## 3 Analysis of Test Results
### Analysis of Strength and Failure Characteristics  
Rock is a typical heterogeneous material, and its failure results from the formation, propagation, intersection, and penetration of micro-cracks, ultimately leading to localized failure. The shape of the fracture section is primarily influenced by the rock's physical properties and stress state. In actual triaxial stress (\(\sigma_{1}>\sigma_{2}>\sigma_{3}\)), according to [PERSON]'s strength theory in [PERSON] and [PERSON] (2018), the \(\sigma_{3}\) minimum means the controlling sandstone \(\sigma_{3}\) direction deformation is the weakest. A large number of micro-cracks will be generated; the concentrated tensile stress derived from the end of the micro-crack will indirectly accelerate the growth of sandstone in \(\sigma_{1}\) compression deformation in that direction. Large expansion deformation occurs in the order of \(\epsilon_{3}\), resulting in multiple shear cracks in the rock. The propagation and penetration of these shear cracks create multiple shear failure surfaces that penetrate the entire rock specimen. In the context of coal mining, as per the mine pressure control theory, the mechanical behavior of deep floor rock is depicted in Figure 10.  
Among them, 1 and 5 are areas with original rock stress distribution areas, the 2 area is a stress reduction zone, the 3 area is a stress increase zone, and the 4 area is the goal; as well, H represents the mining failure depth, \(\theta\) represents the fault dip angle, \(\gamma\) represents the unit weight of the overlying strata, and H  
It can be seen from the crushed rock specimen that under the stress environment of 600 m, the red sandstone mainly experiences shear failure and a minor tensile failure, and the failure crack exhibits a \"V\"-type failure pattern, one main crack runs through the entire test piece, and several other cracks intersect with it obliquely, forming a strike parallel to \(\sigma_{2}\), with the shear failure surface having a certain angle with \(\sigma_{3}\) and generate debris at the failure surface [25]. Moreover, the damage is more serious, and the number of cracks is reduced to \(\sigma_{2}\). Figure 11: Crack propagation pattern at each burial depth: (**a**) surrounding rock failure of 600 m fault; (**b**) surrounding rock failure of 1000 m fault; (**c**) surrounding rock failure of 1400 m fault. cracks increases at the place where the axial stress is applied on the top of the specimen. Under the stress environment of 1000 m, there are both \"V\"-type failures and \"X\"-type failures. Multiple through-cracks are generated in the test piece, and the through-cracks intersect with other micro-cracks, forming a network shape in three-dimensional space. There is a substantial amount of rock powder left by shear friction, as well as a few small flake-like rock blocks inside. The rock failure mode is mainly a shear failure formed by axial compression. In addition to shear failure, the bottom corner of the test piece also experiences damage; this phenomenon is one of the typical failure modes of rock because the rock is a heterogeneous body with joints; it will fracture in the process of compression. Due to the internal cracks, local small-scale cracks appear at the bottom corner of the rock sample. Under the stress environment of 1400 m, the damage degree of the sample is relatively weak. The main crack runs through the whole test piece. There are some small cracks in the lower part of the test piece that are almost parallel to the height of the test piece, and no crack network is formed. Comprehensive analysis shows that with increasing depth, the failure mode of the rock changes from axial splitting failure to simple shear failure, the roughness of the fracture surface decreases, and the proportion of the crushing area increases.