NEW DEVELOPMENTS IN ROCK SLOPE ENGINEERING DEVELOPMENTS IN ROCK SLOPE ENGINEERING: IMPLICATIONS FOR...

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  • NEW DEVELOPMENTS IN ROCK SLOPE ENGINEERING: IMPLICATIONS FOR OPEN PIT SLOPE DESIGN Doug Stead, Department of Earth Sciences, Simon Fraser University, Burnaby, BC. Ming Yan, Department of Earth Sciences, Simon Fraser University, Burnaby, B.C. Davide Elmo, Department of Earth Sciences, Simon Fraser University, Burnaby, B.C. John Coggan, Camborne School of Mines, University of Exeter, U.K. ABSTRACT Recent years have seen considerable advances in both methods of rock slope data collection and the numerical modeling of slope failure mechanisms. This paper will briefly review new methods of data collection and monitoring and their impact on future design of open pit slopes. Conventional methods of rock slope analysis will be contrasted with numerical approaches incorporating techniques to investigate the importance of intact rock fracture and three dimensional structure. Implications of three dimensional structural variations on lateral footwall rock slope failure release mechanisms will be discussed using simple 3-D distinct element models. The role of intact rock bridges in step-path generation will be emphasized and illustrated with reference to major open pit slope failure mechanisms. The potential application of state-of-the-art coupled finite-discrete element models with fracture generation and propagation capability will be demonstrated.

    RSUM Des progrs importants ont t faits ces dernires annes dans les mthodes dchantillonnage des donnes de pentes rocheuses et dans la modlisation numrique des mcanismes de rupture. Cet article passera brivement en revue ces mthodes dchantillonnage et de surveillance, et commentera leur impact sur la conception de mines ciel ouvert. Les mthodes traditionnelles danalyse de pentes rocheuses seront compares avec les mthodes numriques, en particulier des techniques dinvestigation de limportance des fractures et structures tridimensionnelles au sein de la roche intacte. Limplication des variations structurales tridimensionnelles sur les mcanismes de dcollement latraux des surfaces de ruptures sera illustre par des simples modles tridimensionnels aux lments distincts. Un intrt particulier sera porte sur le rle des ponts rocheux dans la gnration de surfaces de ruptures dans des pentes importantes de mines ciel ouvert. Enfin, une application potentielle de modles aux lments finis-distincts avec gnration de fractures et capacit de propagation sera prsent. 1. INTRODUCTION Numerical modelling of open pit mine slopes has over the last 5 years become increasingly common. At the recently held South African Institution of Mining Symposium on rock slopes (S.A.I.M.M. 2006) numerous papers documented the application of both conventional limit equilibrium models and numerical models. This increase in numerical modelling can be largely explained by progressively deeper open pits in which stress concentrations cannot be ignored. Open pits of over 800m have been successfully mined and the Chuquicamata pit in Chile has a planned final pit depth of 1100m - before it resorts to underground mining, (Olavarria et al. 2006). The transition from deep open pits to underground block caving mining pose significant challenges to the rock mechanics engineer in understanding surface - underground interaction mechanisms. Moss et al (2006) and Brummer et al. (2006) describe such a transition from surface to underground and the development of a major open pit wall failure at the 800 m deep Palabora mine in association with block caving at 400m beneath the pit. Increasingly deeper pits, with in some cases a move to underground mining, entails a major paradigm shift in the application of rock mechanics to open pits. Limit equilibrium techniques which were appropriate in the study of single/multi-bench instabilities may not be suitable for the analysis of deep open pit slopes. Similarly it has become essential to modify data collection

    techniques, not only to gather more comprehensive structural and rock mass data but also to allow integration of this data with slope modelling techniques. The importance of in-situ stress and extensile strains was clearly outlined by Stacey et al. (2003) in a parametric elastic analysis of deep open pits. Stead et al. (2004) emphasized the role of fracture mechanics and intact rock fracture in the failure of natural rock slopes; a transition to deeper and/or undermined open pits makes this a critical issue in open pit stability. This paper will review new developments in data collection and analysis, rock slope monitoring and numerical modelling with particular reference to their application to open pit slopes. Figure 1 shows a flow chart summarising the constraints provided by new data collection/monitoring technologies and the range of slope analysis techniques available.

