The influence of soil anisotropy and K₀ on ground surface movements resulting from tunnel excavation

Finite element (FE) analysis is now often used in engineering practice to model tunnel-induced ground surface settlement. For initial stress regimes with a high coefficient of lateral earth pressure at rest, K₀, it has been shown by several studies that the transverse settlement trough predicted by two-dimensional (2D) FE analysis is too wide when compared with field data. It has been suggested that 3D effects and/or soil anisotropy could account for this discrepancy. This paper presents a suite of both 2D and 3D FE analyses of tunnel construction in London Clay. Both isotropic and anisotropic non-linear elastic pre-yield models are employed, and it is shown that, even for a high degree of soil anisotropy, the transverse settlement trough remains too shallow. By comparing longitudinal settlement profiles obtained from 3D analyses with field data it is demonstrated that the longitudinal trough extends too far in the longitudinal direction, and that consequently it is difficult to establish steady-state settlement conditions behind the tunnel face. Steady-state conditions were achieved only when applying an unrealistically high degree of anisotropy combined with a low-K₀ regime, leading to an unrealistically high volume loss.