Abstract: In order to study the softening characteristics of gravel-geogrid interface during and after cyclic shearing, a series of displacement controlled monotonic direct shear tests, cyclic direct shear tests with 2 000 cycles and post cyclic direct shear tests were carried out on the gravel-geogrid interface using dynamic direct shear apparatus. The shear strength characteristics, volume change behavior, shear stiffness and damping ratio of the gravel-geogrid interface under four groups of normal stresses of 20, 40, 60 and 80 kPa were studied. The direct shear characteristics of the gravel-geogrid interface before and after cyclic shear were compared and analyzed. The results show that: in the cyclic direct shear test, the average peak shear stress of gravel-geogrid interface first increases and then decreases, and finally tends to be stable with the increase of number of cycles, and the interface shows shear softening characteristics; the vertical displacement increment of gravel-geogrid interface decreases gradually with the increase of number of cycles; the shear stiffness first increases and then decreases with the increase of number of cycles, and finally tends to be stable; the damping ratio first decreases and then increases with the increase of number of cycles, and finally tends to be stable; under the same normal stress, the peak shear stress increases and the residual shear stress decreases in the cyclic direct shear test compared with the monotonic direct shear test; the cyclic shear makes the interface cohesion increase significantly and the internal friction angle decrease.

Key words: direct shear test, shear stress, shear stiffness, damping ratio