Subsurface settlements of shield tunneling predicted by 2D and 3D constitutive models considering non-coaxiality and soil anisotropy: a case study

Appropriate constitutive models and reliable excavation and support sequences are believed to be the major concern in using Finite element (FE) analysis to simulate shield tunnel excavation. This paper presents systematic 2D and 3D FE analyses employing a number of constitutive models accounting for initial soil anisotropy and non-coaxial plasticity, as evidenced within site investigations from the Tsinghuayuan Tunnel of the Jing-Zhang High-Speed Railway in China. The aim is to assess the effects of both the initial soil anisotropy and non-coaxiality on longitudinal and transverse tunneling-induced surface settlements. It is shown that the excavation procedures combined with the degree of cross-anisotropy are key towards the accurate prediction of maximum vertical displacements from tunneling, matching field data. Knowledge of the initial soil strength anisotropy can further improve the shape prediction of the transverse tunneling-induced surface settlement troughs. When considering n = 0.6 and "β =" 〖" 0" 〗^"°" in simulations, the transverse surface settlement trough obtained is almost coincided with monitored field data. Initial stiffness anisotropy used in the prediction of shield tunnel-induced surface settlements in sandy pebble soils, does improve realism of results significantly. The maximum longitudinal settlement predicted by considering cross-anisotropy is larger than that predicted by its isotropic counterpart.