There are dozens of geogrids and other geosynthetic products on the market ' how do you know which is right for your project's needs, and how do you compare different types of products? When it comes to roadway construction, the keys are proper design and testing to validate performance. Here are some resources to help you make sure your project performs as designed.
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No. Geogrid strength does not correlate to in-ground performance, which has been demonstrated in multiple studies (Watts et al., & Giroud & Han, ). The ability of the geogrid to interlock and confine the aggregate subjected to traffic loading is fundamental to deliver enhanced performance of the mechanically stabilized layer (MSL). This enhanced performance can only be measured through full-scale in-ground trafficking testing. Geogrid characteristics such as rib shape, aperture shape, polymer type and material structure configuration, rib thickness, in-plane stiffness, junction shape, geogrid-aggregate compatibility and many other factors are contributors to the amount of interlock and confinement achieved, and therefore, enhanced performance of the MSL.
No, they don't. Aperture shape and size are key parameters to allow interlock between the particles of the granular fill material and the geogrid. However, other geogrid properties such as the rib height, rib width, rib aspect ratio, rib material, geogrid in-plane stiffness, and relative location of the rib with respect to the center of mass of the granular fill particles are key to delivering improved performance of the mechanical stabilized layer (MSL). Having the right shape of rib is also fundamental to the performance of the system.
Geogrids must have a minimum tensile strength to make sure the product is not damaged during the installation and compaction of aggregate. However, no correlation has ever been shown to exist between tensile strength and the performance of geogrids used for roadbed applications. This has been demonstrated in multiple studies (Watts et al., & Giroud & Han, ).
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No, InterAx geogrids work differently than TriAx or BX geogrids because of InterAx's unique structure, its wide range of aperture shapes and sizes, and the interactive outer layers. By having a geogrid structure with multiple aperture shapes and sizes, particles of different types of fill materials can interlock and interact more efficiently with InterAx. This geogrid/particle interaction is enhanced by InterAx's interactive outer layers, which conform to the shape and angularity of the particles of the fill material. These characteristics enhance the load distribution onto the subgrade, mitigating subgrade deformation and early failure of the mechanically stabilized layer (MSL). Lastly, since different types of geogrids have different structures, material characteristics, and performance properties, geogrids are not interchangeable without revising the design.
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Biaxial geogrid was invented by Tensar in the late s. Biaxial geogrids have square or rectangular openings, called apertures. Different types of biaxial geogrids exist on the market, but for roadbed applications, testing has consistently shown that integral geogrids made by a process known as punched and drawn perform best. In , Tensar introduced multiaxial geogrids, known as TriAx. These geogrids have triangular apertures and ribs with a higher aspect ratio than biaxial geogrids. This geometry provides better interlock with the surrounding aggregate than biaxial geogrids, resulting in improved performance in stabilization applications. In , Tensar introduced InterAx geogrids. InterAx builds on the decades of knowledge and testing at Tensar to deliver a higher level of performance. Through thousands of hours and millions of dollars of research and development, we combined advanced materials science, a revolutionary new geometry, extensive performance validation testing, and an entirely new design approach to deliver better solutions that are more economical, longer lasting, more resilient, and more sustainable.
Geogrids are polymeric (plastic) construction materials used to confine and stabilize granular fill materials to increase their load distribution capabilities while reducing its potential to deform under load. Geogrids have openings called apertures, which allow for rock particles to strike through. This action is known as particle interlocking, and it allows for rock to become confined within the aperture.
Geotextiles are construction materials made from woven or non-woven polymeric (plastic) fibers. They can be used to provide filtration and separation, which restrains soil from mixing with adjacent materials due to dynamic forces. Geotextiles cannot confine granular fill materials, since there are no apertures to achieve particle interlocking.
Unless you are an expert in pavement design, it may be very difficult to evaluate all of the information that may be submitted to you ' let alone find time to do so. Unscrupulous suppliers often try to take advantage of this fact by providing large quantities of information, even though it is not sufficient to show equivalency and may not even be relevant to the application. You can reduce the confusion by writing a project specific specification that includes the information outlined in the checklists we linked above ' depending on the application (subgrade stabilization or designing a pavement).
Yes. There are different design methods and inputs for each design method. The allowable amount of permanent deformation also likely differs for both of these applications. They require different testing, and have significantly different failure criteria. Specifics on what to look for when comparing testing for subgrade stabilization and paved road design can be found in the FAQ below.
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Performance is not based on index properties of the geogrid, measured in a lab. It is determined empirically, using full-scale in-ground trafficking testing. Request a copy of the design inputs and the design method which were used as the basis of equivalency. Also ask for the full-scale testing, with the specific product being submitted (not another 'similar product'), and documentation from an independent authority verifying that the testing and calibration were performed properly. Once you have this information, you will be able to compare performance related sections and evaluate equivalency.
