Ambient vibration characterization and monitoring of a rock slope close to collapse

Abstract

We analyse the ambient vibration response of Alpe di Roscioro (AdR), an incipient rock slope failure located above the village Preonzo in southern Switzerland. Following a major failure in May 2012 (volume ∼210 000 m 3 ), the remaining unstable rock mass (∼140 000 m 3 ) remains highly fractured and disrupted, and has been the subject of intensive monitoring. We deployed a small-aperture seismic array at the site shortly after the 2012 failure. The measured seismic response exhibited strong directional amplification (factors up to 35 at 3.5 Hz), higher than previously recorded on rock slopes. The dominant direction of ground motion was found to be parallel to the predominant direction of deformation and perpendicular to open fractures, reflecting subsurface structure of the slope. We then equipped the site with two semi-permanent seismic stations to monitor the seismic response with the goal of identifying changes caused by internal damage that may precede subsequent failure. Although failure has not yet occurred, our data reveal important variations in the seismic response. Amplification factors and resonant frequencies exhibit seasonal trends related (both directly and inversely) to temperature changes and are sensitive to freezing periods (resonant frequencies increase with temperature and during freezing). We attribute these effects to thermal expansion driving microcrack closure, in addition to ice formation, which increase fracture and bulk rock stiffness. We find the site response at AdR is linear over the measured range of weak input motions spanning two orders of magnitude. Our results further develop and refine ambient vibration methods used in rock slope hazard assessment.

On the seismic response of instable rock slopes based on ambient vibration recordings

“…We are not aware of any other rock site presenting so high amplification levels. However, since comparable amplification levels (factors of 35) were previously observed at another rock slope instability (Preonzo, Burjánek et al 2017), we suggest that this phenomenon might be not rare for instabilities close to collapse. Fig.…”

“…6, the SRSR curves from 2014 (labeled with GON1-…) and 2015 (labeled with GON2-…) are plotted together. One can see that the dynamic behavior did not significantly change, and the measurements with 1-year time gap correspond very well (Bottelin et al 2013;Burjánek et al 2017), the re-measurement in 2015 was taken in very similar meteorological conditions as in 2014. The seismic activity was particularly low in the vicinity of the site in between June 2014 and June 2015 (no events of M > 2 within an epicentral distance of 20 km, no events of M > 3 within an epicentral distance of 60 km).…”

“…GON201 is the most upper station (within the sliding area) with a polarization azimuth of around 50°, GON102 around 40° and GON101 around 20-30°. The main direction of ground motion polarization is usually parallel to the slope deformation (e.g., Burjánek et al 2010Burjánek et al , 2017. Therefore, the slightly rotating strike angle could indicate that the rock-slide movement is not linear in the area of the three stations, but contains a rotation.…”

“…The recording time for all measurements was 90 min, and the data acquisition was done with a digital recording unit coupled to Lennartz 3D seismometer with an eigenperiod of 1 s. The frequency band of interest lies only up to 20 Hz considering the minimum scales (wavelengths) of interest on the order of 10-50 m. This estimate originates from an empirical observation that the response usually becomes highly variable for frequencies above 20 Hz for an interstation distance of this order (tens of meters). It was observed in the past that the seismic response is sensitive to the meteorological conditions, especially during freezing periods (Bottelin et al 2013;Burjánek et al 2017). …”

Modeling the Seismic Response of Unstable Rock Mass With Deep Compliant Fractures

“…Other potential changes in media (i.e., of the seismic response) are neither expected nor have been observed on the timescales of the ambient vibration experiments (Kleinbrod et al, ). Moreover, the site response at one such fractured slope has been observed to be linear over the measured range of weak input motions spanning two orders of magnitude (Burjánek et al, ), that is the response was the same for both ambient vibrations (0.8 μm peak‐to‐peak displacement) and earthquakes (up to 127 μm). This strongly supports the validity of models which adopt linear media.…”