Monitoring and early warning of the 2012 Preonzo catastrophic rockslope failure

Abstract

In this paper, we describe the investigations and actions taken to reduce risk and prevent casualties from a catastrophic 210,000 m3 rockslope failure, which occurred near the village of Preonzo in the Swiss Alps on May 15, 2012. We describe the geological predisposition and displacement history before and during the accelerated creep stage as well as the development and operation of an efficient early warning system. The failure of May 15, 2012, occurred from a large and retrogressive instability in gneisses and amphibolites with a total volume of about 350,000 m3, which formed an alpine meadow 1250 m above the valley floor. About 140,000 m3 of unstable rock mass remained in place and might collapse partially or completely in the future. The instability showed clearly visible signs of movements along a tension crack since 1989 and accelerated creep with significant hydromechanical forcing since about 2006. Because the active rockslide at Preonzo threatened a large industrial facility and important transport routes located directly at the toe of the slope, an early warning system was installed in 2010. The thresholds for prealarm, general public alarm, and evacuation were derived from crack meter and total station monitoring data covering a period of about 10 years, supplemented with information from past failure events with similar predisposition. These thresholds were successfully applied to evacuate the industrial facility and to close important roads a few days before the catastrophic slope failure of May 15, 2012. The rock slope failure occurred in two events, exposing a compound rupture plane dipping 42° and generating deposits in the midslope portion with a travel angle of 39°. Three hours after the second rockslide, the fresh deposits became reactivated in a devastating debris avalanche that reached the foot of the slope but did not destroy any infrastructure. The final run-out distance of this combined rock collapse–debris avalanche corresponded to the predictions made in the year 2004.

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Notes

  1. 1.

    In this paper, the term rockslide is used for all kinds of rock slope instabilities independent of the actual kinematic mode, i.e., this term includes planar, circular and compound slides, topples, spreads, and collapses following the definition of Hungr et al. (2014).

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Acknowledgments

We would like to thank M. Franzi (Dipartimento del Territorio/Sezione Forestale Ticino) for providing data from Preonzo and many fruitful discussions. Several colleagues from ETH Zurich (H. Willenberg, M. Ziegler, R. Seifert, A. Kos) and UBC Vancouver (E. Eberhardt, O. Hungr) provided important contributions to the studies presented in this paper. We would like to acknowledge special support of M. Ziegler for postfailure photogrammetric analysis. We also thank three anonymous reviewers for their detailed comments.

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Correspondence to Simon Loew.

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Loew, S., Gschwind, S., Gischig, V. et al. Monitoring and early warning of the 2012 Preonzo catastrophic rockslope failure. Landslides 14, 141–154 (2017). https://doi.org/10.1007/s10346-016-0701-y

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Keywords

  • Rockslide
  • Early warning
  • Monitoring
  • Acceleration
  • Threshold
  • Evacuation