Abstract

Knowledge of ground stresses is crucial for ground control activities such as the design of underground openings, selection of support systems, and analysis for stability. The most popular of in situ stress measurement methods are over-coring and hydraulic fracturing techniques (Brady and Brown, 2007). Far-field stresses are usually measured far from the underground mine work areas to capture the pre-mining stress tensor. However, it is a known fact that far-field stresses experience changes in orientation and magnitude due to the presence of geological structures and due to the excavations created by mining activities. As a result, mining-induced stresses around drifts, ramps, and stopes in underground mines are quite different from far-field or pre-mining stresses. Therefore, investigating the development of an effective and practical method to estimate mining-induced stresses near a mining face should prove useful in practice. The purpose of this research is to develop a simple and practical methodology for determining mining-induced stresses near underground mine openings such as drifts and crosscuts where the near-field stresses in the sidewalls, the back, and the face could be of concern to the stability of the opening. Strain relief occurs once the rock core is drilled off, and it is a function of the surrounding stress field. This study uses exploration rock cores that are drilled off for the purpose of orebody definition in the underground mine to show how rock core diametrical deformation analysis can be employed to determine the differential mining-induced stresses. The methodology requires knowledge of the elastic properties of the intact rock. Therefore, uniaxial testing constitutes part of the new methodology. A customized test apparatus has been designed and built-in laboratory to enable precise measurements of rock core. The challenge of this approach is estimating the magnitudes of two principal stresses. The proposed method provides the analytical solution to determine the major and minor principal stresses in a plane perpendicular to the borehole axis close to the free mining face. This analytical solution has been validated with published results of laboratory experiments using rock cores extracted while subjected to under uniaxial compression. It is confirmed that the results presented by the proposed method agreed well with the actual applied stress in the experiments and their test results.