Abstract: Applied with a new large-scale interface apparatus, the influence of normal stiffness on 3D monotonic and cyclic behavior of the gravel-structure interface was investigated in detail. Under constant normal stiffness condition, the interface contracts first and then dilates, and the normal stress accordingly reduces and then increases during monotonic shearing. The irreversible volumetric change of the interface grows continuously, the normal stress, peak shear stress, shear strength of the interface decreases overall due to cyclic shearing while the peak resultant stress ratio remains invariable despite of shear cycle. It is indicated that the normal stiffness plays a vital role in 3D monotonic and cyclic behavior of the interface. With larger normal stiffness, the normal stress and the peak shear stress increase more during monotonic shearing; the irreversible volumetric change accumulates more slowly and its ultimate value is accordingly smaller. Meanwhile, the shear strength of the interface decreases faster to zero and the corresponding cycle number at which the shear strength becomes zero is smaller. The normal stiffness has a markable impacts on the value of performance parameters of the interface such as the irreversible and reversible volumetric change, shear stress, resultant shear stress, except the friction angle. In addition, good consistency exists in the relationship pattern between volumetric change, reversible volumetric change, resultant shear stress and resultant stress ratio versus tangential displacement, regardless of normal stiffness.

Key words: Interface, gravelly soil, 3D behavior, normal stiffness, direct shear test