I THINK MINING

  A week of flights and meetings: Vancouver to Fort McMurray to Edmonton to YVR.  Long discussions around tables and papers flying fast and furious.  We argued how to cap an oil sands tailings impoundment.  We were held up by arcane arguments about strength, density, analytical method, material purchase, equipment selection, and the opinions of the local regulators—all of whom seem to think we are doing something new. 

In fact we were only seeking to do what I did nearly thirty-five years ago.  Today we know more, but we have less faith in science, engineering and judgment than we had decades ago.  Or we have less courage to act.  Or we are snowed down by the egos of review and professional competition that demands we criticize everybody else.  Regardless, here is a small part of what we did so long ago that we are seeking to replicate tomorrow. 

Richards Bay was a picturesque inlet on the east coast of Natal in South Africa.  Now it is a bustling harbour.  But when I first saw it, it was pristine: the railway had not yet arrived and there were no harbour walls where ships now dock. 

On the north side of the bay is a bluff of dense sand.  Off the bluff and extending a few kilometres into the lowlands were very soft clays and silts, transported over the centuries by erosion and deposited in the resulting delta.  On the muds were sparse reeds and water grasses.  Someway off-shore was a sand island on which a group of local Zulus had erected rough dwellings of corrugated iron.  They returned to these homes each evening along a series of well-worn paths through the reeds. 

I was there with a shear vane to measure the strength of the muds.  For we had been retained to design and build a tailings impoundment for a fertilizer plant.  The plant processed apatite rock brought by rail from the Transvaal, and turned the rock into fertilizer to export to far African countries—at least those which were prepared to break the embargo on trade with South Africa.  The waste was phosphogypsum, or simply gypsum as we referred to it.  And I had to design the impoundment. 

It was not difficult to measure the shear vane strength of the mud.  It had hardly any, and the vane spun easily to the touch.  At this remove of time, I cannot recall what strengths we measured, but be assured it was far less than 5 kPa. 

    After much site investigation and design we proceeded to construction.  In order to create the conventional starter dike commonly used in tailings impoundment construction, we built a 30-m wide dike of sand over a layer of geotextile.  The starter dike extended one kilometre south from the sand bluffs, one kilometre west, and one kilometre north back to the bluff.  First we cut the reeds, allowing them to lie where they fell.  Over this we laid out the strongest Bidim geotextile available.  We joined adjacent pieces with the wire of coat-hangers bought at the local dry-cleaners. 

Then with a fleet of light scrapers, we brought in and placed a one-meter thick layer of sand.  The underlying mud squeezed and bulged beneath the geotextile.  Waves of mud up to a meter developed ahead of the advancing sand.  The scraper drivers became expert at gauging the speed at which the mud wave developed beneath the scrapers and the delighted in setting their speed to maximize the wave and sometime to out-run it.  It was like breaking mach 1, the speed-of sound barrier.  With time, mud pumping, and the repeated passage of scrapers, the upper layer of clay and silt consolidated and we could advance those scrapers fast and furious out over the sands.

The gypsum was poured into the one by one kilometer polder thus created, raised some 10 to 15 meters, and reclaimed.  Today if you go to Google Earth you will struggle to find this tailings impoundment.  I think they have built a yacht club on it.

  Maybe thirty years hence somebody will say the same of our oil sands tailings impoundment.