HOME   WISE Uranium Project   >   Tailings Dam Safety   >   Issues   >

The Los Frailes tailings dam failure (Aznalcóllar, Spain)

(last updated 27 Feb 2015)


On April 25, 1998, a tailings dam failure of the Los Frailes lead-zinc mine at Aznalcóllar near Seville, Spain, released 4-5 million cubic meters of toxic tailings slurries and liquid into nearby Río Agrio, a tributary to Río Guadiamar. The slurry wave covered several thousand hectares of farmland, and it threatens the Doñana National Park, a UN World Heritage Area.


The causes of the dam failure

Although a final report on the cause of the Los Frailes dam failure has not yet been published, it is rather improbable that it must be attributed to the force majeure implicitly claimed by Boliden Ltd. , the Swedish-Canadian operator of the mine. It seems to be clear so far that a separation dam between two parts of the impoundment experienced a foundation failure, that is, the soil beneath the base of this dam lost its mechanical integrity. The separation dam is located between the basins filled with tailings from pyroclastic rock and from pyritic rock. The foundation failure of the separation dam then initiated the break of the main dam.

The dam break (after El País of May 14, 1998)

Schematic of Los Frailes dam failure

  1. A slab of soil beneath the dam slided approximately one metre towards Río Agrio. The front of the slide is about 20 metres wide, and it is located in the area of the junction of the two impoundments.
  2. The dam cracked and broke abrutly; the wall collapsed and sweeped along the separation dam between the two impoundments.
  3. Between five and seven million cubic metres of contamined water and slurries spilled through the gap.
  4. The bed of Río Agrio rose three metres, changing its course and bringing Tertiary rocks to light.

Note: for a more detailed description, see below.

The reason for this foundation failure is a matter of discussion, still. While Boliden Ltd. maintained that there was no way to foresee it, even the report prepared by Geocisa on behalf of Boliden in June 1996 described the weak point of the dam already two years in advance of the failure: "Sliding surfaces are forming in the marl underneath the foundation, not affecting, therefore, in a general way, the conditions of support of the embankment raised on the land." With respect to the seepage, another report of the Swedish consultant Golder Associates AB recognized that these were of 10 cubic metres per hour and that 85% of these were given back once pumped from wells located in the outer perimeter of the dam. (El Mundo May 25, 1998)

Some unnamed geotechnical experts cited by El Mundo (May 19, 1998) suggest that the foundation failure was caused from chemical attack of the impounded acidic pyritic slurries on the marl forming the dam foundation material. Marl consists of clay and calcium carbonate (CaCO3). The calcium carbonate contained in this marl decomposed under the acid attack, deteriorating the mechanical stability of the soil.

Rafael Baena Escudero of the Department of Physical Geography and Regional Geographic Analysis stated: "In this case, a complete lack of foresight emerged. The dam was built on top of expansive clays. Within these clays, deformations have occured, which were propagated to the soil, readjusting the blocks whenever a movement occured. In this sense, the seepage through the marls has the effect that these layers, the phylosilicates, swell and expand their volume. The opposite happens when they dry out and force the shrinking of the clay. This movement of expansion/contraction is constant and should have been accounted for. Especially, after the inclinometers had become deformed: something was moving. - This is a matter of general negligence and not a problem of nature." (El Mundo, May 25, 1998)
According to a report by Geocisa , deformations of the inclinometers had been observed already in 1997. One of four inclinometers has been out of operation since May 1997, the three others were not in a good condition. (El País Domingo of May 17, 1998)

The acidic seepage, combined with the continued blasting in the nearby open pit mine, is also identified as the most probable hypothesis for the cause of the failure by Luis Berga, expert of the Universitat Politecnica de Catalunya . He presented the results of his study on 19 June in Barcelona at the congress of the International Commission on Large Dams . (La Vanguardia of June 20, 1998)

Another possible factor contributing to the dam failure is the beach width between the decanting water and the dam crest. According to El País Domingo of May 17, 1998, technicians exist who assure that in some photos an unusual water presence was observed where the dry beaches are expected. Insufficient beach width is a very critical situation for tailings dams (see Safety of Tailings Dams). In addition, Boliden had applied for a license to build a water treatment plant that would double its present release capacity. Therefore, some experts raise the hypothesis that the dam was being filled at a rate higher than designed.

