Three-dimensional numerical analyses of the response of a single pile and pile groups to tunnelling in weak weathered rock

Abstract

The effects of tunnelling in weak weathered rock on the behaviour of a pre-existing single pile and pile groups (3×3 and 5×5 pile groups) in weathered residual soil above a tunnel were studied by performing three-dimensional (3D) elasto-plastic numerical analyses. Due to changes in the relative shear displacement between the pile and the soil at the pile–soil interface resulting from the ground deformation occurring with tunnel advancement, the shear stresses and axial pile force distributions change drastically. Upward shear stresses are induced throughout most of the length of the pile except near the tip of the pile, where downward shear stresses are mobilised, resulting in a reduction in the axial pile force distribution with pile depth equivalent to a net increase in the tensile force on the pile. In doing so, the shear transfer mechanism at the pile–soil interface associated with the tunnelling has been fully investigated. A maximum tensile force of approximately 0.20Pa developed on the single pile solely due to tunnelling based on the concrete analysis conditions where Pa is the service axial pile loading prior to tunnelling. Overall, a larger number of piles causes a greater effect of tunnelling on the piles in terms of pile settlement, whereas changes in the axial pile forces for piles in groups causes a smaller effect than for a single pile due to the shielding effect. Empirical methods that estimate pile settlements need to be modified. More deformations and stresses are induced on the tunnelling lining when there is a pile group above a tunnel than when there is a single pile. The reduction of the apparent allowable pile capacity due to tunnelling-induced pile head settlement was significant, in particular for grouped piles. Several insights into the pile response to tunnelling obtained from numerical analyses are reported and discussed.