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Slope Stability 2013 – P.M. Dight (ed) © 2013 Australian Centre for Geomechanics, Perth, ISBN 978-0-9870937-5-2

Slope Stability 2013, Brisbane, Australia 775

M. Sturzenegger Klohn Crippen Berger Ltd., Canada

D. Willms Klohn Crippen Berger Ltd., Canada

K. Pate Seattle City Light, USA

B. Johnston Tetra Tech Inc., USA

This paper reports on projects, which integrate the advantages of terrestrial remote sensing techniques to analyse the performance of rock slopes. The first project is concerned with a bench performance assessment of an inactive open pit mine in British Columbia, Canada; this assessment is part of the design work for the proposed pit expansion. The second project involves characterisation of the rock abutments of an operational concrete hydroelectric dam, in the Washington State, USA.

The bench scale stability of mine slopes in blocky to moderately fractured rock mass is primarily determined by structurally-controlled failure mechanisms such as planar, wedge and toppling failure. Conventional design involves the analysis of adversely-oriented discontinuities, which have the potential to generate unstable blocks. These kinematic analyses are used to develop design parameters, including bench face angle, bench width and inter-ramp angle. Bench performance assessment aids in both bench and inter-ramp design, based on detailed quantification of the previously achieved bench geometry.

The first case study is from geotechnical investigation work at Bell Pit, near Granisle, BC. The bench performance assessment in this case utilises terrestrial digital photogrammetry to quantify the achieved bench geometry of the pit walls. The advantages of using remote sensing data as opposed to measurements made at the outcrop include: a more detailed and arguably more accurate dataset, reduction of issues related to access, and reduction of safety concerns due to rock falls. Similar advantages are advertised in the literature concerning remote sensing-based discontinuity characterisation, which is now commonly used in the industry. In this case study, a geometric correction is applied to the measured back-break and effective bench face angle in order to enhance measurement accuracy. A discussion concerning the validity of the assessment is provided.

The second case study involves discontinuity characterisation of the natural rock slope abutments of the Boundary Dam, a concrete hydro-electric dam located along the Pend Oreille River. These abutments show evidence of past rock block failures where blocks failed along discontinuities. Structural mapping using terrestrial remote sensing techniques allows both the characterisation of these discontinuities and the description of past failure mechanisms, which may highlight potential future failure modes. The procedure is carried out along the Pend Oreille River canyon using a combination of terrestrial and vessel-mounted LiDAR (Light Detection and Ranging) point clouds.

Rock slope performance assessments, on both natural and man-made rock exposures, include measurements of slope geometry and observation of past failure mechanisms. In open pit mines, for example, bench performance assessment consists of the measurements of achieved bench parameters, including effective bench face angle, back-break, bench height and bench width. These parameters can be compared to planned design parameters in order to validate a design based on limited drillhole and rock exposure data (Mathis, 2007; Read and Stacey, 2009). Systematic documentation and evaluation of the

Experience using terrestrial remote sensing techniques for rock slope performance assessment M. Sturzenegger et al.

776 Slope Stability 2013, Brisbane, Australia

performance of benches is an important component of any open pit slope assessment program. During early mine excavation stages, performance analysis of test benches should provide validation of the design model. Slope performance assessment will also be part of the monitoring program during the mine life, and for pit expansion projects.

The availability and reliability of rock slope parameters measured based on field measurement is commonly reduced by limited access to the rock slope and due to safety concerns. The resolution of aerial surveys on steep rock slopes is often too low to provide adequate measurements. Terrestrial remote sensing techniques, such as terrestrial photogrammetry and laser scanning are well suited for this purpose. Validation of bench design and back-break prediction using terrestrial photogrammetry data has been reported by Mathis (2007, 2011). Photogrammetry-based measurement of bench parameters, including the height of failure debris and bench face roughness (morphology), were also carried out by Lee (2011) and Tuckey (2012) to highlight zones where bench failures occurred and left scars along the bench face.

The purpose of this paper is to report on the application of terrestrial remote sensing techniques for rock slope performance assessments for two industry-based projects: the expansion of the Bell Pit in British Columbia, Canada, and the inspection of the Boundary Dam abutments in Washington State, USA. For the bench performance assessment at Bell Pit, a geometric correction is applied to the measured back-break and effective bench face angle in order to enhance data accuracy. In addition, a discussion concerning the validity of the assessment is provided. At Boundary Dam, potential failure mechanisms were highlighted based on LiDAR-based inspection of the dam abutments. Issues related to the application of the techniques at both sites are discussed.

Bell Pit is located on the Newman Peninsula, in the Babine Lake region of west-central British Columbia, approximately 65 km east of the town of Smithers (Figure 1). Bell pit operated between 1972 and 1992 (Dirom et al., 1995). A pre-feasibility study is currently examining the possibility of re-opening and expanding the pit.

