A report commissioned by Brazilian miner Vale SA last year, to look into the stability of a tailings dam that ruptured on Jan. 25, certified it as sound but raised concerns over its drainage and monitoring systems.
The report by German-based TÜV SÜD, reviewed by Reuters on Tuesday and marked as last updated in August 2018, made a variety of recommendations aimed at improving the safety of the structure, It also said the dam adhered to the minimum legal requirements for stability.
Among the issues identified were cracks in drainage channels, which were pictured in the report. The audit also recommended the installation of a new monitoring system able to pick up tiny movements in the soil.
Vale followed recommendations
The findings of the audit were reported earlier on Tuesday in Brazilian papers Folha de S.Paulo and O Estado de S.Paulo. 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 do Feijao iron ore mine in Brazil's mining heartland of Minas Gerais killed at least 134 people, with 199 still missing.
The report appeared at odds with 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 inspection of the dam.
Issues with drainage could be crucial in investigations into the causes of the dam rupture, with a state environmental official telling Reuters that evidence suggested the burst was caused by liquefaction.
Liquefaction is a process whereby solid material such as sand loses strength and stiffness, behaving 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 issue leads to liquefaction?
Drainage issues can cause water to seep into the dried tailings, changing their consistency and stability.
"To increase dam safety regarding failure from liquefaction, it is recommended to adopt measures to reduce the possibility of a trigger," TÜV SÜD said in its evaluation of the dam.
"To that end, (Vale) should avoid vibrations, prohibit nearby detonations, avoid heavy machinery traffic on the dam, prevent higher water levels in the tailings, not conduct work removing material from the feet of the slopes or conduct work that causes an overload of the reservoir of the dam," it said.
"The installation of seismological monitoring equipment is also recommended in the vicinity of the dam."
TÜV SÜD did not respond to a request for comment.