserved. Bench-scale instabilities mostly comprise small wedge failures, with release provided by brittle blast induced fractures. Mine B is an open pit excavated in a sequence of bedded (anisotropic) metasediments, with bedding in one wall dipping adversely into the pit at 20° to 30°. Major sections of the wall resemble three dimensional step-paths, resulting from blasting overbreak and nested wedges failures, with basal sliding along bedding, and

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kinematic release provided by bedding-orthogonal conjugate joints and blast induced fractures. The foliation is effectively fully persistent, and the rock bridges seem to be related to bedding thickness, because slope failure requires step-over fracturing orthogonal to bedding. Mine C comprises a porphyry system of highly fractured and hydrothermally altered granodiorite, and is the most intensely fractured rock mass of all three sites. The rock mass has been progressively damaged during several episodes of tectonic deformation, faulting, and hydro- thermal infiltration, with newer fractures overprinting older discontinuity sets. Rock bridges at this site are centimetre-scale, 3D non- coplanar bridges dispersed throughout the rock mass. In situ block sizes are much smaller than 1 m3, and instability is characterised by bench-scale sliding failures that have resulted in extensive crest loss.

Fig 8. Basic fractography mapping of the Chief.

6. Results 6.1 The Chief A LiDAR survey of the Chief was taken from approximately 400 m. The point cloud covers a region of interest 150 m high and 300 m wide, with a pixel spacing ranging from 10 cm to 20 cm, depending on the straight-line distance between the scanner and the location on the slope. The point cloud resolves major fractography features including fracture arrest marks, which indicate the extent of discrete episodes of fracture growth, and also the length of brittle intact rock fractures, which indicates the typical range of step-path heights. Fig. 8 shows inter-arrest mark distances, which we measured to understand the length of discrete episodes of fracture growth, and the step-surface heights in the major central arches, which indicate the thickness of previous exfoliation slab failures. The height of step surfaces formed by arches, rock fall detachment niches, arrest marks, and bifurcated exfoliation joints may be a useful estimate of near-surface exfoliation joint spacing and, by proxy, the ef- fective length of intact rock bridges which stabilize incipient exfoliation slabs. Fig. 9 summarises measurements of 101 step surface heights taken over the entire LiDAR point cloud, with negative exponential and lognormal functions overlain.

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The maximum step height is 5.7 m, corresponding to the major arches in the centre of the ROI. The minimum step height is 0.2 m, corresponding to the truncation length, and the average is 1.1 m. The exponential and log-normal functions highlight the limitations of applying continuous probability distributions to discrete data sets. Continuous functions may be reliable for characterising step path geometry over a limited range of lengths where frequency appears continuous, but they are poorly suited to predicting gaps where no structures occur, and the occurrence of large outliers, such as the major central arch structure, which suggests the previous detachment of a 6 m thick exfoliation slab. Based on the high intact rock strength (UCS N 100 MPa), the detachment of a 6 m thick slab in the central arch is unlikely to have occurred as an instantaneous limit equilibrium failure along fully-formed pre-existing joints. Glacial processes likely played an important role in the arch formation, with influence from glacial loading and unloading, freeze/thaw cycles, seasonal pore pressure fluctuations, and mechanical weathering by glacial scour. Release of the slab likely occurred progressively. We agree with the conclusions of Matasci et al. (2011, 2015), and Stock et al. (2012), who studied rockfall events in granitic rock slopes in the Yosemite Valley (USA) and the Drus mountain peak in France: stress concentrations occurring at the interface between rock bridges and the tips of pre-existing joints likely caused sub-critical fracture propagation that eventually culminated in critical propagation of (1) incipient exfoliation joints, and (2) orthogonal brittle fracture release structures. At the Chief, we suggest that this progressive fracturing produced the “failed rock bridges” preserved in the major central arches.

Fig. 9. Step path heights measured from the LiDAR scan of the Chief

6.2 The Mine A The overall slope height at Mine A is about 190 m, and the field investigations included modified window mapping with 2D digital trace maps, and three photogrammetry surveys carried out at different resolutions (focal lengths f = 100 mm; f = 200 mm; and f = 300 mm). 6.2.1 Window Mapping

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We focused field mapping on the orientation of major discontinuity sets, trace lengths, discontinuity surface roughness, and evidence of incipient proto joints with terminations in intact rock. The mapping windows were approximately rectangular, with edge dimensions between 10 m and 20 m. The maximum discontinuity persistence measured in close range bench face mapping was approximately 40 m; which represents a censored estimate of persistence of a multi- bench scale joint extending above and below the largest mapping window. The average persistence was approximately 16 m and the minimum persistence was 1 m, corresponding to the truncation length. Table 2 summarises the results from the accompanying digital trace maps, and Fig. 10 shows an example trace map for Window A. Fig. 11 summarises the intensity measurements for pre-existing discontinuities, inferred intact rock bridges, and blast induced fractures for each window, and also shows plots of intensity versus GSI, blast damage intensity B21 versus pre-existing fracture intensity P21 and rock bridge intensity R21 versus P21. The intensity values do not appear to correlate with GSI; however, blast damage is more intense when pre-existing fracturing is greater, in agreement with the findings of Hagan (1982) that blast damage is more intense when pre-existing fracturing is greater. Intact rock bridge trace intensity does not seem to correlate with pre-existing fracture intensity, because massive rock has fewer pre-existing fractures, and thus fewer discrete rock bridges between pre-existing discontinuity tips. 6.2.2 Photogrammetry investigation Camera stations for photogrammetry surveys were positioned along the crest of the pit and selected benches, subject to safety considerations and access constraints. The average distance to the pit walls was approximately 500 m (the diameter of the pit). Each three dimensional photogrammetry model was registered in local mine coordinates by manually identifying at least six geodetic survey prisms as control points. We carried out large scale photogrammetry mapping over multi- bench sections of the pit, by overlaying several DTMs simultaneously. The number of discontinuity measurements increases with survey resolution, due to the corresponding decreases in truncation length. The f = 100 mm survey data included 210 discontinuity measurements; the f = 200 mm dataset included 306 discontinuities, and the f = 300 mm dataset included 411 discontinuities. Fig. 12 presents a normalized histogram of fitted discontinuity diameters for all three photogrammetry surveys. Table 2 Summary of intensity measurements from Mine A window maps

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Fig. 10. Digital trace map for Window A at Mine A

Fig. 11. Summary of digital trace mapping intensity results for mine A

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Fig. 12. Fitted discontinuity diameters, normalized by survey population.

