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Detection of nanoseismic events related to slope instabilities in the quarry district of Coreno Ausonio (Italy)
Full text linkLe cave per l’estrazione di materiale roccioso rappresentano contesti in cui possono aver luogo eventi di instabilità gravitativa causati dalle continue sollecitazioni cui sono soggette le pareti produttive, principalmente connesse alle vibrazioni dovute alle esplosioni necessarie alle operazioni di disgaggio. La necessità di gestione del rischio da frana per la salvaguardia del personale impegnato nell’attività estrattiva ha portato, nel tempo, alla richiesta di attivare sistemi di monitoraggio nelle aree di coltivazione mineraria e di cava.
Nel presente lavoro il monitoraggio nanosismometrico, una tecnica di geofisica passiva recentemente sviluppata per le indagini di microsismicità, è stato impiegato nel distretto di cave a cielo aperto di Coreno Ausonio (in provincia di Frosinone). Il monitoraggio nanosismometrico consente l’individuazione e la localizzazione di deboli eventi sismici, fino a magnitudo locale (ML) nell’ordine di -3, attraverso l’impiego di quattro sensori sismometrici disposti secondo una specifica geometria di array detta SNS (Seismic Navigation System).
Dopo aver individuato una cava in cui erano programmate esplosioni per la volata delle pareti in roccia, nel corso del 2013 sono state organizzate 3 campagne di acquisizione durante tre giornate, pianificate in modo da monitorare l’area in un periodo compreso da qualche ora prima dell’esplosione alle 24 ore successive. Su una parete della cava non più produttiva è stato effettuato un rilevamento geologico-tecnico che ha permesso di individuare 4 principali sistemi di discontinuità e caratterizzarli in termini di giacitura, resistenza, rugosità, apertura, spaziatura e condizioni idrauliche secondo gli standard ISRM (1978). L’analisi di stabilità della parete in esame, tenuto conto della sua orientazione, ha restituito una scarsa propensione ad eventi di instabilità.
Analizzando mediante il software NanoseismicSuite i dati sismometrici acquisiti è stato possibile ottenere i “supersonogrammi”, ovvero particolari spettrogrammi auto-adattanti alle variazioni del rumore sismico di fondo, dai quali sono state definite alcune caratteristiche specifiche di forma d’onda per diverse tipologie di eventi. In base ai supersonogrammi, è stato possibile individuare e localizzare 15 esplosioni, di cui 3 provenienti dalla cava di riferimento e 12 da cave adiacenti del distretto, e 27 deboli eventi di instabilità gravitativa, distinti in 23 eventi di collasso e 4 rotture legate alle fratturazione dell’ammasso roccioso. Le 3 esplosioni avvenute nella cava di riferimento, e quindi aventi coordinate di origine nota, sono state utilizzate per calibrare il modello di sottosuolo, successivamente impiegato per localizzare gli altri eventi registrati.
Le rotture sono risultate originate in diverse zone del distretto estrattivo, mentre gli eventi di collasso sono stati localizzati in una specifica area e risultano essere avvenuti prevalentemente in un limitato intervallo di tempo a seguito delle 9 esplosioni registrate nella campagna del 26-27 luglio 2013. Non è stato possibile, invece, individuare eventi riconducibili ad instabilità negli orari di attività di cava a causa dell’elevato livello di rumore apportato dagli strumenti per l’estrazione e la lavorazione del materiale roccioso. Si è escluso che gli eventi di collasso fossero riconducibili direttamente all’attività di estrazione sia perché registrati al di fuori dell’orario di lavorazione delle cave sia perché, analizzando l’intera registrazione per intervalli orari, le frequenze tipiche dei macchinari di lavorazione non sono risultate energizzante. La zona di origine è risultata essere un’area nella quale sono stati rinvenuti detrito sciolto costituito da blocchi eterometrici ed una parete non in coltivazione con medesime caratteristiche delle discontinuità rispetto alla parete sulla quale era stato effettuato il rilevamento geologico-tecnico, ma con diversa orientazione.
