Abstract

A release of energy occurs during mining-related seismic events such as rock bursts and coal bumps. The magnitude of these events depends partly upon the energy made available by the loading system and partly upon the post-peak softening behavior of the failing medium. In addition, the extent of unstable failure and associated release of energy can be affected by slip along interfaces between dissimilar materials. This paper compares the results of two numerical models from the back analysis of a coal mine collapse which resulted in a 3.9 local magnitude seismic event. Release of kinetic energy is considered in the simulations, which were run using a 2D distinct element software package. The model inputs differ in that the interfaces between the coal and the surrounding rock are defined either through Coulomb slip joint parameters or continuously yielding (displacement-softening) joint parameters. The geometry, loading conditions, and mining sequence are otherwise identical. The coal is modeled as a strain-softening material, while the roof and floor are modeled as continuous elastic blocks. The failure response and magnitude of released energy are compared between the two models. The results of the analysis indicate that the softening coal/rock interface facilitates the collapse of large width-to-height ratio pillars and leads to a release of energy more than one order of magnitude higher than the alternative coal-rock interface.

1. INTRODUCTION

The unstable failure of coal pillars, especially those with large width-to-height (w/h) ratios, involves a great deal of complexity. Although large w/h pillars exhibit hardening behavior under loading and are prone to fail gradually, the combination of certain loading conditions and geologic properties can result in sudden collapse. The nature and extent of such failures depends on many factors, including the geometry of the mine and the presence of stiff overlying strata. Another factor that has a significant effect on the behavior of the pillars is the strength, or rather weakness, of the interface between the coal and the host rock. There is little data available regarding the properties of coal/rock interfaces, but a number of studies have been conducted to better understand the effect that they have on pillar strength [1], [2], [3], and [4]. This paper presents the results of two mine-scale numerical models in which a pillar retreat mine fails unstably, and although the modeling procedures are identical in each simulation, the application of different coal/rock interface parameters results in a different progression and magnitude of failure.

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