Written by: Rich LovdalLet’s talk about how geogrid reinforcing mesh works in a Segmental Retaining Wall (SRW)
such as Allan Block. The most common type of geogrid is a polyester coated
mesh. Lightweight grids are typically Bi-Axial,
meaning that their transverse and longitudinal (in the roll direction) strands
are the same strength and these grids are more common in residential
applications. Commercial strength geogrids are Uni-Axial meaning their
longitudinal strands are stronger than the transverse strands. Using these Uni-Axial geogrids in a retaining
wall means that you must roll the grid layers out perpendicular to the wall so
the strong members are running perpendicular to the wall providing the most shear
strength to the wall structure.
Geogrid is typically placed in horizontal layers and has openings between the directional strands that form a mesh. The mesh interacts with the compacted soil to provide extra lateral strength to the soil particles similar to how reinforcing steel adds the same to concrete. If there is a load applied to the reinforced mass, the force will travel down diagonally through the soil and intersect the grid layers putting them into tension. The geogrid strands resist these forces strengthening the soil. The more layers of grid within a soil structure, the more internal strength the mass has to resist higher loads. This simple video shows how geogrid resists the internal forces from outside forces.
Specifically in SRW design, the grid combines with the Allan Block facing and the compacted soil to form what the industry calls a coherent gravity mass. This mass works as a composite unit to resist any outside forces from surcharges above the wall like roadways or slopes or internal forces from the active soil forces or seismic events.