A 1980s mining story

When the Columbia River was dammed by glacial ice, sediments deposited in the deep valleys of northern Washington State. Amax sought to develop a molybdenum mine just off the Columbia in the Colville Tribe reserve. I was brought across to design the tailings impoundment at the selected site. I was brought across because preliminary surveys had revealed up to 30 meters of soft, unconsolidated mud filling the selected valley, and after all, I had just succeeded with a similar soil deposit in South Africa.

To contain the calculated volume of tailings from the proposed mine, the impoundment would have to be 1,000-ft high, an unprecedented height in 1980, but more common now. A quick calculation revealed that the rate of rise was so fast that no amount of expedited drainage of excess pore pressure would yield a fast enough gain of strength to provide the required stability. We had to find another way.

In concert with the mining engineers, we came up with the idea of building a 300-ft high starter dike of waste rock. Stripping overburden would provide the waste rock and if we dumped it in fifty-foot lifts we could have the starter dike ready before the mill came on stream. If we cycloned the tailings, there would be enough coarse material to fill in behind the dike and then go on to raise the embankment above the rock dike. The fine cyclone overflow would be discharge behind the sand embankment. Sketches and drawings were generated, and stability analyses undertaken (a junior colleague got his master’s thesis completed on the strength of the stability analyses).

We consulted with the experts including a memorable visit to Arthur Cassagrande to discuss piping of tailings through the segregated waste rock. I came across that new American institution, the peer reviewer, and I was eviscerated by them for my early mistakes. Finally the local authorities including Roy Soderberg of the Spokane mining department approved the design and we were ready to go to construction.

But Carter became president, the economy collapsed, mining projects were shelved in every state, and engineers in the mining industry were thrown out of work. House prices collapsed, the interest rate shot up, and Reagan became president. The mine never opened. Rob Dorey, now an independent consultant in Denver, took the ideas, ran with them, expanded them, improved them, and formulated the design of a great tailings impoundment that was built in Idaho.