## 4 Macroscopic Failure Characteristics and Intrinsic Mechanisms of the Specimens
### Intrinsic Mechanisms of Specim Failure  
The effects of the freeze-thaw cycle count and freezing temperature on the strength variations and macroscopic failure discrepancies in coal-measure sandstone are primarily reflected in the changes in the bonding state of the internal particles and the pore-crack structure of the specimens [28; 29]. This influence process can be divided into six stages: the normal stage, saturation stage, first freezing stage, first thawing stage, multiple freezing stage, and multiple freeze-thaw cycle stage, as illustrated in Figure 10.  
Figure 10: _Cont._  
From Figure 10a,b, it can be observed that coal-measure sandstone in its natural state is composed of various matrix particles bonded together, accompanied by some primary pores and cracks. After saturating the sandstone specimens, water fills the primary pores and cracks and partially adheres to the surfaces of the matrix particles, reducing the friction between them. Simultaneously, the free water in the pores and cracks can dissolve soluble minerals within the sandstone, weakening the bonding between particles and slightly reducing their resistance to deformation. As the specimens transition from the saturation stage (Figure 10b) to the first freezing stage (Figure 10c), the free water within the pores and cracks gradually transforms into ice crystals, causing volume expansion. Under the influence of frost-heave forces, the matrix surrounding the pores and cracks undergoes compressive deformation, and the sharp corners of the cracks experience tensile expansion, resulting in initial freezing damage to the specimens. Concurrently, as the freezing temperature decreases from ambient to the design value, the matrix particles contract. Due to the differing thermal expansion coefficients of the particles, the degree of contraction varies slightly, further weakening the bonding properties between the matrix particles. When the specimens transition from the first freezing stage (Figure 10c) to the first thawing stage (Figure 10d), the temperature rises, and the frozen water within the specimens gradually transforms back into free water, migrating into newly formed pores and cracks. As shown in Figure 10e,f, upon repeating the aforementioned freeze-thaw process, and undergoing multiple freeze-thaw cycles, the primary pores and cracks of the specimens further enlarge, and secondary and through cracks gradually increase. The specimens' resistance to deformation and strength progressively decline, with their mechanical properties transitioning from brittleness to ductility, and the failure modes evolve from shear failure to a composite mode of shear and tensile failure.  
Furthermore, as the freezing temperature decreases, the degree of contraction of the internal matrix particles of the specimens gradually increases. Due to differences in the thermal expansion coefficients between the particles, the relative displacement between them increases, further weakening their bonding properties. Consequently, under low-temperature conditions, the specimens exhibit a relatively high number of secondary and through cracks, with an uneven distribution, leading to more severe damage. The surface of the specimens displays a dense distribution of macroscopic tensile and shear cracks.  
Figure 10: Schematic diagram of the internal-structure evolution process of coal-measure sandstone specimens under different freezing and thawing states. (**a**) Normal stage; (**b**) saturation stage, the blue color is the water saturation test; (**c**) first freezing stage, the blue color is the freezing test; (**d**) first thawing stage; (**e**) multiple freezing stage, the blue color is the multiple freezing test; (**f**) multiple freeze–thaw cycle stage.