Rock mechanics in the investigation and construction of Tumut 1 Underground Power Station, Snowy Mountains, Australia
Stress-Strain Curves
The stress-strain curves for the gneiss and the granite in the compression test were linear up to 60–80 per cent of the compressive stress to produce failure (fig. 6). Above the linear range, the strain increased more rapidly for equal-stress increments until failure occurred. In the tension test similarly the stress-strain curve was linear, but the upper limit of this linearity was considerably more variable.
Gneiss sample 3 is similar petrographically to gneiss samples 1 and 2, on which compressive and tensile tests were performed; likewise, granitic sample 4 is similar to granitic samples 1 and 2 on which compressive and tensile tests were performed. Mohr envelopes constructed from the tensile, compressive, and triaxial test data are given in Figure 6.
Specific Gravity and Porosity
The specific gravity of the gneiss ranged from 2.71 to 2.73 and of the granite from 2.70 to 2.71. The porosity of both rock types ranged from 0.3 to 0.7 per cent.
General Remarks
The results show that unjointed and unaltered gneiss and granite are very strong rocks with quite similar strength and elastic properties. These are not necessarily the properties of the rock mass, because this is disrupted by joints and faults. However, because most of the joints are tightly closed and interlocked, it is considered that the compressive propeties may not depart very significantly from those measured on specimens in the laboratory. Ideally the strength and elastic properties of the rock mass should be measured in place. The in-place tensile properties are likely to be very different from the laboratory test results since in tension the rock mass would certainly part readily along existing fractures.
TABLE 5
Poissons’ Ratio
TABLE 6
Triaxial Strength
