Integrated Modelling of Subsidence Mechanisms and Impacts Due to Mine Caving

Montreal Energy & Mines, Montreal, April 29- May 2, 2007

Integrated Modelling of Subsidence Mechanisms and Impacts Due to Mine Caving Elmo Davide1 O’Connor3 C., Vyazmensky1 A., Stead1 D., Dunbar2 S., Eberhardt2 E., Scoble2 M., and Moss4, A. 1

2

Simon Fraser University, Vancouver. University of British Columbia, Vancouver. 3 Itasca Consulting Canada Inc. 4 Rio Tinto plc Abstract

With ever-increasing global demand for mineral resources, numerous mines are moving towards developing and mining deeper, more complex and lower grade ore bodies. In many cases, mass mining methods (e.g. block caving) are being considered to deal with deeper, lower grade deposits. The body of practical knowledge, however, related to the impacts mass mining methods have on the surface environment in terms of rock mass response, subsidence, etc., is limited, imposing both economic risks to the mine and safety risks to mine personnel. This paper presents the framework for and preliminary results from a large collaborative research initiative between Diavik Diamond Mines, Rio Tinto, the University of British Columbia (UBC) and Simon Fraser University (SFU). The project involves the integration of available subsidence, rock mass, mine operation and monitoring data, together with the advanced application of combined finite/discrete-element modelling codes capable of simulating fracture behaviour, cave propagation and surface subsidence above block cave mines. The main objective of this work is to develop advanced numerical modelling methods to characterize complex rock mass behaviour above block cave mines. The research will also include the study of rock deformation mechanisms associated with the transition from surface to underground mining and the potential interaction between simultaneous open-pit and underground mining operations.

1 Introduction Subsidence modelling associated with block caving is complicated by the variety of ground deformation mechanisms that may occur between the underground caving operation and the surface. These progressive ground deformation processes require characterisation both in space and time throughout the rock mass between the cave and the surface. Although significant advances have been made in modelling brittle fracture behaviour directly above the caved zone further work is essential in order to provide robust linkages between the processes involved in cave initiation, subsequent cave propagation and induced subsidence. A comprehensive numerical modelling study focused on block caving related surface subsidence is being carried out as a large collaborative research initiative between Diavik Diamond Mines, Rio Tinto, the University of British Columbia (UBC) and Simon Fraser University (SFU). An integrated approach has been adopted, in which rock mass characterisation and surface and subsurface monitoring will be used as a fundamental constraint on the models. To our knowledge sophisticated modelling of block caving founded on observational data has to date received limited attention and this represents an important and novel aspect of the current project. It is expected that this approach will provide important advances in the understanding of block caving subsidence mechanisms. Numerical modelling will also offer a useful tool to analyse important issues related to crown pillar stability and associated pit slope stability where a transition from open-pit to underground operations is undertaken. State-of-the-art continuum and discontinuum 79


numerical codes will be used in order to simulate these processes and preliminary modelling results are discussed in this paper. 2 Block Caving Mining Block caving mining is characterized by caving and extraction of a massive volume of rock which potentially translates into the formation of a surface depression whose morphology depends on the characteristics of the mining, the rock mass, and the topography of the ground surface (Figure 1). Block caving induced subsidence thus becomes a major concern since it may put at risk mine infrastructure. The ability to predict surface subsidence associated with block caving mining is thus a critical factor for both mining planning and operational hazard assessments. However, owing to problems of scale and lack of access a fundamental understanding of the complex rock mass response in block caving settings remains limited. Because block caving geomechanics is still largely an empirically based discipline, the use of advanced numerical modelling will provide an opportunity to investigate the factors governing caving mechanisms and develop improved methodologies for the prediction of associated surface subsidence.

Figure 1: Conceptual representation of surface subsidence associated with block caving mining and related subsidence characterization terminology (after van As, 2003).

