Brisbane is Australia’s fastest growing metropolitan region, according to the Queensland Government, which expects the population of the South East Queensland region to grow from 2.8M to 4.4M from 2006 to 2031. It’s one of the main drivers behind a massive road infrastructure project the state introduced in 2005 to connect the city with the airport and northern suburbs.

The Airport Link, Northern Busway (Windsor to Kedron) and Airport Roundabout Upgrade projects are being delivered together in a single design and build contract. BrisConnections, consisting of Macquarie Capital Group, Thiess, John Holland, Arup and PB, was awarded the contract worth AUD 4.8bn (USD 4.8bn) in May 2008. It is to finance, design, build and operate the Airport Link toll road for a period of 45 years, while the Northern Busway and the Airport Roundabout Upgrade will both be handed back to the state after construction is completed in mid 2012.

A joint venture of Thiess John Holland (TJH) has been contracted for design and construction on the projects, including two separate tunnels: one for the 6.7km Airport Link and one of about 1.5km for the Windsor to Kedron section of the Northern Busway extension (see side block). The Queensland government planned the two projects in conjunction because they share a corridor between suburbs Bowen Hills and Kedron. Once completed, the Airport Link and Northern Busway combined should reduce traffic on the major thoroughfare of Lutwyche Road by up to 40 per cent.

TJH is also building the 750m flyover bridge and ‘fast-diamond’ intersection that will replace the airport roundabout in the contract’s third project, which opened in November 2010, a year early.

These three projects to accommodate potential population growth are hindered by the already densely populated north side suburbs. “There are 31 000 households along the project alignment, and 8000 of these residents are living next to, or within close proximity to, the nine major worksites we have operating across Brisbane,” says Gordon Ralph, Thiess John Holland project director.

After a number of breakthroughs this year, BrisConnections announced in December that work is 60 per cent complete and on track to finish as scheduled, despite the pressures to reduce community impact during construction, and the discovery of weaker ground requiring adjustments to the Airport Link project.

Airport Link alignment
The 6.7km toll road will be primarily underground and will allow drivers travelling from the city to the airport to avoid up to 18 sets of traffic lights. The alignment was based on three surface connections to (from south to northeast) the suburbs of Bowen Hills, Kedron and Toombul.

Three-lane twin tunnels are being constructed in hard rock using roadheaders between Bowen Hills and Kedron. Two-lane twin tunnels are being excavated by EPB TBMs from Lutwyche to Toombul where ground conditions are predominantly clays and siltstones. In total more than 11km of tunnel and ramps will be constructed for the Airport Link portion of the project, requiring up to 17 roadheaders and two 12.48m diameter Herrenknecht EPB machines with some cut-and-cover work. BrisConnections adjusted the alignment to reduce grades and to take advantage of the Brisbane Tuff that underlies the alluvial soils present throughout the northern suburbs, according to a paper presented at WTC in Vancouver by representatives of the concessionaire. This would help reduce the risk of ground surface settlement. Revisions added the mid tunnel access shaft at Truro Street, extended the TBM excavation and moved the TBMs’ launch box from Kedron to Kalinga Park in Toombul.

Moving the location of the TBM launch avoided interference with a local high school and emergency services in the area, however it introduced other issues, as the park is prone to flooding.

“The challenging ground conditions in the area, comprising mostly of soft clays, meant we had to pump 350t of bentonite into the ground to stabilise it prior to excavation,” says Ralph. “Enormous volumes of concrete have been used during excavation and construction, with more than 12 500 cubic metres for completion of diaphragm support walls alone.”

“Adding to this complexity is the surrounding Toombul flood plain on which the launch box is constructed, requiring a flood wall to be built around the entire perimeter for protection in the event of a 1- in-100-year flood,” he explains.

Revising the alignment also changed the Kedron connection from a surface arrangement done by cut and cover to a network of underground tunnels and ramps built by roadheaders. Two cavern junctions with a maximum span in excess of 25m would be built to link the two tubes of the mail tunnel with the entry and exit ramps.

