## 7 Conclusions  
Based on the characteristics of the actual project and the requirements of deformation control, three underground excavation construction schemes that closely cross beneath the existing tunnel were proposed in this paper, and the implementation process and detailed structure of the three underground excavation schemes were reflected through the establishment of a three-dimensional refined numerical calculation model, the feasibility and control effect of the three schemes were discussed, and the deformation control mechanism of the existing tunnel structure and the undercrossing area under different support methods and underground excavation methods was further studied. The conclusions of this study can be summarized as follows:  
(1) In terms of deformation control of existing tunnels, the L-type beam + full-face excavation method in Scheme I is the largest, the pier column + double-pilot tunnel method in Scheme II is relatively small, and the final settlement in Scheme III, the temporary steel column + double-pilot tunnel method, is the smallest, but the tunnel deformation changes violently during the excavation process, and the maximum settlement exceeds that in  
Figure 16: Deformation curve of the side wall of the undercrossing area during excavation. Scheme II; in terms of soil deformation control on both sides of the undercrossing area, there is little difference between Scheme I and Scheme II, both of which are smaller than Scheme III in lateral displacement, and there is a \"sharp point\" in the soil deformation curve of the side wall of Scheme III. (2) At the end of the tunnel support stage, due to the large free face generated by the excavation of the soil on both sides of the L-type beam structure during the construction process, the settlement near the walls on both sides of the existing tunnel is large, and only the soil in the middle of the undercrossing area plays a jacking role on the tunnel, the bottom of the middle wall bears a large compressive stress, and the bottom plate in the transverse direction is an inverted \"U\"-shaped deformation. The pier column structure has a good effect on the settlement control of the existing tunnel. The deformation of the bottom plate in the transverse direction is uniform, and the overall settlement of the tunnel is small. (3) In the process of rock and soil excavation in the undercrossing area, the settlement of the existing tunnel under the condition of double-pilot tunnel excavation is less than that under the condition of full-face excavation under the same tunnel support mode; when the L-type beam structure is used to support the tunnel, the increase in the vertical displacement of the existing tunnel during the excavation of the subsurface soil is greater than in the case where the pier column support is used. When the double-pilot tunnel method is adopted for excavation, the deformation of the side walls of the undercrossing area tends to be almost stable after the excavation of the two pilot tunnels is completed, and the lateral displacement of the two side walls changes slightly during the excavation of the middle soil mass. (4) Through comprehensive analysis and consideration, the deformation control effect of Scheme III (temporary steel column + double-pilot tunnel excavation) is mediocre, and there are problems such as uneven stress on the tunnel structure, dramatically changing tunnel deformation, high construction requirements, and large construction difficulty, so the applicability is not strong in the actual project. Regardless of the construction period, the pier column + double-pilot tunnel method is the best way to control the deformation. Alternatively, the combination construction scheme of pier column + full-face excavation can be chosen as a secondary option. Conceptualization, X.Z. ; Methodology, Z.Z. ; Software, Z.Z. ; Validation, X.L. ; Investigation, Z.W. ; Resources, L.L. ; Writing--original draft, Z.Z. ; Writing--review & editing, X.Z. and L.W. ; Funding acquisition, X.Z. and X.L.