Effects of Advancing Open Face Tunneling on an Existing Loaded Pile

Abstract

A three-dimensional, elasto-plastic, coupled-consolidation numerical analysis was conducted to investigate the effects of an advancing open face tunnel excavation on an existing, loaded pile. Based on the ground conditions, geometry, and tunnel construction method simulated, a significant zone of influence can be identified one tunnel diameter ahead and one behind the tunnel excavation face. Within this zone of influence, the settlement of the pile is greater than the ground settlement due to the plastic yielding at the pile toe. A computed ground surface settlement profile simulated by a normal Gaussian distribution does not accurately reflect pile head settlement and a significant nonlinear distribution of subsurface settlement with depth is induced by tunnel excavation. Under plane strain conditions, other empirical and analytical methods predicted similar trends in the subsurface settlements, but gave larger magnitudes of subsurface settlements. The open face tunnel excavation induces complex distributions of relative subsurface settlements and both positive and negative side shear stresses along the pile. Within the zone of influence of the tunnel excavation, excess positive and negative pore-water pressures are generated at the pile head and pile toe, respectively. At the pile toe, the pore-water pressure was reduced to 40% of its initial hydrostatic value. At 10 days after the tunnel face passed the pile, the excess pore-water pressures at the pile recovered to within 20% of initial hydrostatic values. Due to the additional settlement of the pile that occurred during tunneling, the soil resistance factor of safety for the pile can be regarded as decreasing from 3.0 to 1.5, according to a displacement-based failure load criterion. The tunnel excavation did not significantly affect the existing bending moment and the axial structural load distribution within the pile.