A Trial Evaluation of Rock Core DCDA Absolute Shear Stress Measurement for Routine Quantitative Mining Hazard Assessment in Deep Underground High Stress Mines

Abstract

It is difficult to implement routine mine-wide programs to measure absolute stress in deep and over-stressed mines because the drilled holes and cores are often damaged during and immediately following the drilling. We evaluated the use of the Diametrical Core Deformation Analysis (DCDA) method, developed by Funato and Ito, to overcome this problem. This non-destructive method can evaluate the absolute shear stresses and measurement errors in the planes orthogonal to the core axes by precisely measuring the ellipticity of the core section orthogonal to the borehole axis. The five readings required to evaluate a single core take only about ten minutes to make. The measurement system is compact enough for a single regular courier parcel or can be checked-in as luggage on a flight. Absolute shear stresses were determined for thirty-five core specimens from fourteen holes drilled in a range of directions in the highly stressed rock mass surrounding a deep South African gold mine. Using the DCDA method we were able to determine the absolute 3D shear stress field averaged over an area of interest. It was consistent with the 3D stress field measured in-situ with an overcoring method, with the maximum principal stress larger than 100 MPa, with a root mean square of residuals of several MPa. Interestingly, the results represent relaxation in shear stress near the fault intersected by the boreholes. The DCDA measurements require a core diameter larger than approximately 40 mm, and a core that is longer than approximately 10 cm. The method assumes that there is no significant inelastic deformation and that the rock is isotropic.