Analysis Of Effects Of Tunneling On Piled Foundations Above The Tunnel

In order to relieve the congestion of ground transportation in the city,large amount of urban subway tunnels are put into construction in China. Because of geological conditions and other factors limit, sometimes the subway tunnel is inevitably beneath the existing pile foundation. Tunnel excavation causes the soil movement around the tunnel, which have adverse effects on the pile foundation above the tunnel and superstructure.In this paper, FLAC3D finite difference program and three-dimensional elasto-plastic numerical analyses are applied to simulate the effect of tunnelling by short bench cut method in the rock on the behaviour of a existing single pile and pile groups above the tunnel. Considering nonlinear of soil and interaction of soil and piles, tunnelling-induced soil deformation, liner deformation and stress, pile settlement and lateral displacement, the relative shear displacement between the pile and soil, pile axial force, pile shaft resistance, the shear transfer mechanism at the pile-soil interface and pile bending moment are computed to analyze in different working conditions. The computed results are compared with that from empirical methods and available papers, and good agreement is obtained. The conclusions drawn from computed results are showed as following:(1) The zone of influence on the pile head displacement in the longitudinal direction of tunnel due to the tunneling can be identified as ±2D(D is the tunnel diameter) from the pile centre, and the pile head displacement increases most significantly when the tunnel face is passing the pile axis in the longitudinal direction.(2) The zone of influence on the relative shear displacement between the pile and soil, pile axial force, pile side friction and pile bending moment in the longitudinal direction of tunnel due to the tunneling can be identified as ±0.625D from the pile centre.(3) A larger number of piles causes a greater effect of tunnelling on the pile settlement, whereas tunnelling-induced changes in the axial pile forces and the relative shear displacement between the pile and soil for pile groups are smaller than that for a single pile due to the shielding effect.(4)The apparent allowable pile capacity decreases significantly due to tunneling, in particular for pile groups.(5) With the tunnel excavation, pile axial force reduces and positive shaft resistance along most of the pile shaft increases constantly, however, downward negative shaft resistance is induced around the pile tip, resulting in an increase in the pile tensile force. The conclusions obtained from this paper provide some guidance for subsequent similar projects.