Experimental Investigation of the Behavior of Monopile under Asymmetric Two-Way Cyclic Lateral Loads

The monopile is the most common form of foundation employed in offshore wind turbines. These foundations are subjected to millions of repeated load cycles, owing to wind and wave action. In this study, a series of six cyclic lateral load tests and two monotonic tests were performed on an aluminum model pipe pile with an outer diameter of 63.5 mm and a wall thickness of 2.5 mm. A model monopile was embedded in medium dense sand (D-r = 55%) and subjected to asymmetric two-way cyclic loading. From the experimental investigations, the effects of embedded length and the asymmetric two-way cyclic loads on the lateral pile head displacements and the cyclic secant stiffness of the soil-pile system were studied. Linear regression analysis was also performed to fit the conventional degradation parameters using the minimum number of critical constraints, which included the loading conditions and the flexibility parameters of the soil-pile system. From the test results, it was observed that asymmetric two-way loading causes a reversal of accumulated displacement for a pile embedded at a greater depth (L > 1.91T) under relatively lower amplitudes (zeta(b) < 0.37). The cyclic secant stiffness was observed to increase at a relatively constant rate (A(kappa)) with the logarithm of the number of cycles. The study also revealed that the magnitude of initial cyclic secant stiffness, in comparison with the monotonic stiffness, exhibited a critical drop near the specific load character, zeta(c) = -0.38. (c) 2021 American Society of Civil Engineers.