The f = 200 mm survey covers the majority of the pit circumference, and a truncation length of 0.8 m was applied. The ground point spacing is approximately 1.6 cm, double the resolution of the f = 100 mm survey. The maximum three dimensional mapping area size used for mapping in the f = 200 mm survey was up to approximately 200 m square, from simultaneous overlay of 12 DTMs. This maximum mapping area is smaller than the overall-slope scale mapping windows used in the f = 100 mm survey. As a result of the decreased mapping area, the maximum measured persistence from the f = 200 mm survey is slightly smaller than observed in the f = 100 mm survey. The highest persistence structure was 45.8 m; the minimum persistence was 0.8 m (corresponding with the truncation length); and the average recorded persistence was approximately 9.6 m. All of the joint sets recorded in the f = 200 mm survey had at least one structure classified as very high persistence (N 20 m). A peak in frequency occurs for discontinuities between 4 m and 5 m, and smaller discontinuities are under-sampled. The higher resolution of the survey, compared with the initial f = 100 mm model, shifts the distribution towards a smaller peak value of persistence; the focal length is doubled, and the peak frequency for discontinuity persistence is approximately halved. The f = 300 mm survey was focused exclusively on the southeast wall of the pit; due to the increased resolution, a full pit model would be prohibitive in terms of memory requirements and mapping time. The ground point spacing is approximately 1.1 cm. The truncation length is 0.3 m. To take advantage of the higher resolution of the f = 300 mm survey, mapping focused on characterisation of bench-scale or smaller discontinuities, from medium persistence (3 m to 10 m) down to the cut-off length of 0.3 m. The increase in resolution corresponds with increased frequency of smaller discontinuities, and censor ing of larger discontinuities due to decreased mapping area.

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6.3 Mine B The investigation at Mine B included long range and close range photogrammetry survey, both using focal length of f = 100 mm. We carried out the long range survey from a distance of 1 km, and the photogrammetry model achieved a point resolution of 6.4 cm. We carried out the close range survey from a distance of 50 m, and the photogrammetry model achieved a ground point resolution of approximately 3.2 mm. We also carried out modified window mapping at six locations near the toe of the slope (Fig. 13).

Fig. 13. Photogrammetry and window mapping locations at Mine B.

Table 3 Summary of intensity measurements from Mine B window maps

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6.3.1 Window mapping Window maps at Mine B were smaller than the windows at Mine A, due to the irregular stepped geometry of the slope, as well as safety and access constraints. Table 3 summarises the intensity measurements for pre-existing discontinuities (P21), intact rock bridges (R21), and blast induced damage (B21). Fig. 14 shows an example of the trace map for window W-04, which was taken on a “bullnose” exposure, perpendicular to the main wall dip direction. The bullnose exposure provides a view of the bedding-orthogonal joint sets, and blast induced damage which includes recent fractures, dilation of pre-existing discontinuities, locally reduced rock bridge content, and increased the potential for step-path slope instability. Fig. 15 summarises the intensity measurements for each window map, and also shows plots of intensity (P21, R21, and B21) versus GSI. The intensity measurements do not seem correlated to GSI, with the exception that the lowest intensity measurements are associated with the highest GSI, in agreement with the results from Mine A: rock masses with high GSI are massive, have few discontinuities, and thus few rock bridges between discontinuity tips. Blast induced fracture intensity seems to increase with intensity of pre-existing fractures, however the correlation appears weaker than observed at Mine A. In contrast to the results from Mine A, the intact rock bridge trace intensity R21 appears to show a weak linear correlation with pre-existing fracture intensity P21; this may be partially attributed to the pervasive foliation: in the trace maps we identified mostly bedding-orthogonal, non-coplanar intact rock bridges, and we noted that as intensity of foliation increases, more intact rock bridges can be distinguished. To limit the influence of censoring bias, we included higher persistence discontinuities extending outside the field mapping windows, by making preliminary multi-bench scale sketches of the mapping area from a distance of 50 m to 100 m from the slope. The trace length distribution (n = 110) approximates a continuous negative exponential distribution (Fig. 16). However, for persistence N 10 m, the discontinuous, discrete nature of the dataset becomes apparent: no trace lengths occur between 10 m and 30 m, but several structures were measured with trace lengths of 30 m and 40 m. The continuous exponential function assigns approximately equal probability for all values of discontinuity persistence between about 10 m and 40 m. Consequently, it is poorly suited to predicting the discrete occurrence of the major outlier structures, and the gap between persistence values of 10 m and 30 m. The highest frequency occurs for trace lengths b 0.5 m, reflecting length or size bias, which results in preferential sampling of approximately “person-size” discontinuities during field mapping and under-sampling of larger structures (N 10 m). 6.3.2 Photogrammetry investigation The long-range photogrammetry survey achieved a truncation length of 4.9 m, because of the relatively low resolution of the DTMs (6.4 cm) and our effort to focus on higher persistence structures. The close range survey, with a much higher resolution (3.2 mm) achieved a truncation length of 0.1 m. The discontinuities mapped in the long-range photogrammetry survey (n = 241) generally confirm the field mapping assessments of persistence. The most persistent feature is the foliation. Clusters of release surfaces comprising blast-induced fractures and pre-existing joints are also present. The foliation has the highest average persistence (28 m), however one major fault has the highest overall persistence (177 m) and is censored in the close range survey (Fig. 17). Fig. 18 presents a normalized histogram of fitted discontinuity diameters (persistence) for the close range and the long range surveys, with a negative exponential distribution overlain. Using 1 m persistence intervals for frequency analysis, the peak frequency is 15 m for the long-range survey and 2 m for the close-range survey. The results show that high resolution, close-range survey can provide useful data on mesoscale (1 m to 10 m) discontinuities which have been truncated in the long range survey.

