Geosynthetics for Erosion Control
BY MARY ELIZABETH WILLIAMS-VILLANO
Unlike some of her peers, my grandmother welcomed new ideas that made her life easier. When vacuum cleaners came along, she put away her broom and never looked at it again. She was equally thrilled to say goodbye to her wood-burning stove and wringer washer when they were replaced by a gas range and an automatic washing machine.
But let’s take that broom out of the closet for a second. It’s an apt analogy, because when a better way of doing something comes along, it sweeps away older, less efficient methods. Similarly, geosynthetics have been almost as gamechanging for the erosion-control field.
Although traditional BMPs such as straw wattles and rock riprap haven’t gone anywhere, geosynthetic materials are finding their way into more and more soft- and hard-armoring jobs. Often, they’re the best solution to some of the most difficult erosion problems.
We’ve had these materials for about 50 years now, and they’ve grown more sophisticated as time has gone on. “They started being used in the late ’60s, early ’70s, and kind of bumped along through the ’80s”, said Todd Anderson, vice president of sales and marketing for Ten- Cate Geosynthetics Americas in Pendergrass, Georgia. “After 1990, things started to pick up.”
“Geosynthetics provide superior resistance to erosion,” said Marco Invernizzi, an engineering consultant and owner of MAIN Geo-Construc tion
Sources and Alpi Engineering, LLC in Austin, Texas.
“They can handle almost any type of environment or pH factor, from lightly aggressive to very aggressive, and that’s invaluable in civil engineering. Years ago, we couldn’t find any solutions that would last longer than ten or fifteen years. These products are expected to stand for hundreds of years.”
However, Anderson adds, surprisingly, the impact of geosynthetics is still pretty small in the overall scheme of things. Only about ten percent of the erosion-control jobs that could benefit from these BMPs actually do.
Cost is one reason. Straw wattles are relatively cheap, easy to get hold of, and work just fine in lots of applications. But when you absolutely have to contain soil, as when property or habitats are at stake (including human ones), geosynthetic solutions are often the best choice of all the BMPs.
According to Dr. Robert M. Koerner, director emeritus of the Geosynthetics Institute, Folsom, Pennsylvania, there are eight main types of geosynthetics: geogrids, geotextiles, geonets, geomembranes, geosynthetic clay liners, geopipe, geofoam and geocomposites. They can be found on steep slopes, in fast-moving water channels, covering waste and ash piles, underneath and behind retaining walls, and sometimes, as the building blocks of entire walls themselves.
Woven and non-woven geotextile fabrics are the most familiar and most commonly used type of geosynthetic; the ubiquitous silt fence, for instance. Both woven and nonwoven geofabrics are porous, but have different roles to play in solving erosion problems. “Think about a paper coffee filter,” said Anderson. It holds the coffee grounds, the water comes through them, and afterwards, you throw it away.”
“Or, you can buy a metal coffee filter that you’ll use for years to come, because it’s very robust. The woven metal filter is a higher-end product, with larger openings so you can design them for different-sized soil particles such as sand, which has a different particle size than silt,” Anderson continued.
“We look at it as an engineered product on the woven side, and as more of a commodity product on the nonwoven side. Both will work, and in many applications, the non-woven is the preferable, more economical solution.”
A common rule of thumb, according to Anderson, is that non-wovens are used when one-way drainage is needed, and wovens, when two-way drainage is desired. Consider wave action; water comes in when waves crash into the shore, and goes out when the wave draws down.
There is a cost differential; a woven geotextile can cost a little more than double the price of a nonwoven. “All we’re trying to do is hold fine particulate matter,” said Anderson. “Both solutions will work, but one does so in a more technical way. They may cost more, but over time, the woven geofabrics save money.”
Geosynthetics were crucial to the success of the Lake Castleberry Dam project in Apex, North Carolina, begun in February of 2015. The privately-owned dam, built back in 1950, had suffered from neglect, and there was plenty of evidence of severe erosion.
This deterioration had to be addressed, not only because of the residential development on the shores of the lake, but because the state’s safety regulations require that slopes must be protected against erosion from wave action, wind and rain. To stay in compliance, the upstream face of the dam needed reinforcing.
Needing protection was an area that extended from three feet above the lake’s water surface to two feet below it along the dam face, a rather steep 2.5:1 slope.
The Cary, North Carolina engineering firm Withers- Ravenel was engaged to design a fix. Normally, for a hard-armor project of this type, riprap would be the traditional choice, and using it was considered. But there was another major factor here.
The view of, and access to, scenic Lake Castleberry is one of the main reasons people buy homes in the residential development it faces. Because of that, riprap was considered to be too unsightly by the development’s owners.
WithersRavenel senior civil designer Blake Ball and project manager Craig Duerr, P.E., LEED AP, contacted Staci Smith, P.E., at ACF Environmental in Richmond, Virginia. They were seeking suggestions on more attractive-looking and user-friendly alternative solutions for stabilizing the upper soil layers on the embankments.
They still needed to achieve robust erosion control, but with the added requirement that it be aesthetically pleasing. In addition, the stabilization needed to be low maintenance.
Riprap didn’t fit that bill. If it was used, the dam face would have to be periodically sprayed with herbicides to keep unwanted vegetation at bay.
Nor was a rock face particularly safe. As Doug Evans, erosion-control division salesperson for the installing subcontractor on the project, Landsaver Environmental of Richmond, Virginia, put it, “Can you imagine how your ankles would feel, trying to walk down a riprapped slope? Or how a small child would navigate that?” Not only that, but the site was a logistical nightmare. It was in a narrow area that would be difficult to get dump trucks into. Whatever BMP was going to be used would have to be hand-placed.
