Stabilizing the Environment
It’s a great analogy for erosion control work and the products we use to do it with. One of the things we do to keep boogeymen away is to lay down protective blankets. Except our boogeymen are called silt, pollution and runoff, our blankets are carefully engineered devices, and what we’re protecting is Mother Nature herself.
Land development can result in a multitude of damaging effects to the environment. Silt and debris runoff from construction sites kills fish and muddies the waters for humans, too.
Even so, human progress isn’t going to stop. We’re going to keep building structures and paving roads.
Fortunately, there are ways to mitigate the soil erosion and resultant pollution created by human activity. Some brilliant minds have developed some effective solutions.
ECBs and TRMs
Two of these solutions are erosion control blankets (ECBs) and turf reinforcement mats (TRMs). If you’ve driven on a highway nestled at the bottom of a green living embankment, then you’ve probably seen a successful installation of ECBs. If you’ve seen vegetation thriving along the sides of a drainage channel, that probably wouldn’t have been possible without the use of TRMs. Or maybe you’ve never noticed anything at all, except completely rehabilitated land. Thanks to these products, you are spared seeing the ugliness that used to be there.
ECBs are manufactured from natural fibers such as straw, coir (coconut fiber), excelsior (wood shavings), or a combination of these things. They’re lightweight and open-weave, to allow vegetation to grow through. The materials are tied together with natural jute or synthetic netting made of geosynthetic plastic, such as biodegradable polyethylene. Sometimes, they’re further reinforced by sandwiching them between layers of more geosynthetic netting or fabric. Usually, they’re biodegradable.
TRMs are heavier-duty versions of ECBs. They may include natural materials too, but they are even more strongly reinforced with geosynthetics. All or part may be non-biodegradable. They’re used for very severe slopes and swifter channels.
Both ECBs and TRMs fall under the category of RECPs (rolled erosion-control products, so called because they arrive in big rolls). They’re classified into two groups: permanent and temporary, although these category distinctions are somewhat relative.
ECBs fall into the temporary group. They’re usually meant to completely biodegrade within one to five years of installation.
TRMs are considered permanent. “A TRM is a permanent solution for an area where an ECB just isn’t strong enough, and we need something permanent to stabilize the area,” said Elsa Pond, of the Washington State Department of Transportation. Some TRMs can last as long as 30 years.
Both types are used right after hydroseeding to keep the seed from blowing or washing away, and to maintain moisture so the seeds can germinate. Once they have germinated, the RECPs provide effective stem and root reinforcement for the maturing vegetation. Trees can be planted right through RECPs.
Eventually, Nature herself takes over, and temporary RECPs put themselves out of a job. A carpet of green is her permanent erosion control, and it’s usually the best.
Proper installation of RECPs is vital. The first step is removing rocks, stumps, dirt clods, tree limbs—anything that would cause a bump under the mat or blanket. You need a smooth surface so there’ll be close contact between the soil and the RECP. “Often, if the surface isn’t prepared correctly, you’ll have a lot of voids,” said Pond. “It takes time to do this well.”
A mat or blanket, once laid, should be trenched in and staked down properly. Exactly how you should do this will vary somewhat, depending on the product you use, but in general, you should excavate at least a one-foot-by-six-inch longitudinal anchor trench, two to three feet over the crest of a slope. Then tuck the top end of the mat or blanket into the trench, and secure it to the bottom, using ground-anchoring devices spaced every foot or so. Backfill and compact soil into the trench, and unroll the rest of the RECP down the slope.
The anchoring devices should be at least 0.20-inch diameter, made of steel, with steel washers at the head of each pin. They should be driven flush to the soil surface. All anchors should be between six to eight inches long, and driven in far enough to resist pullout. Looser soils may need longer anchors; rocky soils may need stakes made of heavier metal.
“If a RECP is installed incorrectly, and it rains, water will get underneath and has no way to get back on top,” explains Samuel Collins, of Kentucky’s Transportation Cabinet, construction division.
“What you want to avoid is the formation of rills (pooled water) underneath a blanket or mat.”
These products do cover everything, so if you don’t have good contact with the soil, “you can get rills and not even know about it until it’s too late, after they’ve caused failure,” continues Collins. To avoid this outcome, an installation should be monitored. After a few weeks, once vegetation starts growing, look for the patches where it isn’t; there may be a problem brewing underneath. “If there’s instability, it might mean that the slope is too steep, so you’ll have to take an alternative measure,” said Collins.
As great as they are, “these products are only as good as their installation,” said Andy Durham, PE, LEED AP, of the Propex Operating Company, LLC, in Chattanooga, Tennessee. And Collins adds, “If good ground coverage is not established, it’s not necessarily the failure of the RECP.” It’s a poor sportsman who blames the ball when he loses the game.
