## 4 Results
### Effect of Moisture Content on Mechanical Properties
#### 4.2.2 Axial Compression and Unloading Confining Pressure (\(\sigma_{3}\)) Stress Path  
Figure 14 shows the stress-strain relationship of the sandstone specimens under axial compression and unloading of the minimum horizontal stress at different water content levels. As the water content increased, the deviatoric stress of sandstone gradually decreased. The deviatoric stresses for the dry, 30% saturated, and saturated specimens were 124.4, 113.4, and 97.4 MPa, respectively. The deviatoric stress decreased by 21.7% from the dry state to the saturated state. Under this loading path, the specimen primarily failed along the direction of the minimum horizontal stress (\(\sigma_{3}\)). As the stress was unloaded, the internal cracks in the rock gradually propagated, forming new tensile fractures along the vertical plane in the unloading direction. The number of cracks increased and extended in parallel at certain angles. As the cracks continued to expand, they eventually coalesced into a macroscopic failure surface, resulting in the fracture of the rock specimen (Figure 15). With an increase in the water content, the number of cracks steadily increased, and the crack propagation process became more complex. Ultimately, the cracks merged into larger  
Figure 12: Three-dimensional deviatoric stress–strain relationship diagrams: (**a**) dry; (**b**) 30% saturated; (**c**) saturated.  
Figure 13: Failure modes of sandstone with different water contents: (**a**) dry; (**b**) 30% saturated; (**c**) saturated.  
2.3 Axial Compression and Unloading Confining Pressures on \(\sigma_{2}\) and \(\sigma_{3}\) Stress Paths  
Figure 16 shows the stress-strain relationships of the sandstone specimens under axial compression and unloading of the confining pressure at different water content levels. As the water content increased, the strength and elastic modulus of the sandstone specimens decreased significantly. The specimens exhibited a ductile failure trend, characterized by a slow failure process after the peak stress and a longer softening phase in the total stress-strain curve, indicating noticeable plastic deformation. From the dry state to the saturated state, the deviatoric stress decreased by 27.4%, which was much larger than that observed under the other stress paths.  
As Figure 17 shows, under this stress path, the sandstone specimens primarily failed along the \(\sigma_{2}\) direction, with the fracture plane perpendicular to the direction of the maximum horizontal stress. The rapid unloading of the maximum horizontal stress induced stress concentrations along this direction, causing cracks to propagate in the stress-concentrated zones. In the dry state, the sandstone exhibited evenly distributed  
Figure 16: Three-dimensional deviatoric stress–strain relationship diagrams: (**a**) dry; (**b**) 30% saturated; (**c**) saturated.  
Figure 14: Three-dimensional deviatoric stress–strain relationship diagrams: (**a**) dry; (**b**) 30% saturated; (**c**) saturated.  
Figure 15: Failure modes of sandstone with different water contents: (**a**) dry; (**b**) 30% saturated; (**c**) saturated.  
throughgoing cracks, dividing the specimen into three major blocks. At a moderate water content, the cracks displayed a combination of brittle and ductile characteristics, with more branched cracks and significant localized fragmentation. In the saturated state, the crack network became more complex, exhibiting ductile failure characteristics with a widespread crack distribution but without full penetration.