When Tony Allen, P.E., was at work on his civil engineering master’s thesis at Oregon State University in the early 1980s, he certainly couldn’t have foreseen the impact it would have during the next 30 years on geotextiles and geogrids in reinforcement applications. Of great importance, the work he pursued then (and since) has led to real understanding of the durability of geosynthetic reinforcements.
The term geosynthetics was, in fact, only a few years old at the time, having been coined in the late 1970s by J.P. Giroud, Ph.D., P.E.
This year, Allen’s work may culminate with the publication of a long-needed geosynthetic reinforcement standard from the American Association of State Highway and Transportation Officials (AASHTO) and ASTM Committee D35 on Geosynthetics — one that will have a strong impact on geogrid selection for critical reinforcement applications.
All of it stems from Washington State Department of Transportation’s (WSDOT) T925 testing protocol, which takes into account numerous reduction factors to provide useful, repeatable data for long-term durability predictions and more accurate factors of safety.
And the influence of T925 is not limited only to practice in the United States.
Then and now
Though a specialty with geosynthetic reinforcement issues may not have been Allen’s intended career path, two things certainly worked on his side, if not forced his hand. Foremost, Allen had the good fortune to work under the guidance of one of the field’s forerunners, Dick Bell, Ph.D., who designed some of the first geosynthetic-reinforced walls.
Also, the U.S. Northwest was a hot zone for reinforcement interest, need, and experimentation. It remains as such.
Today, Allen still lives there. He works for WSDOT’s State Materials Laboratory and chairs the National Transportation Product Evaluation Program’s (NTPEP) Geosynthetic Reinforcement committee.
WSDOT benefitted from his hire in 1983, adding his geosynthetic background to fill a void. (Nearly every state DOT lacked geosynthetics-trained personnel then.) By 1990, a key, pooled-fund study was underway with WSDOT, the Federal Highway Administration, and other partners.
The study lasted 10 years and has helped the practice of geosynthetic reinforcement selection significantly — both for users and manufacturers. Prior to it, geosynthetic reinforcement was approached almost solely from considerations of creep in design and performance analysis. Durability wasn’t able to be considered without dependable long-term data, and the (potential) effect of installation damage on reinforcement performance did not seem to be strongly considered.
Prior to it, geogrid selection was limited largely to high-density polyethylene (HDPE) products, so choices in reinforcement, particularly for government agencies, were limited in many respects to either HDPE or welded wire. T925’s testing protocol, in combination with another key development — the Stepped Isothermal Method — allowed for a quicker, more affordable testing and extrapolation of creep data in polyester (PET) reinforcement materials.
With this data, PET reinforcements could also enter the market. And this made the market more competitive all around (including on price).
Essentially, before WSDOT’s research, which it took national for the subsequent pooled-fund study, and the corresponding development of the Stepped Isothermal Method, there just wasn’t a good way to produce dependable geosynthetic reinforcement designs without being almost prohibitively conservative.
By 1996, WSDOT was using results from the pooled-fund study to develop the first full iterations of T925. Not long after that, it caught the attention of the International Standards Organization (ISO).
ISO Technical Committee 221 (Geosynthetics)/Working Group 5 (Durability), through John Greenwood, locked onto WSDOT’s T925 protocol in 1998. With the help of Allen, TC221/WG5 began moving it into an ISO Technical Report. However, the long process of translation for international use was time-consuming. Drafts were issued in 2002 and 2006, but they drew heavily on the mid-1990s version of T925, which had since more fully incorporated quality assurance measures — consistency of products in a product line, the Stepped Isothermal Method for producing accurate long-term test results, et cetera.
So while ISO’s last use of T925 is out-of-date, it is exemplary of the interest the protocol has generated internationally. Allen reports that T925 is still generating interest, including its adoption in numerous parts of the world as the protocol of choice for evaluating geosynthetic reinforcements. Given the recent passing of a provisional standard by AASHTO and interest expressed at the January 2010 winter meeting of ASTM D35 (January 2010, San Antonio, Texas), it’s fair to say that the influence of T925 will continue.
In your practice
The testing of materials is, in its way, an obscure source of influence on any engineering practice. How a reinforcement geosynthetic, such as a geogrid, is subjected to stress/strain analysis and how those results are evaluated and reported may be beyond the full understanding of everyone in the project approval chain, but those reports certainly play a key role in what options you have in the final design.
The story of geosynthetic reinforcement options prior to the development of the T925 protocol is an exemplary case of how questions raised by engineers and regulators regarding product performance, and how to safely predict it, can limit options.
The expansion of the geosynthetic reinforcement market following T925’s development is exemplary of the effect those efforts in the testing community can have — effect on design options, product approval lists, manufacturing, reinforcement performance, et cetera.
Today, roughly 20 state DOTs lean upon T925 for geosynthetic reinforcement analysis, either in their rules for product approval or through their membership in and acceptance of NTPEP test data.
Currently, product lines from six manufacturers — including Maccaferri, ACE Geosynthetics, Tensar, Tencate, Synteen, and Luckenhaus — have been published in testing reports from NTPEP’s reinforcement program. This is data that NTPEP member states can use for their product approval lists. Two more companies (Strata Systems and Linear Composites) are awaiting results in the final stage of long-term product data reporting.
With the passing of a provisional AASHTO standard and a likely ASTM D35 standard (to be discussed next June 9-11 in St. Louis), it is reasonable to expect more manufacturers will begin the process of product line submissions to NTPEP’s testing program in the near future.
Materials move mainstream
But the development of rigorous testing protocols and standards is a slow process, as the story of T925 attests. Two other interesting developments in testing and standards are also worth mentioning here: one affecting the future of erosion and sediment control systems and the other the future of geocellular systems.
Large-scale testing for erosion and sediment control
However, until now there have not been true, standardized, long-term performance data to show any one BMP is actually best — or just better or even good.
A number of university-affiliated laboratories have developed large-scale performance testing equipment for erosion and sediment control materials, but only one independent lab is currently able to perform a full suite of standardized tests for large-scale performance evaluation: TRI/Environmental Inc.’s Denver Downs Research Facility in Anderson, S.C. It has even earned accreditation from the Geosynthetic Accreditation Institute’s Laboratory Accreditation Program (GAI-LAP). (GAI-LAP is administered by the Geosynthetic Institute.) Tests, such as those for channel performance (ASTM D 6460) and slope performance (ASTM D 6459), are repeatable and give a stronger understanding of expected material performance in the field than bench scale (small scale) tests.
This will impact not only the geosynthetic side of the industry — with its turf reinforcement mats and synthetic netting on erosion control blankets and sediment retention devices — but the rapidly growing arena of hydraulically applied products, as well as the many other types of erosion and sediment control options available.
Only a few manufacturers have begun the standardized large-scale testing process to date, but NTPEP is in the process of incorporating long-term testing data. More manufacturers will take part in this soon, making the next few years perhaps the final maturing phase in this industry as it aims for its erosion and sediment control products to be treated fully as construction materials.
They deserve to be. Many of the products are advanced, worthy of site consideration, and have long-proven performance in the field; but in the absence of large-scale performance data, there has been no way to know what you’re getting until you’ve already got it. In a crowded field of BMP options, the best products do not necessarily stand out in the selection process. They soon will.
A geocell standard
But ASTM D35 on Geosynthetics lacks a standard to help guide use of these systems, which may become problematic given the variety of product constructions.
An inaugural meeting was held in January to begin generating the interest and discussions needed to develop a consensus standard. Further discussion is expected in June at D35’s summer meeting. Given the recentness of the inaugural meeting, and with only two meetings per year, a standard — if consensus can be reached — is probably a long way off.