Case Study: The Failing Mechanism of the Tailing Dam Breach at Kolontár, Hungary

A new case recently published by Jozsef Garai and Imre Kovács at the ISSMGE International Journal of Geoengineering Case Histories, reveals the failing mechanism of the Ajka Alumina tailing dam at Kolontár, Hungary, by examining publicly available images from the internet.

The Kolontár tailing dam breach occurred on October 4, 2010, at the Ajka alumina plant in western Hungary. The dam held a reservoir of toxic red sludge (pH 13+) and released approximately 1.5 million cubic meters of highly alkaline red mud and water into the surrounding areas. The catastrophic event resulted in 10 fatalities and 286 injuries. This was the most severe environmental disaster recorded in Hungary’s history.

Through detailed analysis of publicly available photographs, the study reveals that the tailing dam breach was the culmination of a three-stage failing mechanism rooted in the dam's design and underlying soil conditions.

The key reasons for the dam failure were:

• Improper use of the North dam: The failed North dam was originally designed with a smaller cross-section to separate two reservoirs and not to withstand significant horizontal pressure from stored sludge. The second reservoir was never built, and the North dam effectively became an external wall but was not redesigned or reinforced to handle the increased load.
• Weakening of the Underlying Clay Layer: The dam was built on granular soil overlying Pannonian fat clay. Highly alkaline water from the reservoir leached out, interacting with the clay and significantly reducing its internal friction angle. This chemical interaction also changed the clay's cation composition from calcium to sodium, further weakening the clay's strength at the interface. This process created a weak layer on the surface of the clay with very low shear resistance, which acted as the sliding surface for the dam.
• Sliding Mechanism: The dam's failure ultimately occurred due to sliding along this weakened clay layer. Persistent scattered radar interferometry conducted between 2003 and 2010 detected a 3.9 cm/year outward horizontal displacement in the failed area. The study mentions that the failed area’s granular soil layer, which provided resistance against sliding, was the thinnest (0.5 meters) under the dam. Other areas with a thicker granular layer and bigger cross-sections did not fail from sliding, even though their resistance was nearly exhausted.