An Innovative Experimental Setup for Characterizing Friction Fatigue during Cyclic Jacking of Piles in Dense Sand

Abstract

In this study, an innovative experimental setup for model pile tests was developed to characterize friction fatigue in sand during cyclic jacking. In the experimental setup, a high-performance precision electric linear actuator was used to control jacking of the model piles. P-wave tomographic imaging was used to characterize the associated changes in the distribution of the P-wave velocities in the soil, and tactile pressure sensors were utilized to monitor related variations in stresses in the soil surrounding the pile. Measurements obtained from the experiments allow us to explore the behavior of fiction fatigue from a different perspective, especially from the viewpoint of stress changes in the soil surrounding the pile. It was found that for a soil element surrounding the pile in a jacking cycle, the maximum values of the stationary radial and hoop stresses and the ultimate radial and hoop stresses in that soil element are reached when the pile penetrates to nearly the same depth, which is therefore defined as the transition depth. For the soil elements at the same depth, such a transition depth increases with increasing distance from the pile centerline. When the pile base gradually penetrates through the transition depth, these stresses in the soil elements surrounding the pile will exceed the peak value, which in turn leads to a reduction in the ultimate unit shear resistance (τf), giving rise to friction fatigue. Based on these experimental findings, a simple model is also proposed to explain friction fatigue.