Numerical study on 3D effect and practical design in shield tunneling

Abstract

In practice, different design methods are used in solving geotechnical problems depending on the type of issue such as the tunneling, braced excavation, or bearing capacity of a foundation, that is, the basic mechanism of the design method differs depending on the problems even for the same ground. A numerical analysis using the finite element method has recently become familiar owing to an improved computing performance; however, it is not widely used in the design of geotechnical problems including tunnel excavation owing to the reliability of the constitutive model of the ground material. If a constitutive model of soils can properly express the properties of the ground material, a numerical analysis will play a vital role in solving the geotechnical problems. In this paper, the current state of a numerical analysis and its applicability in tunnel design are discussed. Herein, the simulation of the ground behavior during tunnel excavation is carried out using sandy and clay ground parameters for shallow and deep tunnel excavations. This paper is mainly focused on the effects of tunnel excavation under three-dimensional (3D) conditions, as well as the current design method. Non-linear 2D and 3D finite element analyses have been conducted, in which the elastoplastic sub-loading tij model has been used as a constitutive model of the soil. The performance and acceptability of the constitutive model have already been proven to reproduce the results of various model tests on different geotechnical problems such as the tunneling, braced excavation, and bearing capacity of a foundation, as well as the measured field data. It was found that a 2D finite element analysis where the rate of stress release is considered, can be used for the prediction of the ground deformation and surface settlement; however, it does not provide rational information in the prediction of tunnel lining forces such as the stress, bending moment, and axial force, which emphasize the necessity of a 3D analysis with a proper construction process in a tunnel design.