Discussion on Excavation Methods of Shallow-covered Extra-large Cross-section Urban Tunnels

Hongqihegou station on No.3 line of Chongqing mass rapid transit system is located in the central busy urban area,where the surrounding environment is complex.The maximum cross-section area of the station tunnel is 730 m2,the cover of the station tunnel is 8.6 m and the cover-span ratio of the station tunnel is about 0.4.Therefore,the station tunnel is a typical extra-shallow-covered extra-large cross-section tunnel.Prior to commencement of the works,comparison and contrast are made on several optional excavation methods of the station tunnel and conclusion is drawn that each excavation method has its own advantages and disadvantages and that effective construction measures must be taken for each excavation method so as to achieve successful excavation.The discussions on the optional excavation methods of the station tunnel can provide reference for the construction of similar extra-large cross-section tunnels in the future.