## 5 Conclusions  
The influence of different excavation sequences, second primary lining support time, temporary support disassembly, and secondary lining support time of the double-side heading method on large section tunnel stability were analyzed, and the following conclusions were drawn:  
1. In the double-side heading method, the excavation sequence from the right pilot tunnel to the left pilot tunnel and then to the middle pilot tunnel was the most conservative as its surface settlement was small. This method had a good control effect on the horizontal displacement of tunnel lining at the arch waist. The excavation sequence from both sides to the pilot middle tunnel can protect the concrete safety at the arch waist better, but tensile stress also occurred at the vault. The construction  
Figure 24: The vertical stress of the secondary lining (unit: Pa): (a) Condition 1 and (b) Condition 2. Figure 25: The horizontal stress of the secondary lining (unit: Pa): (a) Condition 1 and (b) Condition 2.  
from the middle pilot tunnel to both sides was conducive to the bearing performance of the concrete at the arch crown but produces tensile stress at the arch waist. 2. The second primary lining supporting time was recommended to rapidly support the second primary lining after the excavation of the current tunnel face because the vertical displacement of the surrounding rock in this condition was small. This method was conducive to controlling the horizontal displacement at the arch waist so that the primary support at the arch waist bore small horizontal stress. The method of supporting the second primary lining of the current tunnel face and the first primary lining of the next tunnel face simultaneously was conducive to controlling the vertical settlement at the vault. 3. The method of temporary support layered disassembly and secondary lining layered support was relatively reasonable because it could effectively control the horizontal displacement of the secondary lining and exerted the bearing capacity of the arch waist concrete. The method of removing the whole temporary support and applying the whole secondary lining caused a large displacement at the arch waist, which did not benefit the safety of the primary lining structure. 4. The excavation sequence, second primary lining support time, temporary support removal, and secondary lining support time should be adjusted according to the actual deformation and stress of the section on site when the double-side heading method is adopted for the construction of large section tunnels. 5. Further study should focus on the extent of yielding zones following each excavation step and the influence of bolt length and lining support thickness on the stability of the surrounding rock. Conceptualization, J.W. ; methodology, J.W. and Z.W. ; software, Z.W. ; validation, L.L., Z.L., J.W., Y.L., A.C., H.L., Y.S. and X.L. ; formal analysis, J.W. and Z.W. ; investigation, J.W., X.L., A.C., H.L., and Y.S. ; resources, J.W. ; data curation, Z.W. ; writing--original draft preparation, J.W., Z.W., and A.C.; writing--review and editing, J.W. and A.C.; project administration, L.L. and Z.L. All authors have read and agreed to the published version of the manuscript. This research was funded by the Shanghai Municipal Science and Technology Project (18 DZ1201301; 19 DZ1200900); the Xiamen Road and Bridge Group (XM2017-TZ0151; XM2017-TZ0117); the project of Key Laboratory of Impact and Safety Engineering (Ningbo University), Ministry of Education (CJ202101); the Shanghai Municipal Science and Technology Major Project (2021 SHZDZX0100) and the Fundamental Research Funds for the Central Universities; the Key Laboratory of Land Subsidence Monitoring and Prevention, Ministry of Natural Resources of the People's Republic of China (No. KLLSMP202101; KLLSMP20201); the Suzhou Rail Transit Line 1 Co., Ltd. (SURT01 YJ1S10002); and the China Railway 15 Bureau Group Co., Ltd. (CR15 CG-XLDYH7-2019-GC01). The study did not require ethical approval. The study did not involve humans.