Jack Caldwell - Mining Engineer - Robertson GeoConsultants

Slimes & Cement

The best cover is no cover. If you can use the upper tailings or waste rock as the growth medium in which a stand of climax vegetation will flourish, you have the best cover.

Most tailings and waste rock will never be suitable as a growth medium: too acidic, lacking in nutrients; prone to dust production, and erosion susceptible.

The first “cover” that I designed and constructed was for a slimes dam within the Johannesburg, South Africa, city limits. The top of the slimes dam was nearly flat, dry & high, and a persistent source of dust as the winds blew across, to transport fine particles to the city besides and below. We tried the commercial dust suppression fluids then available. None worked; very quickly the crust broke up and the dust continued unabated. Professor Geoff Blight urged me to mix cement into the upper 300 mm of the slimes.

“That ought to stabilize them,” he said.

The cost alarmed the dam’s owners, but we proceeded, and the result was little dust for as long as the dam awaited the day it was removed so that the slimes could be reworked to recover gold and uranium left behind by those who processed the material the first time.

Lesson learnt: temporary covers of the tailings itself, plus a bit of cement or lime, may be perfectly acceptable. It is not always necessary to proceed to complex covers.

 

Degrading Waste Rock

The regulators were demanding construction of a cover over a waste rock dump at the Royal Mountain King Mine. I went to the mine, which is near Copperopolis, California, to see the dump and opine on a suitable cover.

At the first sight of the dump, I was amazed by the gracious curves and abundant green vegetation.

“We have a major problem with the local deer that come to graze on the dump,” my host acknowledged.

A short inspection revealed that the waste rock, dumped about ten years previously, was degrading into a fine soil. The miners, on reclamation, had hydroseeded the surface, and now the result was a rocky soil layer well-knit together by the roots of the grasses and shrubs growing on this new, natural cover.

We never did construct a cover on that dump. It would have been a shame to strip the soil and vegetation and replace this natural development with a costly, artificial cover.

Lesson learnt: it is difficult to construct covers on waste rock, and if the rock is likely to degrade over time to a soil that will support vegetation, it may not be necessary to construct a cover at all.

 

Cold Climate Covers

In the far north of the Canadian Northwest Territories is a diamond mine - Ekati it is called. In the past few years, we have filled up one of the cells of the containment facility with fine processed kimberlite. Extracting the diamonds from the ore involves no chemicals to speak off - the resulting material is a fine silt or sand.

The climate is harsh: cold; windswept; faint sunlight. Yet we noticed that on the edges of the cell where no tailings had been deposited in the past two to three years, a rugged grass had established itself naturally. At another area where some years prior a few piles of rock had been randomly dumped, the vegetation was even more sturdy and prolific.

The vegetation expert told us that the tailings are good to support vegetation although addition of some nutrients and fertilizer would help. He liked the random waste rock piles, for they provided protection from the wind which was otherwise prone to cut down new vegetation as the dust sliced through the tender stems.

And so now the trail is on to test covers that consist essentially of the tailings and random rock piles. We argue about the geometry of the rock piles, about the nutrients, about the moose that will traverse the area to graze the vegetation, and about erosion as water meanders slowly over the tailings and between the rock piles.

Lesson learnt: even in the harshest climates some mine waste materials, suitably reinforced with random rock, may make a new, natural landscape over the waste facility.

 

Here is a case study:

A simple cover of rock is what we constructed at the 24 UMTRA piles. Over the radioactive wastes and radon barrier was a layer of rock. Many thought this ugly. Personally I think a pile of rock standing proud in the landscape is beautiful: not natural, but so robust and textured that it impresses the mind and eye.

Rock was used to control, nay eliminate erosion, for 1,000 years and more. The rock was placed to resist the forces of erosion by waters rushing down the cover. The rock type was selected to resist decay for as long a period as we had courage to predict.

 

Technical Approach

We designed the rock cover for the flow of water from the probable maximum precipitation. The details of the calculation of the required rock gradation are best studied by reading the example calculation at this link.

At its simplest the method involves determining the flow rate of water down the cover and then sizing the rock so that the force of the flowing water cannot dislodge or move individual rock pieces.

The durability of the rock was a major issue. We went to local cemeteries and studied the weathering of the tombstones for data on the response of different local rocks to heat, cold, chemical attack, and time.

The thickness of the rock layer was determined by the average size of the rock. We deemed it best to make the layer at least 1.5 time the maximum size of rock particle used. That way the layer did not consist of a single rock piece.

 

The Filter Layer

The rock gradation generally required for a cover is such that the voids are large. The rock could be placed on an underlying layer of soil of clay, silt, or sand. The issue is that when water from precipitation flows rapidly down through the rock and encounters the soil , the water flows laterally through the rock and over the soil. If the flow is rapid enough, the soil will be eroded. This quickly leads to sloughing of the rock and surface irregularities.

To preclude erosion of the underlying soil by water flow laterally through the rock cover, we placed a filter layer of sand between the soil and rock. Carefully chosen using standard filter criteria, the sand layer inhibits erosion.

 

The Long-Term Performance

The Shiprock, New Mexico UMTRA pile is sometimes referred to as the Shiprock National Forest. The cover is a standard rock cover sand over the clay of the radon barrier. Seeds blown in by the wind quickly pass down through the rock and settled atop the sand. The sand is moist—a source of water in an otherwise dry desert. The seeds soon germinate and force their stems up through the rock. Their roots quickly penetrate the sand to find nutrients in the clay of the radon barrier. The result is an abundant vegetation stand in the desert.

We did not think of or anticipate this when we built the cover. It all came as a surprise. Yet I am told there is no detriment to the function of the pile. It is still erosion resistant and there is no exceedance of radon emanation. Maybe we created a long-term stable cover in spite of our misdesign.

 

Advantages and Disadvantages

We may thus note these lessons learnt:

  • Rock covers have the advantage of being erosion resistant and of requiring little maintenance.
  • Some say their disadvantages include poor aesthetics and no contribution to infiltration reduction.
  • And you must be careful that they do not become the focus of growth of invasive vegetation.