Slightly Weathered Granite (continued):

Engineering Properties.---Requires use of explosives for excavation; often suitable for foundations of concrete dams; practically impervious but the rock mass is often highly permeable due to the presence of open joints; unsuitable as concrete aggregates.

Fresh Granite :

Fresh granite lying immediately beneath the various types of weathered granite frequently shows limonite stains along joints; this is described as “fresh granite with limonite stained joints” such staining indicates the rock mass is permeable, and may be loose and require support, for example in tunnels and shafts.

In common engineering usage, completely and highly weathered granite would be referred to as soils, and excavation in them as “common” or “earth excavation”; slightly weathered granite would always be referred to as “rock”, but moderately weathered granite, being intermediate, would often be difficult to classify.

The above classification applies especially to granitic rocks and, with appropriate modifications, to other igneous rocks in which weathering is predominantly chemical weathering. In rock types resistant to chemical weathering, such as quartzites, fine-grained quartz porphyries and acid lavas, a large degree of mechanical break-up of the rock mass along its joints may occur, accompanied by very little decay or softening or reduction in strength of the rock itself. In those circumstances the above classification has but little application.

Different rock types show a great difference in their tendency to weather, and their manner of weathering under similar conditions. The rock type found to be most susceptible to weathering is a medium to coarse-grained grey granite, very rich in biotite mica (often about 20 per cent.), which is one of the most abundant rock types in the Snowy Mountains area. It forms most of the country extending along the Upper Snowy River to its junction with the Eucumbene River, and it also occurs extensively along the Tumut River in the vicinity of T.1 Power Station, and along the Tooma-Tumut Tunnel line. It often has been proved by exploration to be completely or highly weathered to depths of from 60 to 100 feet below the surface, but then may pass somewhat abruptly in a distance of a few feet into fresh rock. Even in the completely or highly weathered zone, there are frequently large residual boulders of fresh or only slightly weathered granite surrounded by completely or highly weathered granite, and such boulders often outcrop on the surface in large clusters, giving a most misleading impression that they are outcrops of solid bed-rock. The section of Piper’s Creek Tunnel (Fig. 2) which is part of the aqueduct system feeding into Guthega Dam, and the Guthega Dam (Fig. 3) on the Upper Snowy River illustrate the extent of weathering in this rock. The granitic gneisses which are abundant and important rock types in the vicinity of T.1 Power Station and along the Tooma-Tumut Tunnel Line, also weather to very great depths, but usually show a more gradual transition through all the grades of weathering down into fresh rock. At Tooma Dam site (Fig. 4) the gneiss on the left abutment is highly to moderately weathered to 140 feet, and contains bands of moderately to slightly weathered rock down to the bottom of the drill hole, a depth of 290 feet.