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Fig. 15. Summary of digital trace mapping intensity results for Mine B.

Fig. 15. Summary of digital trace mapping intensity results for Mine B.

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The foliation at Mine B is of sedimentary origin, and is continuous at the scale of the overall pit, except where interrupted by faults or very persistent joints. True coplanar rock bridges along foliation are unlikely. However, intervals of tight or “over-closed” foliation (Barton, 2007) may separate discontinuity segments that have been opened by blasting or mining-induced relaxation. Barton (1972, 2007, 2013) described discontinuity over-closure as an increase in shear strength observed in laboratory direct shear tests that occurs when samples are loaded to normal stresses exceeding in situ stress, prior to unloading and shearing. Inter-ramp or overall slope stability analysis involving deep-seated failure could consider the cohesive strength benefit of over-closed or tight intervals of foliation based on the results of laboratory direct shear ortriaxial tests on tightly-closed foliation discontinuities. Although co-planar intact rock bridges are unlikely to influence planar sliding, non-coplanar intact rock bridges are an important influence on the formation of step-path type failures. Close-range observations from window mapping and 2D trace mapping indicate peak values of persistence for non-foliation discontinuities of approximately 2 m or less, reflecting the typical length of “step over” fractures which propagate across bedding. This length scale may represent a conservative upper bound value for the size of non-coplanar intact rock bridges along step-path failure surfaces at this site.

Fig. 17. Two overlapping photogrammetry surveys at Mine B.

6.4 Mine C At Mine C we investigated a subsection of the pit wall from the pit crest to about 120 m depth. We carried out limited field mapping, and two LiDAR surveys. We discuss the results with emphasis on the influence of LiDAR resolution on measurements of discontinuity persistence. 6.4.1 Field mapping The rock mass at Mine C is very blocky, with zones of disintegrated and altered rock surrounding major faults. Block shape is sub-cubic, with edge length of 1 m or less and GSI in the range of 40 to 50.

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Discontinuity surface conditions are fair to poor, with frequent occurrence of clay coatings. Extremely persistent discontinuities exceeding 50 m are present, but with the exception of faults, these are difficult to distinguish, due to overprinting of older, more persistent discontinuity sets by intense recent damage induced by fault reactivation and blast induced damage. The field mapping identified mostly medium-persistence discontinuities (3 m to 10 m) with an average persistence of 6.9 m. The largest persistence was N 60 m, for a segment of a major inter-ramp scale fault. We identified bench-scale slope failures including planar sliding, toppling, wedge failure, and rock mass failure resulting from intense blast damage. We also identified a possible multi-bench, non-daylighting wedge (Fig. 19). Due to the high fracture intensity, metre-scale bridges of intact rock are unlikely. Instead, we suggest that rock bridges may be characterised as centimetre scale, 3D non-coplanar intervals dispersed throughout the rock mass. In some cases larger “rock mass bridges” occur, comprising metre scale intervals of fractured rock mass separating major structures.

Fig. 18. Normalized frequency of discontinuity diameters for photogrammetry surveys at Mine B.

6.4.2. LiDAR investigation We carried out LiDAR scans at two resolutions in order to investigate the influence of resolution on truncation length and persistence measurements. The low resolution survey comprises eight overlapping scans that were merged into a single point loud, and the high resolution scan comprises three overlapping scans. Fig. 20 shows the merged point clouds for both surveys, with different colours indicating each constituent scan. Even in the high resolution survey, we found it impractical to identify intact rock bridges at this site. The small block sizes, close to moderate spacing of discontinuities (60 mm to 600 mm; ISRM, 1978) and dominance of low persistence discontinuities suggest that intact rock bridges at Mine C could be more easily treated as a dispersed volume of centimetre scale 3D non-coplanar intact rock bridges, which have a cumulative effect in contributing cohesion and tensile strength to the rock mass. These conditions typify the homogeneous, heavily jointed rock masses where rock mass classification systems such as GSI and the Hoek-Brown shear strength criterion are applicable. We carried out the first survey using a low resolution setting, resulting in point cloud spacing of 5 cm to 20 cm, depending on the straight-line distance between the scanner and each point on the rock face (resolution is higher for sections of the slope that are closer to the scanner). The final low resolution point cloud is

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comprised of 8 overlapping scans, containing 9.5 million points. Each separate scan took about 20 min to complete. We carried out the second survey using a high resolution setting, resulting in a point cloud spacing of approximately 1 cm to 10 cm. Each scan took about one hour to complete, and due to time limitations, we were only able to complete three high resolution scans. The final high resolution point cloud contains approximately 22.3 million points.

Fig. 19. Major structural features and slope failure mechanisms at Mine C.

In the first stage of LiDAR mapping, we mapped 308 discontinuities using the low resolution point cloud; we then overlaid these discontinuities on the high resolution point cloud, in order to avoid duplicate discontinuity measurements. In the second stage, we mapped 644 discontinuities using the high resolution survey. We also examined previous bench scale instabilities, which frequently showed complex failure surface morphology with some component of brittle fracture providing lateral release (Fig. 21). Fig. 22 shows a normalized histogram of fitted discontinuity diameters for the two LiDAR surveys. Using 1 m persistence intervals for frequency analysis, the peak frequency occurs for discontinuities between 1 m and 2 m in the low resolution survey, and discontinuities smaller than 2 m are under-sampled due to truncation and length bias. In the high resolution LiDAR survey, the exponential increase in frequency for smaller discontinuities is abrupt, and the peak frequency occurs for discontinuities smaller than 1 m.