3 Analysis of subsidence associated with block caving Current approaches to assessing surface subsidence associated with block caving mining including empirical, analytical and numerical methods (Vyazmensky et al, 2007) are briefly reviewed in this section. The Laubscher’s method (Laubscher, 2000) is the most commonly used empirical method for estimating subsidence parameters in cave mining. This empirical approach is based on a design chart that relates the predicted cave angle to the MRMR (Mining Rock Mass Rating), density of the caved rock, height of the caved rock and mine geometry (minimum and maximum span of a footprint). However, it is argued that determining the density of the caved rock represents a difficult undertaking resulting in an inherent degree of built-in uncertainty. Furthermore, the approach does not account for the effects of major geological structures which may influence the dip of the cave angle. Estimates of the angle of break need to be adjusted for local geological conditions requiring sound engineering judgment and experience in similar geotechnical settings. Whereas the Laubscher’s design chart may constitute a useful tool for preliminary estimates of

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the angle of break, its application to design and subsidence predictions should be exerted with caution. Analytical methods include limit equilibrium solutions for specific failure mechanisms. Hoek (1974) developed an initial limit equilibrium model for the analysis of surface cracking associated with the progressive sub-level caving of an inclined orebody. Brown & Ferguson (1979), Kvapil et al (1989), Karzulovic (1990), Herdocia (1991), Lupo (1996), Flores & Karzulovic (2004) modified Hoek’s method incorporating various additional parameters and mining geometries. Woodruff (1966) postulated that the tension cracks surrounding a caved or subsidence area do not necessarily represent planes of movement extending from ground surface to undercut level. Therefore the mechanism of failure behind Hoek’s (1974) limit equilibrium approach may be limited to specific conditions, and its relevance would be restricted, for the general case, to the initial estimation of the angle of break. Flores & Karzulovic (2004) summarised the most common analytical methods, failure modes and techniques currently available for block caving mining, with a particular emphasis on the transition from open pit to underground mining. Numerical techniques are inherently suited to complex geometries and material behaviour, therefore providing an opportunity to improve understanding of subsidence phenomena and, potentially, increase confidence in subsidence predictions. Different modelling approaches exist, based on the concept that the deformation of a rock mass subjected to applied external loads can be considered as being either continuous or discontinuous. The main differences between the continuum and discontinuum analysis techniques lie in the conceptualisation and modelling of the fractured rock mass and the subsequent deformation that can occur. Continuum, discontinuum and hybrid numerical techniques (combining finite/discrete approaches) have been applied to the analysis of block caving subsidence and published accounts are summarised in Table 1. It is noted that most of the examples listed in Table 1 considered back analysis or predictive modelling of particular mine sites. In light of increasing use of the block caving mining methods and the importance of increasing knowledge of potential surface subsidence there is a genuine need for a comprehensive numerical study on the general principles of surface subsidence development associated with block caving mining. Author Singh et al (1993) Karzulovic et al (1999) Flores & Karzulovic (2004) Li & Brummer (2005) Gilbride et al (2005) Elmo et al (2007) Vyazmensky et al (2007)

Table 1: Numerical studies of surface subsidence. Approach Type of analysis Continuum - Large Strain Site specific: Rajpura Dariba and Kiruna mines Continuum - Large Strain Site specific: El Teniente mine Continuum - Large Strain Conceptual (Block Caving and open-pit) Discontinuum Site specific: Palabora mine Discontinuum Site specific: Questa mine Hybrid Conceptual (Block caving and open-pit) Hybrid Conceptual (Block caving)

