Load history idealisation effects for design of monopiles in clay

Evaluation of the characteristic behaviour of clay under cyclic loading is desirable and essential for an optimised monopile design of offshore wind turbines (OWTs). In current design practice, an irregular storm history is transformed into an idealised regular cyclic history with loading packages of uniform amplitude in ascending order. This paper investigates the effects of load representations by performing a series of load-controlled cyclic triaxial tests. The load histories investigated consist of a calculated 1 h storm loading on an OWT monopile foundation in the North Sea and its representations with different orders of the idealised load packages. The role of the average shear stress is discussed by comparing the shear strain evolution, pore water pressure accumulation, soil stiffness and strength. Cyclic degradation obtained with and without average shear stress components is studied. The experimental investigations in this paper indicate that the current design practice of load idealisation may not necessarily be conservative. The average shear stress affects the strain accumulation significantly but has relatively minor effects on the evolution of pore water pressure. It is suggested that the soil element response subjected to irregular load history should be investigated further from both experimental and numerical perspectives to develop an improved procedure for monopile design.