Gabions provide affordability, endurance.

By Tara Beecham

In various forms, gabions—largely recognized today as wire baskets or cages loaded with rock—have served as a form of erosion control for thousands of years. Even though blending into the soil or rock wall environments we pass daily in our contemporary world, gabions are working as hard as ever, in enduring forms that can last several lifetimes.

Stable yet versatile, gabions can be placed in land or water areas, altering their setting in a natural, made-to-fit manner.

Whether used independently to stabilize soil or as part of a larger retaining wall project, the appeal of gabions revolves around several considerations. The wire containers, which are fastened together in lengths that meet project specifications, keep the stone in check. While stability is the gabion’s hallmark, using gabions provides a flexibility not available to solid-wall structures such as a poured concrete wall, when a site requires as much.

With a wide variety of applications and uses, including shore and beach protection, channel linings, rockfall protection, counterforts, and culvert headwalls, cost-effectiveness is a primary part of the gabion’s appeal.

“Then they start looking at the advantages: low-end labor requirements, low-end equipment requirement, and the fact that the stone fill material for the gabions is local,” notes Colin Glass of Terra Aqua Gabions, which is based in Fort Smith, AK. Gabion baskets can be easily moved onto a worksite. Workers unfamiliar with gabion installation can quickly learn the installation process involved with gabions.

Gabions can also be an environmentally sensitive form of erosion control. Drainage from the soil or backfill can easily pass through gabions, allowing for a reduction in hydrostatic pressure. At the same time, gabions can maintain vegetation, creating a more natural effect when contrasted with concrete or solid blocks.

“When we do erosion control in this day and age, the goal of the designer is to secure the erosion occurring at the site through the most native means possible,” says Glass. “When designers look at the erosion control applications, they want to implement solutions that are as natural as possible to return the site to its original natural condition. Vegetation is very key in virtually all erosion control solutions these days. A gabion structure is 95% natural stone.”

When planning a gabion project, a foundation surface must be prepared, considering gabions can facilitate vegetation through their construction.

“We vegetate the gabions through multiple methods. The most common is to mix seed and soil with gabion rock,” says Glass. “That way you get soil and seed spread throughout the entire cross section of your gabion structure. The porosity of the gabions allows for a free exchange between the water table and the backfill.”

When working to achieve a natural look, very little works as well as live plantings. Another method of encouraging vegetation involves placing a row of gabions at the site, then driving stakes through the gabion itself into the native slope. This will allow the root structure of the plant, most commonly a willow tree live stake, to root into the actual native slope.

A 3,000-foot-long wall that stabilizes one of the support panels of the Lexington Avenue Bridge that stretches across the Mississippi River from St. Paul, MN, to Lillydale, MN, is one example.

Photo: Craig Olden Inc.

A composite gabion retaining wall structure

Dwayne Stenlund, a certified professional in erosion and sediment control with the Minnesota Department of Transportation (MnDOT), explains that the lower portion of the wall was constructed of limestone rock–filled, coated twisted wire gabions while the top part consisted of Green Terramesh gabions. Based in St. Paul, the 35E Bridge Reconstruction Project as it is known is located on part of the Mississippi floodplain and adjacent to Fort Snelling State Park.

“It varied up to 9 meters high for the rock-filled gabion portion, and up to 8.4 meters high for the Green Terramesh wall above the rock gabion. You’re trying to have living spaces. It’s in a state park. You are trying to blend the hard armor with the soft. The engineers might want the hardscaping; the environmentalists want the green,” says Stenlund. “We couldn’t flatten out the slope. We chose the gabions because we couldn’t extend the toe.”

The wall was also intended to resist being what Stenlund describes as a “taggable space” in an effort to prevent graffiti.

Maccaferri Gabions Inc. based in Williamsport, MD, was involved in the design of the project, and the company’s Green Terramesh gabions were chosen for the upper part of the structure partly because of their use in stormwater management.

“Living spaces treat stormwater,” says Stenlund. “This is a way of having living green spaces where they are engineered to give us the best possible water quality.”

