The permeability k in meters per second [m/s] expresses the velocity of liquid in a porous medium in water-saturated conditions and accordingly is an attribute of soils or aggregates. A certain permeability k has to be made available e.g. by joint fillings or base courses to absorb a certain design storm. For design reasons the permeability ku in unsaturated regions – like with aggregates for pavement structures – has a lower velocity and applies to the relationship ku = k/2.