From shoring up aging infrastructures to redirecting water flow to offering an artistic appearance, gabions and hard armor have become an increasingly important tool in North America’s erosion control efforts.
Municipal officials in Enniskillen, ON, have been slowly rebuilding aging bridge structures since the early 1990s using precast concrete box structures manufactured by International Erosion Control Systems (IECS). Enniskillen has worked on one bridge a year for the last several years using IECS concrete gabions.
“The design has changed slightly, but we’ve been using them for erosion control at least since the 1990s,” says Ray Dobbin, township engineer.
The concrete gabions combine noncorrosive interengageable stackable concrete blocks with transverse ducts and are available in various sizes. The gabions are teamed with geotextile cloth.
Because the product has proven itself reliable over the past decade, Dobbin continues to use it in various applications. “We’ve been using them as end walls,” he says. “We like the drainage holes and the curve on top. We also use them on roadwork. We like that with them being precast, we can construct a small bridge in less than a day.”
Dobbin says the township has used the concrete gabions as wing walls on precast structures, fastening down one layer of the blocks across the top of the structure and using that as an end wall across the bridge to support the cover over the bridge.
In choosing the gabions for the project, Dobbin considers structure. “That’s because it’s on a road, as opposed to a farm culvert in a field along a road. Structurally, the base blocks are on the bottom and then we build the cinder blocks up with the curve,” he says. “We generally don’t go too high with them-maybe three or four high-and with those features, it makes it easier. They work better building the wall and supporting the wall and the traffic. It stands up to the load on the road.”
Drainage issues for the backfill in the road also are a concern, Dobbin says. “The precast concrete will have joints in it. We wrap them with filter fabrics, and that helps to drain the backfilling behind the structure. Because we’re in clay soil here, we backfill with granules to keep it well drained in behind; that works out well.”
Dobbin says he doesn’t like to put any longer culverts in than he must. “We want to put the minimum amount of culverts in and make use of that for the traffic portion on the road. That makes it important for the end walls to be able to stack them and keep them as straight up and down as we can to maximize the road width. These stack very straight and we’re able to maximize that. Because of their design, we can hold up that embankment on the wall fairly straight up and down, get a retaining wall, and put a shorter bridge in.”
For municipalities on limited budgets, Dobbin finds the precast structures cost-effective. “Our concern is we can take a bridge out and replace it with a precast box and these precast concrete gabions,” he says, adding that can usually be done within a week.
“We can take the bridge out Monday, set the precast structure on Tuesday, set the wing walls on Wednesday, the backfill on Thursday, and Friday we clean up and traffic is OK for the weekends,” Dobbin says.
The time benefit is key, Dobbin points out. “With precast, we know what we’ve got to do, so we get it all ready. The blocks come and we stack them and we’re away. We don’t have a bridge down half the summer or for a month.”
Another benefit is that much of the work can be done in-house, Dobbin notes. “We do a lot of the work with our own guys and use our own dump trucks and laborers to build a lot of the bridges,” he says. “That’s another attraction-we just dig, set, and backfill. That’s why we got into precast.”
Appearance is another consideration. “We’ve used broken concrete and riprap with these gabions at the end of the bridges, and we’ve used plain concrete blocks,” he says. “The appearance of these look much more professional and they stay consistent in appearance. It’s visually appealing, more like how new bridges look. It gives the appearance of a more complete, finished job, and that’s important. We don’t like to-and can’t afford to-spend a lot of money on them.”
Aging structures means the town will be engaged in such projects for years to come. “A lot of our structures are old, built in the 1930s, and they are narrow,” Dobbin says. “That’s a big concern because new farm equipment and trucks are wider now. It’s time to replace the bridges.”
Flexibility and Design
Specialty Contractors Inc. in Phoenix, AZ, has used gabions in a variety of applications-from the practical to the artistic. The company is a specialty road contractor that performs all types of erosion control for roadwork, including projects using gabions and riprap.
Gary Waugh, the company’s general manager, has frequently used gabions from Maccaferri. “They’ve always given me excellent support,” he notes. “Their company is more than 100 years old, so they’ve been doing this longer than anyone else.”
Maccaferri gabions are rectangular wire mesh baskets filled with rock at the project site. The double-twisted hexagonal steel wire mesh is reinforced by heavier wire alongside the edges and by transverse diaphragms. They are used for retaining walls and other erosion control applications.
The value of vendor responsiveness to contractor needs was hammered home several years ago when Specialty Contractors was working on one of its largest projects ever-a flood control project for the Flood Control District of Maricopa County in Arizona focusing on beautification and realignment of a portion of the Skunk Creek Channel. Over 4 miles of Skunk Creek Channel, Specialty Contractors installed 450 cubic yards per day of Maccaferri gabions six days a week for nine months.
“They would have at least one truck pull up on the job every week supplying us with gabion materials on an as-needed basis, and we never ran out,” Waugh says of Maccaferri.
The time constraint was “tremendous,” Waugh notes. “At 450 cubic yards a day for six days, everybody was basically freaking out thinking that this could not possibly happen,” he says. “That’s nearly 3,000 cubic yards a week, which is basically a full truckload, and to not have the supplies stop or any interruptions was pretty amazing.
“Normally you end up having to order these things way up front. They never let us down. They were “˜Johnny on the spot.'”
When the project was completed, the entire gabion job was covered with dirt. “You can’t see them, but they’re there,” Waugh says. “They are protecting the site nonetheless.”
Maccaferri’s rapid response to needed materials has been consistent whether the job has been large or small, notes Waugh. “We did another project in northern Arizona where we had a yard so small we could only stockpile a certain amount of gabions there,” he says. “We had to have a truck show up in the far reaches of northern Arizona because there was such a small area we staged out, we couldn’t load up on gabions. We never had a problem running out of gabions. Their supplying us with materials was flawless and it always has been.”
