Multiplex Engineering (NZ) and HEB Smithbridge are removing an accident black spot from busy State Highway 2 with the construction of the Mangatawhiri Deviation.   BY GAVIN RILEY

Mangatawhiri.jpgWhen Transport Minister Annette King turned the first sod on the Mangatawhiri Deviation roading project a year ago, she said: “I have absolutely no doubt this project will save lives.”

Mangatawhiri (population 1900) is in Franklin district, 10 kilometres east of the State Highway 1-State Highway 2 junction. It is also on a section of road that carries about 14,000 vehicles a day and up to 25,000 at peak holiday periods as Aucklanders head for the Coromandel.

Perhaps not surprisingly, the area has a high accident rate. In the five years to the end of 2005 there were 305 reported crashes on the 40 kilometre Pokeno-to-Mangatarata stretch of State Highway 2, 24 of them fatal and 40 involving serious injury.

Five of the fatal and 16 of the serious accidents were within the section that the seven kilometre Mangatawhiri Deviation will cover, from the Mangatawhiri stream bridge almost to the Maramarua golf course (see accompanying map).

A joint venture between Multiplex Engineering (NZ) and HEB Smithbridge began work last November on the Mangatawhiri Deviation. The $45.9 million Transit New Zealand project entails building a two-lane undivided highway with passing opportunities in each direction.

New overbridges are being constructed where the deviation crosses local roads, reducing delays and improving safety. The existing section of State Highway 2 is to be retained as a local road, providing access to Mangatawhiri township and serving as the preferred route for cyclists.

Multiplex-HEB Smithbridge began clearing the deviation route in November and December last year and earthworks began in earnest in January.

Some five kilometres of all-weather haul roads were built to move heavy equipment along the project length. These will eventually be uplifted and used for fill or subgrade improvement.

The discovery of a disused gravel pit at the eastern end of the deviation has given Multiplex-HEB Smithbridge access to valuable filling rock and reduced the need for metal to be trucked to the site.

Long periods of fine weather helped the project to progress swiftly in the first season. Works undertaken during this period involved 68,000 cubic metres of cut to waste, 139,000 cubic metres of cut to fill, 223,000 cubic metres of imported fill, 15,000 cubic metres of imported subgrade improvement, 68,000 square metres of geotextile cloth, and 1200 metres of drainage pipe ranging from 375mm to 2550mm diameter.

Fifteen sediment ponds were constructed, 51 control devices installed, and diversions of Bell, Homestead and Koheroa roads set up to allow the start of structures.

At Koheroa Road bridge 26 piles were driven to an average depth of 18 metres, and two pile caps, two abutments and four columns were constructed. At Homestead Road bridge the numbers were 12 piles, four columns and two pier caps, plus two MSE walls.

Three cattle underpasses were built, and at the Golf Road underpass (a 68 metre-long superspan, corrugated-multi-plate, pipe-arch structure), ground beams, foundations and the floor slab were installed.

Weather experts are forecasting a fine early summer this season and Multiplex-HEB Smithbridge project manager Byron Banks is hoping they’re right.

“Completion is expected by May 2009 but we’re aiming to finish ahead of programme,” he says.  

Works to be undertaken before the finish date include completion of two bridges and the multi-plate underpass, temporary traffic management, completion of tie-ins to the existing highway, pavement and sealing, lighting, guardrails and signage, and landscaping.

The remaining work involves 211,000 cubic metres of cut to waste, 217,000 cubic metres of cut to fill (which includes 100,000 cubic metres from the discovered gravel pit), 45,000 cubic metres of imported fill and a similar volume of imported subgrade improvement, 31,000 square metres of geotextile cloth, and 700 metres of drainage pipe.

Multiplex-HEB Smithbridge is employing up to 40 staff on the project and is making use of a number of subcontractors/suppliers, including: Smithbridge, structures; Gibbons, MSE walls; Evergreen, hydroseeding; Counties Power, lighting; Smith & Tetley, quarry material; Stevenson, lab and site testing; Marin Construction, multi-plate; and Site Surveys.

Though ahead of schedule, the Mangatawhiri Deviation is not without its special challenges.

“Sixty percent of the project is through the southern hemisphere’s largest organic dairy farm so we have to be constantly aware of our environmental impact,” Banks says.

“Also, we’re dealing with extremely weak subgrades – very wet tephra soils with low CBRs.”

When the Managatawhiri Deviation opens to traffic it will have removed one of the country’s worst accident black spots. But it won’t be the last word in major safety improvements in the area.

Included in Transit’s 10-year state-highway plan (up to 2016-17) is a project similar to Mangatawhiri and just a few kilometres east, the Maramarua Deviation.

Both schemes have local as well as regional benefits, and these go beyond safety – as Transport Minister Annette King acknowledged when she got Mangatawhiri underway.

She said local people often had to deal with the trauma of crashes and for that reason the local community would be delighted that the deviation was being started.

She added: “While this project is essentially about safety, it is also part of the wider jigsaw involved in developing and expanding our country’s infrastructure.

“Mangatawhiri is in what is known as the ‘growth triangle’ – the area that encompasses Auckland, Hamilton and one of the country’s fastest growing cities, Tauranga.”

Contractor Vol.31 No.10 November 2007
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