The separation function refers to the separation of two dissimilar soils. The primary function of the geotextile is to prevent intermixing of the two soils throughout the life of the structure. Geotextiles are commonly used for separation when used beneath road-way pavement sections. Although you will not be constructing pavement sections, you may want to use some aggregate over certain sections of the road, in which case the principles described here still apply. Roadway pavements are basically structures for taking the high contact pressures from the vehicle tires and reducing that pressure through the depth of the pavement to a level that can be supported by the underlying soil.
Pressure is dissipated down through the various layers of materials within the pavement.
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.