**3. The Underpinning Scheme of Bridge Pile**

In order to ensure that the Bayi Bridge can still operate normally during the construction, an active underpinning method was adopted to deal with the problem of pile foundation intruding into the tunnel so as to minimize the impact of the construction on the upper bridge structure. The underpinning structure selected was a gantry-type underpinning system consisting of new underpinning piles and underpinning beams. Before the pile foundation underpinning, the new underpinning pile and the foundation pit enclosure structure were to be constructed sequentially. The underpinning beam construction and pile underpinning operation were carried out in the foundation pit. The underpinning steps are as follows:


(5) Considering that the structure above the underpinning beam needs to remain balanced, the original pile foundation is cut off using a wire saw. The cutting height of the pile is controlled within the range of 300~500 mm, and the cutting gap is 5~10 mm. After the first cutting is completed, it is necessary to ensure continuous monitoring for more than 1 h, and the second cutting can only be carried out after the settlement becomes stable. During the pile cutting process and after the pile cutting is completed, the jacking force should be monitored and adjusted to ensure that the original column elevation remains unchanged. The construction process of the underpinning pile foundation is shown in Figure 3.

**Figure 3.** Construction process of underpinning pile foundation.
