Rock mechanics in the investigation and construction of Tumut 1 Underground Power Station, Snowy Mountains, Australia
ROCK TYPES AND THEIR PROPERTIES
At T. 1 power station site Happy Valley granite occurs in sheetlike masses 100–300 feet in thickness intrusive into Boomerang Creek granitic gneiss. These granite sheets strike N. 65° E. to N. 100° E. and dip 40° – 50° SE. Their distribution at power-station level is shown in Figure 3. In most places the contact between the two rock types is gradational over a distance of several inches, but in some places the transition zone is as much as 20 feet wide. Viewed broadly the boundaries are fairly straight for distances of several hundred feet, but in detail they show many irregularities.
A dolerite dike 1–2 inches wide occurs in the northeast corner of the machine hall in the roof but thins and disappears at lower levels. Several larger dolerite dikes up to several feet in width occur in the access tunnel and pressure shafts.
Boomerang Creek Granitic Gneiss
In hand specimen the gneiss is a granular, fine – to medium–grained dark–gray rock. At the power-station site it only rarely shows any distinct foliation of preferred orientation of minerals. Grains of gray glassy quartz, white feldspar, black biotite, silvery muscovite, and altered cordierite can be recognized. In many places the altered cordierite is associated in small dark-gray patches with mica giving the rock a very distinct blotchy appearance.
Under the microscope the gneiss is entirely crystalline. The grain size ranges from 0.3 to 0.6mm in diameter. Cordierite altered to pinite ranges up to 2–3 mm, and some orthoclase up to 6 mm. The pinitized cordierite and orthoclase commonly occur as porphyroblasts enclosing other minerals. Hand specimens are noticeably variable in composition. The quantitative mineralogical composition, determined on five to nine thin sections made from each of two representative samples by the point-counting method, is given in Table 1. Much of the biotite is slightly altered to chlorite and intergrown with muscovite. Minor constituents are sillimanite, apatite, zircon, rutile, sphene, and iron ore.
Boomerang Creek granitic gneiss is considered to be a metamorphic rock probably derived from sedimentary rocks. Farther downstream similar rocks show distinct contorted banding which appears to represent the bedding of the original sedimentary rock.
The nomenclature of these rocks presents some difficulties. Since most of the rock at the power-station site appears massive without gneissic structure, it could more strictly be called granulite.
TABLE 1
Boomerang Creek Granitic Gneiss (Granulite)
(Analyst: D. Lafeber)
