To model the trajectories followed by a block falling along a slope, it is crucial the adoption of rules capable of simulating the changes in the boulder energy content due to impacts. The numerical simulation of impacts is thus fundamental for assessing the risk associated with rockfall events and for designing sheltering structures. The different approaches developed to simulate the impact of a boulder on a soil layer can be classified in three main categories: lumped mass, rigorous and hybrid. Lumped mass approaches consider the boulder as a material point and the boulder-soil interaction is evaluated by means of restitution coefficients. Rigorous approaches simulate the boulder-soil interaction by either continuum mechanics or the distinct element method. Hybrid approaches model the boulder as a point mass but the interaction with the soil is simulated by means of more advanced constitutive relationships. Both the lumped mass and the hybrid models do not generally take the influence of rotation and boulder shape into account. In order to investigate these factors, the authors improved the hybrid model BIMPAM proposed by di Prisco and Vecchiotti (2006). In this renewed version: i) a rotational degree of freedom for the boulder is added; ii) a moment- rotation relationship is defined; and iii) a toppling slider, simulating the occurrence of toppling at the end of the impact phase, conceived. The model is capable of accounting for the influence of the boulder shape. The parametrical analysis, concerning inclined trajectories on horizontal strata, clearly shows that during the impact a boulder spin originates, even in case the initial motion of the block is purely translational.
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