Formation protection layer

A formation protection layer (PSS) is part of the track superstructure . It is a base layer between the subgrade (or any existing frost protection layer ) and the ballast bed , which on the one hand distributes the load and on the other serves to drain off the surface water to the side and prevents the ballast and the subgrade or frost protection material from mixing.

The installation of formation protection layers is generally carried out if there is either a non-weather-resistant rock below the formation or if the subsoil cannot withstand the planned loads of rail traffic. The formation protection layer thus acts as a "shield" for the underlying subgrade .