Evaluation of the geological, geophysical and hydrogeological conditions at Fjaellveden

The Fjaellveden study site has a flat topography and a high per-centage of outcrops. The main type of rock in the area is veined gneiss with a north-east structural strike and vertical dip. The veined gneiss contains sulphide minerals, primarily pyrite and pyrrhotite, in the form of fracture minerals and as impregnations. In conformity with the gneiss structure there are strata of granite gneiss. The rock mass has a fracture frequency of 4.0 fractures per metre within the upper 100 metres. The frequency decreases with increasing depth and below the 300 m level it is 1.8 fractures per metre. The Fjaellveden study site is delimited to the north-east and south-west by regional fracture zones, 80-90 m wide and dipping approx 75degree towards the south-west. The fracture zones contain wide sections of crushed and clay-altered rock, mylonites and breccias. The regional zones delimit a block at least 3times3 km large. This block contains only local fracture zones. Drill hole examinations indicate that the fracture zones are small and of a mean width of 5 m. Within the upper 100-200 metres horizontal fractures can be found. Common fracture minerals in fracture zones are calcite, kaolinite, chlorite and illite. Existing granite gneiss strata possess higher hydraulic conductivity than the surrounding veined gneiss. At a depth of 500 m the granite gneiss has a hydraulic conductivity than the surrounding veined gneiss. At a depth of 500 m the granite gneiss has a hydraulic conductivity of 3times10-9 m/s, the corresponding value in the surrounding bedrock being 2times10-11 m/s. For the rock mass as a whole, the hydraulic conductivity decreases from approx 10-8 m/s at the surface rock to approx 10-11 m/s at a depth of 600 m. The hydraulic conductivity in the local fracture zones in the Fjaellveden area is 5times10-9 m/s at the 500 m level. The flat topography of the area implies that the hydraulic gradients in the bedrock are small. This has also been recorded by means of piezometric measurements. (author)