ABSTRACT:

The general impression after the crosshole measurements and the production of he tomography contrast plots is that the rock at the dam site has remarkable high sound velocities. Low velocity zones are detcted along bedding planes at the marble/mica schist contacts. These zones could be interpreted as karstification.

Surface observations confirm this.

Fracture related low velocity zones, interpreted as possible karst forms, are limited To very few areas, mainly in connection with the appearance of the grey marble, Løset (1987).

Locally, velocity anomalies could be interpreted as very limited small karst forms. The extension of these anomalies is so confined that the appearance of such features are difficult to verify from available geological information.

1 INTRODUCTION

The reported investigtion project is directly related to the hydro power engineering project named Storglomfjordutbyggingen, which The State Power Board of Norway is in charge of.

The aim of the Storglomvatn karst investigation has been to detect widened fractures (grikes), caves and conduits to prevent leakage from the reservoir. The investigation methods used have been both traditional and up to date, and both technological and geoscientific.

Dam Storglomvatn is a projected rock fill dam of 5–6 mill m3 volume. The dam will be one of several dams constructed to establish the hydro power reservoir of Svartisen, i.e. "The Black ice Reservoir" km north of the Arctic Circle.

The rock in the dam site area consists of variably jointed and folded micamarble schist, marble and intermittent layers of sandstone and quartzite. The structure is nearly vertical. In the marble, karst cavities can be seen at the surface. Indications of such cavities are found deep down in the rock mass by means of investigation drill holes.

Cross hole seismic investigation was performed to detect the extent of this phenomenon. It was particularly important to locate major karstic features before grouting. This represents a cost and time-saving factor in the project realization.

Karst as described by Lauritzen (1983) and Løset (1985), often represents a major problem for rock engineering projects in limestone/marble areas. It is usually impossible to detect the depth extension of the karst from surface inspection alone. Karst cavities can be from some centimeters to many meters in diameter, and the water flow through cavities and communication channels can be huge.

Cross hole seismic investigation is considered to be one of the best methods to detect cavities and to give an overall evaluation of the rock properties, (Ballard 1982, 1983; Curro 1983; Franklin 1981).

2 SITE DESCRIPTION

Storglomvatn is located in the Saltfjellet-Svartisen area close to the polar circle on the coastal mountainous area in North Norway. This region is climatic classified as coastal subarctic which gives annually low mean temperature

(Figure in full paper)

Figure 1. Walbøgrotten - Storglomvatn. This cave is only partly explored because the vertical shaft is blocked. The direction of Walbøgrotten is perpendicular to the dam axis. The cave is located about 150 meters north of the end of the weir crest. The entrance is at 570 m a. s. l.

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