Based on analyzing case history data in the Illinois Coal Basin, it was determined that mine collapse resulting in surface subsidence can occur from floor softening. In other words, certain materials present in the mine floor can soften after coal extraction and fail when bearing pillars. The softening mechanisms considered are: 1. slaking/swelling due to moisture exposure, and 2. creep or strain softening due to sustained loads. In some case histories, the effect of floor slaking or swelling on floor stability is a fairly dramatic event by the occurrence of pooling of mine water and subsequent surface subsidence in the area of pooling. In other cases, the time factor is greater. The effect of softening on the ultimate bearing capacity of the mine floor was assessed by modeling representative conditions using FEM and elasto-plastic elements. In the models, the properties of softened mine floors were determined from results from subsurface exploration work in floor areas which were stable and others which had failed. The zone of softening was established on stress-field analysis.
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