Rock-filled cages offer some advantages over other types of structures.

By Lynn Merrill

For more than a century, gabions have been used to prevent erosion in a variety of settings and applications. Their ability to flex and settle and the ease with which they can be installed make them ideal for both waterway stabilization and cut-and-fill situations.

The term gabion is defined as a cylinder of wicker filled with earth and stones used in building fortifications. The modern term originated in Italy but appears to have been derived from a Latin word meaning "woven basket." Gabions were used for erosion control applications along the Reno River near Casalecchio, Italy, in 1894.

Gabions, also known as rock cages, are double-twisted, steel-wire-mesh boxes used as a confinement system for stone. They can be used as hard armor to control erosion in hydraulic and soil-retention applications, but unlike hard armoring, gabion structures are porous and allow for free drainage. This permits silt deposits to accumulate in the voids between the natural stone, which allows vegetation to become established over time.

Unlike rigid concrete structures, gabions have the ability to settle and flex under various environmental conditions. Gabions can be used in such vertical applications as walls and can support such structures as roads or parking lots. In water channels, gabions can help dissipate some of the energy in the water flow through the rough surfaces and voids between the individual rocks within the gabions. In conjunction with other erosion control techniques, gabions can be applied to settings where a natural appearance is required.

Design improvements and advancements in the use of galvanized materials, zinc, and polyvinyl chloride - coated wire mesh have expanded the application of gabions. Factors to consider when determining whether gabions are an acceptable solution for a particular site include the corrosion level of the soil and/or the water, access to the site for equipment to install the gabions, availability of stone fill material, and the project-site geotechnical parameters. Gabions are fairly easy to install and do not require the complicated engineering necessary for the installation of concrete structures.

In deciding whether or not to use gabions in a particular application, it is important to find someone who has experience installing the product, suggests Tony Blatnik with Johnston Morehouse Dickey Company in Brecksville, OH. "There are many companies that sell products and get them specified but have never actually installed them," he says. "Education in the field is the best way to accomplish the goals of your clients. There are Internet and software packages available to help with designs, but they only help with the baskets themselves. Most do not tell the designer the other erosion control products that should be used in conjunction with the gabions. I personally have seen failures because the designer used a software design and never placed a fabric under the basket. With water running under, through, and over the baskets, it was only a matter time before the dirt base was gone."

"Gabions are typically utilized as a long-term or permanent solution to erosion control," says Colin Glass of Terra Aqua Gabions Inc. in Shouteau, OK. "However, there are also several temporary applications in which gabions can be used to abate soil erosion from project sites. These include scour pads, filtration devices, drainage paths, and storm drain protection. Gabions are commonly used in conjunction with other erosion control products. [For example], gabion retaining walls and gabion channel linings are always used with fabric filter to prevent fine soils from being displaced and transported. For long-term solutions, gabions can easily be vegetated at the time of construction utilizing several additional erosion control products to initiate vegetation."

MIT Stata Center Project

At the Massachusetts Institute of Technology in Cambridge, MA, construction of the Stata Center for computer science included the installation of gabions as part of a stormwater drainage system planned to complement the Frank O. Gehry - designed building. "There was a requirement by the City of Cambridge to retain stormwater runoff," says Dick Bailey, project manager with The Welch Corporation in Brighton, MA. "They ended up designing a detention area to retain the water. Above this, they wanted to create a wetlands look with plants and a large boulder field. It's lower than the surrounding site, and on two sides they ended up using gabions to create a stepped look to give it a natural setting."

The construction specifications included the requirement for stainless steel gabion cages. "We actually ended up fabricating the baskets on-site rather than having the baskets made off-site," says Bailey. "It was cost-effective to do that because these things stepped back at various distances. We had to go through a process of getting the stainless steel made because that's not the preferred wire for baskets. These are not the typical gabions just filled with dump rock. The front and top face of the basket originally would have been cut stone to give a stone-wall look. When we did the mockup, we didn't like it, and we ended up using blast rock. We had to cull through and get stone with a flat face, hand-set the front face, and then backfill behind it with smaller 6-inch-minus rock. The top face is also layered with flat-face stone but in a random pattern. This was labor-intensive but only from the factor of putting the stone in."

