The Ohai Coal Mine Lease (CML) area encompasses historic underground and open pit working, waste dumps, workshop and plant sites. Tonkin & Taylor has conducted a staged study across the site to develop landform designs suitable for end of mine life and conversion of the CML to rural land. The focus has been on historic open pits with unstable cut and fills. The approach is conventional with attention to detail, optimising local materials and conditions. The approach has been to maximise ultimate pit lake level, designed for acceptable water quality discharge to the local trout stream.
Slopes above lake level are designed for stability against failure and performance in design earthquakes. Slope angles, heights and drainage are adjusted to provide long term erosion control and ease of vegetation.

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Slope movements in the operating open pit were interpreted in terms of the geological structure present in the pit. Steep seam dips and multiple faults limited the ability to win the identified coal resource. Slope designs were developed to allow winning of the coal and generally provide long term slope stability suitable for post mining landforms. Areas recognised as having potential for short term instability were monitored and revisions made to slope designs as appropriate during operations. At mine closure there will be some reshaping to produce slopes compatible with the long term landform.

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The Golden Cross mine is designed for both open pit and underground mining of gold-bearing ore. The processing of the ore results in a tailings slurry that is pumped to a tailings impoundment area in a nearby valley. The tailings are retained by a “main” dam across the valley and a “saddle” dam to one side.

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Coeur Gold purchased the Golden Cross Mine from Cyprus Minerals as an operating mine (see separate project sheet). The tailings dam was being constructed in stages, as required by level of tailings storage. Investigations and design studies were carried out with a view to raising the tailings dam above design level. During operation of the mine a large deep-seated landslide became evident, requiring design and implementation of stabilising measures. On reaching design height, a closure and rehabilitation plan was designed and implemented for the tailings facilities and silt dam.

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Auckland ’s largest Open Cast Quarry, Hunua, supplys a substantial part of the regions aggregate requirements, including infrastructure such as roading and concrete. The quarrys present extraction point is nearing the end of its current economic life and a new extraction pit, known locally as Symonds Hill, is planned within the quarry zone.

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