On April the 25th, 1998, Spain woke up to be shaken by the news that the dam bounding the Aznalcóllar tailings pond had failed. The disaster released about 1.3million m3 of mine tailings and some 5.5million m3 of contaminated water into the Agrio river, not far upstream from the Doñana national park.
Principia were hired as expert witnesses by Boliden, the owner of the facility, and now, 20 years down the line, it is worth recalling some lessons, lest we forget them.
The failure itself was truly spectacular: some 700m of dam, together with the top 15m of the ground, moved a distance of up to 60m before stopping. Why did the dam fail? And what can we do to prevent such failures in the future?
At a technical level, the reasons for the failure are clear. The dam did not fail because of an external event, such as an earthquake, explosion or flood. It did not fail because of poor workmanship or because the built structure did not conform to the original design. The dam was actually destined to fail from the very start, simply because the design calculations did not incorporate the true characteristics of the underlying ground.
The dam sat on about 4m of alluvial deposits underlain by 70m of blue marl. Two major features of the blue marl were not taken into account:
- The first was their homogeneously low permeability, which implies that it would take centuries before the weight of the dam provided a frictional contribution to its stability against sliding. When the dam failed, some 20 years after the onset of its construction, that weight still contributed very little. Indeed, the failure had been imminent ten years earlier and was averted purely by chance, simply because operational reasons led to a widening of the base of the dam.
- The second ignored characteristic was the brittleness of the mechanical behavior of the marl. What is meant by brittleness is that, when the material is deformed, after it reaches its peak strength , further deformation causes a reduction in the load it can carry. In other words, the residual strength is much smaller than the peak strength. This allows the development of a progressive failure, in which parts of the surface have already gone past their peak strength at some point in the past and now can only offer the material’s residual strength. The end result is that, at the point of failure, the average strength mobilized along the failure surface is well below the peak strength.
If the low permeability had been taken into account, the marl would have never had a chance to display its brittleness.
The two aspects above were the cause of the failure. But how can one explain the fact that, rather than simply developing a few cracks, the failed dam managed to travel a distance of up to 60m? This happened because, upon the failure of the dam, its initial movement caused the tailings behind it to liquefy, which had two consequences:
- The pressures they exerted on the dam suddenly increased by 65%.
- Behaving now as a liquid, the tailings could follow the dam and continue pushing it during its displacement, something that would have been impossible if they had continued to behave as a solid.
No doubt disasters will continue accompanying the history of mankind, but the lessons learned from the past will hopefully make them less frequent and serious in the future.