Expert Report for Brumadinho On January 25th, 2019 1

[GeoEnvGuy]

The expert report has been released for the tailings dam disaster.

http://www.b1technicalinvestigation.com/

In summary, the following history created the conditions for instability in Dam I:
• A design that resulted in a steep upstream constructed slope;
• Water management within the tailings impoundment that at times allowed ponded water to get close to the crest of the dam, resulting in the deposition of weak tailings near the crest;
• A setback in the design that pushed the upper portions of the slope over weaker fine tailings;
• A lack of significant internal drainage that resulted in a persistently high water level in the dam, particularly in the toe region;
• High iron content, resulting in heavy tailings with bonding between particles. This bonding created stiff tailings that were potentially very brittle if triggered to become undrained; and
• High and intense regional wet season rainfall that can result in significant loss of suction, producing a small loss of strength in the unsaturated materials above the water level.

The Panel found that the failure and resulting flow slide was the result of flow liquefaction within the tailings in the dam. The history described above created a dam that was composed of mostly loose, saturated, heavy, and brittle tailings that had high shear stresses within the downstream slope, resulting in a marginally stable dam (i.e., close to failure in undrained conditions). Laboratory testing showed that the amount of strain required to trigger strength loss could be very small, especially in the weaker tailings. These were the main components that made flow liquefaction possible.

The Panel concluded that the sudden strength loss and resulting failure of the marginally stable dam were due to a critical combination of ongoing internal strains due to creep, and a strength reduction due to loss of suction in the unsaturated zone caused by the intense rainfall towards the end of 2018. This followed a number of years of increasing rainfall after tailings deposition ceased in July 2016. The calculated pre-failure strains from this combination of triggers match the small deformations of the dam detected in the post-failure analysis of satellite images from the year prior to the failure. The internal strains and strength reduction in the unsaturated zone reached a critical level that resulted in the observed failure on January 25, 2019.

 

[miningman]

• Water management within the tailings impoundment that at times allowed ponded water to get close to the crest of the dam.........well thats got to be the most important thing to avoid during the life of a dam!!!!!!!

• A lack of significant internal drainage that resulted in a persistently high water level in the dam, particularly in the toe region;
Tats got to be another of the three most important things to avoid as well.

I know that this panel of experts were not asked to allocate blame but both of these operational failures would have been blatantly obvious to anyone walking the dam , without the need for piezometers or instrumentation. This failure was not due to excessive rainfall, it was due to managements failure to adequately monitor the dam , and lower the water level as it rose higher

 

[GeoEnvGuy]

Looked through Appendix H and as someone who knows something about liquefaction of sands and tailings the last three pages of the appendix show what happens when you assign liquefied strengths to a 2D Limit Equilibrium Model. I am not surprised to see that the Factor of safety is less than 0.3, so if something were to trigger liquefaction the dam would fail by a lot. I am surprised to see that they used the Su/sigmav value of 0.01 this is typically lower than the Robertson correlation as stated in Appendix E which has a minimum of 0.03, looking through the CPT plots in Appendix E the highlighted zones all plot with a su/sigmav of less than 0.05. The factor of safety in limit equilibrium wouldn't change much if someone would have increased the strength to that, it would still show that if the loose tailings within the dam liquefied it would still fail by a lot. I assume this is the state for a lot of upstream tailings dams worldwide.

Looking through the peak strength limit equilibrium analysis it shows the stability of a failure on the bottom portion of the slope is 1.2, so this represents the long term factor of safety which is totally inadequate. The experts also state there peak undrained Su/sigmav is 0.37 based on lab tests this is much higher than the interpreted values from the Olson and Stark correlations which maxes out at 0.27 as mentioned in Appendix E. I assume if they would have run the analysis with a lower undrained strength the resulting factor of safety would have been near or less than 1, which would indicate the dam is being held together by areas of dilative materials peak undrained strengths and matric suction. I wonder how many tailings dams around the world are in this position.

I also wonder how many of these mining projects would actually go ahead as feasible if they were to know the cost of making a dam structural shell and foundation able to withstand the large scale liquefaction of the loose tailings pumped upstream of the dam.