An assessment of the groundwater inflows likely to be encountered during construction is important since this is one of the major hazards of tunnelling.

Piper’s Creek Tunnel.

—This small tunnel which is part of the aqueduct system diverting the headwaters of Piper’s Creek into Guthega dam, was constructed in 1954-55. The tunnel is about 1,200 feet long, 7 feet high, and 5 feet wide. It is a free-flow tunnel. The rock in this area is grey biotite granite. A geological examination and seismic refraction survey were made along the tunnel line for the purpose of estimating how much of the tunnel could be left unlined. No drilling or other direct exploration was carried out prior to construction. The layers indicated by the seismic refraction survey are shown by dotted lines on Fig. 2 together with the conditions actually found when the tunnel was driven. The surface seismic layer with velocities of 1,600 to 1,700 feet per second was interpreted as soil; a second layer with velocities of 2,000 to 5,400 feet per second was interpreted as completely weathered granite with residual boulders; the layer with velocities of 8,600 to 10,000 feet per second was interpreted as firm granite, slightly weathered along joints; and the lowest layer with velocities of 13,000 to 14,000 feet per second, as nearly fresh granite, slightly weathered along some joints. Conditions as found were in fairly good agreement with these forecasts.

The granite at each end was completely or highly weathered, with some residual boulders or fresher granite, down to depths of 60 to 100 feet from the surface. There was a rapid transition through chiefly fresh granite, weathered and loosened along joints, into fresh granite with some clay and limonite along joints.

Guthega Tunnel.

—This tunnel is located on the left bank of the upper Snowy River and conveys the water from Guthega Dam to the surface penstocks of Guthega Power Station. It is a pressure tunnel, 3 miles in length and 20 feet in diameter.

The tunnel rock is granite. It is chiefly unlined except at each end where the vertical and side cover is deficient, and the rock is affected by weathering. It is also lined for short distances at a few intermediate points where it intersected narrow zones of crushed and closely jointed granite along faults. The tunnel inlet commenced as a deep open cut in completely to highly weathered granite. The main weathering extended to a depth of 70 feet from the surface. Blocky granite with limonite-stained and c1ay-coated joints persisted through much of the first 1850 feet. In this section the vertical cover was less than 200 feet, and the general looseness of the joints was probably a mechanical effect of weathering. Rather similar conditions of weathering prevailed at the outlet end. Here the granite was extensively weathered for 80 to 100 feet from the surface, and below this it was chiefly fresh but loose and blocky, due to the presence of somewhat open sometimes limonite-stained and clay-coated joints where the vertical cover was less than 300 feet. In the last half mile of tunnel the jointing had an important effect on the tunnel shape and overbreak. The direction of the tunnel was here parallel to a very persistent set of joints spaced 6 inches to 3 feet apart, dipping at about 75°. The flat joint surfaces were often coated with clay films, and divided the granite into thin slabs. Where they were cut by other joints, particularly by flat-lying joints or seams, the slabs tended to fall from the “hanging wall” of the tunnel and from the roof and caused considerable overbreak, producing locally a flat-sided, f1at-roofed tunnel. Roof bolting was effective in such places in pinning slabs of granite together.