The Brisbane Airport Link and Northern Busway project included the construction of more than 12.6 kilometers (7.8 mi) of tunnels. The underground works included twin three-lane mainline tunnels; 5 one- to three-lane, on- and off-ramp tunnels; and a two-lane busway tunnel. The finished span of the mainline tunnels is approximately 13 meters (43 ft), and where the ramp tunnels intersect with the mainline tunnels, the spans of the resulting caverns are up to 26 meters (85 ft). The maximum cover over the tunnels is approximately 40 meters (131 ft), and much of the tunnel alignment underlies residential neighborhoods and light commercial properties.
Delve Underground was engaged by the Parsons Brinckerhoff-Arup Joint Venture to help design the initial tunnel support and final lining. The project was delivered under a public-private partnership, BrisConnections, which contracted Theiss-John Holland to construct the project.
The geologic conditions along the tunnel alignment are highly variable and range from very strong and massive tuff to completely weathered siltstone. The majority of the tunnels were constructed using the sequential excavation method, with most of the excavation being performed by roadheader and drill-and-blast in the more competent rock formations. Two tunnel boring machines were used to mine approximately half of the mainline tunnels, with the remainder being mined by roadheader and drill-and-blast.