Geotechnical advice, Macraes Open Pit Mine, East Otago, New Zealand

OceanaGold operates New Zealand’s largest gold mine at Macraes, located 100 km by road, north of Dunedin in the Otago region of New Zealand’s South Island. The Macraes Gold Project comprises two mines: the open pit which has been operating since 1990, and Frasers Underground which was commissioned in January 2008. The proven and probable mineral reserves currently support an approximate eight-year mine life. The mine currently produces 125-145 Koz of gold per annum.

PSM was engaged to provide ongoing geotechnical advice on pit slope design, slope stability and mining-induced landslide. Of particular concern was the fact that the mine is located on an outcrop of a major fault causing movement of some of the mine’s infrastructure.

The mine is located on an outcrop of a major fault causing movement of some of the mine’s infrastructure. With the highly fractured rock mass, maintaining stability presented a challenge.

Engineering and technical opportunities included:

• Upwards of 50 m of movement which had occurred on the walls of operating open pits
• Access to the underground pit through the open pit resulted in the need to maintain safety and stability, particularly regarding interaction between the open cut and underground mines.

Following rigorous assessment, PSM’s tailored approach included:

• Provision of extensive geotechnical mapping and drill hole investigations
• Numerical modelling using Slide, Phase² and 3D Abaqus, FLAC3D
• Provision of extensive survey prism and crack-pin monitoring
• Development of a unique energy balance program to monitor stability.

PSM`s multi-pronged approach to analysing complex slope stability data enabled OceanaGold to continue safe mining at the Macraes Open Pit Mine. PSM’s reputation for reliability and proven results continues to satisfy OceanaGold, regional council and peer reviewers. A close relationship between PSM and mine staff has developed over the past 20 years based on our extensive historical knowledge of the mine site and PSM’s proven track record.

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