More than 10,000 wick drains have been “planted” beside the causeway on Auckland’s Northwestern Motorway (State Highway 16) to help remove sea water from thick mud as part of the NZ Transport Agency’s Causeway Upgrade Project.
The drains – made from plastic and fabric - are planted 10-15 metres deep in marine mud on both sides of the Traherne Island area. They work like elaborate drinking straws, and are placed 1.3m apart draining water from deep underground to help with the consolidation of the layers of marine materials.
“Intensive ground reclamation like this is an important part of the project’s work to upgrade and improve this section of the motorway,” says the Transport Agency’s Highways Manager, Tommy Parker.
“Draining water from the marine mud needs to be done carefully and takes time to be done effectively. We are mindful to protect the environment while we work, installing new bird roosts for species which inhabit the area, and silt fences,”
A two to three metre-deep layer of fill, or drainage blanket, using more than 26,000 truckloads of marble-sized quarry stones, has now been spread on top of the wick drains to help squeeze water out of the mud below, a process that will take up to 12 months. When the results indicate that the ground is stable, construction of new motorway lanes will start on top of it.
Key features of the 4.8km-long project being delivered by the Causeway Alliance include widening the motorway between the Great North Road interchange and the Whau River bridge at Te Atatu. When finished in early 2017, the causeway will have been raised 1.5m to stop flooding at extreme high tides, there will be four lanes city-bound and four/five lanes westbound with dedicated bus lanes in each direction, and the existing Northwestern cycleway between Great North Road and Te Atatu will be upgraded.
Mr Parker says as much work as possible is done during daytime off-peak hours to minimise any disruption to drivers and to avoid disturbing neighbours with construction noise at night.
“We appreciate drivers’ co-operation while we get the upgrade work done. The motorway runs through a live and busy construction site and it is important that they observe the 80kph speed limit between St Lukes and Te Atatu for their safety and the safety of our workers.”
The Causeway Alliance - the Transport Agency, AECOM, Coffey, Fulton Hogan, Leighton Contractors and Sinclair Knight Merz – is completing the upgrade.
The project is one of five underway or planned to finish the 47km-long Western Ring Route along the Southwestern, Northwestern and Upper Harbour motorways, which is identified by the Government as one of its Roads of National Significance to support New Zealand’s future prosperity. The others are construction of the Waterview tunnels to connect the Southwestern and Northwestern motorways, upgrades of the Lincoln Road and Te Atatu Road interchanges, and improvements to the Northwestern Motorway from St Lukes to Great North Road.
Mr Parker says there will significant benefits for drivers when the Causeway Upgrade Project and the other related works are completed.
“For the first time, Auckland will have direct motorway access between the CBD and the airport, and the Western Ring Route will also better connect people and freight with the city’s rapidly growing areas in the north-west and south west.”
Mr Parker adds that the Western Ring Route will also benefit the Northland and Waikato/Bay of Plenty regions.
“It’s a key part of a massive investment in infrastructure in Northland and Waikato/Bay of Plenty needed to meet the demands of rapid economic and population growth underway in the top half of the North Island,” he says.
For more information on about the Causeway Upgrade Project visit the website(external link).