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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.

RÉSUMÉ Des progrès importants ont été faits ces dernières années dans les méthodes d’échantillonnage des données de pentes rocheuses et dans la modélisation numérique des mécanismes de rupture. Cet article passera brièvement en revue ces méthodes d’échantillonnage et de surveillance, et commentera leur impact sur la conception de mines à ciel ouvert. Les méthodes traditionnelles d’analyse de pentes rocheuses seront comparées avec les méthodes numériques, en particulier des techniques d’investigation de l’importance des fractures et structures tridimensionnelles au sein de la roche intacte. L’implication des variations structurales tridimensionnelles sur les mécanismes de décollement latéraux des surfaces de ruptures sera illustrée par des simples modèles tridimensionnels aux éléments distincts. Un intérêt particulier sera porte sur le rôle des ponts rocheux dans la génération de surfaces de ruptures dans des pentes importantes de mines à ciel ouvert. Enfin, une application potentielle de modèles aux éléments finis-distincts avec génération de fractures et capacité de propagation sera présenté. 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 DEM’s 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

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discontinuity data using discrete fracture networks (DFN’s) 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 DFN’s 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 mass ratings, Monte (2004) have been developed based on laser scanning imagery and may become increasingly important in future engineering practice. An increasing use of the Geological Strength Index, G.S.I., (Hoek et al. 2005, Hoek and Diederichs 2006) is being made in surface mine slope design and engineers now routinely derive strength and

deformation parameters using the GSI and the RocLab code, (RocScience 2005). Both techniques must however be applied in the modelling of mine slopes with caution and must consider sound engineering principles including the potential failure modes and scale effects. Harries et al. (2006) discuss the use of slope stability radar (SSR) in the monitoring of slope movement at an open cast mine. This method is capable of producing sub millimetre precision 24 hour scanning at frequent intervals (15 minutes). Over 70 rockfalls/dump failures have been monitored to date and in all cases precursor warning movements were recorded. Data presentation is either as colour “rainbow-heat” plots of total movement showing both the extent and area of greatest activity and as time-displacement graphs. A heat map of wall movement is shown in Figure 5. Alarms levels may be pre-selected. Cahill and Lee (2006) describe the combined use of prism monitoring and slope stability radar at the Leinster Nickel open pit. Over three years of experience with this method have been achieved and radar is used to continuously monitor selected walls where low confidence in prism or visual observations exists. A scan can be made every 10-15 minutes. Figure 6 shows an example of slope stability radar monitoring at the Harmony pit, S. A. Significant acceleration of movement was observed 12 hours prior to failure. SSR has also been successfully used to monitor repeated slab failures and small failures not picked up by other survey methods. This method has been used in conjunction with laser scanning, digital photogrammetry, survey prisms and microseismic monitoring (Little 2006).

Figure 5. Slope stability radar deformation “heat map” indicating rockfall failure, after Harries et al. (2006) Microseismic monitoring systems have found widespread and successful use in underground mining but until recently have had limited application in surface mining. Recording of microseismic activity in conjunction with conventional borehole and surface monitoring instrumentation may hold considerable promise in furthering our understanding of brittle rock slope deformation mechanisms. Willenberg et al. (2004)

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and Bland et al. (2005) discuss microseismic installations at the Randa slide, Switzerland and the Frank slide, Canada respectively. These installations are aimed both at monitoring ongoing deformation induced seismicity and characterizing the rock slope failure processes.

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Figure 6. Slope stability radar deformation correlated against open pit mining activity, Cahill and Lee (2006) Recent years have seen an increased number of microseismic monitoring schemes in open pit mines. Lynch and Malovichko (2006) state that since 2002 twenty five open pit mines have been instrumented in Southern Africa and Australia and all have indicated through microseismicity ongoing brittle fracturing processes. These systems allow seismic event location and source parameters (fracture dimension-average slip) to be determined with a potential to improve our understanding of both energy changes with mining activity and slope failure. They noted that microseismicity was recorded even in mines with limited depths (as low as 80m). As open pits get deeper, stress-induced fracturing becomes more important and results in increased microseismic activity. Recent work described by Lynch and Malovichko (2006) at the Navachab mine, S.A. has shown an interesting time difference of 1-2 months between slopes indicated to be undergoing deformation by microseismicity and by measured surface movements. Figure 7 shows cumulative microseismic events correlated with mine excavation. By combining continuum numerical modelling using FLAC (Itasca 2005) with microseismic monitoring of a 180m high slope these workers have shown seismic events to be concentrated along major joint sets sub-parallel to the pit wall. Microseismic monitoring of open pit mine slopes holds considerable potential in the future constraint of numerical models that are able to simulate intact rock fracture and step-path failure. Microseismic monitoring is an extremely important technique when transitioning from open pit to underground caving providing critical information on both the caving processes and induced open pit slope movements, Moss et al. (2006).

