Abstract: The subway construction in sandy cobble and loess layers in Xi'an is selected as the background. Applying numerical methods, in-situ monitoring and other means,, in view of the excavation methods during the construction of cross passage by tunneling and back well method, the pre-reinforcement schemes of existing shield pipe segment is proposed, optimized analyzed and evaluated. The results show that the increase of excavation layers will result in the surface settlement and the shortage of construction work space. It is suggested to use the 4 steps, and the core soil method is reserved for excavation. The length of the upper and lower steps should not exceed 3 m. The grouting thickness of the gravel and loess stratum from outside of the cross passage is at least 2.0 m, and the elastic modulus of the stratum should be increased by more than 0.2 times after the reinforcement. The advanced support methods of small and large pipe sheds can be used to enhance the excavation stability of the stratum and the safety of the shield tunnel segments. Before the existing pipe shield segment is broken, the connecting bolts of 4 to 6 ring segments on both sides of the transverse channel should be loosened to reduce the effect of longitudinal compression stress concentration of the shield segment caused by the excavation of the transverse channel. When the horizontal cross passage passes through the existing shield segment, the static breaking method should be adopted to remove the segment vertically from top to bottom. Then the concrete pouring and reinforcement implantation should be carried out at the interface between the air shaft cross passage and the shield segment. The WSS grouting should be used to reinforce the soil stratum and implement the anti-seepage treatment at the joint.

Key words: tunnel first then well method, Xi’an subway, shield lining segments, construction technology, mechanical response