Numerical analysis of offshore monopile foundations in layered clay

Monopiles are often embedded in layered soil profiles, where stratification significantly influences their lateral load response. However, much of the existing literature and proposed methodologies predominately focus on uniform soil profiles which is far from the reality. To address this, three-dimensional (3D) finite-element (FE) analyses were conducted on laterally loaded monopiles with varying slenderness ratios in different clay layering configurations. Three general profiles were examined: a two-layer clay profile, a soft interlayer within a stiff layer, and a three-layer clay profile. By systematically varying layer thickness and position, a broad range of scenarios was analyzed, leading to quantitative design recommendations for monopiles with different slenderness ratios. Additionally, two methods are proposed for estimating monopile capacity in layered soils, based on its capacity in the corresponding homogeneous soil profile. The first, a predictive equation, is more accurate and convenient for two-layer profiles but becomes complex for multi-layered systems. To overcome this, an alternative approach—the layer-coefficient method—was developed. This method differentiates lateral capacity ratio (LCR) curves to generate layer coefficient graphs, providing insight into each layer’s influence on lateral capacity. Compared to the equation-based method, the layer-coefficient approach is more intuitive, offers clearer design guidance, and is more convenient for multilayered soil profiles.