Block caving is becoming an increasingly important mining method for rock masses that were previously considered to be too strong or at greater than usual depth, or as an extension of open pit mining. The behaviour of the rock mass is affected by the strength of the intact rock, the orientation, continuity, spacing and strength of the discontinuities and the in situ stress. The caving rock mass passes through several stages with changes to induced stresses, and at each stage the fragmentation increases. At several block caving sites we are using all available jointing data (from boreholes, photographs, face mapping, etc.) to create integrated 3D stochastic models of the rock mass using the discrete fracture modelling suite FracMan. The results from the discrete models at specific sites will be compared with empirical block cave design methods (e.g. Laubscher, Stability Graph) and with other 3D methods of rock mass/rock block size characterisation.
Block caving is gaining greater importance as a mining method due to the exhaustion of near surface high tonnage/low grade ore bodies that are amenable to open pitting. High tonnage/ low grade ore bodies (e.g. porphyry copper type) often extend to below the depth of the stripping ratio of even very large open pits, and block cave mining allows a relatively cheap and efficient method of extending the mine life. Block caving is also now being applied to rock masses that have previously been considered either too hard and/or too deep for the method to be used effectively. Successful block caving is the cheapest form of underground bulk mining in terms of unit costs. Block cave mining exploits the natural break up of rock masses due to gravity and the redistribution of in situ stress within the crown of an undercut in a rock mass after a void has been created under an ore body (Laubscher 2000). Fragmentation initially occurs due to a combination of movement on pre-existing fractures and extension of these fractures joining together forming whole blocks. As the cave matures new fractures are generated within the solid rock between existing fractures leading to smaller block sizes. The blocks fall into a moving draw column and grind together as the ore column is drawn. This reduces their size (through greater fragmentation) but can also have the unwanted side effect of creating more fines. Once continuous caving has been initiated, the rate of production from the block cave will depend on the rate at which the cave propagates following draw of the muck pile and the creation of a small air gap into which caved material can fall (Laubscher 2000). Duplancic & Brady (1999) used a seismic monitoring system to study the early stages of caving at the Northparkes Mine Lift 1, New South Wales, Australia. From this study, they developed a conceptual model of caving (Figure 1).