Rockfall hazard assessments are increasingly done with rockfall numerical simulations. A key parameter in such models is the modelling of the interaction between the falling rock and the soil. The presented approach focuses on the impact phase modelling in the case of the interaction of a boulder with a coarse granular soil. The soil is modeled as a non cohesive granular medium using the Discrete Element Method. The statistical analysis of numerical results is carried out using Bayesian methods which leads to the definition of a stochastic impact model. The relevance of a predictive use of the model is evaluated by comparing the reflected velocities predicted by the model to results from half-scale experiments.