## 2 Engineering Background
### Problems and Solutions Encountered in Tunnel Excavation  
The shield machine of the Chunfeng tunnel is located between pile #2 and pile #3 of No. 1 bridge of the Hongling Interchange, 40 m after its start. The plane distance between the tunnel and pile #2 and pile #3 is 2.4 m and 4.3 m, respectively. The top elevation of the shield structure is \(-\)4.92 m, and the bottom elevations of pile #2 and pile #3 are \(-\)20.2 m and \(-\)21.2 m, respectively. Relative position relationship is shown in Figure 5. Here, the stratum is mainly plain fill, muddy clay, medium sand, gravelly sand, moderately weathered granite, and slightly weathered granite. The stratum crossed by the tunnel is mainly moderately weathered granite. The bearing stratum at the pile ends of pile #2 and #3 is moderately weathered granite.  
Considering the problem of large-diameter adjacent shield tunnelling bridge piles, combined with the treatment experience of previous similar engineering problems [18], a scheme for strengthening the soil around the pile foundation needed to be proposed.  
The shield passes through the bridge piles of Hongling Interchange No. 1 bridge within about 16 m of Chainage ZK0 + 875-ZK0 + 890. The location of the grouting hole is shown in Figure 6. Before shield tunnelling, the soil around the pile foundation were reinforced by sleeve valve pipe grouting. The grouting holes were arranged in a quincux shape with a spacing of 1 m \(\times\) 1 m. The length of the grouting pipe near the bridge pile must be driven into the moderately weathered granite layer for 1 m. The water-cement ratio of cement slurry used for grouting is 0.5:1-1:1. The initial grouting pressure is 0.5-0.4 MPa. After the grouting pressure gradually rises to 0.5-0.8 MPa, it is necessary to continue grouting for 10 min. The grouting effect verification index is that the unconfined compressive strength of grouting reinforced bodies for 28 days should be higher than 1.0 MPa, and its permeability coefficient less than 1.0 \(\times\) 10\({}^{-7}\) cm/s.  
Figure 4: Engineering geological profile.  
Figure 5: Relative relationship between shield tunnel and bridge piles.  
Numerical Simulation of Mechanical Behaviour of Large-Diameter Adjacent Shield Tunnelling Bridge Piles  
It is known from the current research results that in the process of adjacent shield tunnelling bridge piles, stress redistribution of soil mass is caused by disturbing the surrounding soil layer, which affects the stress, deformation and bearing characteristics of adjacent piles [19]. If the displacement of the pile body is too large, or uneven settlement is large, this would cause cracks in the overlying bridge, which would affect the durability of the bridge.  
Figure 6: Grouting reinforcement scheme. (**a**) Plan of the grouting reinforcement scheme. (**b**) Section of the grouting reinforcement scheme.  
and regular use of the bridge [20]. In addition, the relative positions between the pile toes and the tunnel determine the disturbance of the pile. Excessive settlement usually occurs when the pile toes are above the tunnel. If the pile toes are embedded deeper than the tunnel, the main problem is related to the additional bending of the pile. When the super large diameter shield of the Chunfeng tunnel adjacent crosses the bridge piles, the impact of the bridge piles must be evaluated and predicted in advance. The deformation control standard of piles also needs to be determined to evaluate the safety of the existing bridge in shield tunnelling.