Abigroup uses innovative solution to build environmentally sensitive Ross River Bridge
17 October 2011
The $110 million Douglas Arterial Duplication (DAD) project involves the construction of 5.7km of two lane road to relieve traffic congestion on a section of the Bruce Highway through Townsville in North Queensland. The project, which is being carried out for the Queensland Department of Transport and Main Roads by Abigroup in an Early Contractor Involvement Joint Venture, involves the building of four new dual lane bridges including an eight span bridge across the environmentally sensitive Ross River.
DAD, which is due for completion at the end of the year, involves 211,000 tonnes of earthworks, 43,500 tonnes of pavement gravel and 4200m3 of concrete for the bridges and the placement of 105 bridge girders. The project has a workforce of 175 including sub-contractors.
Abigroup Construction Manager Glen Langfeldt said, “We’re making good progress on DAD, which we started back in April 2010, despite the number of days lost due to our extended wet season and the effects of Cyclone Yasi. “The contract extends to cover duplicating the existing Ring Road and building four multi-span bridges, we’re also putting in place concrete barriers to divide the road, creating hard shoulders and two sets of traffic signals.”
Because of environmental concerns, Abigroup was required to come up with an innovative way to build the Ross River Bridge so as not to risk polluting the river and ensuring its flow is not impeded during the wet season. The river acts as an alternative emergency water supply for Townsville.
To build the bridge Abigroup used a launching truss for bridge girder placement – the first time this sort of technology has been used in North Queensland. Using this method enabled major work on the bridge to be carried out over the river rather than using the more conventional method of cranes based on pier foundations in the river bed or floating barges.
It took ten weeks to determine assembly methodology and then assemble the 90m, 80 tonne, launching truss which was delivered in two shipping containers and 14 semi-trailers. The first stage of assembling the truss involved bolting together segments on 1200 high temporary stands and fitting the hydraulics and electrics. Girder trolley drive units, which are driven on a monorail toothed rack, were then hung under the main truss section. Catenory cabling along the truss allowed the trolleys to be driven via a pendant control unit from beside the truss. The hydraulic components are all within the truss system and Ecoterra 46, a biodegradable vegetable oil, was used throughout the hydraulic system.
The second stage of assembly involved lifting the 90m truss onto three 4.5m high temporary towers which allowed sufficient height for support legs to be attached and all launch supports to be fitted onto the truss monorails.The system has three independent launch support leg assemblies with jack controls which are individually operated from a platform within each leg. This allowed the truss to be jacked up at each of the three launch supports enabling the truss to be driven forward to the next forward bridge pier. Three cameras were mounted on the transverse movement cylinders which fed to a monitor above the control panel so that the operator could control the truss to remain within a 50mm sideways movement tolerance range.
DAD Project Construction Manager Glen Langfeldt with a section of the new road behind him
Senior Project Engineer Greg Lowcock (at left) and Supervisor Alf Panlook in front of the launching truss which was in the process of being dismantled before being moved to Abigroup's Townsville Port Access Road project
After all the sections were carefully checked and extensive testing carried out, the launching truss was ready to move its first girder. Before then the whole structure was tied down to secure it against high winds. Extra tie down to the original design was put in place prior to Cyclone Yasi which struck at the beginning of February. The launch process involved extending the truss across the river over pre-built piers. Support legs were lowered on each of the pre-built piers to support the structure. Once the truss was secure at the forward pier the launch support legs were removed to leave the two main truss spans clear for girder placement. This process involved the truck carrying the girder reversing to the rear of the truss and a platform being lowered to lift the girder into the truss where it is attached to the monorail. Restraint plates and bearings were fitted to the girder from the previously completed bridge deck before the girder was fully suspended in the truss and driven to the required position via the drive unit in the adjoining trolley and the pendant operation from the ground below. When the girder was in position it was lowered from the truss by the lift cylinders at each end. Once all the girders had been placed in a section of the bridge, deck work on that span of the bridge was completed followed by launching the truss forward again to the next pier to begin the cycle again.
Abigroup Senior Project Engineer Greg Lowcock was responsible for the work site development of assembly methodology and bridge construction using the launching truss. “We carried out the work during the wet season so as not to lose any time on the project. There were so many pieces and components it was almost like assembling a giant Meccano set!” he said. “Then, just as the assembly was completed, Cyclone Yasi struck and we were really worried about the stability of the truss. Working with the truss designer we tied the truss down as securely as we could and fortunately it survived the Cyclone and we were able to recommence work.”
It took the truss seven months to move the required 56 concrete girders, each weighing 77-tonnes, into position and the Bridge was fully completed in September. The truss has now been dismantled and is currently being reassembled a few kilometres down the road to help lift support beams on another bridge over the Ross River as part of Abigroup’s Townsville Port Access Road project. JK Watson Engineering designed the truss and also supplied all of the hydraulic and electrical components. Capalaba Engineering manufactured the steel components.