The effect of geological alterations on pillar strength

Abstract

This study investigated the problem of determining hard rock pillar strength when geological alterations are present in the pillars. These alterations substantially weaken the pillars, and a better understanding of pillar strength will allow for improved designs to be implemented in future. The dissertation includes a literature review and describes three valuable case studies of pillar collapses in Southern Africa. This includes a Zimbabwean operation in the Great Dyke, the Wonderkop Mine in the Western Bushveld and Everest Platinum Mine in the Eastern Bushveld. Access to the Everest Platinum Mine was still possible and most of the work in this study focusses on the pillar behaviour at this mine. A geological alteration is present between the hanging wall and top reef contact at this location, and this resulted in a mine-wide collapse and closure of the mine.

Empirical methods are still popular in the rock engineering fraternity to determine pillar strength. The Hedley and Grant formula, which was derived for Canadian uranium pillars, has been used extensively in the South Africa hard rock pillar designs. Surprisingly, very few collapses of hard rock bord and pillars mines have been reported in the country. This pillar strength formulation therefore seems to be mostly conservative, but its application at the three mines mentioned above did not prevent the collapses of the underground workings.

This study proposed an alternative numerical modelling approach to determine the stability of bord and pillar layouts where alteration layers are present. The displacement discontinuity code, TEXAN, proved to be suitable to analyse and simulate the pillar failure. The capability of the code to simulate irregular-shaped pillars on a large scale was indispensable for this kind of study. Furthermore, the built-in limit equilibrium model allows the pillar scaling and failure to be simulated. The model contains an interface at the hangingwall and footwall contacts and this appears to be suitable to simulate the effect of geological alterations. For the Everest Mine, two areas were simulated, namely part of the collapsed area and a second area, with larger pillars, that is still stable. This allowed for a first order calibration of the limit equilibrium model.

The calibrated model was subsequently used to explore alternative layout designs for these ground conditions. Barrier pillars will clearly be necessary to compartmentalise the mine. The numerical modelling predicted that the barrier pillars will remain stable, even for large scale collapses, provided their width exceeds 25 m. Main access routes into the mine can be protected by a double row of pillars of at least 15 m wide to provide for a safe travelling way.

In summary, a key finding of the study is that geological alterations substantially reduce the strength of hard rock pillars and a revised design methodology is required. The traditional South African design methodology of using the empirical Hedley and Grant formula does not work in these cases. A displacement discontinuity numerical model using a limit equilibrium model appears to be useful to simulate this pillar behaviour on a mine-wide scale. After calibration of the model, this can be used to explore appropriate layouts and aspects such as the required width of barrier pillars. Further work includes additional calibration of the model and underground monitoring of future layouts to verify the stability of the barrier pillars. A drawback of the current limit equilibrium model is that it is a symmetrical model with partings at both the hangingwall and footwall contacts. In contrast, the pillars at Everest Mine only has a weak alteration layer at the top reef contact and there is a need to extend the limit equilibrium model to better match this mechanism of pillar failure. The time-dependent failure of the pillars was beyond the scope of this study, and this also needs to be explored in future.