Fracture characteristics and mechanical behavior of sandstone containing a crack at the end surface under true triaxial stress condition with a free face

After tunnel excavation, inherent cracks within rock masses may be exposed at the free surface of the tunnel, which are referred to as end cracks. Crack growth from the end crack occurs much more easily than that from a crack inside the rock mass. This study conducted compression experiments under true triaxial stress condition, along with numerical simulations, to explore the failure characteristics and mechanical behavior of surrounding rocks containing end cracks near the free surface of the tunnel. The results reveal that compared with the inclination angle of the end crack, the failure pattern of rock is significantly affected by the end crack length. As the end crack length increases, the failure pattern of rock gradually transitions from the mixed tensile-shear fracture to the shear fracture. The peak strength of rock initially decreases and then increases as the end crack length increases. While, the peak strength of rock basically remains unchanged with the increase of the inclination angle, suggesting that the capacity of surrounding rocks containing end cracks near the free surface is primarily influenced by the end crack length. Based on the failure characteristics of surrounding rocks containing end cracks, as obtained from numerical simulations, the support measure of "grouting consolidation + local priority strengthening support" was proposed. The results indicate that the support measure can effectively restrain crack propagation and improve the capacity of surrounding rocks containing end cracks near the free surface.