Professor Jere Jennings who taught so many of us now-old civil engineers in mining used to say: “When you have read everything there is to read, when you have done all the calculations possible, then drink a bottle of brandy and exercise engineering judgement.”
Ralph B. Peck wrote more eloquently about judgement in civil engineering. He taught us that you should observe things, read magazines including the adverts, and constantly get a feel for dimensions. He taught us to beware the undisclosed detail, the unexpected stratum that could induce failure, and the unexpected event now considered to be part of a risk assessment. He taught us to work on the basis of average conditions, but to define possible deviations from the expected, monitor to establish actual performance, and to have a plan to deal with the potential adverse response of the site to our actions.
Today this is sublimed into risk management, risk tolerance, and all those commercially-available computer codes that facilitate decision making. Professional judgement is now a commercial enterprise and no longer the essence of professional practice. It is the subject of seminars and conferences, papers, and learned academic talks. It is all bullshit run rampant.
Here are my personal opinions based on long years of practice.
The first was the failure of the Bafokeng slimes dam (thus it was called and I respect precedent.) I walked the site, I clambered up the failure surface, I watched Jennings opine, I heard the Judge determine that it was an act of God, I talked to those involved in the failure, and I pondered. I have read the many papers by eminent professors that say it was the result of piping, static soil liquefaction, and so on.
My opinion, that is contrary to all the rest, is that it was a simple slope failure induced by excessive load and water pressure. Or maybe it was a failure of the penstock induced by water-hammer.
The basis of this opinion is twofold. First the slope failed right above the penstock. Too much of an incidental coincidence. Second, in the years after the Bafokeng failure, I dealt with three more failures of similar neighbouring slimes dams. They all failed at thirty meters height. The Bofokeng slimes dam was thirty meters high when it failed. How can you not conclude that thirty meters is the terminal height at which foundation failure on the high plasticity clays of low strength (ten degrees at most) fail?
So when I designed the new slimes dam, I specified a buttress dam system of no more than thirty meters high. Then behind this, let the dam rise another thirty meters. My original design called for a massive step or set-back of the inner dam at sixty meters high. The “wise guys” who took over after I left did not follow my original concept. Now the dam rises more than seventy meters higher than the buttress. Those who dictated later had none of these things; they used computer codes to calculate factors of safety. They used circular arcs failure surface where I know they were planar failure surfaces. But they had computer print-outs and thus they carried weight.
The slimes dam still stands. I hate the fact that they still use the old penstocks we designed. I know they penstock pipes are corroded and decaying. I know they should long ago have been grouted shut, and replaced by a barge and pump-back system. But they are afraid the barge and pipes will be stolen, And it costs more to pump than to let gravity supply the energy. They have only themselves to blame if it fails and emulates Padcal and the old Bafokeng failure happens again.
I came to the United Stated and designed and built the Cannon Mine tailings dam in Wenatchee, Washington. This is still the highest privately owned dam in the state. It is now closed and the site of the Dry Gulch Riding Stable, This dam proves that you can operate a mine close to a town. My dam proves that you can place a tailings dam close to and indeed above people and succeed. This dam is built to the very best standards of water dams and so it should be, for more than thirty thousand people live downstream.
A fun story about the design of this dam. The downstream slope is 1.73 to 1. In those days we had no computers to calculate an optimum slope. We arrived at this number when drunk on brandy on the basis that nobody would question so precise a number as 1.73. Nobody did; nobody has; and this dam will be stable in perpetuity, to use modern terminology.
Hence to Greens Creek in Alaska. The earthquake potential is very high. It rains incessantly. We could not manage the water or provide for seismic stability. No calculations were needed to establish this. Gut feel and judgement sufficed. So we ultimately persuaded them to go filter pressed tailings. The result is history: the facility still operates successfully and in protection of the environment, and many other mines have followed suite.
The to Pond 5 at the Suncor oil sands mine in Alberta. One new-years eve sans family, but fortified by many a bottle of brandy, we worked out the solution. It is implemented, is working, and offers a solution to all the silly papers and directives 74 of regulators. We simple let the tailings freeze; laid down more geotextile and geogrid than calculations can justify; dumped light-weight coke on the geosynthetics. and installed wick drains to dewater the fluid tailings. It is working perfectly some five years later and many others write learned papers on the calculations they have done.
Correction: We did many FLAC analyses to study the approaches; we did so many computer runs my mind spins; but it all added to and expanded my judgement and confidence that the approach would work. We could not have done what we did if we did not have FLAC on powerful computers.
The point of all these old stories is this: observe what you can; do the calculations you can; drink brandy; and then use judgement. If all else fails, emulate nature—it has been at it for a long time, has experimented, and knows what works.
Find a tailings disposal approach that uses the forces of nature. Recall that the definition of a civil engineer is one who uses the forces of nature for the benefit of mankind. I submit that my solution for the oil sands and compliance with Directive 74 is just this: use the forces of nature.
I call my approach: Optimize Seasonal Deposition. The basics are:
- In spring, place about a meter of tailings that will gain strength due to consolidation induced by subsequent layers.
- In summer, place a meter in thin lifts that dry by sun-heating.
- In fall, place a meter that will freeze and thaw. The freeze-thaw will result is substantial strength gain.
Thus you may use the free forces of nature to dispose of oil sand tailings, comply with that silly Directive 74, and protect the environment to the benefit of mankind. They will demand more data, more computer code masturbation, more long reports, and longer delay.
That is now the way of things. As a consultant you can make money from it. Your professional reputation may even advance. But you will still miss out on the simple pleasures of looking, observing, measuring, reading, drinking brandy, and coming to a deeply-informed judgement.