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UNSTABLE ROCK AND ITS TREATMENT SNOWY MOUNTAINS SCHEME

Use of rock bolts in patterns

Single bolts or small groups of bolts can be made to hold up isolated blocks of rock by pinning them back to more stable rock. It is necessary to calculate the potential rock load, and the bolts necessary to hold it.

However a more significant application of rock bolting is the systematic use of rock bolts in patterns as rock reinforcement. With a suitable bolt spacing together with a suitable ratio of bolt length to bolt spacing the influence of adjacent bolts can be made to interact to create a continuous rock structure of considerable strength out of a loose mass of broken rock or detached joint blocks. This has been demonstated by experiments on models made of rock particles and by photo–elastic experiments (Figs. 7 and 8).

Fig. 6.—Groutable rock bolts with cone-and-shell anchor.

The more important general guides which have been developed for the installation of bolts in patterns in jointed rocks are the following.

1. The ratio of bolt length to bolt spacing should not be less than about 2. This is to ensure an adequate degree of interaction between the bolts in a pattern. The bulbs of pressure beneath the plates interact, with the formation of a zone of uniform compression with a thickness equal to about one third of the length of the bolts.

2. The length of the bolts should be at least three times the width of the joint blocks. This is to ensure that bolts are anchored to joint blocks embedded sufficiently deeply in the rock mass. The outer layer of joint blocks is loose and possibly close to fallout. Its loose appearance is usually the reason why support is considered to be necessary. The second layer of joint blocks may also be considerably loosened.

3. The compressive force applied to the rock mass by the tension in the bolts should be sufficient to hold the loose layers of joint blocks firmly in contact. For example, consider a pattern of rock bolts 8 ft long spaced at 4 ft centres, installed with a tensile force of 15,500 lb in each bolt (Fig. 9). This produces an average compression in the zone of uniform compression, in the direction of the bolts, of about 6-7 lb./sq.in. This pressure would be sufficient to hold together, in contact, a layer some 5-8 ft thick of loose rock or completely detached joint blocks in the roof of an excavation. It is to be noted that since the tensile force in each bolt is constant (15,500 lb in this case) irrespective of bolt length, the average compressive force applied to the rock mass decreases rapidly with bolt spacing. For example bolts at 5 ft spacing would exert a compression of 4.3 lb. sq./in. equivalent to the weight of 3-7 ft of detached rock. Therefore if the bolt spacing is made too wide even if the length is increased appropriately, the pressure exerted on the rock becomes too small to be useful.

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