Solid Stabilization

July 1, 2006
Size matters in many things, but not when it comes to controlling erosion with hard armor.On the large end, consider Natchez Bluffs in Mississippi, “perhaps the biggest waterway stabilization project ever done,” says John Wolosick, director of engineering with Atlanta-based Hayward Baker, which completed three of the four government contracts awarded to reinforce a 2-mile, up-to-74-foot-high section of the Mississippi River waterfront. Using massive amounts of soil nails and shotcrete, the government-funded project spent $25 million shoring up a landslide that claimed Clifton Avenue and a number of homes listed on the National Register of Historic Places, Wolosick says.Now downsize to San Rafael, CA, where Steve Zeiger, senior associate engineer and stormwater program manager, oversaw the installation of 100 feet of Matterhorn California Inc.’s Secura Slope retaining wall next to a home along rain-flooded Mahone Creek. “It has held up very well through a couple of storms,” Zeiger reports. “If we didn’t install it, the guy would have lost his backyard. We spent about $30,000. That cost a lot less than other things we could have done.”
Photo: Matterhorn California Secura Slope retaining wall along flooded Mahone CreekHard armor includes concrete, riprap, stone, blocks, and retaining wall solutions. Uses of these materials range from holding back the erosive effects of the massive Mississippi River to holding back those of tiny Mahone Creek. Hard armor is particularly useful for controlling erosion along waterways. The US Army Corps of Engineers has traditionally used hard armor because it is a permanent and predictable solution. Hard armor, though, has lost some favor with a public seeking softer methods, such as turf-reinforcement mats and coir logs. There are several good reasons for using softer methods. One is they look more natural look and often preserve habitat. Also, entire channels lined with riprap or concrete, as opposed to softer methods, tend to increase the velocity of the water flowing through them, which can lead to increased erosion downstream. However, existing conditions and water velocities sometimes make hard armor the only practical choice, either by itself or combined with softer alternatives.Mattresses and Gabions
Marine mattresses—stone-filled mats, surrounded by plastic geogrid that resists wear—have been gaining in popularity. The mattress’s ability to cover and stay in place on steep slopes is particularly impressive to Travis Carpenter, project engineer with MACTEC Engineering and Consulting, with corporate offices in Alpharetta, GA. Carpenter is based in Portland, ME, and has been involved with two recent projects using Tensar Earth Technologies’ mattresses along that state’s Androscoggin River as well as the Buffalo River in New York.
One advantage, according to Jeff Fiske, Tensar’s coastal waterways product manager, is “the ability to move a large monolithic unit into place and cover a lot of ground in a single pick. They are available in units of 175 square feet and, on a prepared subgrid, can be placed in as little as five minutes apiece. That gives an ability to cover a lot of ground rapidly. It works well along shorelines and inlets and in brackish water conditions. It also has capability on steep slopes. Where many revetments are limited to 2-to-1 or flatter slopes, this has been installed successfully on much steeper slopes, as steep as 1-to-1.” Tensar’s Triton Coastal and Waterway Systems provide different options for use in and around the water. The Triton family includes marine mattresses, marine cells, and gabions, as well as segmental retaining wall systems and geogrids (connectors). Using readily available natural fill material creates flexible cells that are significantly less expensive than conventional solutions, such as riprap. Mattresses also conform to land contours and site configurations while resisting scour better than rigid systems. The structural geogrids resist chemical, biological, and environmental degradation. Along the Androscoggin River, fluctuating water levels were eroding away steep embankments in both cases. “This Tensar marine mattress–type product can be favorable in those types of applications,” Carpenter notes.The Buffalo River project involved 200 feet of shoreline, covering from the top of the slope down to below the waterline, with marine mattresses used along the entire slope. In Maine, by contrast, Carpenter has employed two different systems along 500 feet of bank: mattresses from the toe to the midpoint of the bank and a softer, vegetative system the rest of the way up.The mattresses, he explains, are comparable to gabion baskets. The geogrid material is compartmentalized, stretched over a frame, filled using a bucket loader with small stone, and stacked up onsite. Deployment is accomplished with a crane from the top of the riverbank. Mattresses stretch up to 35 feet long, 5 feet wide, and about a foot thick. Workers placed as many as 20 to 30 per day in Maine.The amount of bed preparation depends on the steepness of the slope. The site is cleared and grubbed of any organic materials. Any discernible holes or voids are filled. The goal is to end up with a surface free of abrupt changes or depressions.