Geogrids are a class of geosynthetic materials made of polymer materials with widespread transportation, infrastructure, and structural applications. Geogrids are now routinely used in soil stabilization applications ranging from reinforcing walls to soil reinforcement below grade or embankments with increased potential for remote-sensing applications. Developments in manufacturing procedures have allowed new geogrid designs to be fabricated in various forms of uniaxial, biaxial, and triaxial configurations. The design flexibility allows deployments based on the load-carrying capacity desired, where biaxial geogrids may be incorporated when loads are applied in both the principal directions. On the other hand, uniaxial geogrids provide higher strength in one direction and are used for mechanically stabilized earth walls. More recently, triaxial geogrids that offer a more quasi-isotropic load capacity in multiple directions have been proposed for base course reinforcement. The variety of structures, polymers, and the geometry of the geogrid materials provide engineers and designers many options for new applications. Still, they also create complexity in terms of selection, characterization, and long-term durability. In this review, advances and current understanding of geogrid materials and their applications to date are presented. A critical analysis of the various geogrid systems, their physical and chemical characteristics are presented with an eye on how these properties impact the short- and long-term properties. The review investigates the approaches to mechanical behavior characterization and how computational methods have been more recently applied to advance our understanding of how these materials perform in the field. Finally, recent applications are presented for remote sensing sub-grade conditions and incorporation of geogrids in composite materials.

A full-scale unsurfaced test section was constructed to evaluate the performance of two recently developed innovative geogrids, referred to as NX-1 and NX-2, having a unique combination of hexagonal, trapezoidal, and triangular aperture shapes, rib aspect ratios greater than 1.0, and a coextruded, composite polymer sheet designed to improve aggregate and geogrid interaction. The test section consisted of a 25-cm-thick crushed aggregate surface layer placed over a weak clay subgrade. Simulated truck traffic was applied using a load cart outfitted with a single-axle dual-wheel truck gear. Rutting performance and instrumentation response data gathered from earth pressure cells and single-depth deflectometers were monitored at multiple traffic intervals. It was found that the geogrids improved rutting performance when compared with an unstabilized test item, and NX-1 was found to be the best performer of the two geogrids. Calculated traffic benefit ratios ranged from approximately 1.2 at low levels of rutting up to approximately 13.0 at higher levels of rutting. Instrumentation response data indicated that the geogrids reduced measured pressure and deflection near the surface of the subgrade layer. Pressure response data in the aggregate layer suggested that the geogrids redistributed applied pressure higher in the aggregate layer, effectively changing the measured stress profile with depth.

Geogrids are commonly used in paved and unpaved roadways to stabilize base and subbase layers by laterally restraining movements of unbound aggregates under applied wheel loading, often referred to as interlocking. The geogrid-aggregate interlock is a function of the match between the geogrid aperture size and geometry and the grain size and shape characteristics of the stabilized aggregates. Open-graded coarse aggregates (OGA) are commonly used as an alternative to dense-graded materials in permeable base/subbase applications as well as in railway ballast to facilitate drainage. This paper describes an experimental study recently conducted to investigate the repeated load-deformation behavior of a geogrid-stabilized OGA material in large-scale triaxial testing. Two multi-axial geogrids with the same aperture patterns but different aperture sizes were tested. The aperture patterns include different sizes of hexagonal, trapezoidal, and triangular openings. The multi-axial geogrid with the larger apertures better matched the OGA material properties to provide a reduction in permanent deformation in the test specimen. The improved interlocking and the related local stiffness enhancement were successfully quantified using embedded shear wave transducers termed bender elements. To stabilize aggregates with relatively uniform grain sizes, for example, AASHTO No. 57 stone studied here and railroad ballast, the ratio between the aperture size and a representative particle size (i.e., D₉₀) served as a good indicator of the effectiveness of the geogrid. The findings of this experimental study therefore provided valuable information on the governing properties and mechanisms that play a role in geogrid-aggregate interaction with multi-axial geogrids.

This study documents field measurements from controlled test sections with in-ground piezoelectric earth pressure cells embedded above and below different polymer geosynthetic materials placed at the interface of soft clay subgrade and crushed limestone subbase. A control section with no geosynthetic layer was monitored for comparison. In-ground stresses were monitored during vibratory roller compaction and trafficking under a heavy loaded truck. One of the geogrids is a relatively new product for which there is less field performance data than the more traditional biaxial geogrids. Results from this study demonstrate better performance of the new geogrid test section compared to other test sections as verified using compaction and rut depth measurements. Improved performance is attributed to higher lateral restraint of the subbase layer and reduced horizontal stresses in the soft subgrade layer under loading.

Geogrid inclusion in low-volume road applications has been shown to improve rutting performance. Although rutting performance and traditional instrumentation response data provide meaningful insights into pavement behavior, they do not provide a direct measurement of material behavior near the geogrid. Recently, the University of Illinois at Urbana-Champaign has developed field-deployable shear-wave transducers, that is, bender elements to measure the localized stiffness enhancement attributed to geogrid inclusion in unbound aggregate layers, which have been verified in laboratory and limited field applications. Bender element sensors were installed in a full-scale accelerated pavement test experiment at the U.S. Army Engineer Research and Development Center that included a recently developed multi-axial hexagonal geogrid. The sensors were installed directly on the geogrid and 10 cm above the geogrid in the unbound aggregate layer to measure the geogrid zone of influence. The test item was trafficked with a dual-wheel truck gear, and bender element measurements were made at select intervals throughout traffic application. To estimate the unbound aggregate moduli, the data were analyzed using two techniques: first arrival time and peak-to-peak arrival time. The bender element sensors were found to be sufficiently robust to survive installation, construction, and trafficking. Calculated modulus values 10 cm above the geogrid were found to be higher than those directly on the geogrid, which was an unexpected finding. However, a forensic investigation revealed moisture and fines migration into the bottom of the unbound aggregate layer, which could have dampened the shear-wave signal, resulting in lower calculated modulus values.

For as long as geogrids have been used in civil engineering applications, the basis for acceptance and rejection of these products has been through the utilization of method specifications, whereby minimum values for mechanical and geometrical properties are defined by the design engineer. With the advent of new technologies such as triaxial geogrids, performance-based specifications offer an attractive, more reliable alternative for design engineers in so much as the performance of the composite structure is defined to yield the desired serviceability. This paper reviews the history of geogrid specifications in the Americas, the means by which performance benefit can be quantified through small- and full-scale testing and offer suggestions on how the designer can specify performance in lieu of material/mechanical properties as the basis for product acceptance and/or rejection.