Comparison of H-B and M-C Failure Criterion at Low Confinement and their Impact on the Prediction of Failure Zone Around an Underground Metal Mine

Abstract

The accurate assessment of rock mass strength and the failure zone near the excavation is important for the rational design of underground structures. To predict the strength and behaviour of the rock mass during excavations, generally, the two most widely used failure criterion namely, linear Mohr-Coulomb and nonlinear Hoek-Brown criteria are used. However, the failure envelopes from the two criteria deviate much, especially, at low confinements. The main motive of this investigation is to understand the stress state around an opening. An underground copper mine of Hindustan Copper Limited (HCL) i.e., Khetri Copper mine in India was considered for this research where the host rock is quartzite schist. The mine is operated at a depth of around 475 m and the ore is extracted using the sublevel stopping method. From the finite element model, it was found that the stress state of the rock mass around the excavation mostly lies in the range between the direct tension and low value of confinements. This is the range where the H-B and M-C failure envelopes differ. The FEM modelling showed a different value of failure zone around the opening, 17.8 m from H-B compared to 8.0 m from the M-C criterion. This research suggested that for the quartzite rock investigated, the support requirements are more if the H-B failure criterion is considered instead of the M-C failure criterion.