Studies On Land Subsidence, Seepage Field And Consolidation Behavior Of Soft Soil Around A Shallow Circular Tunnel

During the process of tunnel construction, regardless of the tunnel depth, tunnel excavation causes soil convergence around the tunnel face and results in short-term surface settlement; meanwhile, soil disturbance and the resulting new boundary conditions lead to re-distribution of stress field and seepage field, and consolidation and creep of the soft soil, which bring about long-term subsidence. Excessive subsidence caused by tunnel construction may affect the safety and normal use of the surface structures, foundations and adjacent pipelines, etc. Therefore, how to predict and control the tunnel settlement becomes an important engineering issue. With the accelerated urbanization process, a large number of tunnels are built and put into use; however, the relevant theoretical study lags far behind the booming engineering practice. To give technique supports for the tunnel construction, the seepage field, consolidation behavior and land subsidence of the soil around shallow tunnel are systematically studied using both analytical and numerical methods in this dissertation. The main original work includes:1. The general ground deformation pattern suggested by Sagaseta, which can consider uniform convergence, pure distortion and vertical translation of tunnel individually, is incorporated into Airy stress function as the complex boundary condition of the displacement around the tunnel section. An elastic solution for the prediction of the tunneling-induced ground deformations for shallow tunnels in the soft ground is derived, and the simplified solution for normal consolidated soil is proposed. The influences of uniform convergence, pure distortion and vertical translation on land subsidence are analyzed. The comparisons of the solutions with the data observed in four different tunnels are made and the reasonable values of the parameters in the solution are investigated. Finally, three deformation types for different soils are suggested for practical purpose.2. A general ground deformation pattern is assumed as the displacement boundary condition around the tunnel section, and the elastic half plane with a hole is mapped conformably onto a ring, a solution for complex boundary condition of the displacement around a circular tunnel is deduced by the complex variable method. The results given by the solution are compared with the ones given by FEM and the data observed. Differences between the complex variable method and Airy stress function method are discussed.3. An analytical solution is derived for steady flow into a lined tunnel in a semi-infinite aquifer firstly by assuming the soil and lining are saturated, homogeneous and isotropic media. Using the conformal mapping, the flow in the soil region, as well as in the lining region, can be described by Laplace equation in a ring domain. Water inflow volume and hydraulic head along the tunnel circumference are solved by Fourier method for the Dirichlet problem. The influences of the ratio of radius to depth of tunnel, the hydraulic conductivity of both soil and lining and the internal water pressure on the water inflow volume and hydraulic head along the tunnel circumference are discussed and compared with numerical stimulation.4. Based on Burges viscoelastic constitutive model and Terzaghi-Rendulic consolidation theory, the governing equation about excess pore pressure is established with drainage and no-drainage boundary. An analytical solution for the excess pore pressure of tunnel in viscoelastic saturated soil is obtained. By this solution, the excess pore pressure of any position can be determined at any time in the considered region.5. Using the FEM code-Abaqus, a part of Shanghai metro line NO.2 shield tunnel is simulated to investigate the behavior of long-term ground subsidence. Burgers viseoelastic constitutive model fitted by prony series is implemented in the code coupled with Biot's consolidation theory, and the multi-layered characteristics of soil are also taken into account. A multiple-step numerical procedure is designed to simulate the mechanical behavior of shield tunnel in different period during the long-term duration. Finally, comparison with the observed data shows that the long-term response of shield tunnel can be predicted by such numerically simulation.