Around noon, two 30-m high tailing retention basins associated with the extraction of fluorite and located one on top of the other broke. A 180,000-m³ wave of effluent containing 95% water swept through the valley all the way to the AVISIO River, submerging within just a few minutes the villages of Stava and Tesero, killing 268 people and causing damage estimated at €155 million.

A commission of experts assigned by Italian judicial authorities to determine the accident causes examined and rejected the hypotheses of earthquake activity or an explosives blast at one of the many mines in the region.

During normal operations, the water contained in basin tailings is collected from within for discharge via a drainage system. The granular phase at the periphery serves to consolidate and reinforce the enclosure. In Stava, discharge was routed through a pipeline running below the basins crossing the body of the retention enclosure. The commission indicated that subsidence had caused the end of one segment of this pipeline to loosen at the level of a previous repair (bypass). The drainage system defect prevented smooth discharge of water and led to an increase in the hydraulic head inside the enclosure body, which was ultimately responsible for the collapse. This defect most likely dated back several months prior to the accident: an opening had formed on the sidewall of the upper basin in January 1985, causing the leak repaired in March. The basins had been entirely emptied in May to prepare for renovation and then placed back into service on 15 July, just 4 days before the accident.

The year’s record rainfall (+22% compared to the average of the 66 previous years) and the considerable snowfall of the past winter contributed to the occurrence of this accident, yet neither constituted the primary cause, since structural disorders had been noticed as of January, earlier than either the snowmelt or the major part of the rainy period.

The investigation also revealed design mistakes: excessive sidewall slope (reaching 40° in spots), an overly marshy foundation soil to provide for both good drainage and wall consolidation offering the required stability. A member of the expert commission declared that the structure “had been built to the very limit of its stability capacity, making just the slightest disturbance sufficient to cause collapse. […] It is actually surprising that the dam had not failed earlier.” A study requested by the township of Tesero in 1975 had already pointed to the inadequate real values of factors of safety for these basins.

In 1992, Rome’s Supreme Court confirmed on appeal the guilty verdict for criminal negligence and involuntary homicide, issuing prison sentences for 10 individuals, including 8 company executives responsible for operating the site since the upper basin was built, and 2 members of the regional council assigned to oversee mine safety. In 2004, €132 million were paid in the form of compensation to the 739 victims by some of the subsequent operators as well as by the Autonomous Province of Trento.

Following the accident, Italy’s legislation regarding tailings dams was strengthened, and the Stava Mine was definitively closed.