Building a stormwater pipe next to a river, during high tide and rain events, is understandably complicated.

In September, Tasman Civil began the process of replacing the 79-year-old stormwater culvert under St Vincent Street. Before work began, the team installed a cofferdam to help manage the flow of water.

If you’re not sure what a ‘cofferdam’ is, Manager Capital Projects Rebecca Dawkins explains.

Heavy rainfall and high tides created challenges for the project, with spring high tides limiting construction to low-tide periods.

Rainfall that cannot be fully cleared by onsite pumps makes it necessary to manage stormwater flow through the site. To address this, a storm gate was built into the cofferdam to allow controlled overflow.

During periods of heavy rain, the storm gate is opened and stormwater is directed through the site under managed conditions, allowing work to continue safely.

Before a project begins, the contractor always undertakes a potholing exercise. This is when small holes are dug to confirm that services are where we expect them to be.

“Potholing is a really helpful exercise that help us map out the design and construction plan,” says Rebecca.

“But we’re never 100% sure where services are until we begin excavating.”

For just over a century, from 1872, Haven Road, near Fountain Place was the home of Nelson City Gas Works. Underground services still show remnants of historical Nelson.

“We’ve found a lot of previously unrecorded underground services including old gas mains, cables and concrete/brick structures.”

As for the known services, they’ve also proved challenging.

Finally, underground infrastructure always add the complication of being.... underground. The scale of the excavation for this project in width and depth has required a specific design to provide a safe working environment. The 9.5m wide and 3.5m deep trench even has a scaffold bridge to provide access.

Despite all these challenges the contractor has worked hard to keep the project tracking on time with this section of the work on St Vincent Street expected to be completed this side of Christmas, and the next section beginning early 2026. This will again require the traffic management to change with Halifax St opening, and St Vincent Street closing.