The Norwegian Geotechnical Institute (NGI) is engaged in research in the field of geodynamic investigation of rock and soil quality. The results from this work can be used to determine the in situ modulus of deformation and other rock mass properties, and to locate major joints and fractured zones. This paper describes how the cross hole seismic method can be used to investigate weak rock. The material under consideration forms the foundation rock for a large rock fill dam called the OddatjØrn dam. More specifically, the quality of the foundation for the central core was evaluated by means of acoustic measurements through 40 meter long boreholes. The OddatjØrn dam is one of several dams constructed to establish Norways largest hydro power reservoir called BiåsjØ. A number of smaller lakes have been regulated into one big reservoir. These lakes are connected by a system of tunnels. The BlåsjØ reservoir is part of the Ulla-FØrre Hydro Power Scheme in south-west Norway. The project utilizes the water power from a number of rivers in a 2000 km2 mountainous region 80 km north-east of the town of Stayanger. Atmospheric low pressures, steadily coming in from the Atlantic, give high precipitation, up to 2000 mm pr. year, in this region. The State Power System, one of the four directorates of the Norwegian Water Resources and Electricity Board, is responsible for the planning and construction work of the project. Construction was started in 1974 and will continue towards 1990. The contribution of the project to the national firm power production capacity will amount to 5663 GWh pt. year. The total cost of the planning and construction work will be approximately US $ 2000 in 1984 prices.
2 THE ODDATJØRN DAM
The OddatjØrn dam is a rock fill dam of 5.7 mill. M3 volume with a traditional central core of moraine material, figure 1. The construction work was started in the beginning of the Ulla-FØrre project and the dam will be completed in 1987-88. The rock in the dam foundation consists partly of a hard, massive gneis. The left abutment is of variable quality and includes some highly fractured pegmatites which are not very suitable for foundation purposes. The quality of the rock which forms the foundation of the central core, is further diminished due to blasting operations immediately above the left abutment in a quarry used for dam material production. Core drillings have located permeable zones down to more than 80 meters be- low the surface. Some major weakness zones cross the foundation of the central core. The smallest distance from the quarry to the dam abutment is down to 50 meters and the maximum charge weight pt. interwall was 4000 kg of explosives. The maximum peak ground vibration level is unknown. However, as the results from the seismic investigations will illustrate, the rock quality was definitely affected by the close proximity of the blasting. The footing for the central core was to be fully grouted. The quality of this anti-seepage grouting is traditionally investigated by means of core drillings and water leakage tests. To complement these tests, the State Power Board ordered cross hole seismic measurements of the rock conditions.