Deutsch (27.2 MB) - Nagra

SUMMARY
In 1980, Nagra (National Cooperative for the Storage
of Radioactive Waste) carried out an extensive geological
investigation programme covering an area of
some 1'200 km 2 in Northern Switzerland. The aim of
the programme was to obtain the geological information
necessary for assessing the suitability of the crystalline
basement as a disposal medium for high-level
radioactive waste. The wide range of investigations
carried out can be divided into a deep drilling programme,
geophysical reconnaissance of the regional
petrophysical and structural conditions, a hydrogeological
programme for investigation of water flowpaths
deep in the bedrock and a neotectonic programme
for identification of active crustal movements
in the area.
The Weiach borehole was the second in the deep
drilling programme. The drill-site (coordinates
676'744/268'618; 368.7 m a.s.l.) lies within the community
of Weiach in Canton Zurich some 3 km west of
Glattfelden. The [mal depth of the borehole was
2'482m.
It was intended that the Weiach bore hole should
penetrate through the sedimentary overburden and
around 1'000 m into the crystalline basement. A wide
range of investigations was carried out in the borehole
itself, as well as on drill cores and water samples.
The most up-to-date methods were used to
draw up a detailed picture of the geology of the
basement and of the deep groundwater regime.
Drilling began on 10th January 1983 and continued,
with numerous interruptions for scientific investigations,
until mid-November 1983. Further tests were
then carried out during the months up to the end of
March 1985. Finally, a multiple packer system was
installed between 30.3. and 3.4.1985 with a view to
earring out long-term observations of the hydraulic
conditions in the upper Muschelkalk, the Buntsandstein,
the Rotliegendes and the Crystalline. The planned
programme was carried out more or less in full
and with a considerable degree of success.
The drilling and investigation programmes were
planned and managed by Nagra employees with the
assistance of external advisers. In total, more than 40
universities, constractors and service companies from
Switzerland, the Federal Republic of Germany,
France, Great Britain, Denmark, Canada and the
USA were involved in the investigations, with a total
manpower of over 150 scientists and technicians.
This report is a summary of the most important
results obtained during the course of the programme.
Similar information and datasets are presented
for each of the boreholes (Bottstein, Weiach,
Leuggern, Schafisheim, Kaisten and Siblingen) in a
series of investigation reports, thus allowing intercomparisons
to be made between the different sets
of results. Conclusions to be drawn from the data
will be reserved for a [mal synthesis report. Detailed
reporting of the individual investigations is provided
in Nagra's series of Technical Reports (see NTB-list
at end of this report).
Geological overview
The Weiach drill-site is located in the vicinity of a
Permocarboniferous through which runs ENE-WSW
and is situated deep in the crystalline basement. This
borehole provided the fIrst defmite evidence for the
existence of the trough by drilling through 457 m of
Permian sand- and siltstones and 572 m of coalbearing
Upper Carboniferous sandstones and clays.
The basement consisting of biotite-plagioclase-gneiss
was encountered only at a depth of 2'020 m and was
drilled down to 2'482 m.
Based on investigations carried out to date, the Weiach
Crystalline would appear to be a regionally
metamorphosed, pre-Cambrian sediment series
which underwent intensive tectonohydrothermal
overprinting due to Late Variscan-Upper Carboniferous
movements and granite intrusions (around 300-
320 million years). There are no directly comparable
rock units in the southern or central Black Forest but
a very similar gneiss series was encountered in the
Urach-3 borehole in the Schwabish Alb.
Drilling techniques
The drilling rig used was an all-electrical National
80B, SMG AS 300/40 with Gulliver mast. In both,
sediments and crystalline rock, almost exclusively the