Friction degradation in rapid sliding: Back analysis of the catastrophic Vajont landslide using DDA

The Vajont landslide is an outstanding case history for the study and back analysis of complex instability mechanisms involved in sliding along planes of weakness in rocks. It moved a 250 m thick mass over a distance of about 500 meters horizontally, reaching velocities of 20 - 25 m/s, during less than 45 seconds. We use Discontinuous Deformation Analysis (DDA) to study velocity - distance evolution of that event by implementing a simple friction degradation algorithm. The slide moved across a basal plane that possessed an estimated friction angle of 12^o only (µ = 0.231) due to previous sliding episodes. When the catastrophic landslide was triggered, friction had to be further degraded to explain the mapped runout distance and the estimated duration of the event. We find that further friction degradation of at least 25% must have taken place both across the basal plane (µ = 0.16) as well as in the rock joints consisting the sliding mass. This degradation resulted in peak velocity of 22 m/s, runout distance of 470 meters, and event duration of 37 seconds.