Physical and numerical modeling of bucket foundations in sands to monitor suction installation process and predict response under combined loading

Abstract

Bucket foundations are increasingly being used as foundations for offshore wind turbines (OWT). The main advantage of the bucket foundation is that it can be installed by applying suction at the top of the foundation. Hence, bucket foundations have become popular alternatives to driven piles because of the high cost of installation and technical challenges associated with driving piles in deep waters. Although the use of bucket foundations has increased, there are many uncertainties in the design of a bucket foundation. The required suction pressure and the influence of suction penetration on the surrounding soil need to be better understood. Estimation of the capacity of the foundation to combined loads is essential for design of foundations for OWT. There is also a need to predict the response of the bucket foundation under repeated loading. Above aspects have been partly investigated in previous studies for clays. However, the response of bucket foundation in sands has not yet been well understood and documented. This study focuses on the response of bucket foundations in sands. Physical model tests are performed on sands to investigate the penetration characteristics of bucket foundations. The suction pressure needed for penetration is evaluated. The amount of soil heave is measured and quantified. It is shown that the relative density of sand has a significant influence on the penetration resistance and soil response. Design parameters to estimate the suction pressure and soil heave are proposed from the test results. Extensive axisymmetric and three-dimensional finite element analyses are performed to calculate the vertical, horizontal, and moment capacities of bucket foundations in uniform sand and sand overlying clay soil profiles. When the vertical load is applied, a unique load transfer mechanism is shown to occur due to arching. A pronounced increase in the shaft resistance and a widened failure surface are observed. If a thick clay layer underlies the sand layer, the shape of the failure surface displays complex pattern conditional on strength parameters and thickness of sand layer. Based on numerical results, predictive equations and charts for the vertical bearing capacity are proposed. Analyses are also performed to determine the horizontal and moment capacities. Predictive equations for the horizontal and moment capacities are proposed. This study also presents capacity envelopes for combined horizontal and moment loadings. The envelopes are developed for a range of vertical loads to cover the wide range of OWT used in the field. The capacity envelopes can be easily used to predict the capacity of the OWT foundation under combined loads. Finally, this study investigates the long-term response of a bucket foundation. An empirical model that predicts the stiffness degradation of soils as a function of load amplitude and number of cycles is implemented as a user-defined code in a commercial finite element program. Three-dimensional finite element analyses are performed to evaluate the permanent response of bucket foundations subjected to repeated loading and recommendations on the design is presented.