Fraser’s openpit gold mine

OceanaGold approached GHD with the challenge of building a bridge to reroute their road and allow access to a new area of their goldmine in Macraes Flat, East Otago.

As one of the largest active mines in New Zealand, the brief for the bridge to re-route this road presented several challenges. For a start, the road had to be designed and built in a way that did not impact ongoing operations and it also needed to be built high enough for mine haul trucks to pass through.

The biggest challenge, however, was meeting OceanGold’s commitment to effective remediation of the site once they had finished their operations. Enabling wildlife to recover and thrive once they have gone. This not only required the design life of the bridge to be less than 25 years, but it also meant the bridge needed to be easily deconstructed from the site without leaving a trace.

Designing and constructing a structure to meet this highly unusual brief demanded out of the box thinking – we needed to investigate alternative methods. Methods that did not rely on the usual concrete or steel piles which are often left in the ground once redundant.

Our Geotech, transport and civil engineers turned this challenge into an opportunity for discovery. The team came up with an innovative solution to construct the bridge abutments out of load-bearing Mechanically Stabilised Earth (MSE).

The decision to use MSE for the abutments is what made this solution so unique. Most integral abutments in New Zealand are built using steel reinforcement, a costly design that usually requires a concrete facing system. As a result, this is one of the first bridges built with MSE in the country.

For the client, the abutments were the perfect solution. Constructing the bridge out of MSE significantly reduced build time and allowed mine haul trucks to pass under the road sooner, ensuring the feed of ore supply to the process plant remained secure.

The bridge can be also deconstructed just as quickly as it was constructed, in just a few weeks, without leaving any material in the ground.

Taking this efficient, cost-effective approach provides a whole lifecycle approach to the bridge and, ultimately, reduced the environmental impact.

This resulting structure is most likely the highest and largest of its kind in the country, it's roughly 12 metres high, spanning 35 metres. Completing the project has proven this construction format can be adopted safely for large structures in our highly seismic environment. It is pioneering the future of bridge design in New Zealand.