Fatal rockfall near to Munich in Germany

Last night a fatal rockfall occurred at Stein an der Traun in Southern Bavaria. According to this report, which is also the source of the remarkable image above, the block was the "size of a house" (probably something of an over-estimate), falling from the 15 m slope behind the building. There were four people in the house at the time, of which two were killed and two were injured. There is no obvious trigger for the failure.

It is of course too early to speculate about the causes of this sad accident, but in my experience the combination of an unfortunately-orientated discontinuity in the rock (often a joint) and cutting of the toe of the slope to create space for the building are often to blame. It will be interesting to find out what caused this accident.

A strange update on the Nachterstedt landslide

A strange update has appeared in the "The Local" regarding the Nachterstedt landslide. This reports that "Scientists at the Collm Observatory at Leipzig University registered an underground movement in the area measuring 1.0 on the Richter Scale, just six minutes before the first call to the emergency services on July 18. An earthquake can be ruled out, the scientists told Der Spiegel magazine, but they suggested it would be feasible that an old mining tunnel collapsed, causing the landslide. "

For info a magnitude 1.0 earthquake is the equivalent of about 30 kg of TNT. I cannot understand why the scientists believe that this was a tunnel collapsing. To me it is far more likely that the signal was the landslide itself occurring. I must admit that I cannot understand why the collapse of an underground mine working would cause this catastrophic slide anyway.

On another matter, the paper also reports that a further collapse is considered likely:
"Surveyors have been pulled back out of the Auf der Halde community, where the ground has sunk by a further 0.4 millimetres and is expected to also collapse. Preparations are now being made for emergency measures to be taken should the remaining houses also fall into the Concordia Lake, which was created by flooding an old open-pit mine."

Update: the Nachterstedt landslide

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.

Intriguing landslide at Nachterstedt in Germany

NB: I have posted an update on this landslide here.

A somewhat intriguing landslide occurred on Saturday at Nachterstedt in Germany (the location is 51.808 N, 11.343 E for those who are interested). The site is the edge of Lake Concordia, a flooded open cast coal mine:

The mine apparently closed in 1991 whereupon it was converted into a recreation area. The lake was apparently created in 1994. Reports suggest that on Saturday the area received some rain, but that this was not exceptional in any way. The slide appears to have been rapid, with a volume of about 1 million cubic metres. Reports suggest that it created a mini-tsunami on the lake (not surprising given the images below), and that two houses were carried with the slide into the lake. It is thought that three adults were killed in one of the houses. The images of the site are quite remarkable:

Image published in the LA Times providing an over-view of the site

Image published in Deutsche Welle providing a more detailed of the landslide

Close up published in the LA Times of one of the houses affected by the landslide

This is quite an intriguing slide, both in terms of the mechanisms of the slide and of the trigger. In particular it is not at all clear to me why this slope has failed when the weather was not wet. Central Europe had heavy rain a fortnight ago, so this might be an interesting starting point to ponder. Earthtimes suggests that the area might have been underlain by old mine adits, though this is not obvious from the images above. The middle image does show what appears to be shallow coal seams on the right side, but I can't see any evidence of coal seams or abandoned workings elsewhere.

The Google Earth image of the site is quite high resolution. I have zoomed in below to show the area of the slide:


A few of things to note. First, the area that has slid appears to have been suffering some erosion of its toe. Second, this area also has extensive herringbone drains around the large gullies that cross it. I wonder why these gullies are there (they seem to be better developed than those elsewhere on the walls of the lake), and why these herringbone drains have been built. Is this an indication of a drainage problem. Third, there is another small slide on one of the images - see the right side of the Reuters image below:


This smaller slide also looks quite fresh (i.e. recent). It could of course be that this was triggered by the displacement wave, but it could also be an indication that there is a general stability problem. Finally, the presence of the ponds on the slide behind obviously displaced blocks suggests that there was some water in the slope.

The Earthtimes article suggests that the mine was still being progressively flooded, so it could be that the key issue here is instability under raised groundwater conditions.

NB: I have posted an update on this landslide here.