Relationship between mining drainage and karst ground collapse

Six karst ground collapses occurred at southern depression of a certain mining area at Zhejiang, which panicked the local residents. The mining area is located about 1.5 km west of Xin’an river dam, surrounding by Xin’an river reservoir at east, north and west sides. The ore body mainly occurs in the core of Songkengwu syncline with the strike of north-east direction. The Linhou depression is located at the plunging end of the syncline at south. The core of Songkengwu syncline is a bare tectonic strong karst area, while the southern depression is a covered shallow-eroded strong karst area almost filled by silty clay and pebbly silty clay. The karst groundwater is recharged by precipitation, the upper pore water and the lateral flow. Mining drainage is the only human impact. The karst groundwater general flows from north-east to the Xin’an river reservoir at southwest.The ground collapse occurred with increased mining drainage in 2010. Lacking of groundwater monitoring data in the mining and surrounding areas, especially the southern depression, it was difficult to conclude directly that the increased mining drainage induced the ground collapse. Thus, after fully considering the karst development and the recharge, runoff and discharge conditions of groundwater, eight karst groundwater observing wells were set, installed with Solinst Levelogger water table recorder (Model 3001, Canada) and GPRS online monitoring system, in the southern depression and the northern mining area to carry out the realtime water table observation. The relationship between ground collapse and mining drainage and the potential causes of ground collapse are discussed through comparing the observation data of water table, mining drainage and local precipitation. It was concluded that the dynamic of water table was consistent at the study area and impacted by precipitation and mining drainage comprehensively. The water tables of the observing wells (GC1, GC4, GC7 and GC10) in and close to the mining area were apparently impacted by precipitation and mining drainage, while the water tables of the observing wells (GC2, GC3 and GC5) at the southern depression were slightly impacted by precipitation and mining drainage. The sharp contrast of the water table dynamic range of the two indicated a weak hydraulic relation. Through combined consideration of water table observation data, water balance calculation and the response of the local government, it can be inferred that there was a close hydraulic relation between the two area in dry seasons. When the water tables of the observing wells GC2, GC3 and GC5 were higher than that of GC1 and GC4, weak reverse recharge would happen. Long-term runoff suffosion led to the appearance and expansion of soil caves in the covering layer right above the limestone roof where karst developed. Rapidly increased mining drainage result in sudden decline of the water table, which formed vacuum in the soil caves, reduced the uplift force of the covering layer, changed the recharge and discharge relationship between the pore water and karst water, and finally caused ground collapse.