Long-term behaviour of ground around tunnel due to groundwater level fluctuations

The soft clayey soil around fully impermeable segmented tunnels can swell if groundwater table gradually rises in several years. It is analogous to the consolidation of cohesive soil under the influence of gradual groundwater drawdown. Under the gradual rising of groundwater, effective stress in saturated cohesive soils decreases by an increase in porewater pressure and volumetric strain of soil elements increase. Also, the degree of saturation of soils increases and suction forces disappear in unsaturated soils. The new saturated soils may continue to swell until the end of the secondary swelling stage. Because of these events, the loads on the tunnel lining may increase or decrease at different locations. Under the effect of these loads, lining deformation may reach a new equilibrium status. In this paper, a coupled soil-water finite element analysis is employed to study the long-term behaviour of ground and tunnel due to the changes in groundwater level by using an old tunnelling case in Japan. The field measurements are used to support the results of analyses. One-ring model is used to model the lining behavior and interface planes are used between linings and surrounding soils to allow relative movement of soil and lining. The relationship between suction, degree of saturation and permeability ratio are used to model swelling behaviour of unsaturated soils. A series of back analyses were performed to fit the analytical results of ground surface settlement with field measurements by changing the anisotropic permeability of clay and equivalent stiffness of concrete lining. The time-dependent behaviour of diameter changes in one specific tunnel lining and ground surface displacement around the tunnel are investigated numerically.