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Performance is not based on index properties of the geogrid, measured in a lab. It is determined empirically, using full-scale in-ground trafficking testing. Require a copy of the third-party testing on which the supplier is basing its claim of equivalency. This testing should include Accelerated Pavement Testing (APT), performed on paved sections, following the procedures defined by NCHRP report 512. There should be multiple sections tested over different subgrade conditions and different pavement section thicknesses. Require a third-party review, by pavement design experts, verifying that the design values and methodology used are supported by testing for that product, and follow the guidance of AASHTO in R50-09 ('Geosynthetic Reinforcement of the Aggregate Base Course of Flexible Pavement Structures'). Also require third-party field validation testing, following AASHTO T221-90 (), to demonstrate that the results obtained in the field support the assumptions used in the design methodology, and that deformations are in line with predictions.
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When dealing with biaxial geogrids, there are two orthogonal sets of polymeric tension-resistant components that are generally referred to as ribs. Junctions are formed at the crossover points of those ribs while the apertures between them allow for particles of the surrounding soil to penetrate between the ribs. Additionally, other testing properties include the ability of the soil surrounding the area to transfer stresses and strains on the surface of the ribs and the junctions, ultimate tensile strength and the ability of the ribs to meet minimum rib thickness.
In order to better stabilize the subgrade, biaxial geogrids confine the compacted aggregate soil into its apertures. Since the geogrid provides strength in both directions, the weight of the load across the surface of the geogrid is supported horizontally and vertically. When more weight is added to the load, the aggregate layers are compacted even more, further increasing the stability of the subgrade.
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Geotextiles are used to separate dissimilar materials which prevents the mixture of granular subgrade materials into softer subsoils. Not only does this ensure that the ground is more stable for rigid pavement sections, but it also controls erosion and reinforces the strength of steep slopes in paved areas. When combined with an area where geogrids are in use, the stability of the subsoil is improved even more, creating a safer, more secure area.
Biaxial geogrids save time and money by simplifying the construction process, reducing the need for sub-excavation and reducing aggregate fill thickness. Once the process is complete, it also cuts down on the need for pavement replacement, which saves time and money in regard to future labor costs and the price of equipment use.
Common Extruded Biaxial Geogrid Questions with an Industry Expert
Q: What are the main conditions on a site that might lead someone wanting to utilize either biaxial extruded or another extruded grid for a project?
A: When we look at the use of Geo grid there's usually two main camps that you're in. The first one being soft soil, you get out there and you have an immediate problem. The contractor must either get equipment across or he's looking for a way to build some sort of working platform, the soils are too soft and so we use the geogrid in conjunction with stone and we build it up high enough thick enough that we can bridge over the soft soils that's the easy the simple hey we have a problem here's the solution. More of an immediate fix.
The second way we use it, or the other most common way is through pavement design where we're taking a complete pavement section usually asphalt on top of aggregate and then we're including a geogrid at the bottom. For one of two purposes, one being to extend the life of the pavement. Maybe your design is for 10 years, and you wanted it to last for 15 or 20, so you'd add the geogrid at the bottom to increase the life. The second way to look at it is we need it to last for 10 years but we can't afford this much asphalt thickness wise or this much aggregate and so we're adding a geogrid at the bottom and reducing either the asphalt thickness and or the aggregate thickness. That is all done through engineering, it's not black box it's all through AASHTO, the way that we do that. Those are the two main reasons or the two main uses of the Geogrid.
Q: Other than the design life are there other things that would help you determine what type of geogrid? What type of information would you need from a customer to point them in the right direction as far as on the design?
A: There's a lot of different ideas that go into this design. The main one being the softness of the soil, you know how soft, and we use a term called CBR, which is California bearing ratio and in layman's terms what that means is can I drive a truck across it without losing the truck or how much is it rutting when I drive the truck across or can I wear my good shoes out there without losing them, you know am I going to sink in. We use that softness of the soil as probably the main one. Then the type of aggregate that we're going to use. You know some places use concrete others use a regular Stone base, others even use a Sandy mix and so depending on the size of the aggregate and the thickness helps determine what strength geogrid we would like to use. Lastly, we're using what are we driving over, is it something like a Dairy Queen which would be 95 percent you know just regular folks like you and me driving over and
then a trash truck once a week so we will not need something as strong or says if we were doing a a railroad or a port or an interstate highway or even some of these major collector roads but you're going to have more truck traffic on it, once we get into construction equipment then we're at a whole different level and we're bumping up the strength of the Geogrid. So, we need to look at the soil, we need to look at the aggregate that's being used on top of it and then what is the actual application of this section.
Q: Do you often see non-woven filter Fabrics or some other type? I know there are some grids out there that have a non-woven fabric bonded to the grid and then some people use them in conjunction. What would be the purpose of that?
