The Otira Viaduct, an engineering feat near Arthur's Pass between Christchurch and the West Coast, opened 20 years ago.

A 445-metre cantilevered bridge that has made travel between the West Coast and Canterbury safer and faster is notching up 20 years.

The Otira Viaduct near Arthur's Pass officially opened on November 6, 1999 and has won numerous awards.

Paul Gorman The viaduct through the Otira Gorge. A major earthquake on the Alpine Fault would badly damage parts of State Highway 73 across Arthur's Pass from Canterbury to the West Coast.

It replaced the "zig zag" – a notoriously rough road featuring many hairpin corners that constantly needed repairs and redesigns due to shifting ground and undermining.

The project was "unusually challenging and demanding", Ian Billings and Richard Holyoake, of engineering firm Beca, wrote in The Structural Engineer in 2002.

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NZTA Tall piers and long spans had to be built.

The $25 million cost seems puny now, said David Bates, project manager for a predecessor of the NZ Transport Agency. But it was one of the biggest roading projects of the time, he said. It was finished on time and on budget.

The bridge was built on four "piers", or legs, up to 45m high. The two-lane deck was built on spans up to 134m long. It was designed to last 100 years.

The first 18 months were spent on the four foundations, Holyoake said. Research showed they would not find bedrock and the foundations sit in "extremely variable" rock avalanche material and silty sand.

The foundations were each 25m deep, 4m in diameter, and complex.

Don Scott/Stuff The curved and sloped Otira Viaduct on the Arthur's Pass highway was "unusually challenging and demanding" to design and build. This photo was taken in April 2000.

Contractor McConnell Smith built the cantilevered spans by extending the T shape by a few metres at one end, then a few metres at the other, Holyoake said. This kept the structures balanced as work progressed.

They also had to account for the curved and sloped spans, which was a "fantastic" challenge.

Holyoake was a young site engineer and visited Otira once or twice a week during the three-year build.

NZTA The notorious zig zag, left, was replaced by the viaduct, right.

Bates recalled the vigorous West Coast and alpine weather, river floods, and risk from rock slides and earthquakes. "We always knew it was going to be a tough job," he said.

Tony Western, 25, was killed in July 1998 when a chain failed and a pump fell on him. It was "extremely unfortunate", Bates said at the time.

Julianne Myers Dave Bates was a project manager for Transit NZ. He's now retired.

An early report predicted a 90 per cent chance the zig-zag road would fail entirely by 1999, cutting the strategic West Coast-Canterbury link. The report largely justified the project and cost, Bates said.

But when the viaduct was officially opened on November 6, 1999, the zig zag had not failed and survived a few years longer.

About 1785 vehicles passed through nearby Arthur's Pass village on average each day in 2018.