Study on the mechanical behavior and subsequent seepage characteristics of rough structural planes in sandstone under dynamic and constant shear rate

To investigate the dynamic shear mechanical response of structural surfaces and the subsequent seepage characteristics, a dynamic constant velocity shear system was used to conduct shear mechanical tests on sandstone rough structural surfaces under different shear velocity conditions. The influence of shear rate v and joint roughness coefficient JRC on the peak shear strength and slip characteristics of structural planes was investigated. In addition, a 3D scanner was used to obtain the damage characteristics of rough surfaces before and after shearing, and seepage tests were conducted under different confining pressures to investigate the subsequent seepage characteristics of damaged surfaces. The shear test results indicated that the dynamic peak shear strength of sandstone structural planes decreased with the shear rate. As the shear rate increased from 50 mm/s to 210 mm/s, the peak shear strength of sandstone structural planes with JRC=12.43±1.06 decreased from 8.49 MPa to 6.88 MPa. Under the same shear rate, the dynamic peak shear strength of sandstone structural planes increased with the JRC. The 3D scanning results indicated that the height frequency of structural planes decreases with the increase of shear rate. Under the same roughness conditions, the damage degree to the structural plane increased with the shear rate, resulting in a decrease in hydraulic opening and subsequently reducing the transmissivity of the structural plane. The results of the seepage test indicate that dynamic shear has a significant impact on the seepage characteristics of structural planes. The hydraulic gradient and volumetric flow rate of damaged structural planes have a nonlinear relationship, which conforms to Forchheimer’s law. The linear coefficient a and nonlinear coefficient b both increased with the shear rate. As the shear rate increased from 50 mm/s to 210 mm/s, the coefficients a and b increased by 68% and 229%, respectively. As the shear rate increased, the hydraulic opening of the structural plane decreased, and the transmissivity of the damaged structural plane showed a decreasing trend.