Experimental study of shear-seepage behaviour of rock joints under constant normal stiffness

In order to study the shear-flow coupling characteristics of joints under constant normal stiffness (CNS) boundary condition, the shear-flow coupling tests under three different stiffness and seepage pressure settings were carried out for duplicate joint specimens with three different joint roughnesses. Meanwhile, the effects of normal stiffness, seepage pressure and joint roughness on the mechanical properties and seepage characteristics in joint shearing process were systematically analyzed. The test results indicate that the peak shear strength of joint increases with the increase of normal stiffness, while the flow rate, equivalent hydraulic aperture and transmissivity decrease with the increase of normal stiffness; and the flow rate of seepage through joint surfaces during shearing process decreases with the increase of joint roughness. A three-stage change rule of flow rate that is similar to the joint dilatancy is shown in the shearing process: rapid growth stage, slow growth stage, and stable stage. The same rule has also been observed in equivalent hydraulic aperture and transmissivity. During the stable stage, the flow rate is approximately linear with the variation of the normal stiffness and seepage pressure, and the joints with higher roughness present lower flow rate as the seepage pressure increases.