## 3 Test Results and Analysis  
### Evaluation of Standard Triaxial Testing Outcomes for Sandstone  
Figure 4a depicts the stress-strain profiles for sandstone samples subjected to conventional triaxial compression tests at different pressures of confinement. In accordance with the curves' characteristics, the breakdown progression of the samples subjected to each confining pressure can generally be segmented into five distinct stages. These are analyzed, in each stage, as follows:  
Figure 3: Stress trajectories during triaxial CLU experiments. Figure 4 illustrates the stress-strain relationships for sandstone that underwent standard triaxial compression examinations at varying confining pressures, revealing consistent pattern traits across the curves. The breakdown process of the specimens is broadly categorized into five phases based on the curve features: Stage I represents crack closure, Stage II is the phase of elastic deformation, Stage III involves crack propagation, Stage IV is the macroscopic failure phase, and Stage V is the residual phase. An analysis of the curve characteristics, using a confining pressure of \(\sigma\)3 = 5 MPa as a case study, is presented in Figure 4b. The distinguishing features of each phase are detailed as follows:  
1. Stage I--Crack Closure: During the initial phase of loading in the experiments, imperfections such as pre-existing fissures within the rock gradually seal up. During this period, stress ascends at a slow pace, whereas strain surges swiftly. The curve during this stage exhibits a non-linear pattern, signifying irreversible modifications in the rock's structure and properties. When stress is sustained over time, the crack enters a stable state, and the stress-strain curve exhibits a predominantly linear relationship. Therefore, through the change law of the stress-strain curve, it can be determined that \(\varepsilon_{1}=0.14\) is the crack closing state [39, 40]. 2. Stage II--Elastic Deformation: Upon the stress surpassing the threshold for crack closure, the pre-existing fissures within the rock tend to seal. At this juncture, the rock transitions into a period of elastic deformation, marked by a linear escalation in the stress-strain curves, indicative of the material's elastic behavior. 3. Stage III--Crack Propagation: As the applied stress surpasses the level at which cracks begin to form in the rock specimen, the previously sealed cracks start to extend, and new fissures emerge. This phase marks the rock's entry into the phase of plastic deformation, during which elastic deformation still plays a leading role; the stress-strain curves start to exhibit non-linear features, indicating the commencement of significant structural changes within the rock. 4. Stage IV--Macroscopic Failure: This stage is reached when the stress within the specimen surpasses the threshold necessary for crack propagation, leading to gradual expansion and merging of cracks within the rock. Such activity significantly alters the rock's internal structure, culminating in the creation of a macroscopic failure plane. As a result, the specimen undergoes damage due to a diminished load-bearing  
Figure 4: Curves depicting stress versus strain for sandstone samples during standard triaxial compression tests, along with the curve characteristics at each stage. capacity. During this stage, the radial strain of the rock sharply escalates, with its rate of increase significantly exceeding that of axial strain, indicating the rock's dilation. * Stage V--Residual Stage: Upon the stress in the specimen achieving its peak value, the rock's load-bearing capability sharply falls as a result of the macroscopic failure plane's formation, leading to a swift decrease in stress down to the residual strength. Despite this, the rock maintains a degree of load-bearing capacity during this phase. In summary, the stress-strain profiles of sandstone subjected to varying confining pressures demonstrate only minor variations, with each curve exhibiting a nearly linear pattern until the peak stress is reached.