EFFECT OF DRAINAGE CONDITIONS ON PORE WATER PRESSURE DISTRIBUTION AND LINING STRESSES IN TUNNELS UNDER GROUNDWATER TABLE

Tunnelling in a saturated soil often produces a long-term interaction between the tunnel lining and the surrounding soil. Inltration and external pore-water pressures are often important factors that must be considered in the lining design. Development of pore water pressure may accelerate leakage and cause deterioration of the lining. This can be particularly troublesome to structural and functional components of the tunnel and can often lead to structural failure. In this paper, an axi-symmetric circular tunnel under the groundwater table is modeled using nite dierence software, and dierent conditions, such as an impermeable tunnel, a permeable tunnel and a lining drainage system with varying permeability are investigated. The structural behaviour of the lining and ow through the tunnel lining were modelled using a combination of beam and solid elements. The eects of tunnel parameters, such as depth and diameter and variation of underground water levels, on the eectiveness of the lining drainage system are examined. Moreover, the deterioration of a drainage system caused by clogging is investigated and the importance of giving careful consideration to lining permeability and hydraulic boundary conditions is highlighted. As a result of coupled analysis, a lining drainage system in a shallow tunnel with small diameter is more ecient, and eective drainage conditions reduce pore water pressure loads, which decreases the bending moments and tensile stresses of the tunnel lining. By performing a parametric study for a tunnel with dierent lining permeabilities, a design curve to evaluate pore water pressure loads on the lining has been proposed. It is shown that the hydraulic deterioration of the drainage system signicantly develops a magnitude of pore water pressure load on the tunnel lining.