Numerical investigation of tunnel excavation effects on single pile

In this article, the effects of tunnel excavation on the behavior of preloaded piles above the tunnel are examined. The analyses were conducted using the finite difference method (FDM) with the FLAC 3D software. A 20-meter pile located one meter above an 8-meter diameter tunnel was studied. Several important parameters, such as ground surface settlement in a free-field condition (without the presence of the pile), pile settlement, variations in axial force along the pile, and the mechanism of shear stress transfer at the pile-soil interface due to tunnel excavation, were investigated. The relative displacement created between the pile and the soil at the pile-soil interface due to tunnel advancement leads to changes in the distribution of axial forces and shear stresses at the interface. As the tunnel advances, the axial force along the pile decreases, and the shear stresses at the interface become active in most of the pile length, preventing further settlement of the pile. It has been determined that existing solutions may not accurately estimate the behavior of the pile, as several key issues have not been included. Due to the change in relative shear displacement between the pile and the soil alongside the pile with tunnel advancement, shear stresses and the distribution of axial forces along the pile change significantly. Downward shear stress is generated at the upper part of the pile, while upward shear stress is mobilized at the lower part of the pile, resulting in compressive forces acting on the pile. Most of the axial force on the pile in the transverse direction (behind and in front of the piles) has developed within ± 2D, where D is the tunnel diameter. Additionally, the mobilization of shear resistance at the pile-soil interface has been identified as a key factor governing the pile-soil-tunnel interaction. The reduction in the apparent allowable capacity of the pile due to tunnel excavation depends on the position of the pile relative to the tunnel position.