Study on the mechanism of groundwater level restoration process to karst sinkholes in karst deposit drainage area：A case of Dachengqiao, Ningxiang county, Hu’nan Province

By the end of 2016, 17 sinkholes have occurred in Dachengqiao drainage area during groundwater level restoration process. Based on previous research results, through monitoring of groundwater level dynamic characteristics, hydrogeological and engineering geological drilling and field investigation,this paper studied the characteristics of groundwater restoration and its mechanism on sinkhole formation, which has provided scientific basis for early warning and prevention of ground subsidence in the study area.The results show that，(1)sinkholes are mainly distributed at river terraces and Mazao gully. Sinkholes firstly occurred at recharge area and then runoff area, which has a close relationship with the groundwater dynamic change. (2)The groundwater restoration process can be divided into four stages.The formation mechanisms of the sinkholes can be summarized as the creep seepage and air burst effect, suction effect, soften and suction effect and soften and degradation effect based on these four stages. The collapse patterns are the combination of gravitation-,erosion-,vacuum cavitation- and burst-collapse. It is found that the groundwater level fluctuatation in the range of 4-5 m to the surface of bedrock may cause sinkhole more easily. Currently, the groundwater level in the recharge area is 63.95-71.80 m, which is 5-6 m above the bedrock surface, and the groundwater level in the runoff area is 42.8-57.7 m which is 4-17 m below the bedrock surface. Therefore, it is necessary to closely observe the changes of groundwater level.