The physical properties of geogrids – how important are they?
by Jonathan Cook, on 19-Aug-2019 11:17:55
When we develop, design and manufacture our geogrids, we concern ourselves more with their intended use, rather than any measurement of their physical properties.
This is because the relative importance of these properties is entirely dependent on the application and may not be relevant to overall performance. A geogrid installed to stabilise the ground beneath a road, for example, requires different characteristics to one used to reinforce a retaining wall.
In fact, most physical properties used in the geosynthetics industry to gauge performance are inappropriate; ultimate tensile strength, for example, is not the most important factor in how well a geogrid will perform, particularly in road applications.
Why ultimate tensile strength is not important for geogrid performance
Ultimate tensile strength 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.
We are often asked for the tensile strength of our geogrids by prospective customers and many geogrid specifications focus on tensile strength, equating high ultimate tensile strength with better performance.
However, in reality, this measure is irrelevant, particularly when Tensar TriAx geogrid is used in the design of roads or temporary working platforms. In these cases, ultimate tensile strength alone is actually a poor indicator of performance, as the geogrid is stabilising, rather than reinforcing the granular material.
Some geogrids do rely on their tensile provide a reinforcement effect in a road pavement but this means they stretch and deform when loaded – which can lead to deformation in the road surface above.
So, in fact, a geogrid’s ability to stabilise a granular fill is a far more important factor in determining the performance of a road or working platform.
An aggregate layer stabilised with Tensar TriAx, on the other hand, performs as a composite, due to the interlocking mechanism and particle confinement that develops between the aggregate and the geogrid, preventing the lateral movement of the granular material.
This Tensar ‘mechanically stabilised layer’:
- Stabilises the granular fill
- Limits deformation under dynamic trafficking load
- Evenly distributes load through 360° degrees.
Junction efficiency is more important than junction strength
Junction efficiency, and not junction strength, is the only parameter officially recognised by the European Assessment Document (EAD) applicable to assessing stabilisation geogrid performance.
This is because the efficiency of a geogrid’s junctions (or nodes) compared to the ribs is a much better indicator of the geogrid’s ‘confinement effect’ – where the aggregate catches in the apertures of the geogrid and pushes up against its ribs, preventing the material from moving laterally. Pressure is put against the ribs, which are held in place by the junctions (or nodes).
Long-term design strength
Long-term design strength (LTDS) is relevant in reinforced soil applications, including retaining walls and slope reinforcement, where layers of geosynthetics, such as geogrids, are placed within the fill used to form the finished structure.
Unlike roads, where in-service load is transient, a reinforced soil structure carries permanent load throughout its operational life, which could be up to 120 years, and so the design of the structure and load bearing components (including the geogrids) should reflect this.
Materials and durability are important
Polymer type has less impact on performance than the way the geogrid is manufactured. Tensar geogrids are manufactured from either polypropylene or High-Density Polyethylene (HDPE) which is ‘punched and drawn’, so are highly durable and robust, allowing them to reinforce most materials in a wide range of environments. They are resistant to:
- Chemical and biological attack (as they are chemically-inert)
- UV radiation
- Damage during installation.
Consider the whole picture when assessing geogrid performance
Geogrid performance should always be considered as part of an overall system—the benefit is in how the geogrid interacts with the layers around it.
Tensar’s methodology for designing road pavements, developed over 35 years, is based on full-scale trafficking research that has quantified the stabilisation effect of the geogrid placed within a granular layer, to increase traffic capacity of the road, reduce pavement thickness or a combination of the two.
When selecting uniaxial geogrids for slope reinforcement and retaining walls, its creep strength is an important factor in determining long term design strength. However, designers also need to consider a number of project-specific ‘partial reduction factors’, which can affect strength, including: The effect of installation damage, environmental effects and consistency of product data.
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