5 Must-Have Features in a Biaxial Geogrid

How Geogrids Work

Geogrids refer to geosynthetic materials made up of interconnected parallel sets of tensile ribs with apertures to allow strike-through for the surrounding aggregates (soil, stone, or geotechnical material). These materials work by building a firm working surface over soft, loose soil. They are designed to interlock the granular or soil materials placed over them into place. Using their open apertures, the geogrids confine the materials within to increase the strength and compactness of the overlying aggregates.

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In essence, geogrids help to redistribute the load over a wider area. This mechanism has proven invaluable to constructing pavement, retaining walls, and reinforcing foundation construction.

How to Lay Geogrids

Geogrids should be installed while observing all local building codes and standards. Geogrids have specific installation methods depending if they are to be used to reinforce a solid mass behind a retaining wall or on a slope or if they are to be used to improve the design of a parking lot or highway. End-use specific installation guides are available but there are a few general rules of thumb that apply to all installation cases. Make sure the geogrid is laid flat and does not have any creases. Ensure that there are no spaces between panels of geogrid so that there is no gap in reinforcement by making sure to overlap and geogrid seams as instructed by the design engineer.

Types of Geogrids

There are three types of geogrids. Each of these grids is specially designed for specific construction needs with different tensile strengths. They include:

1. Uniaxial geogrids - These grids are designed to withstand stress in a single direction. They are made by having the majority of their strength in the longitude direction with their tensile strength originating from the machine direction. They can be knitted, woven, or extruded. They are primarily used for retaining walls, embankments, and landfill liner systems over soft soils and steep slopes.
2. Biaxial geogrid - These grids are designed with equal or similar tensile strength both in machine and cross direction. This means they can distribute loads over wider areas and are better suited for base stabilization applications. These grids are used in constructing foundations for railroad truck beds, roadbeds, pavements, parking lots, weak subgrades, and airport runways.
3. Triaxial geogrids - These grids are very similar to biaxial geogrids and are used in the same applications.

Application of Geogrids in Construction

Applied in construction of retaining walls

In retaining wall construction, geogrids are used to reinforce the soil mass behind a wall or slope. This gives several benefits during construction including a reduction in material being brought in from off site with the construction of traditional gravity retaining walls.

Characteristics of a geogrid reinforced retaining wall system

• They have higher adaptability to foundation deformation.
• They are flexible enough to hold in an earthquake.
• The construction is more economical.

Application in Base Reinforcement

Geogrids are often used to stabilize the soil below highway, road, and parking lot surfaces.

Application in pavement construction

Geogrids are used in pavement construction to solve the problem of a soft subgrade. They are also a great way to reduce base thickness and time taken during construction. By improving the strength of the subgrade, geogrids can effectively increase the lifespan of the pavements.

Advantages of Geogrids in Construction

Geogrids provide stabilisation or reinforcement to enhance the performance of soils, as well as separation between soil and aggregate layers. There are four main types of geogrid, each with different physical properties or characteristics. The most suitable type of geogrid in any given application will depend on both these physical properties along with the compatibility of the aggregate and soil.  

In this guide, we’ll be looking at the physical properties of geogrids and how they impact suitability, before looking more closely at the key properties of Tensar geogrids in particular.

Use the links below to jump to the section you’re most interested in:

• What is geogrid?
• Types of geogrid
• Geogrid size and aperture 
• Geogrid strength
• Ultimate tensile strength
• Junction efficiency
• Radial stiffness
• Elastic modulus
• Long-term design strength
• What are the requirements for a geogrid?
• Physical properties of Tensar geogrids
• Key takeaways
  
  
  

Tensar geogrids can be used to solve civil and geotechnical engineering problems in or on the ground. They help design engineers to achieve substantial time- and cost-savings across a range of applications, from roads and railways to working platforms and foundations. If this could be of use for your upcoming project, speak to our team today.

Geogrid strength

Geogrid strength can refer to a number of different properties, including:

• Ultimate tensile strength
• Wide-width tensile strength
• Single rib tensile strength
• Long-term design strength
  
  

While the design strength of geogrids for the function of reinforcement is a critical property, for the function of stabilisation, other physical characteristics should be considered when assessing their potential performance, such as junction efficiency, rib height, and radial stiffness. We will go into each characteristic in more detail below.

