Failure mechanism of the Aznalcóllar Dam, Seville, Spain

In this paper an explanation is given of the geotechnical causes that induced the failure of the Aznalcóllar Dam in Seville, Spain. On the day of the failure the dam was 28 m high with a crest length of approximately 3000 m. It stored nearly 20 million m³ of mining waste. The construction of the tailing pond started in 1979. On the night of 24 April 1998 a breach developed in the embankment of the dam. To analyse the problem an extensive investigation of in-situ and laboratory tests was carried out. The data obtained have confirmed that the rupture was caused by a failure of the dam foundation at a depth of approximately 14 m below original ground level. This gave rise to an almost flat slide with a horizontal movement of 60 m affecting a section of about 600 m along the dam. The foundation consists of carbonated, slightly expansive overconsolidated silty clays known locally as ‘Andalusian blue marls’. In spite of their low dry density, they are hard and exhibit strain-softening behaviour. These phenomena, together with the presence of bedding planes (not observed by the borehole investigation, but detected by special geophysical techniques) and particularly the pore pressure generated in the foundation by the weight of the tailings and by the weight of the dam itself, have had a considerable effect on the failure process.