Ultrasonic emission related to rocks cracking precursors: firts results from rock sample test

The analysis of stress accumulation and crack-induced vibrations of an unstable slope before failure is considered an effective tool in rock-fall disaster forecasting. The national project SMILAND (Innovative Integrated Systems for Monitoring and Assessment of High Risk Landslides) aims at development of innovative solutions for rocky landslide early-warning based on optical fiber sensors. Within this project, a petrophysical characterization, based on different laboratory methods aims to detect the elastic properties, has been carried out on different type of rock samples. The acoustic emissions (AEs) released during the rock fracturing of laboratory samples has been also studied. Nowadays, the implemented AE monitoring techniques, used both in laboratory and in the field, are mainly based on seismometers, geophones or also microphone arrays. These devices, although accurate and reliable, may be difficult to install in the field and are liable to lighting and electromagnetic interference (EMI). In recent years, the interest in fiber optic-based sensors (FOS) has experienced a rapid increase, fostered by the advantages that these sensors offer in comparison with the classical ones. In particular, their intrinsic robustness to EMI and lightning, their remote operability and distributed sensing ability, make the FOS suitable to the distributed monitoring of several parameters (e.g. strain and temperature) in hard environment conditions too. The application of such sensors to landslides and cliff collapses is nowadays mainly limited to displacement measurements. Nonetheless, the application of fiber sensors to the characterization and monitoring of seismic precursory patterns of unstable rocks or cliffs is still an unexplored research field. Interferometric sensors have shown very high sensitivity, that consists an extremely important feature for detecting small wave amplitude precursor AEs related to initial rock cracking development. Considering that the correlation between the rock behavior to different stress conditions and internal resistance variation is principally due to the mechanical properties of the rock mass, different rock types samples (marble, dolomite and trachyte) has been used in laboratory tests to detect the elastic properties of rocks related to mineral composition, fabric, structure and porosity. Elastic properties of rock samples have been measured by using different laboratory methods such as uniaxial apparatus, ultrasonic static and dynamic multifrequency instrument. The comparison of data results have shown a substantial homogeneity from elastic modulus obtained by different methods. In this frame, all the rocks involved in the testing phase have been characterized from the mineralogical-petrographical and structural point of view. Such a complete characterization has been carried out on three mutually perpendicular sections with respect to the macroscopic fabric elements of the rock. Grain and bulk densities of the rocks will be also measured. Laboratory testing of FOS have been carried out coupling the developed sensors with several rock samples, having different lithological, structural and mechanical features.