The report prepared by the experts of Eptisa implicitly finds that the subsoil was unsuitable to support the mass of tailings piled up. The relevance of other factors, as seismic events, blasting at the nearby mine, or seepage impacts is denied. (El País of Oct 20, 1998)

Information on the tailings dam failure (after El País of Oct 20, 1998)

View of Los Frailes dam failure

According to the Eptisa report, the pressure of the tailings on the ground caused a large displacement of clays at a depth of about 14 meters, which caused the landslide of the dam.

Section of Los Frailes dam failure

On November 6, 1998, Boliden has sent a report to the judge who is in charge of the Doñana case, in which it announces that a new displacement of the dam has taken place. The displacement has been of three millimetres. Sources of the Government assure that, although they do not want to alarm anybody, "there exists the risk of a new spill". (ABCe Nov. 12, 1998)

On November 13, 1998, the advisor of the Environment Protection department of the Junta of Andalusia, José Luis Blanco, warned of the existence of "risks" in the environs of Doñana if the tailings dam of Aznalcóllar filled with water in strong rains. The advisor said that the dam still contains more volume of tailings than it has lost during the April spill. In case of rains, the dam could fill with liquid and, therefore, exert more pressure on the walls of the dam. In his opinion, the best solution is to advance in the sealing of the dam to avoid that water can be accumulated. (El Mundo, Nov. 14, 1998)

In a press release dated Feb. 26, 1999, Boliden for the first time admits that the tailings dam was ill designed. Boliden blames its contractor Dragados y Construcciones , and its associated engineering firms, Itecsa and Geocisa for the failure. Their "incorrect interpretation of the geotechnical properties of the Margas Azules (Blue Clay) Formation [...] facilitated the failure of the tailings dam." (Boliden news release, Feb. 26, 1999 )

On March 24, 1999, the regional government of Andalusia authorized the restart of the Aznalcóllar mine, subject to conditions. Some part of the remaining space in the mine's former open pit is to be used for tailings disposal. This pit has received already 6 million tonnes of contaminated material recovered during the cleanup work of the Guadiamar valley. The authorized disposal volume would allow for the mine operation for further three years only, while Boliden wants to continue operation for 10 years. The regional government did, however, not allow for the disposal of more material, for concerns of anticipated groundwater contamination. (El País, March 25, 1999)

On April 1, 1999, Boliden announced that it has decided to restart the Los Frailes mine. Mining operations resumed on April 6, while the mill has not yet restarted.

New movements of 14 centimeters have been detected in the Aznalcóllar tailings dam, after displacements of 3 millimeters had been detected on Nov 6, 1998. Boliden now urgently demands the licensing of stabilization works which had been postponed by the judge who is in charge of the Doñana case, to allow investigations into the cause of the failure. (ABCe, April 7, 1999)

On April 22, 1999, the Spanish Government extended the claims against the foreign owners of Boliden Apirsa SL, Canadian Boliden Ltd. and Swedish Trelleborg AB. The cleanup costs for the dam failure are estimated at 15 - 30 billion Pesetas (US$ 100 - 200 million). Boliden has paid only 1.9 billion Pesetas (US$ 12 million). The claims are being extended against the foreign owners, since it is feared that the Spanish branch might declare insolvency. (El Pais / El Mundo / La Vanguardia / ABCe, April 23, 1999)

Since April 6, 1999, the environmental ministry of Andalusia has knowledge of new high acidity and metal concentrations in the water of the Guadiamar river near the Los Frailes mine. The subsoil of the dam is contaminated as a result of the dam failure, and the contaminated seepage flows into the channel of the Guadiamar at the rate of 86,400 liters of acidic water per day. (La Vanguardia / ABCe / El Mundo, May 5 and 6, 1999)

On June 29, 1999, Boliden Limited announced that its Spanish subsidiary, Boliden Apirsa SL (Apirsa), recommenced milling activities at its Los Frailes open pit mine. Mining activities had previously recommenced on April 1, 1999. Full production is expected to be reached in the third quarter of this year. Tailings from the plant are being deposited in the depleted Aznalcóllar open pit.

