Technical Report NTB 93-10
Grimsel Test SiteInvestigation of Hydraulic Parameters in the Saturated and Unsaturated Zone of the Ventilation Drift
This report documents the results of Nagra investigations in the ventilation drift during Phase 3 (1991 – 1993) at the Grimsel Test Site. The investigations focused on issues related to the unsaturated zone in the vicinity of underground tunnels induced by the ventilation.
The unsaturated zone was described through point-measurements of water potential and water content. The former was measured with thermocouple-psychrometers and the latter with time-domain reflectometry. Both techniques were specially modified for the low-permeability crystalline rock. The measurements were performed under re- and desaturation conditions; no irreversible effects due to tunnel ventilation were observed.
An approach for estimating inflow into the tunnel from low-permeability rock sections by discrete measurements was also developed. The vapor flux at the tunnel surface is calculated from measurements of the temperature and relative humidity difference within a measurement cell (Evapometer). The results were used to perform the water balance for the first tunnel section of the ventilation drift; comparision with the results of classical ventilation tests from Phase 2 showed that the discharge is slightly lower (about a factor of two).
A first study for estimating the mean water flux into a tunnel section and its variability, assuming geostatistical characteristics for discrete point flux measurements was performed. The study showed that based on a small number of regularly distributed measurements, Iinked to the structural units of the tunnel section, it is possible to calculate the mean water flux with an error of less than 20 %.
