Distribution and influence factors of sinkholes in the Chenzhou area, Hunan Province

The Chenzhou area is characterized by widely distributed carbonate rocks and thin overburden soil. The sinkholes occur frequently due to long-term dewatering of mines. Based on geological investigations, this paper analyzes the distribution and influencing factors of sinkholes in this area. Results show that， (1) most sinkholes took place from 1980 to 1983, with 8.5 sinkholes per year on average. But severe sinkholes occurred since 2012, with 5.88 sinkholes per year. Sinkholes tended to occur during rainy seasons between March and July. (2) Fifty-four sinkholes (55.67% of the total) are distributed in low hilly gullies, and 40 sinkholes (41.24% of the total) are distributed in karst plain. Ninety-seven sinkholes took place in Carboniferous Hutian Group strata and Lower Qiziqiao Formation of Middle Devonian strata. Many sinkholes appeared in fault zones and fold zones. (3) 40.21% of sinkholes are distributed in alluvium layer, and 59.79% of sinkholes are distributed in diluvial beds. Collapse sinkholes were more likely to occur in silty clay because of its large distribution and more prone to subsoil erosion than other types of overburden materials. There are 63 sinkholes distributed in single-layer soil structure areas, and 34 sinkholes occurred in double-layer and multilayer soil structure. Double-layer soil consists of silty clay and pebble soil, whereas multilayer soil is form of silty clay, mud clay and sandy soil. 49.48% of sinkholes occurred in the overburden soil with thickness less than 10 m, and 50.52% of sinkholes took place in the overburden soil with thickness 10 m to 15 m. (4) The basic conditions of sinkholes include well developed karst in the shallow subsurface, and thin and weak overburden soil. The primary triggering factor is pumping karst groundwater for urban water supply and mining.