Geogrids

When Does a Retaining Wall Need Geogrid?

A retaining wall often needs geogrid when the wall height reaches about three to four feet, but that threshold can drop when poor soils, surcharge loads, slopes above or below the wall, or tiered wall layouts increase pressure on the structure. Even shorter walls may need reinforcement when the backfill is weak, when vehicles or buildings add load near the top of the wall, or when site geometry makes the retained soil mass less stable.

What Is Geogrid Used For in Retaining Walls?

Geogrids are used for soil reinforcement when you need to support retaining walls, paving overlays, and other structures over weak or shifting soils. The geometry of geogrids and the variety of strengths, aperture dimensions, and rigidity mean that geogrids are incredibly useful for improving the structural integrity of soil structures and structures built on top of poor soils.

Under load, a geogrid can deform with the surrounding fill without tearing, which lets it handle shifting soils and improve performance during minor seismic movement.

Ground Stabilization With Geogrid

Geogrids have been in use for several decades as a way of stabilizing roads or sidewalks built on poor soils. Soft, sub-grade soil is often inadequate at handling heavy loads. Laying a geogrid provides a solid working surface upon which to build by interlocking with the surrounding fill material. In most cases, using a geogrid is more reliable and cost-effective than pouring concrete and offers better water drainage, too. A biaxial geogrid or triaxial polypropylene geogrid is the most common choice for ground stabilization projects due to its rigidity and strength in multiple directions, making it ideal for supporting heavy loads on soft soils.

Geogrids can extend the expected service life of sidewalks and roads and provide higher stability and stiffness, enabling roads to withstand heavier loads without issues. They're a low-cost and durable way of improving the surface quality of a new road.

Slope Reinforcement With Geogrid

Another common use of geogrid is slope reinforcement and wall reinforcement. In these projects, typically, uniaxial geogrid made from polyester is used to create a reinforced soil mass, which replaces the need for heavy, expensive gravity walls. This is incredibly useful, especially on steep embankments where near-vertical construction is not warranted, but also in retaining structures and geogrid retaining wall systems. Geogrids interlock with the facing units and the fill material in the structural fill zone, creating a giant mass of reinforced material creating long-lasting, structrually sound civil structures.

Benefits of Geogrid Reinforcement

• reduce the need for very thick retaining walls
• cut project costs compared with heavier wall systems
• allow the use of local backfill materials, depending on their quality
• increase load-bearing capacity and overall wall performance
• support quicker natural vegetation regrowth on slopes and embankments
• provide a more environmentally friendly reinforcement option
  

Geogrids are available in a range of sizes and types, and it's important to choose the style that's best suited to your intended purpose. Your engineer will typically specify the geogrid type and strength based on your project requirements. Some geogrids have a high tensile strength in just one direction, while others are designed to be strong in all directions but are relatively stiff. The end use, wall height, type of soil in the area you're working, and the size and angle of any structure or embankment influence your choice of grid.

How to Install Geogrid in a Retaining Wall

For a retaining wall, installation works best when you place each layer at the planned elevation and match the embedment length to the wall height, soil conditions, and loading.

1. Build the wall to the first reinforcement course, place backfill, and compact it in lifts so the base under each geogrid layer is firm and level.
2. Place the front edge of the geogrid just behind the wall face according to the wall system detail so the reinforcement connects to the block or facing unit correctly.
3. Roll the geogrid perpendicular to the wall face so the machine direction runs back into the reinforced zone, where it can resist the wall load.
4. Pull the geogrid taut and remove slack before placing fill so the layer stays flat and engaged.
5. Place drainage aggregate directly behind the wall face and reinforced backfill over the remaining geogrid length without disturbing the grid.
6. Compact the backfill in lifts over the geogrid and repeat the process at the specified vertical spacing for the wall design.
7. Use a single continuous piece for the required embedment length rather than joining two pieces of geogrid together to reach depth, because embedment length should match the wall height, the conditions of the subgrade and soil, and the supported load.
8. Keep heavy equipment back from the wall face until you have enough cover over the geogrid to avoid shifting the blocks or damaging the reinforcement.
   

Types of Geogrid for Retaining Walls

There are three types of geogrids: uniaxial, biaxial, and triaxial.

Uniaxial Geogrids for Retaining Walls

The higher tensile strength of uniaxial geogrids is in the machine direction. Uniaxial geogrids are mainly used for soil reinforcement in a segmental retaining wall or steepened slope. Uniaxial geogrids are usually woven or knitted from polyester yarn into a strong structure with a polymeric or PVC coating, making them resistant to strain, creep, and natural chemicals. Extruded uniaxial geogrids are made from HDPE (high-density polyethylene).

Retaining walls rely on horizontal tensile loads perpendicular to the wall face, which is why uniaxial geogrid is typically the correct specification for soil reinforcement in mechanically stabilized earth (MSE) retaining walls. When you roll the grid back into the reinforced zone, the strong machine direction resists the pullout forces generated by the retained soil mass.

Biaxial Geogrids for Base Stabilization

For raised patios, walkways, highways, parking lots, and access roads, biaxial geogrids are the most common type. Using square holes and a more rigid structure, biaxial geogrid increases soil load capacity since they have similar tensile strength in the two directions and has limited creep. These geogrids are used for reinforcing the base and the subgrade. In unpaved or paved applications, they minimize rutting and maintain the cumulative depth.

Unlike uniaxial geogrid, biaxial geogrid carries similar strength in both directions. You’ll want to install biaxial geogrid as deep into the surface as you can, with at least 6” overlap between the pieces. This lets the aggregate interlock, preventing the base materials and subgrade from moving under the pavement surface’s load.

Keep in mind, however, that there are diminishing returns after about 12”, wherein you’ll likely need to install an additional layer of geogrid. When using this type of geogrid as a separation layer between the subgrade and the base, a non-woven geotextile is recommended to prevent silt from migrating into the base with heavy traffic.

Triaxial Geogrids for Multi-Directional Base Support

With triangular apertures and reinforced ribs, triaxial geogrids may appear differently from biaxial geogrids, but generally perform and are installed in a similar way to biaxial geogrids and are used in the same applications as biaxial geogrids.