Abstract:

Tunnel design in urban areas must account for overburden loads as well as additional loads imposed by future surface infrastructures, called the embankment load. In saturated ground, the embankment load will generate not only mechanical response through stress and displacement changes, but also hydraulic response through pore pressure changes. Because this hydro-mechanical (H-M) response will be transferred to the tunnel liner and because the liner deforms with the ground, the embankment load will correspondingly influence the liner's stability. This paper examines how severely the induced H-M response from the embankment load will affect the integrity of a horseshoe-shaped (NATM) tunnel in saturated ground. Two tunnel models are created in the computer code FLAC. One model is under weak and low-permeability ground (i.e., clay), and the other one is under relatively strong and high-permeability ground (i.e., granite). Each tunnel model is run under different liner drainage conditions: permeable and impermeable. The embankment load is then applied 10 years after tunnel construction. Simulation results suggest that impermeable tunnels will experience the most severe bending moment from the embankment load, particularly for tunnels in clay. Further, the severity does not stop shortly after the load is applied, but continues towards the steady-state.

Introduction

One of the geotechnical issues associated with tunneling in urban areas is excavation-induced deformation which, if not carefully controlled, may cause damage to existing surface structures. While this effect of tunneling has been a major environmental issue and widely studied for decades, research on the reverse situation – the influence of surface structures on an existing tunnel – has been limited. Nevertheless, the extra deadweight from future surface infrastructures, called the embankment load, should be addressed for tunnel design because it can affect the structural integrity of the tunnel.

In dry ground, the embankment load will change the stress and displacement fields of the ground beneath the structure until equilibrium is reached. In urban tunneling, where the overburden is shallow, changes in the stress and displacement in the ground will easily generate additional ground pressure and structural forces on and in the liner, respectively, i.e., increasing the liner axial forces and bending moment (Rezaei et al., 2013; Katebi et al., 2015). Further, embankment loads have also been seen to widen settlement troughs, such as those found above the metro tunnels in London and Frankfurt (Potts & Addenbrooke, 1997).

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