SHALLOW COVER TUNNEL BELOW “POP’S FIG,” A WORLD’S FIRST, SOFT-GROUND TUNNEL DRIVEN ABOVE THE LARGEST HIGHWAY CAVERNS TO DATE IN THE SOUTHERN HEMISPHERE MINED CAVERNS

摘要

This paper describes a number of aspects of the design and construction of the NorthernrnBusway (NB) Tunnel, part of a major infrastructure project ($4.8 billion) in Brisbane,rnAustralia. The NB Mined Tunnel is approximately 495 m long with an excavated widthrnof 15 m and a full tunnel excavation height of 8 m. Tunnel Portals and adjacent tunnelrnconstruction are below a major urban arterial Road. Ground cover over the tunnelrnbeneath the road varies from 3.5 m to 12.5 m. The Airport Link (AL) Mainline Tunnelsrnrun in close proximity below the Northern Busway tunnel with a minimum separationrnof 5 m between the extrados of the AL and the NB tunnels. At the Southern Portal thernground and an area above the tunnel is occupied by a large Pop’s Fig Tree which isrnrequired to be preserved. Key challenges are the low ground cover over the tunnelrnbeneath a major urban environment, and the very variable geology along the tunnelrnalignment, providing mixed face tunneling conditions and ranging from surface residualrnsoils to high strength rock at depth. As a consequence, the predicted settlement valuesrnand their potential to cause damage to the buildings were critical to determining andrnobtaining approval for the selected tunnel alignment. The paper presents the designrnchallenges and tunnel design methodology of this successfully completed project.