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4 Numerical examples of block caving subsidence using FLAC3D The caving process, and the subsequent subsidence that can result, is in part controlled by the joint fabric of the rockmass. Although the rock mass may behave as a continuum at a large scale, the behaviour of that continuum is controlled by discontinuities in the rock mass at smaller scales. The use of uniform properties will be unable to reproduce asymmetric behaviour. FLAC3D (Itasca, 2007), being a continuum code, cannot explicitly model discontinuous behaviour. It was thus necessary to incorporate the effects of jointing for the model to be representative. The Equivalent Rockmass Model (ERM) is proposed based upon Clark (2006). To account for joint fabric, randomly oriented ubiquitous joint planes are distributed through every zone in the model based upon mapping data. This allows for the larger scale behaviour to be affected by small scale effects. In order to calibrate the model, the San Manuel Mine in Arizona has been used as a test site. Caving at this mine started in the mid 50’s and was initially tracked by the U.S. Bureau of Mines as a unique opportunity to understand the caving and subsidence process at a green field site. The entire mine was reconstructed in GoCAD based upon paper plots, air photos, electronic files, and reports into an integrated 3D mine model. Early test results using the ERM have yielded promising results with the subsidence profile and breakthrough matching observations reasonably well. The San Manuel Fault (see Figure 2a) is the boundary between the lower unit of monzonite porphyry and the upper unit formed by the more competent Gila Conglomerate. When caving reaches the San Manuel fault, it rides up the fault and distorts the caving profile updip. In Figure 2b, a 3D iso-surface of displacement is shown. Site observations gave a breakthrough to surface just to the North of panel 7-1 after ~110 days of mining. This is consistent with where and when the model predicts breakthrough to occur. Buildings No. 1 and No. 4 Shafts

North Ore Body Crater

South Ore Northeast Ore Body Crater Body Crater Surface Cracking

Mining Blocks 1415 (400 ft)

2015 (600 ft)

1415 Grizzly & 1475 Haulage Levels

San Manuel, West & East Faults

a

2015 Grizzly & 2075 Haulage Levels

b

Figure 2: Full 3-D reconstruction of the San Manuel mine and subsidence crater up to 1972 (a). Displacement contours on a long section through the orebody after mining of the first 9 panels along with a 3D iso-surface of displacement magnitude highlighting the location of initial breakthrough.

Ongoing work aims to better define the long-term behaviour of the caving process to ensure that the initial results carry forward into late stage mining. Testing is also ongoing to determine the sensitivity of the model to the zone size, ubiquitous joint orientation, and joint distribution.

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5

Initial numerical modelling of block caving mining using a hybrid finite/discrete approach This section presents preliminary results of block caving modelling undertaken at SFU using a hybrid finite/discrete numerical approach (ELFEN code - Rockfield, 2006). An important characteristic of this approach is that failure of the intact rock material can be modelled through fracturing of the initial continuum meshed geometry. Accordingly, caving propagation can be simulated as a result of both failure of the intact rock material and displacement/rotation along pre-defined fracture planes. Detailed descriptions of the constitutive material models and fracture mechanics criteria implemented in the analysis can be found in Klerck (2000), Klerck et al. (2004) and Owen et al. (2004). A pre-defined fracture network of discretised discontinuities is imported into the current geomechanical model using 2D fractures traces derived from a 3D Discrete Fracture Network (DFN) model (FracMan code - Golder, 2006). The pre-defined fracture network adds kinematic controls to the failure mechanism and may provide preferred orientation to the failure. 5.1

Numerical examples of block caving subsidence using a hybrid finite/discrete element approach Figure 3 shows a conceptual, 2D, block caving model. The undercut level is located 200m below ground level and the model assumes that the geometry of the undercut is sufficient for caving initiation (150 long undercut excavated in 5 stages of 30m each). A uniform draw of ore is assumed. The ore extraction is simulated by looped deletion of the discrete elements within the full length of the undercut level. The in-situ stress is defined by a vertical stress proportional to the depth and a horizontal stress specified through a specified in situ stress ratio K . Figures 3(a) and 3(b) show the caving development simulated for two models with different pre-defined fracture networks. The results clearly show the capability of the hybrid approach to capture caving initiation and the subsequent progressive fracturing of the rock mass above the undercut. In the models the cave back eventually progresses to the surface; interestingly, the location at which the cave back breaks to the surface appears to be controlled by the orientation of the predefined fracture networks.

Figure 3: Two-dimensional hybrid FEM/DEM modeling of block caving mining; (a) model with two joint sets inclined at 10 and 80 degrees and (b) model with two joint sets inclined at 30 and 60 degrees respectively. 83


5.2

Initial numerical analysis of a transition from open pit to underground operations by block caving mining As large open pits reach increasingly greater depths and more frequently involve interaction with underground mines, numerical modelling provides a useful tool to analyse important issues related to both crown pillar and pit slope stability. This section presents examples of a hybrid modelling approach investigating the geotechnical aspects of the interaction between open pit and underground block caving mining. A conceptual model was used in the current study (Figure 4). Further details and material parameters are given in Elmo et al (2007).