Waugh says the major advantage to gabions is that they’re a flexible approach to erosion control, allowing for slight ground shifts. “Gabions are far superior to either grouted riprap, gunite channels, or concrete channels being that they won’t break, won’t crack. They flex and they continue to do their job for years and years,” he says.
Environmental considerations also come into play, Waugh points out. “With the environmental consciousness of the nation now, gabions are great because grasses will grow through them and eventually they will fade into the landscape. Plus, you don’t have graffiti; people write on gunite channels or concrete channels.”
Another Specialty Contractors project using gabions was one undertaken for appearance’s sake only: the Huhugam Heritage Center in Maricopa City, AZ, for the Gila River Indian Community.
“We made a huge gabion wall,” says Waugh. “It was a 130-foot radius rising from 12 feet high on each end, with the highest point about 37 feet tall. Each basket was offset from top to bottom and from left to right so that the entire project looked like a pottery shard.”
The wall featured an inner face of a retaining structure; the outer face was a 2:1 slope to the ground level, and tunnels provided access. To reduce the cost, the project made use of Terramesh, a gabion facing unit with an integral soil-reinforcing “tail” element, compacted within the structural backfill embankment material to form a mechanically stabilized earth structure.
“It’s just gorgeous,” says Waugh. “We used angular rock and a special-colored rock, and when we set the rock in the baskets, it looks like a masonry fireplace. When you stand back and look at it, it looks like a pottery shard. That’s been one of the most interesting jobs I’ve done.”
Scour Protection
The Utah Department of Transportation (UDOT) has a program to conduct underwater inspections of bridges every five years. If an inspection shows that scour from the action of the water is eroding soil around the foundation, the state takes action to shore up protection.
Bridges are categorized according to their risk of being damaged because of the scour, says Michael Fazio, deputy director of research for UDOT.
Two Utah bridges caught the attention of inspectors: a bridge over the Green River on State Route 19 in Green River, UT, and a bridge over the Colorado River on State Route 191 in Moab. Scour was caused water that surges down from the Rocky Mountains through the Grand Canyon along the Colorado River, where it picks up momentum before entering the downstream areas at the bridges in Utah.
“Scour had removed most of the protection from the foundation, and they were assessed as possibly critical and due for maintenance repair or for protection,” says Fazio. “We were investigating what would have been the best way to protect the bridges, and we were looking for consultants to help us out, but the cost was higher than we could afford, so we started looking into ideas through vendors.”
A solution was presented by Contech Earth Stabilization Solutions, which proposed the use of A-Jacks. The high-stability concrete armor units are designed to interlock into a flexible, high-performance matrix that disperses energy and resists flowing water’s erosive forces to address scour and erosion.
“The vendor offered information showing us A-Jacks would provide adequate protection if installed properly,” says Fazio. UDOT officials used that information as a baseline to do their own research. They examined the third-party research on A-Jacks conducted by Dr. Chris Thornton of Colorado State University, who specializes in hydraulics, river mechanics, erosion, and sediment and is director of the university’s hydraulics laboratory. UDOT officials also read about the design and installation of A-Jacks in a federal publication and witnessed an installation in another state. The state agency then conducted its own installation of A-Jacks at one of the piers in the summer of 2003.
The installation in the Green River and the Colorado River was challenging. “There is very poor visibility. We had scuba divers who couldn’t see anything and had to feel and touch and then place the A-Jacks accordingly,” Fazio says.
UDOT officials proclaimed the installation a success. “We were quite amazed at the results,” notes Fazio. “Since this was still an experimental feature, we employed the help of the United States Geological Survey [USGS] to monitor the installation.”
The USGS conducted a three-dimensional measurement of the riverbed at the Green River site so UDOT would have a before and after comparison. After the installation of the A-Jacks, the USGS used an acoustic Doppler technique with real-time global positioning and side-scan sonar imaging to chart the channel bottom.
“The A-Jacks were able to slow down the water sufficiently so that deposits around the pier that were determined scour-critical and increasing were shifting the flow from that side of the river toward a different part of the river under the bridge,” Fazio says.
While noting success, UDOT officials are still curious to see how the system will perform under very high flows. “The flow through the Green River has been around 20,000 cubic feet per second and usually can hit up to 40,000 cubic feet per second in very high flood flows,” says Fazio. “When we get there, then we’ll know just how successful this is.”
Essentially, the A-Jacks structure, which is built in a three-dimensional fashion, acts as a “big mattress with a very low center of gravity,” says Fazio. “It’s about 10 feet high by 10 feet wide, so its center of gravity is low enough so the flow of the river wouldn’t automatically pull it out or move it away if a strong flow would come.
“Also, the shape of the A-Jacks on the top would provide enough roughness to reduce the velocity so there wouldn’t be as much force under or around the piers. The A-Jacks act as an energy dissipater around the piers.”
Fazio points out that what makes this installation different from a gabion installation is that the concrete pieces can be interlocked and banded or cabled together into modules to create a large mass flexible enough to conform with a curved channel bottom.
UDOT subsequently installed A-Jacks on other projects in the Colorado River and San Rafael River, Fazio notes. In the San Rafael River project, the A-Jacks had been installed under the flow line of the river. His agency planned to visit the installation-completed in the summer of 2006-to see how the A-Jacks performed in the wake of a high water flow in September 2006.
UDOT intends to continue using the product in future applications, such as in areas where the erosion from the scour action of the river is quite high and especially in the southeast part of the state, where there are rivers that are very dense because of the high sediment loading they carry from surrounding watershed areas.
