Abstract: Based on the pile active underpinning project of Xima Bridge in Fuzhou Metro Line I shield tunnel, considering the complex hyperstatic structures composed by bridge superstructure, bridge piles and soil masses, the numerical simulation and the field monitoring data are utilized jointly to study the inner force and displacement responses of hyperstatic structures under different jacking loads. The influences of superstructure stiffness and substructure stiffness on the determination of jacking load are emphasized. The reasonable jacking loads for two typical conditions (the piles under cap beam and abutment beam) are 1 800 kN and 2 600 kN respectively, which make the axial forces of underpinned piles reduce to zero. The field monitoring is also carried out during construction, and the displacement of bridge superstructure and the axial force of bridge pile from monitoring data are consistent with their counterparts from numerical simulations. Based on these results from numerical simulation and field monitoring, a simplified analytical method is further proposed to calculate the reasonable active jacking load by considering the stiffness ratio between the superstructure and substructure, which can provide some reference for the pile underpinning projects under similar scenarios.

Key words: bridge pile, active underpinning, hyperstatic structure, jacking load, stiffness of superstructure and substructure