The confined and congested urban space in the heart of Brisbane has brought many construction challenges for the city, including for the Cross River Rail Project, the latest major addition to the metro network. Logistics and sequencing of steps becomes ever more important, and the possibilities for using off-site fabrication or precasting – and then, of course, those elements need to be threaded through the city and carefully slotted into the spots they need to go.
Recently, the challenge on Cross River Rail that faced local contractor Kenny Construction was to get 59 large, heavy, concrete trusses to the site and where they needed to go, placed within the space of a below ground station excavation. Each truss has a span of 17.6m and some had a mass of up to 70,000kg, although most were 48,300kg.
While getting so many concrete trusses to site sequentially was a fair enough challenge in itself, once on site then the transport solution for the precast units had to contend with restricted headroom and slim gaps of clearance – only 100mm on either side – to the side walls.
As ever, the clock would be ticking too, while other construction activities had to keep going on the busy project programme.
Options?
Feeding the 70 tonnes precast trusses into the tight site underground space ruled out using tower cranes as the main option. But they would play a role.
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By GlobalDataThe main transport solution was, instead, to employ multi-wheeled, highly manoeuvrable transport trailers – carrying packing structures. A few trusses could then be carried at once, slowly, carefully, down into the tunnel.
Mammoet proposed the solution as a custom packing structure positioned on self-propelled modular trailers (SPMTs). Its solution would be able move up to four of the heavy concrete trusses at once and transport them to the installation points. That would be important to the success of the construction programme.
First, though, the equipment configuration was put through its paces, off site. Mammoet says “a series of dry runs” of the packing structure on the SPMTs were performed. Part of the testing was to ensure the same configuration would serve both sizes of truss.
Then the machines got to work moving the trusses.
And, that is where the conventional site cranes come in: a tower crane had the job of lowering the concrete trusses onto the packing structures, that sat on the SPMTs. Then the self-propelled machine moved into the tunnel.
The first set of trusses to be transported had a combined mass of 193 tonnes. They went the farthest distance underground, to the far end of the station, slowly travelling a distance of 150m.
The hydraulic suspension and electrical multi-steering system of the SPMTs enabled them to carefully position the trusses in their designated positions.
Altogether, in the first phase of work, a total of 55 trusses were placed, each with a mass of 48.3 tonnes. The remaining four trusses were extra-large, at 70 tonnes each, and installed in the second phase of the operation. By taking multiple trusses at a time, each transport journey installed up to four trusses, totalling approximately 15 iterations, says Mammoet.
Fergus McHugh, Project Manager at Kenny Constructions, says, “This was a challenging scope of works, yet everything went according to plan.”