“With traditional riprap, where you must place individual stones on the bank, you are limited to how steep of a slope you can do before those stones simply start rolling down the bank,” Carpenter says. “With this system, the smaller stones are constrained and you can anchor the mass to the slope so it does not slide down.” Costs to reinforce steep slopes are generally less with mattresses than with riprap, even in a stony environment like the Northeast, Carpenter notes. Mattresses also ease deployment below water level. “Riprap generally requires some sort of excavation below the waterline to secure it to the bank below the toe,” the engineer continues. “Doing that below the waterline can be difficult and expensive. Marine mattresses can simply be laid out there and anchored to the slope.” Concrete is often a more expensive alternative and offers less chance of vegetative growth, Carpenter says. “These marine mattresses allow some vegetation to take hold over time,” he adds. “Tensar said you could almost forcefully vegetate it—place the stone-filed mattress on the slope, spread a layer of topsoil on top, wash it down into the stone, and then seed it with a wildflower mix or grass. They said they have been successful in establishing vegetation right off the bat.”Mattresses work well on rivers, in flood zones, and in wetlands, Carpenter concludes. “Many times, regulatory agencies restrict the amount of fill you can place in a flood zone. These marine mattresses are a foot thick. Comparable amounts of riprap would have to be 2 to 3 feet thick or more, meaning you need to place a lot more fill. That may mean excavating down 2 feet as opposed to laying the mattress right on top.”Cement-Based ProductsConsiderable design engineering has gone into two Armortec shoreline-protection products offered by Contech: A-Jacks and Armorflex. “You are using products tested by third-party, independent labs, as opposed to rock riprap, which has become more art than science,” says Contech erosion control specialist Jeff Collins. “And in the end you get a nice, vegetative look. We turn a manmade solution into something that looks natural.”Armorflex mats consist of machine-compressed, cellular-concrete blocks of interlocking shape for easy onsite handling. The open- and closed-cell blocks cable together longitudinally with galvanized steel aircraft or polyester cables. The big advantage, Collins explains, is extreme flexibility, allowing the mat to adhere to creek- and streambanks. Unlike poured concrete, the design permits vegetation to grow through.A-Jacks bank revetments are concrete armor units assembled into a permeable, interlocking matrix. The voids in the matrix are filled with soil and provide a stable base to support woody vegetation above the normal stream-base flow. They provide significant protection at the creek’s toe, the point of highest sheer stress, Collins explains. Delivered on pallets to the job site, they can be easily placed by hand and are effective in constricted areas.Photo: Coleman Moore Company A-Jacks being installed in Johnston, IA “They are used in place of traditional rock riprap or gabion baskets,” Collins says. “They knock the energy out of the water. The water slows and sediment starts to fall on the A-Jacks units. Vegetation will grow right through the middle of them.”Mark Moore of Coleman Moore Co., a Des Moines, IA, erosion control industry supplier, has worked with numerous engineering firms to install these products on various Hawkeye State projects.“We put them [A-Jacks] in on a recent job in Johnston,” Moore recalls. “We put them on the outside bends of a meandering stream, with one row buried halfway [into the creek sediment]. So far they have worked great. We had another job at Brown Deer Golf Course in Coralville, where they were stacked three high on the outside bends. The one in Johnston had ideal conditions. The stream was low and the weather was nice. The Coralville job was done in mid-December, the coldest month of the year, and in running water. It was not an easy installation. But they look great.”Both units allow property owners to fix erosion problems without the use of large equipment. “It can be done with a Bobcat as opposed to a large excavator,” Collins says. “Using riprap would require a big excavator and dump truck. With these, you can find a small staging area. You do not tear up the ground and can drop them near where you are working. In the Midwest the major selling point is the ability to fix the problem without tearing up a lot of existing property.”Don Thieman of ASP Enterprises, a Missouri erosion control and geosynthetic product distributor, says he has written “tons” of A-Jacks and Armorflex units into contracts in the St. Louis area. “Armorflex has always been used with culvert outlets or bridge abutments to protect some type of structure,” he says. “It is generally part of a project where they are putting in something like a box culvert or a bridge. In some cases, these culverts have water flying out of there pretty swiftly. You need hard armor to protect that [area]. You want to have a finish grade of not more than a half-inch per foot. Once you have a good grade, you use a site-specific geotextile fabric. The fabric ultimately does the erosion control. The Armorflex is simply a ballast that holds that fabric down.”Armorflex’s longevity often surpasses that of gabion baskets, Theiman continues. The only other real alternative is riprap, although that is less aesthetically pleasing.