## 5 Discussion  
To reduce the impact on Line 5 during the construction of the Heping Avenue Shield Tunnel, the isolation pile construction scheme was designed based on the simulation analysis of the control effect of an isolation pile, the ground space conditions and the cost of an isolation pile.  
Due to the densely distributed aboveground buildings around the Heping Avenue South Shield Tunnel and Line 5, the space for the implementation of isolation piles from the ground is limited, and there is no site available for large-scale bored pile construction; however, the microcomposite bolt pile requires a small construction site, has a convenient construction scheme, and uses a multirow plum blossom pile arrangement, which can ensure the isolation stiffness requirements. Therefore, the project adopted microcomposite bolt piles, and the pile layout is the same as that used in the simulation, as shown in Figure 8.  
In the isolation pile area shown in Figure 12, microcomposite bolt piles with a diameter of \(\Phi\)180 and lengths of 25 m, 26 m, and 28 m are adopted. The bolt piles are arranged in four rows of plum blossom patterns, the spacing is 0.25 m \(\times\) 0.25 m, and the top of the pile is applied with a 0.5 m \(\times\) 1.2 m crown beam. To monitor the impact of the construction of the backing structure after the isolation piles were applied, some monitoring points were arranged on the surface and buildings near the isolation pile, and the monitoring data are shown in Figure 13.  
As can be seen from Figure 13, the displacement of each monitoring point is not large, and the surface displacement has changed greatly from 10 to 13 December 2021, but the order of magnitude is small, and the overall impact is not great.  
According to the monitoring data, after the application of the isolation piles, the settlement of the surrounding surface and buildings was within a reasonable range, so the effectiveness of the method is proven, and the installation of the isolation piles can ensure the safe and smooth passage of the shield tunnel.  
Figure 12: Plan view of the layout of the isolation piles and the location of the monitoring points.  
In this paper, the impact of super-large-diameter shield construction on the existing Line 5 is studied by means of numerical simulation, and the reinforcement effect of different protection schemes on the existing line is studied, but there are still some problems in this study, which need to be further studied. In this paper, only the pile length and the horizontal position of the single pile are considered when simulating the influence of different isolation schemes on the isolation effect of the existing lines, and the pile spacing and pile diameter of multiple piles should be considered in further research. In this study, a two-dimensional finite element numerical simulation method is adopted; however, in the simulation of the shield construction process, three-dimensional numerical simulation can fully take the shielding process into account, and therefore, it will make the calculation results more accurate. Thus, it is recommended that the three-dimensional numerical simulation is conducted in further research work.