Numerical investigation of monopiles in structured clay under cyclic loading

Offshore wind is undergoing rapid growth and is expected to play a vital role in the transition to a low-carbon global economy. This paper presents the results of implicit three-dimensional finite element analyses investigating cyclic lateral loading on a large diameter monopile in structured London Clay. The cyclic behaviour of the clay was simulated using the subloading tij constitutive model. Symmetric and asymmetric two-way and one-way low frequency cyclic lateral loads were used with an amplitude representing offshore wind turbine production loading conditions. Simulations showed that one-way loads induced greater cumulative pile deformation and rotation relative to two-way loads. For all load configurations, the rate of accumulation of lateral pile displacement and rotation at mudline reduced with each subsequent cycle. In contrast, the cyclic secant stiffness progressively degraded with its rate of decay reducing with each ensuing cycle. Findings are indicative of the incidence of strain softening, reconsolidation and densification during low-frequency cyclic loading. It was established that assuming the clay to be unstructured would be conservative in the design of monopiles. •One-way cyclic loads are more onerous for monopiles in structured marine clay.•Secant cyclic stiffness degrades logarithmically with number of cycles.•Reconsolidation and densification govern during low frequency cyclic loading.