Analysis of Soil-Compacting Effect Caused by Shield Tunneling Using Three-Dimensional Elastoplastic Solution of Cylindrical Cavity Expansion

Soil squeezing effect and formation disturbance caused by tunnel excavation can be simulated by cylindrical cavity expansion due to the comparability between tunneling and cavity expansion. Although most of the existing theoretical derivation is based on simple constitutive model of soil foundation, not only the relation between principal stress components was simplified in the solution process, but also the stress history, initial stress anisotropy, and stress-induced anisotropy of structural soil were neglected. The mechanical characteristics of soil are closely related to its stress history, so there is a gap between the above research and the actual engineering conditions. A three-dimensional elastoplastic solution of cylindrical cavity expansion is obtained based on the theory of critical state soil mechanics and engineering characteristics of shield tunneling. In order to fully consider the influence of initial anisotropy and induced anisotropy on the mechanical behavior of soils, the soil elastoplastic constitutive relation of cavity expansion is described in the course of -based modified Cam-clay ( -MCC) model after soil yielding. An equation with equal number of variables is obtained under the elastic-plastic boundary condition based on the Lagrange multiplier method. By solving the extreme value of the original function, the analytical solution of radial, tangential, and vertical effective stresses distribution around the circular tunnel excavation is obtained. In addition, changes of elastic deformation area and plastic deformation area for soil during the shield excavation have been analyzed. Calculation results are compared with the numerical solutions which usually consider isotropic soil behavior as the basic assumption. In this paper, a constitutive model which is more consistent with the actual mechanical behavior of the soil and the construction process of the shield tunnel is considered. Therefore, the numerical solutions are more realistic and suitable for the shield excavation analysis and can provide theoretical guidance required for design of shield tunneling.