Tunnelling-induced consolidation settlements in London Clay

Abstract

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. 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.