20 July 2009
Update: the Nachterstedt landslide
Posted by Dave Petley
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.
Dave Petley is the Vice-Chancellor of the University of Hull in the United Kingdom. His blog provides commentary and analysis of landslide events occurring worldwide, including the landslides themselves, latest research, and conferences and meetings.
I don't know if you've seen this yet, but Der Spiegal has both an article and a set of photos reporting on this disaster. (I think at least a couple of the latter are repeats- i.e. I've seen them here earlier)
Ok – Now I come back to the wave it would have produced. I thought maybe it was a slower moving slide and maybe did not produce a large wave. But if it was fast…then what about the wave?? Is the overall rise in water level covering up the evidence of a wave…maybe it only had runup that was lower than the new high water level. The smaller slide off to the side may have been caused by fluctuating water levels/wave. Just me speculating….any thoughts?
Thanks for the update and the video link!In the meantime I have collected some geological maps to outline the development of the mining site (in German, sorry). Definitely, the houses were built on a rather old mining tip (in more recent times, this wouldn't have been allowed), so liquefaction is a very likely explanation.There's little to be done about the rising lake level. The pumps couldn't be run forever. Still, something must have gone wrong with the slope stability assessment and the reclamation planning.The water level rise results mainly from diverted surface water of the River Selke and from groundwater. Precipitation in this area is very low.Also, to correct my last comment (on the first post): The Aschersleben salt anticline is too far away and too deep here, so I no longer think of subrosion as a potential trigger.It has been suggested that the slide may have been triggered by the collapse of old (forgotten) mine adits.
The DLR (German Aerospace Center) has published aerial photos after the landslide and from May 2006 (i.e. newer than Google's image). The photos and comparisons (shoreline and top of slope) are also available in English.
For the ones who would like to get a more detailed picture of the Nachterstedt landslide, the Center for Satellite Based Crisis Information of the DLR provides some new high resolution aerial pictures http://www.zki.caf.dlr.de/applications/2009/nachterstedt/174_de.html. They show a quite complex slide pattern.Additionally, the blog GeoBerg.de point to a very nice data source, the drilling data base of the Saxony-Anhalt state http://webs.idu.de/lagb/lagb-default.asp. The SDI visualises reports from various exploration drillings in the area.And a last update: News are reporting that the company responsible for the remediation, plans to use explosives to trigger slides to improve stability, if no further slides occure at the very steep main scarp.