Numerical investigation of the vertical response of bucket foundations supporting offshore wind turbines in sand

Abstract

The vertical response of bucket foundations embedded in medium dense and very dense sands was investigated employing finite element method. The effect of bucket length on the vertical load capacity was studied by considering three different foundation geometries, namely, surface circular foundation, bucket foundation and embedded solid foundation. The effect of soil plug on the end-bearing capacity was explored by comparing the vertical load behaviour of bucket and embedded solid foundations, while assuming zero skirt. The vertical capacities of all the considered geometries are noted to be higher in very dense sand, and the maximum vertical capacity is obtained in the case of embedded solid foundation. For bucket foundations, the ultimate vertical bearing capacity is noted to increase in near-linear manner with increasing skirt length. Due to the presence of soil plug inside the bucket foundation, the percentage reduction in end-bearing capacity compared to that of embedded solid foundation is found to be about 21–22% in medium dense sand and 18–19% in very dense sand. Plastic strain contour diagrams indicate surface flow failure mechanism for surface circular foundation, which then changes to confined deep flow failure mechanism with increasing skirt length. Based on the results of the numerical analyses, design expressions are obtained to estimate the vertical bearing capacity of bucket foundations embedded in sand.