Flawed system ‘ doomed’ tailings dam to failure
Scathing report: Authors say there was little in way of long-term planning or execution
VICTORIA
The tailings dam at the Mount Polley mine was “doomed to fail” and the remedies that could have prevented the reckoning were undertaken “too little and too late.”
Such was the depressing, persuasive conclusion of the trio of experts appointed to review last August’s breach of the dam — an environmental catastrophe that need not have happened at all.
The root cause of the failure, they determined, was literally at the root of the dam: an underlying deposit of glacial till that was never fully mapped nor properly understood. We only know about it now because of the forensic engineering work that was part of their review.
But if that were the whole story, their report would not be as troubling as it is. For authors Norbert Morgenstern, Dirk van Zyl and Steven Vick — all experts in engineering — painted a far from flattering portrait of the Mount Polley operation and the constant raising of the dam that preceded the breach.
“Dam-raising proceeded incrementally, one year at a time, driven by impoundment storage requirements for only the next year ahead,” they write. “More reactive than anticipatory, there was little in the way of long-term planning or execution.”
Those requirements were dictated not only by the volume of waste rock but the need to store the vast amount of water used to process the ore. Moreover, the storage demands began to put pressure on the design of the dam itself.
“The design was caught between the rising water and the mine plan, between the imperative of raising the dam and the scarcity of materials for building it. Something had to give and the result was over-steepened dam slopes, deferred buttressing, and the seemingly ad hoc nature of dam expansion that so often ended up constructing something different from what had originally been designed.”
Something had to give and something did.
The report details a “tortuous, incremental” process that eventually produced an embankment, almost 40 metres high, with no supporting buttress and a steep slope of the kind “reserved exclusively for rock fill dams on solid rock foundations.”
Except the foundation for that section of the dam, far from being solid rock, was a kind of glacial till that would firmly support the growing weight of the dam until the moment it didn’t. As happened shortly after 1 a.m. last Aug. 4.
In a what-if coda, the report details how an engineering plan, submitted just eight days before the dam failure, would have added a supporting buttress in the middle of the section that breached. “Had it been in place, the failure would have been averted,” they write. “The final, fateful instance of too little, too late.”
Not all of the shortcomings identified in the report contributed directly to the failure. Still, the experts were dismayed about the adoption of “design criteria that left little margin for error” and safety standards that “made it harder to gauge just how closely dam raising was approaching the edge of the cliff.”
More site visits by government inspectors wouldn’t have helped because the flaw was under the base of the dam, detectable only by the kind of engineering work brought to bear by the review panel: “By definition, no amount of inspection can uncover a hidden flaw.”
But the panel did recommend best practices to reduce the risk in future, starting with an in-depth examination of other tailings dams to see if there are any heretofore undetected flaws lurking in their foundations.
The government accepted that recommendation, along with a call for third- party reviews of tailings dams, and a full-blown review of construction guidelines, including key considerations like steepness of slopes.
The most controversial recommendation was also the one that would change the industry the most, arising as it did from a concern about the amount of water that was allowed to build up behind the Mount Polley dam.
In the four years leading up to the failure, there was a 10-fold increase in the amount of water impounded with the tailings. The water didn’t cause the breach, but ensured that when it happened, the volume of material spilled was greater and travelled further than if the dam impounded tailings alone.
Hence their call for the province to phase out storing water and tailings together behind dams. “Only this can provide the kind of fail-safe redundancy that prevents releases no matter what,” they wrote.
Options include separate treatment and filtration of water, draining and compacting of tailings, and dry storage below ground. They cited the filtered tailings technology adopted by the Greens Creek silver mine in the Alaska panhandle.
For those who might decry that option as more costly, the report says: “Cost estimates for conventional tailings dams do not include the risk costs, either direct or indirect, with failure potential.” Point taken, given that the tab for the Mount Polley cleanup has been estimated at $200 million.
Based on current statistics, the report says the province can expect two tailings dam failures every 10 years, six every 30 years.
“The panel does not accept the concept of a tolerable failure rate for tailings dams. To do so, no matter how small, would institutionalize failure,” they wrote, building toward the most powerful observation in their report.
“First Nations will not accept this, the public will not permit it, government will not allow it, and the mining industry will not survive it.”
Enough said. Get on with it.
In a what-if coda, the report detail show ane ngineering plan, submitted just eight days before the dam failure, would have added a supporting buttress in the middle of the section that breached.