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Méthodes et outils pour l'auscultation et la surveillance des instabilités gravitaires

Abstract : Rock instabilities are one of the most common natural risks encountered in high alpine relief and, despite this fact, they still remain largely uncharted in Earth Sciences. Given their characteristics, these instabilities need to be investigated and closely monitored in order to improve our knowledge of the stability of rock masses. The main purpose of this thesis was therefore to develop a multi-sensor instrumental device to form a sound basis for the continuous acquisition of the relevant parameters for monitoring the stability of rock masses. This self-contained device is capable of continuously measuring and storing high frequency (up to one kHz) and low frequency data generated by different types of sensor over long periods (several months). Moreover, it can be linked up to a remote-monitoring system for supervising the site and checking the operation of the instrumentation. The measurement station developed can also carry out pre-processing of the data collected and includes monitoring algorithms combining different criteria for an initial warning management procedure. A meshed-network transmission system and client-server oriented remote supervision system have also been developed within the context of this project. In March 2008, this surveillance system was used to monitor fifteen instability sites in France. Within the context of this thesis, particular attention was focused on the variation in deformations affecting an unstable vertical rock slice on the Chamousset site (south-east Vercors, France), up to about one week prior to its collapse and fall. Observations of various parameters (deformation, weather conditions, monitoring of seismic activity) taken over a period of about three months, revealed the following : a clear but moderate acceleration in horizontal displacement of the top of the rock slice, closely related to alternating freezing-thawing episodes; numerous seismic events, due to the progressive failure of the rock slice, some of which had a very low prevailing frequency (5 to 10Hz), probably dominated by slice resonance; individualised spectral modes at frequencies greater than 10-15 Hz on the rock slice and which, as a general rule, tended to increase with time; occurrence of seismic crises which did not degenerate towards the instability. In view of its dynamics, the experimental work carried out on site has enhanced our knowledge of the processes involved and, in particular, revealed new parameters that would be worthwhile including in the study of this type of instability. Based on these findings, it was possible to specify the instrumental characteristics to be achieved.
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Submitted on : Tuesday, January 8, 2013 - 2:38:03 PM
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Philippe Mourot. Méthodes et outils pour l'auscultation et la surveillance des instabilités gravitaires. Géologie appliquée. Université de Savoie, 2008. Français. ⟨NNT : ⟩. ⟨tel-00771338⟩

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