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With the intensive development of cities, the utilization of underground space has attracted more and more attention from industry and academia. Underground rail (metro) in cities has become an imperative mode for people in their daily lives. Meanwhile, the safety of rail tunnel construction has constantly been a challenging issue because of the presence of complex strata containing shallow biogas. Accidents in tunnel construction because of shallow biogas which resulted in massive casualties and property loss have been reported in some recent literature. The excavation of formations containing shallow biogas not only poses a threat to the safety of the earlier stage of tunnel construction but also affects the later operation of metro lines. Therefore, the safety problem caused by shallow biogas should be taken into consideration seriously and avoided in the pre-construction stage. A typical underwater metro tunnel, Hangzhou Metro Line 6, is introduced in this study to suggest the proper approach to deal with the biogas problem during the construction process. The generation mechanism of shallow biogas is clarified and the process of identifying biogas risk during strata exploration is also discussed. A risk identification and control system for shield tunneling through biogas strata is proposed to mitigate the potential dangers of shallow biogas during the construction process. This study provides actual construction experience and countermeasures for other similar metro tunnel projects that encounter biogas strata to diminish the potential risks and avoid severe accidents.

Pipe jacking has been the dominant trenchless technology for constructing small (<2 m) to medium-diameter (<4 m) tunnels. Uncertainties and construction difficulties increase significantly when the diameter of the tunnel exceeds 4 m. This paper presents a case study of the largest concrete pipe-jacking tunnel project in the world, the sewerage tunnel along Jinshan Lake, Zhenjiang, China. In this project, an underwater tunnel with a diameter of 4.67 m was constructed by the earth pressure balance (EPB) pipe-jacking method. The case study reports project background, and geological and hydrogeology conditions. The key techniques such as the selection of pipe-jacking machine, jacking force estimation and control, design of intermediate jacking station, grouting process control, launching, and reception of the tunnel boring machine, trajectory control of pipe jacking, and ventilation and gas monitoring during the construction period were investigated and discussed. Furthermore, to overcome the technical difficulties associated with the oversized jacked tunnel, the corresponding countermeasures were adopted point by point, so that the safety of the whole project could be guaranteed. This study filled the knowledge gap of technical know-how for large-diameter (over 4.5 m) pipe-jacking tunnel and is expected to provide practical guide for future large-diameter pipe-jacking tunnels.

The surface subsidence caused by coal mining is a large area, and computer simulation is a fast and intuitive method, which can help us understand the macroscopic subsidence law. The mined-out area left over by coal mining is not disposed of appropriately for a long time. Thus, it can easily cause ground subsidence, collapse, or spot cracking, especially when mining multiple coal seams, which seriously restricts the construction and safety of the near-surface rock and soil layers. Based on the engineering background of five-layer coal mining in the Beibu Coal Mine of Laiwu City, a “Fast Lagrangian Analysis of Continua in 3D” numerical calculation model was established. The model was used to analyze the surface deformation indexes of four groups with different mining sequences in multiple coal seams, revealing the sequence effects of mining time on the surface deformation law in the goaf collapse areas, hence obtaining optimal mining sequences. The results showed that the four groups of mining sequences (including vertical settlement and horizontal deformation) have stable surface deformation centers, but the deformation ranges and amounts are quite different. The settlement deformation is the main difference. Mining sequence I has the largest deformation of 62.7 cm, followed by mining sequence III. Mining sequences II and IV are basically the same, at only 22% of the value of mining sequence 1. A multi-index analysis of the surface deformation curve including the inflection point, stagnation point, and slope showed that the larger the surface deformation, the more evident the change of the curve (concave or convex) and slope, the more uneven the foundation stress, the more severe the damage to the surface structures, and the less suitable the surface construction. Finally, upon analyzing the indicators of surface stability and adaptability, mining sequence IV was indicated as the optimal scheme. It is suggested that an optimal mining sequence should be appropriately selected before the mining of multiple coal seams. The research results can provide effective guidance for addressing surface deformations under similar geological conditions, and can provide scientific evaluations for the safety and stability of surface buildings and structures, leading to considerable economic and social benefits.

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