Fatal Flaw in ASTM Standard Means Erosion and Sediment Pollution From Your Projects
Re: ASTM D6460-12, Standard Test Method for Determination of Rolled Erosion Control Product (RECP) Performance in Protecting Earthen Channels from Stormwater-Induced Erosion
By Thomas Carpenter, CPESC
Sediment is the number one pollutant in the US. There is a fatal flaw in the ASTM D6460 test protocol that directly impacts erosion potential, and thus sediment pollution, from construction sites into our waterways. Updating the standard as soon as possible is the best means to effective turf reinforcement mat (TRM) performance!
The erosion and sediment control industry is where the rubber meets the road—we have a responsibility to advise others and develop better pollution prevention technology. New EPA discharge limits are basically here, which is one reason for change—because we have to! But the best reason is to better protect our environment because it is just the right thing to do.
ASTM D6460 is the industry standard test for measuring the performance of erosion control materials, and specifically channel protection products. I am sure at the time it was done with the highest professional intention.
The overall science of the standard test is fine, and the professionalism of the labs performing the tests is fine. I feel the most significant problem is the allowable performance limit parameter of 0.5 inch cumulative soil loss. This parameter allows for a measured 0.5 inch soil loss over the entire soil profile of the test section—before the test product is considered to have reached its performance limits.
Here are cross-section illustrations at the conclusion of a typical test for an unvegetated rolled erosion control product (RECP), at three intervals over the length of a 40-foot test section, and indicative of the half-inch soil loss parameter. The first cross-section illustration at 15 feet from the start of the test section shows erosion of the soil profile beginning; the second illustration at 25 feet shows increasing erosion; and the third illustration at the termination of the test section showing considerable gully erosion. These illustrations are indicative of the test-bed profile when the half-inch cumulative limit is finally met.
This profile is indicative of the bed profile at the end of an unvegetated test, and could provide performance data ratings—for example, 10 feet per second and 7 pounds of shear—for a turf reinforcement mat that would be used by a manufacturer in marketing and installation specifications. But clearly the erosion is far beyond any acceptable performance in the field. These significant unvegetated performance levels are then used by engineers and designers to specify erosion control products on their projects.
As we all know, once the erosion process begins, the erosion site does not self-repair. In the first phase of erosion, most if not all fertile soil for supporting vegetation is lost. Additionally, erosion creates future environmental legal liabilities and maintenance issues for the property owners, and often destruction of the infrastructure. On long highway ditches or steeper slopes, for example, hundreds of tons of sediment can erode away and pollute our water resources.
In my extensive national travels and visits to new construction sites, I have seen a high failure rate for TRMs in concentrated flows—I don’t know how there couldn’t be—but I do not have hard data to back me up. I do know TRMs often hide erosion rills and gullies as they “tent” over the eroded area and appear to be intact in the first few months.
Another issue with ASTM D 6460 is the lack of good definition in the test set up, measurement, and analysis. Many years have passed since its inception, and the knowledge of better test-bed preparation and measuring protocols, as well as the understanding of appropriate calculations, has improved to the point where the standard needs to be updated so that all labs will be reporting relative comparative data.
Suggestions for Improvements
I propose changing the standard D 6460 to a cumulative soil loss of 0.1 inch, and to make it a priority by the appropriate ASTM committee to vote within the next year for approval. I would encourage the ASTM committee to discuss updating other internal elements of the standard to reflect the new knowledge and understanding—with the hope politics will not get in the way of doing the right thing—and vote as soon as possible.
Second, I suggest that manufacturers consider increasing their minimum staple installation rates by a factor of three. It is well established and known by those of us experienced in erosion and sediment control that staple rate makes a significant improvement in the performance of an unvegetated TRM.
Why? More staples hold the material against the ground more securely and in more locations, thereby forcing the water to flow on top of the material more consistently and to flow in a disruptive, nonlinear fashion, as well as creating areas where sediment accumulates rather than washing downstream.
I do think it would help if all manufacturers recommended fast-germinating cover grass for installations, irrigation to initiate and facilitate germination and subsequent plant growth, and either fertile soil or growth mediums under the TRMs.
Times have changed, and our industry needs to step up to provide products that meet today’s demands. Regulators, designers, and property owners look to us for guidance and recommendations, and we need to put our best foot forward.
The main goal here is first to protect the environment—by eliminating as much risk as possible by employing effective products and installation practices—and then to constantly be updating our practices with learned knowledge. A second goal might be to start developing the next-generation TRM which performs well unvegetated and simultaneously promotes and sustains vegetation over time.
Out-of-the-Box Twist on Vegetating TRMs
Since I'm already out on a limb here, I’ll take this opportunity to share a new for most people, but proven, twist on TRM performance. Having stated the problems above, one solution used in the industry has been to put sod over the TRM to ensure vegetation—which is effective. But it also comes with the inherent risk of a major storm washing it all away before the sod has established, and the requirement for copious amounts of irrigation (since the roots are not in contact the soil and the grass blades are exposed to the wind, which lose moisture via the transpiration process).
In the many years as a contractor (and occasional inventor) I recognized these obstacles and in response, reversed the sequence—putting the sod under the TRM (only the 100% synthetic types)—with exceptional and documented results.
|TRM over sod (before)
|TRM over sod (after)
The sod under the TRM solves all of the problems mentioned above and greatly improves the TRM performance! First of all, the sod roots are against the soil profile, so they can begin connecting immediately—requiring less irrigation. Second, the TRM over the sod protects the vegetation from the wind and dry air, and thus from losing moisture through transpiration—the primary reason that sod dies.
Third, with the sod under the TRM (which does require extra stapling—approximately at 1 foot O.C. to prevent tenting), the TRM “gains” most of its fully vegetated rating at installation—eliminating the risk of losing the installation during a major storm event (as in the sod over the TRM scenario).
Fourth, this scenario allows significantly less irrigation, and in fact, possibly only the initial installation irrigation (depending on the season and region), which bodes well for both the designer and contractor. Last but not least, the TRM becomes fully vegetated within two to four weeks, providing the intended aesthetics as well.