Numerous research projects over many years have tried to correlate the physical properties of geogrids with their performance in subgrade stabilization, but no correlation has yet been found. Many manufacturer and supplier sales representatives routinely misrepresent this fact in their efforts to claim equivalence under construction specifications, publishing product data sheets that compare selected physical properties between products. The situation is further confused because many public agencies do not have performance-based specifications, so they default to specifying products based on physical properties. However, because physical properties do not correlate to performance, the comparison of physical properties between products is effectively an arbitrary comparison. There are no physical properties of geogrid that are universal inputs into any accepted and validated design procedure for subgrade stabilization.
Comparisons based on physical properties, and / or small-scale tests, are not the same as performance comparisons, nor do they apply to all conditions. Reliance on these comparisons can lead to a false sense of security.
To properly evaluate a product, product testing needs to be performed by a 3rd party. Methodologies derived from that testing need to be calibrated and validated to make sure that the level of performance (usually measured in looking at deformation) is in line with the predicted values. Simply basing performance on one or two case studies does not constitute a design methodology.
(J.P. Giroud and Jie Han, 2012)
For Subgrade Stabilization, the Giroud-Han design method is the most widely accepted method for evaluating the benefit of geosynthetics. This methodology is outlined in the FHWA Geosynthetics Engineering Manual (NHI-07-092). It is generic and applies to all geosynthetics products. Proper testing and calibration of products are outlined in the reports below. (For geogrids used as a structural component of pavement design, different testing and methodologies apply. (Click here for more information)
The original reports outlining this method were published in the August 2004 ASCE Journal of Geo-technical and Geo-environmental Engineering Subsequent papers outlining appropriate testing and calibration to these methods has been published in recent years. Copies of some of these papers are included here.
SHORT ANSWER: The physical properties of geogrids do not correlate directly to full-scale testing results. They are not inputs to accepted pavement design methods and cannot be used to predict in-ground performance or compare geogrid products. More details on physical properties, such as tensile strength, can be found on the Frequently Asked Concerns and Questions Comparing Geosyntehtics for Roadbed Applications page.
SHORT ANSWER: Testing and review needed, as outlined by recognized pavement design experts, includes laboratory testing, full scale 3rd party Accelerated Pavement Testing, 3rd party Expert Review and 3rd party field validation.
How do I know if a product has gone through the proper calibration?
Any manufacturer claiming that its products are calibrated to the Giroud-Han method should be prepared to provide documentation from the authors of this method (letter or report), verifying that testing and calibration have been properly performed for the specific product(s) being processed.