On December 29, 1999, a report on the causes of the dam failure was published by the regional government of Andalusia. The report was prepared by Centro de Estudios y Experimentación (Cedex) - a technical organism that depends on the Ministry of Public Works and the Economy. The report agrees with Boliden that the dam failure was caused from a failure of the Blue Marl formation beneath the impoundment; but it falls behind Boliden's position in saying that this failure was unforseeable. According to the report, the dam was designed in compliance with the state of the art. The Blue Marl formation, however, had been impermeable rather than semipermeable, as anticipated. The report suggests that the safety factor of 1.4 usually applied to the construction of such dams has to be raised to 1.6 - 1.8. (El País / El Mundo / La Vanguardia / ABCe, Dec. 30, 1999)

An expert report prepared on behalf of the Court of the Sevillian locality of Sanlúcar la Mayor concludes that the dam failure was a result of negligence: The dam failed since it was constructed and enlarged in two projects that did not take into account two factors crucial for the stability. First, the fragility of clays and the resulting risk of triggering a phenomenon of progressive failure. Second, the high pressures of the water in the clayey foundation. The report was prepared by Antonio Gens and Eduardo Alonso , university professors of the Polytechnical University of Catalonia in Barcelona. (El País / El Mundo / La Vanguardia / ABCe, Apr. 8, 2000)

On October 2, 2000, Boliden Ltd. announced that its subsidiary Boliden Apirsa has "filed a court application for commencement of 'suspension de pagos' proceedings (the equivalent of Canadian CCAA and United States Chapter 11 proceedings)." This means that Boliden Apirsa is insolvent. The company would not continue the development of the Los Frailes mine after October 2001.

On December 27, 2000, Judge Celia Belhadj-Ben Gómez ruled that there are no indications of penal responsibility in the dam failure. (El Mundo Dec. 27, 2000)

On January 3, 2001, the Andalusian Government filed an appeal against this decision. (El País Jan. 4, 2001)
On January 5, 2001, the Spanish Environmental Ministry filed an appeal against the decision. (El País Jan. 6, 2001)

On February 9, 2001, Boliden Apirsa stopped production at the Los Frailes mine in order to prevent a further rise of the water level above the licensed limit in the former open pit which now contains the tailings recovered from the spill and receives those from current production. (El Mundo, Feb. 11, 2001)

The Court of Sanlúcar la Mayor has accepted the suspension of payments of Boliden Apirsa. (El Mundo/El País July 20, 2001)

On September 5, 2001, Boliden Apirsa announced the definitive closure of the Los Frailes mine and the dismissal of all of its 425 employees. The mining will conclude that week and the mill will cease operation on Sep 20, 2001. (El Mundo/El País Sep 6, 2001)

On November 19, 2001, the Regional Court of Sevilla finally confirmed the decision of Judge Celia Belhadj-Ben Gómez that there are no indications of penal responsibility in the dam failure. However, a civil lawsuit remains possible. (El País Nov. 20, 2001)

On November 20, 2001, the Andalusian Government and the Spanish Environmental Ministry announced to sue for damages. Both Administrations have spent more than Pesetas 40,000 million (Euro 240 million / US$ 210 million) for the clean-up of the spill. (El País Nov. 21, 2001)

On December 14, 2001, Boliden Apirsa signed agreements with the Regional Government of Andalucia and with the workers council and unions regarding environmental restoration plans and severance payments. The mining company had presented a plan of environmental restoration and abandonment of the mine valued in 8,269 million pesetas (EUR 50 million / US$ 45 million). The workers council, however, estimated that at least an additional 5,000 million pesetas (EUR 30 million / US$ 27 million) were required.
In the agreement obtained, the Regional Government had to accept the payment with assets of the company for lack of sufficient funds available. But it reserved the right to claim from Apirsa's Swedish parent company Boliden Ltd any additional funds that might be required in the future. (El País Dec. 15, 2001)

