Within its US$1.4bn works contract, the cost of the tunnelled section to TOJV is reported as approximately $400m. Where much of the surface highway construction was sub-contracted, TOJV managed the tunnel construction itself. Designers of the fast-track design+build tunnel contract are Hyder Consulting, an Australian subsidiary of Hyder UK, in JV with Australian consultant CMPS&F, now part of the Egis Group, and with specialist tunnel design advice provided by Jacobs Associates of San Francisco. Golder Associates completed the geotechnical investigation and Halcrow of the UK, with Kinhill of Australia, are the proof engineers for the JV and also the specialist engineers representing the syndicate of financing banks.

The Office of the Independent Reviewer oversees the project’s design and construction for Transurban and the Victorian state government authority that will take over ownership and operation of the expressway at the end of the concession. Engineers from Parsons Brinckerhoff of the US, as part of its role within the Office of the Independent Reviewer, among others, have been engaged to study the current problems

At up to 65m deep, the CityLink Burnley Tunnel is the deepest and longest tunnel or foundation excavation under Melbourne’s metropolitan area. The hydrogeological regime of the local area was partly known at the start of the project but records show that the course of the Yarra River has changed several times in recent history, creating complex geology between the ground surface and the tunnel. Studies of the groundwater encountered during excavation identified it as salty seawater trapped in the mudstone for up to a million years.

The watertable adjacent to the Yarra rises to within 5m of ground surface. According to Malcolm Short, manager-engineering for the TOJV design+build team, the tanked central section of the 3.5km long Burnley Tunnel starts at 35m below ground surface on the west side and 45m on the east side of the central portion, with a drained tunnel section either side to the surface.

The invert membrane in the tanked sections was installed in two halves with a heat welded join down the centre. Once the membrane was installed, the 1.8m thick slab of mass 40MPa unreinforced invert concrete was placed in 12m long bays, again in two halves. The two-part installation process was needed to maintain programme and sustain vehicular access to the leading bench excavation. The sequence was time efficient but it did make for a difficult work site at the central join in the membrane. Behind the trailing invert, the stations for membrane installation and final concrete lining of the tunnel arch followed using three 12.5m long CIFA forms from Italy.

"Cracks in the 1.8m thick unreinforced invert concrete slabs were identified in mid-1999," said Short. "An investigative testing programme confirmed that some slabs in the 250m mid-section of the Burnley Tunnel were not serviceable. Some six or seven of these slabs have to be repaired and a programme of installing rock anchors to hold the floor down was proposed by TOJV. This work started in Nov 1999 and further studies are under way to test the serviceability of the invert slabs for 700m either side of the identified 250m problem area."