The Fundão Dam failure at the Samarco Mine in Brazil and the Nerlerk berm slope failure in the Beaufort Sea were the result of liquefaction flow events brought on by the extrusion of materials within the foundation of the structure. This process is referred to as liquefaction by extrusion failure. 

In many cases, liquefaction by extrusion failure is difficult to predict because it can develop with no apparent warning (e.g. excess pore pressures will not develop prior to failure). 

While unpredictable, certain conditions make extrusion failure more likely. These include: 

• Soft material exists which can deform laterally with minimal confinement as vertical or shear load increases.
• Overlying material lacks ductility (e.g. clean sand) and cannot strain to match the soft material extrusion.
• Ductility contrast between the soft extruding material and the sand can result in reduction in lateral confinement of the sand.
• Saturated sand can collapse, and dry sand can crack.

Evaluating Extrusion Failure in Tailings Dams

In a tailings dam, the potential for extrusion failure can increase with rising water levels due to post-construction drain failure or flood storage or from foundation deformations in a soft or weak unit beneath the dam.

The following steps can be used when evaluating the potential for liquefaction by extrusion failure in a tailings dam:

Step 1:

Characterize the state parameter of the tailings using cone penetration tests (CPTs). CPTs are in situ tests where a cone on the end of a series of rods is pushed into the ground at a constant rate, and nearly continuous measurements are made of resistance to penetration of the cone and of a surface sleeve.

Step 2:

Identify the material properties of the tailings through laboratory testing, including triaxial tests on reconstituted samples.

Step 3:

Calibrate a critical state soil model to the laboratory and field behaviour, including the stress path of interest. A critical state soil model is a theoretical framework for understanding the behaviour of soil in response to changes in density and stress.

Step 4:

Prepare a 2D deformation model of the tailings dam to assess the potential response of the tailings to increasing pore pressures or foundation deformations. The model can show which conditions are more likely to result in liquefaction by extrusion failure.

Safe and Sustainable Dam Design

Because of the danger associated with these stress paths, it is common practice for upstream-constructed facilities to be designed using a precautionary approach in which the dam is designed to remain stable in the event of liquefaction. In some cases, involving pre-existing or older facilities, it is not possible to take such pre-emptive measures and an understanding of the potential for triggering liquefaction is required. 

In such situations, critical state soil models can be used to analyze the potential development of these stress paths and the potential response of the tailings.