Technical Report NTB 91-01

Studies are being conducted in the Grimsel rock laboratory on the hydrogeological, rockmechanical, geophysical, and geochemical properties of crystalline rocks within the scope of a German-Swiss joint cooperation project between the Nationale Genossenschaft für die Lagerung radioaktiver Abfälle (Nagra; Swiss National Cooperative for the Disposal of Radioactive Wastes), the Bundesanstalt für Geowissenschaften und Rohstoffe (BGR; Federal Institute for Geosciences and Natural Resources), and the Gesellschaft fur Strahlen- und Umweltforschung (GSF; Society for Radiation and Environmental Research). In-situ tests and numerical model calculations are carried out by the BGR within the "Fracture System Flow Test ll project (BK) to investigate the significance of flow and transport processes in fissured rock.

After core drilling, project phase BK I (until 1987) focused on the geological investigation and hydrogeological assessment of the fracture systems encountered in the granite. When the flow testing equipment had been set up, tests with a duration of a few hours or days were conducted. Based on improvements in the test equipment, the tests in phase BK II (until 1990) were extended to several weeks and months. This report deals primarily with the results of project phase II, during which 100 in-situ tests were carried out.

The principle of the Fracture System Flow Test is primarily to determine the hydraulic conditions at several observation points distributed spatially in the section of rock mass selected for the test. Specially developed probes are installed in the boreholes to monitor pressure, temperature, and electrical resistivity. This data is digitized by the computer within the probe before being sent to the main computer outside the borehole. The original binary data is converted into ASCII and is stored in an archive.

A detailed knowledge of the main hydraulic connecting paths is used for three-dimensional model calculations in which the rock is represented as a discontinuum of joint systems and not as a homogeneous continuum. Simplified analytical and numerical calculations are made to determine mass transport and hydraulic parameters. It was found, however, that homogeneous modelling of the rock mass yields only an approximation, because the flow processes are obviously influenced by channels and because in long-term tests, two zones of lamprophyre become evident as hydraulic barriers.

For the preliminary three-dimensional model calculations, the DURST finite-element program system, developed within the scope of the project, has proved itself for simulating flow and mass transport processes. Versions for PCs and mainframe computers are available; the program is being extended to simulate multiphase flow. It is used internationally under the name ROCKFLOW.

This report was prepared within the scope of the research project "Bohrlochkranzversuch (Fracture System Flow Test)" which is financed by the Federal Ministry of Research and Technology (Project no. KWA 53045).