The geology of the area consists of the Intermontane Belt, comprising a number of accreted terranes of Paleozoic and Mesozoic island arc volcanic and sedimentary rocks (MacIntyre, 2001; Dirom et al., 1995). These rocks were intruded by both Cretaceous and Eocene plutons and associated sub-volcanic rocks. The copper-gold mineralisation resulting from hydrothermal alteration is found in the Eocene intrusions. The Bell Pit is transected by the northwest–southeast striking Newman Fault, another ENE–WSW striking main fault and a number of other minor faults.

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A terrestrial digital photogrammetry survey was undertaken using a Nikon 90D digital camera with a 55 mm lens. The photogrammetry survey was undertaken from the top of the pit walls. Photographs taken in the field were processed using the software 3DM CalibCam (Adam Technology, 2012a) and 3DM Analyst (Adam Technology, 2012b) in order to generate the 3D models and measure bench parameters.

Figure 2 illustrates the main bench parameters as defined in Ryan and Pryor (2000) and represents a typical bench from the Bell Pit:

The effective bench face angle (EBFA) is the angle to which the bench face is expected after blasting and subsequent back-break. It represents the angle from bench toe to crest.

The back-break is the horizontal distance between the bench toe and achieved bench crest. In some cases, most of the back-break occurs along non-persistent discontinuities near bench crest; overbreak is the horizontal distance between the planned and achieved bench and may occur along the face, at the toe or at the crest of the bench.

Bench width is the distance between the achieved bench crest and the toe of the following bench.

Bench height is the vertical distance between benches and is usually controlled by the selection of mining equipment.

As most bench toes were covered by rock debris, the apparent toe at the top of the debris was measured. The resulting back-break, bench width and height measurements are consequently apparent and must be corrected geometrically in order to get the true back-break, bench width and height, respectively.

For each design sector of the pit, a cumulative distribution function of EBFA was derived based on measurements along multiple cross sections separated by 20 m interval. This interval was considered adequate in order to provide a statistically significant amount of measurements. It could be reduced for



small design sectors in order to provide more data; however, depending on the persistence of bench−controlling discontinuities, a small sample interval value may result in capturing several times the same failure or a back-break due to the same discontinuity.

At Bell Pit, benches are generally composed of two rock faces with angles ranging between 65 and 80° (Figure 2). Bench geometry is influenced by the double bench mining method, which involves an offset between the lower and upper rock faces (Figure 3(a)). The total back-break is due to a combination of this mining method, rock mass quality and jointing. In some cases, adverse discontinuities generate overbreak near bench crest or at mid-bench height (Figure 3(b)). Some benches were observed to be controlled almost entirely by bench scale discontinuities or wedges (Figure 3(c)); others have a more irregular geometry due to a more complex joint configuration (Figure 3(d)).

Figure 4 illustrates the difference between true and apparent EBFA. Figure 4(a) and (b) show that 80% of the achieved EBFA are steeper than 61 and 57°, respectively.

During bench performance assessment, evidence of bench failures, such as wedges or slabs along the bench faces was reported and the associated failure mechanisms were analysed. In the two sectors presented here, crest overbreak is mostly caused by planar failures along moderately steep joints. It was observed along both sectors that some benches were not wide enough to contain failure debris, because of the presence of these undercutting joints.

The angle of repose of the bench failure debris cones measured using terrestrial digital photogrammetry ranges between 28 and 37°, with an average of 34°.

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Incorporating such performance assessments into bench design is complicated by the fact that bench deformation is controlled by many parameters, including rock mass strength, discontinuity parameters and blast damage. The measurements or estimates of these parameters can be subject to significant uncertainty. For example, the back-break is controlled by different factors in different rock types (Read and Stacey, 2009). In massive competent rock masses, it is controlled by the type and layout of the blast pattern. In blocky to moderately fractured rock masses, its control is likely to be adverse discontinuities. Degradation and unravelling are major contributors to back-break in intensely fractured rock mass and rotational soil-type failures are typically responsible for it in very weak rock masses. The rock type should determine the main back-break controlling factor and consequently the bench design approach.

At Bell Pit, the rock mass is blocky to moderately fractured and back-break is predominantly controlled by adverse discontinuities. It was observed that most of the time, these discontinuities are located near bench crest, although a number of them with bench-scale persistence control entire bench faces. Comparison between the achieved EBFA (61 and 57°, for Sectors 1 and 2 respectively, at 80% reliability) and a suggested design bench face angle (65°) highlights a 4 and 8° difference, respectively. This difference should be taken into consideration in the future design, by either decreasing the design bench face angle and/or increasing bench width. However, it should be noted that future mining practice may differ from the original mining practice at Bell Pit. The observed benches contain an offset at mid-bench height resulting from the double bench mining method (Dirom et al., 1995). Dependant on the mining method used for the pit expansion, the achieved EBFA measured at the current Bell Pit may not be appropriate to validate our design. Consequently, the design should be reviewed following observations of bench performance during the early expansion stages of the pit.

