Rapid sliding and friction degradation: Lessons from the catastrophic Vajont landslide

We use the numerical, discrete element, Discontinuous Deformation Analysis (DDA) method to study velocity distance evolution of the catastrophic Vajont landslide using a newly implemented friction degradation law based on empirical evidence that peak friction is mobilized in rock discontinuities after displacement distance of approximate to 1% of overriding block length. It has been established by previous researchers that during the catastrophic event the slide moved across a basal plane that possessed a friction angle of 12 degrees only (mu = 0.231) due to previous sliding events. When the catastrophic event was triggered, most likely due to excessive filling of the reservoir, 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 25% must have taken place both across the basal plane as well as in the rock joints consisting the sliding mass. This degradation resulted in 25 m/s peak velocity of the sliding mass, about 570 m runout distance, and sliding duration of 37 s. Our back-analyzed friction - velocity relationship expands the range of velocity and displacement values that can be controlled experimentally using rotary shear tests, by an order of magnitude. With the aid of numerical analysis, therefore, catastrophic landslides can become natural experiments at the field scale that provide ultimate friction degradation values for clay filled rock discontinuities. Using the case of the Vajont landslide we find that frictional resistance across clay-filled dolomite interfaces cannot degrade to values lower than mu = 0.16.