Formation mechanism analysis of cover collapse sinkholes in Wugaishan Town, Chenzhou City, Hunan province, China

The ongoing growth of cover collapse sinkhole occurrences that concentrated in Wugaishan Town, Chenzhou City in recent years increased the requirement for detailed analysis of collapses genesis. This study is aimed to understand formation mechanism and evolution process of cover collapse sinkholes by multidisciplinary methods in Manaoshan mining area. These sinkholes mainly were discovered in the rainy seasons ranging from April to June or in the thin overburden less than 10 m. Additionally, the development level of the collapses was correlated well with locations and scales of karst caves within underlying bedrock. The results indicated that mine-related cover collapse sinkholes might be ascribed to dynamic changes of groundwater caused by mine dewatering activity. Analysis of hydrogeological condition and dynamic characteristics of groundwater, the formation mechanism of cover collapse sinkholes is characterized into the following categories: infiltration erosion effect, saturation erosion effect and coupling buoyant weight decrease with load-increased effect, which mostly depends on depth and variation velocity of groundwater level. The maximum variation velocity of groundwater level recorded by monitoring sites was 0.815 m/h and 2.07 m/h at the process of groundwater level rising and falling, respectively, which might be the favorable condition for causing collapse pits.