From Ambient Vibration Assessment of Potential Rock Slope Instabilities To Earthquake Triggered Rockslides

Abstract eng:
Earthquake-induced landslides have a huge social impact, causing many casualties and significant damage to infrastructure. They belong to the most destructive secondary hazards related to earthquakes. However, the impact of strong seismic events is not limited just to triggering of catastrophic slope failures. It involves also weakening of the internal structure of intact rock masses (and therefore generate new potential rock slope failures) or reactivating of dormant rock slides which remain slow and ductile, but can move in a continuous or intermittent manner over long distances. Hazard mitigation of potentially catastrophic landslides and the assessment of the slope state (i.e., how close a given slope is from failure) requires thorough understanding of the mechanisms driving the slope movements and its seismic response. Our recent studies on seismic ambient vibrations of unstable slopes have shown a close link between the seismic response and the internal structure of unstable rock mass. Thus, systematic passive seismic measurements have been performed in Switzerland during the last two years in order to study the seismic response of potential rock slides concerning a broad class of slope failure mechanisms, material conditions, tectonic settings and activity levels. The data acquisition was conducted with a number of threecomponent seismometers which were installed at the sites of interest for several hours. During each measurement a reference station was installed on a presumably stable part nearby the instability. Site-to-reference spectral ratios have been calculated to estimate the relative amplification of ground motion at unstable parts. A systematic analysis of the recorded dataset has shown highly directional ground motion in the unstable parts of the slopes and significantly amplified ground motion with respect to the stable areas. These effects are strongest at certain frequencies. The amplification levels at the investigated sites are rather scattered. Amplification factors up to 70 have been observed, which can obviously increase the transient strains during strong ground shaking, i.e. increase the susceptibility of slopes to earthquake-triggering. In most cases the directions of maximum amplification are perpendicular to observed open cracks and in good agreement with the deformation directions obtained by geodetic measurements. Moreover, numerical modelling is performed in order to simulate the seismic waves propagating through the instable rock slope which is represented as a media of different material contrasts, internal fractures and complex topography. The goal of the simulations is to find a model that fits best to the acquired data and therefore improves the understanding of the observed wave field and the key controlling parameters. In addition, the earthquake induced strain transients are also analyzed.

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