Maintaining Vertical: Techniques for Slope Stabilization

March 1, 2004
Slope stabilization techniques range from vegetation establishment and erosion control blankets to concrete walls and heavy wire-mesh systems. The choice depends on type of soil, drainage, aesthetics, and cost. Pennsylvania Stabilized
In West Mifflin, PA, the site of a new Target store posed a challenge. An old steel-mill slag dump inclined from the store’s parking lot down to a major roadway. To avoid the expense of hauling away the slag, the decision was made to grind the stone and add soil. Slope angles ranged from 3:1 to 1:1. Mark Forse, vice president of JMD Company headquartered in Bethel Park, PA, which worked on the project, explains that the major erosion challenges were to keep the slope from eroding until vegetation could be established and to permanently mitigate erosion with a retaining wall.
The condition of the slope before cutting, bolting, and mesh treatmentThe critical area at the bottom of the slope was reinforced with a segmental block retaining wall and geogrid system from Keystone Retaining Wall Systems. The design of the wall adds strength and stability to the slope. The interlocking blocks are connected to geogrids that run perpendicular to the wall back into the slope face. For temporary erosion control, the 400-ft.-long face of the slope was hydroseeded with a seed mixture of hard fescues, creeping red fescues, and ryegrass. Heavy amendments were added to mitigate the poor soil mix of crushed stone and clay and to encourage quick germination. A 2- to 4-in. layer of topsoil was added, and the slope was covered with 80%/20% paper/wood mulch at 1,500 lb./ac. Tackifier stabilized the mulch and seed. In steeper areas, North American Green S150 double-net erosion control blankets were anchored in place with 6-in. staples. In the area behind the retaining wall, P300 UV-resistant turf reinforcement mats were installed. The area also was seeded with the fescue and ryegrass combination. Installing the erosion control measures was a challenge to the workers. Forse says workers had to be suspended from vehicles parked in the lot above to reach all parts of the slope. But the work paid off; they achieved an 80% germination rate, even in the poor-quality soils. “The site is totally stabilized,” he adds. There has been no silt on the road even after a month of heavy rains in October 2002 and a very wet spring the next year. Controlling Erosion After Southern California Wildfires
Scott Savard of Somis Pacific, a farm management company in Somis, CA, is only too familiar with the aftereffects of the wildfires that swept through the state in the fall of 2003. Once the fires were out, Somis needed fast protection for hillsides in Ventura County that had been denuded by the fires whipping through avocado and lemon orchards. Some areas were seeded, but that didn’t provide sufficient coverage for protection fast enough. Some areas had received several soaking rains of 1–3 in.
As an immediate, short-term solution, crews stretched 100- x 20-ft. plastic sheets on the hillsides and anchored them across the top of the slope and along the seams with Weight Strips from RTM LLC of Ventura, CA. “They’re 20 times better than sandbags,” Savard says. He is surprised that more people are not using them, although he admits, “We were skeptical at the start.” Sandbags are labor-intensive, and sometimes there is no good sandy soil on-site to use as fill. When sandbags deteriorate and break, they must be replaced. Eyelets on the reusable weights allow them to be secured by ropes or stakes, and the weights’ flexibility allows them to have 100% contact with the surface of contours and dips, keeping the sheets in place through heavy rain or wind. The 3- x 5- x 32-in. weights are made of recycled tires, weigh 16 lb., and cost $12.50 apiece. Bob Hitch of RTM says the company recycles about 500 tires per day. The strips are bound together by steel wire and have wire loops for attaching to ropes. They not only provide a useful addition for the erosion control industry but also reduce the amount of tires that end up in landfills. Savard also has used the RTM weights as orchard drain trash racks. Citrus orchards especially release heavy brush and debris. Typical trash racks either are too fine and clog easily or must be welded together out of rebar. Savard decided to use the strips to form barriers around the trash drains. He found that the water flows through but the trash is captured. Orchards lining a blue-line channel of interest to the Department of Fish and Game recently faced an erosion problem. A slope had been rebuilt with a drainage pipe and flume installed and vegetation planted. A heavy rain of 1.5 in. in about two hours caused a washout under the flume. Even after rearranging the pipe and flume for better drainage, the eroded area needed protection. Somis Pacific installed 6-mil plastic and the weights. In spite of heavy rains and winds, the hillside has not eroded and seems to be in place until vegetation can root.The Fay Avenue project in La JollaScupltured Walls
When the excavation usually required for building and reinforcing a retaining wall isn’t a workable option, soil nailing can sometimes provide an alternative. That was the case with the Fay Avenue realignment project in La Jolla, CA; a portion of the avenue near La Jolla High School was to be rebuilt adjacent to a slope that fell from the edge of the school’s athletic field.
