Rockfall hazard assessments are increasingly done using trajectory analysis. A key parameter in such approaches is the modelling of the interaction between the falling rock and the soil. The presented study focuses on this phase in the case of a boulder impacting a scree slope. In the context of trajectory analysis, the available information on the local configuration of soil particles near the impact point and on the incident kinematic parameters of the boulder are not sufficient to perform a deterministic prediction of boulder bouncing. Stochastic approaches are then necessary to properly model the impact phase. Numerical investigtions using a Discrete Element Method are performed in order to investigate the impact process. The statistical treatment of the results leads to the definition of a stochastic impact model linking the block kinematics parameters before and after its interaction with the granular soil. The comparison between the predictions of the model and the results from half-scale experiments emphasizes the relevance of a predictive use of the stochastic impact model and the interest of using stochastic approaches in the field of trajectory analysis.