What is Geogrid?
Geogrid is a strong polymer plastic that looks like a grid or net when laid flat. It can be manufactured by knitting or weaving, or fusing or welding polyester or polypropylene yarns together.
The original type of geogrid became available in North America in 1982. It was made by punching holes into a sheet and then stretching or ‘drawing’ the grid shape.
Compaction
During compaction, geogrids interlock with the granular material or soil that is placed on top of it. This confined space stops the underlying material from spreading or crumbling beyond its confines and creates a much more stable base material. This is also important for the longevity of your construction as it reduces the amount of force that has to be applied over a larger area and disperses the load.
Depending on the type of cellular structure used, the ribs and junctions in a geogrid can withstand a variety of tensile stresses. A biaxial design, for example, has ribs that stretch in both the machine and transverse directions, giving it equal stress resistance in two directions. This feature makes it a great choice for ground stabilisation applications.
A more complex design, such as triaxial geogrid, has additional diagonal ribs to offer increased in-plane stiffness. This makes it suitable for use in slope stabilisation applications, as it can increase the capacity of the sand and improve its performance in heavy traffic areas.
While conventional aggregate fill can be placed over geogrids, it is recommended to place a layer of non-woven geotextile directly beneath the geogrid to prevent silt from migrating up into the base geogrid material during construction activities. With a proper installation, rubber-tired machinery, such as end dumps and belly dumps, can cautiously traverse the geogrid solution at slow speeds (under five mph), dispersing aggregate fill gradually.
Reinforcement
Using geogrid to reinforce soils increases their bearing capacity. This means that they can support structures that are built on top of them, like roads or pavements. It also helps reduce subsidence and differential settlement. This is due to the ability of the ribs and apertures of a geogrid to distribute loads throughout its structure. This allows for a more even distribution of stresses and helps to stabilise soft or unsuitable soils.
There is a wide range of geogrid types available. Choosing the right one for your project can depend on what you are building or reinforcing. Uniaxial designs are good for slope reinforcement, while biaxial and triaxial options are better suited to ground stabilisation projects. These are more complex to manufacture, so they tend to be more expensive than uniaxial geogrids.
Research into geogrid materials has focused on their creep and tensile properties, as well as their long-term durability. A number of factors have been found to influence these properties, including the aperture shape, loading directions, and High Strength Plastic Geocell oxidation. A geogrid’s tensile modulus, junction strength, and flexural rigidity are important when it comes to designing a civil engineering structure. It is also crucial to consider the environment in which a geogrid will be used, as this can have an impact on its performance. A study has shown that HDPE geogrids can retain their mechanical properties for up to 120 years under a cyclic load.
Stabilisation
If you’ve ever been to a public event with an overspill car park on a grassy area, then it’s likely that you have seen some kind of reinforcement grid or mesh laid down to stop the ground from being churned up by the weight and force of vehicles. Unfortunately, those grids aren’t geogrid and they aren’t designed for use above ground.
Geogrids are primarily used to perform a stabilisation function in soil and aggregate materials, adding tensile strength through a system of interlocking vertical and horizontal ribs that create apertures in a grid pattern. Most are made of polymer plastics such as polyester, polyethylene or polypropylene, and can be woven from yarns or heat-welded from strips of material. They can also be extruded or pressed into a thin sheet and punched with a regular grid then stretched longitudinally to form the grid structure.
The most important factor in choosing the right geogrid for a particular application is its stability function. Depending on the size of the grid and its intended use, uniaxial or biaxial geogrids may be chosen.
Uniaxial geogrids are rolled out perpendicular to the wall in lengths dictated by a certified engineer’s recommendations based on the height of the wall, the conditions of the soil and subgrade and the load that the wall will need to support. A layer of geotextile fabric is usually laid over the top to prevent silt travelling up the wall over time. Some manufacturers such as Tensar, offer a composite product that combines both a biaxial geogrid and a geotextile, providing separation and stabilisation in one roll.
Drainage
Whether your project is in a landfill liner system, retaining wall or road construction, geogrids provide drainage and stability. They help to build firm working surfaces over soft ground conditions that can be problematic when bearing loads are applied. This can reduce iterative maintenance costs, and extend the lifespan of paved surfaces.
It also helps to reduce rutting, cracking and other typical issues that occur with paved surfaces over time. This can be caused by a number of factors, such as environmental conditions or the weight of vehicles and structures. Geogrids impact on the behaviour of these materials through soil interlocking, distributing load and Shear Strength Enhancement, reducing soil compression and improving permeability, so these factors do not cause damage to the surface.
There are many different types of polymer geogrids, with differences largely relating to the material used and manufacturing type. It is recommended to always consult with a geotechnical engineer to ensure that the correct product for your project is chosen.
Typically, uniaxial geogrids are chosen for slope and wall reinforcement, as they have rectangular apertures that offer high tensile strengths in one direction. This is more cost effective than biaxial geogrids, which have an equal strength in both directions. However, for applications such as roads and pavements, a biaxial product is preferred due to its improved shear strengths in both directions.