Generally, geology in the 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-tohigh strength). In between investigation 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 found the Brisbane Tuff to be present, but to a lesser extent and quality than anticipated (Figure 1). Besides making excavation more difficult than expected, the new-found geology would require a heavier, more robust support than in the initial design, which would also add delays to the schedule.

In July 2009 TJH sought and received permission from the Queensland government in late 2009 to sink an extra access shaft on Rose Street in Wooloowin, measuring 42m deep and 15min diameter. This shaft allowed road header excavation of the east and west caverns to complete before the TBMs arrived from the launch site at Kalinga Park, as it provided a total of three extra work fronts to overcome any delays.

The consultant hired by the Queensland government to review TJH’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.

Revising the alignment had an impact on other aspects of the Airport Link’s construction. Some of the proposed traditional hard rock tunnelling for the ramps has instead used a slower and more involved NATM approach, according to the BrisConnections WTC paper.

Tunnelling
Despite earlier fears of delays, Airport Link completion is on track. Tunnelling started in March 2009 with the first roadheader—of what would eventually be 17—excavating from Truro Street, south to Bowen Hills. Roadheader excavation varies to depths as much as 35m underground, and is being used for entry and exit ramps, as well.

Ground conditions are primarily Neranleigh Fernvale rock, unique to the Brisbane area. Cutting heads feature up to 72 picks for hard rock and 57 for soft rock. On average the roadheaders being used on the project weigh 135t and measure 18m in length. Manufacturers have included Aker Wirth, Sandvik and Mitsui.

In June 2010 roadheaders from the Bowen Hills and Truro Street sites met in the northbound Airport Link tunnel, as the project’s first breakthrough. By end of November, roadheaders had met in the southbound tunnel as well.

BrisConnections estimates permanent concrete lining of both tunnels should be complete by early 2011, while back-end work such as drainage and smoke-duct installation will continue into the year.

North of the Truro Street site, roadheader excavation in the northbound and southbound mainline tunnels is still operating, 24 hours a day. Roadheader excavation on Kedron exit and entry ramps has completed where cut-and-cover construction will take over on the surface.

Two Herrenknecht EPB machines are each boring 2.5km of twin tunnels from Toombul to Lutwyche at various depths up to 55m from the surface. The biggest TBMs ever to work in Australia, according to BrisConnections, each AUD 45M machine was assembled inside a 105m long x 35m wide x 22m deep launch box at Kalinga Park. Two 160-t cranes and three 40-t cranes were required to lower 250 separate components for the 3600t machines. Each 12.48m- diameter cutterhead, carrying 73, 17-in cutter wheels made of hardened steel, weighed 220t alone.

“Because the machines are 195m long they have to be launched in two stages. Once ‘Rocksy’ and ‘Sandy’ have excavated 130m they will stop for six weeks so the rear of the machine can be assembled,” Ralph explains.

The first machine, ‘Rocksy’, arrived in Brisbane in November 2009 from Germany and started on the eastbound tunnel in July 2010. ‘Sandy’ arrived in April and started tunnelling in August for the westbound line. ‘Rocksy’ had progressed 70m in the eastbound tunnel when ‘Sandy’ started.

The TBMs are installing a steel-fibrereinforced precast segmental lining in a 9+1 configuration, with an internal diameter of 11.4m. Segments have been produced at a casting facility in nearby Eagle Farm.

BrisConnections expects more 1.1 million tonnes of spoil will be generated by the TBMs. TJH is running a 1.8km-long spoil conveyor belt that travels from the Kalinga worksite to a purpose-built spoil handling facility adjacent to an arterial road network. This removed approximately 80 000 truckjourneys from nearby residential streets.

“We understand the construction activities inconvenience and impact some residents,” says Ralph. As the Airport Link project nears completion next year, Kalinga Park will be reinstated and returned to the community with enhanced barbecue facilities, waterways and cycle paths.


The October 2010 breakthrough by an Aker Wirth roadheader T3.20 Fig. 1: Anticipated and actual ground conditions for the Airport Link Fig. 2: Map of the tunneldrive alignments and interconnections