Abstract: Xi'an rail transit project is built in the loess area of China. The subway parking line interval is featured with shallow buried depth,small clear distance and asymmetric distribution of sections. To avoid excessive disturbance between the left and right lines during excavation,we need to determine a reasonable stagger distance of the working face. We took the parking line interval of Xi'an Metro Line 8 project as the research background,and dynamically simulated the excavation under different schemes by using FLAC^3D finite difference software. By comparing the simulation results with the actual monitoring data,we verified the correctness of the simulation. We found that the final deformation and stress of surrounding rock was hardly affected by different schemes,but the deformation rate and the extreme value in the process need to be controlled. The longitudinal influence range of tunnel face on excavated soil reached about 3 times the span. When the offset of tunnel face is 2-3 times the span,the deformation rate of surrounding rock can be effectively controlled. When the offset of tunnel is large,the maximum shear stress of surrounding rock increased significantly during excavation,and meanwhile leading to large relative displacement difference between the middle soil column,unfavorable to the stability of surrounding rock. In conclusion,we suggest to reduce the offset of the working face to 30 m to ensure construction safety and improve construction efficiency.

Key words: tunnel engineering, reasonable offset, numerical simulation, stability of surrounding rock, small clear distance, asymmetrical cross-section