Research On The Instability Mechanism And Surface Subsidence Control Of The Excavation Surface Of The Composite Stratum Subway Shield Construction

With the intensification of urbanization,the urban density is getting higher and higher,the problem of road traffic congestion is prominent,and the ground traffic resources are saturated.In order to seek more efficient and safe travel methods,the development and utilization of urban subway has become an important means of social sustainable development.Shield tunneling has been widely used in the construction of urban subway tunnels due to its unique advantages.With the expansion of the construction scope,shield tunneling method is also faced with many problems and challenges such as diverse geological conditions,diverse structural sections and complex construction environment.The surface settlement and collapse accident caused by the instability of the excavation face is a common problem faced by shield construction.The geological structure of Jinan area is complex and changeable,with many composite strata,and the unevenness of soft and hard is prominent.Safety accidents are more likely to occur when the metro shield tunnel passes through the compound stratum with great difference of geological conditions and frequent soil changes.Therefore,the study on the excavation surface stability of shield tunnel in composite stratum has gradually become one of the core problems in shield tunnel construction.Relying on the Jinan Metro Line 2 project,this thesis uses theoretical analysis and numerical simulation as a means to focus on the instability mechanism of the excavation surface of the metro shield tunnel in composite stratum and the control of surface settlement,research in compound formation of shield tunnel construction of excavation surface buckling failure mode,and analysis of various factors on the excavation face stability and the influence law of the surrounding environment,Propose control measures to ensure the stability of the excavation surface and reduce ground settlement.The main research work and achievements are as follows:(1)Research on the failure mode of shield tunnel excavation surface instability for composite stratum,and a logarithmic helical failure model was proposed,which was in line with the characteristics of composite stratum conditions.The theoretical calculation formula of the limit supporting force of the excavation face was derived based on the upper bound legal principle of limit analysis,and the optimal solution was obtained by using MATLAB software programming.Analyze the influence of tunnel depth,internal friction angle and cohesion on the ultimate supporting force.The correctness of the failure model is verified by comparing with the results obtained by previous scholars.(2)Taking Jinan Metro as the engineering background,the three-dimensional tunnel model of composite stratum was established by ABAQUS software.Considering the construction process of soil disturbance,segment assembly and grouting hardening,the whole process of shield tunneling was simulated.The deformation of the excavation face,the limit supporting force,the development of the plastic zone of the soil and the law of surface settlement of the excavation face are studied.On this basis,the factors that affect the stability of the excavation surface of the composite stratum,such as the tunnel diameter,the buried depth of the tunnel,the friction angle and the cohesive force of the soil body,are analyzed.Obtain the influence law of the above parameters on the soil deformation and the limit support force of the excavation surface,and the influence law of the above various parameters on the ground settlement under the action of the limit support force.(3)A more in-depth analysis of the factors affecting surface settlement is carried out,and the influence of grouting pressure,grouting material elastic modulus and soil disturbance around the tunnel face on surface settlement is analyzed.According to the results of theoretical analysis and numerical simulation,the control measures to ensure the stability of the excavation face of shield tunnel in composite stratum and to reduce the ground subsidence are put forward.