Strength reduction on saturation of coal and coal measures rocks with implications for coal pillar strength

Coal and associated coal measures rocks, including shales, mudstones and sandstones, may lose significant strength on water saturation due to the absorption of water, stress corrosion or mineralogical changes to the clays within the rock matrix. As most coal seams are natural aquifers, water ingress into underground mine workings is common, and is typically controlled during mining by dewatering pumps. After a mine has closed, unless ongoing pumping is maintained, workings will flood and coal pillars and the pillar roof and floor foundations will revert to fully saturated conditions. This paper reports on a study of how flooding effects pillar strength in an abandoned coal mine in Queensland, Australia. The reduction in strength of coal and coal measures rocks between unsaturated and saturated water moisture contents has been quantified through physical testing. These strength data, along with the distribution of lithologies in a coal pillar from the mine, were used as inputs for numerical modelling of the strength reduction of a coal pillar on water saturation. It was found that the modelled pillar strength reduction could be estimated by the average strength reduction of all the lithological components of the pillar, weighted by the thickness of each lithology.