The site was 6,900 ft.2 with a slope of 1:6 at the top section and a slope of 0.75:1 lower down. The soil was a partially cemented sand with some localized sloughing. Several options were considered, but most required too much excavation into the hillside, massive shoring, or huge expense. A soil nail wall was chosen. Soil nail walls can be constructed in areas where there is no groundwater, soils are able to stand unsupported during the nailing application, and good drainage is possible. Such soils as soft clays, organic soils, and coarse sands are not suitable for soil nailing.For the Fay Avenue project, Condon-Johnson & Associates of San Diego, CA, received a design-build contract and chose Boulderscape of Capistrano Beach, CA, to provide the finished look for the project. George Burrough, Condon-Johnson’s San Diego district manager, says the decision was to put money into the wall rather than into temporary shoring. Soil nail walls are constructed in stages from the top down. Less excavation is required because there is no geogrid reinforcement. Soil nail walls are not tensioned but reinforce the soil by creating an equilibrium stability inside the wall. Design of the nail length and placement depends on the site conditions. Usual conditions call for nails about every 6 ft. with a placement angle of about 10–20°. Length of the nails should be at least 60% of the height of the wall, although rockier soils can handle shorter nail lengths. Nails—steel bars—are placed either by being driven into the soil with a pneumatic hammer or by predrilling holes with a rotary drill and inserting nails. The nails either are grouted in place or can be encapsulated in plastic sheaths to prevent corrosion. Soil nail wall constructionConstructing a soil nail wall can mean less traffic obstruction, less construction time, and less expense.The Fay Avenue project consists of two walls with a planting terrace between them. Plants discourage graffiti and help cover what vandalism does occur, according to Burrough. For the first stage of construction, the slope was excavated 12 ft. down, and holes were dug with a rotary drill. The holes were stuffed with encapsulated, pregrouted nails and then were grouted again. Wire mesh was hung on the nails, and a layer of shotcrete facing was applied. Drain panels behind the wall provide weep holes for moisture to escape. After construction was complete, an architectural finish was applied. Boulderscape built mockups of the finish to give the contractors a choice of looks and colors. Once the design was chosen, work on the architectural wall began. Dowels of 3/8 in. were drilled into the existing shotcrete and six-gauge wire mesh was hung. A crew from Boulderscape pumped the shotcrete onto the site, applying it 4–16 in. thick. Artisans waited about 40 minutes to an hour and then began sculpting the design. After the shotcrete had cured for two to seven days, depending on the thickness, a salt-based stain was applied—a base coat with two or three accent colors in the crevices. The stains actually react with the shotcrete, creating permanent color. The components of the shotcrete can be varied for waterproofing or different temperatures. “Each wall is different,” says Steve Jimenez of Boulderscape. “They are made for blending in.” “These walls are an excellent choice, especially in the development environment where we are losing the ideal building terrain,” Burrough notes. “They are a viable, practical option for slopes or where a cut must be made.” Caltrans (the California Department of Transportation) has approved similar walls for several road projects to stabilize cuts and steep slopes. The first project done by boulderscape was a huge 28,000-ft.2 project right along Pacific Coast Highway. After 18 years, it still looks great and most people driving by don’t even realize that the wall is an engineering feat. The Fay Avenue realignment project received the 2001 Award of Excellence from the American Society of Civil Engineers.Safety in Arizona
Wire mesh is another option for stabilizing slopes and providing rockfall protection. Wire-mesh systems can be passive or active. Passive systems consist of mesh draped over a slope to guide rocks that break loose to the bottom of the hill. Active systems employ anchors, mesh, and tension to keep the hillside intact.
