GNGTS 2017 - 36° Convegno Nazionale

GNGTS 2017 S essione 3.3 697 Through a microseismic monitoring in continuous acquisition mode, it is possible to disaggregate and analyse the records in order to distinguish contribution in different frequency ranges, allowing the identification of the natural (low frequency) or anthropic (high frequency) vibration sources. Moreover, the observation of permanent changes in the vibrational behaviour of the monitored rock mass can be related to a rock damage process and, hence, interpreted as precursor of its failure. In this paper, we present preliminary results of spectral and time-frequency analyses, performed by Geopsy software (www.geopsy.org ), obtained in two test sites set for studying variations in the vibrational response of rock masses to induced vibrations. The first site is located in an abandoned quarry (Acuto, Frosinone), where �� the rock mass was shaken by a vibrodyne �� for monitoring a protruding and potentially unstable rock block; the second site is located along the Terni-Giuncano railway line (Terni), where a rock wall, exposed to a hourly transit of trains, was monitored. Acuto test site. The abandoned quarry is located NE of the village of Acuto (Frosinone), in the carbonatic Monti Ernici ridge. This quarry was chosen on �� Autumn 2015 as test site for the installation of a multi-sensor monitoring system on a rock block prone to failure, to investigate long-term rock mass deformations due to temperature, wind and rainfalls. �� The multi-sensor monitoring system consists in: �� one thermometer for the rock mass temperature; six strain-gauges installed on micro-fractures of the rock mass; four extensimeter installed on open fractures; one optical device for the detection of rock fall events on a railway track posed to reproduce hazard scenarios and �� two weather stations, installed at foot and top of the slope wall, equipped with air- thermometer, hygrometer, pluviometer and anemometer for wind speed and direction� �� . �� The sub- vertical quarry wall has a height ranging from 15 m up to 50 m and is composed of Mesozoic wackestone with rudists (Accordi et al. , 1986). A geomechanical characterisation of the rock mass led to the identification of four joint sets, here indicated according to dip direction/dip convention: S0 (130/13) corresponding to the limestone strata, S1 (270/74), S2 (355/62) and S3 (190/64) (Fantini et al. , 2016). The monitored sector is located in the NW portion of the 500- m-long quarry front and is characterised by the presence of a 64 m 3 densely cracked protruding block, separated from the back quarry wall by a main fracture. Field activities. Experimental activities took place on July 2016 and �� consisted in shaking the rock mass with a vibrodyne, an electro-mechanical device able to produce vibrations at fixed frequencies and amplitudes that induce stress-strain effects under controlled conditions. During such experiments, the monitoring system was implemented with the installation of six 1- component Kinemetrics FBA11 accelerometers connected to a Kinemetrics K2 datalogger that was provided with an internal 3-component accelerometer and set �� to record data in continuous mode �� with a sampling frequency of 250 Hz. The accelerometers were installed partly on the quarry wall and partly on the prominent block, in particular: two on the rock wall, two on the block lateral side, and two on the block front side (Fig. 1). Subsequently, s�� ix shaking sequences were generated by the vibrodyne at different frequencies (5 Hz, 10 Hz, 15 Hz, two at 20 Hz and 25 Hz) and time durations (40, 25, 17, 13, 26, 6 minutes respectively). After each shaking sequence the experiment was interrupted for at least one hour, to restore undisturbed vibrational conditions on the rock wall. Preliminary results. A Standard Spectral Ratios (SSR) analysis (Borcherdt, 1970, 1994) was carried out by taking in account the spectra obtained on the �� prominent �� block and on the rock wall, the latter considered as reference. Such an analysis points out an amplification on the block at 25 Hz in case of both 15 and 20 Hz inputs. S�� pectrograms of the recorded signals show that at the lowest generated frequencies (5 and 10 Hz) no energy was received by the rock block and, consequently, no significant induced vibration was detected. On the contrary, starting from the 15 Hz signal, a very low energy response was detected at the rock block. The 25 Hz and 40-50 Hz frequencies are unfortunately also induced by the generator necessary to energize the vibrodyne, so the data derived from the 25 Hz shaking test are of difficult interpretation.