By Jack Caldwell - Mining Engineer - Robertson GeoConsultants

The story of tailings management is the story of application of a few basic principles. All involve control of the forces of nature. Once we worked on the basis of observation and judgment. Today we have computer codes for almost all and the work is done in back rooms by the junior engineers who feed printout to the senior engineers who still rely on judgment, albeit now quantified judgment. Here follows a brief overview of these changes as they relate to the principles that always have and probably always will rule in the art, science, and engineering of tailings management.

Design for the Big Forces

Gravity and hence Stability which dominates all other considerations. I designed the new Bafokeng Dam without doing any stability analyses. All I could do was observe that no local impoundment stood higher than 30 m at which point the outer slopes slide along the low strength clays that were ubiquitous throughout the area. I had a small IBM 650 to do stability analyses for the second impoundment I designed. Today, the junior engineers carry out the most sophisticated slope stability analyses in a few minutes. For a long time we read papers on increasingly clever slope stability analyses. Today we see none, for the job is routine.

Earthquakes which induce liquefaction and flow and failure of tailing facilities. I came to Pasadena in 1978 to a conference on earthquakes. I came to try to find out how to analyze the seismic stability of a slimes dam in the Free State. Mine rock bursts had felled a five-story apartment building and we were concerned about the “seismic” stability of the adjacent slimes dam. I returned to South Africa convinced I would never understand the theory; I knew I could not test the tailings as they did in the United States. So we found another way; basically buttress. Today the juniors run FLAC to quantify seismic stability as a matter of unreported routine.

Erosion which washes away the tailings and will in the fullness of time reduces them to pale geomorphic expressions. I started out trying to cement stabilize slimes dams susceptible to wind and water erosion. I ended up using vast rock rip-rap covers on the UMTRA Project to provide 1,000 years and more of erosion resistance. Now we have computer codes that replicate geomorphic processes and persuade us to create closed impoundments of “natural” contours.

Operate For Least Cost

Gravity is the cheapest form of energy. The tailings engineer seeks to use gravity to move the discharged tailings. Move the fluid tailings as far as you can by getting them to flow as far as possible to the ultimate desired location, before they loose water, and become solid. But keep in mind that gravity is also a great way to consolidate the tailings once deposited. A few meters of additional tailings, and the gravity-induced loads will squeeze water out, consolidate the tailings and cause them to increase in strength and solids content. Thus they are less susceptible to failure, flow, or deformation.

The sun and the wind too are cheap forms of energy. Thin lift deposition is all the rage in the oil sands. The idea is to let the sun and wind dry out the tailings thereby rendering them strong and of high solids content. But never forget it rains and this inhibits evaporation. So no matter what the partially saturated flow theories tell you, there will be times when the sun and wind cannot be relied on.

Finally in cold climates there is freezing. Most soils and tailings will freeze if it cold enough. And most soils and tailings increase in solids content and strength if you can arrange to get the thaw water away as the spring returns.

The skill of the tailings engineer consists in balancing these considerations to produce a stable, cost-effective tailings disposal scheme. Let me have your ideas on these ideas.