20 July 2009
This is an update with the latest news about the Nachterstedt landslide. The original post is here.
Further information has now emerged about the slide. This is summarised below.
1. The slide occurred in spoil from the mining operations.
In a comment in the original post, Florian Jenn (see his blog here) noted that the slide occurred in spoil dumped from earlier mining operations. This is consistent with the pictures, which seems to show a fine grained, reasonably homogeneous material. This type of material is also prone to gully erosion, which has clearly occurred. Thus, a liquefaction failure is quite possible.
2. This is the second failure at this site
A larger slide (6 million cubic metre) occurred at this site in 1959, resulting in a fatality. This will raise questions about the wisdom of the phased increase in lake level.
3. The lake was being filled through natural processes
If essentially the lake level was being increased essentially through rainwater (much of it flowing into the lake from the surrounding ground), then a possible explanation is that the heavy rainfall of late June has caused an increase in lake level over the last fortnight. Groundwater would have risen in response, perhaps triggering the failure.
4. The authorities have concluded that there is no possibility of survival for the three missing people
This is quite correct, assuming that they were in the building. Finding their remains will be a both challenging and dangerous.
Clearly this slide was both rapid and sudden. I guess it is hard to imagine such a slide. The first failure in this video, of the famous Pantai Remis slide in Malaysia, will give an indication of the speed and violence of a liquefaction-induced failure in the walls of an old mine:
There are more details of the Pantai Remis landslide here. Note that the latter part of the video is rather different from the Nachterstedt landslide as the sea was breaking through the wall of the mine.