Abstract: The unloading-rebound and recompression deformation characteristics of soft clay significantly affects the design and structural safety of deep foundations, as well as embankments and culvert passages constructed on preloaded soft subgrades subjected to reverse excavation. To investigate the underlying mechanisms, this study conducted a systematic experimental investigation into the unloaded rebound and recompression behaviors of soft clay from Anxiang, Dongting Lake. The theoretical findings were applied to engineering practice subsequently. The results reveal a clear three-stage evolution of rebound deformation with increasing unloading ratio R: when R≤0.2, the rebound rate  is extremely low, corresponding to a rebound incubation period; when 0.20.8,  rises sharply, indicating an accelerated development stage. The critical unloading ratio Rcr of the soil is 0.2, and the ultimate unloading ratio Ru is 0.8. The rebound rate  is significantly influenced by the preloading history. Under the same unloading ratio, a higher preloading load leads to a higher . The maximum rebound rate max after unloading shows a strong linear correlation with the preloading load pmax. The relationship curves between the rebound modulus Ec and the unloading ratio R (Ec-R and lgEc-R) exhibit two linear segments. The inflection points of these curves, with horizontal coordinates slightly greater than Rcr and close to Ru, respectively, effectively define the stable deformation zone and the significant rebound development zone. The recompression ratio r and the reloading ratio R′ show a nonlinear increasing relationship, which can also be divided into three stages and well fitted by a cubic polynomial without a constant term. A formula for calculating the rebound and recompression deformation of deep foundation pits was established based on the r-R′ relationship. Using Ru and Rcr as boundaries, the soil at the bottom of a foundation pit was categorized into strong, moderate, and non-rebound zones. The critical depth of the rebound zone (i.e., the neutral point depth of piles) was determined, and a formula for calculating the ultimate uplift load of foundation piles at the pit bottom was proposed. These findings providing a quantitative basis for the accurate calculation and risk control of rebound and recompression deformation of soft clay foundation pits, and has been applied in engineering projects such as highway culverts and passages in the Dongting Lake area.

Key words: soft clay, unloading rebound, recompression deformation, settlement, uplift load