Theoretical analysis and fracture process of elliptical tunnel with symmetric cracks subjected to water pressure and ground stress

The water diversion tunnel played an important role in the allocation of water resources in the water diversion project. In the process of actual use, some cracks or joints at the edge of the tunnel could produce further cracking under the action of internal water pressure and in-situ stress, which was dangerous to the safety of the tunnel. The analytical solution formulas of stress intensity factors (SIFs) of the elliptical tunnel with two cracks were derived by using the complex function method, and the external factors affecting the stability of the elliptical tunnel were analyzed. The failure process of the specimens was analyzed by using the high-speed camera and digital image correlation (DIC) method to obtain the evolution laws of crack initiation behavior and tunnel fracture modes. The results can conclude that both sides of the elliptical tunnel and the crack tip were stress concentration areas. As the inclination angle & alpha; of the elliptical tunnel is varied from 0 degrees to 45 degrees, tensile cracks occur at the top and bottom of the elliptical tunnel caused by tensile stress, and the compressive strength is smallest when the elliptical tunnel with & alpha; is 45 degrees. As the inclination angle & alpha; is 75 degrees, two penetrating cracks appear at the edge of the elliptical tunnel and crack tips. The initiation pressure increases with the inclination angle & alpha;. The SIF of mode II rises with the ellipse major semi axis and the crack length.