Study On Environmental Effects Of Shield Tunnel Construction In Multi-layered Stratum

The shield construction technology has been widely used in tunnel construction because of its advantages.During the process of shield tunneling,the soil around the tunnel is inevitably disturbed,which is subjected to a series of complex mechanical behaviors,such as loading and unloading,and the changes of soil's stress state produce different degree of additional stress and displacement deformation.When the displacement deformation exceeds a certain limit,it will endanger the safety and normal use of surface building(structure),underground pipeline facilities and underground structures.Due to the multi-layered characteristics of natural stratum in the process of sedimentary,the existence of multi-layered stratum makes more complicated to determine the value and the control standard of displacement deformation caused by shield tunnel construction.At the same time,compared with sand stratum,coastal saturated soft soil stratum,weathered rock stratum and composite stratum,etc.,for shield tunneling through the loess stratum,there is lack of systematic research.Therefore,the distribution characteristics and development regularity of displacement deformation caused by shield tunnel construction in loess multi-layered stratum are studied to ensure the safety of shield tunnel construction and guide shield tunneling in loess stratum,which has a great theoretical value and engineering application value.In this paper,the environmental effects caused by the construction of shield tunnel in the multi-layered stratum is calculated and analyzed;the main research contents include the following aspects:(1)Analyze the intrinsic relation between the Peck empirical prediction formula and the stochastic medium theory,and then determine the calculation method of the parameter value in the stochastic medium theory formula.On the basis of this,the Peck empirical prediction formula is extended to the prediction of displacement deformation caused by shield construction in multi-layered stratum according to the stochastic medium theory.For the circular shield tunnel,the prediction results of the two methods are the same.(2)The maximum allowable value of ground settlement caused by the construction of shield construction in the multi-layered stratum is analyzed according to the two different positions of the foundation of the building(construction)on the side of the settlement trough and the center of the settlement trough.The maximum allowable value of ground settlement caused by shield construction in the multi-layed stratum is discussed by the Peck empirical prediction formula.(3)The analytical element stiffness matrices for the plane strain of total stress problem and the Biot consolidation problem of multi-layered stratum under loads in the Cartesian coordinate system and for the three-dimension of total stress problem and the Biot consolidation problem of multi-layered stratum under the axisymmetric loads and the general loads in the Cartesian coordinate system are deduced by using Fourier integral transformation,Laplace integral transformation,variable transformation,matrix theory and differential equation theory.On the basis of this,the analytical stiffness matrix method is used to solve the plane stress and the three-dimension of the total stress problem and Biot consolidation problem of the multi-layered stratum in the Cartesian coordinate system,which provides a theoretical basis for the calculation and analysis of shield tunneling in the multi-layered stratum.(4)Based on the solution of the multi-layered stratum under loads using the analytical stiffness matrix method,the calculation method of the additional stress caused by the construction disturbance of the shield tunnel in the multi-layered stratum is established.When shield is tunneling in the multi-layered stratum,the additional stress caused by the additional thrust in the front of the tunnel,the driving friction between the shield and the soil,the grouting pressure is analyzed respectively.Based on the analytical stiffness matrix method for the stratigraphic loading problem,the multi-layered stratum model is coupled with the boundary element method,and the elliptical non-equivalent radial soil displacement is used as the boundary condition of the tunnel hole,which reflects the stress release and ground loss caused by shield tunneling,the displacement control boundary method for instantaneous deformation and consolidation deformation of the soil caused by the disturbance of shield tunnel construction in multi-layered stratum is established.(5)Relying on the Xi'an Metro Line 2 Yongning Gate Station ~ Zhongjie station shield tunnel section,the displacement deformation caused by the shield tunnel construction in the loess multi-layered stratum are analyzed by different methods such as site settlement monitoring,Peck empirical prediction formula and displacement control finite element method.