The dynamic response of four unstable rock compartments in the Alps has been studied using the ambient vibration technique, with the aim of identifying precursors to rockfalls. The test sites present various geological settings (limestone, argillite, and shale-sandstone series), failure mechanisms and volumes. The ambient vibration spectra measured on the unstable compartments systematically showed clear energy peaks at specific frequencies, in contrast with records made on the adjacent stable rock masses. These predominant frequencies were interpreted as resonant frequencies of the unstable compartments, in agreement with 2-D modal analysis. In the horizontal plane, ground motion at the fundamental frequency was found to be systematically parallel to the line of maximum slope gradient, and perpendicular to the main bounding fracture observed at most of the sites. The fundamental frequency of each prone-to-fall compartment shows reversible variations related to temperature fluctuations at different timescales, with a significant contrast in magnitude and phase shift between sites. At the more fractured site, resonance seems to result from a contrast in internal rigidity between the compartment and adjacent rock mass, rather than from decoupling along a rear fracture, which is the mechanism observed at the three other sites. No change in fundamental frequency resulting from damage was observed over the period of study.

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