Experimental investigation of buoyancy distribution characteristics in basement structures founded on clayey soils

• This study addresses the challenge of determining appropriate design groundwater levels for anti-uplift stability of underground structures situated in weakly permeable clayey soils. By integrating large-scale physical model testing with theoretical derivation, the research systematically investigates the pore water pressure transmission characteristics and buoyancy reduction mechanisms under varying hydraulic boundary conditions. The findings confirm a significant hysteresis in pore pressure response within dense clays, where transmission delay and reduction intensify with increasing seepage path and water level. A distance-dependent pore pressure attenuation model was developed to reflect this behavior, providing a more accurate representation of in-situ uplift forces for design purposes. • Experimental observations further revealed that a narrow interface gap between the outer edge of the structure and surrounding soil creates an auxiliary seepage path, resulting in a localized zone with maximal buoyancy reduction. This effect is especially prominent at the sidewalls, where pore pressure builds up more rapidly and deviates from the uniform distribution predicted by classical hydrostatic theory. Based on these insights, the study proposes a zoned buoyancy reduction strategy for basement slabs, whereby spatially variable design uplift pressures are recommended depending on the position relative to structural boundaries. These contributions provide both theoretical and practical value, enhancing the precision and safety of anti-buoyancy design in underground construction projects involving low-permeability soils. to investigate the distribution characteristics and transmission mechanisms of pore water pressure beneath the base slab of underground structures in clay, and to optimize the design groundwater level for anti-floating stability, a series of laboratory model tests were conducted. Two conditions were simulated: with and without petroleum jelly applied to the inner walls of the model box. The temporal variation of pore water pressure at various locations beneath the slab under different water level conditions was systematically analyzed. Test results indicate a significant hysteresis in pore pressure response within clay; the longer the seepage path, the longer the time required for pore pressure to reach equilibrium, and the more pronounced the reduction effect. The pore water pressure distribution beneath the base slab was found to be non-uniform. At point A, located near the model box wall, the shorter seepage path led to pore pressure values approaching the theoretical prediction. This phenomenon is attributed to an additional seepage path formed by a slight gap at the interface between the smooth model wall and the clay. Based on the test data, a theoretical model for pore pressure transmission in clay was developed, and analytical solutions under various groundwater conditions were derived and validated. Accordingly, a zoned anti-floating design strategy was proposed: a dynamic reduction coefficient should be adopted near the outer wall area, while a uniform coefficient of 0.895 can be used for the central area of the slab. These findings provide theoretical and practical guidance for the anti-floating design of underground structures in weakly permeable soil layers.