The Study On Stability Of Excavation Face For Shield Tunnel Under Seepage

With the acceleration of China's urbanization process,traffic congestion caused by the dramatic increase in urban population has become a common phenomenon in major cities during peak travel times.As one of the main modes of transportation for large and medium-sized cities,the underground railway has become the first choice for solving urban traffic problems in China because it does not occupy the surface space and takes into account the advantages of safety,efficiency and environmental protection.Most of the subway tunnels are constructed by the shield method with small impact on the surrounding environment for the foundation and pipelines in the urban underground space are densely covered,the engineering geological and hydrogeological conditions are complex,and the stability of the excavation face during the construction of the subway is high.In addition,the seepage effect of groundwater is an important factor affecting the stability of the shield tunnel face.Therefore,based on Hoek-Brown nonlinear failure criterion and limit analysis upper bound theorem,this paper considers the groundwater seepage and shield tunnel in front of the excavation face.The deformation mode of the soil and the failure mode under the limit state are studied to provide a theoretical basis for the stability analysis and safety assessment of the shield tunnel face.The main research contents of this paper are as follows:(1)According to the characteristics of insufficient and excessively large earth chamber pressure during the construction of shield tunnel,the soil in front of the excavation face is characterized by two kinds of deformations:slumping and extruding.By this,two upper limit failure mechanisms of the soil in front of the excavation face is constructed by the curve of any line type.(2)Based on the above two failure mechanisms,combined with the Hoek-Brown nonlinear failure criterion and the upper bound theorem of the limit analysis,the target of the failure surface curve is obtained by calculating the external force power and the internal energy dissipation power in the failure mechanism on the basis of considering the groundwater seepage.Then,using the variational method to calculate the analytical equation of the soil failure surface in front of the excavation face considering the seepage of groundwater;(3)By the calculated analytical equation of the soil failure surface in front of the excavation face of the shield tunnel,the shape and failure extent of the failure surface of the soil in front of the excavation face are drawn.On the basis of this,conclude the law of the parameters by comparing the scope of damage in the soil front of the excavation face;(4)Based on the measured data of Changsha Metro Line 2,a three-dimensional model of shield tunneling under consideration of seepage is established by using finite difference software FLAC3D.Through the excavation process of the simulated shield tunnel,the failure mode of the soil in front of the excavation face is obtained.The numerical solution and the theoretical solution of the soil failure surface in front of the tunnel face are compared and analyzed to verify the correctness of the theoretical calculation.