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Caractérisation des phases pré-et post-rupture d'éboulements rocheux de taille intermédiaire : Apport des enregistrements sismiques.

Abstract : Mid-size rockfalls (10^3-10^5 m^3) represent a substantial hazard in mountainous areas, because of relative high rate of occurrence and destructive power. Consequently, few protection means can be applied, emphasizing the need for monitoring techniques and early warning prior to the collapse. After the rupture, quantitative information on the rockfall propagation phase is scarce, owing to their suddenness and location in steep and rugged slopes. In this thesis work, an experimental approach is proposed to derive valuable information from seismic records during rockfall pre-rupture and post-rupture phases. The first part of this work aims at testing the applicability of the ambient vibration technique to monitor unstable rock compartments dynamic response during the pre-rupture phase. This technique {commonly employed in civil engineering for structural health monitoring{ reveals the resonant frequencies of a structure, a decrease in frequency revealing potential damage. A previous case study of an unstable limestone compartment brought to light a ~30% decrease in fundamental resonant frequency (f1) two weeks before the collapse, interpreted as a loss in contact stiffness with the adjacent rock mass. Following this innovative work, we selected and instrumented five prone-to-fall medium size rock compartments located in the Occidental Alps, showing various geological contexts (limestones, argillite and shale-sandstone series), deformation patterns and failure mechanisms. Ambient vibrations recorded on-site revealed characteristic seismic noise features. Spectral peaks were observed and attributed to resonant frequencies of the rock compartments, the fundamental resonant frequency (f1) showing clear polarization parallel to the line of maximum slope gradient and perpendicular to the main bounding fracture observed at most of the sites. Similar findings were made for an unstable rock compartment located in a volcanic caldera characterized by rapid morphological changes and intense rockfall activity. The dynamic response of the rear fracture network was explored, showing that spectral content of seismic noise is controlled by the caldera structure in the 0.5-5 Hz range. The direction of vibration is polarized perpendicularly to the fractures, while vibration amplitudes appear linked to compartment uncoupling from the rock massif. In this case, the physical origin of seismic noise amplification may be due to complex 2D or 3D resonance effects. For four alpine sites, the fundamental frequency f1 was monitored over more than one year and showed fluctuations clearly correlated with temperature oscillations. The thermal control over f1 is highly complex, showing both positive and negative correlations depending on site morphology and destructuration as well as on the studied oscillations periods (daily or seasonal). No change in fundamental frequency resulting from damage was observed over this time span. One site, characterized by intense rock fracturing and a deep-open rear fracture, showed high f1 sensitivity to temperature changes. Thermomechanical numerical simulations revealed that both material contraction-dilation and thermal dependency of the elastic modulus control f1 fluctuations. In addition, high amplitude seasonal f1 oscillations were explained by ice formation in the rear fracture. A criterion was developed to separate thermal-induced f1 fluctuations from damage effects, under the hypothesis that thermal sensitivity of a rock compartment increases towards failure. The second part of this work relates to the post-rupture phase of rockfalls. The seismic records generated by two mid-size rockfalls (one natural, one provoked) that occurred in the same place were analyzed, showing complex envelope and spectrogram features. Both events showed close magnitude, duration and spectral content. The seismic signals of the provoked event were calibrated using video shots, allowing estimation of fallen material velocity during the successive propagation phases. The seismic signal appeared mainly controlled by the propagation phase, two dominant seismic pulses being related to (1) the ground impact of fallen material after free-fall, and (2) the kinematics of one individual _10 m3 rock block. These two pulses are characterized by similar low frequency content but very different particle motions. The discrete element technique was tested to simulate the provoked rockfall propagation. The initial velocity field, the resolution of the topography model and contact laws parameterization were found of critical importance for matching the observed propagation characteristics.
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https://tel.archives-ouvertes.fr/tel-01162605
Contributor : Pierre BOTTELIN Connect in order to contact the contributor
Submitted on : Thursday, June 11, 2015 - 2:01:01 PM
Last modification on : Friday, March 25, 2022 - 9:41:06 AM
Long-term archiving on: : Saturday, September 12, 2015 - 10:21:18 AM

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Pierre Bottelin. Caractérisation des phases pré-et post-rupture d'éboulements rocheux de taille intermédiaire : Apport des enregistrements sismiques.. Géophysique [physics.geo-ph]. Université de Grenoble, 2014. Français. ⟨NNT : ⟩. ⟨tel-01162605⟩

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