Engineering Geology for the Snowy Mountains Scheme
- Granitic Soil :
- Definition.—Surface soil derived from granitic rocks, which does not possess any recognizable granitic fabric; the surface layer often contains humus and roots of plants.
- Engineering Properties.—Generally unsuitable for foundations of structures or roads, or for use as fill; it is very important for soil conservation for spreading over the surface of embankments and cuttings to establish plant growth to prevent erosion; in the absence of vegetation, it is highly unstable and liable to slump when saturated.
- Completely Weathered Granite:
- Definition.—Granite completely decomposed by weathering in place, but still possessing a recognizable granitic fabric; the original felspars are completely decomposed to clay minerals which remain as grains of clay; biotite mica may be decomposed to varying degrees; it will disintegate into a mass of sandy clay when immersed in water and cannot often be recovered as cores by ordinary diamond drilling methods; often stained brown with limonite.
- Engineering Properties.—Can be excavated by hand, and by ripping, without the use of explosives. Unsuitable as foundations for concrete dams or large concrete structures; may be suitable for foundations of earth dams (permeability of 5 to 100 feet per year); unstable in high cuttings at angles steeper than 1 : 1; may be suitable for rolled earth fill for earth dam construction (permeability 0.1 to 5 feet per year); readily eroded by water and frost and requires protection against erosion.
- Highly Weathered Granite :
- Definition.—Intensely weathered granite, weakened to the extent that pieces the size of NX drill core (2⅛ inches in diameter) can be broken and crumbled in the hands; does not disintegrate when soaked in water and can often be recovered as cores by careful diamond drilling, but is often lost; often stained reddish brown due to limonite.
- Engineering Properties.—Very similar to those of completely weathered granite.
- Moderately Weathered Granite:
- Definition.—Granite considerably weathered throughout, but possessing strength such that pieces the size of NX drill core cannot be broken by the unaided hands ; often stained reddish brown with limonite.
- Engineering Properties.—Can be excavated with difficulty without the use of explosives ; may be suitable for foundations of small concrete structures; its stability in rock cuttings depends on the structure of the rock mass, especially the jointing, the moderately weathered granite itself being capable of standing vertically; mostly crumbles under 'dozer tracks; may be suitable for semi-pervious fill for dam construction; although the rock itself is practically impervious, the rock mass is often highly permeable due to the presence of open joints.
- Slightly Weathered Granite:
- Definition.—Granite distinctly weathered throughout the fabric of the rock, as shown by slight limonite staining, and some decomposition of the felspars; but its strength approaches that of fresh granite.
