Jump back to the proceedings of the 1979 Uranium Mill Tailings Management conference for the story of a tailings impoundment that was mis-managed and which, in consequence, failed.
Here is the way the paper by John Nelson and Joseph Kane describe the causes of failure (I edit some of the gauche prose the authors thrust on us.):
On July 16, 1979 the Church Rock Tailings Dam failed. The embankment was about 35-ft high and was constructed on a relatively deep deposit of clayey, silty sand. Certain soils were collapsible and laboratory testing indicated collapse in excess of ten percent upon wetting. The impoundment was not lined, and seepage into the foundation soils could readily occur. Along the southern half of the embankment, approximately three feet of settlement had been observed since the beginning of operation in 1977. As a result of the large settlement, differential movement of the embankment would be expected. BOth longitudinal and transverse cracks had been observed in the embankment prior to the failure, and were attributed to the differential movement occurring in the embankment.
In order to protect against internal erosion of the embankment in the cracks, it had been recommended that a sand beach be maintained against the face of the embankment. The purpose of this sand beach was to act as a source of sand that could then be carried into any cracks that might develop. The idea was that passage of sand into cracks would prevent internal piping. The fact that sand was carried into the cracks was confirmed by observation after the failures of sand in the cracks on the walls of the breach.
Just prior to the failure, freeboard at the embankment had been decreased as a result of pond filling–the freeboard had been reduced to the point where tailings solution was in direct contact with the embankment and the sand beach had not been maintained. In that configuration, the sand beach because it was below the tailings fluid, was ineffective and the cracks within the embankment probably filled with tailings solution. The owner’s consultants propose as a probable cause of failure that internal erosion of the embankment occurred under the action of the fluid in the cracks.
Slope instability was ruled out on the basis of a few simple 1979 style stability analyses.
Thus we have an instance of poor design, over-optimisitic assumptions, lousy impoundment management, and failure. These days one cannot but wonder how they could stand b and see the embankment move, deform, crack, and not stop operation. No doubt some unnamed consultant told them the risk was small as long as the kept a sand beach above water. Which of course they did not do.
What was the outcome of this failure? If you know more, please comment.