NZTA project

Transmission Gully taking shape 

It’s just over two-years since enabling work and construction of Wellington’s Transmission Gully Motorway began and it is forging ahead despite the difficult terrain and adverse weather. Richard Silcock visited the project last month and reports on progress.

“PARK YOUR CAR facing out and wear a hard hat, boots and PPE gear,” says Amanda Nilsen, senior communications advisor for Wellington’s Transmission Gully Motorway.

“It’s a health and safety measure.”

Such was the strict adherence to health and safety measures I encountered when I looked in on the Transmission Gully Motorway (TGM) project last month.

This massive project is progressing in leaps and bounds despite the adverse wet autumn and winter experienced last year. Fortunately the hot dry summer has dried the ground and presented ideal conditions for the contractors involved in the earthworks.

The New Zealand Transport Agency’s manager for project delivery, Chris Hunt, says that as a result of the extremely wet winter and the earthquake experienced in late November 2016 there have been some minor impacts on the project’s delivery which may result in an extension of the contract by up to 20 additional days.

This $850 million TGM project will link the Wellington–Porirua Motorway near Linden in the south and SH58 at Pauatahanui with the Kapiti Expressway at Mackays Crossing near Paekakariki in the north.

Physical works started in October 2015, with CPB Constructors and HEB Construction working in a joint venture (JV) to construct the motorway under a PPP contract.

The project has been described as one of the most technically challenging road projects ever undertaken in this country.

Wellington's Transmission Gully
CPB HEB JV’s project director Boyd Knights.

“It’s a ‘greenfields’ project stretching 27 kilometres through some diverse topography, ranging from areas of comparatively flat land, through to undulating former farmland, a regional park and a forest park, and steep hills strewn with bush-clad steep gullies,” says the JV’s project director, Boyd Knights. “At the Wainui Saddle section of the alignment it rises 460 metres above sea level.”

As reported previously (Contractor May 2017), steady progress was being made at several sites simultaneously, with work centred around site clearance, constructing access roads, archaeological digs, relocation of utilities, transfer of native fish from streams, stream diversions, construction of sediment retention ponds, earthworks, bank hydro-seeding and preliminary bridge foundation work.

“We’ve achieved good productivity since then and completed some five million cubic metres of earthworks and expect to ‘move’ up to nine million cubic metres of earth in total by the time the project is completed,” says Boyd.*

“The ground comprises alluvium, colluvium, fractured bed rock and peat swamp. Most of it is being re-utilised as fill for the numerous gullies and stream beds,” he says. “In addition, 130,000 tonnes of rip-wrap and rock armoury is being ‘imported’ from local quarries along with dolomite which is being shipped over from Nelson.

“With this amount of earthworks the alignment is rapidly taking shape and the sheer magnitude of the project and its trajectory can be appreciated,” says Boyd.

The JV is doing the bulk of the earthworks, with subcontractor Goodman Contractors carrying out 15 percent of the total volume of work under contract and providing additional plant and operators under a ‘wet-hire’ contract.

In addition, a number of other earthmoving operators, including Zaxcavate, Silverstrand, Tenga Pickering, Mills Albert, Bryce O’Sullivan, Dirtpro, Brownell, JAD and Peipi Civil have been subcontracted by the JV under ‘wet-hire’ contracts.

Madd-K Contracting, under a subcontract to Goodman’s, has one of the latest Hitachi excavators working the site and is carrying out ‘pockets’ of work along sections of the alignment (refer cover story in last month’s Contractor).

Goodman’s managing director Stan Goodman says they have around 100 staff and considerable machinery including several new CAT 637K scrapers working on site.

“Over winter our boys were working on a section near the southern end of the project which had previously been a pine forest,” says Stan. “We excavated and moved 700,000 cubic metres of earth despite the weather and steepness of the site – which was quite a challenge.”

Under a separate contract with the Porirua City Council, Goodman’s is also involved in earthworks for the 1.5-kilometre Waitangirua link road which will straddle Duck Creek via a bridge before joining the motorway at the James Cook Interchange. These earthworks are well advanced and when the road is completed will provide a link to Whitby.

On any given day there’s a fleet of upwards of 300 excavators, dump trucks and earth scrapers working. The earthmoving plant comprises 40 and 60 tonne excavators, D6 and D10 dozers, rock hammers, and a fleet of dump trucks and earth scrapers.

“With the good weather we experienced over summer we are currently moving around 150,000 cubic metres of earth a week and would expect to have most of the earthworks completed by the end of April next year,” says Boyd.

