Abstract: A series of 3D cyclic large-scale direct-shear tests of a gravel-structure interface was conducted at different stress amplitude ratios using 80 t 3D multifunction apparatus for soil-structure interface (3DMAS). The 3D cyclic interface behavior, including tangential deformation, non-coaxial angle and shear flexibility, was addressed in stress-controlled two-way elliptical shear path and the influence of stress amplitude ratio was discussed in detail. Distinct tangential displacements in the x and y directions, non-coaxial angle, shear flexibility are observed subjected to two-way elliptical cycling of shear stress. The tangential displacement amplitudes in the x and y directions and the shear flexibility of the interface gradually decrease with cyclic shearing and then tends to be stabilized, indicating noticeable evolution characteristics. The non-coaxial angle is significantly dependent on the shear stress amplitude and the shear stress increment direction, and the maximum, minimum and stabilized non-coaxial angles almost remain invariable throughout cyclic shearing. The development of the shear flexibility against rotation angle is contrary to that of the non-coaxial angle, and the rapid growth of the peak shear flexibility will impede the development of the non-coaxial angle of the interface. The stress amplitude ratio slightly influences the relationship patterns of the tangential displacements in the x and y directions and their relationship patterns against shear cycle, and the shear stress-displacement relationship patterns, but significantly affects the tangential displacement amplitudes and their migration extent, the length of the major and minor axis of the elliptical curves of the tangential displacements in the x and y directions and of the shear stress-displacement response, as well as the magnitude and development trend of the non-coaxial angle and shear flexibility. Increased stress amplitude ratio leads to magnified tangential displacement amplitudes and enlarged migration towards the negative direction, increased peak shear flexibility and slowed evolution with cyclic shearing. In addition, the non-coaxial angle and the shear flexibility present different performances when the stress amplitude ratio equals and does not equal 1.0.

Key words: soil-structure interface, 3D direct-shear test, non-coaxial angle, shear flexibility, stress amplitude ratio