In an earlier blog, I questioned whether the tailings dams at Waihi are earthquake safe
I have now received the latest reports on hazard/earthquake risk for all tailings dams. The reports are perfunctory and/or rely on obsolete research. Waikato Regional Council has failed to properly regulate these critical structures. The reports have strengthened my opinion that a fully independent reassessment of seismic safety is urgently required.
The URS Report on Golden Cross
The report contains a very cursory examination of seismic risk. Liquefaction is “dealt with” in just four sentences and seismic risk in one small paragraph.
The URS report rubberstamps two earlier reports regarding liquefaction issued by the same company (2000 and 2004). The design as a “water retention structure” apparently makes the dam totally immune to these risks according to URS.
“Earthquake Induced Failure” is summarily dismissed as “inconceivable” with perfunctory and simplistic reasoning that the dam is a downstream method dam and therefore cannot fail. Reliance is placed on a 20-year-old 1998 report from URS’s predecessor company. According to this 1998 Report “massive cracking, larger movements and major slumping would be required to breach the embankment”. Such as has just occurred thousands of times in and around Kaikoura maybe?
URS’s conclusion is – nothing to see here – move on – no need to even include earthquake induced failure in the quantitative risk assessment.
The URS report is dated August 2016. The Hauraki District Council and WRC held a civil defence earthquake workshop on the GNS Report in November 2015. Astoundingly, URS makes absolutely no reference to the Kerepehi Fault report or the increased seismic risk for the dam site in their report despite having known (or should have known) about it for 9 months, and despite the fact that Golden Cross is only 15km from the nearest segment of the fault.
Oceana Gold Waihi
The September 2016 reports on the two Oceana Gold tailings dams from Engineering Geology (EG) both contain an assessment of seismic safety. Both reports rely on outdated data – a 2007 GNS estimate of seismic hazard, and a 2010 National Seismic Hazard Model which suggested a reduction in the rate of seismicity for Waihi. There is a passing reference to the 2016 GNS Kerepehi Fault report but even though this report was known to the authors for at least 10 months, neither EG report takes any account of its findings.
As a result, the underlying assumptions about the severity and strength of an earthquake at the site are obsolete and underestimate the seismic hazard:-
- The EG reports assume a magnitude Mw 6.7 quake but the Kerepehi report projects earthquakes of up to magnitude Mw 7.4 should several segments of the Kerepehi Fault rupture together. A 7.4 quake is 5 times bigger and 11.2 times stronger than 6.7 quake (USGS calculator)
- Peak ground acceleration (PGA) is assumed to be 0.3 – 0.5 G compared to the PGA in central Christchurch in 2011 which exceeded 1.8g (i.e. 1.8 times the acceleration of gravity), with the highest recording 2.2g, a shaking intensity equivalent to MM X+ (- epicentre 10km from city CBD)
- The distance from the epicentre is assumed to be 10 – 40 km. The distance from the tailings dams to the nearest part of the Kerepehi Fault is approximately 19 km, so the upper limit of this criterion is double the actual distance.
Based on these outdated and conservative assumptions EG estimates that:-
“Permanent deformations of up to 40cm are predicted in the downstream shoulder on section H-H” and states “The deformations would not be expected to affect the integrity and ability of the embankment to safely contain tailings and ponded water”.
Would a Mw m7.4 earthquake- 5 times bigger and 11 times stronger – produce cracks 5 times (2m ) or 11 times (4.4m) as wide, and make the dams vulnerable to failure? How would more true-to-life assumptions about peak ground acceleration and distance from epicentre effect the dam failure modelling? The Regional Council doesn’t think this situation is urgent. Rather than regulate, require accountability and get truly independent expert advice as it should, the Council has decided to kick the can down the road. Even although is has specific up to the minute research on the nearest active fault, it is willing to wait for an updated nation-wide GNS National Seismic Hazard Model which may not be completed for several years.
Dr Ristau of GNS believes a subduction quake, where the entire zone ruptures, would produce an enormous earthquake in the North Island of at least magnitude 8.5 or greater.
- Why has the Regional Council not challenged URS’s attempt to gloss over liquefaction/earthquake risk, by omitting any consideration of these risks in its report? This is an even more egregious omission because the authors of the report ignored the findings of the GNS Kerepehi Fault report.
- Why has the Council not undertaken its own independent peer review of the URS Report?
- Why has the Council not required EG to assess liquefaction risk in Storage 1A?
- Why has the Council not required EG to take account of the latest GNS Kerepehi report?
- Why does the Council have to wait for an updated nation-wide GNS national seismic hazard study when the specifics of the increased earthquake risk in the region are available in the GNS Kerepehi Report?
- Is the Council fulfilling its regulatory obligations by accepting a 4-page Peer Review Report?
- Is the Council satisfied with the EG’s vague and equivocal statement that in a modelled earthquake up to 40 cm deformations “would not be expected to” affect the integrity of the embankment?
- For 20 years or more the same consultants have been engaged in most cases from the design phase through to conducting annual reporting. Why has the Council not ever insisted upon totally independent consultants (from overseas if necessary) carrying out these reviews?
- Why has the Council not required the consultants to take account of Dr Ristau’s estimate of a magnitude 8.5 megathrust earthquake or greater from a subduction quake?
Copies of the Reports are available by clicking on the links:-
- URS Report on Golden Cross / Waitekauri
- Engineering Geology Report on Tailings Dam 1A – Oceana Gold Waihi
- Engineering Geology Report on Tailings Dam 2 – Oceana Gold Waihi
- Riley Consultants Peer Review – Oceana Gold Waihi