Rock mechanics in the investigation and construction of Tumut 1 Underground Power Station, Snowy Mountains, Australia
Figure 6 – Elastic Properties of Granite
At T. 2 site an exploratory tunnel of about the same length as at T. 1 has been driven into the vicinity of the machine-hall site, and a comparable amount of underground diamond drilling has been carried out. The maximum total ground-water inflow was about 2000 gallons per hour, less than 10 per cent of that from the similar T. 1 exploratory works. The ground-water pressures at T. 2 were measured by fitting pressure gauges to the sealed-off drill holes. These pressures ranged from 145 to 305 psi. The maximum pressure of 305 psi corresponds with the hydrostatic pressure developed by a water table close to the ground surface above the site. There has probably been little lowering of the water table as a result of the drainage effect of the tunnel and drill holes; the lower pressures at the other drill holes are due to some leakage through joints into the tunnel. Tests made by opening and closing successive holes show connections of greatly varying permeability between all holes, but evidently the holes have not intersected free-draining open joints as at T. 1. In order to study the ground-water pressure gradient around the excavation, two holes, 10 feet and 20 feet long respectively, were drilled into the tunnel wall in an area where the joints appeared to be tightly closed. The 10-foot-long hole produced a ground-water flow of 1 gallon per hour and, when sealed 4 feet from its collar, gave a pressure of 165 psi. The 20-foot hole had a flow of 55 gallons per hour and when sealed 6 feet from the collar gave 145 psi pressure.
These results demonstrate that, where the permeability of the rock is low and the water table is high, very high ground-water pressure can exist very close to the surface of underground openings.
NATURAL STATE OF STRESS OF THE ROCK MASS
The state of stress in the rock mass prior to disturbance by the excavations is here referred to as the natural state of stress.
The natural state of stress has been partially determined at T. l power station, and at two other localities in the Snowy Mountains, by the following method.
As the first step, the tangential compressive stress in the surface layer of rock around the excavations was measured in horizontal and vertical directions at several sites, using an adaptation of the flat-jack method described by M. E. Tincelin (1953). This method consists of cutting a slot into the rock face, which results in expansion of the rock into the slot, and then applying pressure with a flat hydraulic jack to the walls of the slot to restore the rock to its initial position (Fig. 7). The jack pressure, corrected for the shape of the slot, is assumed to equal the tangential compression in the rock face before the slot was cut, in the direction perpendicular to the slot.
Figure 7 – Flat-Jack Test
