ContractorProjectRoads of National Significance

Chocks away for Transmission Gully

Physical work has started on Wellington’s $850 million Transmission Gully Motorway. It is the first New Zealand motorway to be constructed, financed  and maintained under a Public Private Partnership (PPP) contract.  RICHARD SILCOCK reports.

AFTER COUNTLESS DECADES of discussion and feasibility studies that date back to the 1940s, and following almost a year of preparatory work, Wellington’s Transmission Gully Motorway is finally underway with earthworks having commenced just north of Paekakariki.

At an on-site briefing on October 22 to celebrate the commencement of physical works, the Minister of Transport, Simon Bridges said that the motorway has been designed and would be constructed in a manner that met the government’s Safer Journeys Strategy.

“At $850 million it is one of the most significant single pieces of greenfield’s road construction in the lower North Island,” said the minister. “It is a serious piece of highly spec’d infrastructure – its design and construction reflects the need for an alternative route north that will be both quicker and safer for motorists and also be more resilient to earthquakes. It’s really exciting that we now have tangible progress.”

The government gave its commitment to the motorway back in 2009, prompting a number of geotechnical investigations, route determinations, specimen designs and costings. These were accompanied by a series of public consultations, hearings and at times heated public debates before resource consents were awarded in June 2012.

The contract to build the motorway was awarded last year by NZTA to the Wellington Gateway Partnership (WGP), a consortium of financiers and contractors who will finance, design, construct, operate and maintain the highway for 25 years following its completion.

Leighton Contractors and HEB Construction in a joint venture (LHJV) are undertaking the design and construction for WGP and are calling on a number of subcontractors, such as Goodman Contracting and Higgins Contractors for some of the work in a range of areas including earthworks and maintenance activities.

URS and AECOM are assisting with design aspects, Peters and Cheung is providing geotechnical engineering and Boffa Miskell is providing environmental and ecological advice. Opus International Consultants, which carried out a lot of the early feasibility studies and geotechnical work for the highway several years’ ago, has been engaged as principal advisor by the Porirua City Council for the link roads that will tie in with the highway.

Map showing the route of the Transmission Gully motorway
Map showing the route of the Transmission Gully motorway

CEO of WGP, David Low says construction will adhere to international and domestic best practice in order to safely and efficiently deliver the project on time and within budget, cognisant of all consents and stakeholder requirements, while at the same time meeting all safety practices, undertaking some innovative engineering and environmental mitigation techniques.

The four-lane motorway will run 27 kilometres from the Wellington-Porirua Motorway at Linden to MacKay’s Crossing near Paekakariki, where it will join the Kapiti Expressway (refer Contractor November 2015). There will be five link road interchanges along the motorway, three of which will be grade-separated, and the southern tie-in with the Wellington-Porirua Motorway will be constructed in a manner that will allow for future widening.

Work to date 

Preparatory earthworks at Paekakariki

The focus so far has been preparatory: A project office, housing over 100 engineers, and a work compound was established at Lanes Flat near Pauatahanui several months ago following a ceremonial sod turning by Prime Minister John Key and other dignitaries last year, and a blessing of the site by local iwi earlier this year (refer Contractor October 2014 and March 2015). Other satellite site offices have also been established near Paekakariki (the northern most point of the highway) and Porirua near the southern end.

Other work has entailed geological and environmental investigations prior to establishing access tracks and haul roads to parts of the designated motorway ‘greenfield’ route, constructing temporary stream crossings, preparing for some of the stream diversions and undertaking stabilisation trials.

These trials will confirm the correct methodology is in place to permanently protect exposed cuttings and batters. In conjunction with transverse drainage elements, they will establish monitoring benchmarks and ensure environmental performance once the main earthworks commence.

In addition work has involved vegetation clearance, erecting security fencing and the relocation of service utilities. Transpower has removed and re-routed the high voltage transmission lines and supporting pylons that ran through the gully (after which the route is named) and carried power to the Kapiti Coast.

A midden containing a large number of cockle shells was uncovered in the course of creating a bund for the compound at Lanes Flat, giving credence to the area as having once been a site of an early Maori pa site close to the Pauatahanui Inlet.

The initial physical works for the highway itself have involved building erosion and sediment collection ponds, but will soon move to drainage work including installing cross-culverts prior to diverting a number of streams and the commencement of major earthworks.

The project has been divided into zones and construction will be staged so that teams will be working in multiple areas at the same time across the entire length of the project.


Australian born project director for the LHJV, Mick O’Dwyer, says this motorway’s construction presents quite a few challenges from an engineering perspective.

