Gabions for slope and channel stabilization

By Carol Brzozowski

From a precedent-setting “green” solution in New Jersey to a nail-biting installation in Idaho, gabions are increasingly becoming the erosion control method of choice by engineering firms and construction companies called upon to troubleshoot problems.

In 2003, a solution to an erosion problem set a precedent in New Jersey. A homeowner in Mountainside, NJ, called M. Disko Associates in Kenilworth after repeated storm events caused a tributary that wound through his subdivision to erode at the outer bank near his property, moving the bank closer to his home.

The tributary is fed by a twin 36-inch-diameter storm sewer. It flows into a branch of the Nomahegan Brook and into Echo Lake, which is stocked with fish. Though there was no fish life in the channel, and protection of property was the primary concern, any approach had to ultimately consider environmental concerns.

PHOTO: MACCAFERRI

New Jersey has limits on the amount of bank stabilization allowed. Choices in this case included bioengineering and other structural methods. Because of the high water velocities, no bioengineering choice appeared feasible, so riprap and gabions were considered as alternatives.

“Riprap wasn’t appropriate because of slope considerations,” says Mike Disko Jr., an engineer. “You’ve got to basically make traps or little channels out of that, but the gabions can be built more vertically, which mirrored what was happening here in this channel, because you essentially had vertical banks in many locations due to the erosion. So gabions were the first choice.”

Disko Associates teamed up with Maccaferri Gabions Inc. in Williamsport, MD, to come up with an appropriate erosion control solution and soil bioengineering solutions to integrate with the surrounding environment. The banks of the channel were stable; only erosion control was needed.

Disko Associates met with the state several times, because the firm wanted to exceed the state’s length limits. New Jersey’s Department of Environmental Protection planning regulations call for any erosion control solution protecting more than 200 feet of channel to be a green solution.

“To compromise, we used Green Gabions, which are gabions with plant materials. That was the first in New Jersey that they were allowing,” Disko says. Since the Nomahegan Brook project, his firm has received two more permits for projects using the precedent-setting approach.

Green Gabions from Maccaferri were selected because the PVC-coated woven wire mesh offers the strength and flexibility of a regular gabion, but the 30% to 40% voids between the rocks are filled with topsoil to facilitate vegetation. The gabions are lined with Biomac C, a biodegradable coir mat, to prevent the topsoil from washing out under high-flow conditions.

Beneath the flow line of the channel and below the Green Gabions, regular gabions were used, because they have a greater drainage capacity and provide the same erosion protection. They were embedded 3 feet into the channel bed to allow for future scour and to prevent undermining of the toe of the structure.

“The Department of Environmental Protection requires a 3-foot toe or footing, and we put regular gabions there. We wouldn’t want to put any plant material there because it would be a waste and that’s the active flow area,” says Disko. Imported rocks, ranging from 3 to 6 inches in size, were used inside the gabions. Live willow cuttings were inserted into the face of the gabions.

Disko says his firm has had gabions in the ground for up to 20 years now, so he anticipates the gabions’ longevity in this case to follow suit. Even with the new type of material, he says it was not difficult to teach construction crews to do the installation.

“The contractor was able to pick it up fairly quickly,” he says. “If the contractor has done gabion work, it’s essentially the same thing, because you’re just adding the dirt and the coir fiber in the front and then placing the stakings in the front, so it’s essentially the same as doing regular gabions. It’s just more of a green approach,” Disko notes.

Stairs to the River
Luling, TX, is a city of 5,500, situated in a bed of oil wells near Austin and San Antonio. The San Marcos River that runs through Luling had been eroding a bank and a small road next to a golf course. An ensuing flood resulted in the site being declared a disaster area. The city applied for help from the US Department of Agriculture’s Natural Resources Conservation Service, receiving $750,000 for the erosion control effort, which provided the bulk of the necessary financing for the $1 million project.

Then–City Manager Pee Wee Drake says Luling installed about 700 feet of gabion baskets on the river’s north side in a project that was bid in October 2003 and completed in August 2004. “It’s really a beautiful job,” he notes. “It stopped the erosion. We’d like to have about another mile of it, but it takes a lot of money to do that.”

