Characteristics of triaxial compression damage and energy evolution of saturated red sandstone under different unloading levels

To ensure the safe construction of tunnels in underwater red sandstone strata, and to avoid the occurrence of engineering accidents such as water surge and destabilization in the excavation area. Based on the Ganzhou Rongjiang 4th Road Crossing Tunnel Project, triaxial compression tests with initial unloading levels of 60%, 70%, 80%, and 100% (no unloading) were carried out on the saturated muddy red sand perimeter rock to analyze the changing rules of the mechanical properties of triaxial compression under different initial unloading levels. Combined with scanning electron microscopy, the macroscopic damage characteristics of the rock samples were comparatively analyzed after the tests. Based on the principle of energy dissipation, the energy relationship of saturated muddy red sandstone specimens under different initial unloading levels was fitted to reveal the law between the damage characteristics and energy evolution during the unloading process. The results are shown as follows. The peak deviatoric stress in unloaded triaxial compression damage is much higher than that under the initial unloading level of 60%, 70%, and 80%. With the increase of initial unloading level, the peak deviatoric stress of saturated muddy red sandstone is increasing, the value of the increase in the deviatoric stress and the circumferential deformation is decreasing. The time of expansion of the rock samples due to unloading is much shorter. The fissures and bonded particles of the debris and the fracture lines on the surface of the rock samples are increased and penetration occurs and the more pronounced the damage is caused by tensions. Under the unloading condition, the cracks of the rock samples develop rapidly and the elastic strain energy increases slowly after the triaxial compression reaches the yield strength. The dissipated energy increases sharply, and the elastic strain energy stored inside the rock samples reaches the storage limit during the destructive stage and releases instantaneously leading to the generation of more cracks. As the initial unloading level increases, the total strain energy, elastic strain energy and dissipation energy corresponding to the peak deviatoric stress show a decreasing and then increasing trend. The research results can provide some theoretical references for the excavation and construction of underwater tunnels in the red sandstone distribution area. © 2024, Central South University Press. All rights reserved.