22 May 2012

The remarkable Preonzo landslide in Switzerland last week

Posted by Dave Petley

Whilst the main action in the Preonzo landslide in Switzerland occurred when I was out of the office last week, this event is too impressive to ignore. And I must acknowledge here the help from a wide variety of people in putting this post together, including Colin Stark, Jeff Moore, Guy Bullot, Stefan Tobler, Jens and Patrick Ebeling.  Your help is really appreciated.

To recap, the slope at Preonzo was identified as being unstable some time ago, and was monitored over many years.  The final failure event was anticipated as a result of an accelerating trend in movement rate observed in early May, and monitored with a radar system.  The area below the slope was thus cordoned off before the start of the major failure events.

The slope that collapsed is this shown below in a Google Earth perspective view:

The reasons that the slope had been identified as being potentially hazardous are clear from the high resolution imagery of the crown area, which appears to show a tension crack across the terrace:

One of the main failure events, on (I think) 15th May, was captured in this remarkable video:

This appears to be a major collapse event from the middle part of the slope (i.e. not from the crown), possibly induced by rockfalls from the upper reaches of the slope shown in the image above.  The video demonstrates that there is more material that is yet to collapse.

The video below shows the police closing off the road and some precursory rockfall activity prior to the main failures.

The various newspaper articles suggest that to date 300,000 cubic metres of material have collapsed, with about another 500,000 cubic metres remaining in the unstable area.  There are two wonderful sets of photographs of this event online.  Check out this set, from which the before and after images below are taken:


Whilst this set of images has some great shots of the crown of the landslide: