Earth pressure distribution on laterally loaded offshore monopiles

Abstract

Unlike conventional pile foundations, a monopile is a large diameter open-ended steel tube driven into the seabed. Accurate quantification of the earth pressure distribution along a monopile due to lateral loads plays a pivotal role in the design of offshore wind turbines and remains a problem of great interest. This paper presents an attempt to address the problem by means of three-dimensional (3D) finite element modeling, with focus on the effect of pile slenderness on earth pressure distribution and the associated soil deformation mechanisms. The study shows that as pile diameter increases, the deformation mode of the pile will change from flexural to rotational deflection while the deformation pattern of the surrounding soil varies from being the wedge-type to a combination of the wedge-type and the rotation-type. The magnitude of either normal contact stress or shear stress at a pile section decreases with increasing pile diameter, but the distribution pattern of the normalized contact stress (either normal or shear stress) is nearly the same. The implications of these findings for engineering practice are two-fold: (a) Use of the p-y relationship established from field tests on piles of reduced scales in the design of large-diameter piles may still result in a potential risk of overestimating the lateral bearing capacity if the test pile diameter is not large; and (b) a unified factor independent of pile diameter can be introduced to average the contact stress across the pile section in the development of simplified methods.