We performed a passive seismic monitoring of the La Praz ∼14,000 m3 unstable slope (French Alps) spanning over 10 years. During the last 6 months prior to collapse, we detected a clear 24% decrease ...in the slope's fundamental resonance frequency, f0, caused by a reduction in overall rock mass stiffness. The combined study of f0 and slope deformation suggested the alternating importance of sudden brittle failure processes versus more ductile phases with possible sliding. Seismic monitoring revealed slope damage that remained ambiguous or undetected with ground surface deformation monitoring, and highlighted critical periods with intense damage. Only some of these critical damage periods could be related to clear external forcing factors such as intense rainfall episodes. These new insights into rock slope's structural condition at depth represent an asset for future monitoring systems. Surface deformation and passive seismic stiffness tracking combined could reveal active slopes with ongoing damage processes.
Plain Language Summary
Forecasting the time of rockfalls is of critical importance for risk mitigation operators in order to preserve the safety of persons and the integrity of infrastructure. Most monitoring systems are based on ground deformation measurements, which may fail when the surface motion does not accurately reflect changes in slope stiffness with time. In this work, we used a seismic sensor that passively recorded ground vibrations on top of a ∼14,000 m3 unstable slope. We detected a significant decrease in the slope's first resonance frequency (−24%) during the 6 months preceding complete collapse. We also revealed details of slope damage processes acting within the slope, showing sudden breakage phases alternating with smoother deformation and sliding phases. These processes would not have been suspected with slope deformation monitoring alone. The use of a new parameter that combines passive seismic tracking and surface deformation measurements could help revealing active slopes with ongoing damage processes. This inovative approach represents an asset for future practical rock slope monitoring.
Key Points
We observed a 24% decrease in rock slope fundamental frequency during the 6 months preceding a ∼14,000 m3 collapse
New insights into slope degradation processes revealed the alternate control of two damage regimes with contrasting rate
Results revealed slope damage phases that would not have been suspected on the basis of slope deformation monitoring alone
Objectif Décrire le devenir podologique de patients diabétiques présentant un pied de grade 2 ou 3 et inclus dans un suivi combiné : podologue (rythme recommandé) et consultation mixte ...(diabétologue-infirmière tous les 6 mois) pour revoir les règles de protection des pieds, quantifier l’évolution de la neuropathie par neuroesthésiométrie, valider l’autonomie motrice et visuelle nécessaire à la surveillance des pieds, rappeler de la nécessité de contacter l’équipe dès la moindre lésion. Patients et méthodes 134 patients inclus (76,5 % de DT2, 27,4 ± 13 ans de diabète, 7,8 ± 1,3 d’Hba1c) : 68 sont régulièrement suivis (33+25 mois), 66 ont rapidement interrompu le suivi. Résultats À l’inclusion : 35 % d’AOMI, 22 % de déformation, 24 % d’incapacité visuelle, 22 % d’incapacité motrice pour surveiller les pieds. 65 % des patients suivis ne feront aucune plaie. La neuropathie est une complication évolutive : le seuil de neuroesthésiométrie passe de 12 à 25 volt durant le suivi pour les patients de grade 2 ( p = 0,02). Ce sont les patients qui voient leur seuil de neuroesthesiométrie évoluer le plus vite qui font le plus de plaie au cours du suivi. Les plaies seront minimes parmi les patients ayant eu le suivi régulier, aucune ne débouchera sur une amputation. 7 plaies chez les patients qui n’ont pas de suivi médecin-infirmière conduiront à 4 amputations. La comparaison entre les patients faisant une plaie pendant le suivi et ceux qui n’en font pas montre que seule l’existence d’une déformation les différentie ( p < 0,03) ; par contre l’existence d’une artérite n’est pas un élément distinctif. Conclusion Un suivi mixte podologique et consultation spécifique diabétologue infirmier économise 10 % de plaies graves sur un recul de 33 mois. La neuropathie sensitive est une complication évolutive qu’il est utile d’aborder avec une approche quantitative pour mieux cibler l’évolutivité du pied et son risque. Déclaration d’intérêt Les auteurs déclarent ne pas avoir d’intérêt direct ou indirect (financier ou en nature) avec un organisme privé, industriel ou commercial en relation avec le sujet présenté.