Geological Consultants Inc. in Phoenix, AZ, chose the TECCO mesh system by Geobrugg of Santa Fe, NM, to tame a steep-cut slope next to a parking lot at the Peridot Retirement Community in Prescott, AZ. The unstable rock at slopes of 0.25:1 increased the possibility of rockslides. A close-up of completed TECCO mesh installation in PrescottThe TECCO system includes high-tensile–strength wire mesh, spike plates, and compression claws. Steel anchors normally are provided by the contractor. The active stabilization occurs because of pretensioning against the slope. Some of the advantages for contractors choosing this type of system include the following:Strength and flexibility of the system, allowing yield to loading or expansive soilsAbility to revegetate the slope through the mesh openingsDrainage through the meshLower cost than retaining walls or shotcrete mainly because greater anchor spacing is possible but also because of lower material costsThe wire mesh itself consists of 3-mm, high-tensile steel wire that has been galvanized with zinc and aluminum for greater protection against corrosion. Testing has shown a tensile strength of 150 kN/m in the longitudinal direction. The strength characteristics approach those of cable nets at a lower cost. Minimum anchor spacing for such a system is approximately every 8 ft., but extra anchors were added to cover the irregular terrain at the Peridot Retirement Community site. Work began in late 2001. The site covered about 17,000 ft.2 The slope was cleared of loose rock, and 225 anchor holes were drilled. The anchors, 28-mm threaded bolts ranging from 6 to 20 ft. long, were inserted and grouted. Requirements called for anchor depth to achieve at least 2 m of bonding in solid rock. Mesh rolls were draped over the slope by a cherry picker and cut to size. Panels were connected with compression claws, and anchor plates were installed over the anchors. Torquing the plates tensioned the mesh against the slope to provide the stabilization needed. Erik Rorem of Geobrugg says the cost of this project was about $18/ft.2, approximately half of the original design that called for a concrete facing wall. Drilling and placing the anchors is about 90% of any project, according to Rorem. He notes that the use of the system is growing, with a number of projects for Caltrans and municipalities in the eastern United States. In some projects, the soil under the mesh is hydroseeded, and the final result is a natural-looking green wall. Gentle Golf Course Slopes
Not all slopes are rocky vertical cliffs. Sometimes the slopes that need to be stabilized are the mounds and rolling hills of a golf course.
Garrett Gill of Gill Miller Inc. in River Falls, WI, describes a renovation project at the Village Links Golf Course in Glen Ellyn, IL. The 27-hole course first was built in the 1960s, and a major renovation and expansion began in the fall of 2002. Because the course also is a significant stormwater management facility for the village of Glen Ellyn, that capacity had to be maintained and enhanced.New greens and tees were built, and the overall course was extended. The drainage was revamped, including excavation of new ponds. Because, as Gill says, the “water level would bounce” as the amount of stormwater runoff fluctuated, the pond banks were stabilized with Landlok 1051 turf reinforcement mats from SI Geosolutions. The desire was for the pond banks to look as natural as possible once the planted vegetation had grown in. Other slopes on the course were covered with DS75 Single Net Straw Blankets from North American Green—about 7,200 yd.2 of them. Gill says he prefers to stabilize all slopes over 10%; because of the cost, that can’t always be done, but it is the ideal situation. The cost on this project was about $1.00/ yd.2 of erosion control blanket. He notes that using blankets results in a higher initial cost, but the course can open sooner and begin bringing in money.“The important thing with golf course work is timing,” Gill says. Work began in the fall to be able to hit a seeding window of August 1 through September 15, 2003. The course had excellent germination and complete slope stabilization; vegetation has matured through the fall and winter and will be ready for opening in June 2004. Erosion control blankets have been the method of choice for Gill Miller. “On some courses we have installed 70 acres of blanket,” he says. He prefers the blankets because they are cheaper than sod, and they can be planted with whatever type of seed is necessary to match the rest of the course. In addition, sod must be watered heavily, so he might have a wet area next to a spot that needs to remain dry for work. Hydromulching requires heavy equipment that sometimes leaves ruts in already-landscaped areas of the course, but blankets are light and can be carried on small carts. The blankets also are easy to install; at the Glen Ellyn course, installation was done by the golf course staff.