“We can transition at a certain storm-event elevation [from Armorflex] to a vegetatively reinforced soil slope or to a turf-reinforcement mat,” Theiman says. “In a constant-flow area, we backfill them with a fine, granular material. But above the waterline we can fill in the voids with soil and vegetate them. That gives you a vegetative solution that is better than riprap. I have one such project under construction, two we finished at the end of 2005, and a couple more about to get started. Most have been publicly funded projects.” Photo: Coleman Moore Company A severe-weather A-Jacks installation at Brown Deer Golf Course in Coralville, IA Thieman says the installation process takes little time because the mats are pre-assembled. The manufacturer puts together a plan showing where the mats should go, and workers basically remove them from the truck and put them in place. “We just had a job where the alternatives were riprap or Armorflex,” he notes. “The final prices were very comparable.”Articulating concrete blocks connected with stainless steel cable or polyester revetment rope are another widely used technique for stabilizing slopes and streambanks. Cable Concrete from International Erosion Control Systems is one such product. “It is a hard-armor solution designed for areas prone to extreme erosion because of extremely fast-moving water, steep embankments and water overtopping,” says the company’s Brent Smith.Larry Larson of R.H. Moore and Associates, a Tampa, FL–based distributor of erosion control products, was onsite for the installation of Cable Concrete at East Brook Outfall and the State Road 200 project, both in the Orlando area. East Brook Outfall is a channel-lining project behind an apartment complex, Larson explains. It had high-velocity flows during rain events. Engineers decided to go with a hard-armor system that could articulate, or move with the subgrade, and that could also be vegetated. Cast-in-place concrete slabs or riprap were the other possibilities they considered. However, concrete slabs often crack if there are any holes beneath them. And with Florida’s sandy soils, riprap usually has to be trucked in from out of state, making it a costly alternative. “With kids around, you also have the liability issue with riprap. Either they fall and break a leg or throw it through a window,” Larson says.Photo: R.H. Moore & AssociatesCable Concrete in place at East Brook Outfall (top), and after six months (bottom) Photo: R.H. Moore & AssociatesCable Concrete is assembled in mattress formation, usually in either 8- by 16-foot or 4- by 16-foot units, Larson explains. “They are delivered on flatbed trucks and assembled in the field. You grade the bank the way you want and lay them on top of the prepared surface. It worked very well in East Brook. They want to get a vegetative look and are seeding it. It has been in place about six months.”The Florida Department of Transportation (DOT) supervised the State Road 200 project. The challenge was how best to provide scour protection from the fast-moving water flowing under the bridge. DOT engineers determined either riprap or articulating cement-block mattresses were the best solutions. “During higher flows,” Larson says, “erosion occurs around the piers. That is common along channel linings or bridge piers along rivers. Riprap requires very large stones to handle the water velocities. You can go with a much thinner articulated mattress system installed by barge and divers. The cost to truck that size stone down from Georgia or Tennessee was just too much.” Steel Retaining StructuresOpened in 1994, Prairie Landing Golf Club in West Chicago was about four years old when wind-driven waves along a lake started causing lake-bank erosion to some of its final grades, particularly around putting surfaces, according to head superintendent Tony Kalina. These were architectural features, almost-vertical walls planted with turf. The question was how to maintain the integrity of the design but eliminate the erosive effects of wave action.