Études expérimentales de la dynamique et de l’émission sismique des instabilités gravitaires

Rockfalls are a major natural hazard in steep areas. However, the dynamics of gravitational instabilities is still not well understood and observations of these events are rare. Laboratory experiments of granular flows are then a unique way to get insight into the key parameters that control their dynamics.I therefore chose to adopt an experimental approach during my PhD, focusing first on the dynamics and erosion processes of granular flows. I identified the initial and boundary conditions for which a slow propagation phase appears at the end of the deceleration of granular flows. When the slow propagation phase develops, the dynamics and deposition of granular flows change fundamentally.Secondly, I focused on the seismic signal generated by landslides. I first established analytical scaling laws that allow to retrieve the mass and the speed of an object impacting a surface from the spectral content of the emitted seismic signal. I applied these approach for impacts of rock boulders in a natural site but the associated seismic signal had been recorded with an insufficient sampling frequency. Still, I observed an empirical scaling law for the elastic energy radiated during these impacts.Finally, I showed experimentally that the seismic signal is a consistent tool to get insights into granular flows dynamics. I proposed empirical scaling laws to describe the variation of the elastic energy radiated by a granular flow as a function of the flow physical characteristics.Les éboulements sont un aléa naturel majeur dans les régions à fort relief. Cependant, la dynamique des instabilités gravitaires est encore mal comprise et les observations de ces événements sont rares. Les expériences de laboratoire d'écoulements granulaires sont alors un moyen unique pour contraindre les paramètres clés qui contrôlent leur dynamique.J'ai donc choisi d'adopter une approche expérimentale au cours de ma thèse, en me focalisant dans un premier temps sur la dynamique et les processus d'érosion des écoulements granulaires.J'ai identifié les conditions initiales et aux limites pour lesquelles une phase de propagation lente apparaît à la fin de la décélération des écoulements granulaires. Lorsque cette phase se développe, la dynamique et le dépôt des écoulements changent fondamentalement. Dans un second temps, je me suis intéressé au signal sismique généré par les éboulements.J'ai établi des lois d'échelle analytiques pour retrouver la masse et la vitesse d'un objet entrant en collision avec une surface à partir du contenu spectral du signal sismique émis. J'ai appliqué cette approche pour des impacts de blocs rocheux en site naturel mais le signal sismique associé avait été enregistré avec une fréquence d'échantillonnage trop faible. J'ai malgré cela pu observer une loi d’échelle empirique pour l'énergie élastique rayonnée lors de ces impacts.Enfin, j’ai montré expérimentalement que le signal sismique est un outil pertinent pour nous renseigner sur la dynamique des écoulements granulaires. J'ai proposé des lois d'échelle empiriques pour décrire la variation de l'énergie élastique rayonnée en fonction des caractéristiques physiques de l’écoulement

A physical model of the high-frequency seismic signal generated by debris flows

We propose a physical model for the high‐frequency (>1 Hz) spectral distribution of seismic power generated by debris flows. The modeled debris flow is assumed to have four regions where the impact rate and impulses are controlled by different mechanisms: the flow body, a coarser‐grained snout, a snout lip where particles fall from the snout on the bed, and a dilute front composed of saltating particles. We calculate the seismic power produced by this impact model in two end‐member scenarios, a thin‐flow and thick‐flow limit, which assume that the ratio of grain sizes to flow thicknesses are either near unity or much less than unity. The thin‐flow limit is more appropriate for boulder‐rich flows that are most likely to generate large seismic signals. As a flow passes a seismic station, the rise phase of the seismic amplitude is generated primarily by the snout while the decay phase is generated first by the snout and then the main flow body. The lip and saltating front generate a negligible seismic signal. When ground properties are known, seismic power depends most strongly on both particle diameter and average flow speed cubed, and also depends on length and width of the flow. The effective particle diameter for producing seismic power is substantially higher than the median grain size and close to the 73rd percentile for a realistic grain size distribution. We discuss how the model can be used to estimate effective particle diameter and average flow speed from an integrated measure of seismic power

