45 / 2021-08-18 01:55:13

Determination of mining-induced stresses using diametral rock core deformation

mining-induced stresses,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. 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. 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 (Funato and Ito, 2016). 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 cores (Figure 1). The apparatus uses a Mitutoyo LSM-512S laser scan micrometer having a tolerance of 0.0001 mm to measure diametrical core rock deformations. The rock sample is placed between a pair of parallel rollers to enable measurements over 360º rotation of the rock core sample. Provisions are made to accommodate a wide range of core diameters ranging from 30.5 mm (AQTK) to 63.4 mm (HQ).



Finite element modelling is used to verify the analytical solution of proposed methodology. Two case studies from operating underground mines are presented for demonstration. In the case studies, rock core diametral deformations were measured and rock samples were prepared and tested for Young’s modulus of Elasticity and Poisson’s ratio. Tests were conducted in accordance with ASTM D7012. Using the analytical model, the differential stress, namely the difference between the local principal stresses in the plane perpendicular the core rock axis is calculated using the developed methodology. Secondly, it is shown that if the rock cores are extracted near the mining face, it would be possible to estimate the mining-induced principal stresses and their orientations, which are of primary interest to the ground control specialist. It is also shown that NQ sample size (47.6 mm) provides more reliable results than the AQTK size (30.5 mm).