In a separate MnDOT project on Route 61 in Two Harbors, MN, along the north shore of Lake Superior, gabions were also used for their natural appearance in the Silver Cliff Creek Tunnel Project. The road MnDOT was working on was deemed historic and had to be altered for a similar use, according to Stenlund, who says the organization converted it to a bike trail.

Photo: Terra Aqua

Properly constructed, woven wire mesh gabions can be a permanent solution.

“The gabion wall was built first and filled in. The subcontractor injected compost into the space of the gabion rock-filled basket,” he explains. “I call it compost grouting. We used a native seed mix designed for that habitat: a cool-season native grass mix.”

Once installed, maintaining gabions typically requires minimal, if any, additional labor. “Generally the only maintenance that we see is potential damage to the mesh, which is based on vandalism and/or sometimes hydraulic debris impact,” says Glass. “Any maintenance that we see is basically patchwork on the mesh.”

Terra Aqua manufactures woven wire mesh gabions. “When they are properly designed and properly constructed, they can be a permanent solution,” Glass says, noting that site conditions must be considered when installing gabions. “If you have contaminated soil, use PVC [-coated gabions]. Choosing the material type will be based on the site conditions. If you are taking the proper site conditions into account, and you construct it properly, then you can have a permanent structure.”

The pH level of the soil and the amount of water that will be interacting with the gabion materials should be considered on the project’s outset before materials are selected. A climate with large amounts of rainfall or extreme dryness can also affect the type of gabions chosen for a project.

Photo: Maccaferri

Sometimes, gabions are used as part of streambank stabilization projects, as was the case with the Trinity River Valley Authority’s project at Johnson Creek, in Fort Worth, TX. The bank had poor soil and was being eroded by the creek.

Typically, PVC-coated gabions and galvanized gabions are the most popular types, says Glass, noting the galvanized types are the less expensive of the two. PVC-coated wire is essential if the gabions used are installed in an aggressive environment, adds Brian Roche of Contech Construction Products, a distributor of Albuquerque-based Modular Gabion Systems products and New Mexico’s representative for the International Erosion Control Association (IECA) Mountain States Chapter.

“We anticipate using PVC materials for wet conditions—any time we expect a material to be submerged, any time you see a highly corrosive soil or highly corrosive water quality,” says Glass. “In contrast, we’ll use the galvanized for dry conditions when we expect only intermittent contact with water.

Photo: Maccaferri

 “For marine and oceanic operations we offer a stainless steel material, and that’s new. It doesn’t corrode. Instead of a steel wire and coating, it has a core of stainless steel wire all the way through, so it doesn’t corrode. We also see the stainless steel might offer some advantages for landfill projects where we see a high level of contamination.”

Gabions are available in several types of wire. Choosing the right type of gabion for a site can be an important part of a project’s success.    

“Double-twisted mesh provides strength and a flexible structure where it is very important for bank stabilization,” explains Ghislain Brunet of Maccaferri. “Double-twisted mesh also prevents the wire from raveling. Double-twisted mesh gabions are very good to sustain settlement.”  

Galfan gabions, made of double-twisted hexagonal wire mesh with zinc- and aluminum- (Galfan-) coated steel wire, are another example of gabions that Terra Aqua offers that are used in a variety of purposes, including building retaining walls, slope paving, and drop structures.     

Photo: Maccaferri

Mattresses are another option contractors can consider as an erosion control solution, particularly in sites where lining channels or shore protection is necessary. In addition to being filled with stone and saturated with a soil medium, one of Modular Gabion Systems’ products, the EcoMattress, has a natural fiber mat placed across the gabion top before the gabion lid is closed and fastened. This allows small seedlings to be placed in a way that would allow them to grow through the structure’s mat.

“Reno mattresses, commonly referred to as gabion mattresses, are typically used in applications where full coverage is required and a thickness of 12 inches or less is necessary,” says Roche, noting gabions are typically used when the required thickness for a structure is 12 or more inches.     

For example, Terra Aqua used 1-foot-thick gabion mattress scour protection for drain inlets along a ditch during a Texas Department of Transportation highway project based in Johnson County. 

