Rock mass falls, especially in mountain ranges like the Alps, belong to natural morphological phenomena. If they occur near population areas, they comprise great hazard to human life and to the nearby infrastructure. Thus it is very important to predict any possible occurrence and foresee the likely pathway of rock avalanches, so that precautions could be taken just in time.
In order to evaluate the qualification of the numerical program PFC modified for run out modeling two cases were selected for comparing reality and simulation: Punta Thurwieser rock avalanche with a steep Fahrböschung and Frank Slide showing a gentle Fahrböschung.
PFC3D (Particle Flow Code in 3 Dimensions) is a discontinuum mechanics program developed by ITASCA CONSULTING GROUP. PFC models the movements and interactions of stressed assemblies of spherical particles either in or getting contact with wall elements. The particles may be bonded together at their contact points to represent a solid that may fracture due to progressive bond breakage. Every particle is checked on contact with every other particle at every timestep. Thus PFC can simulate not only failure mechanisms of rock slopes but also the run out of a detached and fractured rock mass. The run out model using PFC consists of two basic elements: two dimensional wall elements which represent the non moved ground and spherical elements representing the rock mass particles in motion.
Parameter variations showed that the parameters necessary to get results coinciding with observations in nature in the particular cases are completely different. The diagrams of mean particle velocities and kinetic energy over time clearly point out that some 30 percent of total kinetic energy is rotational kinetic energy in Thurwieser, whereas the contribution of rotational kinetic energy in Frank Slide is zero. Thus Frank run out is a real "slide" of a coherent mass, whilst Punta Thurwieser run out is a rock mass fall with much internal movement.
Therefore, the prediction of the run out kinematics and the fixing of the parameters is a demanding task in each case when modeling run outs.