    2. TRENDS IN PIT SLOPE CHARACTERIZATION

    AND MONITORING Geotechnical data collection techniques in open pit mining have changed considerably during the last 5 years. The importance of characterizing discontinuity properties for both groundwater flow and slope stability has led to more sophisticated borehole fracture orientation methods including acoustic televiewers and cameras. Surface data collection techniques have undergone a revolutionary change from simple line scan/window bench mapping to

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  • Figure 1. Flow chart showing current rock slope data collection, monitoring and analysis techniques

    include a combination of digital photogrammetry and 3D-laser scanning techniques. Digital photogrammetry uses SLR digital cameras mounted on a tripod which record overlapping photographs. These images are imported into photogrammetric software such as Sirovision (Poropat and Elmouttie 2006) in order to derive a digital elevation model of the rock face. It is then possible to extract discontinuity data and produce stereonets showing joint sets within the mine slope in addition to other discontinuity properties (Figure 2). These digital photogrammetric derived DEMs can be readily imported into mine design packages. The advantage of this technique is that a rapid map of the mine slope is provided with structural data obtained from often inaccessible locations. Read and Ogden (2006) and Porpopat and Elmouttie (2006) describe ongoing developments in the use of a digitally derived Siromodel" able to interface with advanced numerical modelling codes. Three dimensional scanning lasers are also a common instrument at major open pit mines and now routinely incorporated into mine slope characterization, (Little, 2006). The use of 3D laser scanning in surface mines is wide-ranging including blast design, volume calculations, instability monitoring and geotechnical characterization. Figure 3 shows a 3D-laser scan of a rock slope undertaken with an Optech ILRIS-3D laser scanner. The point cloud, x, y, z, data is imported into the SPLIT-FX code (Kemeny and Donovan 2005) and structural data obtained. Figure 4 shows the use of a Quarryman 3D-laser scanner to monitor the development of a structurally controlled failure in a kaolinized granite mine slope, (Stead et al.

    2005). Repeated surveys allowed the retrogression of the failure with time to be monitored. Future integrated use of mine slope stability modelling codes with digitally derived rock slope faces, failure surfaces etc will be an important development in surface mining geotechnics.

    Figure 2. Digital photogrammetric image of rock slopes in slate (top) and granite quarries (bottom) Characterization of rock slope discontinuities is an important element in rock mass classification - characterization methods. Statistical characterization of

    Engineering GeologyMapping/GSIMech Props

    Laser scanningPhotogrammetry

    MonitoringTotal Stations

    Laser scanningPhotogrammetry

    Slope Stability RadarMicroseismic

    Borehole inclin./extens.

    Kinematic (DIPS)Split-FX

    Discrete Fracture NetworksFracman

    Key Block Theory

    Limit Equilibrium

    ContinuumShear Strength Reduction

    FLAC/PHASE2

    Discontinuum ApproachNo Fracture/Rigid or Plasticity

    (UDEC/3DEC)

    Discontinuum Approach: Particle Flow CodePFC2D/3D, ELFEN

    Desig

    n C

    harts

    Discontinuum ApproachCoupled with Fracture Mechanics

    ELFEN

    CONSTRAINTS

    Engineering GeologyMapping/GSIMech Props

    Laser scanningPhotogrammetry

    MonitoringTotal Stations

    Laser scanningPhotogrammetry

    Slope Stability RadarMicroseismic

    Borehole inclin./extens.

    Kinematic (DIPS)Split-FX

    Discrete Fracture NetworksFracman

    Key Block Theory

    Limit Equilibrium

    ContinuumShear Strength Reduction

    FLAC/PHASE2

    Discontinuum ApproachNo Fracture/Rigid or Plasticity

    (UDEC/3DEC)

    Discontinuum Approach: Particle Flow CodePFC2D/3D, ELFEN

    Desig

    n C

    harts

    Discontinuum ApproachCoupled with Fracture Mechanics

    ELFEN

    CONSTRAINTS

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  • discontinuity data using discrete fracture networks (DFNs) has been reported extensively within the literature, (Derschowitz et al. 2004). The use of these techniques in surface mining has yet to be fully exploited. Developments in both digital data collection techniques and computing power for numerical modelling make DFN techniques particularly appropriate. The recognition of the potential importance of stress-induced intact rock fracture and discontinuity controlled step-paths in high mine slopes will ensure increasing use of DFNs integrated with both digital rock slope surface models and numerical codes.

    Figure 3. 3D-Laser scan point cloud of a rock slope and structural data derived using the Split-FX code, Kemeny and Donovan (2005)

    Figure 4. 3D-Laser scan of a mine slope failure in kaolinized granite Digital rock ma