The failure mode of mine faces and pillars is affected by geometry and material properties of the surrounding rock. As mining progresses, the changing geometry affects the distribution of stresses and loading stiffness of the mine. This work builds on previous efforts in modeling underground excavations by investigating parameters that could be used to indicate instances of unstable pillar failure. A mechanically coupled discrete element model of coal is utilized to study the effect of geometry and loading system stiffness on pillar failure. First, the coupled model is verified in stable and unstable loading conditions using unconfined compressive strength tests with variable platen elastic modulus. Unstable versus stable failure of the coal specimen is detected by comparing the stiffness of the loading system to the unloading stiffness of the coal during failure. A transition from stable to unstable failure occurs when the loading system stiffness is less than the post-peak stiffness of the coal. A resultant force based damping mechanism is used to maintain quasi-static conditions during the simulations. Work done by the damping mechanism is low for stable failures, but it increases in tests beyond the stable-unstable transition. This analysis is extended to a series of pillar tests with increasing width-to-height ratios. Stiffness measurements show unstable and stable failures by assigning different loading system moduli. Damping work is consistently higher during unstable pillar failure. Comparing local damping work and bond failure reveals increased localization of failure during unstable failure.

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