Vertical bearing performance of a concrete-filled steel tubular single pile and pile group in sand-rock composite ground

The seabed in the Fujian Sea Area is a soil-rock composite ground that limits the application of steel pipe piles in offshore wind power foundations. Therefore, a new type of concrete-filled steel tubular pile was proposed. However, the vertical bearing performance of concrete-filled steel tubular piles in sand-rock composite ground has not been investigated thoroughly. In this study, a 1-g model test and the finite element method were used to study the vertical bearing performance of concrete-filled steel single and group piles, and the influence of different pile spacings on the bearing performance was further explored. The numerical model is validated using experimental data. Subsequently, the research results demonstrated that the load-settlement curves of concrete-filled steel tubular single and group piles in sand-rock composite ground changed slowly. Moreover, when the load exceeded the limit, the concrete-filled steel tubular pile experienced penetration failure. For the pile groups, as the load increased, connection cracks between the piles appeared on both the sand and bedrock surfaces, which reduced the exertion of pile tip resistance to a certain extent. Furthermore, to effectively avoid the influence of the pile group effect, the pile spacing of a concrete-filled steel tubular pile group should be greater than 4 d ( d is the pile diameter). These results can facilitate the design and application of concrete-filled steel tubular piles in offshore wind turbine foundations.