Analysis of acoustic emission patterns for monitoring of rock slope deformation mechanisms

Acoustic emission (AE) is generated in soil and rock materials by rearrangement of particles during displacement or increasing damage in the microstructure preceding a collapse; therefore AE is appropriate for estimation of slope degradation. To overcome the high attenuation that characterise geological materials and thus to be able to monitor AE activity, a system that makes use of a waveguide to transmit AE waves from a deforming zone to a piezoelectric transducer was developed. The system quantifies acoustic activity as Ring Down Count (RDC) rates. In soil applications RDC rates have been correlated with the rate of deformation, whereas the recent application to rock slopes requires new interpretation strategies. In order to develop new strategies the system was installed at two rock slope trial sites in Italy and Austria. RDC rates from these sites, which have been measured over 5 and 1.5 years respectively, are analysed and clear and recurring trends are identified. The comparison of AE trends with response from a series of traditional instruments available at the sites allows correlation with changes in external slope loading and internal stress changes. AE signatures from the large rock slope in Italy have been identified as generated in response to variations in the groundwater level and snow loading. At the slope in Austria, AE signatures include the detachment of small boulders from the slope surface caused by the succession of freeze-thaw cycles during winter time. The work reported in this paper is contributing to the development of AE monitoring and interpretation strategies for rock slopes. The longer-term aim is to identify approaching failures and derive rules for setting thresholds that can be used to give warning of rock slope failures in time to enable action to be taken.