The present study proposes a time-dependent Hoek-Brown strain softening constitutive model to assess the failure analysis of a coal mass. A methodology for implementing the proposed time-dependent constitutive model in the 3D FDM has been proposed. The constitutive model has three sets of parameters: crack initiation strength properties ( $m_{c}$ and $s_{c}$ ), ultimate peak strength parameters ( $m_{p}$ and $s_{p}$ ), and residual strength parameters ( $m_{r}$ and $s_{r}$ ). Failed cases of bord and pillar mines from the Witbank Coalfield were utilised to estimate the strength properties. The Strength properties of the coal were deduced using back-analysis techniques. A constant tributary area load was applied from the model’s top during the simulation. The simulation results were obtained in terms of the average axial strain with time. The simulation results showed that the average strain of the pillar increased exponentially during the simulation stage of age before failure. The pillar model is considered failed after getting an un-equilibrium condition. At that time, a sharp increase in axial strain was observed. Stable cases from the Witbank coal fields were also simulated with age to validate the deduced strength parameters. The stable cases results showed a negligible increment in axial strain, confirming the pillar’s stability. The simulation results showed that the pillar failure begins at the surface and progresses deeper into the core, ultimately forming an hourglass shape. The pillar strength formula with respect to the age of the pillar has also been proposed based on the statistical analysis.
