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Landslide site one of the best monitored in the world - 20/03/1998

A landslide at the Golden Cross Mine at Waihi, on the east coast of New Zealand's North Island, is one of the most thoroughly studied and most comprehensively monitored landslides in the world.

Huge amounts of data from the landslide have been examined by technical specialists from New Zealand and overseas, said peer-reviewer Dick Beetham of the Institute of Geological & Nuclear Sciences Limited.

Mr Beetham, an engineering geologist, said specialists were called in to the mine site in 1996 when cracks appeared near the tailings dam. Mr Beetham has extensive experience in landslides and land stabilisation including work at Tongariro hydro construction and the Clyde dam project at Cromwell in central Otago.

It became apparent after studying aerial photographs and an initial site inspection, that the tailings dam and silt pond at the gold and silver mine had been built on a pre-historic landslide. A three-year programme costing about $30 million has resulted in the area being stabilised.

'' Before it was stabilised, movement of the landslide ranged from zero during dry periods to several millimetres a day during wet periods,'' Mr Beetham said.

The instability after heavy rain was not surprising with the landslide having a base slip surface in clay derived from old volcanic material. It was not recognised initially that the tailings dam was located on a prehistoric landslide, Mr Beetham said.

Mining operations had resulted redistribution of weight, changes to subterranean water flows, and the placing of waste rock stockpiles near the toe of the slide. These had combined to make the area more unstable.

Mr Beetham said the $30 million stabilisation programme included a 760m-long underground drainage tunnel and 24 pumped wells which removed an average of 12,000 cubic metres of water from the site each week.

The mine owner had also strengthened the tailings and silt control dams, and upgraded perimeter drainage, the mine water treatment plant, monitoring equipment and emergency plans.

''The site is now one of the best investigated and most extensively monitored landslides in the world,'' Mr Beetham said.

Monitoring the site are nine active inclinometers, five shear monitors, six extensometers, three downhole TDR coaxial cable shear monitors, and numerous GPS survey points. Groundwater levels are also being monitored using numerous standpipe and pneumatic piezometers, as are drainage flows from a network of drainage drillholes.