Cyclic Laterally Loaded Medium Scale Field Pile Testing for the PISA Project

Abstract

The PISA project explored the development of improved design methods for offshore wind turbine monopile foundations. Medium scale field pile testing for the project took place at Cowden in the UK, where the soil consists of a heavily over-consolidated glacial till, and Dunkirk in France, where the soil consists of a dense marine sand. The main focus of the testing was on monotonic capacity. However, results were also obtained for cyclic loading tests on medium scale piles of two different diameters (0.762 m and 2.0 m), and a length to diameter (L/D) ratio of 5.25. The testing consisted of uniform one-way cycling on 7 separate piles across the two sites at different amplitudes, including tests of over 20000 cycles, as well as two-way cycling on one pile at each site at different amplitudes and frequencies. The one-way tests explored ratcheting phenomena and associated effects, whilst the two-way tests explored stiffness and damping at increasing load amplitudes. This paper presents a summary of the cyclic pile tests that were completed, illustrated by results from the Dunkirk site. Comparisons are made to the PISA monotonic test results, as well as to observations from model testing reported in the literature. These field measurements support the development of new cyclic modelling approaches for offshore monopile foundations.