Experimental Study on the Effects of Unloading Normal Stress on Shear Mechanical Behaviour of Sandstone Containing a Parallel Fissure Pair

To gain deeper insight into the effects of unloading normal stress on shear mechanical behaviour, laboratory tests are carried out on the red-sandstone specimens containing a parallel fissure pair under the constant shear stress and the unloading normal stress. The results reveal that the trace of the entire rupture surface is mainly controlled by the rock bridge, and the impacts of the rupture surfaces of the unloading tests are narrower than those of the direct shear tests. Tensile failure, tension–shear failure, shear failure, and two-stage failure are observed, the failure rules of rock bridges are further summarized according to the ranges of the length and inclination of the rock bridge. The shear strength of the normal unloading test has a slight increase compared with that of the direct shear test. With an increase in the initial normal/shear stress, the shear strength of rock specimens increase; under the low to medium initial stress conditions, the shear strength increment has a linear growth tendency, but under the medium to high initial stress conditions, the growth trend slows down. The ratio of shear deformation on rupture surface increases with the increase of the initial shear stress but decreases with the increase of the initial normal stress. The ratio of shear deformation on rupture plane increases with the increase of the initial shear stress, and decreases with the increase of the initial normal stress, There is a little difference in the deformation ratio (shear damage deformation divided by tensile damage deformation, ΔD_rs/ΔD_rn) between the observed tensile failure and tension–shear failure. The dominate damage deformation under different geometric conditions is closely related to the failure patterns.