What is Geogrid and how does it work?

What is Geogrid?

Geogrid, also known as geogrid mesh or stabilisation mesh, is a type of geosynthetic material used to provide stabilisation and reinforcement to soils and similar materials. Made from polymer plastics, typically polypropylene, polyethylene, or polyester, geogrids consist of a series of interlocking vertical and horizontal ribs that create apertures (open spaces) in a grid pattern. Often this will be a pattern of square holes, making the geogrid look like a rigid plastic netting, but patterns of rectangles or triangles are also common, depending on the make and the intended application - more on this later.

How does a Geogrid work?

Geogrids are, basically, designed to prevent the movement of soil and other granular materials, be it beneath a pavement to reduce the impact of dynamic loads or behind a retaining wall to reduce the pressure against it. They achieve this through the use of their apertures, which allow the material placed on top of them to strike through the geogrid and create interlocking pockets between the high tensile ribs. This essentially creates a composite material that holds together better and distributes weight more evenly than either material can alone, helping to prevent concentrated loads from causing structural failure or contributing to the erosion of the base material and subgrade.

If you imagine holding a clump of soil in one hand and then pressing down on it with the other, what would happen? The soil clump would lose its shape, either becoming flatter and more spread out, or it would crumble and fall away, depending on its consistency. Now, imagine putting the same clump of soil into a square plastic mould; what would happen then? The pressure of your hand would compact the soil, but the mould would stop it from spreading or crumbling beyond its confines. Thus the soil in the mould scenario would move significantly less than the non-confined soil and create a much more stable base material. In its simplest form, this is what geogrid does but on a larger scale.

Compared to other geotextile products, geogrids can feel quite stiff. This is because the polymer material is effectively stretched out to create a high tensile strength in one or both rib directions, commonly known as the machine (or longitudinal) and transverse (or cross) directions. This, along with the strength of the joints, or nodes, where the ribs intersect, is key to the success of any geogrid. The material that fills up each aperture bears against the ribs that contain it, transmitting the load along the connected ribs via the junctions and distributing the load over a wider area. This only works if the ribs and the junctions are strong enough to withstand the tension.

Keep reading for a more technical description of the forces at play in a successful geogrid installation. Otherwise, skip down to the next section to learn more about how geogrids can be used.

How does geogrid help to stabilise soil?

Geogrid soil stabilisation relies on three things; the creation of a Tension Membrane Effect to improve the Bearing Capacity of the ground and give it increased Lateral Restraining Capability. Let’s take a more detailed look at each of these now.

• Tension Membrane Effect

When used as a geotechnical engineering term, the Tension Membrane Effect describes the stabilising effects of geogrids on a soil foundation. It is based on the concept of vertical stress distribution and the ability of a geosynthetic sheet to be deformed and absorb forces through tension. When a Geogrid is placed over or within the soil, it acts as a framework, reinforcing the subgrade layers and creating a “tension membrane” that creates an even soil distribution. This tension membrane helps to alleviate a number of geotechnical issues that can affect the stability of a soil foundation, such as subsidence or differential settlement. By providing increased strength through the Tension Membrane Effect, geogrids can help to reduce the risk of geotechnical issues and improve the safety and stability of soil foundations.

• Improvement of Bearing Capacity

Bearing capacity is an essential concept in geotechnical engineering, as it helps to determine the load-bearing capabilities of the soil, i.e., the capacity for soil to support loads applied from the ground above. The bearing capacity of a geogrid is defined as its ability to distribute and transfer those loads over an area that extends both within the geogrid itself and beneath it. Soil reinforcement geogrids are, therefore, used to increase the bearing capacity of the soil and help ensure stability for structures built on top. Additionally, geogrids are used to strengthen weak or soft soils and reduce settlement. Most geotextiles and geosynthetic materials can do this to some degree. However, since a geogrid bears load from above and distributes it over a large area below, the bearing capacity of a geogrid is much higher. Depending on the geogrid type and loading conditions, bearing capacity can vary from a few kN/m2 up to hundreds of kN/m2, helping to optimise the design in a wide variety of geotechnical engineering projects.

• Lateral Restraining Capability

The Lateral Restraining Capability (LRC) is a geosynthetic solution that stabilises soil and increases road performance. It helps to ensure the safety of highways, roads, and pavements by providing lateral restraint to geogrid reinforcement systems. In simple terms, the stresses produced by the wheel loadings of vehicles driving over the road surface results in the lateral movement of the aggregates beneath. This, in turn, affects the stability of the whole pavement arrangement. Installing geogrid in the soil beneath helps to increase its ability to resist this lateral movement of material by providing uniform distribution of stress over a wide area which minimises displacement and improves the road’s stability. The Lateral Restraining Capability ensures that geogrids are held firmly in place, preventing them from slipping or losing their stiffness. This helps avoid costly repairs and maintenance needs in the long run.

It is the combination of these three mechanisms that make geogrids so effective at stabilising and reinforcing soil and similar materials.

What are Geogrids used for?

As already mentioned, geogrids are most commonly used in construction, landscaping, and hardscaping projects for a variety of applications. These tend to fall into one of two categories; ground stabilisation and reinforcement or slope stabilisation and reinforcement. Let’s take a look at each one in a bit more depth.

Can Geogrid be used as a grass support system?

