Study On Surface Settlement Law And Construction Optimization Of Double Arch Metro Tunnel Caused By Shallow Buried Excavation In Loess Site Of Xi'an

In the construction of urban subways,the ground subsidence caused by the construction will affect the surrounding roads and buildings to varying degrees.In severe cases,it will cause problems such as subsidence and collapse,leading to huge economic losses.Due to the complication of the double-arch tunnels' construction process and the frequency of supporting structures' force conversion,the mechanism of surface settlement is more complex and difficult to articulate than the single tunnel.In view of this,this paper relies on the Yan-ming Lake parking lot entry and exit project of Xi'an Metro Line 5,using a combination of field monitoring,theoretical analysis and numerical simulation to study the changes in surface settlement during the construction of a double-arch tunnel.The main work and conclusions are as follows:(1)Combined with field monitoring data and finite element numerical simulation,the law of surface settlement changes during the construction of a double-arch subway tunnel in Xi'an loess site was obtained.It reveals that the surface settlement changes of the double-arch tunnel have basically experienced a sharp increase,a slow increase and a gradual stabilization stage.Within a certain depth range and the buried depth of the tunnel increases,the excavation impact and the ground settlement amount is increasing accordingly.Guide by the Peck formula,a superposition formula for predicting the surface settlement caused by the construction of a double-arch subway tunnel is proposed.(2)Based on the numerical simulation,the surrounding rock stress change and the distribution of plastic zone during the excavation of the double-arch tunnel are studied.The surface settlement mechanism of the double-arch tunnel during the excavation process is explained from surrounding rock stress,consolidation settlement and stratum loss.It also analyzes the main factors affecting the surface settlement of the double-arch subway tunnel shallowly buried and undercut,and puts forward the control measures of the surface settlement from the aspects of ground pre-reinforcement,construction method and support.(3)Combined with the project practice on Xi'an subway,the numerical simulation method is adopted to discuss the influence of staggered excavation distance of main tunnel,the step length and the single length of demolition on the mechanical behavior of the double-arch tunnel construction.Based on the evaluation basis of the surface settlement,the arch settlement and the clearance convergence,the site construction scheme is optimized.