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Fig.20. Each LiDAR survey comprised several overlapping scans.

Fig. 21. Complex bench scale instability at Mine C.

Fig. 22. Normalized histogram of fitted discontinuity diameters from LiDAR survey at Mine C.

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7. Synthesis and recommendations Discontinuity persistence is a discrete variable, and the results from field mapping and photogrammetry mapping of discontinuity persistence in this study suggest that for outlier structures with very high persistence, continuous statistical distributions are not well suited to predicting their frequency, or the gaps in the datasets where no structures occur. Each study site is located in a different geological setting, with different lithology, tectonic history, and rock mass structure. These factors result in different mechanisms of slope instability at each site, and different rock bridge characteristics. The modified window mapping results from Mines A and B suggest that it is difficult to correlate rock mass quality (GSI) directly with areal intensity of pre-existing discontinuities (P21), inferred rock bridge traces (R21), and blast induced fracture traces (B21). However, both sites seem to show a weak linear correlation between P21 and B21, supporting the expectation that blast damage is more severe when pre-existing fracturing is greater. Mine A comprises sparsely fractured granite, and there is no apparent trend between inferred rock bridges and pre-existing fracturing. In contrast, Mine B comprises a layered rock mass with fully persistent foliation; in this structural setting, inferred rock bridge trace intensity R21 seems to show weak linear correlation with P21. For highly fractured rock masses (GSI b 40) as encountered at Mine C, the value of intensity data collected from trace mapping may be limited, and discrete rock bridges at the centimetre scale are difficult to re- solve. Instead, the dispersed quantity of 3D non-coplanar rock bridges could be characterised using the intensity parameter R21. For slope failure mechanisms requiring internal dilation, non-coplanar rock bridges may be of significant benefit in increasing the shear strength and stiffness of the rock mass, resulting in increased resistance to dilation and instability. Fig. 23 shows an example bi-planar model of slope failure, using Voronoi polygons to represent fractures. The model incorporates fractured rock mass bridges along the basal sliding surface, and also non-coplanar rock bridges distributed throughout the unstable rock mass. Large fractured rock mass bridges increase the shear strength of the basal sliding surface, and non-coplanar rock bridges add stiffness and shear strength to the slide volume. Three conceptual cases for non- coplanar rock bridge content are depicted. In Case A, non-coplanar rock bridge content is zero: blocks edges are fully formed by pre-existing discontinuities. Cases B and C introduce greater content of non-coplanar rock bridges by increasing the rock bridge intensity factor R21. Distinct element numerical modelling trials using UDEC with Voronoi have shown that increases in R21 for this conceptual model can result in greater resistance to shearing and dilation of the transition zone between the upper active wedge and the lower passive wedge (Tuckey et al., 2013). A limitation of this method is that rock bridge failure is constrained to follow the geometry of Voronoi polygon contacts. Although this approach has previously been demonstrated to be useful for simulation of progressive failure in large rock slopes (Alzo'ubi et al., 2007), in reality fracture propagation paths and coalescence patterns may be more variable and complex, with strong dependence on local stresses and the geometry pre-existing cracks (Elmo et al., 2007; Yan, 2007; Bobet and Einstein, 1998; Wong et al., 2001; Sagong and Bobet, 2002; Gehle and Kutter, 2003). A key goal of rock bridge characterisation should be to identify the roles of importance of coplanar versus non-coplanar intact rock bridges, and also rock mass bridges between major structures. In a layered rock mass with continuous bedding or foliation, as at Mine B, special emphasis should be placed on measurement of spacing and persistence of bedding-orthogonal joints, because these structures provide kinematic release for sliding or toppling. Rock bridge size may be correlated with bedding thickness, because step-path slope failures require fractures to propagate across bedding. In Fig. 24 we suggest four conceptual domains describing the condition of intact rock bridges in a layered rock mass relating discontinuity persistence and spacing.

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Fig. 23. Conceptual model for rock bridges in a biplanar open pit slope failure.

Fig. 24. Conceptual domains for intact rock bridges in a layered rock mass.

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The shear strength of coplanar intact rock bridges may be characterised using microscopic studies of intact rock microcrack fabric to identify incipient joint planes, combined with laboratory direct shear testing of incipient discontinuity specimens (Shang et al., 2015). Tensile strength from laboratory direct tension tests, and shear strength parameters from shear box tests can then be used to estimate cohesion and tensile strength of incipient joints in bench scale stability analysis. The shear strength of fractured rock mass bridges, comprising intervals N 10 m in length, is not feasible to measure empirically. Instead, numerical laboratory tests on synthetic rock mass volumes of 100 m3 to 1,000,000 m3 may be used to estimate equivalent shear strength and stiffness. Fracture network engineering, incorporating intensity methods for characterising P21, B21 and R21 can be applied to fracture mechanics based numerical modelling studies of rock slope failure and help to extend the insights already gained from investigations using hybrid FEM-DEM codes (Stead et al., 2006), distinct element codes incorporating Voronoi tessellation (Alzo'ubi et al., 2007), particle flow models (Lorig et al., 2009), or lattice-spring codes (Havaej et al., 2015). 8. Conclusion Photogrammetry mapping investigations from this study show that discontinuity persistence can easily exceed 100 m for rock masses including blocky to massive granite (Mine A), layered metasediments (Mine B), and very blocky, hydrothermally altered granodiorite (Mine C). The frequency distributions for discontinuity persistence appear continuous over a limited range of trace lengths, but gaps in frequency occur between approximately the 90th percentile, and the top 10% most persistent outlier structures. Modified window mapping methods using digital trace mapping can be useful for measuring the intensity of pre-existing discontinuities, blast induced fractures, and inferred intact rock bridges, but the results must be interpreted in the context of the local structural geology, the intensity of pre-existing damage. Measurements from 2D window maps must be interpreted with consideration for the bias and limitations inherent in 2D sampling, and should ideally be compared with mapping from 3D remote sensing models. Acknowledgements Funding for the research was provided through a British Columbia NRAS grant (NRAS-RTP-2009-SFU-026), a Simon Fraser University FRBC Research Chair Endowment and an NSERC Discovery Grant (36375/2010). We also thank the editor, and the two anonymous re- viewers, whose suggestions have greatly improved the manuscript.

