All You Need To Know About Transmission Gully

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Good progress is being reported on the 27 km, four-lane motorway being constructed from Mackays Crossing to Linden through Transmission Gully, with large-scale earthworks currently the major focus of the project.

Described as a highly complex series of works, the public private partnership (PPP) between the New Zealand Transport Agency (NZTA) and the Wellington Gateway Partnership (WGP) requires navigating difficult and steep terrain, along which 25 new structures equating to over a kilometre in collective length will be constructed. The largest of these, the Cannons Creek Bridge, will stretch 230 m in length and sit 60 m above the valley floor.

Built to demanding specifications, the project is part of the Wellington Northern Corridor ‘road of national significance’ and will also result in four interchanges and two new link roads that will connect the new road to Mackays Crossing, State Highway 58, eastern Porirua and Kenepuru.
CPB HEB joint venture (JV) construction project director Boyd Knights says there are currently over 220 pieces of plant operating across the site on any given day, including:
• Moxys – 30 and 40 tonne Cat, Komatsu, Bell and Volvo models
• Scrapers – Cat 623s, 627s and 637s
• Diggers – 20 to 65 tonne Hitachi, Cat, Komatsu and Hyundai machines

• Dozers – Cat and Komatsu 6s, 8s and 10s
• Compactors – Cat, Bomag, Hamm and Dynapac models.


Among numerous challenges being tackled, Mr Knights highlights the involved processes being employed to build “mammoth” bridge structures over busy State Highway 1 (SH1) near Linden/Tawa. “The bridges are built in such a way that they tie into the four different traffic stages the project team will be implementing throughout the construction of the Kenepuru Interchange on SH1,” Mr Knights says.
“We are currently working under Stage 1 temporary traffic management (TTM) on SH1 which effectively involves shifting southbound and northbound traffic towards the centre median. This allows the team to construct the bridge abutments and piers excluding the centre median piers at Bridge 25 (on-ramp bridge heading northbound onto the new motorway) and Bridge 27 (off-ramp bridge heading southbound off the new motorway to rejoin existing SH1).
“Stage 2 TTM on SH1 then pushes southbound and northbound traffic towards the shoulders, which will create sufficient room for us to access the centre median to undertake the centre pier construction on Bridges 25 and 27. Girder erection for these bridges will be undertaken during Stage 2 TTM and will involve a series of traffic contraflows on SH1 under night shift.
“What makes the bridge construction even more challenging is the coordination of all the other disciplines to facilitate. The associated civil works have involved 12 months of building temporary accesses for piling rigs and cranes, soil-nailing the SH1 embankment to facilitate sufficient access at the Collins Ave bridge, noise wall construction, median widening works to facilitate headstock construction due to the SH1 clearance envelope, bridge abutment embankment construction, mechanically stabilised earth (MSE) wall construction, and longitudinal drainage construction, which incorporates some very challenging works with trenches over 6 m deep and installing 2.4 m diameter pipes.”
A high priority has been placed on safety during the project, with the CPB HEB JV’s ‘Safety Essentials’ stipulating very strict minimum working requirements around live traffic, says Mr Knights.
“At a high level, this involves working to an NZTA-approved site-specific TTM plan and concrete barriers to protect work crews from vehicle impact. A lot of time has been put into the planning of the traffic stages to ensure the interchange can be constructed in a safe and productive way to reduce the effect on the community.
“We develop and undertake comprehensive risk reviews, work method statements and safe-work method statements to ensure all hazards are identified and adequate controls are put in place to protect our staff and the community. We use the ‘hierarchy of control’ system, which requires our teams to eliminate, substitute, isolate or use engineering controls to eliminate high-risk hazards,” he explains.
“For live traffic, we cannot eliminate the hazard. However, we can isolate the hazard by using concrete edge barriers to protect our work crews. We also use truck-mounted attenuators as an engineering control to protect our work crews which are designed to absorb a colliding vehicle’s energy.”


One issue Mr Knights would like to address is recent media reportage suggesting the JV had underestimated earthworks for the project by 50%. “Volumes consented at the time of the June 2012 board of inquiry report, that permitted the motorway, were 6 million cu m. In 2013, the successful tender from WGP proposed 8.4 million cu m, based on a more detailed construction design. Consents are nearly all complete for an extra 3 million cu m applied for, covering the extra 2.4 million under the tender and 0.6 million for contingencies.
“While it is the largest volume to be moved on a New Zealand roading project, the need to find quality material to meet resilience requirements was always allowed for by the JV, and is why its tender volume was higher than original consents. The rock and earth is not being taken offsite, but employed in a wide range of uses, such as foundations, fill, landscaping, bunds and MSEs.”


At the end of the 25-year period when the WGP concludes its responsibility for the operation and maintenance of the motorway, that responsibility will then return to the NZTA.

NZTA project delivery senior manager Chris Hunt says it is anticipated the Transmission Gully motorway would be included in any network operating contract that is in place at that time, or whatever system management contracting model may be operating.

On schedule for delivery in April 2020, the project also remains within its $850 million budget – which will be $25 million less than if it had been procured through conventional means.
“When the PPP contract was agreed and signed with the WGP, the $850 million (2014 net present value) price was fixed,” says Mr Hunt. “At that time, detailed cost estimates and financial modelling were undertaken to determine what the cost to the NZTA would be to design, build and operate, and maintain the Transmission Gully motorway for 25 years using its standard public sector procedures. That work indicated that the cost would be $875 million.
“The cost savings occur through private sector innovation and funding which can increase certainty of delivery, achieve true whole-of-life savings and drive better value for money on all aspects of the project – design, build, maintenance and operational management.”


The Transmission Gully motorway will provide:
• A safer and more reliable four-lane motorway with a central median barrier and additional crawler lanes on steeper sections
• Better resilience to landslides, floods and damage to the motorway from a major storm or earthquake
• Quicker reinstatement than the existing SH1 in the event of a major earthquake

• Peak-period travel time savings estimated at abound 10 minutes per vehicle between Kapiti and Wellington, 15 minutes between Kapiti and the Hutt Valley, and five to seven minutes between Porirua and the Hutt Valley

• Easier access from SH1 to Porirua and the Hutt Valley with shorter and more efficient freight movements to and from Seaview/Gracefield, Wingate and the Wairarapa

• Important arterial connections for residential and light commercial areas in eastern Porirua to the state highway network through two new link roads.


  • A decision on whether or not to establish a tolling scheme for the Transmission Gully motorway is to be made by the new Minister of Transport Phil Twyford following discussions with Cabinet.

However, prior to a recommendation being made to the minister, the NZTA will conduct traffic and financial modelling, develop a business case and carry out public consultation.

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