A preliminary report by a Spanish consulting firm suggests that the Los Frailes tailings dam near Aznalcollar, Spain, failed when a shear zone in the underlying rocks slipped.
In a report to owner Boliden (BOL-T), consulting firm EPTISA Servicios de Ingenieria said the failure started in a shear zone below the dam, rather than in the dam itself. The firm emphasized that its initial conclusions were based on its preliminary test holes and on readings taken in the dam’s monitoring wells up to the time the dam failed.
A program of 1,000 metres of drilling, plus laboratory testing of core samples, is under way. Field and laboratory work for the investigation is about 30% complete, and EPTISA’s final report is scheduled to be released in mid-September. Boliden will then pass the report on to a panel of six geotechnical experts — two from each of Spain, Sweden and Canada — for review.
EPTISA says a shear zone in a marl (calcareous clay) formation 15 to 20 metres deep appears to have been the original locus of the failure. The slippage caused the earth beneath the dam to subside, weakening its foundation.
Normally, when zones of fracturing or shearing fail there has been some increase in pore pressure in the surrounding rocks.
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