April 2013, had landslides when 135million tonnes of rock and rubble moved into
the open pit. slide was of solid bedrock that broke from upper half of slope with
waste rock from mine that has been piled above, weight putting additional stress
on slope. rock slid down shear plane before free falling and flowing into the pit.
speed that rockslide hit the bottom caused a 2.5 EQ.
slow movement was noticed before and sensors/radar monitored for more than 2
months. mining stopped when movement reached 5cm a day and workforce was
evacuated.
landslide was along a weak thin sedimentary layer within quartzite rock, aided by
an old fault line, couldn’t have happened without quarrying steepening the slope.
Effect on lives and property: most expensive landslide because of economic
disruption as workforce was laid off while mine was made safe. buildings and main
access road swept away, and equipment destroyed. company spent US$3million on
bulldozers to clear rubble. production reduced by about half. workforce reduced,
involving redundancy pay-outs. local authority lost money in tax revenue which
company usually pays. landslide shows large rock slides caused by slope failure are
predictable with considerable accuracy if warning signs are noticed and monitored.
slide not expected to fall as far as lower part of open pit.
