Geogrid is typically a synthetic material characterized by woven bands of narrow elements in a regular, grid-like pattern with large voids between the woven bands. It's the tensile strength of the woven bands and the voids between those bands that lend stabilizing strength to the projects they are used in.
When used as part of a retaining wall, geogrid adds strength to the wall by integrating the fill material behind the wall with the structure of the wall itself. The large open spaces or voids between the bands of tensile material allow for the interlocking of the compacted fill materials with the geogrid. Connected by gravity or pinning systems to the body of the retaining wall (geogrid sandwiched between courses of retaining wall units), this interlocked fill material and wall are now more resistant to settling and movement. The grid acts as an anchor for the retaining wall, inhibiting it's ability to separate from the fill behind it, such as during freeze/thaw periods. This extends the serviceable life of the retaining wall as well as makes it capable of holding back greater volumes of material than if retaining wall units were used alone.
In fact, many retaining wall block manufacturers specify that retaining walls over a certain height require the use of geogrid or they risk catastrophic failure (most specify the height above which geogrid is needed as 4' of exposed wall height). Even on our own site there are several examples of large retaining walls that were built incorrectly and subsequently suffered catastrophic failure. Usually this failure occurs because inadequate backfill material was used that did not allow for the release of hydrostatic pressure, that fill material was not sufficiently compacted causing massive settling that lead to high hydrostatic pressures or pressure concentrated at specific points, or that geogrid was not used or was installed incorrectly.
Beyond retaining walls, geogrid is also used as a stabilizing force for pavements. The same voids between woven material that allowed interlock of fill material behind a retaining wall also provide for excellent interlock with pavement base materials. In this application, the interlocking material lends dramatically increased strength to the pavement's base by dispersing loads over a greater area. That load dispersal gives the pavement the ability to withstand greater loads and resist failure.
Geogrids as part of pavements have been used since the days of ancient Egypt, when papyrus mats were used to consolidate and stabilize soils.
Installing geogrid as part of a retaining wall is a relatively straightforward process, but first you need to have a couple pieces of important information: the direction of strength of the grid and the recommended placement and length of the grid within the retaining wall. Many block manufacturers will provide geogrid specifications based on wall height and soil type without charge, but if you're unsure, consult an engineering firm about your project. They can provide a stamped, engineered plan that spells out exactly the type of geogrid to use, the location for placement of each layer and the length each layer should be for maximum earth retention. Armed with that information you now only need to know the direction of strength of your selected geogrid. Geogrids are either uni-axial, providing strength in a single direction (think of this as north/south) or bi-axial, with strength in both directions of the grid (north/south and east/west). The grid must be installed with the direction of strength perpendicular to the face of the retaining wall. If your geogrid is uni-axial and is installed with the direction of strength running parallel to the face of the wall, the abilities of the geogrid will be severely compromised (unsubstantiated reports claim a reduction in strength of up to 80%).
With your design specifications and geogrid in hand, you can now install the grid. Follow the recommendations of the retaining wall block manufacturer; typically they recommend simply sandwiching the grid between courses of block, then extending the grid back into the compacted backfill (which should be roughly level with sandwiched courses of block). The grid should be kept flat and taut until fill can be placed over it and compacted. Some contractors accomplish this by using spikes, rebar or stakes to hold the grid in place while fill is being placed over the top. Care should be taken to ensure the geogrid is not damaged during placement as this can reduce it's capabilities.