Simulation of hydraulic disturbances caused by the underground rock characterization facility in Olkiluoto

The hydraulic disturbances that might result from the open ONKALO tunnel system were assessed by means of site-scale finite element simulations. The details of the construction and operation of the ONKALO was not taken into account in the simulations, but the entire tunnel system was instantly made hydraulically active at the beginning of the simulation and was assumed to be open for 100 years. The inflow of groundwater into the tunnels, the resulting drawdown of the water table, and the upconing of deep saline groundwater were analysed separately by using somewhat different modelling approaches and assumptions. The drawdown of the water table was simulated by employing the free surface approach, in which only the saturated part was included in the modelled volume and the sinking water table constituted the free surface. The tunnel inflows were obtained from the state of equilibrium. The evolution of the salinity distribution was simulated with a time-dependent and coupled (flow and salt transport) model.

The simulations showed that without engineering measures (e.g., grouting) to limit inflow of groundwater into the open tunnels, the hydraulic disturbances would be significantly greater than with these measures implemented. The drifts that made up strong sinks in the model, draw groundwater from all directions in the bedrock. Most of inflow (330-1100 l/min) would come from the well-conductive subhorizontal fracture zones intersected by the access tunnel and the shaft. The water table might sink locally to a depth of about 200 metres and the depressed area extend over the Olkiluoto Island. The results also indicated that the salinity of the groundwater could gradually rise around and below the drifts, and locally concentration (TDS) may rise from 22 g/l up to over 50 g/l in the vicinity of the tunnels.

The disturbances can significantly be reduced by the grouting of rock. In the case of tight grouting the depression of the water table was confined to the immediate vicinity of the access tunnel, the maximum drawdown of the water table remained around 10 metres, and the total inflow to the tunnels was about 20 l/min. Also the upconing of the saline water remained moderate, especially at a depth of 400 metres; although the maximum calculated salinity of the groundwater near the drifts at the depth of 500 metres was still more than 50 g/l.