A physical model of the high-frequency seismic signal generated by debris flows

We propose a physical model for the high‐frequency (>1 Hz) spectral distribution of seismic power generated by debris flows. The modeled debris flow is assumed to have four regions where the impact rate and impulses are controlled by different mechanisms: the flow body, a coarser‐grained snout, a snout lip where particles fall from the snout on the bed, and a dilute front composed of saltating particles. We calculate the seismic power produced by this impact model in two end‐member scenarios, a thin‐flow and thick‐flow limit, which assume that the ratio of grain sizes to flow thicknesses are either near unity or much less than unity. The thin‐flow limit is more appropriate for boulder‐rich flows that are most likely to generate large seismic signals. As a flow passes a seismic station, the rise phase of the seismic amplitude is generated primarily by the snout while the decay phase is generated first by the snout and then the main flow body. The lip and saltating front generate a negligible seismic signal. When ground properties are known, seismic power depends most strongly on both particle diameter and average flow speed cubed, and also depends on length and width of the flow. The effective particle diameter for producing seismic power is substantially higher than the median grain size and close to the 73rd percentile for a realistic grain size distribution. We discuss how the model can be used to estimate effective particle diameter and average flow speed from an integrated measure of seismic power

Measuring Basal Force Fluctuations of Debris Flows Using Seismic Recordings and Empirical Green's Functions

We present a novel method for measuring the fluctuating basal normal and shear stresses of debris flows by using along‐channel seismic recordings. Our method couples a simple parameterization of a debris flow as a seismic source with direct measurements of seismic path effects using empirical Green's functions generated with a force hammer. We test this method using two large‐scale (8 and 10 m³) experimental flows at the U.S. Geological Survey debris‐flow flume that were recorded by dozens of three‐component seismic sensors. The seismically derived basal stress fluctuations compare well in amplitude and timing to independent force plate measurements within the valid frequency range (15–50 Hz). We show that although the high‐frequency seismic signals provide band‐limited forcing information, there are systematic relations between the fluctuating stresses and independently measured flow properties, especially mean basal shear stress and flow thickness. However, none of the relationships are simple, and since the flow properties also correlate with one another, we cannot isolate a single factor that relates in a simple way to the fluctuating forces. Nevertheless, our observations, most notably the gradually declining ratio of fluctuating to mean basal stresses during flow passage and the distinctive behavior of the coarse, unsaturated flow front, imply that flow style may be a primary control on the conversion of translational to vibrational kinetic energy. This conversion ultimately controls the radiation of high‐frequency seismic waves. Thus, flow style may provide the key to revealing the nature of the relationship between fluctuating forces and other flow properties

Measuring Basal Force Fluctuations of Debris Flows Using Seismic Recordings and Empirical Green's Functions

We present a novel method for measuring the fluctuating basal normal and shear stresses of debris flows by using along‐channel seismic recordings. Our method couples a simple parameterization of a debris flow as a seismic source with direct measurements of seismic path effects using empirical Green's functions generated with a force hammer. We test this method using two large‐scale (8 and 10 m³) experimental flows at the U.S. Geological Survey debris‐flow flume that were recorded by dozens of three‐component seismic sensors. The seismically derived basal stress fluctuations compare well in amplitude and timing to independent force plate measurements within the valid frequency range (15–50 Hz). We show that although the high‐frequency seismic signals provide band‐limited forcing information, there are systematic relations between the fluctuating stresses and independently measured flow properties, especially mean basal shear stress and flow thickness. However, none of the relationships are simple, and since the flow properties also correlate with one another, we cannot isolate a single factor that relates in a simple way to the fluctuating forces. Nevertheless, our observations, most notably the gradually declining ratio of fluctuating to mean basal stresses during flow passage and the distinctive behavior of the coarse, unsaturated flow front, imply that flow style may be a primary control on the conversion of translational to vibrational kinetic energy. This conversion ultimately controls the radiation of high‐frequency seismic waves. Thus, flow style may provide the key to revealing the nature of the relationship between fluctuating forces and other flow properties

