A generic mechanical model for rock behavior around advancing stopes has been developed to understand brittle failure mechanisms inherent in deep gold mines in South Africa. The model can reproduce the induced fracture patterns such that fracturing mechanisms can be explored and then related to in-situ conditions. The rock mass consists of finite-thickness horizontal layers separated by zero-thickness parting planes; also, a finite-thickness dyke may be defined ahead of the stope face. The generic stope model consists of a rectangular FLAC model within which may be embedded a rectangular PFC2D inclusion. Both the FLAC-only and the coupled FLAC-PFC2D models produce well-defined fractures similar to those observed in the field.
Gold mines in South Africa are located from 2 to 3.5 km below the surface. Sedimentary deposits (the auriferous conglomerate reefs) extend for hundreds of square kilometers and are mostly tabular, with the reef being less than one-meter thick. The host rock is strong quartzite. Bedding parting planes are inherent features in the rock mass that were developed as part of the rockforming processes. The mining may extend for several kilometers on strike and dip. Excavated areas disturb the surrounding rock mass by inducing significant stress redistribution. In the longwall mining method, the surrounding rock experiences large displacements, and the mined areas can become self-filling, with total closure occurring between the roof and floor of the excavation. Shear fractures exhibit relative displacement parallel to the fracture surface; extension fractures exhibit separation normal to the fracture surface. The expected fracture pattern (see Fig. 1) is based on mapped and postulated fractures (see Fig. 2) described in [1] and [2]. The mapped fractures include shear fractures emanating from the hangingwall and footwall that are slightly curved, dipping toward the direction of mining and then becoming vertical.