Polythermal glaciers can trap considerable volumes of liquid water with the potential to generate devastating outburst floods. This study aims to identify water‐filled subglacial reservoirs from ...ambient seismic noise collected by moderate‐cost surveys. The horizontal‐to‐vertical spectral ratio technique is highly sensitive to impedance contrasts at interfaces, thus commonly used to estimate glacier thickness. Here, we focus on the inverse ratio, that is, the V/H spectral ratio (VHSR), whose high values indicate a low impedance volume beneath the surface, suggesting subglacial cavities. We analyze VHSR peaks from a seismic array of 60 nodes installed on the Tête‐Rousse Glacier (Mont Blanc massif, French Alps); data were gathered over 15 days. Mapping the VHSR amplitude over the free surface reveals the main cavity locations and the basal areas affected by melting within the glacier. Results obtained in the field are supported by a conceptual model based on 3D finite‐element simulations.
Plain Language Summary
Considerable volumes of liquid water may be trapped within cavities in polythermal glaciers. If these cavities rupture, the resulting outburst flood has the potential to cause devastation in populated mountain areas. With the aim of testing methods to locate such cavities, we installed 60 small 3‐component seismic sensors on the Tête‐Rousse Glacier (Mont Blanc massif, French Alps), which is known to contain such cavities. We used these sensors to test a detection method based on ambient seismic noise. For 3 weeks, the sensors recorded vibrations within the glacier. On a glacier without cavities, these vibrations ought to be predominantly in the horizontal direction. In the presence of a cavity, we expect the ice above the cavity to vibrate mostly vertically—like a bridge. In this paper, we highlight areas on the glacier where vertical vibrations were stronger than horizontal vibrations. These areas fit well with the locations of the main known cavities in this glacier, and with areas affected by basal melting. We supported our field observations with modeling based on 3D simulations, paving the way to a new method to locate water‐filled cavities within glaciers.
Key Points
Spectral analysis from ambient seismic noise is complementary to other geophysical methods for investigating glaciers at depth
Results suggest that the vertical‐to‐horizontal spectral ratio is a reliable proxy to locate subglacial cavities
Experimental results were confirmed using a simplified numerical model
SUMMARY
In late June 2016, the Harmalière clayey landslide (located 30 km south of the city of Grenoble, French Alps) was dramatically reactivated at the headscarp after a 35-yr-long period of ...continuous but limited activity. The total involved volume, which moved as sliding blocks of various sizes, was estimated to be about 2 × 10 6 m3. Two seismometers were installed at the rear of the main headscarp in August 2016, on both sides of a developing fracture delineating a block with a volume of a few hundred cubic metres. For 4 months, they continuously recorded seismic ambient vibrations and microearthquakes until the block broke. Five seismic parameters were derived from the monitoring: the cumulative number of microearthquakes (CNe), the seismic energy (SE), the block resonance frequency (fB), the relative variation in Rayleigh wave velocity (dV/V) deduced from noise cross-correlations between the two sensors and the associated correlation coefficient (CC). All parameters showed a significant precursory signal before the rupture, but at very different times, which indicates the complexity of the rupture mechanism in this clay material.
Clay‐rich geological formations are responsible for many landslides, the dynamics of which are still poorly understood and intensely debated. Analysis of landslide motion shows that slow clayey slope ...movements can suddenly accelerate and fluidize as a result of sudden loading or heavy rainfall. This solid‐fluid transition, which involves disorganization of the particle network, is accompanied by a loss in rigidity that could potentially be monitored by S‐wave velocity (Vs) variations. To investigate this hypothesis, two types of laboratory experiments were performed on clay samples originating from an area affected by numerous landslides (Trièves, French Alps). First, creep and oscillatory rheometric tests revealed the thixotropic behavior of the clay with a highly pronounced viscosity bifurcation at a critical stress τc. In relation with this reduction in apparent viscosity, a significant drop in Vs is also observed over τc. Second, at zero stress, acoustic surface wave propagation experiments showed a rapid linear Vs decrease with the gravimetric water content (w) in the plastic domain, and a much lower decay in the liquid domain. The geotechnically‐defined liquid limit then appears as a break in theVs‐wcurve. For water contents in the liquid domain in particular, both experiments gave consistent results. These laboratory results demonstrate that rheological changes in clay can be revealed through Vs variations, offering the possibility of monitoring solid‐to‐fluid transitions in the field.