Fundamental changes of granular flow dynamics, deposition, and erosion processes at high slope angles: Insights from laboratory experiments

International audienceEntrainment of underlying debris by geophysical flows can significantly increase the flow deposit extent. To study this phenomenon, analog laboratory experiments have been conducted on granular column collapse over an inclined channel with and without an erodible bed made of similar granular material. Results show that for slope angles below a critical value θc, between 10° and 16°, the run out distance rf depends only on the initial column height h0 and is unaffected by the presence of an erodible bed. On steeper slopes, the flow dynamics change fundamentally, with a slow propagation phase developing after flow front deceleration, significantly extending the flow duration. This phase has characteristics similar to those of steady uniform flows. Its duration increases with increasing slope angle, column volume, column inclination with respect to the slope and channel width, decreasing column aspect ratio (height over length), and in the presence of an erodible bed. It is independent, however, of the maximum front velocity. The increase in the duration of the slow propagation phase has a crucial effect on flow dynamics and deposition. Over a rigid bed, the development of this phase leads to run out distances rf that depend on both the initial column height h0 and length r0. Over an erodible bed, as the duration of the slow propagation phase increases, the duration of bed excavation increases, leading to a greater increase in the run out distance compared with that over a rigid bed (up to 50%). This effect is even more pronounced as bed compaction decreases

Remote focusing of elastic waves in complex structures using time reversal of acoustic waves

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Experimental validation of theoretical methods to estimate the energy radiated by elastic waves during an impact

To be published in Journal of Sound and VibrationEstimating the energy lost in elastic waves during an impact is an important problem in seismology and in industry. We propose three complementary methods to estimate the elastic energy radiated by bead impacts on thin plates and thick blocks from the generated vibration. The first two methods are based on the direct wave front and are shown to be equivalent. The third method makes use of the diffuse regime. These methods are tested for laboratory experiments of impacts and are shown to give the same results, with error bars from 40% to 300% for impacts on a smooth plate and on a rough block, respectively. We show that these methods are relevant to establish the energy budget of an impact. On plates of glass and PMMA, the radiated elastic energy increases from 2% to almost 100% of the total energy lost as the bead diameter approaches the plate thickness. The rest of the lost energy is dissipated by viscoelasticity. For beads larger than the plate thickness, plastic deformation occurs and reduces the amount of energy radiated in the form of elastic waves. On a concrete block, the energy dissipation during the impact is principally inelastic because only 0.2% to 2% of the energy lost by the bead is transported by elastic waves. The radiated elastic energy estimated with the presented methods is quantitatively validated by Hertz's model of elastic impact

Ambient noise monitoring of nonlinear defects on elastic plates

International audienceSolid structures used in industry damage with time due to their usage in sometimes extreme stressing conditions. Rapidly detecting the apparition of defects such as cracks, delamination or debonding and localizing them is crucial for user safety and machine performance. In this study, we investigate contact acoustic nonlinearities, or non-linear defects. When excited by low-frequency (LF) vibrations, non-linear defects generate harmonics or, more generally, signals of higher frequencies than the excitation, due for example to clapping or friction between two surfaces in contact. A periodic measurement of these high- frequency signals on structures could allow an efficient monitoring of a nonlinear damage apparition and evolution. Recent work has shown that passive methods (e.g. noise correlation) taking advantage of the vibration of the structures induced by the ambient noise can be used to localize linear defects (Chehami et al., 2015). The main advantage of these methods is that they are less power consuming than active methods. We present a signal processing technique applied on the high-frequency noise generated by nonlinear defects excited by ambient noise on plate-like structures. To test the method, we develop an experimental setup composed of a thin aluminium plate on which different types of non-linear defects are created (e.g., unbolted screw, Hertzian or frictional contact). The thin plate is excited by a shaker vibrating at low frequencies (< 500 Hz). The high-frequency acoustic signals generated by the defects are recorded by a network of piezoelectric sensors glued on the plate. The influence of the sensor network geometry, of LF excitation amplitude and of the type of nonlinear defect on the detection are discussed. It is shown that defect detection is possible even when the LF excitation is a realistic jet engine noise