Connell Contractors staff were replacing a 120m section of a major pipeline alongside Bluff Highway, and would then begin an even more complicated project to renew 600m of pipeline in Mersey St, Invercargill City Council solid waste and drain› age manager Malcolm Loan said.
The out›of›town company was hired because it had the specialist staff and machinery to be able to replace a pipeline without excavating the full length of pipe, and had temporary bypass pumps to keep the sewage flowing while the work was under way, he said.
The Bluff Highway job, near Janet St, was costing $300,000 while $1 million had been budgeted for Mer› sey St, he said.
Mr Loan said at Bluff Highway, a regular inspection revealed about 100m of 50›year›old concrete pipe was corroded.
The pipe carried about two›thirds of the city’s sewage from the Mersey St pump station to the city treatment plant at Clifton, he said.
The corrosion had been caused by ‘‘acid attack’’ — something known to happen to concrete pipes when sewage became anaerobic (oxygen free), then mixed with air after it left a rising main.
The contractors, who began work three weeks ago, had installed five large portable pumps and five tem› porary pipes to keep the sewage flowing while the pipes were dug out and replaced with a plastic›lined concrete alternative.
The work was expected to be completed soon.
At Mersey St, a pipeline installed about 100 years ago would be replaced, from the Otepuni Stream to Spey St, Mr Loan said.
The job was complicated because the pipeline was deep and in unstable ground.
‘‘Since it was laid the site has been covered with rubbish and silt from estuary reclamation work. Some of the pipeline is under buildings.
‘‘It would be next to impossible to excavate down to the pipe, so it is being replaced by trenchless meth› ods.’’
The process involves digging large access pits at regular intervals and using a machine to demolish the old sections of pipe and draw new pipe into position.
Because of the complexity of the work there was some uncertainty with costs, he said.
‘‘We have a contract for close to $1 million, but there may have to be some minor changes to this. As is normal with this type of work, contingencies have been built into the contract price in case the job turns out to be more difficult than expected.’’
That project would be started later this month and was expected to be completed by the end of June, he said.