Abstract: In order to study the formation and development mechanism of surface subsidence in Chaganaobao iron-zinc mine, Rongguan mining industry, Inner Mongolia, a detailed engineering geological survey was carried out in the mining area. The occurrence of structural planes of skarn and tuff were statistically analyzed by using software Dips. Based on the investigation results of subsidence pit and underground goaf, the formation and development mechanism of surface subsidence were analyzed in detail. The results show that: (1) there are three and four groups of dominant structural planes of skarn and tuff in the mining area, and the dip angles of structural planes vary from 77°to 81° and from 85°to 90° respectively. (2) The collapse pit forms a steep free face of rock mass on the three sides of the east-west and the south, which is mainly destroyed and collapsed in the form of dumping. Step type landslide is formed in the north side of the collapse pit, which is mainly destroyed in the form of arc-shaped landslide. (3) The formation mechanism of subsidence pit is mainly divided into four stages: natural stability stage - caving formation stage - caving extension stage - surface subsidence stage. The west, south and east sides of the collapse pit consist of three development stages: unloading rebound stage, fracture opening stage and fracture surface collapse stage. The development stages of the north side of the collapse pit include unloading rebound stage, compressive tensile crack bottom-up cracking stage and slip surface through slip stage. (4) A steep slope is formed at the footwall of the ore body, and a large number of fractures with different sizes and widths are generated along the strike of the ore body (NE30°), including some staggered fractures. The formation of surface subsidence is determined by multiple factors, among which the unique geological condition is the internal cause, whereas the underground mining activity acts as the inducement. The internal and external factors together account for the formation and development mechanism of the subsidence pit. (5) Subjected to the active toppling failure and passive traction of the surface soil arc landslide in the north side, the rock and soil masses in the northeast side of the collapse pit have experienced a special form of toppling failure. The findings of this paper provide references for mine safety production and collapse pit treatment.

Key words: mining engineering, geological disaster, surface collapse, collapse mechanism, field investigation