Defensive hardware has changed a lot over the centuries. From the medieval knight’s swords and lances, to today’s drones and night-vision goggles, as technology continues to evolve, so does weaponry.
The same holds true for the “weapons” we use to fight erosion. When fast-flowing water is the enemy, and softer measures just won’t do, we must turn to the tanks and fortifications of erosion control—hard armor.
While hard armor can solve the most extreme erosion-control problems, not every product is the most cost-effective or aesthetically pleasing for every application. As a contractor, you will most likely not be the decision maker of which hard-armor BMP (best management practice) to employ; that will usually be specified by a civil engineer. Your job will be to install it and make it work.
That doesn’t mean, however, that you shouldn’t learn as much as you possibly can about the different options available. Ideally, you’ll sit down with the site owner and the project engineer to discuss the solution or combination of solutions that will best provide long-term erosion control without breaking the budget.
There are many factors to consider when selecting the appropriate weaponry. Before a spadeful of soil is dug, a site’s specific features should be mapped out, on paper. The site’s location and environment, its vehicle access, the angle of its slopes, and the amount of vehicular and foot traffic it will get once completed, should all be taken into account.
Our armaments aren’t the only things that have evolved. Aesthetics and environmental concerns now must be considered with every job. A completed project should blend into its natural setting, if not eventually rendered completely invisible. Materials native to the area, such as local rock and plants, should be incorporated whenever possible.
The classic hard-armor arsenal consists of riprap, gabions and concrete, usually in the form of articulated block. Let’s go over the pros and cons of the Big Three.
For thousands of years, human beings have used loose rock to line rivers, streambanks, and coastlines against the erosive forces or water and ice. Made from one of Mother Nature’s most abundant materials, riprap structures are so natural-looking that even the most carefully engineered system may appear to be just another pile of rocks to the untrained eye.
Riprap, otherwise known as rock armor, rubble, or shot rock, uses varying layers and sizes of sharply angled, interlocking rocks to provide slope and channel protection. One of its greatest advantages is that it can flex along with freeze/thaw cycles. Cracking is completely eliminated. The rocks absorb the impact of rapidly flowing water rather than deflect it.
Commercially-mined rocks are used, as they need to be cut at angles. Smooth, rounded rocks are ill-suited for riprap structures, since they won’t interlock.
Climate may determine what kind of rock to use. In the North and Northeast United States, granite is favored for its ability to withstand even the harshest weather.
Broken limestone and rebar-free concrete rubble is also used.
Because of its relative ease of installation and use of an abundant resource, riprap has been a standard erosion-control application for armoring riverbanks, shorelines, streambeds, bridge abutments, and any waterway with a potential for flooding. However, it’s not utilized nearly as often these days, compared to gabions and concrete block systems.
That’s because there are downsides to even the most welldesigned riprap revetment.
Extreme erosive forces—and humans bent on vandalism—can easily displace its components. A riprap-covered slope is at risk when rocks are removed, because the system is designed to work as a whole.
This vulnerability requires constant inspection and maintenance.
Litter and other debris can also catch on the rocks, and the accumulated trash can make the entire installation an eyesore.
When loose rock won’t do the job, loose rock in restraints just might. Like riprap, civilizations have been using gabions for centuries. Ancient Egyptians lined the Nile River with rock-filled boxes made of hemp rope.
Today’s gabions are large wire mesh baskets that imprison rock so it can’t move. These baskets are then fastened together to form flexible, permeable, monolithic structures. Unlike loose riprap, a gabion structure will retain its overall strength even if one of its components is removed or damaged.
Gabions require virtually no maintenance after being erected, and can be modified in the field to follow any contour.
Gabions are most often utilized in areas where ground conditions are inherently unstable. Because of their unified structure, they can withstand incredibly high velocities of water. They have hundreds of applications—from bridge abutments and gravity walls, to channel linings and streambank stabilization projects.
Enclosures are available in an array of different wire types and sizes. The most common and least expensive is galvanized steel wire coated in zinc. Another type, called Bezinol, has a coating consisting of 95 percent zinc and five percent aluminum. Bezinol is slightly more durable than galvanized wire coated in pure zinc.
However, both coatings will dissolve if they come into contact with salt. For roadside applications in snowy areas where road salt is used, or alongside the ocean, it would be wiser to opt for PVC-coated baskets. If installing gabions in saline environments or waterways with a high amount of wave activity, it’s best to go with stainless steel wire. These baskets are the most durable, but are also usually the most expensive.
Gabions can also vary in shape.
