Contractors Move a River for Whitebaiters

Published: Fri 21 Aug 2009 03:54 PM
Contractors Move a River for Whitebaiters
Engineering and earthmoving company Taylors Contracting has been tasked with moving a river twice as part of a major bridge re-building project on the South Island’s West Coast – but the deadline has been determined by whitebait season.
The joint KiwiRail and NZ Transport Agency project to reconstruct the 120 year old Arahura road / rail bridge began 18 months ago. The new bridge includes a two lane road for state highway 6 between Hokitika and Greymouth and separates the rail line from the road.
HEB Structures are the main contractors for the job, with Taylors subcontracted for the river work. This involved constructing gravel guide bank and rock protections to enable abutments and pier heads to be built.
Taylors Contracting Project Manager Neil McKay says they first shifted the Arahura River 12 months ago, from hard against the North abutment back towards the middle of the bridge. Now they have until September the first to carefully move it back to its original position prior to the start of whitebait season.
“The river is privately owned and whitebaiting is a very important local activity. Part of the contract was to ensure whitebaiting wasn’t interfered with. This condition determined the deadline for the river movements. It’s all about doing the job with the least impact on the river, “he says.
Neil McKay says it’s the first time he’s known of whitebait to be a reason for a contract deadline.
Moving a river is not easy work. When diverting main river flows there are numerous environmental factors to consider such as using a method that allows fish to exit the old river flow and mitigating downstream siltation.
While Taylors used GPS technology on their diggers for underwater rock placement it was also utilised to achieve the new river depths.
“It is quite skilled to learn to work a machine by only looking at a screen in the cab. You’re effectively working blind when you’re under water. It’s muddy and there’s no visibility so we need the best technology to do the job,” says Neil.
Taylors have quarried, carted and placed about 60,000 tonnes of rock in the Arahura River and used a precision pattern to ensure each rock shows a smooth face to help water flow. The underwater walls are laid to an exact height. They had to be 2 metres deep and up to seven metres above the water surface in some places. This meant digging down as much as 4 metres below the riverbed and building rock back up.
“When you drive over the bridge people may not realise that there’s probably as much of the rock wall below river level as there is sticking out,” says Neil.
Neil McKay is confident they’ll make the September 1 deadline and he says it’s been a really great project to work on.
“We employed four locals from the coast and then we brought myself and another driver with the GPS experience. It’s been an awesome challenge, especially with the west coast wet weather!”.
The next stage of the job for Taylors is to complete remaining abutment protection works and from there on it’s all about site rehabilitation as well as assisting HEB and ONTRACK complete a heritage park which will display a section of the old bridge.
HEB Structures has been contracted to build the new 220 metre long bridge, which is due to be completed in January 2010. It consists of nine steel and concrete spans and 20 concrete piles. Work has been carried out in stages to keep State Highway 6 open throughout construction. The contract has been meticulous in its planning and HEB Structures in coordination with Taylors have ensured all works tracked precisely to program.
KiwiRail’s infrastructure arm, ONTRACK is leading the bridge construction and NZTA is responsible for the road approaches.

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