Abstract: The interlayer dislocation zone has the characteristics of large extension scale and poor physico-mechanical properties. After reservoir impoundment, the rise of the water level imposes greater water pressure on the interlayer dislocation zone. Under the action of high water pressure, the shear strength of the interlayer dislocation zone and its evolution law need to be studied. In this paper, the newly developed direct shear test system HMSS-300 is used to conduct the direct shear tests under different hydro-mechanical coupling conditions on the interlayer dislocation zone of Xiluodu Hydropower Station. The characteristics and variation law of shear strength of the interlayer dislocation zone under the hydro-mechanical coupling conditions are discussed. The test results show that under the hydro-mechanical coupling conditions, the shear stress-shear displacement curve of the shear strength of the interlayer dislocation zone has an obvious peak value. The normal displacements mostly show the dilation first and then shrinkage as the shear displacement increases. With the increase in water pressure, both the effective internal friction angle and the effective cohesion of the interlayer dislocation zone decrease in a negative exponential relationship. Under the water pressure of 0−2.0 MPa, the degradation degrees of the effective internal friction angle and the effective cohesion of the interlayer dislocation zone are 7%−22% and 32%−93%, respectively. It indicates that the increase of water pressure has a greater effect on the effective cohesion of the interlayer dislocation zone. Under the coupling action of normal load and water pressure, the interlayer dislocation zone presents a shear-slip failure mode and a mixed shear-slip failure mode of medium and fine breccia particles. Under the hydro-mechanical coupling conditions, the softening and swelling of clay minerals, and water molecular layers formed by the water intrusion into the interlayer dislocation particles and the structural unit layers of clay minerals are the main factors that deteriorate the shear strength of the interlayer dislocation zone.

Key words: interlayer dislocation zone, hydro-mechanical coupling, in-situ direct shear test, shear strength