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The historic Martha pit suffered two slips, in 2015 and 2016, the latter and largest bringing down more than 2million tonnes of the North Wall, blocking access and partially filling the bottom of the pit. It has cost about $US3 million ($NZ4.1 million) in remediation work so far.
At the New Zealand branch of the Australian Institute of Mining and Metallurgy annual conference, Oceana Gold geotech engineer at Waihi Liam Ireland updated delegates on progress this week.The North Wall was now ‘‘stabilised’’ but only after much remediation work, including dewatering and bringing down other threatening slips, and the pit area was now under care and maintenance.
Mr Ireland reconfirmed speculation at the time of the big slip in April 2016, that historic underground works and weakness in the actual rock structure had contributed to the failure.
It has also been further undermined by periods of heavy rain.The 2016 slip had been closely monitored and was predicted and the public in Waihi was warned of the likelihood of an imminent slip.The top of the North Wall slip was just 40m from a public road.
Mr Ireland had time-lapse photographs and video of numerous slips and also the subsequent remediation works, including heavy machinery working on top of the slip, showing little room for mistakes.
‘‘This was a live beast and we couldn’t be complacent about what we were doing,’’ Mr Ireland said.
The most recent remediation had been work on an east block and northwest block, the latter of which required about 10% to 15% of material removed.
To date, Oceana had removed about 500,000 tonnes of rock.
Following the presentation, Mr Ireland said Oceana still wanted to resume mining, given there was an estimated 80,000oz of gold still below the pit’s bottom.
In calendar 2016 at Waihi Oceana extracted 300,000oz of gold, from its underground Correnzo and Empire tunnels.
Waihi has an estimated further 570,000oz of gold to be mined and mine life beyond 2019.
Mr Ireland estimated it would take a further year for Oceana to examine all options, gaining consent could take a further year and site preparation another two to three years.