Home > The Fallacy of Prefeasibility Rules and Guidelines

The Fallacy of Prefeasibility Rules and Guidelines


Here are two true stories that illustrate the dangers of setting limits on the accuracy of information, design detail,and cost estimates in the prefeasibility and feasibility phases of mine development.

These two stories focus on tailings disposal. 
Traditionally, the issue of tailings disposal have played but a small part in prefeasibility and feasibility studies.  The conventional wisdom (assumption) has been that there is always a place for the tailings and the costs are but a small part of the mine—hence pay no particular attention to them in deciding if the mine is feasible.

But, as these two stories illustrate this assumption may be very wrong and very misleading. You may need to do a full design of the tailings facility even at the prefeasibility study stage to get a true picture of the mine’s viability.

The first story concerns a mine on the pre-Cambrian shield—those very old, very hard rocks that are a perfect foundation for a waste rock dump or a tailings facility.  Unfortunately some of those old rocks have been deeply scarred and carved by glaciers. There are deep, narrow valleys in the seemingly flat landscape. 
The valleys are filled with glacial detritus: sands, silts, gravels, and vegetation. Look for the swamps, the wetlands, the peat bogs, and the wet, decaying vegetation.
It makes the landscape flat, and it hides engineering challenges.

At the prefeasibility stage you have hardly touched the landscape.  Maybe you have flown over the area in a helicopter, maybe stood on a ridge and cast a glance over the seemingly flat landscape.   This seems like a perfect area for the rock and tailings. 

Dump the waste rock to form a large embankment around the flat area and place the tailings in the reservoir thus created.  It seems so simple.  And a quick cost estimate tells you this will be cheap and the mine is a go.

But some pesky geotechnical engineer warns that the foundations may not support a high waste rock dump.  Failure,deformation, cracking, and creep may occur, they say.  You rightly ask what is to be done to avoid these undesirable consequences.

“Excavate the bog; remove the peat; get rid of the decaying vegetation,” is the obvious answer.

“How much is to be excavated?” is your reply.

“I don’t know. We must drill to find out.”

But this is a prefeasibility study. There is no budget for detailed geotechnical drilling.  So you demand an estimate/guestimate.

The answer is fed into the cost estimate and the answer comes back that the mine is not feasible—not cost effective.  It will cost so much to excavate the decaying vegetation that the mine cannot possible be profitable.

Thus starts a great debate. 

The owners want the mine to proceed—it is their only viable asset.  The project managers cannot stand to see the demise of this project—it is their only billable project.  The cowboys want to ignore the geotechnical engineer’s advice—for they are employed to make it happen regardless of reality.

Now of course you can ignore the geotechnical engineers—they are conservative and fall back on professionalism.  They are naïve and insensitive to costs and mining risks.  Just categorize this as a project risk and proceed is your instinct.  You can sort all this out during detailed design is you rationale.

Of course the mine never opens.  But that is another story.  The story here is that details during the prefeasibility phase can and it is intended promote or kill a mine, even though detail worthy of a detailed design is involved.

The second story involves the question of how high to make the starter dike of the new tailings impoundment. 

The original designers assumed an as-deposited density of 1.4.  The peer reviewers cautioned that 0.9 to 1.0 is more realistic.

“But if we assume 1.0 then the cost of the started dam is exorbitant—we cannot afford that.  We have to work with 1.4 to make the project feasible.  ”

“Then you do not have a mine.  Or you had better come up with a better plan. ”

The specialists do an exhaustive study of the issue of the initial density of deposited tailings.  There are no tailings just like those at this planned new mine.  The data are vague and contradictory.  But it is clear that 1.4 is far too high, and 0.9 maybe a bit low.

So the search for a better plan is put on the fast track. 

Maybe you can keep building the starter embankment at a rate that keeps it ahead of the rising tailings.  There is a risk: if the tailings density is low, you may soon enough run out of deposition capacity.  If the embankment construction is delayed, you will run out of deposition capacity.

These are manageable risks.  Theissues are but prefeasibility issues and can be refined in detaileddesign.  It is worth proceeding knowing that there is a risk, but knowingthat it can be managed.

Thus whatever you read about the limits of prefeasibility or feasibility studies, take none to heart.  Examine each in turn and examine each in detail, for the devil is in the details and some mine may not be feasible depending on the details.  Do not ignore the details until it is too late and you have spent large sums on vacuous studies.

*This article appears courtesy of Jack Caldwell's I Think Mining blog.


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