Gabion sacks, which are created from a single piece of galvanized steel wire mesh, are also a form of gabion contractors can consider for hydraulic or emergency uses.
Limestone and granite are the most common type of rock fill used in gabions because they are both quarried stones. “Availability of stone is a primary factor,” says Glass.
The stone type, as with the gabion structure itself, allows a certain flexibility as well. “The type, not the size, is generally dictated by the locally available stone. The type of stone that is used must be non-erosive and durable,” says Roche, citing a US Army Corps of Engineers requirement that states the stone must be suitable for the worksite’s climate, must be free of defects that could increase the rate at which it deteriorates within the baskets, and must have a dry weight of no less than 165 pounds per cubic foot.

Gabions also allow for permeability that can eliminate the need for a drainage system as well as prevent hydrostatic pressure buildup that could fracture concrete structures, according to a statement by Modular Gabion Systems, which also reported that gabions address the design concerns of headwalls by allowing for climate changes, withstanding water pressures, and dissipating the energy of water flow.

Highway Help
Erosion control beside roadways is a common gabion use. “The NRCS Nashville office has been specifying gabions for the last couple of years on a series of county road erosion repair projects in far east Tennessee in the Appalachian Mountains. A lot of these rural roads have mountain creeks running alongside them and have been seriously eroded due to high flows from seasonal runoff and heavy rains,” says Ed Tinnin, who works in Midwest Construction Products Corp.’s Nashville office. “They have been armoring the road shoulders using gabion gravity walls with mattresses on the bottom for scour protection, along with wingwalls every 50 feet turned into the road perpendicular to the gabion wall. The road surface is cut back for the wingwalls and then re-paved by the county after the gabions are completed.”

Gabion mattress aprons can reduce turbulence by absorbing energy, and slow the velocity of an outlet’s flow.

 Appearance matters, particularly when erosion control is needed in a prominent location, as well as tenacity. “Because of the natural beauty of these areas, some other type walls, such as concrete, would not be desirable; nor would they stand up to the high volumes and velocities coming down these creeks,” says Tinnin. “Riprap would have been aesthetically acceptable and a little less costly, but would not have stood on its own either. The gabions, together with the wingwalls, have the weight and strength to withstand the flows and retain the earth while letting the water flow through the walls.”

The company currently uses PVC-coated gabions on these projects, he explains, because these sites are each partly covered by water. Midwest Construction fabricates and supplies welded wire gabions. The eastern Tennessee projects required the use of different sized rock in their gabions and mattresses.

“The rock size for the gabions is 4 to 8 inches, and for the mattresses it is 3 to 5 inches,” says Tinnin. “The rock fill should be in the specified size range, but if there is a problem with getting a lot of rock smaller than specified—smaller than the mesh openings—then the smaller rock should be used in the interior of the baskets and the larger rocks used for facing to hold the smaller rock in.”

Compared with the use of riprap, gabions are less expensive in most instances because “when you use gabions because they are wire enclosed you are not using as much rock,” explains Roche. Gabions allow the use of smaller rock within mattresses, he says, because 4- to 6-inch stone is often used within gabions.

The Modular Gabion Systems product is a welded-wire gabion that allows for a rigid containment, compared with a flexible containment system. While shear stresses and velocity must be regarded when formulating an erosion control plan, if there is a ready supply of rock in an area, articulated block will be more expensive than gabions, according to Roche. However, he notes, in areas such as Clark County, NV, where rock that could be used to fill gabions is difficult to find, gabions can be costly.

Roche is familiar with instances where customers will use hand-placed flat stone on the facing layer of the gabion to improve gabion appearance, and that can also increase the cost of a gabion project.

“You’ve got to basically handpick rock,” he explains, noting precast facings can also be used for a different appearance.

Earlier this year, Craig Olden Inc., a gabion design and building specialist based in Little Elm, TX, used gabions in a project it was constructing for the North Texas Tollway Authority based in Garland, TX.

“The project consisted of a composite gabion retaining wall structure composed of a gabion facing combined with additional structural elements including reinforced concrete beams and a tie-back anchor system,” explains Trevor Bray, vice president of Craig Olden Inc. “The composite gabion structure was used to stabilize the creek banks downstream from an existing concrete-lined channel.

“PVC-coated woven wire gabions made of 8-by-10 mesh were used to construct this project. A 4-by-8 limestone rock was used to fill the gabions.”

