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Block cave mining is a mass mining method that allows for the bulk mining of large, relatively lower grade, orebodies. This method is increasingly being proposed for a number of deposits worldwide, thus the scope for a better understanding of block caving behaviour. Because many existing large open pit mines are also planning to extend their operations underground by block caving, research is undergoing to investigate the rock deformation mechanisms associated with the transition from surface to underground mining operations.

In general terms block cave 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 cave mining can be used on any orebody that is sufficiently massive and fractured; a major challenge at the mine design stage is to predict how specific orebodies will cave depending on the various geometry of the undercut.

Block caving has been applied to large scale extraction of various metals and minerals, sometimes in thick beds of ore but more usually in steep to vertical masses. Examples of block caving operations include Northparkes (Australia), Palabora (South Africa), Questa Mine (New Mexico), Henderson Mine (Colorado) and Freeport (Indonesia).

Another way to understand what block caving is all about is to examine this figure:

It shows the essential aspects of block caving: an underground tunnel leading to draw points where the overlying rock, broken by gravity more or less flows to the draw point, to be gathered and taken away for processing.

Mining of low grade deposits at deeper levels defines the future of mining. The method of block caving is for now the only underground hard rock mining method capable of mining these low grade deposits present deep underground and capable of achieving production rates equivalent to that of the surface mines. Several top mining companies around the world, like Rio Tinto, Newmont Inc., etc., have already planned block caving as one of their prime future mining methods. CareerMine provides a good list of .

BLOCK CAVING INDUCED SUBSIDENCE

The ability to predict surface subsidence associated with block caving mining is a critical factor for both mining planning and operational hazard assessments (Figure 2). Current approaches to assessing surface subsidence associated with block caving mining, including empirical, analytical and numerical methods 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, thus requiring sound engineering judgment and experience in similar geotechnical settings. Whereas the Laubscher’s design chart constitutes a useful tool for preliminary estimates of 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. For instance, 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. 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. Figures 3 and 4 respectively show examples of preliminary numerical simulations of subsidence associated with block caving. Both figures constitute part of a presentation given in May 2007 by Dr. Davide Elmo at the CIM Conference in Montreal. The scope of the talk was to illustrate an undergoing collaborative research initiative between the University of British Columbia (UBC) and Simon Fraser University (SFU), funded by Diavik Diamond Mines, Rio Tinto and NSERC (Natural Sciences and Engineering Research Council of Canada) with the scope of investigating block caving subsidence and surface to underground mining interaction.

Figure 3: FLAC3D simulation of full 3 D reconstruction of the San Manuel mine and subsidence crater up to 1972 (Left). 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 (Right); work by C. O’Connor, Itasca Canada.

SURFACE TO UNDERGROUND MINING INTERACTION

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. Further details and material parameters are given in Elmo et al. (2007). 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 potential impact of block caving mining on existing open pit operations. The scope is to provide the reader with a list of recent publications, including books and technical papers, which would form a reference background for the person who intends to further explore the subject, understanding how the world has changed and how much the science of block caving has advanced in recent years.

Characterization and empirical analysis of block caving induced surface subsidence and macro deformations, by Woo, Eberhardt Van As. 2009

Geomechanical evaluation of caving macro block options at Chuquicamata Underground Project in Chile using three dimensional numerical modelling, by Hormazabal et al. 2009

Progressive caving induced by mining an inclined orebody, by Hoek E. 1974. IMM Section A: A133 A139.

Evaluation of angle of break to define the subsidence crater of Rio Blanco Mine’s Panel III, by Karzulovic A. 1990. Technical Report, Andina Division, CODELCO Chile.

Caving subsidence at El Teniente Mine (in Spanish), by Karzulovic A., Cavieres P. and Pardo. C. 1999. In: Proceedings of SIMIN 99, Santiago. 2000. Prepared for International Caving Study. JKMRC and Itasca Consulting Group, Inc: Brisbane. 2003. Published by Julius Kruttschnitt Mineral Research Centre Isles Road, Indooroopilly, Queensland 4068, Australia.

Subsidence Definitions for Block Caving Mines, by Van As A., 2003. Technical report.

Geotechnical guidelines for a transition from open pit to underground mining. Project ICS II, Task 4, by Flores G. and Karzulovic A. 2004.

Evaluation of a hybrid FEM/DEM approach for determination of rock mass strength using a combination of discontinuity mapping and fracture mechanics modelling, with particular emphasis on modelling of jointed pillars, by Davide Elmo (2006)

Kinematic model for quasi static granular displacement in block caving: dilatency effects on drawbody shapes by F. Melo et al. (2006)
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