Evaluating Effects of Cyclic Axial Loading on Soil-Pile Interface Properties Utilizing a Recently Developed Cyclic Interface Shear Test Device

Offshore wind turbines (OWTs) are subjected to long-term cyclic loading during their service life. Long-term cyclic axial loads cause changes in the soil-pile interface properties (i.e., strength and stiffness). To investigate these effects, a cyclic interface shear test (CIST) device was developed by the research team. The CIST device allows for directly measuring the shear stress–vertical displacement response (static and post-cyclic t–z curves). Furthermore, the automated CIST device allows for investigating the effects of cyclic loading characteristics (i.e., pattern, amplitude, frequency, and number of cycles) on the soil-pile interface properties, especially interface shear resistance. In addition, the CIST system allows for conducting interface shear tests with two control modes: pressure control (constant load on the interface) and a pressure control with displacement monitoring (control of plates radial displacement before shearing), which is equivalent to constant normal displacement/volume confinement condition. Moreover, the CIST test setup allows for performing scaled 1 g pile tests subjected to long-term cyclic axial loading and allows for the use of instrumentation within the soil and at the interface (such as pore pressure sensors). This study presents the first phase of this research and presents preliminary test results. In this study, 3D printed smooth plates simulating conventional steel piles were used and the soil-plate interface was subjected to cyclic loading. The soil used in the experimental program is a normally consolidated kaolinite clay mixed with sand to reduce consolidation time. In this paper, a short description of the new device is provided and the results from a series of displacement-controlled one-way cyclic interface preliminary tests on cohesive soil–smooth plates interface are summarized and compared to a reference baseline static interface test (no cycles). The preliminary tests presented in this paper show that a contractive behavior was observed at the interface during the application of loading cycles and the interface shear resistance increased with the number of cycles. This increase ranged from 40% to 75% when compared to the static interface shear resistance (no cycles).