Figure 7. Cumulative microseismic events associated with surface mining activity, Lynch and Malovichko (2006) 3. CURRENT TRENDS IN SLOPE ANALYSIS

Coggan et al. (1998) described the application, advantages and limitations of conventional limit equilibrium, continuum and discontinuum approaches in quarry slope design. Although further developments in slope analysis techniques have occurred these observations are still valid. Limit equilibrium techniques continue to be in widespread use in the design of surface mine slopes with an increase in the use of probabilistic techniques. A suite of limit equilibrium codes including RocPlane, Swedge, RocFall and Slide (RocScience 2005) are in routine use. Available limit equilibrium methods have been extended to allow coupling with groundwater flow models, probabilistic analysis, sensitivity analysis and important practical aspects including potential failure geometries and reinforcement. Although these codes prove useful in shallow mines with single and multi-bench operations, they are less appropriate in deep open pit mines where high stresses and intact rock fracture must be considered. The importance of step-path geometries was first recognised in early work by Jennings (1970). This was extended by probabilistic limit equilibrium step-path approaches by Baczynski (2000). Realistic simulation of fracturing of intact rock bridges in major open pit mine slopes will require use of advanced numerical models. Continuum numerical modelling codes are now routinely used in the design and analysis of major rock slopes. Jing and Hudson (2002) provide an excellent

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review of currently available methods. At the recently held 2006 SAIMM conference the continuum finite difference code FLAC (Itasca 2005) appeared to be the most commonly applied numerical code in case studies presented. Figure 8 shows the use of the FLAC 2D and 3D codes in the modelling of surface mine slopes, (Stead and Eberhardt 1998 and Coggan et al. 2000) respectively.

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Interval = 2.5e+000 Figure 8 – Continuum finite difference models of a footwall slope (FLAC2D) and quarry slope (FLAC3D) The main limitation of continuum codes is their inability to consider the blocky behaviour of jointed media. Although continuum codes are extremely useful in the early stages of modelling to characterize the stress distribution and locations of potential stress induced failure it is important to consider the kinematics within discontinuum controlled mine slopes. A major advance in continuum codes is the development of shear strength reduction techniques to allow a critical factor of safety for a surface with in a mine slope to be derived by progressively reducing strength properties, Dawson et al (1999). This approach provides a useful transition between conventional limit equilibrium techniques and numerical methods. It is important however to fully consider the assumptions inherent in the approach and the continuum vs. discontinuum characteristics of the mine slope. Facility exists to assume a direction of weakness within continuum codes using a ubiquitous approach and/or discrete interfaces however this should always be followed by discontinuum modelling where appropriate.

3.1 Discontinuum modelling Discontinuum codes now used in rock slope analysis include the following:

· Conventional 2-D distinct element methods · 3D- distinct element models · Particle flow codes · Coupled finite element -discrete element

codes (with fracture propagation) · Hybrid approaches.

The most common two dimensional distinct element method in rock slope engineering is the UDEC code (Itasca 2005). This code is now used routinely in surface mine analysis particularly to investigate footwall failure mechanism, toppling and complex failures in two-dimensions, Figure 9.

Figure 9. 2D-numerical modelling of toppling/footwall failures (Stead et al. 2005, Stead and Eberhardt 1998) Although allowing for jointed media the limitations of two dimensional representation and non-simulation of intact rock fracture have led to increasing research in the application of both three-dimensional discontinuum models and codes capable of simulating intact rock fracture. 4. THREE-DIMENSIONAL VS. TWO DIMENSIONAL

FAILURE

Although the importance of lateral release surfaces have long been recognized the precise mechanism of lateral release has to a large extent been neglected. Similarly the assumption of simple translational movements may be an over-simplification. Often blocks fail by a rotational mechanism along the apparent dip.

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Figure 10 shows examples of lateral release surfaces, intact rock fracture and the development of rotational block failures.

Figure 10. Lateral release and intact rock fracture Figure 11 shows a preliminary analysis (Stead et al. 2005) using the three dimensional distinct element code, 3DEC. Although failure through conventional down-dip translation may not be kinematically feasible; rotation of blocks may occur where the dip of the longitudinal cross joints is greater than the angle of friction. In highly deformed tectonic areas the need to allow for not only the dip of fold limbs but the plunge of the fold itself becomes especially important in slope design.

