Determination of mining-induced stresses using diametral rock core deformations

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. 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, in-situ stresses around drifts, ramps, and stopes in underground mines are quite different from far field or pre-mining stresses. The purpose of this research is to develop a simple and practical methodology for determining in-situ stresses. Stress relief occurs once the rock core is drilled off. Such relief 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. The method measures and analyzes the diametral core deformations in laboratory. Two case studies from operating underground mines are presented for demonstration. In these case studies, rock core deformations are measured with a customized test apparatus and rock samples were prepared and tested for Young's modulus and Poisson's ratio. The differential stress, namely the difference between the local principal stresses in the plane perpendicular to the core rock axis is calculated. It is shown that this methodology is useful for determining the brittle shear ratio in the rock mass, which is of primary interest to ground control studies.