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.