The mud/debris flow of the Stava (Italy) tailings dams break.

The wastes resulting from the chemical and mechanical processes of mining extraction (tailings) are mostly accumulated in basins retained by a dam that usually, after the construction of the starter dam, are raised sequentially as the impoundment fills with an upstream or downstream or centerline method. Tailing dams are particularly vulnerable to failure mainly due to: (i) poor quality of local material used for the starter dam; (ii) dam construction with solid material mixed with high quantity of water; (iii) lack of specific design criteria; (iv) lack of extensive and continuous monitoring; (v) high cost of remediation works, after the closure of mining activities. The assessment of tailings facilities has mainly concentrated on the stability of tailings dams, while relatively few studies have investigated the flow of tailings released from a dam failure due to the its complex rheological behavior. Furthermore, as many changes in the rheological values along the run-out path have been observed, different interpretations of the flow behavior exist. On July 19, 1985, a fluorite tailings dam failed at Stava, Trento, Italy. About 180,000 m3 of tailings flowed 4.2 km downstream killing 268 people and destroying 62 buildings. The tailings dams consisted of two partially overlapped basins built on a slope. The failure started at 12.22:55 with the collapse of the up-slope basin that caused the overtopping and subsequent collapse of the lower basin. The resulting slurry wave travelled along the Stava Creek reaching a speed as high as 100 km/h, until it reached the Avisio River. Different Authors [e.g.: R.J. Chandler and G. Tosatti, 1995; R. Genevois ant P.R. Tecca, 1993] concluded that the dams were constructed with an unacceptably low factor of safety and that the failure probably was triggered by a blocked decant pipe located within the tailings. In particular, the main causes of instability were found to be: (i) the under-consolidation state of the deposited material; (ii) the spreading of the upper dam on the lower basin; (iii) the excessive height and slope of the dams; (iv) the use of the upstream method, which is the cheapest, but also the most dangerous one; (v) the wrong installation of the drainage pipes. This paper, after a short history of the dams, will present a geotechnical analysis of the dam's failure and an analysis of the flow along the Stava valley.