PHOTO: MACCAFERRI

According to the C.E. Shepherd Co. in Houston, TX, which supplied the Modular Gabion Systems gabions, the project used 12-gauge galvanized PVC-coated gabion mesh with a 3-inch square opening and with stones of a similar size. Varying widths of mesh (15, 18, 36, and 72 inches wide) and precut 3-foot panels were used for the diaphragms, spiral binder connections, and lacing wire. The roll-stock option allowed the project to be constructed in continuous lengths as long as 300 feet, as opposed to using individual baskets, while the various widths provided for the appropriate setbacks at the face.

“We got nine lifts on it,” Drake notes. “We put a bucket down alongside the water and put the first basket against that bulkhead and then stepped up as it came up. It looks like a big set of stairs as you come up to the top of the bank.”

Working With Safety Nets
Gabions have withstood the test of time in many applications. Consider the challenging 1990 installation on the Banks-Lowman Highway, located 22 miles east of Banks, ID. The Western Federal Lands Highway Division found the US Forest Service road was eroding and needed to be restored to contemporary standards.

Grant Heaton is one of the owners of Western Construction in Boise, ID, the company that handled the installation of gabions used to combat erosion problems on the steep and rugged 7-mile highway.

“In many places, it was a one-lane gravel or dirt highway,” Heaton explains. “The Federal Highway Administration let out a contract in three different sections to put in gabions to widen out the narrow travel lanes.

“A lot of these places were steep canyons. There weren’t a lot of wide areas, especially on corners,” Heaton notes. “We would dig down, get a grade established, start with bigger baskets, and start backfilling and working our way back up to make the road wider for two-lane traffic.”

Although metal or concrete walls were possible alternatives, nothing would help make a radius corner like a gabion, Heaton says. “I have put in different kinds of walls where I used 15-foot sections of concrete and tied them with wire baskets, but they don’t make radius very well. That’s why gabions were chosen—because there were so many installed on sharp curves.”

Additionally, the Payette River, which at some points is located 800 feet below the road, dictated a solution that was environmentally friendly and well planned and executed. “We had to put in safety nets,” Heaton explains. “We designed a net system that hooked into the gabions so if somebody fell, they would be able to catch themselves on it.”

Heaton’s company sometimes had to excavate 25 to 30 feet to get the gabions started. “Some had to be drilled, blasted, and then dug out with a big excavator. We were putting the material in the rock truck, hauling it out, and then bringing the material back as we packed all the gabions up,” he says. “You could only put in one section of gabions and then you had to backfill it, do another tier, and stair-step your way up.”

More than 1.8 million cubic yards of rock were excavated over a two-year period. Some of the excavated cuts extended 200 feet above the highway elevation on 0.5:1 and 0.25:1 slopes.

PHOTO: MACCAFERRI

“Some were actually started by hand with jackhammers,” Heaton says. “Five air track drills were used to drill the rock in preparation for blasting.” Controlled blasting techniques minimized fly rock and overabundance material, with excavated materials needing to be contained in the existing roadway without spilling rock and debris into the canyon or over the bank within the high water line of the river.

The 4- to 8-inch gabion rock was created by crushing some of the rock that had been knocked from the hillside; it was run through crushers onsite. The rock was used to fill the 6,625 cubic yards of Eureka, CA–based Hilfiker Retaining Walls’ 9-gauge ArtWeld Gabions.

After the highway was excavated to subgrade elevation with the design done in such a way to minimize rock excavation on adjacent slopes, retaining walls were constructed at various locations to gain the necessary roadway widths. With a 12-person crew and working in three phases, Western Construction built 21 walls, and the Federal Highway Administration added four more.

One challenge of the project was that it called for workers to coordinate their movements carefully. Part of the crew had been assigned to backfill completed gabion baskets while the others placed and filled a row of baskets on the next wall. “We would have as many as three or four gabion sites in operation at one time to be efficient,” Heaton says.

Using a crane and a backhoe, a type of chute was created so that rock could be picked up by the crane and lowered to the workers below. One chute would fill a couple of gabion baskets. “It was labor-intensive,” Heaton notes. “You could only fill it up a third of the way because you had to place the rock by hand inside the basket. There was a minimum weight that could be placed in the basket itself.” Each gabion had to meet weight specifications of a given number of pounds per cubic foot, and every one of 20 baskets was weight-tested by an inspector.

