Experimental study on the failure behaviors of sandstone specimens with two fissures under triaxial loading and unloading conditions

Crack development in the locked section induces the occurrence of three section rockslides, and thus, the study of the crack evolutionary process is of great importance for the instability prediction of rock slopes. However, the cracking mechanics in rock bridges under unloading have not been fully investigated. In this study, sandstone specimens containing a vertical fissure and an inclined fissure were fabricated to simulate the three-section structure, and conventional triaxial compression and confining pressure unloading tests were carried out. Based on the experimental results, the stress-strain curve, strength property and deformation parameters of two-fissured sandstone specimens were analyzed. The internal fracture characteristics of two-fissured sandstone specimens were investigated by using CT scanning. The experimental results show that the peak strength of the two-fissured sandstone specimen decreased with increasing fissure inclination angle, and the strengths under confining pressure unloading were lower than those under conventional triaxial compression. The peak strength of the sandstone specimens with prefabricated fissures increased with the confining pressure. As the confining pressure increased, the upper part of the two-fissured sandstone specimen was easily broken, presenting a V-shaped failure together with the vertical fissure, and the coalesced crack in the rock bridge induced the failure of the rock specimen. Under low confining pressure, the failure patterns of fractured specimens under unloading were more complicated than those under triaxial compression, and there were more transverse cracks. Under high confining pressure, the failure inside the specimen under unloading was simpler than that under triaxial compression. This study helps to elucidate the fundamental nature of rock bridge failure of fractured rock under loading and unloading.