Construction time of secondary lining of ultra-small spacing parallel overlapping tunnel

Abstract Due to the unique spatial structure, stress transfer mechanism, and construction disturbance characteristics of ultra-small spacing parallel overlapping tunnels, systematic research on the timing of secondary lining construction remains lacking. In this regard, based on the ultra-small spacing parallel overlapping tunnel project of Chongqing Line 17, this study employed FLAC3D numerical simulation to analyze the influence mechanism of different construction times for the secondary lining in the pre-built tunnel on the synergistic effect of the surrounding rock-support system. The analysis focused on three dimensions: vault settlement, stress level, and plastic zone distribution. The optimal construction time for the secondary lining of the pre-built tunnel was determined accordingly. The results show that the vault settlement of the pre-built tunnel presents a three-stage evolution characterized by initial settlement-displacement rebound-secondary settlement, while the post-built tunnel shows a two-stage mode of 'initial settlement-excavation settlement'. Constructing the secondary lining for the pre-built tunnel before excavating the post-built tunnel effectively inhibits the pre-built tunnel's secondary settlement. If construction is delayed until after the post-built tunnel excavation, the settlement increases to 4 times that observed before the post-built tunnel excavation. The maximum principal stress in the pre-built tunnel concentrates in the 45°area of the interface with the post-built tunnel, while the minimum principal stress mainly appears in the arch bottom area. The stress distribution of the post-built tunnel resembles that of a traditional single tunnel. Delaying the construction of the pre-built tunnel's secondary lining leads to the gradual release of surrounding rock stress; however, constructing it after the post-built tunnel excavation causes the structural stress to rise significantly. Constructing the secondary lining of the pre-built tunnel after the post-built tunnel excavation promotes the transfer and concentration of the pre-built tunnel's shear plastic zone towards the junction area, significantly increasing the risk of penetration failure. Based on these findings, an evaluation system of 'stress-displacement-plastic zone coordinated control' is proposed. The optimal construction time for the secondary lining of the pre-built tunnel is determined to be when its vault settlement reaches 80% of the development occurring before the excavation of the post-built tunnel. These results provide theoretical support and practical guidance for selecting the secondary lining construction time in this type of tunnel.