## 3 Uniaxial Compression Strength Characteristics of Fissured Sandstone
### Strength Characteristics with Fissures of Five Different Inclinations  
The strength, failure strain, and elastic modulus of rock are influenced by the inclination of fissures. In the frozen-saturated and dried states, the UCS curve exhibits a decreasing trend followed by an increasing trend with increasing inclination; its minimum value is observed at an inclination of 30\({}^{\circ}\). However, in the saturated state, the curve shows continuous growth (Figure 4a). The UCS values decrease in the following order: frozen saturated state > dried state > saturated state. The failure strain initially increases, then decreases, and finally increases again in both the dried and frozen-saturated states. However, in the saturated state, it first increases and then decreases (Figure 4b). Similar to the UCS values, the failure strains also follow a decreasing trend in the order of frozen-saturated state > dried state > saturated state. The elastic modulus experiences an initial drop followed by a  
Figure 3: Stress–strain curves under (**a**) dried, (**b**) saturated, and (**c**) frozen-saturated conditions. rise in both the dried and frozen-saturated states, but demonstrates continuous growth in the saturated state. [PERSON] and [PERSON] reported a strong correlation between the initial crack stress and strength with the full-field strain evolution as inclination changed [6]. Figure 5 illustrates the strain localization area and its corresponding stress variation with changes in inclination, using digital image correlation (DIC). It can be observed that as inclination increases, the strain localization gradually shifts from the middle to the tip of the fissure. The stress \(\sigma\) at which strain localization occurs also increases with inclination and approaches the specimen's strength \(\sigma_{p}\), but only reaches 39.6% of \(\sigma_{p}\) at an inclination of 30\({}^{\circ}\). This indicates that an inclination of 30\({}^{\circ}\) is most conducive to generating strain localization. As the inclination angle becomes higher, it becomes more challenging for strain localization to occur near the fissure, implying greater difficulty in deformation. Consequently, in both frozen-saturated and dried states, there is a downward-upward trend in UCS curve with increasing inclination, reaching its minimum value at an inclination angle of 30\({}^{\circ}\). The strength, failure strain, and elastic modulus are also influenced by temperature and saturation state. In the dried state, the UCS of fissured sandstone is higher than that in the saturated state at the same inclination angle, indicating a softening effect caused by water. However, in the frozen-saturated state, the UCS of fissured sandstone is stronger than that in the saturated state, suggesting an increase in UCS due to freezing (Figure 6a). The ranking of UCS can be described as follows: frozen-saturated state > dried state > saturated state. Similarly, the failure strain follows a similar trend: frozen-saturated state > dried state > saturated state (Figure 6b). On the other hand, for inclinations of 45\({}^{\circ}\) and 60\({}^{\circ}\), there is a similarity between elastic modulus trends and UCS variations (Figure 6c). Pore water has a softening effect on sandstone after it is saturated at room temperature. It not only dissolves soluble cementation among mineral grains, but also acts as a lubricant on the rock matrix. These two effects decrease the bonding strength and friction among sandstone mineral grains, leading to a lower UCS in saturated specimens compared to  
Figure 4: Compressive strength (**a**), failure strain (**b**), and elastic modulus (**c**) vs. inclination. Figure 5: Strain localization areas at different fissure inclination angles in the rock materials. dried ones. Saturated sandstone exhibits increased UCS after freezing due to the changes in the phase composition of pore water, which reflects the relative amounts of unfrozen water and pore ice. Mechanical interactions among the water, ice, and rock matrix also occur.