At one surge tank site, a vertical drill hole was located adjacent to a very large outcrop of fresh granite, and passed through 385 feet of moderately to highly weathered granite before it was abandoned. Another power station site which was very favourable from the engineering standpoint was found to be in a fault zone in rock of very poor quality.

At the site now adopted (Fig. 12) surface geological mapping and diamond drilling established the presence of sound rock and enabled a site for the station to be tentatively fixed. An exploration tunnel (to be used finally as the cable tunnel) measuring 8 feet by 8 feet was driven from the Tumut River in a distance of 1,100 feet to the machine hall, and from a small chamber at the end of this tunnel six diamond drill holes were drilled up into the roof of the machine hall and across towards the tailrace surge chamber. A transparent model constructed of perspex and with drill holes represented by pegs, greatly assisted interpretation of the data. Two kinds of rock were present at the site—a medium or coarse-grained, broadly jointed, biotite granite and fine-grained, closely jointed granitic gneiss. The granite occurs as roughly parallel sheet- like masses 150 to 300 feet in thickness, inclined at about 45°, intrusive into the gneiss. The exploration showed that no major faults intersected the site, although occasional very narrow crushed seams occurred where the rock was sheared. The rock was extensively and rather irregularly jointed in four main directions— sloping at about 20° to the horizontal, and the others mostly steeply inclined. Individual joints were generally not very persistent, and the joint blocks appeared to be generally well interlocked. Most joints were very tightly closed.

However, the exploratory tunnel and the six underground drill holes intersected water flows totalling between 20,000 and 30,000 gallons per hour entering through occasional open joints. The granite is distinctly more broadly jointed and therefore of somewhat better quality than the gneiss, and it was possible to make some rearrangements of the design layout to take advantage of this.

The surge tank is in deeply weathered granite, and it is also intersected by a wide fault zone. There are very few surface out- crops in the vicinity, but the cuttings for the original access track and the final road gave good exposures of the rock and, together with several diamond drill holes, enabled the configuration of the fault zone to be worked out. The surge tank was finally located so that the circular domed roof of the 50-ft. diameter upper chamber was below the main zone of weathering, and the fault zone, which could not be avoided, intersected the surge tank well below its roof.

Concrete Aggregates:

Substantial deposits of natural sands and gravels are lacking along the Upper Tumut and Upper Snowy Rivers, and in most of the mountainous country, because of the narrowness of the valleys and the steepness of stream grades. Finding suitable aggregates for concrete represents a considerable problem in these areas.

River gravels form fairly extensive flats and terraces 10 to 20 feet in thickness along parts of the Eucumbene River and the middle reaches of the Tumut River. They are a mixture of a variety of igneous sedimentary, metamorphic and volcanic rock types.