Mining activities for silver-bearing minerals have been carried out in the Stava-Valley, in the Italian province of Trento, since the 16th century. In the early 20th century, especially since 1934, the interest shifted to fluorite, an important mineral for the chemical industry. The most important mine was situated at the base of the mountain of Prestavel, overlooking the entire Stava-Valley.
To isolate and to concentrate the fluorite the Prestavel mine in 1961 shifted from the simple gravimetric procedure (based on the different density of minerals) to the more effective flotation-process, where a mixture of water, rocks and foaming agents are used to extract the fluorite from the rock debris. However this process produces great quantities of toxic and semi-liquid waste, this waste has to be collected, stored and dried.
To deal with the large amount of mud the construction of a storage basin was initiated on a nearby slope. The dams of the basin were built by separating the gravel and sand from the water and mud, a cheap construction method adequate however only for small basins.
The basin was situated 150m above the valley floor, in a distance of 800m to the last houses of the small village of Stava. In 1969 the first dam reached a height of 25m, even if previously only 9m were planned, and it was decided to build a second basin above the first basin. The dam of the second basin was 34m high (the two basins after a contemporary photography, from ANNOVI).
Fig.1. Schematic reconstruction and cross-section of the two basins of the mine of Prestavel - situated on an impermeable slope of glacial origin, the dams of the basins were constructed with layers of sand and gravel, containing water and the liquid waste-mud/silt. Drainage and desiccation of the mud was however difficult on the impermeable swampy slope.
In 1974 the local administration of Tesero (distant ca. 5 kilometres) demanded a security and stability control for the two basins. The (today as incomplete regarded) study was carried out in the year 1975 and concluded that the gradient of the dams was "extraordinary" and the stability of the construction "at the limits". Nevertheless the mining company provided to the authorities a positive security report, and so the second dam in the following years could be heightened, with the only compromise that the gradient was limited to maximal values of 39 to 40°.
In the years 1978 to 1982 the two basins were not used. In all these years the dams and basins were never again controlled, not by the mining company, neither by the public administration responsible for mining activities in the Province of Trento.
The dam(s) collapsed on July 19, 1985 at 12.22.55 (the remains of the two basins after the collapse, figure from LUCCHI 1995). The resulting mudflow caused one of Italy's worst disasters, killing 268 people, destroying 62 buildings and demolishing 8 bridges.
The upper dam collapsed first, slumping in the subjacent and triggering the collapse of the lower dam. Around 180.000 cubic metres (from the total volume of the two basins of 300.000) of mud, sand and water were released into the Stava-Valley and toward the village of Stava at a speed of 90 km/h. Additionally 40.000 to 50.000 cubic metres were eroded by the primary wave from the underground and incorporated in the mudflow. The mudflow reached the village after 50 seconds, crashing through the buildings, the flow then continued for three minutes until it reached the Avisio River - 4.2 km away - engulfing and destroying everything in its path (Aerial imagines of the Stava-Valley before, and after the disaster; from W.I.S.E.).
Fig.2. The destruction in the valley of Stava caused by a mudflow generated by the collapse of two storage basins of the mine of Prestavel (image in public domain from Wikipedia).
The legal investigation into the disaster confirmed that the dams were poorly maintained and the factor of safety (a value calculated considering the angle of repose of sediments and the slope angle) was to small.
Fig.3. Schematic angle of repose of various materials, this value depends of the grain size and water content of the sediments. Considering the material used to build the dams (sand with lots of silt and clay, saturated with water) of the Prestavel mine, the chosen value of 39° was extraordinarily high.
As one of the last triggers of the collapse of the upper dam is considered a leak of water caused possibly by the rupture of a pipe used to drain water from the liquid mud (the swampy underground was to impermeable to allow desiccation of the mud) - probably the pipe had been bent by the weight of the accumulating sediments. The increasing water pressure of the bunged basin in combination with the water saturation weakening the sediments of the dam caused finally the catastrophic collapse.
LUCCHI, G. (1995): Stava perchè. Editore Cuca & Genovese. Trento
ANNOVI, A.(): La frana di Stava. Accessed 18.07.2011
World Information Service on Energy (13.05.2009): The Stava tailings dam failure (Trento, Italy). Accessed 18.07.2011