Vale, BHP Billiton Brasil and Samarco jointly commissioned Cleary Gottlieb Steen & Hamilton to investigate the immediate cause of the failure. To assist with the investigation, Cleary Gottlieb retained the Panel to advise on the technical aspects of the failure and to conduct testing and analysis.
The panel’s findings are the outcome of a 10-month investigation and scientific analysis involving experienced geotechnical engineers and seismologists, led by Dr Norbert Morgenstern, a university professor of civil engineering and an authority in the field of geotechnical engineering.
In structuring its investigation process, the panel systematically identified and evaluated multiple causation hypotheses. It further imposed hypothesis testing by identifying key questions that the candidate failure mechanism should be able to explain: why did a flowslide occur; why did the flowslide occur where it did; and why did the flowslide occur when it did.
The forensic methods adopted by the panel integrated multiple lines of evidence, which included: observations from eyewitness accounts; data and imagery in geographic information system (GIS) format; field evidence from subsurface exploration by the panel and others; advanced laboratory testing; and sophisticated computer modelling. The executive summary of the report said that responding to the above three questions for hypothesis testing demanded a high level of quantification and exhaustive detail in each of these aspects of the investigation’s evidence-based approach.
The executive summary concluded: “The failure of the Fundão tailings dam by liquefaction flowsliding was the consequence of a chain of events and conditions. 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. As a result of this setback, 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.”
A full overview of the panel’s findings, explanatory materials, detailed modelling, input data, a video and technical diagrams can be found on a dedicated website.
Dean Dalla Valle, chief commercial officer at BHP Billiton, said that the dam failure had a devastating impact. He added: “In November 2015, BHP Billiton committed to making the findings of this investigation public, and we are determined to learn from this tragedy. This important technical study will improve our understanding of what happened at Samarco. We have shared these findings so that the sector can learn from the dam failure and develop and implement further standards that can help prevent a similar event like this happening again.”
Dalla Valle noted that, separate to the external investigation, BHP Billiton had also conducted its own reviews of other significant dams at its operations and of its non-operated minerals joint venture arrangements.
He said: “In the wake of this event, we have separately undertaken a comprehensive review of our significant dams, which has confirmed that those dams are stable. We will take a number of actions to further enhance risk management at these facilities. We have looked comprehensively at tailings dam management and benchmarked to global leading practice. We have assessed our portfolio of dams against these global standards and are implementing actions to enhance the management of our dams.
“We also conducted a review into our non-operated minerals joint venture arrangements and have identified a number of opportunities for improvement. This is consistent with our commitment to continuously improve risk management and our determination to learn from, and share the lessons from, this tragedy.”
BHP Billiton has also led an in-depth review of significant dams in its portfolio, drawing on both internal and external expertise to confirm the structural integrity of these facilities. This included a detailed review of dam design, construction and operations.
The company has decided to create a centralised dam management function that will bring additional specialist expertise in-house at BHP Billiton. In addition, it will assess technology options to further enhance dam management across the portfolio.
BHP Billiton has also assessed guidelines across the globe to determine leading practice in dam management. As an immediate step, BHP Billiton will apply the process for dam safety reviews developed by the Canadian Dam Association at all of its minerals assets. The company stated that this process is regarded as the most rigorous in the industry.
Separately, BHP Billiton has contributed to a review initiated by the International Council on Mining and Metals (ICMM) into tailings management that it said will provide the industry with guidance to better manage the risks associated with tailings facilities. BHP Billiton will continue to work with the ICMM as it develops its framework, and stated that it will share the panel’s findings and its lessons learned with the ICMM.