Rheological effects related to neo-fracturing processes in rock masses

Rockfall hazard is one of the main natural hazards in mountainous areas and along transportation routes. Roads and railways interruptions, as well as damages of buildings, are among the main inconveniences due to the detachment of unstable sectors of highly jointed rock masses. Rockfalls are the result of the combined action of the rock mass creep and of the natural and anthropic solicitations, which lead to the accumulation of inelastic strain within the rock mass and to the formation of new fractures or to the extension and movement of the pre-existing ones (rock mass damaging phenomena). The understanding of rock damaging processes through the microseismic monitoring of rock slopes predisposed for instabilities events can help in defining proper risk mitigation strategies. With the aim of assessing rock mass damaging phenomena, this PhD thesis proposes an analysis of the damping ratio associated with the microseismic emissions recorded in two test sites located in central Italy. Three monitoring campaigns have been conducted: two at the Acuto quarry test site, where the vibrational behaviour of a 12 m3 rock block partially detached from the back rock wall has been investigated; one at the Terni-Giuncano railway test site, where a rock mass close to a railway was studied to analyse the effects produced by the repeated trains transit. A STA/LTA event detection algorithm has been implemented for the recognition of the microseismic emissions from the seismic datasets acquired in continuous mode and with a sampling frequency of 2400 Hz. The damping ratio of the microseismic emissions, filtered in monofrequential waveforms, was evaluated; in the following, the daily mean damping values for each frequency were compared in respect to the environmental parameters monitored on site. No irreversible trend variations were observed, but significant variations related to transient processes were detected. It is deemed that the proposed approach can be applied on yearly seismic dataset and environments exposed to natural and anthropic forcing actions, to be furtherly tested and validated. Additional analyses were carried out on the train transit recordings. Each train passage was analysed in terms of: a) RMS value of the recording; b) cross-correlation between couples of sensors. The observation of the RMS and cross-correlation time series over time confirmed the unvaried long-term vibrational behaviour of the rock mass, according to the results of the damping analysis. In addition, the seismic noise was investigated by computing the average noise in one-minute intervals filtered at specific frequency bands. The cumulative of the filtered intervals allowed to determine the main energised frequency band, which resulted to be the one comprised between 500 and 1000 Hz. The derivative of the averaged noise was compared with the environmental parameters recorded in the two test sites. A marked correlation between the variation of air and rock mass temperature and the derivative of the noise in the frequency band comprised between 0.5 and 30 Hz was noted, while the correlation is feeble or lost for the other frequency bands considered. The correlation observed between thermal cycles and ambient noise variations is in agreement with previous bibliographical studies; moreover, a differentiation in the vibrational response for the different frequency bands has been detected.