Pipe-threading operation set to go

LIKE an unbroken strand of liquorice, more than half a kilometre of plastic pipe has been joined together, ready to be pushed under the Whanganui River next week, ticking off another chapter in the city's massive wastewater treatment scheme.
The pipe ? technically it is called high-density polyethylene ? has a slightly smaller diameter than the current concrete pipeline that links the Beach Rd pumping station on the city side of the river, to the South Beach outfall.
The new plastic pipe will be inserted into the current line by machinery pushing and pulling from either side of the river.
Rick Grobecker, deputy infrastructure manager for the Wanganui District Council, said water was being pumped into the pipeline under extreme pressure yesterday to test the welds in the line.
On Monday the Beach Rd pumping station will be shut down and all the water pumped out of the existing under-river pipeline. A drilling machine will then push through a much smaller diameter pipe from Beach Rd to Landguard Bluff.
Mr Grobecker said engineers would then attach a submersible pump to that pipe before pulling that pump halfway back along the concrete pipe to clean out any water still in it.
Then a 12m length of the high-density plastic pipe will be pushed into the pipeline from Beach Rd across to the other side of the river.
"In the river crossing there are four bends, and we know there'll be a lot more friction created on the new pipe. By pushing the small section of pipe through, we'll be able to gauge what sort of pressures we can expect."
"After that test plug pops out the other side, we'll attach the drill rods to the 600m length of new pipeline and start pulling from the Beach Rd end and pushing from the Landguard Bluff end to get the new pipeline in position."
The engineers reckon it will take about 10 hours to get all the pipeline in place, with the new pipe moving at the rate of about a metre a second.
The new pipe is a metre in diameter and will "float" inside the existing pipeline, which is just under 1.2m in diameter. "In essence, there's about seven inches of clearance between the new and the old pipeline," Mr Grobecker said.
The high-density pipe, which weighs 120 tonnes, is being used because the existing line doesn't have the capacity to handle the increased pressures the wastewater scheme demands.
"The new treatment ponds behind the airport are 25m higher than the current South Beach outfall, so greater pressures are needed to move the wastewater.
"But, more importantly, if the power fails, you get a pressure wave coming back down the pipe.
"That pressure wave could be enough to smash through the current concrete pipeline, so that's why we've had to sleeve in this new line," he said.
The new pipe is thicker than needed too, because the engineers expect a little exterior damage to occur when they push and pull the pipe inside the existing under-river line.
"And this option is quite a bit cheaper. If we were to lay a new pipeline, it would cost $5 or $6 million-plus. This option is costing us about $1.5 million," Mr Grobecker said.
While the new pipeline is being sorted out, the city's domestic wastewater will have to be discharged straight into the Whanganui River until April 14.
Waste will continue to be pumped into the Tasman Sea via the South beach ocean outfall until the treatment ponds are commissioned by the end of June.