Abstract
Because of the surrounding environment restriction, some sections of the Lanzhou Metro Line 2 were forced to change the original open excavation method to the concealed excavation method. Among them, the ingate is the transitional section connecting the shaft and the cross passage, which is accompanied by the force conversion of the structure during the plan change process, and accidents are prone to occur. Combining this project situation, a construction scheme for turning the shaft from the cross passage into the ingate of mine line was proposed, and the finite element software is used to simulate the scheme. Numerical analysis shows that the stability of the tunnel supporting structure, shaft, and cross passage all meet the design requirements. The final ground settlement caused by the proposed construction method is 7.1 mm, and the settlement of the vault is 11.97 mm. It also turns out that the proposed construction method causes less deformation, and the method can be applied to the construction of the ingate conversion. The rationality of the method and numerical model was further verified by the comparison between the monitored data of surface settlement and vault sinking and numerical simulation results. Finally, the modified concealed excavation method and the original open excavation construction method is compared. It is concluded that the concealed excavation method is superior to the original open excavation method in terms of construction schedule, construction safety, economic benefits, and impact of construction on the surrounding environment. It indicates that it is a good choice to replace the construction method early if necessary. The successful implementation of this project can provide a reference for the design and implementation of similar urban subway projects.
Highlights
In recent years, subway construction has developed rapidly
The concealed excavation method includes drilling and blasting method, shield method, TBM, and pipe jacking method. e selection of construction method usually takes into account the construction conditions, surrounding rock conditions, burial depth, construction period, and surrounding environmental conditions
When the foundation pit is deep, measures should be taken to prevent foundation pit deformation and surrounding ground settlement [3, 6, 7]. erefore, when the open excavation method is limited or the construction conditions are not allowed, the open excavation is often changed to the concealed excavation to ensure the construction period
Summary
Subway construction has developed rapidly. Nowadays, the common methods in urban subway construction include open excavation, cover excavation [1,2,3], and concealed excavation [4, 5]. Jiang [13] proposed to change the open excavation to the concealed excavation according to the characteristics of the interlaced section of a tunnel in Changsha, such as small interval, poor geological conditions, and surrounding environment interference. Wei et al [19] used FLAC 3D software to numerically simulate the construction process of ingate and pointed out that, under deep burial conditions, the uplift of the surrounding rock of the ingate floor is relatively large, and it is necessary to take targeted measures. Numerical simulation and field monitoring were used to analyze the stability of the modified construction scheme It can provide reference for the design and construction of shaft and cross passage turning into the mine line in subway tunnel engineering
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