The environmental group Ecologistas en Acción has decided to draw the case on the penal responsibility for the tailings dam failure before the Constitutional Court. (El País Feb. 1, 2002)

On April 23, 2002, the advisor of Environment, Fuensanta Coves, indicated that the legal services of the Regional Government are completing the statements of civil claims against Boliden-Apirsa, to demand a part of the funds used to repair the damages of the accident. The Andalusian Administration has invested more than 152 million Euros (around 25,000 million Pesetas) in the recovery, and it anticipates to spend another 10 million Euros in 2002. (El País April 24, 2002)

On July 2, 2002, the Environmental Council of the Andalusian Government approved the initiation of civil actions against the mining company to try to recover part of the 152 million Euros (25,000 million pesetas) spent to decontaminate the affected zone. (El País July 3, 2002)

On July 31, 2002, the Environment Council of the Andalusian Government concluded the removal of the 10,000 cubic meters of muds that still were stored in the river basin of the Guadiamar. The Environment Council furthermore announced that it will come to the reforestation of the affected zone in October 2002. (El País August 1, 2002)

On August 2, 2002, the Council of Ministers imposed a penalty of 45 million Euros on Boliden, the highest ever by environmental damages in Spanish history. Nevertheless, the fine covers only about one sixth of the cleanup cost of 276 million Euros spent by the administrations so far. (El País / El Mundo, August 3, 2002)
Boliden announced it is not willing to pay a single cent. (ABCe August 5, 2002)

The Andalusian Government plans to impose another penalty of 86 million Euros on Boliden to recover the cost it has spent on the cleanup. (El País August 6, 2002)

Boliden claims damages from the Spanish construction company Dragados: Boliden's Spanish subsidiary Boliden Apirsa has filed a notice of litigation against Dragados y Construcciones S.A., a member of the construction company Dragados S.A., listed in Spain, in connection with the failure of the tailings dam at the Los Frailes mine, Spain, in 1998. Bolidenīs claim against Dragados amounts to a minimum of 1 billion SEK (107 million Euro). The formal claim will be presented to a Spanish court in October. (Boliden Sep 26, 2002)

On Nov. 16, 2002, the regional government of Andalusia filed a civil suit to recover from Boliden 89.8 million euros ($89.9 million) in damages and cleanup costs. (Reuters Nov. 22, 2002)

On Jan. 2, 2003, the Primera Instancia número 11 court of Seville rejected the civil demand of the regional government of Andalusia against Boliden. (ABCe Jan. 4, 2003)

The regional government of Andalusia now has decided to demand from Boliden recovery of 89.9 million euros in damages by the administrative route. (ABCe Nov. 5, 2003)

"As previously announced, Boliden's Spanish subsidiary Boliden Apirsa filed a notice of litigation against the Spanish company Dragados y Construcciones S.A. Now Boliden Apirsa has filed the final claim in a court in Madrid. Boliden's claim against Dragados amounts to around EUR 115 million." (Boliden Jan. 23, 2004)

The Spanish Supreme Court has slightly reduced one of the three sanctions that the Cabinet had imposed on Boliden. The sanction that the Executive initially had fixed at EUR 41,606,316.75, was reduced by EUR 1,352,772.
The Court, nevertheless, maintains the other two fines imposed on Boliden Apirsa SL: EUR 601,012.10 for an infraction of the Water Law, and EUR 2,780,181.66, value of the damages caused to the hydraulic public domain. (ABCe Nov. 26, 2004)

On June 6, 2006, the judicial hearing is to begin that was initiated by Boliden Apirsa against Geocisa, Intecsa, Dragados (now ACS), and Banco Vitalicio y Zurich. The hearing is expected to last until June 30, 2006. (El País June 5, 2006)

On Nov. 24, 2006, the Court of First Instance Number 9 of Madrid dismissed the suit filed by Boliden Apirsa against Geocisa, Intecsa, Dragados (now ACS), and Banco Vitalicio y Zurich for payment of EUR 250 million. The court acknowledges the great difficulties to assess the properties of the subsoil and finds that Boliden has not demonstrated that the companies have not used the current technology. (El Mundo Nov. 24, 2006)

