Subsidence risk assessment of decommissioned bord-and-pillar collieries

Underground mining of coal results in voids with remanent pillars to support the overlying strata. Failure of any individual pillar overloads adjacent pillars and may result in failure propagation with damaging surface subsidence. It has been shown that coal pillar strength can be estimated from the attributes of effective width and height, however the risk for pillar panel failure resulting in surface subsidence cannot be estimated by consideration of individual pillars in isolation. In this paper we show the calculation of a panel failure probability density function from a statistical distribution of the pillar geometrical attributes reflecting mine design uncertainties and environmental degradation with time. We first calculate the static then dynamic safety factors. In a piece-wise fashion over the surface area we estimate the probabilities that 80% of pillars in a circular area of diameter equal to the cover depth have a safety factor less than 1.6. These probabilities then form the risk matrix likelihood and estimated surface tilts form the consequence in a thematic surface map of risk. In studies of an urban residential site experiencing multiple incidences of mine induced surface subsidence we demonstrate the methodology and show that when every pillar, numbering more than 1100, is evaluated for stability and the interaction between these pillars is considered, then both the location of observed failures and the resultant measured surface tilts are predictable. Based on this assessment, geotechnical monitoring by a seismic array, extensometers and piezometers of a mine panel identified as at risk is currently being undertaken while remediation options are evaluated.