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References Adam Technology, 2010. 3DM analyst 2.3.4 user's manual. Available from: http://www. adamtech.com.au. Adobe, 2010. Adobe illustrator version CS5. www.adobe.com/Illustrator. Alzo'ubi, A., Martin, C.D., and Cruden, D.M., 2007. A discrete element damage model for rock slopes. In:

Proceedings of the 1st Canada-US Rock Mechanics Symposium, Eberhardt, Stead, and Morrison (eds.), (Vancouver, Canada, 27–31 May 2007).

Baecher, G.B., Lanney, N.A., 1978. Trace Length Biases in Joint Surveys. Proceedings of the 19th US Symposium on Rock Mechanics (USRMS), 1–3 May, Reno, Nevada vol. 1, pp. 56–65.

Baecher, G.B., Einstein, H.H., Lanney, N.A., 1977. Statistical description of rock properties and sampling. Proceedings of the 18th U.S. Symposium on Rock Mechanics (USRMS), 22–24 June, Golden, Colorado, 5C1.1 5C1.8.

Barton, N., 1972. A model study of rock-joint deformation. Int. J. Rock Mech. Min. Sci. Geomech. Abstr. 9 (5), 579–602.

Barton, N., 2007. Thermal over-closure of joints and rock masses and implications for HLW repositories. In: Olalla, Grossman, Ribeiro e Sousa (Eds.), The Second Half Cen- tury of Rock Mechanics, Proceedings of the 11th Congress of the International Society for Rock Mechanics. Taylor and Francis.

Barton, N., 2013. Shear strength criteria for rock, rock joints, rockfill and rock masses: problems and some solutions. J. Rock Mech. Geotech. Eng. 5, 249–261.

Barton, N., Choubey, V., 1977. The shear strength of rock joints in theory and practise. Rock Mech. 10 (1–2), 1–54.

Barton, N., Lien, R., Lunde, J., 1974. Engineering classification of rock masses for the design of tunnel support. Rock Mech. Rock. Eng. 6 (4), 189–236.

Bieniawski, Z.T., 1976. Rock Mass Classification in Rock Engineering. In: Bieniawski, Z.T. (- Ed.)Exploration for Rock Engineering vol. 1. Balkema, Cape Town, pp. 97–106.

Bieniawski, Z.T., 1989. Engineering Rock Mass Classifications. Wile, New York. Billaux, D., Luvison, B., Darcel, C., Paris, B., 2006. 3FLO, calculs d'écoulements tridimensionnels. bases

théoriques. 3FLO Version 2.31, User's Manual. Itasca Consultats SAS, Lyon, France. Birch, J.S., 2006. Using 3DM analyst mine mapping suite for rock face characterization. In: Tonon, F.,

Kottenstette, J. (Eds.), Laser and Photogrammetric Methods for Rock Face Characterization. ARMA, pp. 13–32.

Blin-Lacroix, J., Thomas, A., 1990. Fracture field modelling according to various data structures. Eng. Geol. 28 (1), 171–190.

Bobet, A., Einstein, H.H., 1998. Fracture coalescence in rock-type materials under uniaxial and biaxial compression. Int. J. Rock Mech. Min. Sci. 35, 863–888.

Brideau, M.A., Stead, D., Kinakin, D., Fecova, K., 2005. Influence of tectonic structures on the hope slide, British Columbia, Canada. Eng. Geol. 80, 242–259.

Crosta, G., 1997. Evaluating rock mass geometry from photographic images. Rock Mech. Rock. Eng. 30 (1), 35–58.

Dershowitz, W.S., Lee, G., Geier, J., LaPointe, P.R., 1998. FracMan: interactive discrete fracture data analysis, geometric modelling and exploration simulation. User documenta- tion. Golder Associates Inc., Seattle.

Dershowitz, W.S., 1985. Rock Joint Systems (Ph.D. thesis) Massachusetts Institute of Technology. Dershowitz, W.S., Einstein, H.H., 1988. Characterising rock joint geometry with joint system models. Rock

Mech. Rock. Eng. 21 (1), 21–51.

23 June 2016

SRK Consulting



Dershowitz, W.S., Hermanson, J., Follin, S., Mauldon, M., 2000. Fracture intensity measures in 1-D, 2-D,

and 3-D at Aspö, Sweden. Proceedings of the 4th North American Rock Mechanics Symposium (NARMS), Seattle, 31 July–3 August, pp. 849–853.

Elmo, D., Yan, M., Stead, D., Rogers, S.F., 2007. The importance of intact rock bridges in the stability of high rock slopes: towards a quantitative investigation using an integrated numerical modelling-discrete fracture network approach. Proceedings of Slope Sta- bility 2007, Perth, pp. 253–266.

Elmo, D., Clayton, C., Rogers, S., Beddoes, R., Greer, S., 2011. Numerical simulations of potential rock bridge failure within a naturally fractured rock mass. Proceedings of the 2011 International Symposium on Slope Stability in Mining and Civil Engineering, Vancouver, 18–21 September 2011.

Elmo, D., Liu, Y., Rogers, S., 2014. Principles of discrete fracture network modelling for geotechnical applications. Proceedings of DFNE2014 – 1st International Conference on Discrete Fracture Network Engineering, 19–22 October, Vancouver.

Frayssines, M., Hantz, D., 2006. Failure mechanisms and triggering factors in calcareous cliffs of the subalpine ranges (French Alps). Eng. Geol. 86 (4), 256–270.

