ROCK TYPES AND CAUSES OF INSTABILITY

Rock types

The underground works are located in two broad groups of rocks (Fig. 1).

1. Granitic rocks. These comprise granitic gneisses. granodiorites and granites of Palaeozoic age. About three-quarters of the tunnelling and both underground power stations are in these rocks.

   The granitic rocks are intruded by dykes of porphyry and lamprophyre related to the granites, and by dykes of basalt related to surface flows of basalt of Tertiary age which occur in the northern part of the area near Kiandra, and to the east of the Snowy Mountains.

2. Sedimentary rocks. These are chiefly thoroughly compacted, well cemented, fine-grained sandstone, siltstone and shale of Upper Ordovician age. In many places they are slightly recrystallized due to regional metamorphism, and grade into quartzite, slatey shale and phyllite. They are intruded by the granodiorites and granites and adjacent to the boundaries of the intrusions may be further re-crystallized into quartzite and hornfels. These beds are mostly highly folded, with dips usually steeper than 40°

Rock defects causing instability

All these rocks, when free from defects, are strong and durable. Large tunnels can be driven through them without support and they are unaffected by water. However, defects of various kinds are frequently present causing actual or potential instability.

Weathering

Much of the Snowy Mountains is covered by a thick mantle of weathering. In the granitic rocks weathering attacks felspars and biotite converting them to clay minerals and chlorite causing the rocks to become soft and weak and ultimately to disintegrate into clayey sand. The intensity of weathering decreases with depth. The surface layer of clayey sand passes down into a zone containing residual boulders of more or less weathered granite, then down into a zone consisting of angular joint blocks separated by seams of disintegrated granite, then down into fresh granite with stained joints.

Weathering involving considerable disintegration of granitie rocks, frequently extends to depths of 100 ft or more from the surface. It therefore creates problems in establishing portals and tunnelling under shallow cover. Deep open cuts in weathered rock have been generally found to be unstable unless cut back to flat slopes. Where possible they are avoided by limiting their depth to that necessary to obtain cover over the tunnel equal to the tunnel height, accepting the difliculties associated with tunnelling through weathered rock.

Hydrothermal alteration

Zones of decomposed granite closely resembling highly weathered granite have been intersected at depths well below any possible influence of surface weathering. For example several zones 15 to 20 ft wide were encountered in the Eucumbene-Snowy tunnel at depths of between 900 and 1100 ft below undulating terrain. These weak zones are associated with faults and closely jointed granite, and on the ground surface they are usually indicated by prominent lineaments.