Spanish builder Dragados has rejected a report blaming it for the failure of the Los Frailes zinc mine tailings dam failure in April 1998. The dam failure spilt millions of tonnes of acidic solids and water into surrounding farmland.

The owner of the mine, Canadian-Swedish company Boliden, has issued a report suggesting the dam’s failure was the result of errors in the design of the dam and deficiencies in its construction, by Dragados y Construcciones and its associated engineering firms Intecsa and Geocisa.

Dragados group rejected the findings of the report commissioned by Boliden as biased.

Boliden acquired the mine when it took over the Spanish company Aprisa in 1996.

The study of the failure, conducted by Principia EQE under the direction of Professor Joaquin Marti of the University of Madrid, identified fundamental deficiencies in the original 1977 and subsequent 1996 design and construction works, which affected the stability of the dam.

Principia EQE said that the assignment of optimistic shear strength parameters to the relatively brittle Margas Azules (Blue Clay) formation in the foundation was a principal cause of the failure.

EQE also said shear strength parameters from laboratory tests appeared to have been disregarded in the design, although the tests suggested the use of more conservative friction angles for the Blue Clay.