What is a Geogrid?

Geogrid

A geogrid is a geosynthetic grid that reinforces soil and aggregate by carrying tensile load.

What a geogrid is

The FAA lists four main geosynthetic products: geotextiles, geogrids, geomembranes and geocomposites. A geogrid is the grid type. FHWA guidance lists two main types: HDPE geogrid, which is uniaxial, and PET geogrid with a PVC or acrylic coating.

A geogrid is extensible: it can deform about as much as the soil before it fails. It holds soil through friction on its surface and through passive soil resistance against its transverse ribs.

Where it is used

Most segmental block retaining wall systems use geosynthetic reinforcement, and geogrids are the most common type. In a pavement, a geosynthetic can be the first layer of mechanical stabilization over soft fine-grained soils. It then forms a working platform for the pavement layers above.

Geogrid in airport pavement

FAA AC 150/5320-6G calls for subgrade improvement where the mean subgrade strength is below CBR 3. The methods include chemical, mechanical and geosynthetic stabilization. The same AC gives the FAA position on geosynthetics:

• The FAA gives no reduction in pavement structure for any geosynthetic.
• The most common use on airports is a layer that stops fines from migrating into a base or subbase.
• The geotechnical report must record the type of geosynthetic and its expected long-term benefit.

AC 150/5370-10H has a geogrid paragraph in Item P-154 Subbase Course. A project uses it only when the designer fills it in. A geotechnical engineer must design the geogrid, the FAA must accept it, and the material is accepted against ASTM D4759. The FAA is still researching geosynthetics under aircraft loading.

For inspection

A geogrid lies under the pavement layers, so the pavement surface does not show it. To learn whether a pavement has a geogrid, read the geotechnical report. A surface survey records the distress of the surface layer only.