Lateral force-displacement (P-y)-based Winkler spring models are commonly applied for the design of piles, P being the soil lateral reaction and y the lateral displacement. Despite their relative simplicity, P-y models can capture important aspects of pile behaviour including non-linear soil stiffness. Several P-y models based on cone penetration tests (CPTs) have been proposed over the last two decades, developed largely using empirical curve fitting to results of field tests, centrifuge modelling and finite-element analyses on relatively flexible piles installed in calcareous sand. However, major uncertainties exist when attempting to extrapolate empirical models for use with soil types and pile geometries outside the database on which they were formulated. There is an urgent need for a reliable P-y method for application to the design of rigid monopiles used extensively for offshore wind projects. A series of field lateral load tests performed on open-ended steel pipe piles driven in dense siliceous sand is reported here. The pile embedment length and load eccentricity were varied to investigate the behaviour of rigid and flexible monopiles. The measured pile response was used to evaluate the performance of a number of recent CPT-based P-y models and an update to an existing power-law model is suggested for rigid monopiles in siliceous sand.

To date, monopiles are the most popular foundations used in the offshore wind sector, accounting for over 75% of existing turbine foundations. The cyclic lateral loading response of these foundations is one of the most important issues to be considered during the life-cycle design of monopiles. Current design guidelines rely on methods developed from field tests performed on long slender piles typically used in the oil and gas industry. Monopiles used for wind turbines tend to be relatively short and stiff. In order to investigate the response of a rigid monopile to long-term cyclic lateral loading, a series of field model experiments were conducted on instrumented steel pipe piles. The piles, which had a diameter of 340 mm and slenderness ratio of 6.5, were driven into dense sand at the University College Dublin geotechnical test site. Lateral loading tests were performed on two separate piles. In the cyclic lateral load tests, one pile was loaded with circa 5000 cycles at two load amplitudes (approximately 20% and 50% of the static resistance). In the second cyclic load test, more than 3000 load cycles were applied at three load amplitudes (30%, 40% and 70% of the ultimate resistance). The ability of current design models to predict the monopile response is considered.