Geogrid in Flexible Pavement

Geogrid technology has developed steadily since the products werefirst introduced in the early 1980’s. The initial geogrid products rapidlygained popularity within the civil engineering industry, principallydue to their ability to provide simple, cost-effective solutions in variousroadway and grade separation applications.They have gained widespread acceptance over the last 25 yearsas a solution to problems associated with roads constructed on softor problematic subgrades, but their use with roads on competentsubgrades has been less common. Clear, well-established, design methodology from the AmericanAssociation of State Highway and Transportation Officials (AASHTO)is now available that allows the design engineer to quantify the benefitsof using geogrids to extend pavement design life. This approach canbe applied for the design of major highways or light duty pavementsassociated with local housing or retail store developments.

Geogrid Composition

A geogrid is a regular grid structure of polymeric material used toreinforce soil or other geotechnical engineering related materials.Products are generally classified as either Uniaxial Geogrids orBiaxial Geogrids, depending on whether their strength is predomi-nantly in one or two directions. Uniaxial Geogrids are principally used in grade separation appli-cations for retaining walls and steep slopes. Biaxial Geogrids areused mainly in roadway applications to either stabilize a soft subgrade, or to provide reinforcement to the unbound base coursematerials (referred to as base reinforcement). Benefits of usingBiaxial Geogrids for base reinforcement are typically a reduction ofrequired base course material thickness and/or a significant extension in service life of the pavement structure. In base reinforcement applications, the existing subgrade is of afirm nature or has been rendered such through the use of a subgradeimprovement technique. One of the principal failure mechanisms ofa pavement under these firm subsoil conditions is rutting–resultingfrom progressive lateral movement of the aggregate base courseduring traffic loading. The amount of lateral movement can be reduced greatly byincluding a Biaxial Geogrid within or at the bottom of the basecourse layer. Partial penetration of coarse aggregate particlesthrough the geogrid apertures and subsequent compaction, resultsin “mechanical interlock” or “confinement” of the aggregate particles.

Application Benefits

The principal benefit of using a geogrid within the unbound aggregatecomponent of a flexible pavement is less rutting at the surface. Thisis due to reduced lateral spreading of the unbound aggregate. However, an additional feature of the reinforcement is that thegeogrid confined aggregate results in a much stiffer base courselayer and a lower dynamic deflection of the pavement structureduring traffic loading. Fatigue cracking of the asphalt is thereforealso reduced due to the presence of the geogrid reinforcement.In order for geogrids to work successfully in base reinforcementapplications, they must have the capacity to facilitate efficient loadtransfer between the aggregate and the geogrid. Webster (1992) reported a large-scale research program undertakenby the U.S. Army Corps of Engineers to investigate and determinethe key physical properties of a geogrid required to create optimalinteraction and load transfer. A summary of the key material propertiesdetermined in the study are presented in the table below.Key geogrid properties as determined by the U.S. Army Corps of Engineers.

Expanded Use

As the population of our towns and cities continues to expand rapidly,new or recently constructed housing in the form of sub-division devel-opments are becoming increasingly commonplace. One of the morefrequent problems associated with the roads in these developmentsis adirect result of their method of construction.Phased construction has become an extremely common practice,particularly in residential developments. In order to build a roadwayto gain site access, contractors will initially placethe aggregate component of the pavement and,usually, a thin asphalt layer on top. This techniqueis particularlyuseful when local trenches arerequired for installation of utilitypipes and cables. Pavement distress in the form of asphalt crackingat the surface is common on phased roads withinsub-divisions. In many cases, these “alligatorcracks” start to appear within a very short periodof time following construction – perhaps as little asone or two years. The simple solution to this problem is a layer ofBiaxial Geogrid installed at the bottom or withinthe base course during initial construction. Forrelatively little additional expense at the start of construction, thelifetime of their road is extended enormously, while expensive anddisruptive rehabilitation or reconstruction activities are avoided.Another use of geogrid technology can be found in the developmentof pavements around retail stores. Typically, thicker heavy-duty pavements are adopted in the loading areas around suchstores, while thinner lighter duty pavements are used for the carparking areas.One of the main problems associated with this approach is thepotential for a “bath tub” effect – this is where the subgrade is at alower level in the areas of the heavy duty pavements. These areas areprone to water ingress and build up resulting in a reduction in thelong-term strength of the pavement. In colder regions, these areas are also more susceptible to theeffects of freeze-thaw activity. Both of these situations result in areduction in the design life of the pavement but there are additionalpractical problems for the contractor associated with this more complicated method of construction.In addition to offering protection against the “bath tub” problemsdescribed above, the reinforced sections offer significant material cost savings. Additional benefits result from increased speed of construction – fewer stake out procedures, less undercut/disposal offill, simpler construction, etc.

A Glimpse Into The FutureCurrently, AASHTO provides guidelines for the design of flexible pave-ments in their current design guide (AASHTO,1993) and in InterimStandard PP46-01 (AASHTO, 2001). New pavement design approaches,based on advanced mechanistic-empirical (M-E) principles, are beingdeveloped and refined by AASHTO and other entities. A few DOT’s have already made the leap to improved M-E designmethods, but most are still awaiting official publication of AASHTO’snew design guide which will advocate adoption of this approach topavement design. Official publication of the new AASHTO designguide may still be several years out, but the availability of a M-E baseddesign method incorporatinggeogrids within the pavement structureis currently being finalized by The University of Illinois atChampaign-Urbana.

Source :

R.D. Holtz, Ph.D., P.E., Geosynthetics Soil Reinforcement, Department of Civil & Environmental Engineering, University of Washington