Experimental study on mechanical characteristics and permeability evolution during the coupled hydromechanical failure of sandstone containing a filled fissure

In this study, triaxial seepage tests on sandstone specimens containing a preexisting fissure were conducted for three different fissure filling states to investigate the effect of fillings on the mechanical characteristics, fracture behaviors and permeability evolution during the seepage stress-induced failure of jointed rocks. Furthermore, the final 3D crack networks were reconstructed using the X-ray CT technique to qualitatively and quantitatively reveal the internal damage characteristics. The results show that the enhancing effect of fillings on the stress thresholds of specimens with a low-dip fissure is greater than that of specimens with a high-dip fissure, especially for the peak strength, while the axial stiffness is more sensitive to the fissure inclination than the filling state. The mud and sand fillings obviously affect the crack initiation angles and change the failure types around the fissure from tension to shear, especially in specimens with a high-dip fissure. Interestingly, the types and initiation locations of the main cracks may change from those observed for unfilled specimens due to the presence of sand fillings. The reconstructed 3D internal failure models show that the crack paths and damage extent inside the specimens are both affected by the fillings and water pressure. Moreover, the permeability evolution is dependent on both the cracking behaviors and fillings. The permeability in the crack incubation stage is more affected by crack propagation than fillings, but the plugging effect of fillings is fully exerted after the crack networks and shear fractures are generated, especially in specimens with a low-dip fissure.