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UNSTABLE ROCK AND ITS TREATMENT SNOWY MOUNTAINS SCHEME

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.

Details for this article:

Unstable rock and its treatment in underground works in the Snowy Mountains Scheme

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Author: Moye, D. G (1965)

Article Title: Unstable rock and its treatment in underground works in the Snowy Mountains Scheme.

From: Proc Eighth Commonwealth Mining and Metallurgical Congress, Aust and N.Z. Vol.6 p. 429-441

Other Available Articles

Engineering geology for the Snowy Mountains Scheme

Moye, D.G. (1955)

Engineering geology for the Snowy Mountains Scheme.

J.I.E.Aust., Vol. 27 No.10 pp287–298

Rock Mechanics in the Investigation and Construction of T.1 Underground Power Station, Snowy Mountains, Australia

Moye, D.G. (1958)

Rock Mechanics in the Investigation and Construction of T.1 Underground Power Station, Snowy Mountains, Australia

In Engineering Geology Case Histories No.3 123–54 Geological Society of America 69 (12) p.1617

Existence of high horizontal rock stresses in rock masses.

Moye, D.G. (1962)

Existence of high horizontal rock stresses in rock masses.

Proc. Third Australia-New Zealand Conference on Soil Mechanics and Foundation Engineering. pp 19–22

Seismic Activity in the Snowy Mountains Region and its Relationship to Geological Structures

J. R. Cleary, H. A. Doyle, D. G. Moye (1964)

SEISMIC ACTIVITY IN THE SNOWY MOUNTAINS REGION AND ITS RELATIONSHIP TO GEOLOGICAL STRUCTURES

Journal of the Geological Society of Australia

Unstable rock and its treatment in the Snowy Mountains Scheme.

Moye, D.G. (1965)

Unstable rock and its treatment in the Snowy Mountains Scheme.

Proc. 8th Commonwealth Mining and Metallurgical Congress, Australia & New Zealand. Vol. 6, p. 423–441.

Diamond drilling for foundation exploration

Moye, D.G. (1967)

Diamond drilling for foundation exploration.

Paper 2150 presented at I.E.Aust. Site Investigation Symposium, September 1966. In Civil Engineering Transactions, with Discussion, April 1967.

Geology in Practice

Moye, D.G. (1970)

Geology in Practice. Presidential Address Section 3, Geology, ANZAAS Meeting.

Australian Journal of Science, 32 (12) June, p454–461.

* This paper was presented when Dan had been Director of Exploration of BHP for 3 years.

Field and Laboratory Tests in Rock Mechanics

Alexander, L. G (1960)

Field and Laboratory Tests in Rock Mechanics

Proceedings, 3rd Australian-New Zealand Conference on Soil Mechanics and Foundation Engineering, Sydney Australia, 1960, pp. 161–168.

Discussion at Technical Session No. 9—Rock Mechanics

Alexander, L. G. Moye, D. G. (1960)

Discussion at Technical Session No. 9—Rock Mechanics

Proceedings, 3rd Australian-New Zealand Conference on Soil Mechanics and Foundation Engineering, Sydney Australia, 1960, pp. 254–250