Trajectory analysis of falling rocks during rockfall events is limited by a rough modelling of the impact phase. In order to develop an accurate impact model, numerical investigations are held for a boulder impacting a coarse granular soil in the case of a boulder size equivalent to the mean size of particles. The soil is modelled as a non cohesive granular medium using the discrete element method. Numerical simulations emphasize phenomena involved in energy exchanges between the impacting particle and the granular medium. These results highlight the role of local grain spatial layout, boundary conditions and incident particle energy on the impact processes. The stochastic reflecting laws are finally defined further to a complete statistical analysis of the results. These laws are valid for a boulder and a medium type whatever the grain spatial layout and the incident energy.