T.1 Project Tunnels.

—The longest tunnel for this project is the Eucumbene-Tumut Tunnel, 13.8 miles long and 24 feet in driven diameter. It has been designed as a lined tunnel throughout. Approximately 40 per cent. of the tunnel will be in granite together with some porphyry and diorite, 40 per cent. will be in highly folded and lightly metamorphosed sedimentary rocks---hard compact mudstones, shale, slate, sandstone, and quartzite---and the remainder in highly folded chert, compact tuff, andesite lava, slate and phyllite. The tunnel line is approximately at right angles to the folding of the sedimentary rocks and to the boundaries of the granites. It also makes large angles with the strike direction of known faulting, which it therefore tends to intersect at a favourable angle. Maximum vertical cover over the tunnel is about 1,700 feet, and the cover over almost all of it is more than 500 feet. Except adjacent to the portals all of it lies below the influence of surface weathering.

Tunnelling is well in progress from Eucumbene Portal, and very fast tunnelling speeds (more than 400 feet in 6 days of 3 shifts) have been achieved at this heading both in sedimentary rocks and in granite.

The remaining tunnels for T.1 Project will all be in granitic rocks.

Power Stations:

For many of the proposed power stations topographical and geological conditions favour an underground rather than a surface location. These stations are in steep river valleys where there is rarely sufficient level ground for the construction of a surface station and associated works. The rock in the valley walls is usually very much decomposed by weathering to great depths and provides poor foundations for surface penstocks.

The investigations for underground power station sites have two main objectives. First and most important is to locate a sufficiently large mass of sound rock in which to place the machine hall and associated excavations. The machine hall of T.1 Power Station, for example, measures 60 feet in width, 305 feet in length and 105 feet in maximum height, and it is essential to find the best rock possible in which to construct such very large chambers. Since the power stations are usually at least several hundreds of feet below the surface, the rock is well below the influence of surface weathering, and the chief considerations are the rock type, its geological structure and ground water. Fault zones need to be avoided. The best orientation for the machine hall with respect to the jointing system of the rock requires study; in general it is considered undesirable to have the machine hall oriented so that its long axis is parallel to sets of steeply inclined persistent joints.

The headrace surge tanks are necessarily close to the under- ground power stations. They usually need to be high up the valley. walls close to the surface or reaching up to the surface, and hence are very likely to be within the zone of intense weathering of the rock. The second main objective of the investigations is therefore to find a location for the surge tank, at an elevation fixed by the engineering requirements, where the depth of weathering is shallow and the rock sound.

T.1 Power Station.

—This station is at present under construction. Several sites were investigated over a distance of 1 1/2 miles along the right bank of the Tumut valley before the present one was adopted. The rock throughout this area is biotite granite and granitic gneiss, both of which are weathered to great depths.