## 4 Discussion  
Due to groundwater migration and seepage, pore water in natural rock is not uniformly distributed, resulting in significant variations in rock saturation [3]. When the temperature drops below 0 \({}^{\circ}\)C, the pore water in the rock begins to freeze into ice. The pore ice remolds the pore structure, leading to dramatic differences in mechanical properties (e.g., strength, deformation). Under impact testing, the pore ice reduces the stress concentration in the crack tips, preventing the formation of some cracks caused by compressive stress waves. For example, as saturation increases, the plastic deformation of the sample first decreases and then increases. Moreover, the enhanced phenomenon is observed in static tests, such that the strength, elastic modulus, and brittleness are usually greatly increased at low temperatures [7; 32]. However, as saturation increases, the rock interior is damaged by the expansion of pore ice, weakening the strengthening effect [9]. When the two effects coexist, the saturation of the rock determines which process dominates by affecting the ice content in the pore. In other words, saturation is crucial for enhancing or damaging pore ice at low temperatures. Consequently, the influence of saturation on the mechanical properties of the samples needs further discussion. In the dry or low-water-content frozen samples, where the internal pores are mainly occupied by air, the changes in dynamic properties during freezing are related only to the shrinkage properties of the mineral particles. As the saturation increases, the presence of pore ice becomes the main reason for the change in mechanical properties [6]. When the saturation of the sample is low, the growth of pore ice volume is limited due to the lack of pore water [37]. At this time, the pore ice in the rock has a strengthening effect, which mainly consists of three aspects: the supporting effect increases the dynamic strength or deformation resistance of the rock; the crack-filling effect reduces the stress concentration at the crack tips; and the adhesion forces between the unfrozen film water with the rock matrix and the ice improve the mechanical properties of the pore in terms of tensile and shear strength (Figure 18). When the pore water freezes into ice, the dynamic mechanical properties of the sample tend to strengthen, while the degree of fracture decreases. Moreover, the plastic deformation of the sample under dynamic compressive stress decreases with increasing saturation, which is due to the limitation of microcracks development inside the sample. However, when the saturation gradually increases (>50%), the dynamic strength of the sample decreases significantly, especially at 100% saturation. The continuous expansion of the pore ice gradually leads to the germination of microcracks in the specimens. Therefore, microcracks are more likely to develop earlier during impact loading, leading to an increase in plastic deformation (Figure 8) [1]. When saturation exceeds 50% in this work, there is sufficient water in the rock to provide a basis for the growth of frost heave during freezing. Therefore, the role of pore water in the strengthening or weakening effect needs further investigation. As shown in Figure 18c, the pore distribution characteristics of the sample mainly include main pores, secondary pores, branch pores, and isolated pores. Since the freezing point of water in pores is inversely proportional to the pore radius, the water in the main pores freezes first and then grows into the secondary pores [27]. When the water content of the sample is low (\(S_{\rm r}\) = 30%), most of the unfrozen water migrates through the water film into the main pores and ice growth stops after a certain volume because there is insufficient water [25]. Consequently, the \(T_{2}\) spectrum of the red sandstone in Figure 7 shows no significant change. At this time, the pore ice mainly has a strengthening effect.