Experimental Study of Mechanical and Permeability Behaviors During the Failure of Sandstone Containing Two Preexisting Fissures Under Triaxial Compression

Rock masses are typical inhomogeneous geological materials that contain many fissures and cracks. The coupling effect of the crack propagation and seepage evolution in rocks is very important to the safety of rock engineering. However, hydromechanical coupling behavior during the failure of fissured rocks has rarely been investigated. In this research, hydromechanical coupling tests are performed to fully explore the behaviors in strength, deformation, permeability and failure mode of sandstone samples with two preexisting fissures. The experimental results show that the ratio of crack initiation threshold/peak strength, the ratio of crack damage threshold/peak strength and the value of the elastic modulus decrease by at most 31.8%, 12.2% and 18.4% due to the existence of the two fissures, while the Poisson’s ratio increases by at most 45.6%. Furthermore, the values of permeability before the sudden increase stage range from 2.1% to 17.6% of the maximum permeability value. The influence of bridge length and angle on permeability is more significant under lower confining pressure or higher water pressure. Five failure modes are observed in the double-fissure samples under hydromechanical coupling conditions. Additionally, the “wing cracks + indirect coalescence” failure mode is generated only when the ligament length is shorter than the fissure length. The corresponding strength is lower than that for other failure modes. CT images show that the expansion of cracks inside the samples is more restricted than that at the surface of the samples, especially near the rock bridge region. The effects of failure modes on the mechanical and permeability properties, from greatest to least, are as follows: crack initiation threshold, peak strength, crack damage threshold, elastic modulus, Poisson’s ratio and permeability. This research is contributed to analyze the stability of water-bearing rocks in underground caverns with many preexisting fissures.