Bailey says construction of the gabion system, which used materials from Modular Gabion Systems, went smoothly. "We were able to roll out pieces that were 100 feet long and then put the sides and spacers in. It has the look of standard baskets, but it does have an architectural feature to it that you don't normally see." Specifying stainless steel wire also was unusual on this project but was designed to complement the building's material palette of brick, metal panels, and curtain walls. "I think the architect wanted something that would be a little out of the ordinary, something that probably would last longer and not end up having a plastic coating to it."

Fabricating the baskets on-site allowed various activities to be performed simultaneously. "There were several operations going on that had to interface together," notes Bailey. Liners for the rain storage and wetland areas had to be installed while the gabions were being fabricated.

Bailey suggests relying on the manufacturers of gabions for technical assistance in designing and installing the system. "That will help you out immensely. We considered having the baskets made and sent to the quarry [to have the stone work done there]. We talked with the gabion people, and they said, 'Well, these aren't really baskets that you can pick up and move everywhere.' After we talked with them, we actually had the stone cut in the quarry and brought it up on pallets to cut down the amount of stonecutting done on-site. But understanding the application and talking to the people who supplied the gabions really worked well for us because they put us onto a different track."

El Malpais National Monument

At El Malpais National Monument in New Mexico, strong seasonal rains every August were causing a significant erosion problem at a 6.5-ft. culvert located in the Sandstone Bluffs Overlook area. "Water would be coming out like a fire hose, and that caused an erosion problem because it was a very fine sandy area," explains Clifford Walker, a civil engineer with the National Park Service's Santa Fe office. "That sand was getting picked up and creating what we call an 'arroyo,' a deep-cut wash on the downstream side of a culvert."

The initial design for a mitigation project was done in 2001, and a funding request was submitted to perform the work. Authorization to proceed with the project came two years later. "I had just started working for the Park Service, and they handed me the sketches and said 'Okay, finish off this project,'" recalls Walker. "We went out there and got a more accurate survey, and we found out that in the two years that had passed, it had cut another 7 feet deeper. We had to modify the design a bit and expand it; unfortunately that put us a little bit over budget." The project was constructed in 2003.

All of the erosion at the site occurred downstream, and the impact of the water through the culvert caused a large volume of erosion. "When you looked upstream from the culvert, there was no erosion whatsoever," says Walker. "It was just a nice sagebrush-type desert field. Downstream, there was just this gash that cut about 27 feet below the level of the surface right below the culvert. The culvert was literally sticking out from the wall that had been cut back about 6 feet and was in danger of collapsing. There was only about another 3 feet of earth to be cut back, and then the road would have collapsed."

A large gabion structure was designed and constructed below the culvert using Maccaferri gabions. "At the end of the culvert, we placed the floor of gabions in a triangular shape to spread out the flow of water," says Walker. "It has 6-foot gabion walls on either end, and then at the end of the triangular shape, it starts stepping down to 3-foot gabion steps. Below that, there is a huge gabion apron, 40 feet wide by about 30 feet long, and this is in the flow line of the culvert. About 40 feet farther from the end of the apron, we put in a gabion check dam to try to trap sediment. At the culvert level, we put some dissipators sitting on top of that triangular mat to try to get the water to spread out and decrease its velocity."

The project was completed before the 2003 rain season and appears to be working well. "So much sediment has settled out that it has almost filled up the entire area behind the check dam," comments Walker. "Downstream, we were really concerned about the water blowing over the check dam because maybe it would cause some erosion there. It seems at that point the velocity has slowed and is not causing any erosion further downstream."

The project required removing the remnants of a previous gabion structure that had been installed in 1986, as well as large chunks of volcanic rock that had been dumped in the area in a temporary erosion control effort. "The contractor had to get in there with his excavator and spread those around," says Walker. "Then, to reestablish the flow line, he had to cut into one wall and spread that dirt around. Once all of the earthwork was done, he switched to a smaller bucket on his excavator and used that to handle the rock and dumped it into the gabions. He had a crew of six or seven guys out there filling and sewing the baskets and forming the structures."

