Numerical investigation of the effect of partial drainage on the cyclic response of a monopile in porous elastic soil

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In design practice, laterally loaded monopiles installed in sandy soil are typically analysed under drained conditions. However, the diameter of next-generation monopiles is projected to increase, leading to longer drainage paths for which the fully drained assumption may not hold true. Under these conditions, the soil response could be partially drained. This paper studies the effects of drainage on the cyclic response of a monopile embedded in a porous elastic soil, using fully coupled soilfluid analyses conducted with the finite element code OpenSees. The complete range of drainage conditions between undrained and fully drained is examined by altering a dimensionless time factor that depends on the loading frequency, the stiffness of the soil, the permeability of the soil, and the diameter of the pile. Linear elastic soil is selected to isolate the effects of drainage from those of soil plasticity and dilatancy. Analyses involving cyclic loading applied at different normalised periods are presented in increasing order of complexity. A simplified case using a pile segment of unit length is analysed first, followed by a range of cases involving monopiles with rotation prevented and rotation permitted. The effect of the degree of drainage is discussed, both for the excess pore pressures calculated in the soil and for the loaddisplacement behaviour of the monopile.

5th International Symposium on Frontiers in Offshore Geotechnics (ISFOG2025)

9 - Monopile design to cyclic loads: quasi-static, dynamic and seismic loads