Uplift resistance of under-reamed open caisson shafts in sand

Open caisson construction often involves excessive material use to provide sufficient dead weight to counteract buoyancy forces (uplift) in the permanent state. Under-reaming, which enlarges the caisson base to form a continuous annular anchor, offers a means to reduce material usage by increasing uplift resistance. However, uptake of such techniques in practice remains limited due to a lack of research in this area. This paper provides an improved basis for designing under-reamed caissons in sands using finite-element analysis. A key contribution is the development of a novel workaround using Python and the PLAXIS Remote Scripting Interface, which enables straightforward implementation of adaptive remeshing in PLAXIS 2D. The study employs both elasto-plastic and Hypoplastic material models, ensuring the influence of soil stiffness and soil state dependence are effectively considered. The first part of the numerical study considers ultimate limit state analyses, concluding they are only useful for shallow depths in dense sands. The second part of the numerical study focuses on serviceability limit state analyses, providing a basis for design at greater depths and all soil densities. Five key parameters are investigated: anchor width, caisson depth, anchor roughness, soil relative density, and the inclination of the under-ream. Based on the findings, a design chart is developed to streamline preliminary design in practice. A realistic design case demonstrates the significant material savings achievable through under-reaming in sands.