It was shown, in Section 2.1, that apparent measurements of bench parameters should be corrected, because the true bench toes are often hidden by rock debris. Figure 4 shows the difference between apparent and true effective bench face angle distributions. For the two Bell Pit sectors illustrated in this paper, the difference between the apparent and true back-break would represent a 2° variation of the inter-ramp angle.

Rock debris at the bench toes was also reported, including a note about whether the debris was contained or not by the bench; such observations could be facilitated using the methods presented by Lee (2011) and Tuckey (2012). However, it is difficult to evaluate the reliability of these observations, since Bell Pit was closed and has been left without wall maintenance for a number of years, and consequently the debris was not regularly cleaned off or monitored; consequently the debris could be the results of single or successive rock fall events.



Boundary Dam is located on the Pend Oreille River, in Pend Oreille County in Washington State, just south of the border with British Columbia (Figures 1 and 5). The assessment of the Boundary Dam abutments is part of an inspection program, which takes place every five years. It is conducted in order to provide recommendations if stabilisation work is required.

The main geological unit is the Metaline Limestone, which contains an upper layer of massive limestone, an intermediate unit of bedded dolomite and a lower sequence of thin-bedded limestone and shale (Dings and Whitebread, 1965). At a regional scale, these sedimentary rocks have been folded and faulted.

At the site, the Metaline Limestone is described by Haneberg Geoscience (2010) as weathered and jointed with steeply dipping bedding. The dolomitic limestone includes solution collapse breccias and several prominent faults. Major joints are described in Jacobs Associates (2010) as often iron stained, ranging from planar to undulating, and from tight to open with some clay/shaly seams up to 3 inches in thickness.

Point clouds in three dimensions were provided by Tetra Tech Inc., based on a combination of vessel−mounted LiDAR (VML) and terrestrial laser scanner (TLS) surveys. Discontinuity characterisation was achieved using the module IMSurvey of the Polyworks software (Innovmetric Software Inc., 2012) and based on the methodology described by Sturzenegger and Stead (2009a), in order to measure discontinuity position, orientation and persistence. When the resolution of point clouds is high enough, it is also possible to quantify discontinuity spacing and block sizes, but a number of limitations, mostly related to 3D model resolution, can arise as shown in Sturzenegger et al. (2011).

During discontinuity characterisation along both east and west abutments (Figure 6), evidence of past failure was recognised when exposed discontinuity surfaces suggest a block has fallen or been scaled (e.g. Figure 6(a)). Figure 6(b) illustrates how the likely failure mechanism could be described based on kinematic analysis. Potential unstable rock blocks formed by one or a combination of discontinuities were also observed and similarly described.

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The Boundary Dam project illustrates how the use of terrestrial remote sensing techniques is well-suited to observe and characterise past and future potential failure mechanisms, even in steep terrain, where rock slopes are difficult to access. The authors have built experience and successfully applied the same technique in other remote and mountainous areas. The particularity of this project is that, due to the presence of a river at the base of the rock slopes, terrestrial remote sensing data were combined with vessel-mounted LiDAR data.

When characterising discontinuities, sampling bias should be minimised. A detailed review of the potential bias, which should be accounted for when using terrestrial remote sensing techniques, can be found in Sturzenegger and Stead (2009a, 2009b) and Sturzenegger et al. (2011). The bias associated with discontinuity persistence quantification is particularly difficult to deal with and still requires further research. Statistical sampling techniques based on window mapping providing trace length distribution estimates were presented by several authors, including Mauldon et al. (2001) and Zhang and Einstein (1998), but may be of limited use for quantification of potentially persistent discrete discontinuities. In certain situations, it may be possible to use of a combination of techniques in order to estimate the true persistence of discontinuities. This proved to be successful in another project, where the authors were able to recognise along a drillhole a minor thrust fault previously mapped on a rock face; this finding allowed the authors to estimate the inclination of the fault and determine that its orientation is kinematically favourable with respect to the rock slope orientation.

This paper presents two case studies illustrating the application of terrestrial remote sensing techniques for the assessment of rock slope performance on open pit mine benches and dam abutments, respectively. The first study focuses on the description of the Bell Pit benches and observation of their controlling factors. Terrestrial photogrammetry allows quantification of the achieved effective bench face angle, and comparison with the design bench face angle for use in planning pit expansion. It is advised that comparison should be made carefully, because the mining techniques used while the pit was in operation, between 1972 and 1992, may be different from the ones chosen for a future project.



The second study presented in this paper demonstrates how terrestrial remote sensing-based discontinuity characterisation can be used to describe the failure mechanism present along a rock slope and to highlight the presence of potential instabilities. This approach is becoming more frequently used in the industry, especially for the characterisation of rock slope in challenging environment. This is illustrated here along the abutments of the Boundary Dam.

We thank Arie Moerman at Xstrata Copper Canada, and Seattle City Light for granting permission to use project data in this paper and providing useful comments on the paper.

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