Willis Street works nearing connection stage

The Team from Wellington Water

The first stage of the work to connect a new wastewater pipe in Willis Street, replacing the temporary overland pipe installed after the original tunnel collapse in December, is expected to be complete by the end of March.

Wellington Water Group Manager Tonia Haskell says the final stage of pipe laying along Willis Street is being completed using trenchless methods to lay the pipe underground and below existing services at the Ghuznee Street end.

“This means we’re now approaching one of the last and most complex parts of the job, to connect the new permanent underground pipe to the network and decommission the overland pipe along Willis Street.

“To achieve this we will need to shut down the wastewater pumping stations close to the harbour that send flows up to and along the overland pipe. We’ve been doing a lot of planning for this as it is risky and at all costs, we want to avoid spilling any wastewater into the harbour.

“We need low tide, dry weather, low wastewater flows and ensuring that all existing city wastewater storage is empty so it can take the flow. We also have contingency planning in place including sucker trucks, and ensuring the old interceptor is clear to support management of the wastewater flows.”

Ms Haskell says all this is expected to take place in the week of 23 March pending having optimal conditions.

“If we get heavy rain for example, we wouldn’t be able to turn off the pump stations to disconnect the overland pipe due to the risk of overflowing in the harbour. So in that instance the work would be delayed.”

This phase of the work is expected to take between 6-8 hours, however, it will require Dixon Street between Willis and Victoria Street to be closed to through traffic for up to three days. This is to allow the underground pipe to be exposed, the connection to be made, and then the road to be reinstated. One lane of Willis Street will be opened for for local traffic, travelling one-way from Ghuznee Street entrance.

The pipe ramp will still be in place so the road cannot be used as a main thoroughfare and there will still be no parking along the section of Willis Street between Ghuznee and Dixon streets. Traffic controllers will be onsite to assist throughout.

“Once the permanent pipe is connected, we can remove the overland pipe and bring in contractors to reseal the road. We are talking with Wellington City Council now about reinstatement of the road, which we’d expect to take one to two days. The focus will then shift from Willis Street to laying new pipe in the adjoining slipway down the motorway. This is expected to be complete mid-year but will have minimal traffic impacts.”

Meanwhile, investigative drilling work is being done to determine the best approach to sealing the tunnel at the separate Dixon Street site.

“Work to drill the final boreholes will continue this week and once that is completed, we’ll be able to do a thorough geotechnical assessment of the ground conditions to allow us to complete design work.”