In definitiva, la fase di sperimentazione ha restituito dei risultati di indubbio interesse consentendo di mettere in evidenza alcune limitazioni del monitoraggio nanosismometrico nel contesto preso in esame, in particolare legate all’eccessiva rumorosità registrata nelle ore di attività di cava. La tecnica appare, comunque, un utile strumento di monitoraggio per i fenomeni gravitativi di debole intensità, in grado di contribuire alla gestione del rischio da frana in aree ad elevata attività antropica ed in ambienti naturalmente predisposti ad instabilità gravitative che possono interessare pareti in roccia.Nanoseismic monitoring is a passive geophysical technique used to identify and locate weak seismic events (down to local magnitudes, ML, around -3). This technique was applied in the open-pit quarry district of Coreno Ausonio (central Italy) to detect possible gravity-induced slope instabilities resulting from quarry rock blasting. After identifying an active quarry, an engineering-geological survey was carried out to characterise the jointed rock mass on an abandoned wall in front of the quarry. Four main joint sets were surveyed and their geometric and mechanical properties were measured in order to carry out stability analyses that evidenced scarce proneness to failure of the investigated wall. The analysis of seismic records obtained during three monitoring surveys, performed through the NanoseismicSuite software, made it possible to detect and characterise 15 blasts, of which 3 from the reference quarry and 12 from nearby quarries within the district, as well as 27 weak slope instability events (23 collapses and 4 failures). While failures originated from different areas of the quarry district, collapses occurred in a site characterised by an abandoned quarry having a wall more prone to gravity-induced instabilities than the one previously characterised
Preliminary evidences of slope instabilities through seismic measurements at Cala Rossa bay (Italy)
No full textRock mass characterization coupled with seismic noise measurements to analyze the unstable cliff slope of the Selmun Promontory (Malta)
Full text linkIn the Mediterranean area, cliff slopes represent widespread high-risk landforms as they are highly frequented touristic places often interested by landslide processes. Malta represents a significant case study as several cliffs located all around the island are involved in instability processes, as evidenced by wide block-size talus distributed all along the coast line. These diffused instabilities are related to the predisponding geological setting of Malta Island, i.e. the over-position of grained limestone on plastic clay deposits, that induces lateral spreading phenomena associated to falls and topples of different-size rock blocks and is responsible for a typical landscape with stable plateau of stiff rocks bordered by unstable cliff slopes. The ruins of Gƫajn ƪadid Tower, the first of the thirteen watchtowers built in 1658 by the Gran Master Martin de Redin, stand out in the Selmun area. Currently the safety of this important heritage site, already damaged by an earthquake on October 12th 1856, is threaten by a progressive moving of the landslide process towards the stable plateau area. During autumn 2015, a field campaign was realized to characterize the jointed rock mass. A detailed engineering-geological survey was carried out to reconstruct the geological setting and to define the mechanical properties of the rock mass. Based on the surveyed joint spatial distribution, 58 single-station noise measurements were deployed to cover both the unstable zone and the stable area. The obtained 1-hour records were analyzed in the frequency domain for associating vibrational evidences to different instability levels, i.e. deriving the presence of already isolated blocks by the local seismic response. The here presented results can be a useful contribute to begin to asses defense strategies for the Selmun Promontory, in the frame of managing the landslide risk in the study area and preserving the local historical heritage
Investigating rock mass failure precursors using a multi-sensor monitoring system. Preliminary results from a test-site (Acuto, Italy)
Full text linkIn the last few years, several approaches and methods have been proposed to improve early warning systems for managing risks due to rapid slope failures where important infrastructures are the main exposed elements. To this aim, a multi-sensor monitoring system has been installed in an abandoned quarry at Acuto (central Italy) to realise a natural-scale test site for detecting rock-falls from a cliff slope. The installed multi-sensor monitoring system consists of: i) two weather stations; ii) optical cam (Smart Camera) connected to an Artificial Intelligence (AI) system; iii) stress- strain geotechnical system; iv) seismic monitoring device and nano-seismic array for detecting microseismic events on the cliff slope. The main objective of the experiment at this test site is to investigate precursors of rock mass failures by coupling remote and local sensors. The integrated monitoring system is devoted to record strain rates of rock mass joints, capturing their variations as an effect of forcing actions, which are the temperature, the rainfalls and the wind velocity and direction. The preliminary tests demonstrate that the data analysis methods allowed the identification of external destabilizing actions responsible for strain effects on rock joints. More in particular, it was observed that the temperature variations play a significant role for detectable strains of rock mass joints. The preliminary results obtained so far encourage further experiments
Engineering geological zonation of a complex landslide system through seismic ambient noise measurements at the Selmun Promontory (Malta)
