*2.3. Position of Bridge Pile Foundation and Shield Tunnel*

The structures corresponding to the pile foundations that need to be underpinned for the south approach bridge of the Bayi Bridge are all multi-span reinforced concrete box girder structures. The pavement of the bridge deck consists of 6 cm expanded metal mesh on the lower layer, C40 concrete and 4 cm thick asphalt concrete on the upper layer, and the horizontal width of the guardrail on both sides is 0.44 m. The lower structure is a single circular column and single pile structure. Among them, the upper structure of ramp C is a double-boxed reinforced concrete continuous girder bridge with a bridge deck width of 11.88 m. The bridge deck layout and related size parameters are shown in Figure 2. The bridge piles on ramp C are relatively complex, and there are different distances between new underpinning piles and the old piles, resulting in eccentricity. In this paper, the bridge pile C15 with the most complex working conditions was selected for research. The column diameter is 1.5 m and the column height is 9.7 m. The pile diameter is 1.5 m, the pile length is 24.9 m, and its intrusion length is 3.5 m.

**Figure 1.** Engineering background of the shield tunnel of Nanchang Metro Line 4 passing through the bridge pile foundation of Bayi bridge: (**a**) location of the underpinning section; and (**b**) layout of the underpinning piles.

**Table 1.** Classification of surrounding rock in shield tunnel section.


**Figure 2.** Pile underpinning scheme of C15 bridge pier and the corresponding parameters.