One aspect intended to reduce labor and equipment time was the choice of 9-gauge zinc-coated welded gabions over 11-gauge twisted wire. “Those would be harder to assemble,” Heaton says of the latter. Of the 9-gauge gabions he notes, “These were fairly easy. You use hog rings, where with the other ones you would screw down spiral sections in the corners. These 9-gauge gabions were heavier and didn’t need all of the structural support you’d have to provide with the others.”

Hilfiker custom-built each wall using special 6-foot-wide by 6-, 7.5-, 9-, 10.5-, 12-, and 13.5-foot-deep baskets to minimize the number of baskets and simplify the wall layout for the crew. The walls ranged from 9 to 21 feet high, with a maximum single wall area of 2,800 square feet. The larger gabion baskets were used on the bottom and shorter-length gabions were stair-stepped up to the top, where there was a single row.

With the stability of the back slopes in some areas of the project very poor, some rockslides occurred and some equipment was lost. There were a few injuries. In fact, the job was so challenging that Heaton was grateful that injuries were few; he had anticipated there could be some.

“We did lose a loader,” he says of the equipment. “A 45-ton rock broke free from the slope and landed behind a loader bucket. It ruined the loader and the guy had some knee damage, but, other than that, miraculously he lived.”

And the area became safer for everyone. After construction was completed in December 1990, the widened road made it safer for drivers to pass through.

The project has held up well over the years, Heaton says. His company has been called in on two occasions to deal with slides in the wake of moisture that follows a heavy winter. “It’s over a newly constructed road, and you’re going to find that whenever water or moisture gets in there in slopes that are steep, it happens,” he says. “They’ve had to put some wire to slow down rocks in some areas.”

If he had to do the project over again today, there is nothing he’d do differently, Heaton says. “It’s just tough terrain,” he notes. “We had put in gabions before, but not on such a tense project.”

The learning curve was not too much for his crew. “There’s nothing like time in the sea,” Heaton says. “After you get established and get a crew going, within a couple of weeks you are getting pretty efficient at it. Projects like this are hard to make money with, but being able to open up three or four at a time makes it a lot more efficient where you can utilize your people well, and that’s what we tried to do.”

PHOTO: INTERNATIONAL EROSION CONTROL SYSTEMS

Saving a Roadway in Brunei
In the Sultanate of Brunei in Southeast Asia in 1995, a hillside kept sliding down around a curve in a roadway. Officials tried different ways to control the erosion before enlisting the help of International Erosion Control Systems in West Lorne, ON, Canada. By this time 3 feet of material had slid down to the road, which is a major artery linking different parts of the country.

“We built the toe and built [a wall] 12-feet high,” says Louis Arvai, owner and operator of International Erosion Control Systems. “It stabilized the hillside from coming onto the roadway.”

The interlocking concrete gabions used were constructed of precast block, generally 2 meters long, 0.5 meters wide, and 1 meter high. Holes ran from the front to the back of the gabions, allowing water to drain. Once they were installed, geotextile cloth was used to allow hydrostatic pressure to drain uniformly through these drainage ducts. “We don’t need to put extra drainage behind the blocks because the blocks themselves will handle all the drainage needed,” explains Arvai.

PHOTO: INTERNATIONAL EROSION CONTROL SYSTEMS

The bottom row, called a base block, was supported with a special footing poured underneath it, allowing for a 5% lean-back once the gabions are stacked. Those base blocks weigh about 5,000 pounds and the top blocks weigh about 4,000 pounds. The gabions are expected to last about 80 years, Arvai says.

A pattern was put on the face of the block to give the wall an attractive appearance. “Because it was a roadway, the local government wanted something that was appealing to the eye,” notes Arvai. “Also, because it was on a curve, the blocks had to be flexible enough to go around the bend of the road.”

With very little shoulder on the road and the gabion being well suited for a vertical structure, no digging back into the hillside was required. Earth anchors were driven into the ground and tied off. The blocks are configured to allow the anchors to be tied to a cable that protrudes up the backside, so no excavation is required behind the gabion for tie-back.

International Erosion Control Systems worked with a joint-venture partner in manufacturing the gabions, which were installed by a local company after International Erosion Control Systems conducted the training. “It’s a simple product to install,” says Arvai, “because once you get the elevation for the base and put in some granular base, it’s just a matter of stacking the blocks on the granular. Once you put the first row in, there’s a curve on top of the blocks, and when you set the next one in, it rests up against the curve.”

Carol Brzozowski is a journalist in Coral Springs, FL.

EC - July/August 2005