Chances are, most of us will either walk over a geogrid or pass by one in our day-to-day comings and goings without ever realising it. It’s another one of those fascinatingly helpful products that get hidden away out of sight to do an important job that often goes unnoticed. In this article, we intend to address this injustice and shine some light on the many benefits of geogrid.
Surprisingly, geogrids have actually been around since the 1950s, thanks to Dr Brian Mercer, who invented a way to create polymer meshes by extruding molten plastic into grid structures. This revolutionary process, which was developed here in the United Kingdom, was later picked up by a company called Tensar in the late 1970s and developed further to increase the strength and durability of the extruded structures.
First trialled in 1980 to aid the construction of a temporary retaining wall for a railway in Yorkshire, the Tensar geogrids were a great success and soon became commonplace in construction and landscaping projects. As is the way of all great inventions, the concept of geogrids would later resonate with other companies around the world, many of whom then began to develop their own versions. Thus we now have a wide variety of makes and models available to choose from on today’s market. But just what is geogrid, and what can it do? These are the questions we shall now try to answer.
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
Improvement of Bearing Capacity
Lateral Restraining Capability
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.
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.
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.
Using Geogrid for ground stabilisation
In ground stabilisation projects, geogrids are typically deployed for three reasons:
- To provide a firm working surface - Building on soft subgrade soils can have serious load-bearing consequences if not properly reinforced. Installing geogrid in these conditions can improve the strength of soft subgrade soils, giving you a solid base for your construction.
- To enhance service life - Pavements built on highly expansive subgrade soils can be subject to environmental cracking, which reduces their service life. Installing geogrid under a pavement can help to stabilise the subgrade and reduce its susceptibility to this problem.
- To reduce the structural cross-sectionTraditionally, roadways need to be built on thick bases to increase their stiffness and stability against the loads they will be subjected to. Doing so, however, can be both costly and time-consuming. Reinforcing the soil with geogrid can reduce the thickness of the base required to achieve the same level of stability, thus reducing time and material costs while also increasing the service life of the construction.
Geogrids can be used in this way to stabilise and reinforce patios, walkways, driveways, and roads.
Using Geogrid for slope reinforcement
In slope reinforcement projects, geogrids are typically deployed to increase the structural integrity of soil backfills and prevent sliding on steep embankments. One of the most common applications for geogrid slope reinforcement is in the construction of retaining walls. Using geogrids to hold together and reinforce the soil backfill helps to prevent any ground movement behind, and the subsequent force that would be applied to, a retaining wall. As well as confining the backfill soil, which is especially useful in areas with soft soil, installing geogrids helps to distribute the load and enables retaining walls and landfills etc., to be built higher and steeper more safely.
Using geogrids in slope reinforcement projects can help to make them:
- More economical - Traditionally, the higher a retaining wall needed to be, the thicker it would have to be to hold back the soil behind it. Extra thickness means extra materials and extra cost, all of which can be reduced by reinforcing the soil with geogrids.
- More environmental - Geogrids can help to minimise the environmental impact of embankment and earth wall constructions, allowing you to reuse poor quality in-situ fill material that can help to encourage the slopes back to their natural vegetated state.
- More flexibleRetaining walls built with geogrid systems in place are also better equipped to adapt to deformations in the foundations of the construction. This means they are more resistant to the effects of earthquakes*.
*Fun Fact: Typically, 200 to 300 earthquakes are detected in the UK by the British Geological Survey each year. While most of them can’t be felt, the vibrations they send through the ground can be strong enough to shake the soil a bit. Over time and hundreds of repeated shakings, these small tremors can cause enough movement in the soil to weaken its load-bearing capabilities. However small a problem this might be in the UK, having Geogrid in place can still protect your structures from it.
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:
- 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.
- 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.
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?
Uniaxial geogrids are designed to offer maximum stress resistance in a single direction, hence being called UNI-axial. This type of geogrid is formed through the stretching of ribs in the machine direction, giving it a high tensile strength along that axis with, typically, rectangular apertures rather than square. Uniaxial geogrids are usually more economical than the other types of geogrid because they are the simplest design to manufacture. However, they aren’t suitable for ground stabilisation applications, such as under pavements and roads, where stresses must be dispersed in both directions. What they are perfectly suited for, however, are wall and slope applications where the primary reinforcement needs to be against the forces pointing either towards the face of a wall or down a slope. Structures such as retaining walls, steep earthen slopes, landfill liner systems, and soft soil embankments can all benefit from geogrid reinforcement.
