Importance of Shear Stress Anisotropy and bottom drainage on Tailings Dam Stability: a Case History

In February 2007, a tailings dam used as sedimentation pond of lime particles failed. As a result, an estimated volume of 150,000m3of tailings flowed from the breach in the dam slope. Forensic investigations were carried out to determine the causes of failure, so that suitable changes could be made for the future safe operation of the tailings disposal facility. The investigation identified several features of the tailings material and dam behaviour that it is considered contributed significantly to the slope failure. Among them, a marked anisotropy in the effective shearing resistance of the tailings material and the water accumulation in the tailings were identified as important factors in the failure. The origin of the anisotropy is the method of tailings deposition per layer, and results in a much lower shearing resistance on horizontal planes. The water accumulation was the consequence of a permeability of the tailings being higher than expected due to shrinkage fissures and a significant reduction in drainage efficiency from below the tailings. This loss of drainage was due to precipitation of calcite that lowered dramatically the permeability of the granular soils. The objective of the paper is to illustrate through back-analysis of the tailing dam failure how the shear resistance anisotropy and the efficiency of the drainage system influences the safety factor of the tailings dam.