Criteria for Predicting the Presence of Voids in Abandoned Subsurface Mine Workings in Zones of Intensive Weathering

Empirical criteria (K₁–K₂ > 0 and η–K₃ > 0) for predicting the presence of voids in abandoned shallow underground workings in Donetsk city (Ukraine) located in the zone of intensive weathering are proposed. The voids 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 void existence is decreased. It is also established that a statistical parameter η 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 void is decreased. The proposed criteria confirm that a thick immediate roof and sandstone layers contribute to the existence of voids of unknown stability. On the other hand, the probability of existence of these voids decreases with increasing thickness of alluvium and total thickness of argillites and sandy shale layers in the rock mass which in turn suggests a decreased probability of void migration and surface subsidence in the future as this has likely already occurred. The use of the proposed criteria in engineering practice is discussed.