- Rectangular aperture design
- High tensile strength in one direction (machine direction)
- More economical manufacturing process
- Increases soil confinement lateral restraint
- Ideal for wall and slope reinforcement applications
Biaxial geogrids are designed to offer an equal balance of stress resistance in two directions, hence being called BI-axial. This type of geogrid is formed through the stretching of ribs in both the machine and transverse directions, giving it a high tensile strength along both axes with the more typical square aperture shapes. This gives biaxial geogrids the ability to distribute loads over more expansive areas than usual, allowing for an increased capacity in base stabilisation applications. While suitable for use in wall and slope applications, the more complex design of the biaxial geogrid requires a more costly manufacturing process, which in turn makes for a more expensive, and therefore less economical, product than the uniaxial design for that purpose. Biaxial geogrids are much better suited to base stabilisation applications such as in the construction of car parks, working platforms on weak subgrades, construction haul roads, foundations for roadbeds, permanent unpaved roads, railroad truck beds, and airport runways.
- Square aperture design
- High tensile strength in two directions (machine direction and transverse direction)
- More rigid than uniaxial geogrids
- Helps reduce aggregate layer thickness
- Ideal for subgrade stabilisation and road/pavement optimisation applications
Alternate Geogrid Design Options
While Uniaxial and Biaxial are the two primary types of geogrid that you will find supplied by most manufacturers, there are also two alternatives worth noting:
- Triaxial Geogrid
- Geogrid-Geotextile Composites
An evolution of the biaxial design, triaxial geogrids have been developed by Tensar for improved performance in soft soil applications. Using a triangular aperture design instead of squares, the TriAx® geogrids have additional diagonal ribs that form a hexagon pattern of triangles that increases its in-plane stiffness.
In heavy silt applications, a layer of geotextile fabric is generally recommended to separate the subgrade and base material to prevent the silt from travelling up over time. To save time during installation, some geogrid manufacturers offer a composite product that combines a biaxial geogrid and a geotextile fabric, providing both separation and stabilisation in one roll.
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.
Extruded geogrids, also known as “punched and drawn” geogrids, are the descendants of the original unitised geogrids invented by Dr Mercer. They are made from flat sheets of extruded plastic, usually high-density polypropylene or polyethylene, that are passed through a punching machine to cut out the desired aperture pattern. After this, the material is then stretched in the desired direction (machine direction for uniaxial geogrids and both machine and transverse directions for biaxial geogrids) to develop the required tensile strength. Extruded geogrids are more rigid than their counterparts and can be more expensive, but they do perform exceptionally well in high-tension applications.
Woven geogrids are a more flexible, textile-like alternative to extruded geogrids. They are made from polyester or polypropylene yarns (hundreds of continuous fibres gathered together) that are woven (or knitted) into longitudinal and transverse ribs to create the geogrid apertures. The resultant grid structure is then typically given a protective coating of either bitumen, latex, or PVC. Woven geogrids are available as either uniaxial or biaxial designs and offer high strength at low elongation. They tend to be a cheaper option than extruded geogrids and can therefore offer cost savings where high performance isn’t required.
Bonded geogrids are a relatively new development. They are made from flat ribs of extruded polyester or polypropylene that are passed through rollers to bond them together, typically using heat. This is done with automated machines that can run at different speeds to stretch the ribs in the desired direction for the desired tensile strength.
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.
Installing geogrids for use in retaining walls
Uniaxial geogrids need to be rolled out perpendicular to the wall in lengths that should be dictated by a certified engineer’s recommendations based on the height of the wall, the conditions of the soil and subgrade, and the potential load the wall will have to support. The direction at which the geogrid is laid in relation to the wall is the most important thing to get right, however. If it is laid parallel to the wall, or you fail in any other way to follow the exact instructions given by the manufacturer, a uniaxial Geogrid will not provide the strength and stability needed to make the retaining wall safe.
Installing geogrids for use in ground stabilisation
Biaxial geogrids should be rolled out and allowed to follow the natural contours of the soil. The direction, depth, and whether or not multiple layers are required are all decisions that should be made off the back of a qualified engineer’s recommendations. In most cases, the geogrid can be laid either parallel to the road or pavement centreline or at right angles to it. Using a biaxial geogrid, which has the required tensile strength in both directions, means the direction is less important than it is for uniaxial geogrids. If multiple layers of geogrid are recommended throughout the depth of the fill, they should be evenly spaced with, typically, no more than 500mm between them. Always check the manufacturer’s instructions to be certain, though.
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.
And that’s it for our introductory guide to geogrids and their applications. Whether you're working on a construction project or landscape design, geogrids can be a helpful addition. If, after reading up on all the information available, you decide this could indeed be the solution you need, check out our range of Wrekin Biaxial Geogrids available to buy online.
If you have any questions or need further advice about geogrids, contact JDP today. Our team of experts are always on hand to help with all of your civils and drainage needs.