Travelling south from Mackays Crossing the motorway will climb at a gradient of eight percent (similar to Wellington’s Ngauranga Gorge) to cross the Wainui Saddle. High-sided and benched rock batters will be a feature of this section as the motorway slices through the fractured rock terrain.

“We’re working night shifts and made good progress on cutting sections of the hillside to bring the level down to the final alignment and this will continue for some time,” says Boyd.

Soil nails and shotcrete will add stability to the seven-tiered batters, which will rise 90 metres above the motorway level.

Wellington's Transmission GullyA number of concrete culverts and large pipes have also been constructed and installed to divert several streams and creeks. Possibly one of the most unique diversions is the elevation of Te Puka stream, where 1.7 kilometres of stream bed has been shifted sideways and moved between 20 and 30 metres up above its original level.

To achieve this, the stream was progressively diverted through large pipes and the gully filled with rock and earth up to the same level as the motorway alignment.

“It was a one-of-a-kind design, made doubly difficult by the steepness of the gully,” says Boyd. “But we’ve embraced the challenge and it’s a nice part of the project with engineers and environmentalists recreating what nature had created.”

Twenty-eight bridges are being constructed in total. Most are along the motorway primarily to cross the various deep gullies the alignment follows. In addition bridges are being built on some link roads which will connect with the motorway.

The largest structure and the most unique will be the 230-metre-long, 60-metre-high Cannons Creek Bridge (Bridge 20), which will straddle a deep gully stream that is cloaked in dense native vegetation.

“Excavation and retaining work for the bridge pier foundations has been completed,” says Boyd. “Work on the two, 24-metre-high reinforced concrete piers for the three-span bridge has commenced and these will be socketed into the foundations to a depth of 12 metres to allow for any lateral ground movement in the event of an earthquake.”

Forty-four steel prefabricated girders in rows of four will be placed over the bridge spans. These, up to 19.3-metre-long girders, along with most of the bridge deck, will be incrementally launched across the gully towards the end of this year.

“This bridge is being constructed in this way due to the steepness of the terrain and the close proximity of high voltage transmission lines, both of which preclude the use of large cranes,” says Boyd.**

Over the Christmas period steel girders for a new bridge at Kenepuru (Bridge 28), which crosses over the Wellington–Porirua Motorway and the main trunk railway line, were lifted into place by a 600-tonne crane while the line was shut down for several nights. This bridge will connect the TGM with Kenepuru Drive and the Porirua CBD.

Traffic heading north from Wellington will join the TGM via a purpose-built, two-lane ‘on-ramp’ which will exit the left-hand lane of the Wellington–Porirua Motorway near Linden before crossing back over it from west to east via a bridge (Bridge 25) which will connect with the start of the TGM. These bridge piers have been constructed and the embankments built.

Keystone concrete cruciform panels and reinforced earth walls are almost complete in preparation for a bridge (Bridge 15) that will be constructed to carry the TGM traffic over SH58 at Pauatahanui. The earthworks for the realignment and joining of SH58 at this interchange are well advanced.

Where the motorway joins the Kapiti Expressway at the Mackays Crossing Interchange, SH1 will be realigned to provide access to the TGM for traffic coming from or going to Paekakariki and Pukerua Bay. This section of SHI will be renamed the Coastal Road.

“Before this section can be built extensive ground improvement works had to take place, including the removal and replacement of liquefaction-prone material,” says Boyd. “To achieve this, the area has been drained and pre-loaded to compress what was originally a peat swamp and a market garden.”

There are upwards of 600 people involved in this project, most are local, with some specialists brought in from Australia and the Philippines. Boyd says their staff turn-over is pretty minimal and he attributes that to good staff relations, attention to working safely and the variety of work the project brings.

Over two million native trees and shrubs are being progressively planted along various parts of the alignment making it one of the largest replanting programmes associated with a road construction project in the country.

TGM is expected to be completed by April-May 2020 and will comprise two lanes running each way. There will be a third, ‘slow-lane’ for large trucks and heavy vehicles heading south and negotiating the gradient at the northern end. The lanes will be separated by a central median barrier.

* Estimates consented prior to tender in 2012 were for six million cubic metres. In 2013 it was revised post-tender to 8.4 million cubic metres based on a more detailed construction design, however, consents for up to nine million cubic metres in total have since been granted.

** Contractor will be covering the Cannons Creek Bridge construction in a separate, more in-depth article when the bridge super-structure is launched across the gully.


This article was first published in Contractor‘s April issue.

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