Rock mechanics in the investigation and construction of Tumut 1 Underground Power Station, Snowy Mountains, Australia
Figure 12. – Measurements of Strain, and Movements in the Machine Hall
The first case occurred in the machine-hall roof excavation in the projecting corner of rock between the pilot tunnel and the first 120 feet of completed roof excavation. This rock projection was converted into a temporary isolated rectangular pillar by construction of a roof abutment tunnel and a cross drive through it; all lateral support was thereby removed, and the vertical load evidently increased to the point where the isolated pillar commenced to fail by spalling.
The second case occurred in the lower branches of the penstock tunnels, Figure 4 shows that a single penstock tunnel leads from the base of each of the two pressure shafts; each of these bifurcates produces four tunnels close together that join the machine hall at right angles in the base of the upstream wall. Between each bifurcation and the machine-hall wall there is an isolated pillar of rock. These penstock tunnels were mainly driven at an early stage when only the upper part of the machine hall was excavated (Fig. 12).