Boliden Apirsa has filed an appeal against the decision of the Court of First Instance Number 9 of Madrid. (El País Dec. 5, 2006)

In connection with the bankruptcy proceedings concerning Boliden's Spanish subsidiary Apirsa, the Commercial Court of Seville has granted a preliminary injunction against Boliden AB and Boliden Mineral AB for the seizure of assets up to Euro 141 million, as security for claims relating to the April 1998 dam failure. Boliden disputes the court's order. (El Mundo / Boliden, June 21, 2007)

The Superior Court of Justice (Tribunal Superior de Justicia - TSJA) has annulled the resolution of the Council of Government of the Regional Government that declared that the companies of the mining group Boliden, responsible of the toxic spill of Aznalcóllar, were obliged together to reimburse the costs assumed by the regional Government, stated as EUR 89 million. (El País Dec. 14, 2007)

Ten years later, nobody has paid for the spill of Aznalcóllar: Boliden has avoided to pay EUR 90 million prolonging the fight in the courts. (El País Apr. 24, 2008)

The Supreme Court (Tribunal Supremo - TS) exempts Boliden from the payment of EUR 89.9 million for the reimbursement of costs assumed by the regional government for the cleanup of the spill of Aznalcóllar. (El País Dec. 6, 2011)

Supreme Court makes Boliden responsible for the spill of Aznalcóllar: The Supreme Court attributes the responsibility of the spill of Aznalcóllar to the mining company Boliden. (El País Dec. 28, 2011)

The Supreme Court exculpates the construction companies that raised the Aznalcóllar tailings before the failure: The Supreme Court has rejected the appeal of the Spanish branch of Boliden, Boliden Apirsa, against the constructors (and their insurers) in charge of the construction and raising of the raft, whose breakage triggered the disaster of the toxic spill of Aznalcóllar in April 1998. Boliden was the owner of the dam, but it understood that the companies ACS, Intecsa-Inarsa and Geocisa, that were in charge of the project to raise the raft of residues, were the people in charge of the breakage.
The Swedish Boliden demanded from these companies 115 million Euros in damages. In addition, it asked that they meet the amounts demanded by the regional government of Andalusia (89.9 million) and the central Government (43.7 million) for the environmental damage caused in the area. And so, altogether, Boliden demanded 248.6 million Euros from them. (El País Jan. 26, 2012)

Fourteen years later, back to square one: the Supreme Court ordered the judge of the Primera Instancia número 11 court of Seville to resume the claim of the regional government of Andalusia against the multinational Boliden to pay the EUR 89 million repair cost for the ecological damage in the surroundings of Doñana National Park, a claim the same court had rejected in 2003. (El País May 22, 2012)

On July 11, 2013, the regional government announced it wants the Aznalcóllar mine to reopen. The government plans to issue an international invitation to tender. (El País July 11, 2013)

The Andalusian company Magtel and Grupo México won the tender for reopening the Los Frailes mine. (El País Feb. 21, 2015)
Environmental organizations recall that in August 2014 Grupo México caused a spill of 40 million liters of copper sulphate mixed with sulfuric acid and high concentrations of heavy metals in the Buenavista del Cobre mine in Cananea, Mexico. (abc.es Feb. 22, 2015)

The regional government has reinstated in the Primera Instancia número 11 court of Seville its claim against Boliden to pay the EUR 89 million repair cost for the damage resulting from the dam failure. (El País Feb. 26, 2015)




News + Infos:


On November 29, 2001, the shareholders approved the redomiciliation of the public company parent of the Boliden group of companies from Canada to Sweden. From December 5, 2001, the company is listed in Sweden as Boliden AB rather than in Canada as Boliden Ltd.



Environmental NGOs



> For Spanish-English translations, check Babelfish ! <

> See also: The September 2000 tailings dam failure at Boliden's Aitik mine (Gällivare, Sweden)

HOME   WISE Uranium Project   >   Tailings Dam Safety   >   Issues   >