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Block Caving 

 

Summary

This review describes the underground mining method of block caving. Several publications, websites, links, and software are given, and a number of blogs related to block caving are mentioned.

overview

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.

Diagrammatic representation of a typical block cave mining operation. (Image copyright: Atlas Copco)

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.


BLOCK CAVE MINES AROUND THE WORLD

This image from the Edumine Course Introduction to Block Caving provides a quick view of different block caving operations around the world. Block cave mines around the world

Block Caving requires a huge amount of developments. A block caving mining project called New Afton, belonging to New Gold Inc., located near Kamloops, British Columbia is under construction and is expected reach production in 2012. 4,477 m of developments were done in 2008 alone. You can find more details here.


CAREERS

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 career opportunities in block caving.

BLOCK CAVING INDUCED SUBSIDENCE

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.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.


ELFEN 2D conceptual simulation of block caving for two different jointing conditions; work by D. Elmo, SFU Vancouver.

Figure 4: ELFEN 2D conceptual simulation of block caving for two different jointing conditions (Elmo et al., 2007a)


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.

(Top) conceptual models with pre-inserted discontinuity sets dipping at 10-80 and 30-60 degrees respectively; (Bottom) simulated horizontal displacements of the pit walls analysed as a function of numerical time, showing the potential impact of block caving mining on existing open pit operations, discontinuity sets dipping at 30-60 degrees (Elmo et al., 2007).

Figure 5: (Top) conceptual models with pre-inserted discontinuity sets dipping at 10-80 and 30-60 degrees respectively; (Bottom) simulated horizontal displacements of the pit walls analysed as a function of numerical time, showing the potential impact of block caving mining on existing open pit operations, discontinuity sets dipping at 30-60 degrees (Elmo et al., 2007).


PUBLICATIONS

This technical review is not intended as a comprehensive review of block cave mining. 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.

Block caving manual, by Laubscher D.H. 2000. Prepared for International Caving Study. JKMRC and Itasca Consulting Group, Inc: Brisbane.

Block caving geomechanics by Brown E.T. 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.

Modeling block cave subsidence at the Molycorp, Inc., Questa Mine, by Gilbride L.J., Free K.S. and Kehrman R. 2005. In: Proc. 40th U.S. Symposium on Rock Mechanic, Anchorage.

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)

A conceptual sequence for a block cave in an extreme stress and deformation environment by Beck et al. (2006)

Integrated Modelling of Subsidence Mechanisms and Impacts Due to Mine Caving, by Elmo D., O'Connor C., Vyazmensky A., Stead D., Dunbar S., Eberhardt E., Scoble M., and Moss, A. 2007. CIM Conference and Exhibition, Montreal. The presentation is available at this link.

A hybrid FEM/DEM approach to model the interaction between open-pit and underground block-caving mining, by Elmo D., Vyazmensky A., Stead D. and Rance J. 2007. In: Proceedings of 1st Canada-US Rock Mechanics Symposium. Vancouver.

Combined finite-discrete element modelling of surface subsidence associated with block caving mining, by Vyazmensky A., Elmo D., Stead D. and Rance J. 2007. In: Proceedings of 1st Canada-US Rock Mechanics Symposium. Vancouver.

Characterising the in situ fragmentation of a fractured rock mass using a discrete fracture network approach, by Rogers S.F., Kennard D.K., Dershowitz W.S and van As. A. 2007. In: Proceedings of 1st Canada-U.S. Rock Mechanics Symposium. Vancouver.

The easiest way to understand what block caving is all about, is to read the magnificent 1965 publication Block-Caving Copper Mining Methods and Costs at the Miami Mine, Arizona. To my mind, this is the best exposition of the topic I came across. In spite of, or maybe because of its vintage, it succeeds in describing the use of block-caving in a way that both technically informative and readily understood.

In between 1965 and now, there are a few papers on the topic - see InfoMine's Publications Database. One worth looking at is An Application of Linear Programming for Block Cave Drawdown by Guest, A.R. et al., published in 2000.


WEBSITES

Other website have some information on block caving: Here are a few:


SOFTWARE

Gemcom PC-BC Block Caving is a software I found that apparently enables you to model the block caving process.
Other similar softwares are ELFEN (Rockfield Software, Swansea, UK), PFC2D and PFC3D (Itasca Consulting Group Inc, Minneapolis, US), FLAC and FLAC3D (Itasca Consulting Group Inc, Minneapolis, US) and ABAQUS.


VIDEOS

Here are a few videos that show the block caving method:

There are also videos in the Edumine course Introduction to Block Caving showing different stages of block caving.

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