Nelson City Council has praised the efforts of contractors working on the St Vincent Street infrastructure upgrade project, who they say successfully pulled 1800mm diameter stormwater pipes through the 79-year-old box culvert in just five minutes thanks to a new innovative winch system.
“This was a major milestone for the project and a great example of an approach developed through ECI (Early Contractor Involvement) saving time and money,” says Manager Capital Projects, Rebecca Dawkins.
“This methodology meant we completed six metres of pipe installation in just five minutes, without demolition or excavation, saving approximately $1 million and significantly reducing construction time.”
By creating a custom tug-and-pull winch system, Tasman Civil were able to pull 18 twin 1800mm pipes through the existing culvert. Traditional trenching would have required substantial additional cost, resources, time, demolition work, landfill disposal and road reinstatement, which would have added weeks to the project length.
The biggest challenge to the overall project so far has been the discovery of unexpected underground services.
“We were aware a historic brick-lined sewer may have been in the area,” says Ms Dawkins.
“We potholed where records indicated it may have been and found nothing. It turns out we potholed in the one location it had been removed, and once trenching began, we found it.”
Potholing before construction allows contractors to locate underground services and better inform the design. While Council wasn’t 100% certain of every service in the area, there was certainty around the location of major power cables.
“Excavating a large hole directly outside Nelson Electricity came with its own challenges,” Ms Dawkins said.
Six major power assets exit the substation and cross St Vincent Street. These were fully encased in concrete above the box culvert and needed to be carefully broken out. Due to the power load and risk to the network, restrictions were put in place.
“The six cables needed to be isolated and work near the cables also needed to be completed outside the winter months, one 11kV cable needed to be rerouted down Halifax Street, and a second temporarily diverted around the worksite.”
Tasman Civil worked closely with Nelson Electricity and Delta during the ECI phase to develop the methodology for the excavation and throughout construction.
Before the pipes could be manoeuvred through the culvert, the team cleared half a metre of silt that had built up over decades. Once the culvert was clear and mats had been laid to reduce friction, the pipe installation began.
Each pipe section was lowered into the trench and slowly winched through the culvert. A second pipe was then lowered, welded to the first, and the process repeated until all 18 pipes had been installed, with less than 100mm clearance on either side.
The Council says the project remains on track for completion later this year.
