**4. Numerical Simulation of Pile Foundation Underpinning Process**

During the underpinning construction, the stress and deformation of the new underpinning pile, the underpinning beam and the underpinning pile are relatively complex, and at the stage of jack loading, unloading and pile cutting, the deformation control of the new pile and underpinning beam, such as settlement, inclination and horizontal displacement of pile top is the key to pile underpinning technology [16]. In many previous studies in the literature, finite element method software ABAQUS 3D has been used in the numerical simulation of shield tunnelling and pile foundation underpinning process, which has proved the validity in capturing the mechanical behaviors of the tunnels and the surrounding pile foundations [16,19]. Hence, in this part, ABAQUS 3D finite element simulation was used to analyze the deformation of the bridge substructure of the C15 pile foundation in the three stages of jacking, unloading and pile cutting so as to verify the feasibility and safety of the underpinning technology.