“We considered riprap, gabions, and similar products,” Kalina says, but ultimately opted for steel reinforcement. “Using steel walls allowed us to maintain the severity of the slope, keep the water near the putting surface, yet strengthen the support, augment the slope, and keep the water hazard near its original location.”The materials were provided by Riverwalls Ltd. of Barrington, IL. CEO Darryl Burkett specializes in steel and masonry retaining walls, particularly for golf courses. Burkett is also an inventor; he has developed a machine that holds a 1,000-pound vibrating hammer that can be moved about on a single-axle trailer with a turf compaction of less than 2 pounds per square inch, which is very important on delicate, tightly mown putting surfaces. The hammer grabs and holds a piece of steel and vibrates it into the ground, reducing the possibility of a wall blowout at the bottom, which may be impossible to fix. He has patented a crane that can be driven onto a course using a 1-ton truck and does no damage to the turf. He also has a patent pending on a method to apply stone, outcrop rock, or cut drywall in front of a steel wall to make it more aesthetically appealing.At Prairie Landing Golf Club, the submerged shoreline was extremely steep, about 1-to-1 with a nearly vertical bank as high as 4 feet. The lake was an irrigation reservoir, allowing the superintendent to drop the water level down over a couple of nights. Burkett placed his machinery directly on the putting surface. “He was able to install vertical sheaths of steel and the anchors with surprisingly little damage,” Kalina remembers. “It is basically a skid loader with tracks. It is a little daunting to see it on your putting green at first. But after 15 minutes of watching it operate, it is a little less daunting.” Burkett drove sheets as long as 62 inches into the shore bank, leaving anywhere from 16 to 36 inches of steel exposed above the waterline, Kalina explains. The sheets are 16 to 18 inches wide, and each interlocks with the one next to it. Workers drive them home one at a time and then weld them to a backing bar made of rebar with Manta Ray anchors, from Foresight Products, pounded into the ground to keep the wall stable. The cost was about $85 per linear foot for the 900 feet of stabilized shoreline, which was completed seven years ago and still looks great today.“Do not skimp on the length of the steel,” the superintendent advises. “If 5 feet of depth is required to support the materials above, it would behoove you to go 6 feet deep if possible. Things move. Water still gets behind the wall. The soil and drainage behind the wall still move. Err on the side of caution and drive longer sheets. It will cost more, but the steel wall will remain intact and stay as plumb as possible.”The amount of lake-bank erosion was one of the first things that struck Ted Fist, director of grounds and golf course operations at Windstone Golf Club in North Barrington, IL, when he took the job in 1995. “They had lost as much as 15 feet of shoreline in some areas,” he recalls. “We had sprinkler heads and piping stranded out in the lakes. We looked at riprap, cellular confinement, vegetative rolls of coconut fiber, and shoring bulkheads as possible solutions. The members wanted a cleaner edge. So we looked into vinyl and wood. A stone wall would have been the most striking. But with our soils being unstable, putting in a footer, dewatering the lake, and ordering stone would have been monumental. We felt steel was the most economical of all the options.”Green complexes were more of an issue than fairways because of the need for a cleaner edge. That made Riverwalls’ low-compaction equipment a godsend in Fist’s eyes. “Darryl’s equipment allowed him to get up on the green with a crane and not cause any turf damage at all,” Fist notes. To date, Riverwalls has reinforced about 1,200 feet of Windstone shoreline, with another 1,600 planned this fall. “It has not moved,” Fist says. “It is in exactly the same place where we put it originally.”Burkett was able to complete a green complex every three to four days. In climates similar to Illinois, spring and fall are good times to undertake such a project because play has slowed.Photo: R.H. Moore and Associates Cable Concrete helped complete the Orlando-area State Road 200 project. Steel can be used in combination with other materials. It is possible to vegetate the front of it to hide it. “Steel has an industrial look,” Fist says. “The members have not been really keen on that. We tried to provide the lowest profile possible. It is just 15 to 18 inches above the waterline in most areas. On steeper slopes we use steel. On more gradual ones we can use a vegetative control with very good success.”It is critical to backfill the wall with a washed stone above the normal waterline, Fist counsels. Backfilling with just soil leaves the supporting area wet, with little stability. Filling the void between existing soil and the steel wall with stone creates a solid footing.“The quality of steel is important,” he continues. “It can rust and break down. You want to make sure there is the right amount of magnesium in it.” A back waler is vital to keep the wall stable. “Just hammering in steel without installing deadmen [anchors] or reinforcement to hold the wall vertical will let the soil just push the wall over,” Fist explains. “Some people will just hammer the steel in and leave it. But you have to put some deadmen in and a waler to connect the deadmen to the wall. That keeps the wall from blowing out.”Big and Small
As in the Natchez Bluffs project, Hayward Baker recommended soil nails on another major stabilization project called the Elbow Slide Repair along the Snake River Canyon in west-central Wyoming. “In the spring with the snowmelt, the river swells, which causes erosion at the river line that undercuts the toe of the slope and causes landslides,” says John Wolosick.
