Fracturing in block cave mining

Block caving method is a cost-effective mining method and has been used for many years. It started in coal mines and then in metalliferous mines. Because of its low cost, it is widely used for low grade orebody. The block cave mining process in metalliferous mines consists of constructions of tunnel networks at an upper undercut level and parallel extraction drifts (tunnels) with short draw drifts between the extraction drifts at a lower extraction/production level. Blasting holes are drilled upwards from the tunnels at the undercut level and the orebody is blasted. Draw-bells are created at draw drifts from the extraction level upwards through to the undercut level for the caved orebody to move down into the draw drifts at extraction level. The piled broken orebody in the draw drifts can be transported/extracted from drawpoints for processing. Nowadays, hydraulic fracturing technology is used to pre-condition the orebody before blasting. Hydraulic fracturing may also be employed as post-conditioning if the caving process stopped. 

The success of block caving mining, or cavability, relies on the self-sustained propagation process of the fractured/failed zone over the caving cavity after the broken orebody from blasting has been removed. Prediction of orebody fracturing is very important in design of block cave mining. The Team has demonstrated that the in-house code FRACOD3D can predict the fracture initiation and propagation in block caving mining due to hydraulic fracturing (Figure 1) or due to in situ stresses (Figure 2).

Figure 1. Propagation of hydraulic fracture at 10 m above the caving dome cavity.

Figure 2. Fractures initiated around the top of caving dome cavity under in situ stresses, viewed from different directions (a) along the y-axis, parallel to the minimum horizontal far field principal stress direction, (b) along the x-axis, parallel to the maximum horizontal far field principal stress direction and (c) along vertical down direction.