Have you ever been at a public event with an overspill car park on a grassy area and seen some sort of reinforcement grid or mesh laid down to stop the ground from getting churned up? Well, that probably wasn’t geogrid. While it does look like they could do the same job, geogrids simply aren’t designed for use above ground, where they can be subjected to the direct force and weight of vehicles.

In terms of overspill car parks, pedestrian grassed areas, golf course buggy routes etc., there are two potential alternatives you should look for instead:

1. A permanent panelled system that is virtually invisible from the surface once in-filled, such as Wrekin’s CellTrack range. These are designed for quick and easy installation with a simple interlock panel system that incorporates small ground spikes for anchorage.
2. A versatile grass support system, which can be installed on already established lawns and park areas, such as Wrekin’s Turf Mesh range. These provide great versatility as a temporary system or can be left in position to become a permanent and integral reinforcement mesh.

If you are more interested in this sort of solution than geogrid, call or email us today to see how we can help.

What types of Geogrid are available?

Since its inception back in the 1950s, geogrid design has constantly been evolving to better suit the needs of the construction and landscaping industries. There are now several different types of geogrid available on today’s market that are designed and manufactured for specific applications. But how do you know which kind of geogrid is right for your project? Well, let’s start by taking a look at the different designs available.

Geogrid Designs

As of November 2022, there are currently two main geogrid designs, each with different geometric and structural index properties. Understanding these differences is crucial for selecting the most appropriate geogrid for your project.

The primary concern when it comes to the suitability of a geogrid is the direction of its tensile strength, i.e. upon which set of ribs will the stress of the application need to be absorbed?

Geogrid manufacture types

The other decision you have to make when choosing the right geogrid for your project is the manufacturing type. Typically, most polymer geogrids are either extruded, woven, or bonded, but what is the difference, and does it really matter? Let’s find out by taking a closer look at each one.

Which type of Geogrid is best?

Despite there being a fairly wide variety of geogrid types available to choose from, which may seem a little bit daunting at first glance, the choice of geogrid design largely depends on your application - choose uniaxial for slope reinforcement and biaxial or triaxial for ground stabilisation. In terms of the manufacturing type, they all produce high-quality products that are fit for use in all geogrid applications, so it can largely come down to price and availability. For roadbed applications, however, extruded “punched and drawn” geogrids have consistently tested well and are often considered the best choice. If you have any doubts about which type of geogrid would be best for your project, however, it is always best to speak to a professional. Knowing the right tensile strength required for the intended application is the key to success, and it can be tricky if you aren’t 100% sure what you are doing.

What are the benefits of using Geogrid?

As already discussed, there are a great many benefits to using geogrids in construction and landscaping projects. Some of the key points to remember about geogrids, however, are that they:

• Can be easily installed in any weather conditions.
• Are highly resistant to chemicals, UV radiation, soil microorganisms, and mechanical damage.
• Promote soil stabilisation.
• Provide tensile reinforcement for foundations.
• Offer greater resistance to settling and erosion.
• Enable a more efficient distribution of loads.
• Increase load-bearing capacity of soft sub-soil.
• Increase the longevity of projects and reduce the need for maintenance.

In all types of construction and landscaping applications, installing geogrid can help to:

• Reduce project costs by allowing you to use less expensive fill materials without compromising the integrity of the build.
• Ensure proper land utilisation by increasing the strength of unsuitable soils and allowing them to meet the required properties for construction.
• Reduce asphalt maintenance needs and increase the lifespan of roads and pavements by reducing the effects of differential settlement.
• Reduce the aggregate layer thickness in unpaved roadways without a loss in performance.
• Make construction and landscaping projects safer by increasing the bearing capacity of weak subgrades.
• Enhance the safety of slopes by increasing the soil strength.
• Reduce the excavation depth required on otherwise unsuitable subgrades
• Reduce the amount of foundation material required in railway projects and limit rail ballast movement and displacement by increasing the bearing capacity.
• Provide a more cost effective alternative to poured concrete for working platforms.

For a better idea of the potential savings geogrids can provide, check out the helpful geogrid savings calculator on Wrekin’s website.

How do you install a Geogrid?

Installing geogrid is relatively simple. The key is in ensuring that the desired reinforcement or stabilisation properties are achieved through the correct interplay between the subgrade, the geogrid, and any aggregate that may be required. The difficulty in writing an installation guide is that every project will be different and require site-specific considerations to be taken into account. For that reason, this will just be a very brief overview of the basics.

Once in place, each length of geogrid should be hand-tensioned (pulled tight) to make sure the joints are taught, and there is no slack in the grid. It is often a good idea to hold the geogrid in place at this point with small deposits of fill to avoid losing that tension. Depending on the type of soil the geogrid is being laid upon, various amounts of overlap between the lengths will be required. If this is not done correctly, it can weaken the unified strength of the geogrid installation. Again, it is essential that you follow the manufacturer’s instructions on this and any guidance offered by the engineer. The same then goes for the final steps, which involve placing the cover fill and compacting it down.

Do Geogrids need maintenance?

Given that geogrids are typically installed underneath pavements and roads, behind retaining walls, etc., maintaining them would be a little tricky. For this reason, most geogrids on the market are designed to have a long lifespan, usually between 40 and 100 years, depending on the application. Once installed, it’s virtually impossible to maintain a geogrid without digging it up. It is vitally important, therefore, to ensure that you get the right type of geogrid for your project and install it correctly to get the most benefit.