Experimental study on strength properties, fracture patterns, and permeability behaviors of sandstone containing two filled fissures under triaxial compression

Rock masses generally contain many fissures filled with weak materials. At present, the interaction mechanisms of crack coalescence and seepage in rock with filled fissures are still poorly understood. Therefore, this study conducts systematic laboratory tests on sandstone samples with two filled fissures under hydromechanical coupling conditions. The influences of the fissures, filling, ligament length, and bridge angle on the strength, fracture pattern, and permeability are explored. The results show that the strength properties of the sandstone are weakened more significantly by fissures than by water pressure. The increase of strength properties induced by the fillings is more remarkable in the samples with either a longer ligament length or a lower bridge angle. Unlike for mud filling, for sand filling, most of the sand particles in the fissures are crushed after loading, reflecting that the bearing capacity of the sand filling is fully utilized. Thus, sand filling generally has a greater effect on the strength properties than mud filling does. Furthermore, the failure of double-fissure sandstone can be divided into five modes. With the exception of the samples with a high bridge angle, the samples undergo failure modes that change with the existence of fillings, and for samples with a long ligament length or a low bridge angle, the change is more sensitive to the filling type. The weakening effect of fillings on permeability is associated with the failure mode, and it is more remarkable in samples with the “wing cracks + indirect crack coalescence” or “wing cracks + direct crack coalescence” failure modes than in other samples.