A discussion on causation mechanisms for overburden bumps as distinct from coal bursts

The entire subject area of micro-seismic events due to stored strain energy, as distinct from gas-driven coal outbursts, can be readily sub-divided into firstly events with their energy source from within the coal seam (termed “bursts”) and secondly, event with their energy source outside of the coal seam in either the overburden and/or floor strata. The reason for sub-dividing micro-seismic events in this manner is that if the causation mechanisms and associated geotechnical conditions are materially different, then effective pre-mining predictions and subsequent operational controls may also differ. Attempting to explain a multitude of micro-seismic event types without consideration of varying source mechanisms will inevitably lead to inadequate causal explanations and effective controls. The paper outlines several different causal mechanisms for bumps emanating from both the overburden and/or floor of a coal seam by reference to both theoretical treatments and known associated case histories. These include massive pillar collapses (including the Coalbrook disaster in 1960), large-scale shear slip along fault planes/other geological discontinuities, the compressive failure of thick and strong strata units and finally, multi-seam stress effects. The objective of the paper is to provide an initial “cause and effect” list of geological and geotechnical circumstances that can and indeed have resulted in large magnitude micro-seismic events during underground coal mining activities, being able to predict the likely propensity for such significant events prior to mining being the first requirement in an effective prevention or consequence mitigation process