New Zealand Engineering 1999 September

Infrastructure

Otira: The Green Viaduct

Nursery
Gradient

The spectacular Otira Viaduct, which repairs an ancient link between the east and west coasts of the South Island, is more than a marvel of modern engineering. It is a monument to sensitivity, doffing its cap to cultural considerations and ecological concerns.

From the scarred edges of the oft-eroded Zig Zag section of State Highway 73 in the dense heart of Arthur’s Pass National Park, the large stone masses which lie in the shadow of the sleek new concrete structure occasion only cursory attention from the uninitiated onlooker.

Lumps on a landscape, they nestle amid the scree and scrub stubble, forgotten by the eye as it wanders the towering of the geological gash which is the Otira Gorge. Yet, when Transit New Zealand briefed those contractors submitting tenders for the ambitious project, the seemingly inconsequential boulders were accorded sober reverence.

Long before Europeans had pushed west in quest of gold, Maori had wended their way through the gorge with the prize of greenstone awaiting them on the other side. The boulders, some as large as houses, had acted as markers by which the parties charted their passage. Known as kaitiaki, the huge stones hold special significance for the tangata whenua.

The path chosen for the viaduct took account of the kaitiaki and measures were taken to ensure they suffered no damage during the construction process. However, one 140-tonne boulder at Death’s Corner could not be avoided in any of the options considered by the Draft Road Management Plan.

Accordingly, in consultation with the relevant iwi, a whakamaraama, or clearing the way ritual, was conducted in February 1997 by Te Runanga O Kati Waewae to permit the destruction of the impeding kaitiaki.

Nursery
Studies on the impact to fauna and flora were conducted before work began and a detailed plan to restore the site to its former state was devised.

An important part of the process was the storage and re-use of humus, soil, moss, small rocks and leaf litter collected before contractors moved in.

Among the material preserved were seeds and organic matter vital to the area’s recovery. However, most of the revegetation will be derived from more than 100,000 seeds and cuttings collected from plants at the site and which have been reared at a specialised nursery.

There are clear differences in plant communities between the north and south ends of the project and the integrity of the local ecosystem was dependent on the use of such local material. Seeds and cuttings were propagated in Waimate and hardened at Arthur’s Pass in preparation for planting out.

During construction, the Otira River, which supports invertebrates and algae which sustain fish and the blue duck inhabiting the area, has been protected from pollution generated by the contractors’ equipment. Regular monitoring has been conducted to ensure levels of sediment are not unacceptably high.

Cooperation and consideration for all the issues which affect a national park have been paramount in producing an environmentally sound structure that serves practically and efficiently. Transit New Zealand’s regional highways engineer, David Bates, who is the Otira Viaduct project manager, says consultation during the design phase with local iwi, the Department of Conservation and other interested parties was a key ingredient in achieving an outcome which was acceptable to all.

The $25m Otira Viaduct, to be opened by the end of the year, will replace one of the most insecure stretches of State Highway 73 between Arthur’s Pass and Otira.

Gradient
Its vulnerability is greatest over a 3 km portion from Peg Leg Creek to Candys Bridge, during which it climbs 65 m up from Peg Leg Flat over a major rock avalanche and scree slope before dropping 165 m in a series of hairpin bends to the Candys Bridge site.

Gradients are typically 13 percent and rise as high as 18 percent. In addition to the steep grades and tight corners, which restrict the length of vehicle able to use the road to a 13 m maximum, the overriding concern is the high risk of losing the road through erosion of the slip face below by weathering and the scour action of the Otira River.

Changes in the slip over the last 70 years have required several major realignments, moving the road upwards 20 m and horizontally 65 m into the hill. The limit of this approach has now been reached. The further the highway moves up the scree slope, the steeper and less safe the highway becomes.

Studies undertaken in 1986 predicted there was a 90 per cent chance the road would have slipped into the Otira River by 1999. Confronted with this grim proposition, Transit New Zealand initiated the Arthur’s Pass Roading Project. After assessment, the final document identified the preferred options for improvements in five priority areas, with the Zig Zag being the most urgent one.

It is a challenging site for design engineers, being located in one of the most seismically active areas of New Zealand, less than 20 km from the main New Zealand Alpine Fault.

There are 10 active faults within 50 km of the site capable of generating large earthquakes. The 1929 Arthur’s Pass Earthquake, with a magnitude of 7.1 on the Richter Scale, caused a section of the highway to subside into the river and the road had to be subsequently realigned.

The presence of unstable rock outcrops on either side of the route makes the road and viaduct prone to rockfalls, with boulders in excess of 100 tonnes plummeting into the area.

The high rainfall in the Otira Gorge causes the river to rise from time to time without warning.

Scouring of the viaduct piles will occur because of the steep river bed and fast flows. It was important to create a viaduct which would not only blend with the landscape but provide piers, or supports, which would suffer least from the effects of scour or flood. Among special scour protection measures built into the design are extensive areas of large rocks and depth allowances for the piers.

In the absence of bedrock below the river bed, it was necessary to sink the piers 25 m into the layers of gravel. The extreme hardness of the greywacke gravel and the variable size of the boulders meant that drilling and excavation has been difficult and time consuming.

Special, state-of-the-art, large diameter percussion drilling equipment was used to construct a ring of piles in the ground. Inside this ring, a 4 m diameter single pile was constructed at each pier.

This section of highway is subject to extreme weather conditions and, in winter, snow and ice can make driving treacherous. Consideration was given to heating the viaduct deck but the cost of getting power to the site and high running costs precluded this option.

The unusually steep gradient of 11.75 percent, or one metre fall or rise in every nine metres, will require even greater attention by those negotiating the viaduct when the weather is at its worst.

"There will be a coarse surface on the bridge to minimise the opportunity for sliding on the ice and heavier guardrails on the sides than is usual," Mr Bates says.

A weather station hanging from the structure will monitor conditions and sound an alarm in Transit New Zealand’s Otira maintenance depot when the threat of ice occurs. Grit will be applied on such occasions as is necessary.

"We don’t know at this stage how big a problem, if any, it is going to be. It is inevitable that sooner or later there will be ice on the deck so it is a risk we are going to have to manage on the bridge," he says.

"After all, this is an alpine route and it will be closed at different times because of adverse weather conditions. People should realise that while this project has been designed to secure the route for 95 percent of the year and a lot of money was spent to this end, it does not mean it is weatherproof."

The progress of construction has been encouraging and it is expected the viaduct will be completed at least one month ahead of programme and, accordingly, several hundred thousand dollars under budget.

Brian Cowley is a freelance journalist from Christchurch now based in Seattle