Effect of Relative Density on P-Y Backbone Curves for Cyclic Lateral Load on Pile Foundations in Sandy Soil

Pile foundations installed to support offshore structures are primarily subjected to cyclic lateral loads due to wind, and waves. The p-y curve method, which represents a nonlinear relation between soil-pile reaction and lateral pile deflection, has been used to design cyclic laterally loaded piles. Recommended by the American Petroleum Institute (API) [10] and generally adopted to evaluate the behavior of static and cyclic laterally loaded piles installed in sandy soils, the API p-y curve contains a reduction factor for the initial horizontal subgrade modulus in order to take cyclic effects into consideration. When pile foundations are subjected to cyclic lateral loads, however, the initial horizontal subgrade modulus can both decrease and increase according to the relative density of the soil. In this paper, a series of cyclic lateral load model tests were performed on a preinstalled aluminum flexible pile to examine its cyclic lateral response under different relative density conditions. Model piles were embedded in sandy soils with relative densities of 40%, 70%, and 90% and were subjected to static as well as cyclic lateral loads. From the test results, cyclic p-y backbone curves were derived and compared with static p-y curves in identical soil conditions. Test results showed that the initial horizontal subgrade modulus increased for the model pile installed in sandy soil of 40% relative density, while decreased in relative densities of 70% and 90%. In addition, the infinite depth, above which cyclic lateral loads were supported, was evaluated and the test results were compared with the API p-y curve.