Gehle, C., Kutter, H.K., 2003. Breakage and shear behaviour of intermittent rock joints. Int. J. Rock Mech. Min. Sci. 40, 687–700.

Gerber, E., Scheidegger, A.E., 1969. Stress-induced weathering of rock masses. Eclogae Geol. Helv. 62, 401–415.

Gibson, W., 2015. Frac_Rock Program. SRK Consulting Australia Pty Ltd., Perth. Gischig, V., Amann, F., Moore, J.R., Loew, S., Eisenbeiss, H., Stempfhuber, W., 2011. Com- posite rock

slope kinematics at the current Randa instability, Switzerland, based on remote sensing and numerical modeling. Eng. Geol. 118, 37–53.

Grøneng, G., Nilsen, B., Sandven, R., 2009. Shear strength estimation for Åknes sliding area in western Norway. Int. J. Rock Mech. Min. Sci. 46, 479–488.

Gudmundsson, A., 2011. Rock Fractures in Geological Processes. Cambridge University Press, Cambridge UK.

Hadjigeorgiou, J., Lemy, F., Cote, P., Maldague, X., 2003. An evaluation of image algorithms for constructing discontinuity trace maps. Rock Mech. Rock. Eng. 36 (2), 163–179.

Hagan, T.N., 1982. Controlling blast-induced cracking around large caverns. Proceedings of the International Society for Rock Mechanics International Symposium, Aachen, Germany, 26–28 May, pp. 1155–1167.

Hagan, T.N., McIntyre, J.S., Boyd, G.L., 1978. The influence of blasting in mine stability. In: Brawner, C.O., Dorling, I.P. (Eds.), Proceedings of the 1st International Symposium on Coal Mine Stability, Vancouver, BC, Canda, 1978, pp. 95–122 (paper 7).

Havaej, M., Wolter, A., Stead, D., 2015. The possible role of brittle rock fracture in the 1963 Vajont Slide, Italy. Int. J. Rock Mech. Min. Sci. 78, 319–330.

Hencher, S.R., Lee, S.G., Carter, T.G., Richards, L.R., 2011. Sheet joints: characterization, shear strength and engineering. Rock Mech. Rock. Eng. 44, 1–22.

Hodgson, R.A., 1961. Regional study of jointing in Comb Ridge-Navajo mountain area, Ar- izona and Utah. Bull. Am. Assoc. Pet. Geol. 45 (1), 1–38.

Hoek, E., 1994. Strength of rock and rock masses. ISRM News J. 2 (2), 4–16. Hoek, E., 2012. Blast damage factor D. Technical Note for RocNews, February 2, 2012, Win- ter 2012 Issue,

RocScience (Available from: http://www.rocscience.com/assets/files/ uploads/8584.pdf). Hoek, E., Diederichs, M.S., 2006. Empirical estimation of rock mass modulus. Int. J. Rock Mech. Min. Sci.

43, 203–215.

23 June 2016

SRK Consulting



Hoek, E., Karzulovic, A., 2000. Rock mass properties for surface mines. In: Hustralid, W.A., McCarter, M.K.,

van Zyl, D.J.A. (Eds.), Slope Stability in Surface Mining. Society for Mining, Metallurgical and Exploration (SME), Littleton, Colorado, pp. 59–70.

International Society for Rock Mechanics (ISRM), 1978. Commission on standardization of laboratory and field tests: suggested methods for the quantitative description of discontinuities in rock masses. Int. J. Rock Mech. Min. Sci. Geomech. Abstr. 15 (6), 319–368.

Itasca Consultants SAS (ICSAS), 2006. 3FLO, Version 2.31 (Lyon, France). Jaboyedoff, M., Oppikofer, T., Abellán, A., Derron, M., Loye, A., Metzger, R., Pedrazzini, A., 2012. Use of

LIDAR in landslide investigations: a review. Nat. Hazards 61, 5–28. Jennings, J.E., 1970. A mathematical theory for the calculation of the stability of slopes in open cast mines.

Proceedings of the Symposium on the Theoretical Background to the Planning of Open Pit Mines, Johannesburg, pp. 87–102.

Julius Kruttschnitt Mineral Research Centre (JKMRC), 2000. Jointstats Program (Brisbane, Australia). Karami, A., Greer, S., Beddoes, R., 2007. Numerical assessment of step-path failure of northwest wall of

A154 Pit, Diavik Diamond Mines. In: Potvin, Y. (Ed.), Proceedings of the 2007 International Symposium on Rock Slope Stability in Open Pit Mining and Civil Engineering. CSIRO, Perth, Australia, pp. 293–305.

Kemeny, J., 2005. Time-dependent drift degradation due to the progressive failure of rock bridges along discontinuities. Int. J. Rock Mech. Min. Sci. 42 (1), 35–46.

Kulatilake, P.H., Wu, T.H., 1984a. Estimation of mean trace length of discontinuities. Rock Mech. Rock. Eng. 17, 215–232.

Kulatilake, P.H., Wu, T.H., 1984b. The density of discontinuity traces in sampling windows. Int. J. Rock Mech. Min. Sci. Geomech. Abstr. 21, 345–347.

Kulatilake, P.H., Wathugala, D.N., Stephansson, O., 1993. Joint network modelling with a validation exercise in Stripa Mine, Sweden. Int. J. Rock Mech. Min. Sci. Geomech. Abstr. 30, 503–526.

Lajtai, E.Z., 1969a. Strength of discontinuous rocks in direct shear. Geotechnique 19 (2), 218–233. Lajtai, E.Z., 1969b. Mechanics of second order faults and tension gashes. GSA Bull. 80 (11). Lajtai, E.Z., 1969c. Shear strength of weakness planes in rock. Int. J. Rock Mech. Min. Sci. Geomech. Abstr.