Photo: Maccaferri

Going Green
In New Jersey, the state department of environmental protection requires a “green” erosion control method when that solution safeguards more than 200 feet of channel. Maccaferri used Green Gabions in the fall of 2003 when working on a project at Nomahegan Brook in Mountainside, NJ, which had been eroded by several storm events. The brook feeds into fish-stocked Echo Lake, requiring the erosion control solution to be an environmentally sensitive one.

“On this project, gabions were combined with vegetation to increase the environmental aspect of the project,” says Brunet. “As a hard-armor solution, the gabion is comparable to and more economical than other solutions. It allows vegetation to grow, compared to other structures like block wall.” The open spaces may also provide a form of shelter for aquatic creatures.

Photo: Maccaferri

Strength and flexibility remained considerations, as did the effect of water on the wire over time. “All the gabions used in this project were made with PVC coating over a zinc revetment,” explains Brunet. “The PVC coating will increase considerably the life of the structure and reduce the maintenance.”

Allowing for future scour, the gabions were placed 3 feet into the channel bed. This was also done to prevent any future undermining at the toe of the structure. Rock with a standard size of 4 to 8 inches filled the gabions. After topsoil was placed within the gabions to facilitate vegetation, willow cuttings were positioned in the face of the gabions in an attempt to improve the site’s appearance. As early as six months later, the vegetation was well established.

Employee training also aided with the project’s success. “Maccaferri always provides site assistance for gabion installation. It is very important for the appearance of the structure but also for the stability,” says Brunet. “The design stability of the structure relies on laboratory tests, and we have to be sure that the installation reflects the design parameters.”

Photo: Maccaferri

Cape May Challenges
In 1996, a project began to protect a small channel that runs beside a stretch of the Intercoastal Highway in Cape May, NJ. The picturesque tourist destination located in the far southern region of the state had been threatened by natural and manmade forces, and a plan to protect the waterway began. From the outset, the site had many influences working against it.

“There is a lot of commercial and pleasure boat traffic that runs through there,” explains Maury Shepherd, executive vice president of C.E. Shepherd Co., the parent company of Houston-based Modular Gabion Systems, whose products were used in the project. “They had pretty serious erosion problems in an area with valuable real estate.”

Trevcon Construction Co. served as the contractor for the project, where tug and barge traffic coupled with currents created significant wave action against the project’s bank.
“They needed something that would provide protection for the bank, but also be compliant with whatever movement there was in the soil,” says Shepherd. “The wave action energy was absorbed by the rock. But the gabion keeps the rock in position. Gabions perform much better than riprap.” Welded-wire mesh gabions were selected for use in the project.   

“They have PVC powder coating and are very corrosion resistant,” notes Shepherd, adding that the salt water can’t get under the fuse-bonded coating. “They are also extremely strong.”

Production records indicate that the welded-wire gabions were able to be installed 71% faster than other gabion materials that were tested during a comparative study for the project.

“The comparison was on identical cross sections of the channel with the same crew, which had experienced installation with other types of gabions,” explains Shepherd. “One of the interesting points about this is that by using the welded-wire mesh gabions, they actually finished the job very early.”  

Future damage to the setting was also a consideration. “In any kind of canal they have barges that get loose. They run up on the bank. If any of the gabions are damaged, it’s easy to replace panels in the project because they don’t unravel,” says Shepherd. “They can take out the damaged part, and the joint is as strong as it was originally.”

A decade later, the resulting stabilization has maintained a natural effect at the site. “It looks like a natural embankment. [Because of] the natural deposition of seeds by birds and tidal influence, they eventually revegetate themselves once things stabilize,” says Shepherd. “The project is basically completely revegatated. It has held up extremely well.”

As new erosion control products emerge on the market, improving soil containment methods and site stability, a component as basic as the gabion that can continuously address complex site issues with one application retains a secure place in the industry.

A permanent link with the erosion control industry’s immediate past and its position as an affordable erosion control option requiring a small amount of labor and, in rare cases, maintenance has helped the gabion endure.

Based in Morgantown, PA, Tara Beecham writes frequently for Erosion Control.

EC - September/October 2006