# Failure Mechanism of the Yizhuxiang Collapse under the Joint Effect of Freeze-Thaw and Mining
## 4 Deformation and Failure Mechanism  
### Analysis of Inducing Causes  
Geological impact: the stratigraphic dip of the studied area Yizhuxiang was 95\({}^{\circ}\). The slope in the perilous rock area was a tangential slope, and the rock attitude was relatively gentle (15\({}^{\circ}\)). The upper part of the studied area was Dengying Formation dolomite, which has a strong weathering resistance and tends to form large and steep cliffs with free faces. The rock mass was relatively fragmented due to the impact of fracture cutting. The lower part is Doushantuo Formation siliceous dolomite, shale, and phosphate rock bed, which has weak weathering resistance and relatively low strength and tends to form a gentle slope (Figure 2). Under this rock stratum combination, soft layers such as shale at the bottom are prone to plastic deformation under the gravity of the overlying rock mass and lead to the cracking of the hard rock stratum prone to brittle fracture in the upper part. The structure of hard strata in the upper part and soft strata in the lower part created the geological environment and material conditions for the formation of collapse [49; 50]. In 2015, the widths of the cracks on the rear edge of the perilous rock mass and the road cracks were positively related to the daily precipitation (Figure 7b). That is, the infiltration of precipitation increased the water content of the rock mass, thereby softening the structural plane of the rock mass, changing the mechanical state of the structural plane, and decreasing the antioverturning moment, so that the fractures on the rear edge of the rock mass became gradually connected [51; 52; 53; 54]. Similarly, the fracture water pressure will cause additional deformation of the rock block, resulting in an increase in the opening of the fracture and an increase in the permeability coefficient. The shearing strength of dolomite decreased rapidly with the increase in water content so that the plastic deformation increased and the ductility after failure was significantly enhanced [55]. In 2022, extreme weather events occurred, with the snowfall much higher than those in previous years and the temperature significantly lower than those in previous years. As a result, the collapse occurred before the rainy season, indicating that the rock mass was continuously subjected to repeated freezing and thawing. In the process of freeze-thaw cycles, the damage of the water-ice phase change on the rock mass was gradually accumulated, specifically with the temperature change, the water-filled structural plane repeatedly froze and thawed, resulting in progressive rock mass damage caused by gradual splitting, and the elastic modulus, hardness, and fragmentation of dolomite were negatively related to the number of freeze-thaw cycles [56; 57; 58; 59; 60]. Especially for rock masses with fissures, the frost-heaving force caused by the freezing process of the fissure water is an important reason for the deterioration of the rock mass structure [61; 62]. The lower the freezing temperature, the earlier the frost heave force will appear. The closer it gets to the freezing point [63]. Under the action of multiple freeze-thaw cycles, during the process of continuous accumulation, explosion and release of frost-heave force, a single fracture surface develops into multiple fracture surfaces, and the accumulated damage of cracks leads to further expansion and penetration of cracks [61]. The aforesaid causes ultimately led to a collapse. Mining was one of the main causes causing the collapse. During the mining process, the rock mass is under the action of unloading, which is mainly subjected to tension damage and shear damage [64], which induces cracks in the rock mass, and the security pillars left after mining provide sufficient support for the top rock mass. After mining operations ended, the pillars began to spall and collapse [65]. Under the action effect of pressure, the safety pillars were subjected to stress concentration at the corners and then broke (Figure 9a) [66].