Abstract: When slurry shield tunneling in sandycobble strata, many problems will be encountered, such as cutterhead jamming, pipe blocking, stagnation, and abnormal cutter wear. Hence, the factors induce the above problems in Zirui tunnel of Xihuan road in Chengdu, China, are analyzed. The project difficulties are predicted to conduct specific design. Based on the highstandard pressure and surface subsidence requirements, the techniques of air cushion slurry shield equipped with screw conveyor and screw conveyer mucking + closed pipeline mucking are proposed and adopted. The cobble excavated by the cutterhead are timely transported to the outside of the excavation chamber by the screw conveyor, and further transported to the outside of the tunnel through slurry pump and closed pipe, so as to avoid the pebble accumulation and guarantee smooth shield tunneling. The engineering application validates that the doublemode shield is superior to conventional air cushion slurry shield in aspect of adaptability to sandycobble strata.

Key words: doublemode shield, sandycobble strata, stagnation, screw conveyer mucking, closed pipeline mucking