Addressing such challenges, RECP manufacturers have been stepping up their game, developing innovative ways to ensure the success of their products. For instance, Evansville, Indiana-based Tensar/North American Green has software to help clients determine which erosion-control products are best suited for specific projects. It takes into account various project parameters, such as types of soils, slope gradients and shear stresses. It does the complex engineering equations for you, and then tells you exactly which product or products will meet your site’s needs.
Durham revealed that his company will soon go to market with an assurance program that includes a certified installation and a performance guarantee. The process starts with design and product selection. “You need to have a lot of technical expertise on the front end. You need to know exactly what product to use, what that product is supposed to be used for, and what’s required to secure it to the ground. That’s step one.”
The next step includes training the people installing the product in the proper installation techniques. Then there’s a final inspection, followed by an insured performance guarantee. “This is very new,” says Durham, “the first time its being done in the industry.”
In the introduction of his book, Designing with Geosynthetics, Robert M. Koerner, Ph.D, professor emiritus at Drexel University and director emeritus of the Geosynthetic Institute, writes, “Since 1977, geosynthetics have emerged as exciting engineering materials in a wide array of civil-engineering applications. The rapidity at which the related products have been— and continue to be—developed is nothing short of amazing.” Geosynthetic materials have been incorporated into erosion-control mats and blankets for decades now. They make them stronger and more permanent. And as mentioned earlier, many geosynthetics are biodegradable.
But different people may mean different things by a term such as ‘biodegradable.’ Years ago, consumer-protection advocates had a similar problem with the term ‘lite’ when applied to foods. What does lite really mean—lower calories or lower saturated fats, and if so, how much lower? And lower as compared to what? You see the confusion undefined terms can cause?
“There’s an industry-wide problem with the lack of definitions, specifically when it comes to the term ‘biodegradable,’” said Pond. “There’s no national standard for this term. Erosion-control products are held to standards set by the American Standard Testing Method (ASTM), but there’s really nothing in those standards for what ‘biodegradable’ means. Manufacturers have been using this term, but are being held to no standards that are testable. This is a serious problem for us.”
As Durham explains, “Most all ECBs have biodegradable components, sure, but a lot of them also have plastic netting. That netting is photodegradable, meaning it’ll eventually fall apart in sunlight, but it’s not actually biodegradable. If those plastic materials do biodegrade, it’s a very, very long process.”
Pond is also concerned about what’s left behind once these synthetic materials do photo- or biodegrade. “Are there weird plastic contaminants left in the soil? This is a problem, because we’re starting to see lots of impairments. Water bodies are compromised by different contaminants, and we want to avoid adding to that problem. We need to have full confidence that an RECP will fully degrade within a few years, and leave no chemicals in the soil.” Degrading blanket netting has also trapped snakes, birds and lizards.
At the other extreme are TRMs that break down too quickly. “We use temporary blankets when we want things to biodegrade; with TRMs, we want just the opposite,” said Durham. “We want our materials to last for as long as they possibly can. But when these materials are exposed to sunlight, they begin to degrade. It’s a very complex technical process to make sure that the UV inhibitors are properly incorporated into these products when they’re manufactured.” If a TRM falls to pieces before it’s done its job, it’s worthless.
Durham spoke of a major airport-relocation project in Ecuador where close to a million square yards of ‘permanent’ TRMs are wasting a way in the sun. This is happening be cause the correct UV inhibitors were not properly included when they were made. However, he says this kind of error is usually made by newer manufacturers, not long-established ones.
They work, they’ll keep working (and so will you)
Originally developed to reduce the construction costs of conveyance structures, green-armoring techniques such as RECPs have proven invaluable in preventing water pollution. “A conservative estimate would say that a properly-designed vegetated swale may achieve a 25- to 50-percent reduction in particulate pollutants, including sediment and sedimen-attached phosphorus, metals and bacteria,” states the United States E n v i r o n m e n t a l P r o t e c t i o n Agency’s ‘Stormwater Technology Fact Sheet; Vegetated Swales.’ Even before the publication of Silent Spring, the preservation of our natural environment has been a growing global concern. Recently, greenhouse gas emissions have been added to the worry list, and the term “carbon footprint” has become a household phrase.
It’s been said that the entire erosion control industry was legislated into existence. Federal and state governments and regulatory agencies passed environmental laws and regulations, and a whole bunch of jobs and products were created as a result.
Those regulations are only getting tighter. At one time, the National Pollutant Discharge Elimination System (NPDES) required permits only when disturbing five acres of land or more.
Currently, Phase II of NPDES requires permits for construction activities that disturb one acre or more. In addition, most states now require developers to file Stormwater Pollution Prevention Plans (SWPPPs) before a single shovel of earth is turned. This means more work for you, the erosion-control professional, but it also means a healthier environment.
You can be proud to be among the men and women whose job it is to pull the blankets up over Mother Earth, tuck her in, and keep her safe from harm.