Stage one of the project - replacing the 110-year-old pipe from Anzac Parade to Moana St for $2.39m - is nearly finished.
The next stage involves replacing the pipe from Moana St to the five-road junction at the Raine St, Wakefield St, Nixon St, Burton Ave and Duncan St intersection.
Without the additional funding, it would need to be deferred.
Councillor Kate Joblin asked if the presence of timber could have been foreseen when “working up a big infrastructure project like this”.
Venter said staff couldn’t see what was under the ground and could only go on samples.
“The other alternative was to come up with a completely different method of construction or allow the line to collapse and then try to do a retrospective rehabilitation.
“That comes out to three times [the cost] to what we are now. I believe we are still getting really good value for money.”
The Chronicle first reported on the logs in July, with council three waters contracts manager Stuart Cawley sending a sample to get age-tested.
The results made his “jaw drop”.
Venter said workers had “broken the back of this job” and the funding would allow them to get out of the street by mid-next year at the latest.
Parts of Nixon St have been off-limits to traffic since the project began in 2022.
According to his report to the council, the poor soil quality meant “expensive soil stabilisation” was needed for the preparation of the bore-tunnelling work.
This was done by injecting cementitious grout at regular intervals along the pipe alignment.
The preserved logs had been encountered at random locations at a depth of the tunnelling work - six to seven metres, with the bore tunnelling machine unable to drill through the timber.
That required the sinking of new shafts to locate the bore-head.
Speaking to the Chronicle, Venter said the second part of the project would follow a more traditional open trenching method as the pipe wasn’t so deep.
The one-of-a-kind Vermeer AXIS laser-guided boring machine - operated by contractor Parkinson & Holland - would only be used in stage one.
“I do think we are out of the layer where we find that [timber],” he said.
“The pipe needs to flow from a gradient, so we’re going from the bottom and working our way upwards. It’s gradually getting shallower and shallower.”
Mike Tweed is an assistant news director and multimedia journalist at the Whanganui Chronicle. Since starting in March 2020, he has dabbled in everything from sport to music. At present his focus is local government, primarily the Whanganui District Council.
