This paper focuses on pore-water pressure accumulation in loose sandy tailings during three (3) earthquakes within four minutes and the accompanying decrease in effective stress to assess the dynamic stability of the Fundão Tailings Dam, which failed on 5 November 2015. Results of laboratory cyclic direct simple shear tests are used to illustrate the accumulation of pore-water pressures with closely spaced low-level cyclic events. The seismically-induced pore-water pressures in the loose sands below the left abutment downstream slope and setback area are used to estimate the decrease in factor of safety with time until failure occurred within 20-30 minutes of the earthquakes. Field cone penetration and standard penetration test conducted prior to the failure are used to calculate the factor of safety against liquefaction to estimate pore-water pressures generated during the fore, main, and aftershocks in the sandy tailings. The effective stress stability analyses utilise a liquefied shear strength ratio for the sand tailings below the left abutment downstream slope and toe of the plateau because the sand tailings reached the critical state condition due to a reduction in the effective stress.

This paper analyzes two tailings dam databases and recent failures to assess the local and global failure rates of upstream, downstream, and centerline raised tailings dams. Since 2000, the failure rate for all three raising methods has decreased except in Brazil, which helps explain the recent Brazilian regulations banning upstream raising. However, the failure rates also show that the downstream and centerline raising methods of tailings dams are not immune to failure, so, if mining continues, one of these three raising methods will be used, and all have been involved in prior failures. The paper also presents data on failure mechanisms, showing that slope stability, earthquakes, and overtopping are the three primary causes of tailings dam failures. To continue decreasing the failure rate of tailings dams, the following features and practices should be used in all types of tailings dams: drainage systems, engineering analyses, instrumentation, monitoring, inspection, and qualified external peer review to further reduce the failure rates, especially in Brazil. Finally, a section containing data on release volume as a function of dam height and total storage volume is presented.

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