Figure 4: Basic geometrical definition for the model used in the numerical analysis representing an open pit excavation followed by the simulation of block caving mining.

The effect of the open pit on the stress field is investigated using an excavation sequence assuming 100m stages. The undercut level is located in a central location below the bottom of the pit. The model assumes that the geometry of the undercut is sufficient for caving initiation (150 long undercut excavated in 5 stages of 30m each). The use of 2D fracture traces derived from a 3D DFN model allows for a parametric study on the effects of varying fracture orientation and intensity. Similar to the example described in Section 5.1, the DFN model included two orthogonal joint sets. Model A1 had two joint sets inclined at 10 and 80 degrees respectively, whilst in Model B1 these joint sets were inclined at 30 and 60 degrees respectively. The initial scope of the modelling was to characterise the potential effects of block caving mining on the stability of the pit slopes. Simulated horizontal and vertical displacements of the pit walls were analysed as a function of numerical time. Figure 5 shows the variation of horizontal displacement (positive in the pit slope direction) for two points located on the right hand side of the pit wall at 0m and 300m depth from surface respectively. For comparison purposes, similar models were run without simulation of block caving mining. The results clearly show that progressive block caving resulted in an increased inward horizontal movement of the pit slope with time. For Model B1, this ultimately resulted in a relatively large slope failure characterized by a combined sliding-toppling mechanism (see Figure 6). Comparatively lower horizontal displacements were observed for Model A1, which assumed a 10 degrees dipping predefined joint set. In summary, the results clearly show that, without the block caving, the open pit walls appear relatively stable and reflects the potential impact of block caving mining on existing open pit operations.

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(i) (ii) Figure 5: Effects of progressive block caving for Models A1 and B1 expressed in terms of horizontal displacement for two points located (i) on the top right-hand margin of the pit and (ii) at 300m depth on the right-hand side of the pit slope.

Figure 6: Progressive slope failure observed for Model B1; stages 13sec (Left) and 16sec (right) respectively.

6 Discussions and conclusions This paper describes the framework and initial modelling results from a large collaborative research initiative whose main objective is to explore and develop the ability of advanced numerical modelling methods to characterize complex rock mass behaviour above block cave mines. The use of state-of-the-art continuum and hybrid finite/discrete numerical techniques forms a fundamental part of the ongoing research. Early numerical tests have shown the effectiveness of an Equivalent Rockmass Model (ERM) approach coupled with an advanced 3D continuum modelling in simulating block caving mining. The ERM allows the simulation of randomly oriented ubiquitous joint planes through every zone in a continuum model based upon mapping data. Although currently limited to 2D analysis, preliminary results also suggest that the use of a hybrid finite/discrete approach represent a powerful tool for characterising caving initiation and the subsequent progressive fracturing of the rock mass above the undercut. The authors also illustrate how the approach can be extended to simulate the interaction between surface and underground operations. It is noted that the fracture intensity parameter used in the current hybrid models determines what portion of the natural occurring fractures will be modelled. Since not all pre-existing fractures are represented by the model, the unfractured rock in the equivalent fractured rock mass properties were defined for the initial conceptual models presented in this paper (Elmo et al, 2007). In future analyses, it is intended to investigate the use of specific fracture intensity parameters as meaningful tools to derive rock mass properties for the 85


