How earthquake safe are the mine tailings dams at Waihi?

The two tailings dams near Waihi are some of the largest structures in the country.

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Size of Martha Mine Waihi tailings dam compared to Mount Eden suburbs Auckland

Should either of these dams fail in an earthquake, we could be facing potentially New Zealand’s worst ever environmental disaster, and loss of life and livelihoods on a significant scale.

Therefore it is perfectly reasonable to ask the question – would these structures withstand a large earthquake?  The question is even more pertinent because –

  • the Kaikoura quake unleashed thousands of huge landslides
  • a new GNS Science report on the nearby Kerepehi fault suggests a 7.4 magnitude  earthquake is possible near Waihi, and also estimates that the interval between large quakes is 1000 years rather than 6000-8000 years  as previously assumed.
  • the catastrophic failure of a tailings dam in Brazil in 2015 was due in part to a very small 2.6Mw quake acting as a triggering mechanism for the dam collapse

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Landslides

The huge number and the scale of the landslides triggered by the recent  Kaikoura earthquake is astounding. 

 

In 1996 a major landslide under the Golden Cross tailings dam at Waitekauri near Waihi threatened to cause the dam to fail and spill its contents into the valley below.  A civil defence emergency was declared.  The mining company – Coeur Gold – was forced to  closed the mine permanently and to spend around $NZ30 million  on stabilization work.

The Report on the Kerepehi Fault

The conclusion to the  2016 GNS Science report on the Kerepehi fault states:

” In the towns of the Hauraki Plains, in close proximity to segments of the Kerepehi Fault, the earthquake intensities are expected to reach MMI 9 (“destructive”) on some occasions and perhaps even MMI 10 (“very destructive”). …… The risk is therefore at a level where buildings with high occupancy rates or which house critical facilities should undergo strength checks”.

A map presented to a Hauraki District Council civil defence workshop shows the proximity of Waihi and Waitekauri tailings dams to the nearest segments of the Kerepehi fault.

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The design of the tailings dam at Waihi is based on a seismic hazard study undertaken in 2007, which was updated in 2010 resulting in a reduction of the perceived hazard for this site. However the 2016 GNS report suggests there is a significantly greater earthquake risk in the region than previously assumed. The fact that the Hauraki District Council held a workshop even before the GNS report was made public, is testament to that.

The past assessments of seismic safety for both tailings dam are outdated. They fail to take account of the latest science and events in Brazil and Kaikoura, and may significantly underestimate the earthquake hazard. (A 7.4 Mw quake is 8 times bigger and 22 times stronger (energy release) than a 6.5Mw quake)

The Brazil Tailings Dam Failure

The Fundao tailings dam failure is Brazil’s worst ever environmental disaster.   The scale of the devastation is horrendous.  36 million cubic meters of toxic tailings (similar to the amount stored at the Waihi Martha mine) swept down 600 km of the Doce River and into the Atlantic Ocean.  The rivers ecosystem has been poisoned, whole towns destroyed, 11 people were killed and thousands have lost their livelihood. Watch this 1 minute summary of the disaster  or this 43 minute Australian Four Corners documentary

Brazilian authorities have lodged a claim against Australian mining giant BHP et al for NZ $61 billion in damages – (note: billion not million).

[Compare this real life NZ $61 billion figure with the bonds posted for the Waihi mine – a paltry $26.2 million rehabilitation bond, and a $10.4 million capitalization bond.  The mining company can uplift the rehabilitation bond upon “satisfactory mine closure”.  Therefore any failure of the dam after “closure” would mean even that bond is not available. The bond for the Golden Cross mine is believed to be $12 million.  The taxpayer will almost certainly have to pay most of the costs, should either of these tailings dam ever fail.]

The report on the Brazil dam collapse says three minor earthquakes with an intensity of just  2.2Mw, 2.6Mw and 1.8Mw which occurred 90 minutes before the dam breach were a “triggering mechanism” for a “liquefaction flow slide”. 

Other causes of failure related to poor maintenance and management leading to the tailings becoming over-saturated.

In August 2016 I raised concerns with the Waikato Regional Council, and the Minister of Energy about the seismic safety of the Waihi dams.  I suggested that an immediate independent peer reviewed investigation was justified.  The Minister replied that this was a Regional Council matter and gave no substantive response.  The WRC replied that it was satisfied with the two mining companies concerned carrying out their own in-house review, with no specific time-frame.  The WRC did not consider there was any urgency.

I wrote to WRC again in October drawing attention to the conclusion in the report on the Fundao mine collapse that very small earthquakes were a triggering mechanism for failure, and pointing out that the 2016 GNS Science report had specifically recommended “the location of important facilities with respect to potential liquefaction-driven ground deformation and landslide risk should be mapped with urgency”.

I noted that a high-intensity tropical storm could cause the tailings to be saturated and if followed quickly by an earthquake could make the tailings more prone to a liquefaction flow slide –  a “perfect storm”.  (Wellington suffered major flooding the day after the recent earthquake).

The people living in Waihi, the Karangahake Gorge, Thames, Miranda and in around the Hauraki Gulf – in fact all New Zealanders –  are entitled to the assurance that these two tailings dams are seismically safe.  Given the potential catastrophic environmental and human losses which would arise from a tailings dam failure, it is reasonable to insist that the Regional Council and/or central government should carry out their own independent peer-reviewed study.  Office buildings and houses are routinely re-assessed by independent engineers for earthquake safety, even although the probability of a large quake is low.  Why should  tailings dams be treated differently?  The stakes are far too high to let the companies self-regulate or to simply assume that all is well.

 

Update 7 December : Waikato Regional Council have advised that “GNS is currently undertaking updated estimates of seismic hazard, which once completed, will be taken into account as part of the annual Tailings Storage Facility Monitoring Report provided each year.” Also, the latest reports on the tailings dams at Waihi and Golden Cross had been received by WRC, and I have been promised copies.  It will be interesting to see whether the seismic assessment is based on outdated 2010 data, or whether the latest GNS Kerepehi Fault report is taken into account?

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One thought on “How earthquake safe are the mine tailings dams at Waihi?

  1. Oceania Gold – The owners of the Martha mine at Waihi say in a July newsletter that a failure of their tailings facility “is not possible” , because it is constructed differently from many overseas dams. No one is denying that the Waihi facility has been constructed using international best practice. But there are a few places as earthquake prone as New Zealand, and therefore it is reasonable to ask whether the seismic safety at the Waihi mine can still be assured. Particularly in the light of events in Kaikoura, and the latest GNS Science report on the nearby Kerepehi fault. The “experts” who engineered the tailings dam at Waitekauri said it would be built on stable ground, and and no doubt they too would have been confident that it was “not possible” for the dam to slide, only to later discover a large landslide right under the tailings dam. http://nzresources.com/attachments/8855/Waihiupdate.pdf

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