Distinct element model analyses of unstable failures in underground coal mines

Abstract

Underground mining may induce shear failures along large rock discontinuities and sizable compressive failures of rock materials (e.g. coal) in sidewalls and mining faces. These failures can be in stable or unstable manner. When such failures occur in an unstable manner, they may be accompanied by a significant energy release from the surrounding rock and potentially create coal burst events. This thesis focuses on identifying failure stability in terms of their manifestation as a stable and unstable manner in underground coal mining settings using numerical modeling. Both discontinuity shear and compressive failures are considered. The influence of the shear failure stability on the compressive failure stability of sidewalls and mining faces is also considered. The study uses the numerical modeling program UDEC with its optional constitutive models continuously yielding joint model and the Mohr-Coulomb strain softening model. A laboratory scale numerical model of a double shear test setup is developed and used to assess the ability of UDEC in detecting the failure stability modes (i.e. stable or unstable failure) of discontinuity shear failures. The studies performed using this model confirmed that UDEC can identify the stable and unstable failures when using a previously defined failure stability criterion based on the relative stiffnesses of loading and failing systems. Using the double shear test model, methodologies are developed for identifying the failure stability. Further and detailed analyses of failure stability are performed in in-situ scale models. The results show that the failure stability is governed by the relative stiffnesses of failing discontinuities and loading stiffness of wall rocks. In-situ scale numerical models are also developed and used to study the failure stability in compression in coal mining settings. The results show that both stable and unstable shear failures can occur at the rock-coal interfaces depending on the post-failure characteristic of the interface and the shear loading stiffness of the interface. It is also shown that unstable coal sidewall and mining face failures can occur when a sudden de-confinement is triggered by an unstable failure at the rock-coal interfaces or the existence of weak contact regions along the interfaces.