corresponding numerical modelling of block caving and open pit/underground mining interactions. A series of numerical experiments is currently being conducted to evaluate the factors influencing block caving induced subsidence including rock mass properties, in-situ stress ratio, jointing, major geological structures, surface topography, and mining sequence. References: Brown E.T. and Fergusson G.A. 1979. Prediction of progressive hanging-wall caving, Gath’s mine, Rhodesia. In: Trans. Instn Min. Metall. 88, A92-105. Clark I.H. 2006. Simulation of rockmass strength using ubiquitous joints. In the 4th International FLAC Symposium on Numerical Modelling in Geomechanics Elmo D., Vyazmensky A., Stead D. and Rance J. 2007. A hybrid FEM/DEM approach to model the interaction between open-pit and underground block-caving mining. 1st Canada-US Rock Mechanics Symposium. Vancouver. (in press) Flores G. and Karzulovic A. 2004. Geotechnical guidelines for a transition from open pit to underground mining. Geotechnical guidelines. Project ICS-II, Task 4. Gilbride L.J., Free K.S. and Kehrman R. 2005. Modeling block cave subsidence at the Molycorp, Inc., Questa Mine. In: Proc. 40th U.S. Symposium on Rock Mechanic, Anchorage. Golder Associates. 2006. www.fracman.golder.com. Herdocia A. 1991. Hanging wall stability of sublevel caving mines in Sweden. PhD thesis, Luleå University of Technology, Luleå, Sweden. Hoek E. 1974 Progressive caving induced by mining an inclined orebody. IMM Section A: A133-A139. Itasca. 2007. FLAC3D. Fast Lagrangian Analysis of Continua 3D. Itasca Consulting Group. www.itascacg.com/index.html Karzulovic A. 1990. Evaluation of angle of break to define the subsidence crater of Rio Blanco Mine’s Panel III. Technical Report, Andina Division, CODELCO-Chile. Karzulovic A., Cavieres P. and Pardo C. 1999. Caving subsidence at El Teniente Mine (in Spanish). In: Proceedings of SIMIN 99, Santiago. Klerck P.A. 2000. The finite element modelling of discrete fracture in quasi-brittle materials. Ph.D. thesis, University of Wales, Swansea. Klerck, P. A., Sellers, E. J. & Owen, D. R. J. 2004. Discrete fracture in quasi-brittle materials under compressive and tensile stress states. Comp. Meth. Appl. Mech. Eng. 193, 3035-3056. Kvapil R., Ceccarelli B. and Lonergan J. 1989. Quantitative Analysis of Subsidence at El Teniente Mine. Technical Report, El Teniente Division, CODELCO-Chile. Laubscher D.H. 2000. Block caving manual. Prepared for International Caving Study. JKMRC and Itasca Consulting Group, Inc: Brisbane. Li H. and Brummer R. 2005. Analysis of pit wall failure mechanism and assessment of long-term stability of pit walls Palabora mine. Itasca Consulting Canada Ltd. Technical report. Lupo J.F. 1996. Evaluation of deformations resulting from mass mining of an inclined orebody. PhD thesis, Colorado School of Mines, Golden, Colorado. Owen D.R.J., Feng Y.T., de Souza Neto E.A., Cottrell M.G., Wang F., Andrade Pires F.M. and Yu J. 2004. The modelling of multi-fracturing solids and particulate media. Int. J. Num. Meth. Eng. 60(1): 317-339. Owen, D. R. J., Feng, Y. T., de Souza Neto, E. A., Cottrell, M. G., Wang, F., Andrade Pires, F. M. & Yu, J. 2004. The modelling of multi-fracturing solids and particulate media. Int. J. Num. Meth. Eng. 60(1): 317-339. Rockfield. 2006. Rockfield Software Ltd. Swansea. UK. Elfen Version 3.85 Build9. www.rockfield.co.uk. Singh U.K., Stephansson O.J. and Herdocia A. 1993 Simulation of progressive failure in hangingwall and foot wall for mining With Sub Level Caving. In: Trans. Instn. Min. Metall., A102:A188-A194. Van As A. 2003. Subsidence Definitions for Block Caving Mines. Technical report. Vyazmensky A., Elmo D., Stead D. and Rance J. 2007. Combined finite-discrete element modelling of surface subsidence associated with block caving mining. 1st Canada-US Rock Mechanics Symposium. Vancouver. (in press) Woodruff S. 1966. Methods of working coal and metal mines, Vol 3. Pergamon Press: Oxford.

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Integrated Modelling of Subsidence Mechanisms and Impacts Due to Mine Caving

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