Following a tunnel excavation in low-permeability soil, it is commonly observed that the ground surface continues to settle and ground loading on the tunnel lining changes, as the pore pressures in the ground approach a new equilibrium condition. The monitored ground response following the tunnelling under St James's Park, London, shows that the mechanism of subsurface deformation is composed of three different zones: swelling, consolidation and rigid body movement. The swelling took place in a confined zone above the tunnel crown, extending vertically to approximately 5 m above it. On the sides of the tunnel, the consolidation of the soil occurred in the zone primarily within the tunnel horizon, from the shoulder to just beneath the invert, and extending laterally to a large offset from the tunnel centreline. Above these swelling and consolidation zones the soil moved downward as a rigid body. In this study, soil–fluid coupled three-dimensional finite element analyses were performed to simulate the mechanism of long-term ground response monitored at St James's Park. An advanced critical state soil model, which can simulate the behaviour of London Clay in both drained and undrained conditions, was adopted for the analyses. The analysis results are discussed and compared with the field monitoring data. It is found that the observed mechanism of long-term subsurface ground and tunnel lining response at St James's Park can be simulated accurately only when stiffness anisotropy, the variation of permeability between different units within the London Clay and non-uniform drainage conditions for the tunnel lining are considered. This has important implications for future prediction of the long-term behaviour of tunnels in clays.

Changes in stresses and pore pressure induced by tunnel excavation in low-permeability soil inevitably lead to further ground displacements following the construction. This can be referred to as consolidation ground displacements, which usually take many years to reach their long-term steady-state values, depending on the permeability and compressibility of the soil. As the number of case histories on both the magnitude and the extent of consolidation ground displacements is limited, there is no empirical formulation available to predict the magnitude and extent of such displacements. This paper describes a suite of finite-element analyses carried out as a parametric study aiming to quantify rationally the extent and magnitude of consolidation settlement for both transient and steady-state long-term conditions. The study focuses only on a tunnel constructed in open-face mode in heavily overconsolidated London Clay. The model parameters considered are tunnel geometry, soil anisotropic permeability, volume loss and tunnel drainage conditions. Based on the results, normalised consolidation settlement-relative soil-lining permeability curves are developed. The applicability and limitations of the curves are presented and discussed using published case histories.

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