Abstract: In this study, Chengchao western iron mine is taken as a case to systematically analyze the variation of groundwater and the sudden collapse caused by underground mining by considering the lithology of rock mass and the influence of underground mining. The surface collapse is mainly controlled by underground mining. The relationship between goaf height and the mining depth is linear when the surface subsidence occurs under certain geological and hydrological conditions. The magnitude and extension range of surface deformation are closely related to lithology of strata around the goaf. Meanwhile, continuous rainfall may induce a sudden surface collapse. The surface collapse could be divided into 4 stages: intermittent upward caving stage, cracks connectivity and expansion stage, drainage collapse stage and formation stage of surface collapse pits group. Moreover, the collapse of rock mass above goaf is not sufficient. The accumulation of rock mass cracks with time and the dynamic change of rock mass collapse show intermittent and leaping upward development. The caving height of the overlying rock mass is related to lithology of strata. When rock mass strength decreases and rock mass integrity becomes poor, the single caving height becomes smaller while the total caving height becomes larger. A funnel-like water level drop gradually forms with the orebody exploitation. The decrease of groundwater level is not only related to the relative position of the goaf, but is controlled by the lithology and structure of surrounding rock. Furthermore, the formation and development of cracks in the rock mass provide favorable channels for the downward flow of groundwater. Soluble rocks are eroded continuously, clay and debris are washed away by groundwater, consequently, a hidden space is easily formed underground. The deformation and failure of the whole gypsum may cause the groundwater inrush which generates an instantaneous high negative pressure in the subsurface area, inducing the surface collapse.

Key words: underground mining, surface collapse, groundwater, lithology of strata, anhydrite