Numerical analysis of the interface shear transfer mechanism of a single pile to tunnelling in weathered residual soil

Three-dimensional (3D) numerical analyses have been carried out to study the behaviour of a single pile to adjacent tunnelling in the lateral direction of the pile. The numerical analyses have included comparisons between the current study, previous elastic solutions and advanced 3D elasto-plastic analyses. In the numerical analyses, the interaction between the tunnel, the pile and the soil next to the pile has been analysed. The study includes the axial force distributions on the pile, the relative shear displacement between the pile and the soil, the shear stresses at the soil next to the pile and the pile settlement. In particular, the shear stress transfer mechanism along the pile related to tunnel advancement has been analysed by using interface elements allowing soil slip. It has been found that existing solutions may not accurately estimate the pile behaviour since several key issues are not included. Due to changes in the relative shear displacement between the pile and the soil next to the pile with tunnel advancement, the shear stresses and axial force distributions along the pile change drastically. Downward shear stress develops at the upper part of the pile, while upward shear stress is mobilised at the lower part of the pile, resulting in a compressive force on the pile. A maximum compressive force of about 0.25–0.52Pa was developed on the pile, solely due to tunnelling, depending on the pile tip locations relative to the tunnel position, where Pa is the service pile loading prior to tunnelling. The majority of the axial force on the pile developed within ±2D in the transverse direction (behind and ahead of piles) relative to the pile position, where D is the tunnel diameter. In addition, mobilisation of shear strength at the pile–soil interface was found to be a key factor governing pile–soil–tunnelling interaction. The reduction of apparent allowable pile capacity due to tunnelling was dependent on the pile location relative to the tunnel position. Some insights into the pile behaviour in tunnelling obtained from the numerical analyses will be reported and discussed.