The Loess Stratum Under The Subway Shield Tunnel Excavation Face Stability Study

With the continuous development of underground space,the shield tunnel construction technology has been more and more widely used,in which earth pressure balanced shield tunneling technology,with its safe,efficient,environmental protection and the advantages of less impact on the environment and is widely applied in tunnel engineering.Shield construction process,due to the constant change of the soil conditions and tunneling parameters,makes the actual tunneling shield machine in the process of supporting force are not completely meet the requirements of excavation surface balance.So as to ensure the excavation surface stability and the influence of construction on the surrounding soil is small,need to study the change of the supporting force as well as the influence on the stability of excavation face of stratum,the influence of ground surface deformation,guarantee the stability of excavation face to define the limits of supporting force,and provide reference for practical engineering.In this paper,based on xi 'an metro project,the shield tunnel excavation face by finite element software ABAQUS support simulation,the supporting force is insufficient and the big state of surface instability caused by excavation and the characteristics of soil deformation calculation and summarize,the main factors which influence the stability of excavation face such as tunnel buried depth,tunnel diameter,formation,sensitivity analysis,the conclusion is given.In view of the above problem,this paper mainly to the following research work:(1)Expounds the soil pressure of shield construction method of main technical links,It is determined that the most important technical link to the stability of excavation surface is the control of excavation surface support.Shield construction is expounded main influence on the surrounding environment,the results of numericalsimulation in this paper,the output is mainly for the excavation surface and the horizontal and vertical displacement,plastic zone of surrounding soils and strata,the surface deflection.Expounds the setting principles of supporting pressure,without considering resistance and water pressure,only consider the excavation surface soil behind the calculation formula of soil pressure under the condition of pressure.(2)Compared with the commonly used soil constitutive model,choose more commonly used and reasonable Mohr-Coulomb strength criterion,design a reasonable excavation face stability simulation program,the in-situ stress balance,excavation area stress release,excavation of soil and assembled segment,reduce the pressure of the excavation support four step analysis.(3)By using the finite element software ABAQUS,based on xi 'an metro line 6 a phase of the geotechnical investigation report and select reasonable formation and segment parameters,establish 3 d tunnel model of loess stratum under simulated respectively when the support pressure is insufficient and big,The pattern of instability of excavation,the stress and deformation of surrounding soil,the plastic zone development regularity and strata,the surface subsidence rule.And the excavation surface is given minimum limit support pressure determination standard.(4)Selection under the condition of loess stratum,influential factors on the stability of excavation face of tunnel buried depth,diameter,the soil,the impact analysis,the limit support pressure along with the variation law of above parameters.And study the limit support pressure,the excavation surface horizontal displacement,excavation surface plastic zone,formation and the surface subsidence with the variation law of above parameters.