A measure of the compactness or degree of interlocking of the joint blocks of the rock mass is given by the manner in which it breaks during excavation, In the gneiss the surfaces of excavations consist almost entirely of flat joint faces indicating that under the effects of blasting the rock mass breaks more readily along joint planes than across joint blocks. In contrast, in the granite a large proportion of these surfaces are fresh fractures where the rock mass has broken across joint blocks rather than along existing joint planes. The relative ease with which the gneiss parts along joint planes is revealed also in open-cut excavation for the access-tunnel portal and in natural outcrops where the rock is disintegrating because of weathering.

GROUND WATER

Hand specimens of granite and gneiss have very low porosity and permeability, but ground water can circulate in open joints and fractures in the rock mass. The permeability of the rock mass varies considerably from place to place, according to the presence of absence of these openings.

The water table determined by measurement of water levels in drill holes was originally 100–150 feet from the surface under the steep valley slopes.

The exploratory tunnel and the six exploratory diamond-drill holes put in from the end of the tunnel through the site encountered rather large flows of ground water from several open-jointed zones; the total inflow ranged between 22,000 gallons per hour and 35,000 gallons per hour (the latter followed periods of high rainfalls and snow melt). These flows caused rapid drops of 250-500 feet in the water table above the site and apparently formed a cone of depression.

During construction of the access tunnel and main excavations, there was very little inflow from most of the rock mass, but at several places there were large water flows from open joints. These were almost invariably joints of set c. Considerable inflows also occurred from the flat-dipping fracture zone D. This zone occurs rather low in the excavations, and flows probably would have been much larger if the overlying rock had not been drained before this zone was reached.

When new flows were encountered, existing flows from joints and drill holes tended to decrease; thus, the total combined inflow did not increase very much. The maximum flow into the power station, including flows from the pressure shafts and associated excavations, was about 75,000 gallons per hour, an increase of little more than twice the maximum flows into the exploratory tunnel and associated drill holes. The flows caused further slight lowering of the water table above the site, and there probably also has been some lateral extension of the drained area.

It is evident that the drainage of the rock mass and subsequent lowering of the water table is due almost entirely to the intersection of the rock mass by the relatively few, widely spaced, open joints mainly of set c. It is clear that the four longest of the early exploratory drill holes intersected almost all the water-bearing joints of set c in the power-station area; this would account for the very large drainage effect of these few holes as compared to the relatively small increase in drainage achieved by the main excavations.

It is of interest to compare the ground-water conditions at T. 1 with those so far disclosed at T. 2 power-station site, which is in similar rock types under about 30 per cent less cover.