A biaxial geogrid is a geosynthetic compound made out of polypropylene polymers and are primarily used to reinforce and stabilise soils. They are made through a series of procedures including crossway stretching, lengthway stretching and extruding. This production process ensures that all the geogrids have high tension resistance along with a high tensile strength, which therefore increases the overall bearing force that the geogrids are capable of.
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It is important to also understand that there are three different types of geogrids available on the market; uniaxial, biaxial and triaxial. In our case we are focusing on the Biaxial Geogrid which is designed to ensure that the subgrade is strong in two directions, both length and width. This is why these types of geogrids have been used for projects like pavement and road stabilisation for over thirty years.
We only stock the Biaxial geogrids here at EasyMerchant as the others are more specialist for uses such as airport pavements and hardstand areas etc. Biaxial geogrid is the most commonly sold type as it serves a variety of different purposes.
If the soil in the area where you will be adding a dense load (such as slabs, railway sleepers, trucks or lorries) is too soft, the pressure will force the soil to warp. A Geogrid will help to secure the soil through compacting the aggregate which furthermore enhances the bearing capacity of the underlying ground. In short, it strengthens the ground and stops it from disappearing in heavy traffic, preventing rutting – etc.
Unlike when the load is introduced to the soil alone, the geogrid forces the heavy load’s pressure to be spread over the entire surface area of the grid, due to the subgrade having strength in both directions. It’s a similar reason to why, if you were walking on thin ice, you are better off laying down to spread your body weight over a larger surface, it’s the same principle here.
This essentially means once the Biaxial geogrid is set out and the heavy load is put in place the soil beneath the grid compacts and the subrade’s strength is further enhanced. This means that it is very cost effective by saving both labour and maintenance expenses.
Works really well for farm tracks etc or country lanes that lorries drive up and down. It helps to stop the ruts caused by the weight on the tyres which otherwise would need regularly refilling.
Geogrids can be installed underneath the surfaces of various construction projects to stop deformation caused by subsidence build up over time. Examples of this may include:
Geogrids are also used behind retaining walls as a reinforcement layer to keep the soil in a fixed position.
If the soil in a specific area has a high moisture content, the site may be experiencing ‘pumping’. This is where various soil types mix and weakens the soils core bearing ratio. In other words, wet soggy ground will pit or trench even quicker – In this case a non woven geotextile membrane can be used to separate the aggregate from the geogrid. This will filter the soil participles and it helps to separate them from the aggregate layer which helps to further compact the subgrade. Strengthening the ground and helping to remove water.
Before placing any geogrid down you must first prepare the subgrade (the aggregate or base soil beneath). The subgrade must be leveled out with compliance to any standards in order to ensure the whole job can be performed correctly and efficiently, and within any guidelines set out by construction standards specific to your project. It is also advised that heavy duty hand protection is worn when handling Biaxial geogrid as it can be sharp, especially when cut.
The geogrid can firstly be placed parallel to the central line of where your project is taking place or in the crossways direction. If you are needing to use a geotextile membrane for reasons such as your site experiencing ‘pumping’, then this should be laid down first, before you place the geogrid.
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There must be an overlap between the adjacent rolls. The size of this overlap will depend on the grading and the thickness of the fill and type of subgrade. As standard a minimum overlap tends to be 300mm and and a maximum of 600mm. Commercial projects may vary with contracted agreements. Usually overlaps would just be overlapped with the aggregate being placed over the top to weigh it down, however it can be stapled into place too.
During the fill process it should be placed in small layers and not directly tipped onto the geogrid. Care should be taken when handling the geogrids, to minimise the risk of damage.
There are standards associated with every different project and the compaction must be carried out in accordance with the rules set out.
We stock two types of biaxial geogrid, the 20/20s and 30/30s. The difference between the two is the tensile strength. The 20/20s has a tensile strength of 20KM/m and the 30/30s has a tensile strength of 30KN/m. Each of them have the same grid opening size (the size of the holes in the grid) of 40mm and both come in the same sizes with the same weight.