6 (5). Lato, M., Vöge, M., 2012. Automated mapping of rock discontinuities in 3D lidar and photogrammetry

models. Int. J. Rock Mech. Min. Sci. 54, 150–158. Lato, M., Diederichs, M., Hutchinson, J., Harrap, R., 2009. Optimization of LiDAR scanning and processing

for automated structural evaluation of discontinuities in rockmasses. Int. J. Rock Mech. Min. Sci. 46, 194–199.

Laubscher, D.H., 1977. Geomechanics classification of jointed rock masses - mining applications. Trans. Inst. Min. Metall. Sect. A Min. Ind. (Lond.) 86, 1–8.

Laubscher, D.H., 1990. A geomechanics classification system for rating of rock mass in mine design. J. South. Afr. Inst. Min. Metall. 90 (10), 257–273.

LeBaron, A.M., 2011. The Effect of Rock Bridges on Blasting Fragmentation in Sedimentary Rock (M.Sc. thesis) University of Utah.

Lee, S.G., 2011. Characterizing Highwall Slopes at the Line Creek Mine, British Columbia, Using Terrestrial Photogrammetry (M.Sc. thesis) Simon Fraser University.

Leith, K.J., 2012. Stress development and geomechanical controls on the geomorphic evolution of alpine valleys (PhD. thesis) ETH, Zurich.

Lemy, F., Hadjigeorgiou, J., 2003. Discontinuity trace map construction using photographs of rock exposures. Int. J. Rock Mech. Min. Sci. 40, 903–917.

23 June 2016

SRK Consulting



Lévy, C., Baillet, L., Jongmans, D., Mourot, P., Hantz, D., 2010. Dynamic response of the Chamousset rock

column (Western Alps, France). J. Geophys. Res. 115 (13 pp.). Lorig, L., Stacey, P., Read, J., 2009. Slope design methods: three-dimensional model for rock slopes based

on micromechanics. Guidelines for Open Pit Slope Design. CSIRO Publishing, Collingwood, Victoria. Mas Ivars, D., Pierce, M.E., Darcel, C., Reyes-Montes, J., Potyondy, D.O., Young, R.P., Cundall, P., 2011.

The synthetic rock mass approach for jointed rock mass modelling. Int.J. Rock Mech. Min. Sci. 48, 219–244.

Matasci, B., Carrea, D., Jaboyedoff, M., Pedrazzini, A., Stock, G.M., Oppikofer, T., 2011. Struc- tural characterization of rockfall sources in Yosemite Valley from remote sensing data: toward more accurate susceptibility assessment. Proceedings of the 14th Pan- American Conference on Soil Mechanics and Geotechnical Engineering Conference, Toronto.

Matasci, B., Jaboyedoff, M., Ravanel, L., Deline, P., 2015. Stability Assessment, Potential Col- lapses and Future Evolution of the West Face of the Drus (3,754 m asl, Mont Blanc Massif). Engineering Geology for Society and Territory vol. 2. Springer International Publishing, pp. 791–795.

Mauldon, M., 1998. Estimating mean fracture trace length and density from observations in convex windows. Rock Mech. Rock. Eng. 31 (4), 201–216.

Miller, D.J., Dunne, T., 1996. Topographic perturbations of regional stresses and conse- quent bedrock fracturing. J. Geophys. Res. 101 (B11), 25523–25536.

Moffitt, K., Rogers, S., Beddoes, R., Greer, S., 2007. Analysis of slope stability in strong, fractured rock at the Diavik Diamond Mine, NWT. Rock Mechanics: Meeting Society's Challenges and Demands. Proceedings of the 1st Canada-US Rock Mechanics Sympo- sium, Vancouver, 27–31 May 2007, pp. 1245–1250.

Monte, J.M., 2004. Rock Mass Characterization Using Laser Scanning and Digital Imaging Data Collection Techniques (M.Sc. thesis) University of Arizona.

Optech Incorporated, 2008. ILRIS 3D, information available at product homepage. http:// www.optech.ca/i3dprodline-ilris3d.htm.

Ortega, O., Marrett, R., 2000. Prediction of macrofracture properties using microfracture information, Mesaverde Group sandstones, San Juan Basin, New Mexico. J. Struct. Geol. 22 (5), 571–588.

Ortega, O., Marrett, R.A., Laubach, S., 2006. A scale-independent approach to fracture intensity and average spacing measurement. AAPG Bull. 90 (2), 193–208.

Pan, E., Amadei, B., Savage, W., 1994. Gravitational stresses in long symmetric ridges and valleys in anisotropic rock. Int. J. Rock Mech. Min. Sci. Geomech. Abstr. 32, 201–214.

Paronuzzi, P., Serafini, W., 2009. Stress state analysis of a collapsed overhanging rock slab: a case study. Eng. Geol. 108, 65–75.

Pettitt, W., Pierce, M., Damjanac, B., Hazzard, J., Lorig, L., Fairhurst, C., Sanchez-Nagel, M., Nagel, N., Reyes-Montes, J.M., Andrews, J., Young, R.P., 2012. Fracture network engineering: combining microseismic imaging and hydrofracture numerical simulations. Proceedings of the 2012 American Rock Mechanics/Geomechanics Symposium, Chi- cago, June 24–27.

Priest, S.D., 1993. Discontinuity Analysis for Rock Engineering. Chapman & Hall, London (473 pp.). Priest, S.D., 2004. Determination of discontinuity size distributions from scanline data. Rock Mech. Rock.

Eng. 37 (5), 247–386. Priest, S.D., Hudson, J., 1976. Discontinuity spacings in rock. Int. J. Rock Mech. Min. Sci. Geomech. Abstr.

13, 135–148. Priest, S.D., Hudson, J., 1981. Estimation of discontinuity spacing and trace length using scanline survey.

Int. J. Rock Mech. Min. Sci. Geomech. Abstr. 18, 183–197.

