Forecasting of hollows in abandoned shallow coal mines in zone of intensive weathering

Empirical criteria (K1-K2> 0 and η-K3 >0) for predicting the presence of hollows in abandoned shallow underground workings in Donetsk city (Ukraine) located in the zone of intensive weathering are proposed. The hollows exist when both inequalities K1-K2>0 and η-K3 >0 are satisfied. The statistical analysis has established that the value of criterion K2 is inversely related to rock quality designation (RQD) and that a decrease in RQD leads to a decrease in K1-K2 and, hence, the probability of the hollow existence is decreased. It is also established that a statistical parameter in the left-hand part of inequality η-K3>0 decreases according to logarithmic law with increasing standard linear intensity of weathering-induced fractures (P10). When the fracture intensity (P10) increases, the probability of the presence of a hollow is decreased. The proposed criteria confirm that a thick immediate roof and sandstone layers contribute to the existence of hollows, whereas weak alluvium, argillites and sandy shale allow a collapse of the rock mass.