## 6 Conclusions  
To examine the traits of energy development and the principles governing energy distribution in sandstone during the processes of loading and failure, the MTS-815 electro-hydraulically operated servo-testing process apparatus was utilized for both conventional triaxial compression and CLU experiments on sandstone specimens, conducted at six distinct confining pressures (1, 5, 10, 15, 20, and 25 MPa). The key findings from these tests are summarized as follows:  
1. Shear failure predominates as the primary mode of macroscopic failure under both tested stress paths, influenced by varying levels of confining pressure. When subjected to elevated confining pressures, the tensile stresses within the rock specimens are restricted to levels equal to or lower than the external pressures applied. Consequently, rather than slipping, oblique fractures within the rock preferentially extend along the trajectories of pre-existing fissures, eventually merging to form a continuous shear fracture that penetrates the specimen from the top surface to the bottom. The elevated confining pressure substantially restricts the formation of new secondary fractures. Consequently, during the construction and excavation of deep caverns in high geostress environments, support measures focused primarily on controlling shear failure must be implemented to prevent significant damage to the intact surrounding rock. 2. In the process of triaxial CLU tests conducted on sandstone under different levels of confining pressure, patterns of energy transition were consistently observed, showcasing a distinct peak-shaped outline punctuated by a critical juncture of energy shift. In the initial stages, there is significant accumulation of elastic strain energy. Following this accumulation phase, a transition occurs in which the stored elastic deformation energy is converted into other energy forms that are subsequently released and dissipated as the rock fails. Beyond the peak, during the residual phase post-failure, the dynamics of energy storage and dissipation reach a gradual equilibrium, significantly influenced by the moderating effect of confining pressures. 3. Initially, for rock samples subjected to varying degrees of confining pressure, the energy expended due to internal damage and fracturing exceeds the stored elastic strain energy. As the number of loading repetitions increases, the samples undergo phases of both elastic and plastic deformation. During these phases, the proportion of elastic strain energy initially shows a linear increase, eventually reaching a plateau. With continued cycling, the ratio of expended energy gradually rises, approaching the critical point of peak stress. At this point, the accumulated elastic deformation energy within the samples is rapidly released, resulting in a significant increase in energy dissipation and a notable rise in the curve of the dissipated energy ratio. Under these conditions, the rock specimens experience sudden failure. 4. Employing the energy consumption ratio Ke, defined as the ratio of energy dissipated to energy stored during the loading of rock, acts as a surrogate indicator for assessing the extent of damage within the rock. This metric effectively illustrates the rock's comparative stability when subjected to load. A Ke ratio of 1 signifies a critical equilibrium state of the rock, typically observed near the peak stress level or during the residual phase. When the energy consumption ratio exceeds 1, Ke > 1, it indicatesa phase of instability within the rock, occurring around the peak stress level. The point of inflection observed on the Ke curve can be leveraged as a precursor signal for impending rock failure. J.Z. : Conceptualization; Resources; Data Curation. Y.L. : Formal Analysis; Methodology; Project Administration. H.G. : Formal Analysis; Software. X.Z. : Supervision; Validation; Writing--Review and Editing. S.Z. : Visualization; Writing--Original Draft. All authors have read and agreed to the published version of the manuscript.