Transmission Gully Motorway update

In this fourth Contractor article on the construction of the Transmission Gully Motorway Richard Silcock interviews the construction director about the progress made this year on this five-year ‘greenfields’ project.

The $850 million Transmission Gully Motorway is progressing well says the projects construction director, Justin Redelinghuys, who also reaffirmed that it will not be a tolled motorway.

“It was intended to have the project completed by April next year, but realistically we are now looking at the latter half of next year” says Justin. “However the actual construction end date and official opening has yet to be determined.

“Like any large road construction project of this magnitude several unpredicted issues did crop up and have impacted on our completion date,” he says.

“We’ve had heavy rain last winter, more than anticipated and it has continued well into spring resulting in a hold-up of some earthworks, but we have managed to ‘claw’ back some of that lost time.

“We also encountered some geotechnical and subsidence issues with some of the cut embankments and on the northern bank of Cannons Creek.

“Inspection revealed rainwater was seeping in behind the face of the cut slopes which essentially comprises soft clays overlaying hard rock.

“On the northern bank of Cannons Creek we’ve had to excavate part of the bank as it showed signs of movement which was outside the acceptable design tolerance. This necessitated finding a solution to the problem, scoping and redesigning the earthworks, shotcreting and installing wick drains – all of which took extra time.

“It did not affect construction of the bridge itself – the sequential launching process across the gully proceeded to plan with only a few delays due to strong winds gusting up to 120 kilometres per hour. The bridge superstructure has now reached the northern abutment and we are well ahead in deck construction and expect to have it completed by mid-January next year.” (1)

Some ongoing consenting and approval processes have also impacted on several aspects of the project, which Justin says has held them up on occasions as the way consenting is done here is quite slow by comparison to other countries.

“For example,” he says. “There were a number of the spoil dumping sites that were initially approved but required new consents due to access difficulties, or when the sizes of the sites were not big enough and had to be enlarged or additional sites located and consented.”

At the time of Contractor going to print, of the 23 bridges along the alignment, only one small bridge near the mid-point of the 27-kilometre-long project had yet to start construction.

Most mid-sections are on track with some in the initial stages of paving and a number of interchanges are also nearing completion with work on the Linden interchange ramping up over the coming summer.

At the northern end of the project work has progressed to plan. Where it will join the Kapiti Expressway, the carriageway is well formed and initial paving is commencing. However, there is still a considerable amount of earthworks to be completed on the Wainui Saddle section, which when complete will rise some 243- metres above sea-level at a gradient of eight percent.

“We have struggled a bit in this area due to the variability of ground conditions, with not a metre of ground being of similar composition,” says Justin.

“While considerable compaction has taken place around the peat area at the northern extremity of the project where it joins the Kapiti Expressway, we also had to address the propensity for some instability through the Wainui Saddle cutting which is 90-metres high and the largest of the entire project.

“While the initial geotechnical investigations prior to commencement of the project showed some variations in this section, it was not until a closer investigation was carried out that the true extent of the variation was revealed and this required a change in some of our planned methodology.

“We were, if you like, literally pioneering in this section.”

So far some 700,000 cubic-metres of earth has been excavated for this cutting which is due to be completed by February.

Transmission Gully aerials – view looking down Wainui Saddle to the south.

The motorway will have various paving finishes along its length to meet differing surfacing and gradient requirements.

“Our paving sub-contractor, Beaumont Stabilising, is using granular paving for the long straight sections, because it provides a superior surface and their large Titan paving machines, which were brought in from Australia, are able to deliver the required application thickness very quickly and efficiently,” says Justin.

“This is the first time this type of paving has been used in NZ to the extent we are talking about for this project and it will provide a high quality result.”

Near the southern (Linden) end of the project, where the on/off ramps and bridge 25 interchange with the Wellington/Porirua Motorway, structural asphalt will be used to transition into that motorway.

Other sections will be multi-layered chip/bitumen and will involve additional sub-contractors (2).

Over 720,000 tonnes of aggregate will be needed for the paving. To ensure this amount of quarried and screened rock (AP40) is available an old quarry at nearby Willowbank, off SH58, is being reopened to source much of this material.

“Sourcing most of the aggregate locally will be cost effective and will minimise the impact of trucks hauling aggregate long distances on the open road,” says Justin.

Environmentally, the project has progressed well within the scope of the resource consents with the control of sediment run-off minimised and offset by a number of improvements.

Over one million native trees and grasses have been planted along the alignment, with a further half-million to be established as the project approaches completion. The plantings along the embankments and the diverted streams have all ‘taken’ well and according to Justin, “are looking very natural.”

“With the help of our planting contractor, Evergreen Landcare, we are running one of the largest native vegetation and reforestation projects in the country,” he says (3).

Due to the steep terrain, over 50 percent of the trees and shrubs were brought in by helicopter with most of the planting done over winter months.

Steep terrain on the Wainui Saddle.

Safety and adherence to good safety practices has been paramount throughout construction and Justin says he is pleased there have been no significant accidents in a workforce of between 650 and 900.

“We actually received a written report from WorkSafe recognising and commending the safety practices we instituted and the impact that’s had on safety practices throughout the Wellington region, with some of our practices being copied and implemented elsewhere on other roading projects.”

Asked about some of the challenges the project has thrown up, Jason says the project is unique in terms of its sheer size and scale, its skilled labour requirement, its difficult and variable terrain and its susceptibility to seismic activity.

“The non-availability of enough skilled workers from within NZ due to the number of other large projects under construction around the country at present necessitated the ‘importation’ of around 450 workers on temporary work visas, with most from Australia, South Africa, the Philippines, India and Russia.

“We have around 29 nationalities working on this project, of which forty percent are from New Zealand. We also have around 220 women, who work in a number of roles, from administrative and surveying to excavator operators and Moxy drivers.

“Addressing some aspects of the physical construction has also brought some engineering challenges and it has been a matter of preparedness and thinking well ahead in our forward planning,” says Justin. “Once you have identified an issue, it’s been about finding a solution, getting approval and then fixing it.

“New Zealanders have their own unique way of doing some things and I have found when entering into discussions on the best solution, the ‘softly softly’ approach works far better than the ‘bulldozering – we’ll do it my way’ approach.

“But everything has been manageable and there has not been anything that has not been negotiable,” he says.

The Transmission Gully Motorway is being constructed in a CPB/HEB Construction joint-venture under a PPP contract with the Wellington Gateway Partnership for NZTA and once complete will form part of the Wellington Northern Corridor (4).


  1. Refer April 2019 edition of Contractor for details on the 230-metre-long Cannons Creek Bridge construction, the longest bridge of the project that was incrementally launched across a 60-metre deep gully.
  2. It is expected the successful tenders for this paving work will be announced as Contractor goes to print.
  3. The planting programme is contributing to the Greenroads Certification of the Transmission Gully Motorway. This international accreditation measures conservation, habitat, community engagement, health & safety and archaeology.
  4. The four-lane Wellington Northern Corridor from Wellington to Otaki will encompass the Porirua Motorway, Transmission Gully Motorway and the Kapiti Expressway and will provide a far safer route than the current SH1 route. It is predicted it could save motorists around 20-25 minutes in uninterrupted travel time.

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