Ultimate tensile strength of geogrids

Ultimate tensile strength (UTS) is the maximum amount of load a geogrid can handle before performance is compromised.

This is established through tensile testing, where the geogrid is stretched until it breaks.

Many geogrid specifications focus on tensile strength, equating high UTS with better performance. In reality, however, UTS is irrelevant, particularly when a geogrid is used in the design of roads or temporary working platforms.

In these cases, the ultimate tensile strength of a geogrid alone is actually a poor indicator of performance. This is because the ‘tensioned membrane’ effect—the strength that a geogrid offer when stretched or strained—does not offer appropriate support for the layers above it.To work, the geogrid has to be stretched. But when a load is placed on it, the geogrid will curve to accommodate this load, similar to what happens when a person sits on a hammock.

Consequently, the pavement will suffer deformation at the level where the reinforcement geogrid is placed. Deformation will also likely appear at the road surface level in the form of rutting, cracks and potholes, reducing its operational life.

For more information, please visit Plastic Geogrid For Roads.

Learn more about Why tensile strength is not a good measure of stabilising geogrid performance.

 Geogrid junction efficiency

The physical  property of geogrids known as junction efficiency is the measure of the strength of the node compared to the strength of the rib, expressed as a percentage and indicates the ability of the geogrid to transfer loads from one rib to other ribs in different directions:

Junction efficiency—and not junction strength—is a parameter the European Assessment Document (EAD) characterises and associates with stabilisation geogrid performance.

The EAD sets out parameters the European Organisation for Technical Assessment (EOTA) confirms are specific to the distinct function of stabilisation. No such link has been made between any junction parameters and reinforcement geogrids.

Junction efficiency vs. tensile strength

Where stabilisation geogrids are used to take advantage of the ‘confinement effect’, junction efficiency is the more important physical property in relation to performance in road and temporary working platforms than tensile strength.

The confinement effect—where the aggregate is locked into the apertures of the geogrid and pushes up against its ribs, preventing the material from rotating or moving.

Load is put against the ribs, which are held in place by the junctions. As a result, the efficiency of the junctions compared to the rib is one characteristic that will influence the performance of a stabilisation geogrid.

Radial stiffness

The next physical property of geogrids, known as radial stiffness, is in-plane stiffness measured in a single direction across the geogrid. The mean radial stiffness is the average stiffness measured in multiple directions, while the radial stiffness ration is an indication of the uniformity (isotropy) of radial stiffness. These two characteristics provide indicators of the ability of a stabilisation geogrid to evenly distribute a load through 360 degrees, without deforming elastically.

Mean radial stiffness and radial stiffness ratio are characteristics of stabilisation geogrids associated with the stabilisation function, with the geogrid acting as a component of a mechanically stabilised layer in road applications. The overlapping hexagonal structure of Tensar’s TriAx and Tensar InterAx geogrids provides a more uniform response to the load traffic imposed on the road than geogrids with square or rectangular apertures.

Elastic modulus

Elastic modulus is another physical property of geogrids related to stiffness, but not related to performance in ‘normal’ applications.

Finite element analysis (FEA) sometimes requires a geogrid’s elastic modulus. However, Tensar’s own research has confirmed that the effect of a geogrid should not be modelled on individual product parameters. We have developed an FEA module—for more information, contact the Tensar Technology team.

Long-term design strength

Long-term design strength (LTDS) is a relevant property of geogrids in reinforced soil applications—including retaining walls and slope reinforcement—where layers of geosynthetic materials (such as geogrids) are placed within the fill used to form the finished structure.

Unlike roads, which bear a constantly changing load, reinforced soil carries a permanent load throughout its operational life, which could be up to 120 years. Consequently, the design of the structure and load-bearing components (including the geogrids) should reflect this.

Factors used to calculate LTDS

Creep strength

Polymers are viscoelastic, meaning their strength and stiffness are affected by temperature and how frequently or long they bear a load. Under a constant load, polymer geogrids will stretch very gradually as their physical properties change.

Creep strength can be assessed by subjecting geogrids to long-term loading. This involves suspending different-sized weights from the geogrid in temperature-controlled conditions and measuring and recording the strain for a standard duration of 10,000 hours, just over a year.

Partial reduction factors

Partial reduction factors—such as environmental effects and the effect of damage caused during installation—should be applied before using creep strength in LTDS calculations.

Uniaxial geogrids are also available in different grades, but their geometry is similar across the range.