Design flaws led to deadly Brazil mine disaster

Design flaws and a series of problems over six years led to the collapse of a mine dam and the loss of 19 lives in Brazil's worst environmental disaster, mining giant BHP Billiton said on Tuesday.

A technical report commissioned by the world's biggest miner and Brazil's Vale -- the co-owners of the mine's operator Samarco -- found a chain of events from 2009 to 2015 caused the November 5 catastrophe, which unleashed a tsunami of toxic waste and buried a nearby village.

Samarco is facing billions of dollars in legal claims for clean-up costs and damages.

The 76-page report by an expert panel led by Canadian geotechnical engineer Norbert Morgenstern did not assign blame.

"It is a very detailed, technical report 10 months in the making," BHP's chief commercial officer Dean Dalla Valle said in a media teleconference.

"Given the legal proceedings, it wasn't appropriate that we actually try to use this process to attribute blame," he said, adding that "we have no reason to believe that anyone at BHP had any information that indicated that the dam was in danger of collapsing".

The report said the original dam design was robust. But construction difficulties led to design changes that eventually caused "liquefaction" -- where sediments lose their strength and act like a liquid -- and the dam's rupture.

"Together with the revised design there was a fundamental change in the design concept whereby more widespread saturation was allowed and accepted," the report said.

"This increase in the extent of saturation introduced the potential for sand liquefaction."

A series of three small seismic shocks that occurred some 90 minutes before the rupture was also "likely to have accelerated the failure process that was already well advanced", the report added.

Valle added that there was "no evidence that I'm aware of that anyone put production over safety" when asked if cost-cutting measures in recent years -- which have been taken by BHP and other miners amid a commodity price slump -- affected the dam's management.

"The events that started in 2009, it was a long chain of events that built up... I don't think that that was an issue," he said.

BHP said last month it would appeal a Brazilian court's decision to reinstate a 20 billion reals (US$6.2 billion) civil claim for clean-up costs and damages. The company has argued the claim was superseded by a separate government settlement.

BHP has so far booked a US$2.2 billion charge relating to the failure and conducted a separate review of its other significant mining dams.

Work at the Samarco mine has been halted since the disaster and BHP and Vale have said operations are unlikely to restart this year.