The clean up at the Ajkai Timfoldgyar tailings debris flow accident site continues today, with continued concern about the potential for pollution of the downstream rivers from the environmentally-damaging debris.  I do not wish to speculate on the cause of this accident, but will note that foundation failure is a well-established failure mode for tailings dams – see for example the excellent wise-uranium.org site, which has a section on typical tailings dam failure modes here.  Top of the list is “hazard from weak foundation”, which describes a situation when “the soil or rock at shallow depth below the dam is too weak to support the dam, movement along a failure plane will occur. This may result in partial or complete failure of the dam.”

The classic example of this occurred on 25th April 1998 at the Los Frailes tailings dam in Aznalcollar in Spain.  Good images of this event are hard to source, but this one is helpful (source):

 
The sequence of the failure at this site was quite complex (see description here), with the initiation occurring as a result foundation failure in a separation dam between two parts of the tailings pond.  This occurred as a result of a failure on a weak plane in the foundation of the dam at a depth of about 14 metres.  The dam slid forwards and cracked, allowing the release of the tailings, as this image (from here) shows:

As can be seen above, the failure of the foundation of the dam caused it to be pushed forward by up to 60 metres. This accident was essentially a failure in design – i.e. the designers did not adequately consider the strength of the marl layers below the dam foundation and the likely build up of pore pressures in this area.

It is interesting to note the similarity between the images of the Ajkai Timfoldgyar and the Los Frailes events (compare with the image here), and also that the event in Hungary on Monday occurred after a prolonged period of heavy rainfall.

There is an excellent gallery of the downstream impact of this failure here.

Whatever the cause of this accident, far too many tailings accidents are happening around the world.  As an illustration see the following previous posts on this topic:
Tailings dam failures and the price of commodities
Another Chinese tailings dam failure
A useful tailings dam failure resource
Another Chinese flowslide?
China tailings dam disaster death toll

The authorities really need to start to regulate this bette.