A: Well 95 of the time we do not use a fabric. A fabric is great to use as a factor of safety you know if you don't have all the field conditions, or they change throughout your area and it's great for separation. If for nothing else adding that in adds a degree of separation.
Now with geogrid based on the sizing of the aggregate and the type of soil will naturally separate the two materials. For example, the Corps of Engineers though always recommends I shouldn't say always but most time recommends a non-woven fabric under the grid, that can never hurt. Most of the time if you're including it there's a reason for it such as separation or maybe a little bit of added strength and like you said before we can connect the two. We can either stitch it or bond it or weld it where you're not having to roll out two different products but just one product with both applications on it.
Q: Where would the extruded geogrid be placed? Is it at the bottom of the aggregate or do you put it kind of in the middle of the aggregate? How do you decide where to place the grid?
A: Good question, we have done, when I say we, the industry, has done lots of tests on where we should put that geogrid in the aggregate to give you the greatest benefit. It has come back that the greatest benefit is when you put the geogrid right on top of the soil, the existing or the prepared soil, and the aggregate right on top of that so it would be right at the interface of the soil and the rock. That being said, sometimes the thickness of the stone is 20 inches, obviously that would be at a port or an airport or something else that's receiving some ungodly amount of heavy-duty traffic. If you have, usually the rule of thumb is 12 to 14 inches, if you have more than 12 to 14 inches of stone you should either move the geogrid up about halfway or just add a second layer of grid which is what we do. The problem is when the grid gets too far away from the surface whether that be the surface of the asphalt if you're doing a paved application or the top of the aggregate section the benefit that it's showing goes down considerably if that grid is too far away from the surface because it's naturally spreading out right the load and by the time it gets down to the grid it's not doing as much as if you had moved it up halfway. Nine out of ten times you're putting it right on top of the soil and putting the grid right on top of that.
Q: On the site visits and everything that you've done are there common installation mistakes or best practices that you see as far as actually putting the geogrid in the ground?
A: I like to tell people when I'm out there I should be able to teach your 15-year-old how to put Grid in in about 30 minutes. It is a very simple product it comes in rolls you can kick it out and what they say is kick it and then you stack it, so you'll roll the grid out and then you put an aggregate on top of it. That's the key to installation, I mean it's not hard but, that being said there are some it's amazing what people will do to try and mess up a project. So, when you're putting rock on top of it you'd like to back dump onto the edge of the grid and kind of fan it out, if that makes sense, with your equipment. It is a very stiff product and so you'll get waves in it, and you want those waves to be able to escape to the edges and the back of the roll. If you're just doing it on one side and working your way down, you could have a little bit of ripple which could pop through the aggregate depending on how thick it is.
Other common-sense things we always recommend are for people to use gloves because it's a stiff plastic and without gloves it'll rip your hands wide open. There's just no getting around that, it can cut you up something fierce. So be sure to have gloves and long sleeves. When you're cutting a roll, you can easily use a chop saw that'll cut a roll right in half, unlike a fabric for instance where the fabric is so thick and everything tends to melt, the grid has those openings and so you can cut right through it. Again, that edge is going to be very dangerous, I guess is the right word. With other people there's instances where you're on site and the weather is not helpful and it's windy, well this product will blow all over the place so some people will zip tie two rolls together. Your overlap is usually one to three feet. The wider the overlap the softer the soil. On the competent ground you just overlap it one foot so that it doesn't separate during installation, but some people will zip tie those together. You do have to worry about the waves not being able to get out, some people when they're starting will put big rocks on the edge of the grid and then take those off as that roll of geogrid has been covered up. It's all very simple but some common sense is required and the first time you put it down it's not bad to have someone out there just to make sure or answer any questions like that.
Q: Is there anything else you wish more people knew about the extruded geogrid and its use and just the industry in general? What are some places it's not being utilized as well as it should?
A: Well, that's the big thing, the rule of thumb or what people in the industry say is it's only being used on about 20% of the projects where it could be used. If you have some sort of construction need or interest most of the time, we can include some type of geosynthetic, specifically Geogrids for this one, to extend the life of your pavement, to get over some of those issues you have with soft soils, and or to save you money where we're taking the thickness and pushing it down. For instance, if you went through the engineering and your engineer says 'hey you need 20 inches of stone to mitigate these soft soils' we can, this is rule of thumb, cut that in half with the use of the right geogrid where you're saving 10 inches of stone. If it's super expensive or you're having to haul it a long way another option where you can look at grid is chemical stabilization. Which works fine, chemical stabilization is a viable option, but you have issues with weather and availability which you don't have with extruded biaxial Geogrid. The opportunity to use grid, Extruded geogrid is out there, and definitely not used as much as it could be so I would encourage you if you're watching this video to at least put a call in the worst thing that we can say is in this instance you know the grid isn't going to do anything for your project but we'd be willing to bet that we can do something to at least give you an option where you can cost it out.