Immersion Ratio–Driven Degradation and Load Upshift of Belled Uplift Piles under Groundwater Effects: Model Tests, Field Benchmark, and Design Relations

To clarify how evolving groundwater degrades the capacity of belled uplift piles through the entire loading process, a series of scaled physical model tests with a geometric similarity ratio of 1∶60 was conducted using a prototypical pile from the Beijing Sub-Center, and the results were benchmarked against a field static load test under a consistent load similarity. A dimensionless state variable, the immersion ratio of soil surrounding the pile (Sps), was introduced to unify the representation of groundwater effects. For varying Sps, measurements included the load versus displacement response, axial force along the pile, lateral earth pressure around the shaft, and ground-surface deformation. The ultimate uplift capacity decreased smoothly and nonlinearly with Sps, with accelerated degradation observed for Sps≥0.5. Using the test data together with consistency-checked numerical simulations, an empirical relation was established between Sps and the normalized ultimate capacity (R2≈0.998), and reduction curves suitable for design are provided. Empirical expressions are also presented for the stiffness-inflection threshold and for a load-upshift index. The findings identify a coherent mechanism under groundwater influence in which effective stress in the surrounding soil is reduced, interface mobilization weakens, load redistributes upward, and displacement concentrates in the near field. The proposed reduction curves support antiflotation checks of belled uplift piles in water-rich sites and provide data and theoretical guidance for designs that explicitly account for groundwater evolution.