Jack Caldwell, P.E. has a B.Sc. in Civil Engineering, an M.Sc. (Eng.) in Geotechnical Engineering and a post-graduate law degree. He has over 35 years engineering experience on mining, civil, geotechnical and site remediation projects. He has worked on numerous projects throughout southern Africa, Europe, Canada and the United States.
It is always a very sad day when a major tailings facility failure occurs. As one commenter noted to me: “Whatever the facts, the failure casts a poor light on the industry, and makes all of our jobs more difficult in the future.”
Today we write about the failure of the Mt Polley tailings facility right here in British Columbia. I write only on the basis of the information readily available on the web. I have no other source of knowledge of this facility.
My first impression is that this is in a direct line of failures from Bafokeng, through Merriespruit, to now. Nothing has changed, this failure looks just like the failed Bafokeng dam looked when I went out to see it the week after it failed way back in the earlier 1970s.
We still cannot agree if the cause of the Bafokeng failure was overtopping of the crest of the perimeter embankment by too much water in the dam, piping of water through a sandy layer in the embankment, or slope instability via sliding of soft foundation clays.
The pictures I see, leave me wondering if the failure at Mt Polley was overtopping, piping, or basal failure. As at Bafokeng, I suspect it was all three in a perfect triad of a storm.
We will probably never know for sure as the evidence is washed away to the lake or strewn on the ground downstream of the failed facility. Thus some still claim that at Bafokeng the bulldozer sent to raise the embankment induced liquefaction of the wet tailings. Can’t help but notice the stranded bulldozer at Mt Polley—had it gone up there to raise the embankment?
Maybe the driver of the bulldozer we see perched on the edge of the failure zone saw something. If he did, you can be sure he is safely under wrap and key at this time.
Was the mine aware that something might happen? Some early reports I read which I cannot now find, say that there was a “breach” last year. Some reports talk of a Vancouver environmental company recommending more monitoring. One person I chatted to said they had heard that a young student had spent the summer doing QA/QC work on construction at the dam. Some reports say that the engineers of record are at the site, but their name is not given.
I cannot work out how to access the files of the relevant BC authorities so as to get a look at any dam safety reports that may have been submitted in the past—do such sites exist in BC. They do in many other jurisdictions. If there is no such site, maybe the BC authorities can release such files at this time to enable the profession to learn from what happened.
Many are asking about the chemicals in the materials released. While not a trivial issue, it seems to me that the bigger issue is the apparently large quantities of tailings that are now settled downstream and are probably sedimenting in the lake. It is going to take a lot of work to clean up the solids.
At Bafokeng, soon after the failure, the mining company went in and closed the breach and started tailings deposition soon after–you have to keep the mine running afterall. I cannot conceive how today, that will be allowed, but clearly the breach has to be closed up soon, for there are still lots of tailings waiting to be eroded once the rains start.
I do not know if the mine has a second tailings facility–probably not, so for now the mine is down. And so too I suspect are plans for opening new mines by Imperial Metals.
At this link, is a picture showing the tailings facility full of water. That is just how Bafokeng was the day it failed. In one of those earlier reports I read that the mine had been negotiating with local tribes to release water, but the tribes would have none of it. Pity for now all is released. But then the mine should have been insistent, for holding as much water on the dam as the link picture shows is bad practice as we know from Bafokeng and Merrispruit.
It is difficult to tell from the pictures how high and how steep the perimeter embankment is. But to judge by the size of the bulldozer in one picture I guess about thirty to fifty meters high and about two horizontal to one vertical. It is difficult to tell what the embankment is constructed of. Looks pretty blocky in some pictures, but surely this was not all borrow material. Did they use some cyclone underflow sand?
For the rest of this column, click here: http://ithinkmining.com/2014/08/05/mt-polley-mine-tailings-failure/