Brazil dam mishap: Vale audit report raised concerns over drainage, monitoring system

A report commissioned by Brazilian miner Vale SA last year to look into the stability of the tailings dam that ruptured Jan. 25 certified it as sound but raised concerns over its drainage and monitoring systems, newspaper Folha de S. Paulo reported on Tuesday. The report by German-based TÜV SÜD, which Folha said it had access to, made 17 recommendations aimed at improving the stability of the structure and was dated September 2018.

Among the issues identified were cracks in drainage channels. The report also recommended the installation of a new monitoring system able to pick up tiny movements in the soil, according to Folha. Vale said in a statement it had followed the recommendations in the report, which it described as "routine."

The disaster at the tailings dam at Vale's Corrego does Feijao iron ore mine in Brazil's mining heartland of Minas Gerais killed at least 134 people, with 199 still missing. The report appears to contradict a statement from TÜV SÜD the day after the spill which said "based on our current state of knowledge, no damages were found" during their inspections of the dam.

Issues with drainage could be crucial in investigations into what caused the dam to rupture, with a state environmental official telling Reuters that evidence suggested the burst was caused by liquefaction. Liquefaction is a process whereby a solid material such as sand loses strength and stiffness and behaves more like a liquid.

It is a common cause for the collapse of upstream dams holding mining waste, known as tailings because their walls are mostly built with dried tailings of sand and clay-like mud. Drainage issues can cause water to seep into the dried tailings, changing their consistency and stability. 

(With inputs from agencies.)