Cyclic Simple-Shear Behavior of Sand-Steel Interfaces under Constant Normal Stiffness Condition

This paper presents the results of an experimental investigation on the cyclic behavior of a sand-steel interface under constant normal stress and normal stiffness conditions. The experiments were performed by using an automated interface testing apparatus called C3DSSI. A simple shear-type soil container was used to measure simultaneously the shear deformation of the sand mass and the sliding displacement at the interface. Two-way cyclic tangential-displacement-controlled experiments were performed under a constant normal stiffness condition to study the phenomenon of reduction in the maximum shear stress with cycles. This phenomenon is known to be the cause of degradation of shaft resistance in piles subjected to cyclic loading in the axial direction. Effects of the tangential-displacement amplitude and the magnitude of the normal stiffness on the cyclic response were investigated. It was concluded that the reduction in maximum shear stress was not only due to the reduction in the normal stress, but also to a greater extent due to the increased amount of mobilized sliding displacement at the interface with an increasing number of cycles. The reduction in maximum shear stress with cycles was dramatic when failure was experienced at the interface, i.e., when the shear stress-normal stress ratio (τ/σ_n) reached a peak value and subsequently approached a residual value.