Waste Disposal Area: Embankment Design
The embankments are designed as earth/rockfill water-retaining structures. This is conservative, as with time the tailings will consolidate and become essentially soils with inherent shear strength, unlike water. Following the cessation of tailings deposition and completion of tailings consolidation the risk of release of tailings is almost inconceivable because of its inherent strength. TSF 2 was decommissioned in 2005.
Surface runoff from upslope of the operating tailings pond TSF 1A is diverted away from the area. The only water other than that contained in tailings which is added to the tailings pond is direct rainfall. The embankment crest level is designed to provide a safe height above the tailings plus stored water level. Storage capacity is provided for a 1200 mm rainstorm, (the Probable Maximum Precipitation), plus an additional 1.0 metre minimum freeboard. This is to ensure that the water in the tailings pond, which contains some cyanide, cannot overtop and find its way into surrounding rivers and streams in a severe rain event. The target crest levels are reviewed on an ongoing basis. It is important to ensure that the height of the embankment structure increases at a sufficient rate to 'keep up' with tailings production.
The downstream shoulder of the embankment is considerably flatter than would normally be the case for water retaining embankments because of the need to dispose of large quantities of waste rock. The slope of the downstream shoulder of Storage 2 is typically 4:1 (horiz:vert), but at the steepest section is up to 3.2:1. The slope of the downstream shoulder of Storage 1A is typically 4:1. This compares to 2:1 for the Benmore Dam which is a large water storage dam used for hydro-electric power generation, and the highest hydro dam in New Zealand. The flatter than normal embankment slopes allow development of high quality pasture for livestock farming. Additionally there are reduced risks of surface erosion and decreased maintenance requirements.
The Martha project is located in an area of historically low seismicity. Earthquakes have been felt in Waihi, but none in the last 150 years has caused high levels of shaking. The embankments have been designed to resist the effects of earthquake shaking from the Maximum Credible Earthquake (MCE) based on a seismic hazard study of the site in accordance with international guidelines. The MCE has been conservatively assessed to be a magnitude MW 7 earthquake at a distance of nine kilometres from the site.
From time to time the embankment pasture is seeded with earthworms to help improve pasture quality. For further information on earthworm seeding click here.
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