23 June 2016

SRK Consulting



Rasband, W., 2008. ImageJ: Image Processing and Analysis in Java. National Institutes of Health, Research

Services Branch, National Institute of Mental Health, Bethesda, Maryland, USAhttp://rsb.info.nih.gov/ij/.

Ristau, J., 1994. Field Verification of a Step-Path Simulation Model for Rock Slope Stability Analysis (M.Sc. thesis) University of Idaho.

Sagong, M., Bobet, A., 2002. Coalescence of multiple flaws in a rock-model material in uniaxial compression. Int. J. Rock Mech. Min. Sci. 39, 229–241.

Scavia, C., 1990. Fracture mechanics approach to stability analysis of rock slopes. Eng. Fract. Mech. 35 (4), 899–910.

Shang, J., Hencher, S.R., West, L.J., 2015. Tensile strength of incipient rock discontinuities. In: Schubert, W., Kluckner, A. (Eds.), Future Developments of Rock Mechanics: Pro- ceedings of the SRM Regional Symposium EUROCK 2015 & 64th Geomechanics Col- loquium, 7–10 October, Salzburg, Australia.

Sjöberg, J., 1999. Analysis of Failure Mechanisms in High Rock Slopes. In: Vouille, G., Berest, P. (Eds.), Proceedings of Comptes-Rendus/9 Congrès International de mécanique de Roches, Paris, France vol. 1. Balkema Publishers, A.A. / Taylor and Francis, The Netherlands, pp. 127–130.

Stead, D., Coggan, J., 2012. Numerical modelling of rock slope instability. In: Clague, J., Stead, D. (Eds.), Landslides: Types, Mechanisms and Modelling. Cambridge.

Stead, D., Wolter, A., 2015. A critical review of rock slope failure mechanisms: the importance of structural geology. J. Struct. Geol. 74, 1–23.

Stead, D., Eberhardt, E., Coggan, J.S., 2006. Developments in the characterization of complex rock slope deformation and failure using numerical modelling techniques. Eng. Geol. 83, 217–235.

Stead, D., Coggan, J.S., Elmo, D., Yan, M., 2007. Modelling brittle fracture in rock slopes—experience gained and lessons learned. In: Potvin, Y. (Ed.), Proceedings of the 2007 International Symposium on Rock Slope Stability in Open Pit Mining and Civil Engineering. CSIRO, Perth, Australia, pp. 239–251.

Stock, G.M., Martel, S.J., Collins, B.D., Harp, E.L., 2012. Progressive failure of sheeted rock slopes: the 2009–2010 Rhombus Wall rock falls in Yosemite Valley, California, USA. Earth Surf. Process. Landf. 37 (5), 546–561.

Sturzenegger, M., 2010. Multi-Scale Characterization of Rock Mass Discontinuities and Rock Slope Geometry Using Terrestrial Remote Sensing Techniques (Ph.D. thesis) Simon Fraser University.

Sturzenegger, M., Stead, D., 2009. Close-range terrestrial digital photogrammetry and terrestrial laser scanning for discontinuity characterization on rock cuts. Eng. Geol. 106, 163–182.

Sturzenegger, M., Stead, D., 2012. The Palliser rockslide, Canadian Rocky Mountains: characterization and modeling of a stepped failure surface. Geomorphology 138, 145–161.

Sturzenegger, M., Yan, M., Stead, D., Elmo, D., 2007. Application and limitations of ground- based laser scanning in rock slope characterization. Rock Mechanics: Meeting Society's Challenges and Demands: Proceedings of the 1st Canada-US Rock Mechan- ics Symposium, Vancouver, 27–31 May 2007.

Sturzenegger, M., Stead, D., Elmo, D., 2011. Terrestrial remote sensing-based estimation of mean trace length, trace intensity and block size/shape. Eng. Geol. 119, 96–111.

Terzaghi, R.D., 1965. Sources of error in joint surveys. Geotechnique 15, 287–304. Tuckey, Z., Stead, D., Eberhardt, E., 2013. Combining field methods and numerical modelling to address

challenges in characterising discontinuity persistence and intact rock bridges in large open pit slopes. Proceedings of Slope Stability 2013, Brisbane, Australia.

Turner, B., Kelman, M., Ulmi, M., Turner, T., 2010. Sea to Sky GeoTour, Geology and Land- scapes Along Highway 99 from Vancouver to Whistler, British Columbia (34 pages (1 sheet) Natural Resources

23 June 2016

SRK Consulting



Canada (Available online: http://www.nrcan.gc.ca/earth- sciences/org/vancouver/geotour/7811 (Accessed 21 April 2012)).

Wong, L.N.Y., Wu, Z., 2014. Application of the numerical manifold method to model progressive failure in rock slopes. Eng. Fract. Mech. 119, 1–20.

Wong, R.H.C., Chau, K.T., Tang, C.A., Lin, P., 2001. Analysis of crack coalescence in rock-like materials containing three flaws – part I: experimental approach. Int. J. Rock Mech. Min. Sci. 38, 909–924.

Yan, M., 2007. Numerical Modelling of Brittle Fracture and Step-path Failure: From Labo- ratory to Rock Slope Scale (PhD thesis) Simon Fraser University.

Zhang, G., Zhao, Y., Peng, X., 2010. Simulation of toppling failure of rock slope by numerical manifold method. Int. J. Comput. Methods 7 (01), 167–189.

Zhang, L., Einstein, H.H., 2000. Estimating the intensity of rock discontinuities. Int. J. Rock Mech. Min. Sci. 37 (5), 819–837.

Zhang, L., Einstein, H.H., 1998. Estimating the mean trace length of rock discontinuities. Rock Mech. Rock. Eng. 31 (4), 217–235. Ziegler, M., 2014. Age and Formation Mechanisms of Exfoliation Joints in the Aar Granites of the Central

Alps (Grimsel Region, Switzerland) (PhD. thesis) ETH, Zurich.

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