20 September 2019
The 1985 Stava tailings dam disaster
I’m currently undertaking some work examining the runout of mine waste failures, during which I’m looking back at some old case studies. This is a follow up to the paper that Melanie Froude and I published last year that suggests that loss of life from slope failures in the mining industry, most particularly where there is poor regulation, is increasing with time. This has led me to look back at one of the worst failures of this type in modern history, the 19th July 1985 Stava Tailings Dam failure in Italy.
The location for the disaster was the village of Stava, in Trento. At the site, two tailings dams, built to contain the waste from a fluorite mine, had been constructed. The site, and the collapse, is described in a post-event analysis (Chandler and Tosatti 1995), available online, whilst there is also a nice reflective piece about the role of regulation (Luino and Degraff 2012), also available online. The latter includes this image of the tailings dams prior to failure:-
The dams collapsed on 19th July 1985 at 12:22. The initial failure occurred in the upper tailings dam, which then induced collapse of the lower facility in a domino effect. At the time of failure, 300,000 m³ of material was stored in the two tailings dams. Of this, 180,000 m³ was released in a single event, which mobilised into a very rapid mudflow. Eyewitness accounts suggest that the rate of movement was sufficiently great to generate an air blast that shredded the trees along the path of the flow. There is a good seismic data for the landslide, which suggests that it reached a peak velocity of 27 metres per second (about 100 km/h or 60 mph). The mudflow struck the houses located directly below the tailings dams before sweeping down to the village of Stava, located about 800 m below the lower dam. Luino and De Graff (2012) report that Stava was struck at 12:25, and the 20 or so buidings were completely destroyed in just 13 seconds.
The flow then travelled down the valley, ultimately sweeping through parts of the town of Tesero, located about 3 km downstream. The before and after image below, also from Luino and De Graff (2012), shows the extraordinarily destructive power of this flow:-
The mudflow from the Stava tailings dam failure finally stopped when it reached the main channel of the Avisio river at Tesero. The seismic data suggests that the movement event arrested just ten minutes after the initial collapse. The village of Stava had been completely destroyed, and there was extensive damage in Tesero. In total 268 people were killed, and 56 houses, six industrial buildings and nine other buildings were destroyed.
Subsequent analysis of the site (see Chandler and Tosatti (1995) suggests that the stability of the tailings dams was unacceptably low, primarily because the underlying ground was poorly drained, the construction meant that the dams lacked adequate drainage (allowing high water pressures to develop, and preventing proper consolidation of the tailings), the ponds were being recharged with runoff from the adjacent drainage basins, and the upper dam was unacceptably steep, with a part of the retaining structure being sited on tailings from the lower pond.
The failure led to legal action against those responsible for the dam. In June 1992 a total of ten individuals were convicted of crimes that included culpable disaster and manslaughter of multiple individuals, and were jailed.
The Stava Foundation has a very detailed website that documents the disaster in order to remember the victims, the aim of which is to promulgate the lessons from the Stava tailings dam failure. Sadly, the industry has yet to heed the lessons adequately.
Chandler, R.J. and Tosatti, G. 1995. The Stava tailings dams failure, Italy, July 1985. Proceedings of the Institution of Civil Engineers Geotechnical Engineering, 113 (2), 67-79.
Luino, F. and De Graff, J.V. 2012. The Stava mudflow of 19 July 1985 (Northern Italy): a disaster that effective regulation might have prevented. Natural Hazards and Earth System Sciences, 12, 1030–1042.