Evaluation of Load-Displacement Relationships for Non-Slender Monopiles in Sand

Monopiles are an often used foundation concept for offshore wind turbine converters. These piles are highly subjected to lateral loads and bending moments due to wind and wave forces. To ensure enough stiffness of the foundation and an acceptable pile-head deflection, monopiles with diameters of 4 to 6 m are typically necessary. In current practice these piles are normally designed by use of the p-y curve method although the method is developed and verified for small-diameter, slender piles with diameters up to approximately 2 m. This paper investigate the behaviour of two non-slender aluminium pipe piles subjected to lateral loads at a given vertical eccentricity. The piles are heavily instrumented with strain gauges in order to obtain p-y curves and bending moment distributions along the piles. In order to minimise scale effects the tests are successfully carried out in a pressure tank at varying stress levels. The tests are evaluated with the following main findings: The lateral pile deflection consists primarily of rotation as a rigid object; normalised load-displacement relationships indicate that the lateral load is proportional to the embedded length squared and the pile diameter; in current design the initial stiffness of the p-y curves is considered independent of the pile diameter. This recommendation is questionable as derived p-y curves indicate a strong dependency on pile diameter with a higher initial stiffness related to the largest pile diameter.