ABSTRACT
Prior to most catastrophic slope failures, only sparse information on long-term evolution and history of the instability are available. For large retrogressive rock slope failures, the clock is reset at the time of failure and a three stage evolution of slope displacement rates leading to the next catastrophic slope failure is assumed to describe displacement history. We conducted an integrated analysis of slope instability indicators and induced damage at the Preonzo rock slope instability complex between the last 2 major slope failures in 1702 and 2012, based on historical documents, aerial images, in-situ observations and displacement measurements. An increase in rockfall activity was observed for both large events about 5–10 years before catastrophic slope failure. It is suggested that a key sign of warning predating a catastrophic rock slope failure is the propagation of tension cracks in the head scarp area. We observed a period of linear crack growth since 1962 followed by accelerated non-linear crack growth in 1995 leading to catastrophic failure in 2012. The growth of these tension cracks might be related to the progressive evolution of a basal rupture plane, but does not correlate with other signs of activity systematically mapped on aerial images since 1939.