“Hand-placing riprap would have been astronomically expensive,” said Ball. “We’d have needed a rubbertired backhoe at minimum, to be able to haul the stone and dump it, and then, put every rock in by hand.”
The answer proved to be a cellular confinement system. Its six-inch deep cells could be infilled with soil and aggregate to allow complete vegetation. Slots in the cell walls allow the quick threading of tendrils.
This geosynthetic solution was much lighter, and could be transported by a skid steer. It’s shipped packed flat, and its sections open up into three dimensions, like one of those crepe-paper balls. The eightfoot panels were connected with patented devices made of an inert polymeric material, a technique that’s faster than stapling.
The cells were filled with #57 aggregate, a size that would provide enough voids to eventually fill up with fine particulate matter that would help support the growth of the vegetation. The stone would be strong enough to stand up to the anticipated wave action. Once the grass was fully grown in, the slopes could be mowed; even better, the cellular confinement system would simply disappear from view.
Sounds like the installation went smoothly, doesn’t it? Well, not quite. Ice storms and snow bedeviled the installation crew. “The ground became completely saturated,” said Ball. “If you walked out on the site, you’d be six inches taller when you came back, because of all of the mud and clay that stuck to the bottom of your shoes.”
And yet, the 700-linear-foot installation was finished in about two weeks. As of August, 2015, the area was fully vegetated, providing the natural appearance desired by the owner.
Now we’re going to enter The Matrix—a geosynthetic matrix, that is, that was central to solving a severe erosion problem in Nachitoches, Louisiana. It was caused by two major rainstorm events in late October of 2006 that pounded the Cane
River Creole National Historical Park, which is home to several National Historic Landmark sites.
When seventeen inches of rain fell within a 36-hour period, the subsequent erosion limited access to the park. It expanded the large gully along the riverbank slopes near Cane River Lake and impaired the access to two state highways.
The erosion also threatened visitors’ ability to reach the park’s natural resources and 29 historical buildings. When the problem was finally addressed in January of 2012, officials wanted a green, environmentally-friendly solution that would allow for total revegetation, but still be cost-effective.
Designers at Maccaferri USA, Williamsport, Maryland, proposed using a high-strength, reinforced geomat consisting of thick, UV-stabilized, non-degradable polyamide filaments that form a three-dimensional matrix, extruded onto a double-twisted steel woven mesh. Used for long-term erosion protection applications, it’s designed to be secured to vulnerable slope faces to prevent surface erosion from runoff.
“The mat is bonded together with twisted wire mesh, which gives it anti-unraveling properties and high tensile strength,” said Invernizzi. “If you cut a hole in it to plant a tree, you’d lose no strength whatsoever. That was of great value in this case, because we planted a lot of five-gallon trees in buckets.”
And the result? According to Invernizzi, the installation was a great success, and the owners are very satisfied with the final results.
One of the most fascinating forms of geosynthetics is geofoam. This innovative material was instrumental in solving a serious erosion problem that was a hazard to both safety and the environment.
U.S. Highway 26, a major corridor to two national parks, Yellowstone and Grand Teton, had fallen into disrepair. A $23.5 million reconstruction project was ordered, taking a total of six years to complete.
In the course of that rebuilding project, during the summer of 2009, it was discovered that there was a severe slide threat from an unstable embankment in Togwotee Pass, about ten miles west of Dubois, Wyoming. It would have to be stabilized, and quickly, in under 30 days.
Further complicating this endeavor was the fact that U.S. 26 runs through protected wetlands. The engineers involved in the project needed to find a BMP that would minimize the environmental impact on those wetlands, stabilize the roadway’s embankment and do it within the timeframe specified. “Up and over this pass, there is a lot of live slide activity, with lots of moisture and weight bearing down,” said Bradley Olson, a civil engineer, and a project manager/estimator at Oftedal Construction, Inc., of Mile City, Montana and Caspar, Wyoming.
“WYDOT (the Wyoming Department of Transportation, the agency overseeing the highway project) was very concerned about the wetlands just below those slide plains,” Olson said. “We had to try and keep the mountain from sliding down and burying them.”
A retaining wall, a rock toe-berm, or a rock buttress—traditional solutions to such problems—were rejected, due to the 45-degree angle of the slide plains and the sheer weight of the dirt. It was determined that a lightweight fill was needed to keep from activating the slide.
Due to the fact that it was an environmentally-sensitive area, Oftedal’s crews only had so much room on which they could build. “In order to employ other applications to stabilize the embankment, we would have needed to widen the right-of-way in order to allow for a bigger area of construction,” said Olson. “The 94,000 cubic feet of geofoam we installed allowed for a minimal footprint and a fast installation.”
If they had been able to widen the roadway, a different lightweight fill would have been used, straw bales or wood chips. But that installation would probably have to be revisited in 20 to 30 years.
“This way, unless a gasoline tanker wrecks on top of it and the fuel seeps down through, that’s the only thing that would take the foam away,” said Olson. Barring such an occurrence, this solution may last for 100 years or longer.
The work we do would be much more difficult, costly and timeconsuming without the help of geosynthetics. Fortunately, continuing innovation means that we can look forward to more of these products being developed for those times when outside-of-thebox solutions are needed to help solve the most difficult erosioncontrol problems.