For example, a “Reno mattress” is flatter, and uses less rock material than a conventional gabion. Reno mattresses and gabions aren’t that different, except that gabions are stacked on top of each other to build walls, whereas Reno mattresses are used mainly as surface coverage in channels.
Gabions retain the flexibility of riprap with the added strength and longevity wire provides. Another advantage is their naturally porous drainage system. Concrete walls can often suffer from a buildup of hydrostatic pressure that can cause the wall to fail. It’s a non-issue for gabions, as the gaps between the stones allow water to pass through.
“A gabion structure, over time, will look more pleasing to the eye as opposed to a concrete structure, where you’ll get some staining near the drainage areas,” says Zach Titus, technical sales manager at Terra Aqua Gabion, Fort Smith, Arkansas. Gabions are easy to assemble and take advantage of the abundant availability of rock while using less material than riprap. This is an important consideration in light of transportation and materials costs.
Perhaps the biggest advantage gabions have is their ability to selfvegetate. They collect silt and plant seeds from stormwater
runoff. Over time, gabions will be covered with vegetation and become
invisible (if hydraulic flow is not too great). Of course, hydroseeding
speeds up this process considerably. The voids between rocks create a
protected environment for plant growth, making gabions a great choice
for environmentally sensitive applications.
Concrete block systems
When dealing with fast-moving water in a concentrated flow area, you may need something sturdier than riprap or gabions. This is when you turn to concrete block products. It used to be that hardarmoring with concrete meant lin- ing the bottoms and sides of channels with poured concrete.
has fallen out of fashion for environmental and other reasons. For one,
concrete linings aren’t flexible; any movement in the earth can result
in damage. One substantial crack in a wall, and the whole foundation might need
to be rebuilt, at great expense. Concretelined channels are not
natural- looking, and certainly not aesthetically pleasing. They also
speed up water, causing greater erosion.
There’s another option that combines the strength of concrete with the flexibility and durability of crete block (ACB). Similar to gabions. It’s called articulated con- riprap, an ACB barrier is comprised of an interlocking matrix. The individual blocks are placed together on a geotextile fabric base that’s compatible with the subsoil. The blocks are interconnected through a combination of forms and cables. Concrete blocks aren’t porous, so they require a suitable filter or drainage layer for proper design.
The blocks can be formed on the jobsite or bought prefabricated. If it’s going to be difficult to get cement mixers to the site, then prefab blocks might be the best option. That has to be weighed versus the cost of shipping them.
systems will conform to changes in the subgrade, and can be built over
varying contours and grades. However, they’re not intended to achieve
slope stabilization. That must be established prior to installation.
ACB is considered a hard armored solution that is also somewhat “green.” The voids between the blocks allow for vegetation to establish over time, improving the final appearance, and decreasing flow velocities.
According to Lee Smith, president of Houston, Texas-based Erosion Prevention Products, LLC, 18 to 23 percent of an ACB consists of void areas that can be filled with growth media. “You have the ability to hydromulch it and reestablish grass vegetation,” says Smith, “which is good for the carbon fixation, and it’s also good for aesthetics. You don’t have to see miles and miles of concrete slope paving.”
ACBs are also much more hydraulically stable than rock riprap. “A 24-inch-thick section of rock riprap has been known to start moving at just 10 pounds of shear stress,” said Smith. “That means it doesn’t take that much to become unstable.” By contrast, ACBs can withstand shear stress as high as 25 to 30 pounds, making it two-and-a-half times more stable.
Although ACB structures aren’t as easily displaced as riprap, they can still be vulnerable to hydraulic pressure. If water is allowed to cause gullies at the top of a bank, behind an interlocking concrete system, it can cave in. And because ACBs are interconnected, a cave-in can take out an entire system.
Learning proper installation techniques can be intimidating when trying a new hard armor product. But Alan Akowicz with Maccaferri USA, in Williamsport, Maryland, believes that the biggest learning curve comes from the site itself.
“A product is a product. On paper, we can build anything,” he says. “But we’ve all been on jobsites where if something can go wrong, it will go wrong. It really all comes down to being on the site, seeing the actual conditions, and then trying to place the materials based on that.”
Akowicz often travels to jobsites to train and work with contractors to ensure that his company’s products are installed correctly, and to try to speed the learning curve as much as possible. “These products are very easy to assemble,” he says. “A lot of it’s just like building Legos.”
Cost can be a challenge when considering any type of hard armor. With tight budgets, the emphasis needs to be on cost-effective, durable options that’ll continue to perform with little maintenance. “No one is spending a lot of money to have something work for only two or three years,” says Akowicz.
If rock is needed, the biggest budget item may be the cost of continued on page 30