9 October 2011
Summary: More information has emerged about the cliff collapse video from Cornwall, SW England
The amazing cliff collapse video that I featured on Friday has generated considerable interest. The video was shot by Brian O’Connell, a Senior Geoenvironmental Engineer at Cornwall Council. He posted the following on the Engineering Geotech list:
My thoughts are is that it’s a progressive failure, but would welcome some comments. I appreciate the footage is a little grainy, so I’ll try to through a little more light… As I note in the description, there were a number of failures which increased in size and volume until the cliff collapsed. The cliff is formed from Devonian metasediments – turbidite sequences of mudstone, siltstone and fine grained sandstone. There are normally 5 discontinuity sets of varying persistence (I wasn’t prepared to go and measure them!); observations of a nearby cove indicates unfavourable joint failure which could lead to planar failure. Prior to the failure there were a number of tension cracks approximately 30 m in-land which had been steadily growing in size over the previous 4 days. However the way the failure presented itself, with material spalling-off mostly in the lower third of the slope, opening of large tension cracks within mid to upper section of the cliff face and the ‘sagging’ off the slope geometry as it failed, does seem to indicate a progressive failure?