Abstract: The slope of the phosphorite mine in Guizhou exhibits the characteristic of “upper hard and lower soft”. The complex geological environment and the influence of underground mining activities contribute to the widespread occurrence of slope collapse hazards in the mining area. Through engineering geological investigation and numerical simulation methods, this study explores the formation mechanism of slope collapse in the mountainous area with the “upper hard and lower soft” mining type. The results indicate that such collapses are controlled by the “upper hard and lower soft” slope structure and deep unloading fractures, with the unloading fractures determining the collapse boundaries. After the phosphorite layer is mined, the top plate firstly collapses to the mining hollow area, followed by a gradual compaction of the hollow area, deformation of the overlying rock, and then deformation and destruction of the mining slopes. The collapse involves stages of subsidence deformation, toppling and pulling apart at the slope’s peak, and overall slope failure, summarized by the mechanism: “upstream flow pulling apart – toppling”. Understanding the formation mechanism of such collapses is of guiding significance for the prevention and control of geological hazards.

Key words: Collapse, Mining Slope, Mechanism of Formation, Upper Hard and Lower Soft