Rock mechanics in the investigation and construction of Tumut 1 Underground Power Station, Snowy Mountains, Australia
The great advantage of this method is that it is not necessary to know the elastic properties of the rock, nor are laboratory measurements on rock samples needed. lt can only be used to measure compressive stress. No movement of the rock when the slot is cut would indicate zero or tensile stress.
One test gives the pressure in the rock face in one direction only. The usual procedure at each site was to make measurements in pairs at right angles and, if possible, in three mutually perpendicular directions (for example, by making measurements in the walls and roof of a tunnel), One of the directions was vertical, and the other two were horizontal. Usually several measurements were made in each direction.
As the next step a two-dimensional photoelastic analysis was performed on model openings of the same shape as the tunnels where the measurements were made. From these analyses stress-concentration factors were derived, which when applied to the measured stresses were assumed to give the magnitude of the original natural stresses in the directions of measurement. These of course are not necessarily the principal stresses.
The results for the three localities are shown in Table 7 and diagrammatically in Figure 8.
Table 7 – Natural Stresses at Three Localities in the Snowy Mountains
Figure 8 – Natural Stresses at Three Localities in the Snowy Mountains
Both T. 1 and T. 2 sites are below a steeply sloping wall of the Tumut Valley. The latter is like a deep V notch cut into the plateau. It was expected that this topography would have an effect on the stresses, and to avoid such effects a third site was selected in Eucumbene-Tumut Tunnel 8 miles southeast of T. 1 power station and about 1100 feet below Happy Jacks Plain, an extensive undulating region of moderate relief. The tunnel is nominally circular, 24 feet in diameter, and driven through broadly jointed granite. The granite is generally very sound, except that there is slight but widespread spalling. Four tests were made in the vertical direction (normal to the longitudinal axis of the tunnel), and four in a horizontal direction (parallel to the longitudinal axis of the tunnel).
ln the Eucumbene-Tumut Tunnel the average natural compressive stress in the horizontal direction was computed to be 2⋅6 times the stress in the vertical direction; the vertical stress was about 0⋅8 of the stress caused by the weight of overlying rock. The horizontal stress is thus about 10 times greater than the horizontal stress caused by the weight of overlying rock, assuming the rock to be elastic and isotropic and with a Poisson’s Ratio of 0⋅2 (Jaeger, 1955, p. 120).
