The influence of vertical loading on the lateral behaviour of rigid monopile foundation in clay soil

Abstract

Monopiles with large diameter (larger than 6 m) and low aspect ratio (less than 6) are increasingly used in offshore wind farms. These foundations demonstrate a rigid response under lateral loading. The validity of the existing design methods, that are based on small diameter flexible piles, has been questioned by both the industry and researchers. In addition, the monopiles are subjected to both lateral and vertical loads. The influence of vertical load on the lateral design of short rigid monopiles in clay soil is not clear. This study aims to perform a comprehensive study on the influence of vertical load on the lateral response of monopile foundations in clay soil.

All analysis in this study was performed using 3D finite element modeling in PLAXIS 3D software. The NGI-ADP constitutive model was adopted to simulate the nonlinear mechanical behaviour of clay. Considered in the analysis is a short rigid pile with a diameter of 10 m (L/D = 3) and a long flexible pile with a diameter of 2 m (L/D = 15). The analyzed clay soil profiles consist of a normally consolidated clay soil and an overconsolidated clay soil with a constant undrained shear strength profile equal to 30 kPa. For each pile in each type of clay soil, a pure lateral loading scenario is performed first to assess the validity of current design methods. Subsequently, a combined loading scenario is performed to assess the influence of vertical loading on the lateral behaviour of rigid monopile in clay soil.

Results of the pure lateral loading scenario suggest that current design methods heavily underestimate the lateral capacity of rigid monopile foundations in both clay soil profiles analyzed. According to the findings of this study, it can be concluded that current design methods are not fit to provide an accurate assessment regarding the lateral load response of rigid monopile in clay soil. In order to correctly assess the lateral load response of rigid monopile in clay soil, a method consisting of a 3D finite element model akin to the model used in the research or a PISA design model is advised. A potential third design method, the 1D rotational spring model, is also proposed.

Results of the combined loading scenario suggest that the presence of vertical loading causes a decrease in lateral and moment capacity of the rigid pile in both clay soil profiles analyzed. However, the influence is negligible when the vertical load magnitude is smaller than 50% of its bearing capacity. To quantify the influence of vertical load on a monopile foundation, a series of load analysis were performed on a real offshore wind turbine with a 5MW power capacity. It was found that the vertical load on a typical monopile foundation in clay is around 27% of its bearing capacity. According to the findings of this study, it can be concluded that the influence of vertical load on the lateral response of rigid monopiles in clay soil is limited and can be ignored in foundation design.