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Rockfall sources: toward accurate frequency estimation and detection

English title Rockfall sources: toward accurate frequency estimation and detection
Applicant Jaboyedoff Michel
Number 127132
Funding scheme Project funding (Div. I-III)
Research institution Inst. de Géomatique & d'Analyse du Risque Fac. des Géosciences & de l'Environnement Université de Lausanne
Institution of higher education University of Lausanne - LA
Main discipline Geology
Start/End 01.06.2010 - 31.05.2013
Approved amount 183'948.00
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All Disciplines (3)

Discipline
Geology
Geomorphology
Other disciplines of Earth Sciences

Keywords (6)

Rock fall; Rock wall; Engineering geology; LiDAR; Rock fall frequency; DEM Analysis

Lay Summary (English)

Lead
Lay summary
The urgent need of more scientific based analysis of rockfall hazard is demonstrated by recent events along the Gotthard highway (May 2006 and February 2009; Switzerland) and along the "Sea to Sky" highway between Vancouver and Whistler (July 2008; Canada). The most important questions to answer are:o Where are located the most active rockfall sources? o What are their frequencies? This project focuses (1) on the improvement of the knowledge of rock fracturing characteristics in relationship with topography orientation or the rock types; and (2) on the detection of the external factors that are increasing rockfall triggering. Usually rockfalls are linked to rock weaknesses that are mostly represented by discontinuities. As a consequence, the discontinuities or joint sets must be correctly characterized from a geometrical point of view, which means that their persistence and spacing has to be known. The inspection of fallen blocks and rockfall scars will give information about the discontinuity spacing and rock bridges. The link between block size and fracturing will be studied in detail. This will be achieved using terrestrial laser scanning (TLS) and field survey on specific sites. This new technique permits a complete geometrical characterisation of rock faces. A second very important aspect is the relationship between structures and local topography. It is clear that the ability of a rocky outcrop to produce blocks and the instability mechanism depend on the frequency of unfavourable structures relative to the topographic orientation of slope. The geological and structural conditions that are necessary to create rockfall-prone sources will be also inspected, especially when they are linked to faults and thrusts zones. If these are the main factors to create the instability conditions (intrinsic parameters), the rate of blocks production is controlled by the external factors such as precipitations, freeze and thaw cycles, etc. In addition, the relationship of rockfall location with larger rock instabilities such as deep-seated slope gravitational deformation (DSGSD) has not yet been investigated in detail. It appears that in many cases that rockfalls are located within most deformed part of large landslides, i.e. along its limits or where differential movements are most important. Recent works have shown that the rockfall events possess volume-frequency distributions that can provide erosion rates and permit rockfall hazard assessment. In this project, the rockfall source areas and the related rockfall frequency, will be provided in a similar way based on structural and statistical analysis of discontinuities and stability parameters.
Direct link to Lay Summary Last update: 21.02.2013

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Associated projects

Number Title Start Funding scheme
146426 High resolution vertical geology for rockfall hazard assessment and 3D geological mapping 01.06.2013 Project funding (Div. I-III)
144040 Characterizing and analyzing 3D temporal slope evolution 01.12.2012 Project funding (Div. I-III)
121360 Long-range terrestrial laser scanner for characterization and monitoring of natural hazards 01.09.2008 R'EQUIP

Abstract

The urgent need of more scientific based analysis of rockfall hazard is demonstrated by recent events along the Gotthard highway (May 2006 and February 2009; Switzerland) and along the "Sea to Sky" highway between Vancouver and Whistler (July 2008; Canada). The most important questions to answer are:•Where are located the most active rockfall sources? •What are their frequencies? These two parameters are needed to assess rockfall risk. Several attempts to improve rockfall hazard mapping have been made in the recent years at regional and local scales. They are mostly based on rock mechanics that lead to susceptibility indexes, including stability considerations. Only a few are dedicated to structural analysis and spatial discontinuity characterisations. This project focuses (1) on the improvement of the knowledge of rock fracturing characteristics in relationship with topography orientation or the rock types; and (2) on the detection of the external factors that are increasing rockfall triggering. Usually rockfalls are linked to rock weaknesses that are mostly represented by discontinuities. As a consequence, the discontinuities or joint sets must be correctly characterized from a geometrical point of view, which means that their persistence and spacing has to be known. The inspection of fallen blocks and rockfall scars will give information about the discontinuity spacing and rock bridges. The link between block size and fracturing will be studied in detail. This will be achieved using terrestrial laser scanning (TLS) and field survey on specific sites. This new technique permits a complete geometrical characterisation of rock faces. A second very important aspect is the relationship between structures and local topography. It is clear that the ability of a rocky outcrop to produce blocks and the instability mechanism depend on the frequency of unfavourable structures relative to the topographic orientation of slope. The geological and structural conditions that are necessary to create rockfall-prone sources will be also inspected, especially when they are linked to faults and thrusts zones. If these are the main factors to create the instability conditions (intrinsic parameters), the rate of blocks production is controlled by the external factors such as precipitations, freeze and thaw cycles, etc. These external conditions will also be studied. In addition, the relationship of rockfall location with larger rock instabilities such as deep-seated slope gravitational deformation (DSGSD) has not yet been investigated in detail. It appears that in many cases that rockfalls are located within most deformed part of large landslides, i.e. along its limits or where differential movements are most important. Recent works have shown that the rockfall events possess volume-frequency distributions that can provide erosion rates and permit rockfall hazard assessment. In this project, the rockfall source areas and the related rockfall frequency, will be provided in a similar way based on structural and statistical analysis of discontinuities and stability parameters.
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