The ballast bed distributes the loads of the trains uniformly onto the track substructure and assures a firm, unshifting position of the sleepers. To withstand these dynamic impacts the ballast bed has to be very elastic. The good function depends on the depth of the ballast bed, the size of the ballast stones and the degree of fouling.
Fouling occurs under normal operational loads. This causes the edges of the ballast stones to break off, settlement of the stones and friction processes. Other causes of fouling are material rising up from the subsoil and external environmental influences. Overall, this causes an increase in the proportion of fines which reduces the elasticity of the track, the water permeability and the durability of the track geometry. As a consequence, there will be irregular settlement of the track which tamping can compensate only for short periods. From a certain point in time it will be more economical to clean the entire ballast bed.