This paper presents the results of numerical studies to determine the influence on stability of various support systems used in the panels of tabular, pillar supported hard rock mines based on experience at an underground monitoring site. A distinct element numerical code was used to simulate the observed underground failure and to identify the fundamental failure mechanisms. The influence of variations of support types and support spacing on hangingwall stability was established. The results have provided insights into the mechanisms which are taking place and which influence rockmass behaviour and the interaction between support units and the rockmass.
The Bushveld Complex has been subjected to a long and complex tectonic history which has resulted in areas of dense jointing related to faulting and the emplacement of dykes. The situation is further aggravated by the fact that rocks in the vicinity of the reef have been subsequently altered, significantly reducing cohesion across surfaces and discontinuities. Platinum and chrome mines in the Bushveld Complex, at shallow to intermediate depths, experience instability problems of stope hangingwalls and to a lesser extent of pillars. Local falls of ground and larger collapses in these mines have been identified as the major rock related safety hazards. To ensure the stability of the hangingwall in disturbed areas, the structural integrity of the immediate hangingwall beam needs to be maintained and simple gravity induced falls of ground need to be reduced. Design procedures to provide estimates of safe spans between pillars must take into account the variability in beam thickness encountered in the mines, rock strength, intensity and orientation of jointing, horizontal stresses in the hangingwall as well as the loading of the beam. Blocks bounded by unfavourably oriented joints and partings are observed to readily fall out. Currently, the design of safe hangingwall spans in most mines in the Bushveld Complex is carried out using an empirical approach. This can be adequate where mining conditions are consistent but seriously deficient when dealing with conditions which vary such as in the Bushveld Complex. In such situations pillar spans can be either over designed, reducing ore reserve or under designed, leading to hangingwall collapses and associated casualties and loss of production. In addition design procedures for face and back area support need to be developed which take into account the size and shape of the joint bounded unstable blocks and which result in cost effective, support systems of adequate support resistance and areal coverage for the prevailing condition. Since 1993, a comprehensive research programme aimed at improved design methodologies for ensuring stable spans between pillars, support systems and pillar types and dimensions has been in progress. The methodology and preliminary results obtained from the underground monitoring programme were detailed in a separate paper (Aref et al., 1995). As part of the programme, numerical models are used as an aid to obtain a better understanding of the governing failure mechanisms and support requirements for the tabular ore body at shallow to intermediate depths.
