Distribution features and forming conditions of the Mashankou karst collapse in Huaining County, Anqing City

The Mashankou karst collapse in Huaining county is a large-scale geological hazards site in Anhui Province. On the basis of systematic analysis on geological environment and space-time distribution in this area, the formation mechanism and distribution characteristics of the Mashankou karst collapse are summarized as follows, (1) The Mashankou karst collapse is formed in 1987 and enters the outbreak period during 1988 - 1992 when 39 collapse pits, account for 43.8 % of the total collapse. Especially in 1992, the collapse pits are up to 24, but after 1993 the average annual collapse is only 2.6, (2) Geomorphologically speaking, the collapse pits are mainly distribute in the low-lying floodplain, then in the low hill slope; lithologically, collapse pits are mainly distributed in T sub(1) n, T sub(2)y super(1) and T sub(2)y super(3); hydro-geologically, collapse pits are mainly distribute in the groundwater depression cone range with special focus on groundwater run off direction, and in valleys where water alternates strongly and in area where surface water and groundwater contact closely with each other. The necessary conditions which cause the Mashankou karst collapse include soluble stratum, thin debris cover (generally within 10 m) and frequent fluctuation of the groundwater table. And among them, dramatic change in groundwater dynamic field caused by mine dewatering is the most important factors, which is considered the major cause of the karst collapse.