How to Compare Performance in Construction
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 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 designed.
Frequently Asked Questions
Do all geogrids of the same strength work the same?
No. Geogrid strength does not correlate to in-ground performance, which has been demonstrated in multiple studies (Watts et al., 2004 & Giroud & Han, 2006). 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.
Do all products with the same aperture shape perform the same?
No, they don’t. Aperture shape and size is a key parameter to allow interlock between the particles of the granular fill material and the geogrid. However, other geogrid characteristics 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 deliver improved performance of the mechanical stabilized layer (MSL). Having the right shape of rib is also fundamental to deliver performance of the system.
Does greater tensile strength correlate to better performance?
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., 2004 & Giroud & Han, 2006).
Does InterAx geogrid work and perform the same as TriAx geogrid and BX geogrid?
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.
What are the differences among the different types of geogrids?
Biaxial geogrid was invented by Tensar in the late 1970s. 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 2007, 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 2021, 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.
|InterAx geogrids were introduced in 2021, representing a significant leap forward in geogrid design and materials
|TriAx geogrids were introduced in 2007, which marked an advancement in geosynthetic technology
|BX geogrids have been around since the late 1970s
|Triangular, Hexagonal, Trapezoidal
|Coextruded - Three Layers
|Monolithic - Single Layer
|Monolithic - Single Layer
|Longitudinal, transverse, and diagonal
|Llongitudinal, transverse, and diagonal
|Longitudinal and transverse
|Best load distribution and interlocking capabilities with various types of fill materials
|Adequate load distribution and interlocking capabilities
|Limited load distribution
What is the difference between a geogrid and geotextile?
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.
How am I supposed to be able to evaluate all the testing presented to me?
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).
Is the testing for subgrade stabilization different than the testing needed to quantify the use of a geogrid for a paved road design?
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.
What should I ask for in comparing geogrids for subgrade stabilization?
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.
What should I ask for in comparing geogrids used in pavement design?
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 basin 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 (2012), 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.
- ASTM D1195. "Standard test method for repetitive static plate load tests of soils and flexible pavement components, for use in evaluation and design of airport and highway pavements." (2021).
- Berg, Ryan R., Barry R. Christopher, and Steven W. Perkins. "Geosynthetic reinforcement of the aggregate base course of flexible pavement structures." GMA White Paper II, Geosynthetic Materials Association, Roseville, MN, USA 130 (2000).
- Giroud, J.P., & Han, J., 2006, “Closure to Design method for geogrid-reinforced unpaved roads. I. Development of design method”, Journal of Geotechnical and Geoenvironmental Engineering, Vol. 132, No. 4, pp. 549-551.
- Saeed, A., and J. W. Hall. "NCHRP Report 512: Accelerated Pavement Testing Data Guidelines." National Highway Cooperative Research Program (2003).
- Watts, G.R.A., Blackman, D.I., & Jenner, C.G., 2004, “The performance of reinforced unpaved sub-bases subjected to trafficking”, Proceedings of the Third European Geosynthetics Conference EUROGEO 3, Munich, Germany, pp. 261-266.
Geogrids are not interchangeable in pavement design: Just as a change to your asphalt mix will affect pavement performance, so will switching the geogrid in a stabilized pavement design. To protect your designs, you need to know how to quantify the benefits of geogrid.
When comparing geogrids for stabilizing poor soil, the key question is, “how much granular material will I need to create a stable surface?” The answer is not the same for different geogrids, and on a big site, an inch or two of thickness can add up to real money.
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Need Assistance in Evaluating Equivalence?
Tensar geogrid products are the most rigorously tested geogrids in the world. Each year, Tensar invests millions of dollars to document the performance of our products and provide engineers with the best possible solutions for their projects. Many additional research documents, design tools, and software are available. For more information, please reach out to your local Tensar Regional Sales Manager or contact us directly.