Other approaches were explored, including the use of shotcrete, but there were concerns that such a rigid structure would be undermined by the water flow. "The gabions are kind of flexible, so they can settle and the structure is not compromised," says Walker. "Another thing we liked about the gabions was that they allowed the water to filter down through the baskets, and you can control the infiltration rate with geotextiles. The shotcrete is impermeable, but by allowing the water to filter through the structure, you have a pressure valve and don't create a pressure gradient between the dry soil and the end of the structure."

One of the largest challenges Walker faced during the design phase was to develop asymmetric drawings to facilitate construction. "When the technicians started drawing the plans, they had trouble visualizing some of the sections," he recalls. "Our solution was to go out and buy a bunch of Lego bricks. We actually made a model, which worked great. They correspond almost exactly to the size of the gabions that are available. Once you build your Lego model, then you can make your plan and your cross sections and everything but also include a nice symmetric drawing. Originally I was criticized for including isometrics in technical plans, but the contractors and everybody were thankful that the sheet was there so they could visualize the end product and how it was supposed to come out."

Fort Worth Alliance Airport

Expansion of the facilities at the Fort Worth Alliance Airport in Fort Worth, TX, caused an increase in the amount of drainage area, resulting in a need to expand the capacity of the drainage channels. Gabions were chosen as a cost-effective solution to prevent sidewall scouring of the channel while ensuring an aesthetically pleasing structure.

"The project that we worked on was a relief channel that drained large amounts of water off the runway systems," explains Mark North of Mark North Erosion Systems in Fort Worth. "Basically they were large gravity walls that varied in height from 6 to 18 feet tall. The channel bottom was a 3-foot-thick gabion mattress that was [needed] for the velocity and the amount of water that was coming through the channels. [There were concerns] about the scouring along the embankment and also about the flow in the bottom of the creek channel."

Installation of the gabions was fairly straightforward. "We excavated out the gravity portion of the wall and had to lay the slope embankments back to an angle of repose that would be stable during the construction process," says North. "Then we brought in the gabion wire and fabricated the baskets on-site. We brought in large amounts of limestone for the installation of the gabions, and then behind the structure, we filled in [the area] with granular material for drainage. It was something of a high-profile site, so the customer wanted the baskets very symmetrical and very full. We had to take angular, graded limestone and place it by chipping and facing to make it look like it was a flat profile of fitted stone."

Gabions were chosen as opposed to a concrete structure. "The gabion structure has about 28 to 30% void between the rocks; therefore it's very permeable. You have less hydrostatic load behind the wall." With an impervious concrete structure, North explains, "You have to have a very elaborate drainage system; plus, you have more cost relating to steel reinforcing and more excavation to handle lateral earth pressures that are pushing against that structure trying to make it fail in both sliding and overturning."

North feels that gabions are appropriate structures for wetlands and creek settings. "There are a lot of things that you can do with gabions. You can use some different types of facing stone and face the outside of the baskets with the premium stone and then infill with a less-expensive limestone. Because gabions have a void between the rocks, you can inject topsoil and then plant species, such as vines, to get a very green wall." He points out, however, that "gabions may not be appropriate for use in high-traffic areas" where people coming in contact with them run the risk of "snagging their clothes on the wire. Gabions are much more appropriate down in a creek setting instead of in an area that's going to get lots of human touch."

Installing gabions is a much faster process than building a concrete structure. "I believe that the structure at the airport had approximately 1,300 square yards of gabions in it, and we built that project in 15 to 18 working days," comments North. "The advantage of the gabion in a creek-type setting is that water control is not as sensitive as it is for concrete. You've got to have a very dry area and 40°F and rising temperatures to pour concrete. In concrete applications, you have to form and tie the steel, and when you get ready to pour, you have to make sure that you have a long enough weather window to make it happen. In building a gabion structure, if you get a storm event and the water rises, you can't work while the water is up, but as soon as the water goes back down to its normal flow, you can get back to work. It doesn't matter how wet or cold the site is."

Author Lynn Merrill is director of public services for the City of San Bernardino, CA.

EC - May/June 2004