Roadheaders and tunnel boring machines will help deliver the $4.8 billion Airport Link and Northern Busway project as quickly and quietly as possible.

THE second of 11 roadheaders to be used on the Airport Link and Northern Busway recently broke ground as the project commenced tunnelling at its Bowen Hills site.

The Airport Link, Northern Busway (Windsor to Kendron) and Airport Roundabout Upgrade, all built by BrisConnections, are estimated to cost a total of $4.8 billion. Upon completion, they will be a necessary addition to Brisbane's roads and will provide a critical new link between the northern suburbs, the airport and the city.

The second roadheader, which is 15m long, is one of four which will operate at Bowen Hills - the project's southern most tunnelling site. They will excavate two mainline tunnels for the road and Busway, along with tunnel ramps. Weighing approximately 140t each, the roadheaders can tunnel between three and six metres per day, depending on ground conditions, and each machine will be excavating between 1,800 and 2,300 cubic metres of
soil per week. Brisbane Tuff, a rock unique to the Brisbane area, will mostly be excavated.

The roadheaders are made up of Sandvik MT720s, which are operating from the Truro street portal, and SCB306s. Four of the machines were purchased from Japan, and seven are from Austria.
 
Seven operators are required to operate the roadheaders for each shift, excavating a total of 9 km for the Airport Link and Northern Busway tunnels.

Sandvik's MT720 is 18.2m long and 4.8m high. It is designed for the economical excavation of rock exceeding 120 MPa uniaxial compressive strength. It has a stabilised telescopic cutter boom for optimum cutting power application and a proportional load-sensing hydraulic system for optimum utilisation of energy.

"These machines weigh around 135t and the main cutting head is powered by a 300 kW electric cutter motor. On the Airport Link project these units will be used to create the entrance and exit ramps and caverns to the main tunnels," said Greg Paxton, Sandvik's construction sales and marketing manager.

Construction of these tunnels, the deepest section of which will be 55m, will be undertaken using a combination of the roadheaders, two tunnel boring machines (TBM) and cut and cover techniques, ensuring that the project will be delivered ahead of the state government's original schedule.

Each TBM, costing approximately $45 million, will come from Herrenknecht AG in Germany.

"We are expecting the project's first TBM to arrive later this year and commence tunnelling early in 2010. All of the project's tunnelling to date has been undertaken by roadheaders," a project spokesperson said.

By using two tunnel boring machines instead of cut and cover methods through the Toombul section of the project, a number of advantages will be made. It will reduce the overall construction period and consequent impact on Kalinga Park, reduce impacts from noise and dust on local communities and also eliminate the requirement for the construction of a major launch chamber at Kedron.

"The units provide almost no vibration via the smooth cutting process when compared to other options such as Drill and Blast. This means very little impact for residents in the tunnelling area which is of great importance to all. Given the high profile location of the job it is imperative that the units reduce any environmental impact while in operation," Paxton said.
 
Approximately 2.8 million bank cubic metres (bcm) of spoil will be removed during construction of the tunnels. Close to 340,000 bcm will be reused on the site for the back-fill of cut and cover tunnels, retaining walls and other earth bearing structures, while 520,000 bcm of spoil from TBM construction will be transported via a conveyor.

"All of the spoil from tunnel boring will be brought to the surface at the Toombul work site. A conveyor system will be used to transport spoil from the site to the Brisbane Airport Corporation site. This method will eliminate up to 80,000 truck movements from local roads during the construction period, significantly reducing impacts on the local community and environment," the project spokesperson said.

This conveyor system will also reduce vehicle operating costs, improve environmental outcomes including noise and dust reduction and reduce the overall tunnel construction period by six months, minimising community impact.

Completion of the Airport Link project is expected in late 2012.