Three-Dimensional Behaviors of a Gravel–Steel Interface Considering Initial Shear Stress

Initial shear stress is crucial in the three-dimensional (3D) behaviors of soil–structure interfaces but has seldom been examined. This research presents 3D monotonic and cyclic behaviors of a gravel–structure interface, considering initial shear stress from large-scale direct-shear tests. Tangential displacement is distinctly induced by initial shear stress due to the coupling effect, and the shear distance in the $x$- and $y$-directions exhibits a linear formation. The tangential displacement angle shows a close relationship with the initial shear stress and the orientation shear angle, and it can be expressed using a formula. The distinct noncoaxiality of the interface occurs during the yielding phase. The interface deforms toward the shear stress, and the unit tangential displacement increment reaches the peak when the shear strength is mobilized. The critical orientation shear angle and critical initial shear stress are found and determined by test results and formula calculation. Initial shear stress remarkably influences the magnitude of tangential displacement, shear stress, and irreversible and reversible normal displacements, while it slightly affects the shear strength of the interface, which behaves in an isotropic manner.