The rebound phenomenon, which occurs during rockfalls when a rock block impacts with a slope surface, is influenced by several parameters related not only to the slope material but also to the block and the kinematics. The paper presents a small-scale testing campaign undertaken at the LMR (EPFL) to better understand and quantify the bouncing of blocks on granular slopes. The influence of some factors related to the ground material, the block and the kinematics is examined.

Bei Steinschlagen wird die Flugbahn eines stuerzenden Felsbrockens maßgeblich durch den Abprall vom Hang bestimmt. Der Rueckprall hangt von verschiedenen Parametern ab, wobei nicht nur das Hangmaterial sondern auch Blockeigenschaften und Kinematik eine große Rolle spielen. Zum besseren Verstandnis dieses Phanomens und seiner Quantifizierung werden am LMR (EPFL) kleinmaßstabliche Versuchskampagnen auf granularen Materialien durchgefuehrt. Der vorliegende Artikel stellt die Versuche vor und untersucht den Einfluss verschiedener Boden-, Block- und kinematischer Parameter.

Le phenomène de rebond qui se presente lors de l'impact d'un bloc rocheux sur la surface d'un versant depend non seulement des caracteristiques du terrain, mais aussi de facteurs relatifs au bloc et à la cinematique. L'article presente une campagne d'essais sur modèle reduit entreprise au LMR (EPFL) afin de mieux comprendre et quantifier l'impact de blocs sur des materiaux granulaires. L'influence de divers paramètres relatifs au sol, au bloc et à la cinematique est mise en evidence.


In the context of rockfall problems and trajectory analyses, the bouncing of rock blocks, which occurs when the falling block impacts with the slope surface, represents the part within a trajectory that is the less understood and therefore the most difficult to predict.

Most of the numerous rockfall models developed all over the world represent the bouncing phenomenon in a simplified way by (one or) two overall coefficients, called the restitution coefficients. The most common formulations are written in terms of velocities or energies indicating the amount of velocity or energy dissipated during the ground impact [Hungr & Evans, 1988; Giani, 1992; Azzoni & de Freitas, 1995]:

(Equation in full paper)

The subscripts "i" and "r" stand for "incoming" and "rebounding", characterising the velocity or energy before and after the impact respectively (Fig. 1).

In general, the restitution coefficients used for trajectory calculations and stated in literature are estimated based on a rough description of the slope material (e.g. rock, scree, soil) sometimes completed by information regarding its roughness and compaction degree and the vegetation cover. However, as mentioned by several authors [e.g. Azimi & al, 1982; Falcetta, 1985; Hungr & Evans, 1988; Giani, 1992; Azzoni & de Freitas 1995] who have experienced natural and/or artificial in situ rockfalls, the analysis of impacts shows that the motion during and after impact is governed by three factors:

  • the slope characteristics (strength, stiffness, roughness, inclination, water content, vegetation…),

  • the block characteristics (strength, stiffness, weight, size, shape) and

  • the kinematics (translational and rotational velocities, collision angle, configuration of the block at impact).

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