Water flow FEBEX

Based on the available geological, hydraulic and mechanical characterizations of the Site as well as on results of hydraulic tests performed on boreholes, a hydro-mechanical model for the zone around the FEBEX tunnel was to be prepared. Using this model, changes in water pressure induced by the boring of the FEBEX tunnel in the near vicinity, as well as the total water flow rate to the excavated tunnel was required. 

As part of the "DECOVALEX" project, we were asked to predict the total water flow rate to the excavated tunnel over the last 17.40 meters of FEBEX tunnel section (between 54.00 and 71.40 m). A purely-hydraulic computation, using the previous calibration, yields an estimated steady-state flow of between 6 and 7.6 ml/min. This size scale is to be compared with total water inflow estimated from a semi-quantitative hydrogeological map of the FEBEX tunnel 7.8 ml/min (with an estimation of about 27% of inflow water coming through the matrix). In comparison with the continuous prediction (Alonso et al, 2001), this estimation is rather good and the discontinuous approach allows localizing water output with greater precision. (This kind of comparison was not included within the "DECOVALEX" task.)... 

In the FEBEX project blocks made of Ca-bentonite are tested for their suitability in radioactive waste isolation. This material has been tested in various laboratory experiment to identify the material parameters. The modelling of gas and water flow in engineered barriers demands beneath the intrinsic permeability and porosity constitutive equations for capillary pressure and relative permeabilities. 

The first technique involved discrete measurements at select points on the FEBEX tunnel by means of absorbing pads. The absorbing pads were weighted before and after their placement in order to determine the volume of leaked water. With this information, it was possible to know the distribution of water input flow on the wall of the FEBEX tunnel. 

The numerical model ROCMAS was applied to predict coupled THM processes in a bentonite barrier at the FEBEX in situ test. The results indicate that numerical modeling can provide highly reliable predictions for temperature distribution, and reasonably reliable predictions for moisture flow and stress in a bentonite barrier. Moreover, field observations and modeling shows that resaturation of the buffer was controlled by the properties of the bentonite barrier whereas the permeability of the rock was sufficiently high to act as an unrestriced water source. Therefore, the wetting of the bentonite took place uniformly from the rock and was not impacted by the permeability difference between the Lamprophyres dykes and surrounding rock. 

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