Abstract: This paper presents a case history of the Line A section tunnel of an urban subway passing through the existing Line B train section. Throughout the field tests, the geological problems existing in the construction process are analyzed, and the engineering overview and potential geological risks of the construction are discussed. The safety assessment is performed at Line B. Reinforcement of the typical section plan is also presented. Results shows that in this project, the groundwater within the depth of impact of the project mainly consists of upper perched water and confined water; during the construction process, the fracture zone and sand liquefaction are ignored, but the release rate and volume of shallow toxic gas should be controlled.

Key words: Reinforcement, Sand Liquefaction, Toxic Gas, Fracture Zone