A numerical study of the effect of groundwater movement on long-term tunnel behaviour

Soft ground tunnelling beneath the water table induces pore water pressure changes and alters the hydraulic regime in the ground. There are various factors that control the tunnel/soil interaction behaviour during the equilibration period after tunnel construction through to the long-term steady state. Through the presentation of results from coupled non-linear finite element analyses, this paper considers the differences in predicted ground and lining responses in the long term for different post-tunnelling flow conditions. A 4 m diameter circular tunnel constructed 20 m below ground level is adopted for the geometry, with the soil conditions based on those found in London, UK, and Seoul, South Korea. The lining permeability is varied from fully permeable to impermeable, and an approach to modelling finite lining permeability is presented and assessed. For the permeable lining analyses two flow regimes are modelled, one in which the water table is maintained at its original elevation and one in which the water table is drawn down towards the tunnel. The importance of giving careful consideration to lining permeability and hydraulic boundary conditions is highlighted. The effects of groundwater movements on numerical predictions of tunnel behaviour are also identified.