Geosynthetics are placed in pavement bases to perform one or more of the following functions:

Base reinforcement results from the addition of a geogrid or composite at the bottom or within a base course to increase the structural or load-carrying capacity of a pavement system by the transfer of load to the geosynthetic material. The primary mechanism associated with this application is lateral restraint or confinement of aggregates in the base. Where very weak subgrades exist, geosynthetics can increase the bearing capacity by forcing the potential bearing capacity failure surface to develop along alternate, higher strength surfaces. Geogrids may also be considered for use in locations where chemical stabilization of the subgrade is not desirable due to possible reaction with sulfates in the subgrade, or not practical because of expedited construction concerns, particularly in urban settings. Geogrid has also been used in multi-layered repair of roadway embankment slope failures, and is described in the USDA publication

0577 1204 – SDTDC, “Deep Patch Road Embankment Repair Application Guide,” October 2005

(see Figure 7-3).

There have been assertions that the resultant increase in restraint or confinement should allow for design of thinner structures using these products versus structural designs which do not; however, their benefits may only be noticeable over the long term, and there appears to be an absence of long-term controlled monitoring. For purposes of geosynthetic reinforcement, the Maintenance Division, Pavement Asset Management Section, recommends that its application be viewed as an “insurance policy” rather than a “modulus- multiplier” or structure-reducing product.

Geosynthetics used for separation have classically been applied to prevent subgrade soil fines from migrating into the unbound base (or subbase) or to prevent aggregates from an unbound base (or subbase) from migrating into the subgrade. A small amount of fines introduced into the granular base can retain moisture and significantly reduce the internal friction angle, rendering the flex base weaker. Potential for these circumstances increases where wet, soft subgrades exist. Typically, a geocomposite will be used for this application, placed at the subgrade/unbound base interface. Geotextile separators act to maintain permeability of the base materials over the life of the section, and they allow the use of more open-graded, free-draining base and subbase materials. Another form of separation is being increasingly explored in Texas where there is a high potential for reflective cracking originating in the subgrade or chemically-bound base/subbase. A grid or composite is used to dissipate stresses induced by the opening crack. Longitudinal edge cracking is particularly an issue in areas where moderate to high plasticity index (PI) soils are exposed to prolonged cycles of wetting and drying. Geogrids will typically be employed at the subgrade/bound base interface, or if a flex base is placed above a bound base (e.g., full-depth reclamation/recycling [FDR] projects), the grid may be placed at this location. Grids should be a minimum of 10 ft. wide to reduce the potential for longitudinal cracking due to edge drying.

The function of filtration is to allow for in-pavement moisture transfer but restrict movement of soil particles; hence, composites or fabrics that are placed for the classical purpose of separation will usually incorporate this function as well.

Geosynthetics used in pavement drainage have been limited to addressing problematic locations, typically in a reactive manner. Retrofitted pavement edge drains often used when the structure cross section changes transversely (e.g., rigid pavements with flexible pavement shoulders, widening using different structure) is an example of using geosynthetics to expedite lateral drainage of trapped moisture from within the pavement structure. Vertical moisture barriers using an impermeable membrane have been used to prevent moisture ingress through permeable seams from adjacent land into the roadway substructure.