矿坑排水与岩溶地面塌陷的关系

Relationship between mining drainage and karst ground collapse

浙江省某岩溶矿区南部洼地先后发生6处地面塌陷,引发了当地居民的恐慌。矿区排水是唯一地下水开采活动,为分析塌陷区与矿区之间的水力联系,遂在研究区全面布置地下水监测系统。通过对比分析观测水位、矿坑排水量及当地降雨量数据,探讨地面塌陷与矿区排水之间的关系及其潜在成因。研究表明,研究区岩溶地下水动态变化整体保持一致,受降雨、矿坑排水等综合因素影响。北部矿区和南部洼地存在岩溶连通管道,两者在雨季水力联系微弱、旱季水力联系密切。经长期径流潜蚀作用,导致区内地面塌陷区灰岩顶板岩溶发育处上覆土层出现土洞,当矿区排水量急剧增大时,南部洼地岩溶水位大幅度下降,使得土洞中出现负压、上覆土层所受浮托力降低、孔隙水与岩溶水补排关系改变,在多种效应共同作用下,土洞最终形成地面塌陷。

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, sur- rounding 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 I.inhou depression is located at the plung- ing 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 monito ring 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 groundwa ter observing wells were set, installed with Solinst I.evelogger 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 col lapse are discussed through comparing the observation data of water table, mining drainage and local precipi- tation. 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 drain- age, while the water tables of the observing wells （GC2, GC3 and GCS） 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 G-C2, G-C3 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.