Geotextile material provides separation, which preserves the integrity and extends the life of the road surface layer. Over time, vehicle load pressure causes subgrade soils to migrate into the aggregate base of the pavement section. Contamination of the aggregate base by the subgrade results in the reduction of the effective base thickness to less than originally designed. This concept is illustrated in Figure 30. Geotextiles prevent the subgrade materials from migrating into the aggregate base, thus increasing pavement life.
Filtration is one of the functions most widely performed by geotextiles.
In both the filtration and separation functions, water is permitted to pass through the geotextile. Occasionally, some confusion arises between the separation and filtration functions in this regard. A distinction may be drawn between the two with respect to the quantity of water involved and the degree to which it influences geotextile selection. In the filtration function, the volume of water moving through the fabric is a key design element specifically addressed in the design and selection of the geotextile. It must be able to convey a certain quantity of water across the plane of the fabric through-out its design life to prevent the buildup of water pressure.