Key Points
Liquid limit and Bifurcation in viscosity were related to Vs variations
S‐wave velocity could be used as an indicator of rheological changes in clay
S‐wave velocity measurements could be used in systems monitoring clay landslides
Continuous ambient seismic monitoring of potentially unstable sites is increasingly attracting the attention of researchers for precursor recognition and early warning purposes. Twelve cases of ...long-term continuous noise monitoring have been reported in the literature between 2012 and 2020. Only in a few cases rupture was achieved and irreversible drops in resonance frequency values or shear wave velocity extracted from noise recordings were documented. On the other hand, all monitored sites showed clear reversible fluctuations of the seismic parameters on a daily and seasonal scale due to changes in external weather conditions (air temperature and precipitation). A quantitative comparison of these reversible modifications is used to gain insight into the mechanisms driving the site seismic response. Six possible mechanisms were identified, including three temperature-driven mechanisms (temperature control on fracture opening/closing, superficial stress conditions and bulk rigidity), one precipitation-driven mechanism (water infiltration effect) and two mechanisms sensitive to both temperature and precipitation (ice formation and clay behavior). The reversible variations in seismic parameters under the meteorological constraints are synthesized and compared to the irreversible changes observed prior to failure in different geological conditions.
This paper presents an experimental and numerical analysis on friction-induced vibrations arising from the frictional contact between two bodies in relative motion. The sliding contact has been ...reproduced within a mechanical system characterized by a simple dynamics, in order to better distinguish between the dynamic response of the system and the broadband excitation coming from the contact. The effects of some parameters, mainly relative velocity, roughness and normal load, on the magnitude and frequency content of the induced vibrations are investigated, also by comparing results from experimental measurements and simulations. A distinction between weak and strong coupling is recovered by the experimental results. Finally, accounting for the contribution of the roughness to the contact-induced vibrations, a conceptually innovative method to implement the effect of the roughness in the numerical simulation is proposed.
•An experimental and numerical analysis on Friction Induced Vibrations is proposed.•The effect of different boundary conditions has been investigated.•A distinction between weak and strong coupling is recovered.•A new method to implement the roughness in the numerical simulation is addressed.
Friction-induced vibrations are a complex phenomenon, arising when two surfaces undergo relative sliding. During the last decades many studies on friction-induced vibrations have been carried out, ...where the simulation of the contact dynamic excitation has always been a challenge to face. This work proposes a new method to reproduce the local dynamic excitation from the contact and its effect on the vibrational response of the system. A friction law including a perturbative term of the friction coefficient is introduced. The evolution of the perturbative term, recovered by dedicated experiments, allows for simulating and analysing the contact excitation mechanisms. The comparisons between numerical and experimental results show good correlation between the measured vibrations and the ones simulated numerically, validating the proposed friction law.
•Numerical reproduction of Friction-Induced Vibrations by contact excitation.•Introduction of a contact law with a perturbative term in the friction coefficient.•Analysis of the spectral distribution of the contact excitation term.•The new friction law is validated by comparison with experiments.•A change in the surface roughness and in the sliding velocity is considered.
A potentially unstable limestone column (∼1000 m3, Vercors, French Alps) delineated by an open rear fracture was continuously instrumented with two three-component seismic sensors from mid-May 2009 ...to mid-October 2011. Spectral analysis of seismic noise allowed several resonance frequencies to be determined, ranging from 6 to 21 Hz. The frequency domain decomposition (FDD) technique was applied to the ambient vibrations recorded on the top of the rock column. Three vibration modes were identified at 6, 7.5 and 9 Hz, describing the upper part of corresponding modal shapes. Finite element numerical modelling of the column dynamic response confirmed that the first two modes are bending modes perpendicular and parallel to the fracture, respectively, while the third one corresponds to torsion. Seismic noise monitoring also pointed out that resonance frequencies fluctuate with time, under thermomechanical control. For seasonal cycles, changes in frequency are due to the variations of the bulk elastic properties with temperature. At daily scale, increase in fundamental frequency with temperature has been interpreted as resulting from the rock expansion inducing a closure of the rear fracture rock bridges, hence stiffening the contact between the column and the rock mass. Conversely, the rock contraction induces a fracture opening and a decrease in resonance frequency. In winter, when the temperature drops below 0 °C, a dramatic increase in fundamental frequency is observed from 6 Hz to more than 25 Hz, resulting from ice formation in the fracture. During spring, the resonance frequency gradually diminishes with ice melting to reach the value measured before winter.
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