Geotextiles perform the functions of filtration and separation in the construction of many hydraulic engineering works. They are permeable and allow water to seep through, while blocking the passage of soil particles. In road construction, this separator/filter function also prevents the penetration of granular sub-base layers into soft subgrades which can cause failure of the pavement surface or structure.
Typical filter applications include French drains, coastal protection structures, revetment construction, slope-surface protection and drainage layers in earth dams. The use of geotextiles reduces the need for layers of sand or gravel and reduces the environmental impact of construction.