Abstract:Three surface subsidence have occurred in Zuozhuang gypsum mine in Pingyi county. According to underground survey, these subsidence are not caused by the minedout collapse. According to geological and environmental conditions, mining methods, the layout of alley and other factors, it is regarded that subsidence has been caused by not enough depth of the top columns in the first mining section. Thus, loose covering beds of the Quaternary system and in the middle of working out section become too thin. In the period of underground mining, the top columns have been penetrated, and the loose covering beds of the Quaternary system have been connected. Quaternary pore water carried sand to mines, and induced potential erosion, and formed "sky light" type hour glasses between the top columns in worked out sections and the loose covering beds of Quaternary system. Thus, soil caves in the loosing covering beds of Quaternary system have been gradually formed, enlarged and caused surface subsidence.
