Accurately Developing Design Parameters and Model Performance of Geogrids in Roadbed Applications

Geogrid has a long history of use by the Federal Highway Administration (FHWA), state DOTs, county and municipal agencies in roadway design. Recognizing the value, many of these entities, along with Tensar, have invested in research to define geogrid performance and develop design methods for roadbed applications. But what are the steps to properly test and confirm the data is accurate and that the design methods are implemented properly?

Step 1: Laboratory and Full-Scale Testing

Developing design parameters for geogrid in pavements begins in the lab. Starting with laboratory testing gives a much better understanding of how different geogrids interact with different materials. Once this is interaction is better understood, geogrids should next move to full scale testing to validate the laboratory assumptions under actual wheel loads. At this stage, it’s important to maintain rigorous quality assurance and quality control procedures to accurately measure and record performance. Changes in asphalt temperature, climatic conditions, pavement materials, loading conditions, layer thicknesses, and other conditions will affect performance, and therefore the measured benefit of the geogrid. The National Cooperative Highway Research Program (NCHRP) has published detailed procedures and requirements for this type of testing in NCHRP Report 512, Accelerated Pavement Testing: Data Guidelines.

Keep in mind that field testing is not as precise as Accelerated Pavement Testing (APT). In fact, variances in materials, thicknesses, environmental conditions, and subgrade non-uniformity can lead to significant inaccuracies regarding performance. Significant oversight by pavement experts while conducting field tests is required to make sure appropriate data is captured and analyzed correctly.

Step 2: Third-party Review

Once appropriate testing has been performed, and a methodology for incorporating the test data in design has been developed, the research and design methodology must be reviewed by an independent third-party pavement design expert. The review should provide:

• An evaluation of the design procedures and the actual products tested, and a statement of compliance when accepted design methodologies and good engineering practice
• Boundary conditions defining where the products are appropriate for use

This review ensures that values being used for design are sound and results will be reliable and predictable.

Step 3: Field Validation

The final step in properly incorporating geogrid in pavement design is field validation testing, verifying that the constructed values are in line with the predicted values. This testing may range from basic quality assurance measures (e.g., verifying that the proper geogrid is furnished, checking layer thicknesses, following proper installation procedures) to advanced techniques such as measuring in situ strength and resilient modulus for the prepared subgrade and the aggregate base layer to verify that design values have been achieved. Standard processes for verifying the proper mix, placement, and compaction of asphalt should also be followed for every project.

Each of these steps is essential to protect the integrity of the pavement design, ensuring that the project delivers the intended traffic capacity and design life.