The failure of the tailings dam at Aznalcollar in Spain caused damage estimated at US$200-250M, according to L Berga, chairman of the Spanish Committee on Large Dams, and there are lessons that should be learned from the failure.

Berga was speaking on 19 June, at at the SpanCold meeting on Dam Safety, when he summarised current knowledge about the collapse of the tailings dam.

The slurry pond at the site was around 1.5M m3 in size, Berga said, and was enclosed by some 4100m of dikes, which included a single transverse dike that split the pond into two.

The dikes were made of rockfill covered in a clay layer, 1.84t/m3 in density and 7m thick, with a bentonite cutoff wall on the upstream side that extended 6m into the foundations. The crest width varied around an average 40m, and the slope was 1:2 on the upstream side and 1:1.3 on the downstream side. The dike had been in place for around 20 years and in 1996 a programme of progressive heightening began that was to increase the height to 72.04m. By 1998 the height had reached 68m.

The dikes had been subject to monitoring in the form of 22 levelling plates, four piezometers and four inclinometers, although these were not all in operation at the time of the 1998 incident. Indications during 1997 showed leakage of around 107litres/min, but there were said to be no indications of abnormal behaviour.

During the incident on 25 May, Berga said, the first massive movement was noted at around 2am. At that time some 800m of the front of the dike underwent a horizontal displacement that reached 54m at its maximum point.

The resulting dike breach was open and well defined, measuring 40m at its base and 70m at the crest. The right side of the breach was well defined with slopes approximately vertical, while the left side displayed more general sloping. The movement caused the transverse dike to become separated for about 30m at one end, causing a second breach. Large cracks appeared in the transverse dike and there were partial slidings in the clay.

The breaches in the dike allowed around 6M m3 of mud to pass through, Berga said. This was partly sedimented in the river. Measurements 14km downstream of the flood and mudflow revealed a peak flood of 3.94m one hour after the breach, with a peak flow of 900m/sec.

Consequences and lessons Berga noted that the flood had caused no loss of human life, but went on to say that the environmental impact had been great. The consequences were particularly bad in the Donana national park, where around 4400ha of land had been badly affected by toxic mud.

Berga noted that tailings dams were specifically excluded from new legislation designed to assure the safety of large dams in Spain (see IWP&DC, June 1998, p11). However, he said, there were some safety issues relating to such dams that should be addressed. These issues included poor compactness of the dam material; the specifics of the upstream construction process; seismic safety; and the toxicity of the pond contents.

There were three lessons that should be learned from the Aznalcollar failure, Berga said: •Techniques must be improved in the design, construction and operation of tailings dams.

•The inventory of existing tailings dams should be updated, and the downstream hazard and environmental risk of each dam should be assessed.

•Noting that there was currently no specific legislation in Spain on tailings dam risk, Berga said that it was necessary to regulate their safety.