Rock mechanics in the investigation and construction of Tumut 1 Underground Power Station, Snowy Mountains, Australia
INTRODUCTION
At a power-station site in rock deep under ground, a system of stresses exist, that is initially in a state of equilibrium. The excavation of openings in the form of tunnels and chambers disrupts this equilibrium and causes complex reactions in the rock around the openings; this results in displacements of the rock and a redistribution of stresses that tends to establish a new state of equilibrium. In jointed rock the displacements may be elastic or in the form of sliding along joints and rotation of joint blocks. Fracturing of some of the rock may also occur.
The location, shape and arrangement of the openings, and construction techniques can often be varied, especially in the early stages of the project and thus can be adapted to a certain extent to suit the natural conditions. The most important of these are the rock types, their strength, elastic properties and structure, ground water, and the natural state of stress of the rock mass at depth.
A primary objective of the engineering work is to obtain openings of a specified size which will be permanently stable. To prevent undesirable rock movements and rock falls, it may be necessary to assist the rock mass to reach equilibrium by the use of artificial supports.
The investigation and construction of T. 1 underground power station is described to illustrate these principles.
T. 1 underground power station is situated on the upper Tumut River in the Snowy Mountains of South Eastern Australia (Fig. 1). It is part of a comprehensive scheme to develop the water resources of the mountains for hydroelectric power and irrigation. Descriptions of the engineering features of the Upper Tumut section of the scheme have been given by Campbell, Pinkerton, Bray and Frost (1956), and of some of the geological features affecting the works by Moye (1955).
The Snowy Mountains are the source of three major rivers - the Murray and Murrumbidgee which eventually drain to the west and the Snowy which drains to the southeast. The Tumut is a large northward-flowing tributary of the Murrumbidgee River.
The combined waters of 4 rivers - the Tumut, Eucumbene (a tributary of the Snowy), upper Murrumbidgee, and Tooma (a tributary of the Murray) - are to be regulated by a system of dams and tunnels and used for generation of power first at T. 1 power station, then at T. 2 power station, and at other stations farther downstream along the Tumut Valley. The water finally will be used for irrigation along the Murrumbidgee and Murray rivers.
Figure 1. – Location Map
