Investigations into the residual strength of a 2.5 m wide Bushveld Merensky Reef crush pillar

Abstract

The first crush pillars on the Bushveld platinum mines were introduced at Union Section in 1978. Before the introduction of crush pillars, serious problems were experienced when stoping advanced to a point 30 m to 40 m on both sides of the centre gully. At least three to four stopes were collapsing per month. Increased incidence of bursting has been reported on some mines at deeper levels but most of these bursts occur in the back areas. Nevertheless, the incidence of bursting increases the risk of falls of ground in already dangerous areas, and the larger pillars decrease the extraction ratio. This paper describes the evaluation of stress measurements conducted in two boreholes over a crush pillar with dimensions 2.5 m x 4.0 m, and a height of 1.2 m.The residual strength of the crush pillar with a width of 2.5 m was determined from a series of strain measurements in two boreholes and a Boussinesq matrix inversion. The calculated peak and residual stresses of the pillar were surprisingly high at 280 MPa and 48 MPa respectively. These high stress conditions are suspected to be associated with the narrow pillar geometry on the up-dip side and the stiff loading conditions under which failure took place. In order to get a more realistic evaluation of residual strength, a series of such measurements is recommended to establish a range of residual strengths for narrow crush pillars.