## 6 Conclusions  
This paper focuses on the mechanical responses of existing large-diameter tunnels induced by a close-distance shallow tunneling method (STM) undercrossing in sand and gravel strata in Beijing, China. The following are some of the significant conclusions from this study:  
1. The subsidence evolution of the existing large-diameter tunnel during the undercrossing twin tunnels showed six stages, including (1) the advanced settlement (Stage 1) caused by the construction disturbance in far-field excavation (>2 D), (2) the heave induced by the effect of the grouting pressure, (3) the vertical unloading caused by volume loss at the stratum during tunnel passage having a significant impact on the second settling, (4) the rise induced by the left tunnel deep hole grouting and right tunnel wall back grouting, (5) the third settlement resulting once again in state disturbance during the left tunnel passing, and (6) the settlement development of the existing large-diameter shield tunnel slowly decreasing and then gradually stabilizing in this stage. In conclusion, grouting measures have a great influence on the existing tunnel deformation.
2. The subsidence profile of the existing large-diameter shield tunnel caused by left line tunnel crossing is asymmetric relative to the central line of the second tunnel. The maximum settlement point was approximately 3-6 m away from the center line of the left line tunnel of subway Line 12, which may indeed be related to the soil disturbance of the first tunnel's construction and grouting reinforcement measures. The subsidence and volume loss due to the second tunnel's construction was found to be less than the volume loss due to the first tunnel's construction. This may be due to the reinforcement of the middle part of the left and right lines before the construction of the left and right lines or Line 12, which caused many reinforcements and greatly changed the stiffness of the soil. The final subsidence profile of the existing large-diameter shield tunnel was a \"W\" shape after the second tunnel passed through.
3. When the right line tunnel passed through the existing tunnel, the circumferential stress changed dramatically, with a size of roughly 0.4 MPa at the left arch line of the existing tunnel. The reason for this could be that the undercrossing architecture caused some unloading, which increased the tensile stress at the arch bottom.
4. The opening and compression of the existing large-diameter shield tunnel's joints were inextricably linked to the new tunnel's excavation process. Undercrossing construction will lead to the opening of the joints, and thus the effectiveness of the waterproof function at the joint is significantly reduced. However, the undercrossing effect can be weakened by secondary pressure grouting for the existing large-diameter shield tunnel before excavation.  
J.L. and Q.F.: conceptualization, methodology, project administration, and writing the original draft; X.L., J.D., G.W., and J.W.: data collection, plotting curves and analysis, and review. All authors have read and agreed to the published version of the manuscript.  
The authors gratefully acknowledge the Key Project of High-Speed Rail Joint Fund of the National Natural Science Foundation of China grant number UI934210.  
Not applicable.  
The data used to support the findings of this study are available from the corresponding author upon request.