Characterisation of Residual Shear Strength at the Sheringham Shoal Offshore Wind Farm

Abstract

For offshore foundations, the residual shear strength is an important soil parameter for the evaluation of installation resistance and axial pile capacity (for jacket foundation). Estimation of residual shear strength can be conducted in a shear box test in the conventional way, or with the introduction of an interface to evaluate the change in residual shear strength under influence of friction between soil and the interface. In addition, the residual effective friction angle can be measured in the ring shear test using the Bromhead apparatus. In this study, the three above-mentioned methods are employed to estimate the values of residual shear strength of two soil units: the Swarte Bank Formation and the Chalk Unit sampled from the Sheringham Shoal offshore wind farms. The Swarte Bank Formation is dominated by heavily over-consolidated stiff clay, while the Chalk Unit is characterized by putty white chalk which behaves in a similar manner to stiff clay if weathered, or to soft rock if unweathered. These soil units are located at the bottom of the soil profile at the Sheringham Shoal wind farm and hence are important in providing axial capacity to the foundation. Samples from the two soil units are tested and compared at different rates of shearing to evaluate the change in axial capacity and installation resistance of the offshore wind turbine foundations under various possible loading and drainage conditions. Comparison is also made between residual shear strength with and without a reconsolidation period to assess the potential for soil set-up and its influence on the soil capacity. The results show that, for both the clay and the chalk, the estimated residual shear strengths are quite similar between the conventional and interface shear tests and tend to increase with increasing shearing rate. This can be attributed to the increasing dominance of the turbulent shearing mode. Relative to the peak shear strength, the values of residual shear strength are approximately 5 to 35% lower in most cases. Reconsolidation for a period of 24 hours appears to have, if any, marginal positive effect on residual shear strength of the two soils in both shear box and interface shear box tests. The residual friction angles derived from the shear box and ring shear tests are comparable and fall in the immediate range of shear strength. The various test results imply that the pile foundations at the Sheringham Shoal would have considerably large axial capacity, assuming that the horizontal stress is similar to the normal stress used in testing. The test data however should be used with caution and combined with piling experience in comparable soils where possible. The study aims to provide a source of reference for design of pile foundations for sites with similar soil conditions.