Frictional characteristics of the interface between a structure and surrounding soil, which can commonly be found in many geotechnical works, play an important role in evaluating the bearing capacity. The objective of this paper is to present some results of an experimental series on sand – smooth steel interface under constant normal load (CNL) and constant normal stiffness (CNS) conditions. First, the monotonic interface direct shear tests were performed in order to determine the peak shear stress ratio or peak friction angel of sand – smooth steel interface. Then the cyclic test campaigns were designed to perform. When subjecting to cyclic loading, the interface evidently showed the contractive behavior. Under CNS condition, this contraction led to the degradation in normal stress acting on the interface and then the stress state moved towards the peak (or residual) stress ratio line in the stress plane.