Are you thinking of using geogrids in your pavements to improve the performance of the pavement or perhaps reduce the pavement thickness but still achieve your performance targets? Here are five quick things you need to know before you make your decision.
This may seem intuitive, but it is quite difficult to impress on the engineering community that geogrid index properties such as strength and stiffness are difficult to correlate with the performance benefits. The only real way of knowing if a geogrid WILL work is to put it into a pavement and measure or observe its performance. Due to the number of variables involved in pavement design and construction: particle size, geogrid interaction, subgrade strength/stiffness, variation in wheel loads, etc, continuous testing supported by extensive case studies are needed to build confidence in a particular solution with geogrids.
A Tensar mechanically stabilised road subbase layer, which resulted in a thinner pavement but 60% average increase in roads service life at the Great Yorkshire Way
Point 1 feeds into Point 2, since it begs the question: if lab test results can’t be used to indicate performance, how do we incorporate the performance in the design? Fortunately, geogrid technology has a long history and although we are continuing our research; we know enough that we can make reliable estimates of improvements. However, it is acknowledged that we can do better. In fact, publications, such as AASHTO R50-09, state that improvements from geogrids are derived empirically and should be derived from full-scale trafficking tests. Tensar InterAx geogrids were developed in but the knowledge was built upon years of experience with biaxial and triaxial geogrids, and we are still learning new things about geogrids every day. Some day we expect that pavement design with geogrids will be fully quantified and the effect would be modelled directly, but until then, we can already make substantial savings in pavement designs. This Information Bulletin will illustrate how we use our research to develop existing design methods.
Accelerated Pavement Testing carried out by the US Army Corps of Engineers confirms geogrids improves pavement performance.
Geogrid manufacturers use terms interchangeably, terms such as stabilisation, reinforcement, interlock, flexible geogrids, rigid geogrids, stiffness, radial secant stiffness, strength, radial secant stiffness ratios and the list goes on. There is very little consensus on what each of these terms mean in the context of a marketing publication. To help distinguish from different geogrids, terms are introduced, sometimes at a rather rapid rate which confuse the market. The recent publication of ISO has helped to define some mechanism of actions which point to the intended application of a geogrid. This publication can help consultants have clear discussions with manufacturers.
Which makes things understandably difficult for the end user. If I cannot use index properties as a measure of performance, how can I design with the right geogrid? There are so many confusing manufacturer publications which use the same jargon, how can the designer be sure? Well, the simplest answer is to understand that specification helps to guide the selection of an appropriate geogrid. Just because the geogrid matches the physical characteristics, that does not necessarily mean the geogrid will as intended. We can improve specification to include performance targets which puts the onus on manufacturers to back their claims of performance improvement.
This is because no single property is indicative of performance, it is a combination of many factors. By specifying the required performance, the geogrids proposed should provide sufficient evidence that these numbers can be achieved. We would suggest that the geogrids shouldn't be specified beyond the index properties required to identify the product. The performance targets will be the real test and the main required metric. Click here to learn more about our premier accelerated pavement test with the US Corp of Engineers.
The pavement sections were subjected to more than 800,000 Equivalent Standard Axle Loads (ESALs), the same load imposed by 690 million cars passing over them at the US Corp of Engineers
All geogrids are manufactured to function for a particular application, and using a geogrid for other than its intended purpose can result in disappointment and scepticism as to the efficacy of geogrids in pavements. You wouldn’t use a fork to drink soup, so don’t use a geosynthetic filtration product to improve pavement performance. You might get some benefit, but that is not what it is for. Full-scale trafficking trials, continuous, rigorous and independent testing will pave the way to greater confidence and credible designs and ultimately better pavements for everyone.
Find out how our award-winning Tensar+ design software can aid your road construction projects to incorporate geogrids into your design effectively.