Impact vibration test of monopile foundation model in dry sand

Measurements on full-scale offshore wind turbines have shown that the horizontal foundation stiffness for monopiles is underpredicted by current design methods. This paper presents a 1:20 model scale test of a monopile foundation for offshore wind turbines, installed in dry laboratory sand. The test is performed under fully controlled laboratory conditions, with low-stress soil effects minimised by overburden pressures applied by a vacuum system. Soil–pile interaction stiffness is indirectly measured by eigenfrequencies in a free vibration test, with vibration initiated by a horizontal impact load to the top of the pile. The purpose of the test is to provide a benchmark for evaluation of calculation methods for laterally loaded monopile foundations. Results for system eigenfrequency and system damping are presented along with measurements of small-strain stiffness for the laboratory sand. Pile displacement profiles are derived for different oscillation amplitudes, on the basis of a numerical beam model, accelerometer and strain gauge measurements. A relationship between small-strain soil stiffness and eigenfrequency is identified. The measured system eigenfrequencies and system damping are found to depend on the applied surcharge pressure and the displacement amplitude.