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T. I Power Station is 1100 feet underground under the eastern wall of Tumut River gorge in the Snowy Mountains of southeastern Australia. The machine hall is 306 feet long, 77 feet in maximum width, and 104 feet in maximum height.

In selecting the site major faults in the region were avoided. The rock is granite in steeply inclined sheets up to 300 feet thick intrusive into granitic gneiss. Both rocks have compressive strength of approximately 20, 000 psi, tensile strength 1100 psi, and Young!s Modulus 7-10 x 10 psi. Jointing in both rocks follows a similar pattern but is much more closely spaced in the gneiss.

Original natural stresses in the rock mass, which are an important factor in the behavior of openings, were computed from measurements of stress in the excavations. These are higher, particularly in the horizontal plane, than can be accounted for by the weight of overlying rock alone.

The openings were located mainly in the granite and oriented to avoid main joint directions parallel to the walls. Photoelastic studies were made to determine the stress concentrations caused by the shape of the openings and by different arrangements of multiple openings.

Comprehensive investigations were made to study how rock bolts function in hard jointed rock, and during excavation pattern of rock bolts was used to stabilize the jointed rock in the roof and walls. The excavations were made without major rock falls. Some instrumental observations of the behavior of the walls and roof of the machine hall were made during construction.

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