Experimental investigation on the strength characteristics and fracture mechanism of rock with orthogonally crossed cracks

Rock mass is a typical kind of engineering geological materials with the heterogeneity and anisotropy. The failure disasters of tunnels (e. g. ,roof falling and wall collapse) are related with the joints and cracks in rock,which may lead to serious casualties and economic losses. Cracks in natural rock are distributed in cross shapes. In order to inves-tigate the effects of crossed cracks on the mechanical properties and failure characteristics of rock,a wire cutting equipment is employed to make the rock specimens with different crossed cracks,then an acoustic emission system an digital image correlation technique are used to study the fracture process of rock specimens under uniaxial compres- sion. It has been found that the length of pre-existing cracks has little effect on the rock strength while the angle be- tween the crack and loading direction significantly affects the rock strength. When the angle of primary crack is 90°, the values of rock mechanical parameters reach minimum. Crack initiation is prone to occur at the tips of pre-existing primary and secondary crack,and the crack initiation location is closely related to the angles of pre-existing cracks. The load-ing-induced cracks for the specimens with crossed cracks are Wing or oblique Wing cracks. When the angle of primary crack is smaller than 45°,the pre-existing primary crack determines crack initiation while the pre-existing secondary crack affects the crack propagation. When the angle of primary crack is larger than 45°,crack initiation is more prone to occur around the pre-existing secondary crack and loading makes rock fracture in serval times. The crack initiation stress is about 0. 22-0. 34 times of rock strength. Rock failure process is most significantly affected by the pre-existing cracks perpendicular to the loading direction.