Abstract: This study aims to address the problem that the normal stress at the interface between cushion and concrete face slab in face slab dislocation zones is extremely low and the mechanical mechanism of the interface under such stress conditions remains insufficiently understood. Taking the construction materials of a concrete-faced rockfill dam under construction as the research object, a self-developed tensile-type in-situ direct shear apparatus capable of accurately measuring the normal stress was employed to conduct shear tests on the coarse-grained soil-concrete interface under extremely low normal stress, and in-situ mechanical response data were obtained under actual service conditions. A discrete element numerical model of the interface was then established to investigate particle displacement localization and shear band development, revealing the macro- and meso- mechanism of interface failure under different normal stress levels. A unified interface failure criterion applicable to a wide range of normal stresses and an incremental constitutive model for the interface were proposed and verified against in-situ mechanical response data. The results show that under extremely low normal stress, the interface exhibits a frictional sliding failure mode markedly different from the classical understanding, in which the interface remains without macro-displacement under static friction before the shear stress reaches a critical value, whereas the displacement increases continuously but the stress ceases to rise once the critical value is exceeded. This behavior is fundamentally different from the progressive nonlinear deformation observed under higher normal stress. Under extremely low normal stress, the normalized shear displacement is nearly uniformly distributed along the specimen height with no shear band formation, and the particle assembly tends to translate as a whole, revealing the failure mechanism for the rigid-body sliding mode of the interface. The unified failure criterion describes a continuous transition of interface strength from the frictional type at low normal stress to the cohesive-frictional type at high normal stress; the ultimate shear stress ratio in the criterion approaches the intrinsic sliding friction coefficient of the interface under extremely low normal stress and the criterion asymptotically approaches the Mohr-Coulomb form under higher normal stress. The incremental constitutive model can uniformly characterize the strength and deformation behavior of the interface over a wide range of normal stresses, and the model predictions agree well with the experimental data. The findings not only deepen the theoretical understanding of the macro- and meso-mechanical mechanisms of the interface between coarse-grained soil and concrete, but also provide a theoretical basis for face slab dislocation risk identification and safety assessment of concrete-faced rockfill dams.