Design of the Gongbei tunnel using a very large cross-section pipe-roof and soil freezing method

The Gongbei tunnel, as a critical part of the Zhuhai connection highway of the Hong Kong-Zhuhai-Macao Bridge project, is being constructed in Zhuhai, China. The tunnel, close to the Lingdingyang bay, is located in a very environmentally sensitive location with poor geological conditions and is extremely difficult to be constructed. To ensure no interruptions to customs clearance at Gongbei Port, an innovative long-distance curved pipe roof with soil freezing method is adopted to allow a super-shallow-buried large sectional double-decked six-lane tunnel to traverse under the Port. The area of excavation cross-section is up to 337 m2, however the burial depth is only 5-6 m and only 0.5 m away from the pile foundations of buildings at the Port. The pipe roof around the tunnel is made up of 36 steel pipes with a length of 257 m and diameter of 1620 mm at intervals of about 356 mm, and the soil freezing method is applied for sealing. There are daunting technical challenges such as controlling the jacking precision of the long pipe roof group, guaranteeing the precise thickness of long horizontal soil freezing, controlling the ground surface deformation of the underground excavation section, etc. The paper will elaborate on the selection of the construction method for traversing under Gongbei Port, on the basic principles of the pipe-roof and soil freezing underground excavation method, on the design of a long-distance and large sectional curved pipe roof, and the design of long-distance horizontal precise soil freezing curtain.