The Brisbane Airport Link and Northern Busway project has been given permission to sink an extra 42m deep shaft to avoid a potential eight-month delay to the construction programme after unexpectedly poor ground was met in part of the project.

The Government of Queensland, Australia, last month gave the green light for the additional, 15m dia. shaft to be sunk at Wooloowin, on a state-owned site set aside for a future road upgrade.

JV contractor Thiess-John Holland plans to have two roadheaders working from the shaft, taking the total number of machines at Kedron to six. The roadheaders will bore the permanent access ramps and open up two junction caverns to the main tunnels.

Kedron is about halfway along the main tunnel alignment and the caverns need to be completed before the TBMs arrive at the location. Two 12.48m diameter Herrenknecht TBMs are scheduled to be launched in mid-2010, in Clayfield.

The original plan was to use the ramps to open up the caverns and also provide access for final concrete lining works. However, due to the more extensive ground support needed in weaker strata, continuing with the plan would have severely delayed cavern construction with a knock-on to the TBM drives and completion of the entire project.

The JV began excavation work early this year on the fixed-price, fixed-term contract to complete all construction by 2012. The project involves construction of 11.8km of main tunnel, including 5.7km long twin tubes.

In layout, all the project’s ramps require four cavern junctions to link to the two tubes of the main tunnel – two in the Kedron area and two at Chalk St, where the TBMs will eventually be dismantled. Only the Kedron caverns are affected by the change to construction plans.

Generally, the geology in the overall project area is mainly regionally metamorphosed rocks, including hardened shales, greywackes and quartzites.

At Kedron, the anticipated strata ranged from Brisbane Tuff at the cavern invert (high strength) and crown (low to high strength). In between the early data suggested intersecting sediments and Tuff, overlain by highly weathered material and residual soils. Above the crown the low strength Tingalpa formation was predicted.

Additional site investigation was done after the project had commenced, and the geology was found to be more variable and weaker, in general.

The consultant hired by the Government to review the JV’s application to change plans said that normal levels of site investigation for the size of project had been performed prior to the start of the job.

The Wooloowin shaft will also be used for access in the final concrete lining works, said the JV.

BrisConnections, the client and concessionaire for the new link, is a consortium of the JV partners with Arup, PB and Macquarie.