Ambient vibration classification of unstable rock slopes: A systematic approach

Abstract

In this paper, we are comparing the seismic response of 25 different rock slope instabilities with diverse geological properties, activity level, failure mechanism, fracturing and volumes. We classify them according to their dynamic behaviour. In the dataset, we found two main classes of unstable rock slopes: depth-controlled sites and volume-controlled sites. For depth-controlled instabilities, the seismic wavefield is controlled by horizontally propagating surface waves in the highly fractured and weathered material. At such sites, surface-wave dispersion curves can be retrieved and inverted into velocity profiles of the underground. The lateral borders of depth-controlled instabilities are not obvious and the seismic properties mainly change with depth. The seismic response of volume-controlled sites is dominated by the eigenvibrations of the rock mass itself. Such instabilities have clear lateral and vertical borders and show highly amplified ground motion in limited frequency bands. The observed polarisation of the ground motion is perpendicular to deep open fractures that do not allow surface waves to propagate. A special case of the volume-controlled sites is represented by tower-like rock masses showing strong amplification and directionality. Their dynamic behaviour might not only be related to the internal structure, but also to their geometry that is similar to high-rise buildings. Another type of rock instabilities is represented by block structures, which are difficult to identify by the proposed ambient-vibration methods. A clear relation between geological properties and seismic response was not found.