THE EAGLE GOES DOWN UNDER

This article describes the design and construction of twin three-lane tunnels under the Eagle on the Hill near Adelaide in South Australia. They are Australia's first road tunnels, and the central feature of an upgrade of the Mount Barker Road, which winds steeply through mountainous terrain 15km south east of Adelaide. The highway project consists of the construction of 8.3km dual three-lane road with extensive earthworks, tunnels, and a bridge. The two parallel tunnels are each 480m long, 15m wide, and 8m high. The ground consists of slates and very strong quartzites, and has many cleavages and a high fracture ratio giving frequent stability problems. Construction began in late 1997, with the first tunnel breakthrough in August 1998. To speed excavation, a temporary rockbolt and shotcrete support system was devised, with additional strengthening near the portals, and followed by a permanent concrete lining. The tunnel was excavated by a high-capacity hydraulic hammer, and a roadheader was used to trim the perimeter of the tunnel profile. The steel support ribs had railway sleeper profiles, favouring the application of shotcrete and ensuring excellent contact between the concrete and steel sections. Due to effective design and working methods, the tunnels were completed on budget ahead of schedule.