UNSTABLE ROCK AND ITS TREATMENT SNOWY MOUNTAINS SCHEME
10.—Area of partly decomposed granite 20 ft wide in roof of Eucumbene-Snowy Tunnel. Supported by grouted rock bolts and steel channels and heavy wire mesh spanning between bolts.
Final treatment (continued)
Thereafter it is subject to continuous saturation with water, to the erosive action of flowing water, and in some tunnels, occasionally, to rapidly fluctuating water pressures. The kinds of final treatment are concrete lining, grouted rock bolts, pneumatically applied mortar both plain and reinforced with mesh.
Concrete lining
As mentioned previously, the practice is to concrete line all sections of tunnels in which steel rib supports are installed. The timber blocking which transmits the rock load between rock and steel cannot be relied upon indefinitely to remain firm and strong and not to crush. Failure of blocking could lead to large scale collapse of the rock. Also, in most sections of tunnels where steel ribs have been used, there are extensive seams of weak erodible material which require protection against water.
Grouted rock bolts
Where rock bolts are used for permanent support in unlined tunnels they are grouted to protect them against rusting which could lead to loss of tension and failure (Leech and Pender, 1961). Cement grout is injected to fill the space in the hole between the bolt and the rock. The problem of how to remove the air displaced by the grout in upward sloping holes was solved initially by attaching to the bolt a separate small diameter air-vent tube about the same length of the bolt. A more efficient method has been recently developed by the Authority in which hollow-core bolts are used, the displaced air passing out down through the centre of the bolts. Grouting pressures are kept to the minimum necessary to fill the hole, to avoid the risk of causing jointed rock to fail by hydraulic jacking action.
Pneumatically applied martar
This consists of mortar composed of a graded mixture of natural sand or crushed-rock sand up to 3/16 in. size, cement and water applied to rock, surfaces through a nozzle by pneumatic pressure (Fig. 11). Current specifications provide for mortar to be applied in layers such that the finished coating will cover steel mesh reinforcement to a depth of at least one in. or if reinforcement is not used will have a thickness appropriate to the local conditions, but not less than one in. Surfaces to be treated are cleared of loose or shattered rock and washed with a stream of water and high pressure air.
Pneumatically applied mortar was used initially in the Murrumbidgee-Eucumbene and Tooma-Tumut Tunnels and is now used extensively for the final treatment of a variety of potentially unstable conditions in unlined tunnels.