No full textThe cliff slope of the Selmun Promontory, located in the Northern part of the island of Malta (Central Mediterranean Sea) close to the coastline, is involved in a landslide process as exhibited by the large block-size talus at its bottom. The landslide process is related to the geological succession outcropping in the Selmun area, characterized by the overposition of a grained limestone on a plastic clay, that induces a lateral spreading phenomenon associated with detachment and collapse of different-size rock blocks. The landslide process shapes a typical landscape with a stable plateau of stiff limestone bordered by an unstable cliff slope. The ruins of Għajn Ħadid Tower, the first of the 13 watchtowers built in 1658 by the Grand Master Martin de Redin, stand out on the Selmun Promontory. The conservation of this important heritage site, already damaged by an earthquake which struck the Maltese Archipelago on 1856 October 12, is currently threatened by a progressive retreat of the landslide process towards the inland plateau area. During 2015 and 2016, field surveys were carried out to derive an engineering geological model of the Selmun Promontory. After a high-resolution geomechanical survey, the spatial distribution of the joints affecting the limestone was obtained. At the same time, 116 single-station noise measurements were carried out to cover inland and edge of the limestone plateau as well as the slope where the clays outcrop. The obtained 1-hour time histories were analysed through the horizontal to vertical spectral ratio technique, as well as polarization and ellipticity analysis of particle motion to define the local seismic response in zones having different stability conditions, that is, related to the presence of unstable rock blocks characterized by different vibrational modes. The results obtained demonstrate the suitability of passive seismic geophysical techniques for zoning landslide hazard in case of rock slopes and prove the relevance of anisotropies in conditioning the polarization of vibrational modes for dislodged rock masses
Seismic monitoring of the gravity-induced deformation involving the Peschiera Spring slope (Italy) for the management of a main infrastructure
No full textThe Peschiera Spring slope (Central Apennines, Italy), which hosts the tunnels of the main drainage plant of Rome aqueducts, is involved in a gravity-induced slope deformation. An average aquifer discharge of about 18 m3/s is responsible for an intense limestone dissolution corresponding to the main kinematic elements that dislodge the jointed rock mass. Since 2008 a seismometric network was installed within the tunnels of the drainage plant for detecting local underground instabilities consisting of failures and collapses. About 1000 signals due to instabilities were recorded up to now. A control index (CI), based on both the frequency of occurrence and the cumulative energy of the recorded local instabilities was tested for providing three levels of alert. During the 2014 a total of 32 collapse crises were recorded, that caused several “emergency” states of the plant
High-resolution geological model of the gravitational deformation affecting the western slope of Mt. Epomeo (Ischia)
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Geological constraints for a conceptual evolutionary model of the slope deformations affecting Mt. Nuovo at Ischia (Italy)
Full text linkischia island was the scenario of several Holocene slope in- stability events occurred at different scales, from shallow mass movements, triggered by meteo-climatic forcing, up to massive rock slope failures such as large debris avalanches these last ones related to the volcano-tectonic dynamics of a resurgent caldera. the present study focuses on the gravitational deformation that in- volves Mt. nuovo, located in the western portion of Mt. epomeo resurgent block. a high-resolution engineering-geological model was reconstructed according to a multi-modelling approach sup- ported by field geo-structural evidences and constrained by pas- sive seismic investigations. it revealed a complex morpho-struc- tural setting and led to the identification of a multiple compound mechanism, involving a rock mass volume of about 190 million of cubic meters.
the obtained geological model shows a partial structural control of the pre-existing tectonic pattern on slope deformation mechanisms, highlighting geometric and volumetric similarities between the Mt. nuovo ongoing deformation and an already oc- curred rock avalanche. the defined conceptual evolutionary mod- el allows to hypothesize the role of inner pressures constraining the shear zone initiation and propagation and making reliable a future scenario of generalized collapse.
Starting from these new field and laboratory data, numerical models will be reconstructed in order to depict the evolution of the gravitational slope deformation, evaluate its sensitivity and constrain future evolutionary instability scenarios
Experiment of an innovative nanoseismic monitoring applied to gravity-induced slope instabilities in a karstified rock mass
No full textThe Peschiera Springs slope (RI), which hosts the drainage plant of
the Rome’s aqueduct, is affected by a gravitational slope deformation
process, that is responsible for a rock-mass lateral spreading,
associated to several landforms including sinkholes, subvertical scarps
and trenches. An experiment, based on an innovative method of
monitoring, was carried out to record microseismic events generated
by underground instabilities such as failures and collapses. At this aim
a SNS (Seismic Navigation System) array was installed during the
year 2013 inside a the tunnel of the drainage plant in order to carry out
a continuous monitoring. The nanoseismic monitoring, allowed to
record 37 microseismic events. The seismic records were managed by
NanoseismicSuite software that allowed to identify and characterize
two different typologies of events: 19 underground collapses and 18
underground failures. The locations of these events are in good
agreement with the evolutionary geological model of the ongoing
gravitational slope deformation, i.e. with the spatial distribution of the
main trenches and scarps on the slope
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