Hard Armor, Riprap, Gabions, TRMs
WHEN YOU’RE FIGHTING A BATTLE against soil erosion, sometimes diplomacy just won’t work. When softer solutions fail, it’s time to suit up, pick up your shield and lance, and get going. That’s where hard armor solutions come into play.
you don’t want to fight a war with slingshots when the other side has machine guns. That’s kind of what you have to keep in mind when you consider your choices. The tough thing in the business of erosion control is that nothing is one-size-fits-all. In many cases, there’s no obvious “go-to” that one quick glance will give you.
So what do you choose; more accurately, what did the civil engineer who wrote the specifications choose, and why? And was it the correct choice?
“For every given project, there’s a long checklist of things that have to be considered. you have to look at what physical location you’re in. What do you want it to look like in the end? How many people will be using it (the channel, stream, bridge, etc.?) How long do you want your erosion control solution to last?” These are questions that have to be answered, said Vanessa Hatcher, senior engineer, hard armor, for Contech Engineered Solutions, West Chester, Ohio. And let’s not forget the local, state and federal agencies that may be involved. They are the 800-pound gorillas who may dictate what measures have to be put in place— whether you think they’re
the correct ones or not. Let’s consider a project in an area known as
“The Rim,” along State Road 260 near Tyson, Arizona, a bridge with a
water cross- ing running underneath it. “The road designer wanted to
cover almost a near-vertical slope, 1:1, using straw wattles, stapled
down with seed blown onto it,” said John Baker, regional environmental
manager Riprap for Ames Construction, in Scottsdale, Arizona, the
one thing the road designer didn’t take into account (or didn’t know)
was that this area can get as much as two to three inches of rain in one
storm. “We would go ahead and do exactly what the engineer said, but
whenever a storm would come through, it would blow away every one of
their Best Management Practices (BMPs). They were having us put in check
dams or 20-inch coir logs, that sort of thing,” said Baker.
There just happened to be an abundance of natural rock in the immediate vicinity. “So we came up with the idea: why don’t we just rockpile this whole thing? We have to purchase it; all they have to do is the rock right here, we don’t have pay us for placing it,” said Baker, so he and his workers gathered up the riprap.
Naturally, this unauthorized change in plans didn’t go over well with the people in charge, who just happened to be from the state highway department. “They told us to go back to what they originally wanted. We ended up redoing the first section about six or seven times before we finally convinced them that (riprap) was the way to go,” explained Baker A less scrupulous contractor might have been happy to keep doing the same ineffective thing over and over again—and keep getting paid for it. But Baker felt otherwise. “I prefer to do something one time and get it done right.” He had some credentials backing him up, too. He is a certified professional in both erosion and sedimentation control and stormwater quality.
In this case, riprap saved the day, eventually. But like all erosion control solutions, it has its advantages and disadvantages.
John Moss, director of business development for Houston, Texasbased Construction EcoServices, a stormwater quality design, management and maintenance firm, agrees. “When you have a riprap solution, it’s virtually impossible to keep growth of weeds and small trees out of there,” said Moss. “This becomes a maintenance—or in some cases, even a safety issue.” This story illustrates that if you’re looking for a specific formula, you won’t find one. you have to look at what’s actually happening in that body of water or on that slope.
Turf R einforcement Mats (TRMs) are rolled, matrix-like products composed of non-degradable synthetic fibers, filaments, nets, wire mesh and/or other elements. Placed over bare soil areas, they are useful for erosion control along steep slopes, channels, riverbanks, vegetated waterways, arid areas, and other locations where tough erosion control is required. While considered hard armor, they can be vegetated.
This is what was chosen by the Army Corps of Engineers as a channel lining for a large irrigation canal in El Paso, Texas, that runs parallel to our border fence with Mexico. The irrigation water, which flows at a moderate to high rate, was in danger of eroding the border fence.
The TRM product used for the lining was a high performance woven geotextile. This particular product was chosen for its UV protective coating which would help it withstand the arid conditions of the area, plus its ability to perform on sandy soil.
Signature Contracting Services, Grand Prairie, Texas, was awarded the contract. Their crew had to install 1,428,000 square feet of TRM over a seven-mile stretch of sandy soil. And they had just ten days to do it in, while the water flow was temporarily dammed. If that wasn’t hard enough, the crew was also plagued with severe sandstorms during the installation.
“Come hell or high water—literally—we had to get the job done,” said project manager Jason Bell. “That water was coming, and there was no stopping it. We only had ten days in which to install the TRM, stop the channel from eroding and getting into the footer of the fence, and the fence (subsequently) collapsing. We put all the manpower and resources we had into the project and got it done.” The job had to be finished before the dammed-up water was released from its source in New Mexico to flow down to pecan farmers who couldn’t wait any longer for their only source of irrigation water.
Construction EcoServices recommends the GreenArmor system from Profile Products, Inc., in Buffalo Grove, Illinois. This consists of a TRM called Enkamat, made of polyamide (nylon) filaments thermally fused at their intersections. Ninety-five percent of this TRM is open space, making it easy to vegetate. It’s then hydraulically infilled with a growth medium that contains polymers and tackifiers to encourage turf establishment and longterm root reinforcement.
The root structure combines with the TRM and it works extremely well. In some applications, it’s a one-to-one replacement for riprap. It can even be used in channelized flow situations, drainage ditches and swales, where they have called for a hard armor solution.
In an industrial area, where aesthetics aren’t a consideration, you can often use concrete entirely on a channel or slope. Then it’s a choice of blocks or poured concrete, and which method is more cost-effective. If it’s going to take a great deal of effort to get cement mixers into the area, then prefabricated concrete blocks are probably the best alternative.