Soil nails were the preferred choice as opposed to a tieback wall. “Soil nailing eliminates the use of steel soldier piles. Soil nailing allows you to stabilize ground without the use of huge steel beams,” Wolosick explains.US Highway 26-89 through Snake River Canyon in northwest Wyoming is a two-lane road constructed in 1947. The entire 24-mile section through the canyon is under reconstruction so that it can accommodate increased traffic volumes and be brought up to current design standards. Most of this route is in a narrow corridor between the Snake River and steep mountainous terrain. One of the narrowest portions is a section approximately 1,000 feet long near a point where the Snake River makes a nearly 90-degree bend, with flow changing from a southerly to a westerly direction. This large bend is locally termed the “Elbow” and is prone to landslides and mudslides. The upper and lower portions consist of weathered and decomposed clay shales, which are very moisture sensitive. Throughout the canyon, there are frequent landslides and mudflows within the Bear River Formation or in colluvial and residual soils derived from rocks of the Bear River Formation. To widen the road, significant amounts of cutting and filling were required. However, two conditions limited this work. First, the Snake River at this site has changed course within recent times and begun to erode the toe of the highway embankment. This has caused a 65-foot-high fill slope to become marginally stable, resulting in a slow-moving landslide designated the “Elbow Slide” and extending from its toe at the Snake River to approximately 1,200 feet above the road. The slide has caused continual minor road damage, requiring patching once or twice a year. The unstable fill lies next to a slowly creeping large landslide extending 1,500 feet above the roadway. A consultant recommended the road be moved as far toward the river as possible, to avoid all cutting through this area and to create a toe berm to resist movement of the slide. In addition, this would require installation of some type of retaining structure to provide the additional required roadway width and the building of retaining walls out toward the river. A second challenge was poor foundation conditions for placement of a wall. The existing embankment would not provide adequate support, and building a wall without some type of foundation improvement would increase the risk of a slope failure below the wall. Furthermore, the road had to remain open to at least single-lane traffic during construction. The proposed Hayward Baker design consisted of a system of tiered, soil nail walls combined with a mechanically stabilized earth (MSE) wall. Soil nailing is a construction technique in which passive, tension-resisting elements are installed in the ground to create a reinforced soil mass in situ. The reinforced mass provides lateral and/or vertical support for excavations or slope stabilization and resists ground movements. Reinforcing elements typically are steel bars that can resist tensile, bending, and shear stresses. Nails are placed in a drilled hole and grouted along their entire length. Earth pressures and external loads are transferred directly to the nails in the form of tensile forces. Nail forces are then transferred into the surrounding soil through friction mobilized at the soil/nail interfaces.A lower soil nail wall, varying in height from 6 to 10 feet with 40-foot-long soil nails, reinforced the existing embankment and acted as the foundation for the MSE wall. An upper soil nail wall, varying in height from 13 to 25 feet with soil nails extending 33 feet into the slope, provides support for the existing roadway during and after construction of the MSE wall. Construction began in August 1998 and was completed the following December. Soil nailing was selected because it was more economical, based on construction costs, than a soldier pile and drilled and tensioned tieback anchor system. Soil nails were installed at 15 degrees from horizontal. Each soil nail consisted of steel bars. The bars were placed in 7-inch-diameter drill holes and grouted using neat cement grout. The soil nails were installed by placing the steel bars directly in boreholes whenever possible. However, when caving conditions were encountered, the holes were drilled using steel casing, and the nails were placed within the casings. The casings were then withdrawn, and neat cement grout was injected from the bottom of the hole using tremie tubes, filling the drill hole from the bottom up. The steel casings were then withdrawn as the drill holes were kept filled with grout.The lower wall, which was constructed first, is located 30 feet below the roadway elevation and is 725 feet long with 308 soil nails that are 40 feet long. The facing for the lower wall consists of reinforced shotcrete, 8.5 inches thick. The bottom of the lower wall is located approximately 42 feet below roadway level and a few yards above the Snake River. The upper wall is located just below the elevation of the existing roadway and extends to an elevation of about 25 feet below the existing roadway elevation. The upper wall is 743 feet in length along the road and contains 418 soil nails that are 33 feet in length. Readings indicate the soil nail walls are effectively controlling slope movements.

Rod Moresco is deputy director of public works in Vacaville, CA. He has overseen a dozen Alamo Creek restoration projects, all of which were muc