![\"Progressive](\"https:\/\/blogs.agu.org\/landslideblog\/files\/2015\/04\/15_04-Progressive-1.jpg\")
<\/a>Before and after shots of the Puigcercos scarp, from Royan et al. (2015)<\/p><\/div>\n
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The resolution of the terrastrial LiDAR is such that the movement of the rockfall prior to collapse could be calculated from successive scans, and from this the velocity could be determined, which in turn permits investigation of the nature of progressive failure at the site.\u00a0 This is in reality far from trivial, and required the developed development of new approaches by the project team.\u00a0 They divided the rock face into a series of zones and then looked at the velocity in each of them.\u00a0 In three of the zones, the Saito effect was clearly observed:-<\/p>\n
![\"Progressive](\"https:\/\/blogs.agu.org\/landslideblog\/files\/2015\/04\/15_04-Progressive-2-e1428908019153.jpg\")
<\/a>The evolution of progressive failure, from Royan et al (2015)<\/p><\/div>\n
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The arrow in figure (b) indicates the time of the large failure event.\u00a0 The best fit line for areas 6 and 9 correctly predicts this time of failure remarkably well, with the linear trend developing more than a year before the collapse.\u00a0 The linear trend for area 7 predicts a later failure event – the authors hypothesize that this was developing into a separate failure, but that the major collapse precipitated the collapse of this section as well. \u00a0\u00a0 Interestingly, the project team also saw an increase in the number and rate of rockfall events in these areas prior to the major rockfall.\u00a0 I would think that this is a sign of an increasing rate of deformation of the rock mass as failure developed, which is also an indication of progressive failure.<\/p>\n
Finally, the project team looked at the major rockfall event in relation to potential trigger events, such as heavy rainfall.\u00a0 In their words:<\/p>\n
“No clear external triggering factor was detected for this event. Therefore, the evolution over time of the precursory indicators suggests that just stability reasons seem to be the most probable cause of the final failure. The continuous increase in deformation of areas 6 and 9 created a critical situation in terms of stability, and these areas eventually fell without any external trigger. This approach is supported by the hypothesis exposed in Rosser et al. (2007)<\/a><\/cite> in which when the instability is in the tertiary creep phase, the environmental forces have little or no discernible influence in the evolution of the instability and therefore in the final failure.”<\/p><\/blockquote>\n
I think that this is an extremely interesting and important paper that makes a substantial contribution to our understanding of the development of progressive failure.\u00a0 That once again failure is seen to be a long term process rather than a one-off response to a single trigger is fascinating, and paves the way for improved early warning systems.<\/p>\n
References<\/h5>\n
Roy\u00e1n, M.J., Abell\u00e1n, A. and Vilaplana, J.M. 2015. Progressive failure leading to the 3 December 2013 rockfall at Puigcerc\u00f3s scarp (Catalonia, Spain). Landslides<\/em>. http:\/\/dx.doi.org\/10.1007\/s10346-015-0573-6<\/a><\/p>\n