Pile Lifting and Load Transfer Mechanisms Induced by Capsule Expansion Technology in Soft Clay: A Field Trial and Numerical Analysis

Capsule expansion technology (CET) is innovatively developed to protect subsurface structures from underground engineering construction. However, it has hardly been reported in pile settlement control using CET. In this study, field trials were conducted to verify the feasibility of pile lifting using CET in soft clay, compared to the Tube-a-Manchette (TAM) grouting. Meanwhile, numerical simulations were used to further explore the pile–soil–capsule interaction and load transfer mechanism during pile lifting. The result showed that CET could effectively lift the pile by 1 mm after consolidation, with the lifting efficiency of CET at 62.5% and that of TAM grouting at −200%, proving the feasibility of CET for pile lifting. The ultimate bearing capacity of the pile was improved by 25%, which is attributed to the reinforcement of the soil at the lower part of the pile by the expansion. Pile tip resistance and skin friction near the lower part of the pile play the primary role in bearing the pile head load after CET processing. Moreover, CET changes the load transfer mechanism of the pile, a downward load transfer mechanism after expansion and an upward load transfer mechanism after consolidation. Accordingly, it confirms that the pile is lifted by the coupling effect of pile tip resistance and skin friction during expansion. Then, reducing pile skin friction and tip resistance leads to pile settlement during the consolidation stage.