Evaluation of the Rock Mass Strength for Hard Rock Pillar Design Using Bonded Block Models

The estimation of rock mass strength is a crucial step in pillar design. While the tri-linear (or S-shaped) failure criterion has been proposed to estimate the strength of massive to moderately jointed hard rock masses at low confinement, it remains unclear whether this criterion is applicable to designing wide pillars at great depths, where the pillar core is highly confined. This study employs a plane strain Block Bonded Model (BBM), consisting of polygonal blocks that can break at their contacts, to examine the applicability of the tri-linear strength criterion for estimating the strength of hard rock pillars with various width-to-height ratios (W/H). Furthermore, this study aims to identify a BBM with suitable constitutive laws for blocks (i.e., zones) and contacts that realistically captures the strength and failure mechanism of hard rock pillars under compressive loading conditions. The numerical simulations are based on the case of pillar failures at the Quirke Mine, ON, Canada. The simulation results have revealed that the model calibrated to the tri-linear strength envelope overestimates the strength of pillars with W / H greater than 0.5. However, the pillar model with the Cohesion-Weakening Friction-Strengthening (CWFS) behavior for blocks and Cohesion- and Friction-Weakening (CFW) behavior for contacts realistically captures the expected failure mechanism and post-peak response when calibrated to the empirical pillar strength data. The rock mass strength back-analyzed from this model was found to follow the in situ damage initiation stress level at low confinement but significantly underestimates the confined rock mass strength estimated using the tri-linear strength criterion.