Open Session - SWISS GEOSCIENCE MEETINGs

12.11
Borehole Evapometer measurements: Detection of zones with different
hydraulic conductivities in the Opalinus Clay
Lukas Glur*, Paul Bossart*, Martin Herfort**, Edi Meier*** & Christophe Nussbaum****
* Swisstopo, Route de la Gare 65, CH-2882 St-Ursanne.
** Swiss Federal Nuclear Safety Inspectorate HSK, CH-5253 Villigen-HSK,
*** Edi Meier und Partner AG, Jägerstrasse 2, CH-8406 Winterthur.
**** Institut Géotechnique, Route de la Gare 65, CH-2882 St-Ursanne
The estimation of physical parameters such as hydraulic conductivity or evaporation rate in different structural and lithological
units of the Opalinus Clay is very important for assessing safe radioactive waste disposal. This study was carried out in
the Mont Terri rock laboratory in the framework of the evaporation logging experiment. New equipment, the borehole evapometer,
has been developed by swisstopo in recent years. The borehole evapometer is now ready for in-situ hydraulic testing
in very low permeable formations.
The present aim of this evaporation logging experiment is to detect zones with varying hydraulic conductivities in the
Opalinus Clay, which were measured by HSK and swisstopo. Of particular interest is the comparison between the excavation
damaged zone (EDZ) and undisturbed Opalinus Clay. The measurements were carried out at several depths along three boreholes
cutting through the EDZ.
The borehole evapometer is a tool for estimating the evaporation rate and the hydraulic conductivity in a packed-off borehole
test interval of 1m length. This method uses a constant air flow through the borehole test interval. By measuring differences
in relative humidity and temperature, the evaporation rate and the resulting hydraulic conductivity can be calculated.
The integrated evaporation rate over the whole test section can be obtained, as well as local evaporation rates along
fractures cutting the borehole wall.
Several experimental set-ups and procedures were applied and compared, including variations in duration, frequency, spatial,
temporal and instrumental resolution, air-flow direction, imposed humidity difference, borehole inclination and time of
day. The results were compared with drillcore mappings, high-resolution optical televiewer (DOPTV) and seismic measurements
of the respective boreholes (Yong 2007) and mineralogical analysis of core samples.
The measurements of temperature, humidity as well as the derived evaporation rates and hydraulic conductivities (k) depend
strongly on the experimental procedure and the borehole history. A correlation with the extent of the EDZ and with borehole
depth is visible. The calculated k values range from 10-13 to 10-15 m/s and are comparable to undisturbed Opalinus Clay.
Temperature decreases and absolute humidity increases with borehole depth in all tested boreholes. The method is suitable
for determining small-scale k values in low-permeability formations.
REFERENCES
Yong, Salina. 2007. A three-dimensional analysis of excavation-induced perturbations in the Opalinus Clay at the Mont Terri
Rock Laboratory, PhD thesis, ETH Zurich.
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Symposium 12: Data acquisition, Geo-processing, GIS, digital mapping and 3D visualisation