The potential for a floor to soften is dependent on geologic conditions. Here, sufficient strata lay beneath the coal bed (or other mine ore), which are susceptible to deterioration when exposed to moisture. These floor rocks are commonly classified as claystone, mudstone, or clayey shale. An example of the effect exposure to water can have on these materials is shown in Figure 2. But, the degree of the effect that moisture will have on these rocks depends on the mineralogical content. It is, however, important to note that this geological condition is not prevalent under coal beds in many areas. Where there are more durable floor rocks present, floor failure is rarely a concern. In a mine, the area which is most exposed to moisture from groundwater inflows is in the entries (openings), where the water pools onto the floor. Being unrestrained in the mine opening, these non-durable rocks can swell upwards and reach a fully softened state. The pooled groundwater not only soaks the floor materials in the opening, but also those under the adjacent pillar. Here, fine-grained rock beneath the pillar absorbs the pooled water, causing it to soften to some degree (i.e. a partial softened condition). See Figure 3. Note that these rocks can exert a swell pressure when exposed to moisture, which is related to a suction potential that can be greater than 500 psi. The suction potential and in turn the degree of softening depends on the rock composition, as well as the restraint applied by the pillar and an underlying resistant rock zone.
Conclusion
Where weak, non-durable strata are present in the mine floor, pillar floor bearing instability can be the critical mode of failure. These rocks can dramatically soften or reduce in strength when exposed to moisture. Consequently, this softening condition must be addressed if mine instability and subsequent surface subsidence is to be abated.
