The improvements to the Wellington Northern Corridor – Levin to Wellington Airport, is being constructed across eight sections. The latest, NZ Transport Agency’s project heading north is Peka Peka to Otaki (PP2O) which is a four-lane highway that will bypass Otaki. It includes the construction of interchanges, a rail overpass, a river overpass and a huge amount of concrete from the local Firth teams.

Firth was the principle concrete supplier to the Alliance for the 18km Mackays to Peka Peka (M2PP) section, which opened to the public in February 2017, so was delighted to also be awarded the concrete supply for the next stage of the project – Peka Peka to Otaki (PP2O).

“We have plants in Otaki, Kapiti and Levin that have been supporting the sections of the Wellington Northern Corridor project from the beginning,” says Ian Morby, Area Operations Manager (Wellington & Manawatu) for Firth. Firth upgraded their plants in Otaki and Kapiti with improvements to the batching plants and an increased fleet in early 2014 to enable supply of the high volumes of concrete required for the project.




The work includes high performance concrete for piles of up to 3m in diameter, cross heads, piers, abutments and bridge decks. “We have also been supplying the concrete for the ‘Super T’s’ to Brian Perry Civil which are being made off-site at their Otaihanga pre-cast yard,” says Ian. “Our Kapiti plant has been supplying that side of the work which to date totals about 4,000m3. ”With an additional 5,000m3 of insitu concrete supplied since October, which represents less than half the projected requirement, and over 40,000m3 already supplied to the M2PP section of the corridor, project demands have been keeping the local Firth teams on their toes for the past few years.

And that's not all...

“In addition to the high volumes supplied to PP2O our Kapiti plant continues to also supply high strength spray concrete to the Transmission Gulley project,” says Ian. “We have supplied 3,000m3 of a 5,000m3 order which is testament to the flexibility and ability of the guys at the plant and especially of Gerard Howard, the Plant Supervisor, who is successfully managing both projects on a daily basis.”

“Over the last 18 months there has been an ongoing requirement for Cement-Treated Base (CTB) for the *stream diversions across the Transmission Gully section of the Wellington Northern Corridor, (south of the completed M2PP section)” explains Ben Jury, Major Projects Delivery Manager for Winstone Aggregates. “We were approached by the Northern Zone Manager to provide a flexible solution to provide a stabilised GAP20 product for their stream diversions. Our partnership approach with the CPB HEB JV on this project ensured we were their first port of call for a solution.”




“The obvious solution was to engage Firth, our sister company, as Firth Belmont has a central bowl mixer and we have previously worked with them on another stabilisation project,” said Ben. “This requires very good coordination and communication between all the parties to mix this product in amongst Firth’s normal work and keep our customer supplied with what they need.”




“The Winstone Aggregates team liaise with us as to what other work we have on and when we can process the CTB and at the volume they need,” says Ian. “We have been able to choose quieter times to process the mix and have the flexibility to switch our normal concrete deliveries to our Aotea plant which has been working really well and which means we can keep the CPB and HEB JV supplied with their requirements.”

“It has been the logistics of having the plant available, trucks ready and the weather playing ball as this material can only be placed on dry days,” says Ben. “This involves meticulous planning and we work with the Transmission Gully team on a daily basis to ensure we can help them meet their targets. Ian and the team at Firth have been great to deal with and understand the flexibility required for these works. They have made the plant available to make this product during an exceptionally busy period for them.”

“The stream diversions are a critical part of the project and there is a large team of Ecologists and Hydrologists at Transmission Gully who ensure the design creates the best conditions for a new habitat to establish,” adds Ben. “The two largest sites for diversions are the Te Puka and Horokiwi streams.” (See more below.)

“Up to the end of January we have processed about 7,500m3 of the CTB mix plus their other requirements,” says Ian. “It’s been busy but that's good -that's what we’re here for.”

Principal: New Zealand Transport Agency (NZTA)
Customer: CPB HEB JV / Fletcher Construction
Location: Kapiti Coast, North Island
Product: Certified Concrete


*NZ Transport Agency’s Environmental Approach at Transmission Gully

A big part of the Transmission Gully motorway project is about achieving good long-term environmental outcomes. Much of the landscape along the project route has been highly modified by farming and forestry, and the resulting increased erosion and run-off into local waterways has been clogging up the Pauatahanui Inlet and the Onepoto Arm of the Porirua Harbour ever since.

NZ Transport Agency is working with a range of specialists to help understand and manage the potential impacts of the works on these surroundings. As a result, the most comprehensive package of ecological mitigations and sediment management controls ever seen in New Zealand are in use on this project.

The aim is to minimise the effects of construction on waterways and then offset these by a number of environmental improvements that will see the harbour water quality improve over time. In less than 10 years after the new motorway opens, the water quality and surrounding environment will be in a better state than before the motorway was built.


Future Proofing The Environment

  • 4,500 fish and eels have been relocated from the Te Puka Stream into the upper reaches of the Wainui Stream in one of the biggest fish relocation operations in New Zealand. Ecologists removed the stream inhabitants from a 2.3 kilometre stretch of the stream that’s being diverted. Once the new Te Puka Stream channel has been constructed, fish including koaro, redfin bully, and banded kokopu, will repopulate the new habitat.                                                                               
  • Fish habitats are being improved by building structures that allow native fish to travel upstream and under the motorway to breed. We do this by using bridges, minimising culvert lengths and gradients and replacing existing ‘perched’ culverts with waterways that once again allow fish and eels to move naturally up traditional migratory routes.

View a video of the construction of the Te Puka Stream Diversion