Raising the road: Causeway Upgrade

The $220 million Causeway Upgrade project is vital to the improvement of Auckland’s roading network and its completion date is just a few short months of summer asphalting away. 
By MARY SEARLE BELL.

THE CAUSEWAY CARRIES the Northwestern Motorway and a cycleway across the Waitemata Harbour, providing a direct connection between the western suburbs and the rest of the city.

It was originally constructed in the 1950s and subsequently sank by up to two metres. As a result, during extremely high tides and bad weather, the causeway would flood – closing lanes for motorists and leaving cyclists and runners to battle water up to knee deep.

The upgrade project, which is being undertaken by the Causeway Alliance, began in 2013 and is due to be completed early next year. However, much of the work is done, and users are already enjoying extra lanes and a better view from the higher and wider causeway.

Over 1,500,000 tonnes of quarried material has been imported, handled and spread to build up and widen the causeway by up to 2.5 metres for half its 4.8 kilometre length. The new size allows for the road to be configured with five lanes heading north and four lanes southbound into the city, where there used to be just three each way. There will also be dedicated bus shoulder lanes and the cycle path has also been improved.

The project has had to address a number of complex challenges to get to where it is today.

The causeway runs beside the Motu Manawa – Pollen Island Marine Reserve, and the construction team had to ensure the work had little or no impact on it. The reserve includes intertidal mudflats, tidal channels, mangrove swamps, salt marshes and shell banks. It is an important feeding ground for a number of wading birds, such as the endangered banded rail, the shy and vulnerable native fernbird, and migratory birds such as godwits and sandpipers, which fly in each summer from Asia.

The mudflats and swamps are, as you would expect, soft soils, which is why the original causeway sank. The variable, deep, weak subsoil includes Jurassic-period greywacke from 200 million years ago, marine mud that is 25 million years of age, 30,000-year-old basalt from the Mt Albert lava flow and more recent shell banks. The project has been described as building earthworks and a road over material with the consistency of toothpaste.

Dr Daiquan Yang, principal – geotechnical lead, of engineering consultancy Coffey, says the Causeway Upgrade project is one of most challenging projects he has ever been involved with in his 26-year career in Asia, Europe, Australia and New Zealand.

“The level of geotechnical risks, complexities and challenges is found to be far greater than previous other projects. During the tender design phase, many experts from New Zealand and overseas [Australia] were inspired by the geotechnical design team that we were challenging something almost impossible.”

The geotechnical design solution involved the use of 24,000 wick drains, to remove low level pore water from the marine muds to allow consolidation. Preload was used to achieve the required ground strength, as opposed to other solutions such as deep soil mixing and the introduction of cement stabilising, which was a risk next to such a sensitive marine environment.

The various phases of reclamation works included construction of a temporary stability berm allowing the installation of wick drains, a temporary rock cofferdam that protected the site from the sea and the sea from sediments, and construction of the permanent embankment.

Senior project manager Simon Paton says the chief engineering challenge was to get the stiffness underneath the new section of causeway the same as the old so that the two behave similarly over time.

However, he says the other major challenge was working in a live motorway corridor with 90,000 vehicles going up and down every day.

“It’s a very confined space and we had to get it 1.5 metres higher – we weren’t just working on the horizontal but the vertical plane as well,” he says.

Numerous traffic reconfigurations have been required to keep the three existing motorway lanes running in both directions during construction. As of October 2016 more than 50 major traffic switches had been successfully delivered that enabled the motorway to remain open while it was being upgraded.

“The causeway has horrendous detour routes,” says Simon. “So we had to create work areas without closures.”

In addition, because of its location in the harbour, work has been planned and executed around the tidal cycles.

The cycleway that runs alongside the motorway has 450 daily commuters, and Simon says it was a condition of the contract that it remained open throughout the works. The Causeway Alliance constructed temporary paths, paid close attention to traffic management around cycleway detours and worked closely with cycle interest groups. Consequently, Simon says the team has received “quite a bit of positive feedback” from the cycling community even with all the disruption.

Still ahead of the Alliance team are 45 shifts of paving. Simon says all lanes should be open in their new alignment during October, and then the final surfacing will happen. This is scheduled for late spring/early summer. He says an Open Grade Porous Asphalt (OGPA) will be used on the motorway while a Stone Mastic Asphalt (SMA) will be used on the ramps, as it is better able to cope with breaking and accelerating.

“Ideally, we’ll have two pavers working in tandem to get twice as much done in a single night,” says Simon. “The final surfacing should be completed by the end of January.”

A few final pieces of the project due following the paving works are inherently linked to the neighbouring Waterview Connection project, with its twin tunnels and impressive new interchange. Things such as signage regarding the tunnels can’t be installed until the link is about to open.

“There’s been a lot of coordination between the two projects,” say Simon. “It’s fortunate both projects are being delivered as alliances as alliances have more collaboration anyway.”

The Causeway Alliance comprises AECOM, Coffey, Fulton Hogan, Leighton Contractors, Jacobs and the client, the NZ Transport Agency.