Filter, Graded

a device in headworks that consists of several layers of unconsolidated material with increasingly coarse particles in each layer. The material may be sand, gravel, rubble, or pebbles; the coarseness of the particles increases toward the seepage line. Graded filters are used mainly in earth dams and other hydraulic-engineering installations erected on soft foundations. Such filters prevent seepage from washing fine soil particles from the body and base of the installation.