“The nature of the terrain is our biggest challenge as it ranges from very steep hillsides and gullies [such as the Wainui Saddle], through rocky outcrops to rolling farmland and semi-urban areas,” says Mick. “There are 11 different geological terrains along the alignment, each requiring different construction techniques, including ripping rock with bulldozers through to using excavators, scrapers and dump trucks depending on where we are working.

“It will require large-scale earthworks in the vicinity of 6.5 million cubic metres over the length of the project which in itself present’s challenges. Most of the earth will be used within the project, for example, to fill some of the gullies and provide base layers for the highway.

“To achieve this efficiently will require a highly coordinated mass haul strategy, with the weather, geology and the machinery selection all critical factors in achieving efficient haul rates,” he says. “Progressive stabilisation as the earthworks progress will help minimise any impacts on the environment and ensure good ground compaction.

“Controlling the flow of rainwater run-off from the many steep hills and gullies that run perpendicular to the alignment is also a special challenge,” says Mick. “We will control this through the installation of temporary culverts and overland flow paths during the construction phase.

“For the permanent works we have to consider both 10-year and 100-year storm and flood events and will address this by combining the stream diversions, overland flow channels and buried culverts that will run both parallel and across the highway alignment. The large transverse culverts have been designed to also carry any debris flows from the hills above.”

As there are several fault lines that run through the gully the motorway also needs to withstand the effects of earthquakes. As such, one of the engineering requirements is that the design and construction of the motorway and the various structures meet New Zealand’s earthquake safety standards and are resilient to severe movement.

Twenty-seven structures are to be constructed for the motorway, ranging from small underpasses to a 270-metre long bridge across Cannons Creek. Most will be built using reinforced concrete mounted on hollow-core and super-T bridge beams, with reinforced concrete abutments and piers. Four will be steel structures, of which two will be constructed by incrementally building out across Cannons Creek and Duck Creek.

An impression of the bridge over Cannons Creek
An impression of the bridge over Cannons Creek


In meeting the work safety requirements, LHJV is integrating new technology within established methods to reset ‘the norm’ and carry out tasks both safely and efficiently.

“An example of this is where we will utilise drones [UAVs] to provide fast, real time GPS and survey information for alignment, volume calculations and three-dimensional data,” says Mick. “Rather than sending surveyors in, we will be able to overfly areas to ascertain this information, saving a lot of time and providing real-time calculations.”

Transmission Gully will be the first motorway and road of national significance (RoNS) constructed in New Zealand to achieve Greenroads certification. This is an international sustainability rating system for road design and construction that meets a recognised environmental performance standard.

NZTA say the PPP contract model will help encourage the most advanced technology and innovation is brought to the project and that it will drive value for money, reduce financial risk and ensure a high quality outcome, while also retaining the intellectual property rights and ownership for government.

NZTA chief executive Geoff Dangerfield says, “The Transport Agency will consider PPPs for other projects that have the scale and complexity that will permit superior value for money to be achieved by using a PPP approach.”

Together with the upgraded Wellington-Porirua Motorway and the new Kapiti Expressway, Transmission Gully will provide a seamless, 110 kilometre ‘Wellington Northern Corridor’ route from Wellington to Levin when completed in 2020. It is expected over 22,000 vehicles will use it each day by 2026.

Sections of the current SH1, not incorporated into the new route will be re-designated as an alternative coastal route.

What is a PPP contract?

A Public Private Partnership (PPP) is a long-term contract between a public
authority and a private sector organisation. It covers the financing, design, construction, operation and maintenance of public infrastructure and services over a period of time. Full ownership of the infrastructure remains with the public authority.

In the case of Transmission Gully, the contract is between NZTA (for the
government) as the public authority and Wellington Gateway Partnership as the private organisation.


• It is the first motorway to be constructed under a PPP contract in NZ.

• It will also be the first to attain GreenroadsTM certification in NZ.

• It will comprise:

• A 27-kilometre, four-lane motorway from Linden to Mackay’s Crossing.

• 27 bridges and underpasses, the longest being 270 metres.

• Around 110 cut slopes, the highest being just over 70 metres.

• Three grade-separated interchanges.

• A maximum gradient of 8.3 percent.

• 7 kilometres of swales.

• 17 kilometres of silt/sediment collection ponds.

• 26 kilometres of stream diversion and enhancement work.

• Construction challenges include:

• Building specifications that ensure a high degree of
seismic resilience.

• Steep gullies and hills will necessitate considerable earthworks with
over 6.5 million cubic metres of earth moved to fill gullies.

• Controlling high rainfall run-off.

• 11 different geological terrains along the alignment.

•  It will take five years to complete, with the completion date set at April 2020.

•  The 25-year operational and maintenance contract will be managed by the Wellington Gateway Partnership once the motorway is completed.

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