There’s been some significant progress made constructing the $850 million Transmission Gully motorway that is beyond the public gaze. RICHARD SILCOCK checks out progress.
SCHEDULED FOR COMPLETION in 2020, work on this four-lane, 27-kilometre motorway north of Wellington is progressing steadily at a number of sites despite unseasonal wet weather over the spring/summer period.
Since the start of the physical works in October 2015 (refer Contractor December 2015) significant earthworks and construction has taken place. With the exception of the northern end, where the motorway will cross over SH1 via a bridge and join the recently completed Kapiti Expressway near Paekakariki at Mackay’s Crossing (refer Contractor April 2017), most of this work is largely unseen by the public travelling State Highways 1 and 58.
“The Transmission Gully (TG) project has transitioned over the past eight months from a ‘pioneering’ phase to now largely bulk earthworks and the commencement of some structures,” says Boyd Knights, the project’s new construction director.
“Most of the current work taking place is pretty much in the ‘greenfield’ areas and has involved quite a bit of preparatory work such as the building of access roads, drainage, stream diversions, tree-felling and relocating some utilities such as the main Kapiti gas pipeline – 80 percent of which is now complete.
In addition to the weather, another of the challenges facing the contractors has been gaining access to a number of extremely steep gullies.“With the wetter than usual summer, ground moisture has been an issue and we are using a number of methods to ‘dry out’ the ground,” says Boyd. “This has included ‘discing’ with tractor-drawn agricultural harrows and applying lime stabilisation.”
“There are a number of really steep gullies which are tight and confined, and can only be accessed by single lane declines of up to 35 degree slopes,” says Boyd. “Once these gullies are cleared of vegetation intricate earthworks using small excavators are utilised to establish a suitable foundation base before they are filled in.
“The earthworks being carried out are currently taking place at a number of sites along the alignment immediately south of the existing SH58 interchange at Pauatahanui. Some one million cubic metres of earth has been excavated to date.
In addition, significant earthworks are underway for the construction of the James Cook Interchange to the east of Whitby and Porirua where connections will be made with local link roads.
At the southern end of the project near Linden, soil nail walls are being constructed to support the approach to the interchange bridge that will span SH1 and provide access to the motorway for traffic heading north off the highway. Piling has also commenced for the bridge foundations, and a large pine plantation in the path of the motorway alignment is being harvested nearby.
Over six and a half million cubic metres of earth is expected to be excavated during the motorway’s construction. It comprises silty clays and alluvium, however highly fractured rock is now being encountered as excavators approach the specified depth for the alignment.
Work has also started on the project’s most complex and longest bridge. This three-span structural steel bridge over the Cannons Creek gully will be 230 metres long and 60 metres high. Pier excavations, rock bolting and shotcrete work for the foundations is well advanced.
The piers will be constructed of reinforced concrete and the two bridge abutments will be constructed on top of 10 1.5-metre diameter reinforced concrete piles. The piers will be socketed into the substrate to a depth of around 12 metres.
A large 220 tonne crawler crane, which was imported to work on the Kapiti Expressway bridges, has been mobilised to assist in this bridge construction.
At Mackay’s Crossing, the main reticulated gas pipeline has been largely relocated. Once completed, preloading of the ground will take place and work will commence on the abutments for a four-lane bridge over SH1 which will link the motorway with the Kapiti Expressway.
A new water filtration plant has also been constructed nearby to replace an old filter bank which was in the path of the new motorway.
To prevent sediment laden water to run-off into streams and nearby Pauatahanui Inlet (a large tidal estuary and home to some 50 bird species) sediment retention ponds have been constructed. These use a process of flocculation and ‘sieve socks’ to filter the water and trap the sediment. Clear ‘treated’ water is then decanted off the top of the ponds before being released back into the streams.
At both the Horokiri and Te Puka streams ecologists have temporarily relocated native fish species to a similar habitat. The fish will be returned to the streams once stream diversions have been completed. In addition skinks, geckos and some rare worms have also been relocated.
As an indication of the size of this massive project, some 220 machines are operating on any given day along the alignment, ranging from 65-tonne excavators through to D6-D10 tonne dozers, along with motorscrapers, compactors and a flotilla of dump trucks.
The motorway is being constructed under a PPP contract between the government (NZTA) and Wellington Gateway Partnership, which has contracted CPB (formerly Leightons) and HEB Construction in a joint venture to design and construct the motorway, with URS and AECOM assisting with design. Geotechnical engineering is being done by Gaia Engineers and Boffa Miskell is advising on ecological and environmental matters.
In addition, some 150 subcontracts have been let for the project and there are currently 20 subcontractors involved in the provision of earthworks and drainage through either dry or wet hire or schedule of rates packages. Other services being provided by subcontractors include surveying, ecology, manufacture of precast concrete items such as bridge beams etc, geotextile supply and labour hire.
When completed the motorway will form a key part of Wellington’s Northern Corridor and along with the Kapiti Expressway provide a more direct and safer route for traffic heading to and from Wellington, which by 2026 is predicted to reach traffic volumes in the vicinity of 20,000–22,000 vehicles per day.
Transmission Gully will be the first motorway constructed in New Zealand to achieve a ‘Greenroads’ certification, an international sustainability rating system for road design and construction.