Experimental study on active freezing scheme of freeze-sealing pipe roof used in ultra-shallow buried tunnels

The excavated section of Gongbei Tunnel of Zhuhai Link of Hong Kong-Zhuhai-Macao Bridge is an ultra-shallow buried tunnel located in water-rich area, and a new pre-support system named freeze-sealing pipe roof (FSPR) is first adopted. Based on the diverse combinations of “circular master freezing-tube” within the solid jacking pipes and “profiled enhancing freezing-tube” within the empty jacking pipes, three kinds of active freezing modes are proposed, i.e., solid jacking pipes, combination of solid jacking pipes and empty jacking pipes, and combination of solid jacking pipes as the principal and empty jacking pipes as the complement. The above three freezing modes are analyzed and verified by means of two different analysis methods of temperature-time curve and thickness of frozen curtain according to the requirements of the design program. The results show that the first freezing mode cannot form a 2 m-thick frozen soil curtain in 60 d, the second one can form 2 m-thick frozen soil curtain within 30 d, and the third one can also form 2 m-thick frozen soil curtain in 60 d under lagged running of profiled enhancing freezing-tube of 12 d or 39 d; and the requirements of sealing water between jacked pipes can be achieved after 1 to 3 d when the profiled enhancing freezing-tube is started to work.