Investigation of unstable failure in underground coal mining using the discrete element method

Abstract

Unstable failure in underground coal mining is the sudden and violent ejection of coal from mine walls and pillars into the mine opening. This thesis demonstrates the use of the discrete element method to simulate stable and unstable modes of compressive failure of a western U.S. coal. Two discrete element models are evaluated for their ability to simulate unstable and stable compressive failure using the discrete element program Particle Flow Code in Two Dimensions (PFC2D): the bonded particle model and the displacement softening model. Compressive strength tests show that the displacement softening model is better suited for unstable failure studies based on consistent behavior in stable and unstable modes of failure and a post-peak softening characteristic that is independent of the loading rate. A set of model behaviors, called indicators, are analyzed on their ability to distinguish the stability of failure in a series of unconfined compression tests and then a series slender pillar compressive strength tests. Generally, the indicators show consistent values for stable failures and increasing magnitude with increasing levels of instability. A grid based measurement technique is used to observe indicator behavior and model damage spatially. The work by the damping mechanism, kinetic energy, and the mean unbalanced force are used to analyze pillar edge failure in a model with excavation induced loading conditions. The indicators reveal unstable failure events, and a comparison between stable and unstable mining steps show that the indicators can be used to detect local instabilities on, such as pillar rib failure. Grid based measurements show that the unstable failure is initiated due to a single mining step and that failure occurred along a diagonal failure plane originating from the mine face similar to that seen in practice. Unstable failures show highly localized planes of failure while stable pillar failure is more dispersed. Future application of the techniques developed in this thesis include more in depth study of factors influencing unstable failures in coal mines including the mine/coal seam contact condition and depth.