Time Effect of Buoyant Force Reduction for Underground Structures in Clays: Model Test and Case Study

Abstract

Underground structures experience transient and sustained hydrostatic uplift forces throughout their service life. Conventionally, Archimedes’ principle is used to conduct uplift design. The estimated buoyant force is often employed directly for foundations in coarse backfills, but a reduction factor is suggested to reduce the conservatism in design for foundation in clays. There is no consensus concerning how to choose the reduction factor. Different mitigation measures are proposed empirically, and it is imperative to provide evidence for uplift design optimization. Novel model-scale laboratory tests are carried out in this study to measure the time effect of buoyant force reduction. It is found that the reduction factor for buoyant force is around 0.55–0.66 initially, and increases to a plateau of 0.82 with time. The measured excess pore water pressure underneath the foundation is approximately 80% of the earth pressure, which explains the reduction in buoyant force for foundations in clays. In the end, a case history of a large-scale underground parking garage in Shenzhen, China, is presented to demonstrate the need for reducing the buoyant force in uplift design.