Surveillance acoustique des cavités à risque de fontis et d'effondrements localisés

National audienceIt is very difficult to monitor sinkholes and local collapses from underground using the classical geotechnical instrumentation since the location of such pre-existing phenomena cannot be easily approached or forecast in time in wide and complex underground cavities. INERIS developed and tested an acoustic method to detect, localize and characterize rock falls with the help of a few sensors.Les cavités souterraines de faible profondeur, naturelles ou anthropiques, peuvent être à l'origine de risques de mouvements de terrains par fontis ou par effondrement localisé. Ce phénomène touche l'ensemble du territoire national. Dans l'attente d'un traitement, une surveillance peut permettre de gérer le risque. Jusqu'à présent, cette surveillance était essentiellement réalisée par inspection visuelle et par instrumentation géotechnique conventionnelle. Cette démarche présentant plusieurs limites dans le suivi des phénomènes dans la continuité et d'exposition des équipes intervenantes, il était important d'examiner de nouvelles solutions instrumentales

Analysis of climatic influences on slope microseismic activity and rockfalls: case study of the Matterhorn peak (Northwestern Alps)

Rock instability and rockfalls are commonly preceded by the initiation and propagation of cracks. This process is accompanied by the release of microseismic energy, which can be detected by means of an appropriate monitoring system. Because an increase in rockfall events have been observed in the Matterhorn Peak area since 2003, in 2007 a microseismic monitoring system and a thermometric monitoring system were installed on the Italian side of the Matterhorn peak, close to the J. A. Carrel hut, as part of the Interreg IIIA Alcotra "PERMAdataROC" project. The objective of the installation was to determine whether this instability increase was connected to climatic fluctuations. Detailed analysis of the recorded microseismic data shows a spatial concentration of microseismic activity on the western side of the investigated slope, and a correlation of these data with thermal information shows that the temporal concentration of the microseismic activity could be traced back to the transitions from warm to cold periods. Cold periods, characterized by a continuous and rapid temperature decrease in time, incurred a higher average daily number of microseismic event

Application of a multiplet-location coupled technique to microseismic data for identification of rock slope active surfaces

In rock slope analysis, formation and growth of microcracks are usually coupled with the propagation of elastic waves that can be detected by a suitable microseismic monitoring system. The correct analysis and interpretation of the recorded activity can provide information on the size and the type of the rupture mechanisms. A combination of the coherency computation and the source location techniques, typically used in seismic signal processing, is here proposed to this aim. The application to a microseismic dataset recorded by the microseismic monitoring system installed on the Matterhorn Peak, has evidenced the effectiveness of this approach. In particular the coherency has allowed to identify a set of similar appearing events, while the location of the corresponding hypocenters has evidenced the alignment of the sources on a planar surface, whose orientation is closed to the orientation of a discontinuity system resulting from a morphostructural surve

Cross-analysis of temperature and slope microseismic activity for the Matterhorn peak study site (North-Western Alps)

A microseismic and a thermometric monitoring systems were installed, in the framework of the Interreg IIIA Alcotra "PERMAdataROC" project, on the Italian side of the Matterhorn peak, close to the J.A. Carrel hut. The analysis of a one year temperature recording has evidenced the influence of the slope exposure (north vs. south) on data recorded during different periods of the year and at different depths. In particular, given the Matterhorn peak geometry, approximately a pyramid, it emerges that the thermal gradient north/south and surface/depth can make possible an increase in stress concentration on weakness surfaces that pass through the rock mass and promotes instability events. The numerical correlation between a 6-months microseismic dataset and thermal data has evidenced a temporal concentration of the microseismic activity when temperature falls rapidly. While, no particular activity emerges when temperature raises. Integration of the microseismic monitoring with a thermometric system can then contribute in investigating rock response to climate forcing. Once fully developed and tested, this technique could become a helpful tool for warning in advance when climatic conditions that can favour a rock slope instability set u

Acoustic emission investigation for rock slope stability analysis in high mountain areas

ISSN: 1029-700

Influence of freeze-thaw cycles on microseismic activity on a mountain slope in the Italian Alps

The risks associated with rockfalls at high altitudes have grown in recent years. In order to better comprehend the causes of this increase in mountain slope instability and to establish whether particular climatic conditions can be counted among these causes, microseismic and thermometric monitoring systems were installed around the J.A. Carrel hut (Matterhorn Peak, Italian Alps) as part of the Interreg III Alcotra “PERMAdataROC” project. A detailed analysis of data from a 6-month recording period has highlighted a spatial concentration of the microseismic activity to within 10 m of the surface on the western side of the investigated slope, while a correlation of these data with thermal information has shown that the temporal concentration of the microseismic activity could be traced back to a transition from warm to cold periods. As alpine permafrost has been encountered in the area, with an active layer of between 0 m and 7 m, the results of this investigation are also useful to document the type of activity that is underway in the active layer of the slope

Microseismic activity analysis for the study of the rupture mechanisms in unstable rock masses (Matterhorn, North-western Alps)

ISSN: 1029-700