Abstract

The mechanisms of subsidence over coal mines in which troughs develop above longwall workings or crown-hole collapse occurs over room-and-pillar workings are sufficiently widespread to be quite well understood. Other mechanisms whereby room-and-pillar systems fail are known from stratiform workings—in particular, from evaporite mines, where deterioration of the pillar supports is more rapid. Documented examples from coal workings that involve delayed room-and-pillar failure are, however, relatively rare.A case history of a row of terraced properties whose location coincided with a developing subsidence trough affords evidence of room-and-pillar failure. The terraced properties provided a reference, in the form of a level damp-proof course, from which the progress of the settlement could be monitored and interpreted. Mining records indicated that the centre of the trough was located over a line of 2.2 m high split pillars in room-and-pillar workings at 44 m depth. Borehole evidence showed that some of the pillars were either much smaller than shown or possibly absent altogether. The seat-earth floor was inundated with groundwater and the roof strata were weak mudstones, making the extended pillar system vulnerable to deterioration and failure. The mudstone roof had collapsed extensively over the voids and the resulting debris limited the residual void space available for subsidence. Consequently, some support was provided to the strata as subsidence developed and the process ceased after about 260 mm of settlement at the surface.A large volume of ground was lost into the subsidence trough at the surface. Subsidence at the level of the workings was not conclusively proven by monitoring or measurement, but other possible mechanisms within the superficial deposits that could account credibly for this loss of ground could not be identified. Failure of the room-and-pillar system was the only mechanism consistent with the available evidence and, in view of the condition of the mine workings at that location, is considered to be the cause of the surface subsidence.The absence of other case histories suggests that, although there may be circumstances in which such failure can occur, they are relatively infrequent and, unless there is a means of recording the subsidence and evaluating the mining position with reasonable confidence, room-and-pillar failure is likely to continue to be difficult to identify.

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