Numerical Modelling of Stiffness and Damping Evolution of Offshore Monopile Foundations Under Lateral Cyclic Loading

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4. Numerical Modelling of Stiffness and Damping Evolution of Offshore Monopile Foundations Under Lateral Cyclic Loading

Numerical modelling is fundamental to validating and improving monopile design approaches and the soil constitutive model is a key component. Here, a three-dimensional finite element model of a large diameter monopile is developed based on an advanced soil constitutive model for clay, which has been calibrated with experimental results. The kinematic hardening soil model can capture the accumulation of plastic strains, build-up of pore water pressure, and degradation of shear stiffness and initial structure that characterise the response of natural clay. The monopile is subjected to one-way lateral cyclic loading and the effect of loading intensity and overconsolidation ratio on the cyclic secant stiffness and damping ratio are investigated. The monopile showed ratcheting, where rotation accumulates with increasing numbers of load cycles, and the formation of hysteresis loops in each cycle. In normally consolidated clay, the stiffness decreased, and the damping ratio increased due to excess pore water pressures causing a softer soil response. In contrast, in overconsolidated clay, the stiffness showed an increase because negative excess pressures were generated in the soil deposit. The results demonstrate the importance of pore pressure generation in controlling monopile response and provide some guidance in the engineering design of monopiles in clay deposits.

10th European Conference on Numerical Methods in Geotechnical Engineering (NUMGE2023)

12. Offshore geotechnics