Iron ore miner BHP Billiton said that a panel of four geotechnical specialists published its findings into the immediate causes of the failure of the Fundão tailings dam at the iron ore operations of Samarco Mineração S.A (Samarco) in Minas Gerais, Brazil, on 5 November 2015.
BHP Billiton had also conducted its own reviews of other significant dams at its operations and of its non-operated minerals joint venture arrangements.
According to a review panel, the failure of its Fundão tailings dam by liquefaction flowsliding was “the consequence of a chain of events and conditions” starting with a drainage failure.
“A change in design brought about an increase in saturation, which introduced the potential for liquefaction. As a result of various developments, soft slimes encroached into unintended areas on the left abutment of the dam and the embankment alignment was set back from its originally-planned location,” the review panel said.
Explaining the “setback,” the specialists said slimes “existed beneath the embankment” and were “subjected to the loading its raising imposed.”
“This initiated a mechanism of extrusion of the slimes and pulling apart of the sands as the embankment height increased. With only a small additional increment of loading produced by the earthquakes, the triggering of liquefaction was accelerated and the flowslide initiated,” it explained.
The specialists said a “flowslide” occurred, requiring three conditions to develop: saturation of the sand, loose sand that didn’t compact, and a trigger mechanism.
“Depositing sand tailings by hydraulic means resulted in loose conditions,” the report said.
“The initiation of a flowslide requires not only the presence of saturated tailings but also a trigger mechanism to initiate the process that mobilizes undrained shearing and hence flowsliding. Following an evaluation of potential trigger mechanisms, the panel concluded that lateral extrusion initiated the failure.”
Commenting the “earthquakes” that occurred prior to the collapse, the report said that given the proximity of the dam to collapse due to prior construction loading, this likely accelerated the failure process that was already well-advanced.
BHP Billiton has decided to create a centralized dam management function that will bring additional specialist expertise in-house at BHP Billiton. In addition, the company will assess technology options to further enhance dam management across the portfolio.