<p>Soil–structural interfaces, such as those between foundations, tunnels, retaining walls, and slopes, serve as critical zones for stress and deformation transfer in geotechnical systems. Although extensive research exists on two-dimensional interface mechanics, the zonal characteristics, non-coaxial behavior, and reversible volumetric deformation mechanisms under three-dimensional (3D) loading remain poorly understood. To address this gap, a series of discrete element method (DEM) simulations focused on the phase-dependent response of shear stress, normal displacement, and microstructural interactions were conducted in this study. The influence of the tangential displacement phase difference <i>D</i>_p on the 3D cyclic shear behavior of gravel–structure interfaces under circular shear paths was investigated based on DEM. The controlling role of shear path geometry in governing non-coaxial behavior and reversible volumetric deformation is explicitly revealed. Specifically, it is demonstrated that stable, hysteretic stress-displacement loops with minimal volumetric hysteresis are promoted by circular shear paths (<i>D</i>_p = <i>π</i>/2, 3<i>π</i>/2), whereas abrupt stress reversals and amplified dilatancy-recompression cycles are induced by bidirectional linear paths (<i>D</i>_p = <i>π</i>, 2<i>π</i>). Furthermore, periodic oscillations of non-coaxial angles (<i>α</i>₃) are observed, transitioning from 90° under pure rotation to peak fluctuations exceeding 30° under phase-shifted shear. These mechanistic understandings are highlighted as being crucial for the advancement of 3D interface constitutive models.</p>