Tailings accident was avoidable: study

An independent consulting firm has concluded that design and construction deficiencies contributed to the failure of the Aznacollar tailings dam owned by Boliden‘s (BOL-T) Spanish subsidiary, Boliden Apirsa.

Boliden suffered a public relations disaster last spring when a breach in the tailings dam resulted in a major spillage of water and solid waste over farmland and into the Agrio River basin. However, it now appears that the seeds for the disaster may have been sown 10 years before the company purchased the mine facilities.

Last fall, investigators from the Spanish firm EPTISA (Empresa de ingenieria y consultoria) found that the accident was the result of more than 60 metres of lateral movement of a 700-metre section of the dam along a bedding plane in what is called the Blue Clay Formation at a depth of 14 metres below surface. The independent firm directing the investigation concluded that there was “no doubt” that the accident was caused by a fault in the formation, as a result of surplus pressure in the interstitial water of the clays and pressures due to the weight of the dam and the deposited tailings.

The EPTISA study was recently reviewed by another firm, Principa EQE, under the direction of Joaquin Marti, a specialist in geotechnical engineering and a professor from University of Madrid. This review found that the dam’s stability was affected by three main deficiencies in the design and construction work carried out by a Spanish contractor and its associated engineering firms.

The first deficiency was described as “excessive optimism” in the values used to calculate the pressure distribution of the formation, given its low permeability and thickness. Next, laboratory tests demonstrating “more conservative friction angles than those used in calculating the weight that the formation could support without movement” were disregarded in the design of the dam. Third, the study found that the brittle nature of the formation was not sufficiently recognized.

The study by Principa EQE concluded that the accident could have been avoided if a realistic evaluation of the overall characteristics of the formation had been used in the 1996 and 1997 studies. The results of the study were provided to a Spanish court investigating the cause of the dam failure. The Spanish contractor disputes these findings.