Otira Valley Viaduct — twenty years on

The viaduct in the Otira Valley ‘flows with the grain of the land’, following a collaborative effort between Boffa Miskell landscape architects and the engineering team.

The Otira Valley viaduct, which bypasses unstable slopes in the heart of Arthur's Pass National Park, was officially opened in November 1999. It replaced a notoriously rough road featuring many hairpin corners that constantly needed repairs and redesigns due to shifting ground and undermining.

Boffa Miskell ecologists and landscape architects were part of the team led by Beca Consultants, which assisted (then) Transit New Zealand to plan, design and build the metres of raised highway. The build cost of $25 million seems insignificant now, but at the time it was one of the biggest roading projects undertaken.

Boffa Miskell’s role was to assess the visual and ecological effects of various bridging options, as well as the design and oversight of the restoration programme and kiwi and weed monitoring, in this highly sensitive alpine environment. The visual assessment assisted the engineering and design team to evaluate the range of options for the final structure. Their final choice was a design which ‘flowed with the grain of the land’, yet met structural, financial and feasibility criteria. The project placed an emphasis on minimising effects of the construction on native plant and animal communities.

At the time, monitoring of the local kiwi population (just five birds) appeared to indicate that the project had not affected the birds. The revegetation plan produced by Boffa Miskell and approved by the Department of Conservation, concentrated on getting the conditions right for natural revegetation to take place over time, while starting the process with seedlings and saplings collected from plant communities nearby. The Otira viaduct construction was been hailed as a major success, and twenty years on is still regarded as an excellent example of engineering works in a special environment.