“you definitely need a hard armor solution in high velocity water flows,” said Ken Wilson, president of Midwest Construction Products, Inc., in Fort Myers, Florida, manufacturer of Tri-Lock Block. Large rain events can cause that kind of high velocity flow. “Back in the early 1990s, when the Mississippi River flooded, the water table was actually more than 20 feet above the flood stage in many places. That large a volume of water carries a tremendous amount of weight, and the weight of the water itself keeps pushing the water down. It causes extremely severe erosion. In a case like that, you need a really good hard armor solution.”
An articulated concrete block system was used to reconstruct a flood-damaged Mississippi River levee in Hannibal, Missouri. They could have also used gabion baskets for this job. “If you have a steep slope, and you’re looking to do a gravity wall, you might want to use gabion baskets,” said Wilson. “Channel linings are really good places to use gabion baskets, or something like concrete block. On a two-to-one slope or less, you should probably use concrete revetment or articulated concrete blocks.”
MSEs, articulated concrete block or poured concrete might be the only way to go for steep slopes. “Our company almost always prefers a green solution,” said Moss. But he concedes that sometimes that really isn’t possible.
When faced with such a choice, what is the determining factor?
“It may be that there has been a longtime concern about the adjacent weight-bearing ability of a slope,” says Moss, “For instance, if you’ve got a wall that needs to be built at the top of a roadway that gets a lot of heavy truck traffic. The force-pressure on that roadway might cause failure of the wall that’s right next to it. In those cases, you might have to use a hard armor.”
Gabion baskets or mattresses are essentially riprap in wire cages, natural rock or concrete rubble that is encapsulated inside a basket of galvanized or PVC-coated wire that holds it in place. Geotextiles are used underneath the riprap or gabions to further keep it in place.
To see how gabion was used in an actual project, let’s look at one project that was done by Maccaferri USA, Williamsport, Maryland, and DCI Contracting, Inc., Decatur, Texas, for the city of Dallas.
There are many creeks and tributaries in the Dallas/Fort Worth metropolitan area. When it rains, these creeks rise dramatically. Property owners in a Dallas suburb that adjoins Alice Creek were losing their backyards and facing potential foundation damage to their homes whenever there was a heavy rainstorm. Increased storm-water runoff from a nearby residential subdivision added to the problem.
This happened to be one of Dallas’ poorer neighborhoods, without much tax revenue to back a reclamation project. Residents contacted a pastor from a local church.
The group approached the city, and funding was found to stabilize the banks of the creek. The residents could reclaim their backyards and, hopefully, prevent future damage to their homes.
Jason Fernberg of Maccaferri USA was present at the jobsite. “The problem with Alice Creek is the same problem that we have all over Dallas. It’s the clay soil. It erodes pretty fast, since there’s nothing to hold it together. Because of that, when it rains, the creek rises several feet. When the rain stops, it drops back down to only about six inches deep. So there is this very fast rising-and-sinking, which causes a lot of erosion.”
This particular section adjoining Alice Creek had depended upon tree roots and natural vegetation to hold onto the soil. But it was no longer working. “They dug out the old plans that were originally drawn up (to stabilize the shoreline) a decade ago,” said Fernberg, “and looked for ways to economize on them.”
The original design called for PVC-coated galvanized, black steel gabion mattresses on the sides of the walls. To save money, instead of covering the walls completely with the more expensive PVC-coated gabion baskets, they were used on only the bottom six feet of each wall. Everything above six feet was constructed using plain galvanized gabions and gabion mattresses.
The reason for this specific design was that most of the erosion was occurring at the bottom, causing a slumping effect. Since water levels in this part of the creek don’t typically rise above six feet, except in rare, major flood situations, the design was extremely effective and kept costs low. “Using this method saved about 25 percent on the cost of the materials,” said Fernberg. DCI Contracting hand-placed the rock and ensured that the internal tie wire was in place, keeping the cells of the baskets straight with clean lines.
Moss’ company has installed rock gabions and gabion baskets with either galvanized or poly-coated wire in many applications. “It just depends on what the civil engineer on a job dictates,” he says. “you’ll see gabions used to stabilize the soil around piers, footers, bridges, along shorelines . . . anytime you need to stabilize an almost vertical wall. you can see them along Buffalo Bayou here in Houston,” continued Moss. “It’s stabilized with gabion baskets or gabion walls on both sides where it runs alongside a bunch of big buildings.” There are standard sizes for gabions, but for walls, especially very steep walls next to roadsides, custom sizes might be needed.
What if, after your analysis, you believe that the specified erosion control methods or materials are not really the correct ones? As we’ve seen, you can’t do much of anything without the approval of the engineer (or engineers) in charge. “Before the change is made, sometimes you can do what we call ‘value engineering’,” said Moss. “you come up with another way to satisfy the needs of the project in a way that saves money.”
In most value-engineered opportunities you have to get the approval of the civil engineer that specified the first solution that you are now trying to value-engineer your way out of. you have to look at what the physical circumstances are, how many cubic feet per second the water is flowing, the course of the water, and if peak events are erected to be going through the area?
Many different erosion control products targeted for specific applications are available. It’s a truly dizzying array, and the technology behind them is impressive. Figuring out which ones to use for any given situation depends on a number of factors, including cost, practicality and safety concerns, plus the physics and hydraulics involved. Experience and careful analysis will determine the closest match for your particular erosion control assignment.