Evaluating Geogrid Equivalence in Subgrade Stabilization

Statements from Giroud and Han Regarding Equivalence Between Products

“Geosynthetic index tests of physical or mechanical properties are not universal indicators…”
“…higher strength geosynthetics do not necessarily perform better…”
“Calibration based on small-scale tests and the index properties of the geosynthetic could lead to a false sense of security…”
“… designers should always verify that geosynthetic specific full scale testing along with case histories, for which a calibrated and validated G-H equation was utilized, resulted in satisfactory performance….”

You might also be interested in our FAQ on Comparing Geogrids resource page.

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Design values need to be based on full-scale, highly controlled and monitored testing. Values and design methods should be reviewed by independent authorities with expertise in pavement design and geosynthetics.

What properties matter in a geosynthetic for subgrade stabilization?

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.

"...a geosynthetic improves an unpaved road through complex mechanisms that mostly do not involve the strength of the geosynthetic..."

(J.P. Giroud and Jie Han, 2012)

How should performance be compared? What methodology should be used?

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.

What properties matter in a geosynthetic when it is being incorporated into a pavement design?

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.