Study On Ground Settlement Caused By Metro Shield Construction

The shield tunneling method is widely used in urban subway construction,because of its high degree of mechanization,wide range of application in different soil,fast construction progress and less influence on the surrounding environment,.But the shield construction will cut the soil,will inevitably produce disturbances to the soil and the settlement on the surface,which not only affects the safety of construction,but also affect the ground roads,buildings,underground pipelines and other facilities safety.Based on the shield tunneling of the north extension line of Paris Metro Line 14,the design of subsidence monitoring system,the law of surface subsidence and the measures to reduce settlement are studied in the construction of subway tunnels using earth pressure balance shield(EPB).These works have certain practical value in the construction of metro.Based on the study of working principle and construction process of subway shield method,the paper discusses the soil deformation and consolidation settlement caused by the influence of construction mechanical behavior on soil pressure or stress and pore water pressure,analyzes the temporal and spatial effect of formation movement.The influences factors of soil movement such as digging of shield cutter,back of shield,grouting material,soil,design axis and deformation of segment were studied.According to the flat and vertical section design and engineering geological conditions of the north extension line of Paris Metro Line 14,considering the characteristics of metro shield construction,the ground subsidence monitoring system is designed.The design principle,monitoring content,point arrangement,monitoring data acquisition and feedback are studied.Utilizing total station instrument,prism,inclinometer and settlement instrument to monitor the buildings,roads,and soil internal displacement are studied.Its monitoring purpose,monitoring method,monitoring frequency and alarm threshold are discussed.Based on Plaxis 2D software,using the ideal elastic-plastic constitutive model and Morh-Coulomb yield criterion,considering the tunnel structure,geological condition,segment,grouting and pressure of the earth chamber,the Plaxis 2D software is applied to the north extension line of Paris Metro Line 14 to simulate the construction of tunnel.The deformation of soil around the tunnel is analyzed,and the comparison between the software simulation results and the measured data of the surface monitoring system shows that there is not much difference between them.It is verified that the finite element numerical analysis software is effective in engineering,it is feasible and reliable.As long as a reasonable analysis model and calculation parameters are selected,the finite element method can provide reference for the design and construction of shield engineering.It is an effective analysis tool,and is more convenient,quick,flexible,economical and accurate than the traditional experiment and model test.The surface subsidence data of fine silt,sand-clay and pebble clay are analyzed.The settlement curve of horizontal and vertical ground surface is studied by comparing the settlement curve of sedimentation trough and measured settlement curve simulated by software.It is found that the settlement of the ground surface in the fine silt layer is larger than that of the other two,and the settlement in the pebble clay layer is the smallest.The surface subsidence in a single soil layer is more moderate,while in the composite strata there will be fluctuant,so it should be more closely monitored in the process of composite ground surface excavation,combined with monitoring data,reasonable adjustment of construction parameters to minimize the land subsidence,to ensure the safety of tunnel construction.According to the study of the settlement mechanism and law,the control methods such as the pressure of the fore cabin,the speed of propulsion,the simultaneous grouting,the amount of the screw conveyor,the secondary grouting,the use of modifier and shield correction are put forward to control the surface settlement.And this study provides some references for the future shield project.