Structure
The tunnel cross section is in the form of an arch. The approximate height and width of the arch is 11.2m and 15.3m respectively. The geology comprises coarse volcanic ash tuff on the west side and medium grained granite on the eastern side. Both of these lithologies are intruded by Rhyolite dykes together with some porphyrytic granite and occasional basalt dykes.
The two tubes were constructed using the drill/blast method. Temporary support for the tunnels have been provided by rock-bolts. Permanent supports have been provided by a concrete lining of varying thickness depending upon the stability of the ground.
Three main types of support have been used: 400mm un-reinforced, 500mm un-reinforced, and 600mm reinforced.
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