Field trials on monotonic behaviour of a driven pile at an offshore wind farm in China

Abstract

The existing database of the loading response of offshore pile foundations needs to be enriched, so that safety and economic considerations can be optimally balanced in design. This paper details a programme of field tests on an open-ended steel pile. The pile with a diameter of 2 m was driven to a depth of 72.8 m in a stratified soil site and then subjected to monotonic compressive, tensile and lateral loadings. Resting periods were allowed before each test to enable strength recovery, and a period of about one month was found to be sufficient (i.e., the axial bearing capacity increases from about 10 MN to 25 MN). The test results were interpreted in terms of the internal forces and displacements/deformations of the pile, and the more detailed pile-soil interactions (i.e., qs-s and p-y curves). It is found that, for cohesive soils, the unit shaft resistance qs remains constant through each soil layer regardless of the almost linearly varying undrained shear strength su obtained from CPTu tests, and the soil layer with overall higher su does not guarantee greater qs owing to different proportions of silty and clayey components. Upon reaching the ultimate uplift load, the shaft resistance within the cohesive soils shows a clear softening response. Extensive comparisons were made between these results and the predictions from existing design methods wherever applicable to discuss the accuracy of each.