Figure 11. 3DEC models of a 250m high slope with 3 discontinuity sets showing rotational displacements and lateral step-path release. 5. MODELLING INTACT ROCK FRACTURE IN

ROCK SLOPES FAILURES The excavation of large open pits (with/without underground interaction) and the need to assess the hazard posed by major mountain slopes has resulted in an increased awareness of the importance of intact rock fracture as an integral component of rock slope failure mechanisms. Traditional continuum and discontinuum codes may not be appropriate where intact rock fracture accompanies internal dilation, basal

shear, lateral release and transport. Two lines of approach have been used, and show promising results. 5.1 Particle Flow Codes and the simulation of slope

failures. Particle flow codes are able to represent intact fracture of rocks and flow of mobilised debris though the use of two dimensional particles or three dimensional spheres, Bonds may be specified between the particles which are broken when the imposed stress exceeds their strength. Simulation of waste dump failures, debris flows, piping and structurally controlled rock slope failures incorporating intact rock fracture have all been published in the literature. Figure 12 shows examples relevant to mining. Read and Ogden (2006) describe the projected use of particle flow codes in association with digital imaging and statistical techniques (SIROMODEL) in the analysis of large open pit slopes.

Figure 12. PFC2D models. a. Bilinear rock slope, Stead et al. (2005) b. Hard rock over soft layer, (Preh and Poisel 2002) c. Footwall failure, (Wang et al. 2003) 5.2 Combined finite-discrete models incorporating

fracture mechanics.

A wide variety of rock slope failure mechanisms have been simulated using the combined finite-discrete element code, ELFEN, (Rockfield Ltd 2005). These include planar, biplanar, ploughing, buckling, toppling, wedge and step-path. (Stead et al. 2004, 2005) The progressive failure of complex landslides has been simulated through step-path development. The ELFEN code employs a Mohr plasticity constitutive criterion coupled with a Rankine rotating crack cut-off. When the designated constitutive parameters are exceeded by induced stresses eg. the tensile strength, a crack is formed within the finite element mesh. Re-meshing occurs, cracks are able to grow and particles form. In this way the code allows a

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Figure 13. a.) Simulation of footwall buckling failure: through-going discontinuities. b.) Numerical model of 900m high Randa rock slope: step-path limited persistence discontinuities c.) Preliminary numerical model of rock slope using generated DFN and showing internal deformation/crushing within slope transition from continuum to discontinuum behaviour. ELFEN modelling of rock slopes has progressed through three stages: i) simulation of varied failure mechanisms assuming through-going persistent discontinuities, Figure 13a. ii) simulation of selected case studies assuming non-persistent coplanar or step-path discontinuities, Figure 13b. iii) preliminary simulation of rock slopes assuming discrete fracture networks (Fracman) and non-persistent discontinuities, Figure 13c. The first stage emphasized the ability of ELFEN to simulate the transition from intact rock blocks to failure debris and the potential for a “total slope analysis” approach, (Stead and Coggan 2006, Stead et al. 2006). The second stage has shown the important role of step-path failure through intact rock bridges in the analysis of the 900m high Randa rock slide. This work is of fundamental importance to large open pits. The third stage is in progress and preliminary work indicates the important role of internal deformation, dilation and crushing prior to eventual rock slope failure. This stage of the research entails generation of discrete fracture networks using Fracman and the simulation of the required intact rock fracture in order to develop a through-going failure surface. 6. CONCLUSIONS Rock slope analysis in surface mining can be considered at scales varying from single to multi-benches to large open pit slopes (600m to 1100m in height). As the scale of open pit slopes has increased the need to realistically consider persistence of discontinuities in modelling becomes increasingly important. The experience provided through recent step-path simulation of the 900m high Randa rock slope in Switzerland is consequently highly relevant. With the transition from large open pit slopes to underground

mines a realistic analysis of slopes must involve representative discontinuity networks and intact rock fracture simulation. The authors argue from previous research that a consideration of intact rock fracturing is appropriate at all scales – from the shearing of asperities of individual discontinuities through to large open pit slope failures – the larger the scale the more essential it is to use codes that are able to realistically allow for limited discontinuity persistence. These techniques albeit at the research stage today will over the next decade become increasingly routine in application. It is essential that sophisticated numerical techniques are constrained by sophisticated-rigorous data collection and monitoring practices. This paper has attempted to show how recent developments in these areas are providing a promising foundation on which to extend our understanding of the failure of large rock slopes – both natural and mined. 7. ACKNOWLEDGEMENTS This work was funded by an NSERC Discovery grant and a Forest Renewal of BC Endowment to the second author. 8. REFERENCES

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Moss, A., Diachenko, S. and Townsend, P. (2006). Interaction between the block cave and the pit slopes at Palabora mine. In Stability of rock slopes in open pit mining and civil engineering situations. Johann., S.A., S.A.I.M.M., pp. 399-409.

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