The exploration proved that the site was free from wide fault zones. The tunnel and drill holes intersected some zones of close slickensided joints, but it was not possible to correlate them with any certainty at this stage. They later proved to be the minor faults A and B and similar structures.

From the exploration it also was apparent that the gneiss and the granite were hard strong rock types with similar elastic properties; the principal difference was that the gneiss was more extensively jointed. It was judged that the power station could be constructed in either rock type, but because of the difference in jointing it was considered that large openings would be easier to construct in the granite and would require less permanent support. For this reason most of the station was located in the thickest granite sheet.

Orientation of the Station

In determining the orientation of the power station, the directions of jointing and shearing were the most important geological considerations. In general it is undesirable to have persistent joints parallel or nearly parallel to a large surface, such as long high walls. The more closely the strike of such joints approaches a direction that is normal to the surface, the less effect the joints have in causing instability.

The cores from the sloped diamond-drill holes were of very limited value for determining the direction of joints and minor faults; the pattern had to be determined from measurements in the exploratory tunnel and on surface exposures. The joint pattern so obtained later proved to be quite similar to that in the actual power-station excavation. However, the narrow, very persistent, minor faults A and B were not discovered during the exploratory stage; on the other hand, shearing was found in several localities at the boundary between granite and gneiss, but not in the actual excavations.

The orientation adopted was a compromise designed to make as large as possible the angle between the main long high vertical walls of the machine hall and the various directions of strike of the sets of steeply dipping joints together with the gneiss-granite boundary.

J, Talobre (1957,p. 39-44) has recently indicated an elegant quantitative approach to the problem of finding the best orientation of an underground opening with respect to jointing by using the stereographic projection. This method permits the weighting of the sets of joints to take into account differences in friction along the joints.

Final Location

Construction commenced in June 1955, with the driving of the permanent access tunnel 21 feet high and 20 feet wide from the floor of the Tumut Valley to the machine-hall site. A pilot tunnel of the same size was then driven to the end of the machine hall. The part of the pilot tunnel in gneiss was supported with steel sets and rock bolts, but the part in granite was unsupported. It was evident that the quality of the granite was distinctly better than had been expected from the exploration. Full advantage was taken of this development, by relocating the station; it was moved parallel to its former position, 70 feet farther in the direction of its longitudinal axis. Subsequent experience with the excavation of the openings in both rock types confirmed that there was a definite correlation between rock type and mechanical properties; the granite usually was distinctly superior to gneiss.

Photoelastic Studies

The excavation of underground openings disturbs the natural state of stress in the rock mass and causes stress concentrations to develop in the zone surrounding the opening. In elastic isotropic rock at great depth these stress concentrations depend on the shape of the individual openings, the arrangement of multiple openings, and the initial natural state of stress of the rock. At moderate depths the ratio of the dimensions of the openings to the depth below the surface also affects the stress concentrations (Terzaghi and Richart, 1952, p. 70), but it was assumed that T. 1 power station was sufficiently deep for such effects to be neglected.

The effect of various shapes and arrangements for the power-station openings was studied by two- dimensional photoelastic analyses. Studies were made not only on the final shapes, but also on the varying shapes at different stages of excavation.