Optimal Design And Numerical Simulation Analysis Of Shield Tunneling In Soft Layers

Shield construction technology is widely used in urban rail transit construction and has played a very important role in promoting urban development.However,at present,shield tunneling in soft soil layer seems to have encountered a bottleneck,that is,the situation that the supporting stress and the earth pressure of the excavation face often cannot be balanced,which affects the stability of the excavation face and the surface uplift.In severe cases,the excavation face may become unstable and the surface may collapse.Therefore,only to slow down the construction speed to ensure the safety of construction.As a result,the construction efficiency has always been in a relatively low state,which has seriously delayed the development of urban traffic.Therefore,it is of great engineering significance to study the influence of the change of earth chamber pressure on the stability of excavation face and soil disturbance,and to optimize the design of shield tunneling in soft strata,so as to improve the construction safety and speed up the shield tunneling speed and improve the tunneling efficiency of earth pressure balance shield tunneling.Based on the background of a subway tunnel project in Fuzhou,this paper uses numerical simulation method to compare the monitoring results of ground surface subsidence,theoretically analyzes the causes of ground surface subsidence,and discusses the relevant shield parameters that affect the excavation progress.On the basis of the existing structure of earth pressure balance shield machine,an optimal design scheme of shield cutter head suitable for excavation in soft soil is put forward in order to improve the speed of shield tunneling.The main work and research results are as follows:(1)Through literature of research,the working principle of the earth pressure balance shield machine and the main construction technology links are discussed,and the law of surface settlement caused by shield tunneling construction and several methods currently studied are summarized.The relationship between the advancing speed of the earth pressure balance shield and the pressure of the soil bin is analyzed,and it is further clarified that the key technology of shield construction to control the stability of the excavation face and the speed of the excavation is the control of the pressure of the soil bin and the parameter setting of the support stress ratio.(2)A three-dimensional numerical simulation of the tunneling process of the shield in soft soil is carried out by Midas/GTS NX finite element software.The result shows that when the support stress ratio is less than 1.0 or greater than 1.0,the influences of earth pressure balance shield tunneling on surface settlement,instability and failure of excavation face and stratum disturbance range have been studied.The support stress ratio range and the limit support ratio to keep the excavation face stable are analyzed and determined.The results show that when the support stress ratio is in the range of 0.6 to 1.5,the surface settlement above the excavation face and the surface settlement behind the shield tail are both small,and the change in the ground settlement is not large.When the support stress ratio exceeds this range,the variation range of ground settlement increases significantly.Finally,according to the deformation laws of the soil and the surface,the minimum limit support stress ratio is determined to be 0.1,and the maximum limit support stress ratio is 2.0.(3)Based on the cutter head machine of current earth pressure balance shield,an optimized design scheme of shield cutter head suitable for tunneling in weak soil is proposed,that is,an arch cutter is added to the cutter head machine.Through numerical simulation,the magnitude of the surface settlements before and after the optimization of the cutter head and the change of the limit support stress ratio are analyzed under the same construction conditions and tunneling parameters.The results show that under the condition of the same supporting stress ratio,the law of surface settlement after the installing of arch cutters is basically the same as that without arch cutters.However,after the arch cutter is installed,the ground settlement and the range of soil disturbance are significantly decreased,and the effect of decreasing with the increase of the arch cutter is more obvious.And the longitudinal horizontal displacement of the soil on the excavation face with a support ratio less than 1.0 is decreased by more than 70%,and the longitudinal horizontal displacement of the soil on the excavation surface with a support ratio greater than1.0 is decreased by nearly 60%.When the 1.5m arch cutter is added,the range of the supporting stress ratio to maintain the stability of the excavation face is expanded from 0.6 to 1.5 to 0.4 to 2.0.(4)Combined with a certain section of the Fuzhou Metro tunnel project,based on shield tunneling related parameters and surface settlement monitoring data,the relationship between the excavation face support stress ratio and surface settlement is analyzed.It is found that the change in soil pressure was very sensitive to the impact of surface settlement.Comparing the measured surface subsidence data with the simulated data and dividing the development process of the five sections of longitudinal surface subsidence,it is found that the numerical simulation results are basically consistent with the actual monitoring settlement curve law.This conclusion verifies the rationality of the numerical simulation results,and also fully demonstrates that the use of optimized cutter heads has a positive impact on ground settlement,which creates conditions for the pursuit of high-efficiency shield construction efficiency.