Aerial view of Transmission Gully motorway construction progress

Cannons Creek bridge. Photo / Rosalie Willis

It's a hive of activity along the Transmission Gully motorway construction project north of Wellington. A heck of a lot has been completed. There's lots more to do. Will it open by the end of 2020? Only time will tell. Kāpiti News editor David Haxton takes a trip with Kapiti Heliworx over the much-talked-about project to see how things are going.

You see glimpses of the Transmission Gully motorway construction from the road but from the air the project is breathtaking.

Dennis Young, the managing director of Kapiti Heliworx, runs scenic trips over the mammoth project in the company's seven-seater Airbus AS350 B2 helicopter commonly known as a Squirrel.

The 25-minute tour features various highlights including the Cannons Creek bridge which is the largest structure of the project.

When finished the bridge will be four lanes wide, 226m long and about 60m high.

Other highlights include the Wainui Saddle, the Kenepuru and Pauatahanui interchanges — but overall it's just the mega scale of it all.

"The tours have been getting really positive feedback," Dennis said.

Kapiti Heliworx managing director Dennis Young. Photo / David Haxton

The $850m four-lane project is being built from Mackays Crossing at Paekākāriki linking to the current State Highway 1 at Linden in Tawa.

"It is a highly complex project being constructed on 27km of very difficult terrain," NZ Transport Agency project delivery senior manager Andrew Thackwray said.

The motorway project started in July 2014 with the signing of the Private Public Partnership with the Wellington Gateway Partnership (WGP).

"Under the terms of the contract, WGP will design, construct, finance, operate and maintain the motorway."

CPB Contractors and HEB Construction were sub-contracted by WGP to carry out the design and construction contract, and started the detailed design as soon as the contract was set.

Although its operation and maintenance will be managed by Ventia for 25 years, the motorway is a public asset and is owned by the New Zealand Government.

The original opening date was April 2020, then August 2020, but it is now anticipated to be later in 2020.

"Progress has been slower than anticipated due to numerous weather and seismic events including the need for resources to be diverted for the response to the 2016 Kaikoura earthquake."

Wainui Saddle. Photo / Rosalie Willis

The agency was working with WGP to complete the project as quickly as possible, however this must be done safely and to a high standard.

"As with all projects there are a number of factors which can influence an opening date and there is still a considerable amount of work to do."

How much of the project has been completed

• 80 per cent of the motorway construction is now complete.

• 89 per cent of structures are now complete.

• Over 90 per cent of the earthworks is now complete.

• 14km of the new motorway at pre-pavement level.

• 61 per cent of project planting works completed.

• 7.6 million hours worked by the project team since the project started.

The project is a huge undertaking from a personnel perspective.

"On a fine day 600 to 700 workers are currently onsite at Transmission Gully," Mr Thackwray said.

Kenepuru interchange. Photo / Rosalie Willis

"Roles include earthworks, services, intelligent transport systems, pavements, structures, design, administration, finance, procurement, communications, planning, environmental management, surveying, quality control, traffic management, landscaping, human resources, health and safety specialists, mechanics, cleaners, labourers and management."

As well as the construction of the motorway itself, the project has made significant progress in environmental management including fish relocation and permanent stream diversions, construction of culverts and environmental mitigation.

Pauatahanui interchange. Photo / Rosalie Willis

"The Porirua Harbour and the Pauatahanui Inlet are a significant consideration for the Transmission Gully project.

"Environmental mitigation for the harbour includes the installation of 17 kilometres of silt fences, 37 kilometres of construction run-off channels and over 100 sediment retention ponds that settle out sediment from construction run-off."

The project has a team of ecologists, and erosion and sediment control experts who ensure procedures are in place for controlling and monitoring sediment run-off into watercourses and streams.

Cannons Creek bridge. Photo / Rosalie Willis

"These extensive mitigation plans will ensure that within seven to 10 years of the opening of Transmission Gully, the Porirua Harbour and Pauatahanui Inlet are in a better environmental condition than before the construction of Transmission Gully commenced.

"It is important to note that our mitigation is specific to the impact of Transmission Gully and does not take into account other factors such as the impact of housing development, farming and deforestation on the harbour."

Key features of the motorway project

• Scheduled to open later in 2020.

• The 27km four-lane motorway will run from Mackays Crossing at Paekākāriki to Linden, through Transmission Gully.

• Four interchanges and two link roads will connect the motorway to Mackays Crossing, SH58, eastern Porirua and Kenepuru.

• Earthworks — consents for 9 million cubic metres of earthworks. Cuts up to 70m.

• 25 structures (bridges and major culverts) equating to a total length of more than one kilometre will be constructed along the route.

• The largest structure is the bridge over Cannons Creek — 230m long and 60m high.

• Approximately 10km of stream diversions (Te Puka, Horokiri, Pauatahanui steams and various tributaries).

• Over 534 hectares of new vegetation and 27km of stream remediation, restoring native landscapes and mitigating sedimentation waterways.

• Transmission Gully is the first New Zealand project to apply for the silver Greenroads accreditation, an internationally recognised environmental sustainability accreditation.