Analysis On Unstable Failure Of End Soil Of Shield Tunnel In Loose Weak Stratum

In recent years,with the vigorous development of different forms of tunnels such as transportation pipelines,subways,and urban underground pipe corridors,the shield construction method has been widely recognized and applied by its advantages of safety,high efficiency,and ability to cope with complex geology.The launching and arrival of the shield is an important part of the construction of the shield tunnel.During this process,the instability of the end soil is an important source of risk in the construction of the shield.In the loose and weak strata,the unstable failure is more likely to occur.In this paper,we study the instability and failure of end soil in loose soft soil,and use Particle Flow(PFC)software as the main means of numerical simulation.The software can not only reflect the true deformation of the bulk material,but also analyze the meso-mechanism of the model during the failure.At the same time,the indoor triaxial test was used to determine the mesoscopic parameters of the model.The numerical models for the end of sand and clay are established respectively,combined with the indoor model tests,the influencing factors of the instability and failure of the soil are analyzed.The main research contents of this paper are as follows:(1)Using PFC2D to establish a biaxial compression numerical model,the type of wall restraint was improved,and a flexible wall formed by sticky ball was used in the model.The test proved that the flexible wall not only brings the boundary conditions closer to the actual test,but also eliminates the need for traditional wall servos and saves computing time.(2)Through the indoor triaxial test,the physical and mechanical properties of the loose sand and soft clay were obtained respectively,and with the aid of the PFC biaxial test,the effect of the model parameters on the macroscopic properties of the simulated material was analyzed.In the sand-soil biaxial model,when the horizontal pressure is small(less than 150 kPa),the damage peak has a positive correlation with the friction coefficient of the particles,and when the friction coefficient is greater(greater than 0.3),the impact on the strength is significantly enhanced.The friction coefficient and cohesion in the model have an approximately linear relationship with the friction coefficient of the particles and the bond strength.(3)A PFC numerical model was established to analyze the process of instability and failure of the end soil through numerical simulations in both macroscopic and microscopic terms.Finally,it was found that the instability of the end soil in the sand layer is of a staged nature.Each stage is divided into three stages:"looseness-arching-collapse";The destruction rate in the clay strata is slow,the destruction surface consists of an approximately vertical straight line and a smooth curve,and the surface displacement is step-like.(4)An indoor similarity model experiment was designed and test materials were prepared based on similar experimental principles.The shape of the failure surface of the end soil was obtained through image acquisition and analysis,and the results were compared with the results of numerical simulation.(5)The PFC numerical model was used to analyze the factors influencing the instability of the end soil.With the increase of c and(p,the stability of the end earth was enhanced.When the depth of the tunnel is shallow(17.4m to 23.4m),the tunnel diameter has the greatest effect on the stability of the end soil.The larger the tunnel diameter is,the greater the possibility of tunnel instability will be.In clay formations,the influence of tunnel burial depth is greater than the effect of soil cohesion.In sandy soil layers,the burial depth has little effect on the stability.(6)The PFC numerical model was used to analyze the factors influencing the instability of the end soil.With the increase of c and φ,the stability of the end earth was enhanced.When the depth of the tunnel is shallow(17.4m to 23.4m),the tunnel diameter has the greatest effect on the stability of the end soil.The larger the tunnel diameter is,the greater the possibility of tunnel instability will be.In clay formations,the influence of tunnel burial depth is greater than the effect of soil cohesion.In sandy soil layers,the burial depth has little effect on the stability.