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SIGNALS DAMPING ANALYSIS AS A TOOL FOR INVESTIGATING ONGOING ROCK MASS DAMAGINGD. D’Angiò1, R. Iannucci1, L. Lenti2, S. Martino1

1 Department of Earth Sciences and Research Centre for Geological Risks (CERI), “Sapienza” University of Rome, Italy2 French Institute of Science and Technology for Transport, Development and Networks (IFSTTAR) - Paris East University, France

Introduction. Microseismic monitoring represents an affirmed diagnostic tool to detect vibrational signals in several contexts, like in structural monitoring, mining excavation, tunnelling, and in slope stability assessment as well (Colombero et al., 2018). Due to the resolution and sensitivity of modern seismic devices, it is possible to record very weak signals with a broad frequency resolution. As it regards applications devoted to rock fall and rock slide risk prevention (Spillmann et al., 2007), the signals recorded by microseismic monitoring systems can be related to the ongoing damaging of the rock mass, i.e. formation of new fractures, thus indicating a worsening of its stability conditions, that can lead to rock failures (Loew et al., 2016). By considering a microseismic dataset collected in an experimental test site, this study proposes an analysis of microseismic events focused on investigating damping coefficients from recorded signals, since damping could be regarded as a possible indicator of variations in rock mass micro-fracture network.

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Acuto test site: microseismic monitoring. Since 2015, an abandoned quarry located NE of the village of Acuto (Frosinone) has been individuated as test site for the installation of a multi-sensor monitoring system on a rock block prone to failure, to investigate the long-term rock mass deformations due to temperature, wind and rainfalls. The multi-sensor monitoring system consists in: 1 thermometer for the rock mass temperature; 6 strain-gauges installed on fractures of the rock mass; 4 extensimeters installed on open fractures; 1 optical device for the detection of rock fall events on a railway track posed to reproduce hazard scenarios and 2 weather stations, installed at foot and top of the slope wall, equipped with air-thermometer, hygrometer, pluviometer and anemometer for wind speed and direction (Fantini et al., 2016). Several experimental activities have been carried out so far, in order to force the 12 m3 protruding rock block (separated through a main open fracture from the back quarry wall) by thermal and dynamic actions to study inelastic strains induced by external actions (Fantini et al., 2017).

On February 23th 2018, a microseismic monitoring system has been installed in order to study the vibrational behaviour of the protruding rock block and of the quarry rock wall. In particular, 6 one-component micro-accelerometers Brüel & Kjær type 8344 have been deployed along an horizontal alignment with a spacing of about 15 cm: in this configuration 3 sensors are placed on the rock block (ID: 1, 2, 3), 2 are located on the rock mass wall (ID: 5, 6) and the last one is positioned at the passage between the rock block and the rock mass wall (ID: 4) (Fig. 1). The sensors were connected to a datalogger SomatXR CX23-R of HBM, powered by a supply system constituted of a solar panel and a backup battery. The acquisition was set in continuous mode with a sampling frequency of 2400 Hz; data were periodically downloaded on site until the end of May 2018, when the microseismic monitoring was stopped.

Data analysis. Microseismic events exceeding a user-defined threshold set at 0,001 g have been detected on the three-months dataset collected at Acuto. A typical microseismic event is characterised by a very short duration in time, spanning from 0.05 to 0.2 s, and a frequency content generally ranging from 80 to 500 Hz. The daily events detected are variable from 0 to several hundreds, with a concentration of the events in rainy days. With the aim of investigating

Fig. 1 - Acuto quarry test site. The monitored sector of the quarry wall and the 12m3 protruding rock block is shown in a); the squared area is zoomed in c), where positioning and ID of the accelerometers is reported; datalogger and power supply system (b).

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the variability of dynamic parameters associated to incoming vibrations on the monitored rock mass during time, the damping coefficient associated to each event was derived for some frequencies of interest. Subsequently, the values obtained for the considered frequencies (30, 50, 100, 200, 500, 1000 Hz) were averaged for each day in order to be compared. Even if no relevant fluctuations have been observed in the analysed dataset, since at the Acuto quarry there are only weak external actions mainly referable to temperature ranges and wind intensity that does not facilitate the detection of variations in damping values, it is deemed that the analysis performed can be applied in environments characterised by frequent and intense vibrations.

Future perspectives. The application of damping analysis over time for those rock masses subjected to recurrent and intense external vibrations, as those located in proximity of the infrastructures, could show a variation in damping coefficient estimated, thus indicating a changing in the dynamical behaviour of the monitored system, which could be related to microfracturing processes occurring in the rock mass. For this reason, another dataset acquired on a rock mass located along a railway line will be treated with the presented tool, by taking in account the different typologies of train transited and the different environmental conditions occurred on site.Acknowledgments. The Authors wish to thank the Municipality of Acuto for the authorization provided to the experimental activities carried out at the abandoned quarry. The research was carried out in the framework of the project “Rock failures in cliff slopes: from back- to forward-analysis of processes through monitoring and multi-modelling approaches” (University of Rome “Sapienza”—Year 2016, P.I. Prof. Salvatore Martino)

ReferencesColombero C., Comina D., Vinciguerra S., Benson P.M.; 2018: Microseismicity of an unstable rock mass: From field

monitoring to laboratory testing. Journal of Geophysical Research: Solid Earth, 123, 1673–1693. https://doi.org/10.1002/2017JB014612

Fantini A., Fiorucci M., Martino S., Marino L., Napoli G., Prestininzi A., Salvetti O., Sarandrea P., Stedile L.; 2016: Multi-sensor system designed for monitoring rock falls: the experimental test-site of Acuto (Italy). Rendiconti Online della Società Geologica Italiana, 41, 147-150.

Fantini A., Fiorucci M., Martino S.; 2017: Rock Falls Impacting Railway Tracks: Detection Analysis through an Artificial Intelligence Camera Prototype. Wireless Communications and Mobile Computing, vol. 2017, Article ID 9386928, 11 pages, 2017. doi:10.1155/2017/9386928

Loew S., Gschwind S., Gischig V., Keller-Signer A., Valenti G.; 2016: Monitoring and early warning of the 2012 Preonzo catastrophic rockslope failure. Landslides, 14, 141–154.

Spillmann T., Maurer H., Green A.G., Heincke B., Willenberg H., Husen S.; 2007: Microseismic investigation of an unstable mountain slope in the Swiss Alps. Journal of Geophysical Research, 112.