## 2 Loading Test after Confining Pressure Unloading
### Results Analysis  
The specimens were subjected to uniaxial compression at \(\sigma_{0}\) = 86.5 MPa, 25 MPa confining pressure triaxial compression at \(\sigma_{25}\) = 167.8 MPa. All failed when the confining pressure was unloaded to approximately 6.3 MPa and 5 MPa confining pressure triaxial compression at \(\sigma_{15}\) = 142.3 MPa. Evidently, from Figure 2, the loading triaxial stress-strain curves of the specimens under different confining pressure unloading are similar and can be roughly divided into four stages: fracture compaction, elastic deformation, plastic deformation (deformation and failure development), and post-peak. (a) In the fracture compaction stage, the primary cracks and pores in the specimens were compressed and closed by the deviatoric stress. (b) In the elastic deformation stage, the stress-strain curve is linear and the properties are stable. (c) In the plastic deformation (deformation and failure development) stage, the specimen entered the plastic stage and the stress-strain curve began to deviate from a straight line, with the slope decreasing gradually as microcracks in the specimen continued to develop until the peak stress state was reached. (d) In the post-peak stage, the internal cracks propagated after the specimen reached its peak stress and converged rapidly to form macroscopic fractures, leading to failure of the specimen. Figure 3 shows the peak strain-confining stress curve. Evidently, the peak strain increased with the confining pressure unloading, with the axial peak strain increasing by 5.2, 15.0, and 20.0% and the circumferential peak strain increasing by 20.0, 29.6, and 42.5%, respectively, compared with the case in which the confining pressure unloading was zero. The circumferential peak strain was more sensitive to the response of confining pressure unloading damage effects. Figure 2: Triaxial reloading stress–strain relationship curves of confining pressure unloading specimens. The crack volume strain model proposed by [PERSON] [34] can be used to analyze the crack initiation stress (\(\sigma_{cl}\)) and damage stress (\(\sigma_{cd}\)), with the axial stress corresponding to the starting point of the stable crack propagation stage assumed to be equivalent to the initiation stress (\(\sigma_{ci}\)) and the axial stress corresponding to the starting point of the accelerated crack propagation stage assumed to be equivalent to the damage stress (\(\sigma_{cd}\)). As shown in Figure 4 and Table 1, the initiation stress (\(\sigma_{ci}\)) decreased by 8.8, 20.6, and 41.1%, whereas the damage stress (\(\sigma_{cd}\)) decreased by 9.6, 19.9, and 37.8%, respectively compared with the case in which the confining pressure unloading was zero. In contrast, the initiation stress (\(\sigma_{ci}\)) decreased more when the confining pressure unloading was larger; it was more sensitive to confining pressure unloading. The differentials between the initiation stress (\(\sigma_{ci}\)) and damage stress (\(\sigma_{cd}\)) also decreased. This indicates that microdefects such as microcracks, grain boundaries, and lattice defects in the rock specimens will propagate and develop when the stress environment changes. Consequently, the stress of the specimens increased rapidly to the damage stress value after reaching the initiation stress value; the stable crack propagation stage was shortened and the bearing capacity was reduced. Figure 4: Initiation/damage stress–confining pressure unloading curve. Figure 3: Peak strain–confining pressure unloading curve. Figure 5 reveals that confining pressure unloading affects the strength, which decreases as the confining pressure unloading increases. When the confining pressure unloading was 15 MPa, the decrease was more evident, with the uniaxial and triaxial compressive strengths decreasing by 18.6% and 13.4%, respectively, compared with the case in which the confining pressure unloading was zero.