Engineers in Hungary were this week frantically finishing work on a protective wall to protect villages from toxic sludge released by the partial collapse of a metals factory dam.
Hundreds suffer chemical burns from slurry
As NCE went to press government officials feared a further collapse of the dam was imminent following the failure of the dam’s north west corner on Monday.
The reservoir wall burst on 4 October releasing a flood of toxic red mud onto local villages, killing at least five and putting the area into a state of emergency.
More than 100 people are thought to have suffered chemical burns from the spill, which devastated the towns of Kolontár, Devecser and Somlóvásárhely.
Up to 700,000m³ of the slurry – made up mostly of iron and aluminium oxide – has escaped from the nearby Mal Alumina Plant and spread across an area of 40km². The sludge’s high alkalinity has triggerd environmental concerns.
Hungary’s National Directorate General for Disaster Management (NDGDM) is now overseeing the clean up and construction of the new wall, which it hopes will protect the villages near Ajka in western Hungary which are most under threat.
Since the first collapse, major cracks have appeared on the reservoir’s northern wall next to the breach along a 50m stretch. The wall has subsequently sunk 500mm, prompting officials to say its collapse is imminent.
“These walls will definitely fall down. Why? Because 2.5M.m³ of red mud is pushing on the wall and on the other side is nothing,” said Hungarian environment minister Zoltan Illes.
Over 1,000 emergency workers are helping the relief effort, which is now focused on completing the protective wall where work started last Saturday.
The new protective wall is sited between 200m to 300m downstream of the breached reservoir. It will be about 600m long, between 3m and 6m high, 26m wide at the base and 7m wide at the top. The barrier will comprise a crushed dolomite core with clay and gravel.
It will work by cutting the village of Kolontár in two to protect the largely unaffected zone to the south and the bigger settlement of Devecser a further 2km downstream. Most of the water has drained from the slurry in the reservoir meaning although the remaining slurry is more toxic, it is slower moving, allowing the authorities more time to prepare defences.
“Seepage underneath, or a structural fault within, the dam wall is the most likely cause of failure”
While the clean-up is underway the Hungarian authorities have launched an investigation into causes of the initial collapse.
“Seepage underneath, or a structural fault within, the dam wall is the most likely cause of failure,” said Imperial College London professor of geotechnical engineering Richard Chandler after seeing images of the collapsed dam.
“Overtopping is generally the most common type of failure but in this case but there is no evidence of staining on the reservoir walls.”
Engineers have also focused on the differences in the way the upstream and downstream sides of the breached reservoir wall were built. The walls are made of rolled and compacted concrete. This is a typical construction material for dam walls. However, the sides of the walls are irregular.
“The downstream face of the western wall seems to feature a berm about halfway up and a moderate slope. The northern embankment does not feature a berm and seems steeper, particularly over the upper parts,” said MWH International Dams & Hydropower technical director Peter Mason
“The [structure] suggests there is a difference in either design or construction concepts between the two embankments.”
The collapse has also raised questions about how rigorous the inspection regime was on the dam. Government officials said it was checked three weeks ago.
Inspection regimes for the dam are covered by the European Union Mining Waste Directive 2006 which was implemented in Hungary in 2008-2009.
This requires “a competent person” to carry out regular checks on the structure.
Hungary, however, does not have an equivalent of the UK’s Health & Safety Executive to enforce regulations.