Samarco failed after some small earthquakes caused an already stressed tailings dam to liquify, according to the that released its findings today.
The November 2015 dam failure resulted in 19 deaths. The owners of Samarco, BHP Billition and Vale, have earmarked US$1.3 billion to pay for damages.
The panel review was formed by the law firm Cleary Gottlieb Steen & Hamilton LLP, which was retained by Vale and BHP Billiton. Chair of the panel is , who also conducted a review into the cause of the failure of the Mount Polley tailings dam in 2014. The panel did not assign liability or fault for the Samarco failure.
The panel found that three small seismic shocks triggered the collapse, but the dam鈥檚 failure was 鈥渁lready well advanced鈥� caused by聽a string聽of design and maintenance failures.
The tailings dam became stressed through a series of 鈥渃onstruction defects鈥� stretching back to 2009 that 鈥渋ntroduced the potential for sand liquefaction.鈥�
鈥淭he first incident occurred in 2009 shortly after the Starter Dam was completed,鈥� writes the panel in its report.
The starter dam was eventually subsumed by the larger dam as more capacity was added.
鈥淒ue to construction defects in the base drain, the dam was so badly damaged that the original concept could no longer be implemented. Instead, a revised design substituted a new drainage blanket at a higher elevation.
鈥淭ogether with the revised design there was a fundamental change in the design concept whereby more widespread saturation was allowed and accepted. This increase in the extent of saturation introduced the potential for sand liquefaction.鈥�
The Fund茫o tailings dam in 2015 before its failure
The report finds later聽mishaps: deposition of slimes in areas that were not not intended and structural problems with the concrete conduit that caused the dam to be raised over the slimes.
By 2015 parts of the dam were already in a precarious state and eventual failure was already 鈥渨ell advanced鈥�. Three small seismic shocks caused the dam to fail:
Computer modeling showed that the earthquake forces produced an additional increment of horizontal movement in the slimes that correspondingly affected the overlying sands. Although the movements are quite small and the associated uncertainties large, this additional movement is likely to have accelerated the failure process that was already well advanced.
Hence the failure of the Fund茫o 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.
There is an showing the series of mishaps leading to the failure.
The panel chairman sums up the findings of the panel in the video below.
Image of .