Holding soil in place with concrete mats, hydraulically applied materials, rockfall netting, and grout

By Carol Brzozowski

From the mountains to the ocean, slope stabilization is an ongoing erosion control activity throughout North America as slopes present ongoing dangers due to usual erosion, unanticipated weather activities, or construction jobs.

Watch for Falling Rocks
Colorado hosts many notable areas, but there's one that has always made state officials nervous. That area—located west of Denver—is a 2-mile stretch of Interstate 70 between Georgetown and Silver Plume. Its notoriety is rooted in the fact that many people have been injured or killed by rocks sliding down the steep slopes of Georgetown Hill in the heavily trafficked area, notes Mick Muller, a chief designer and project manager for the Yenter Companies in Arvada, CO.

PHOTO: PROFILE PRODUCTS

One rockfall from the 40% slope in April 2004 took out a 12-foot-high fence on a slope above the Georgetown Loop Overlook. Muller's company was assigned the task to create safer conditions through the installation of two rockfall fences: one to replace the damaged fence and another located higher up on the slope in an effort to slow the progression of falling rocks before they reached the lower fence.

Working through the winter of 2004 to ensure completion before the spring 2005 runoff caused more rockfall, the company used helicopters, a capstan wrench, and a pulley system and also carried materials by hand up nearly 400 feet to build a solution favored by the Colorado Department of Transportation for the rockfall mitigation project: attenuator fences.

"It's a little different than most rockfall fences in that rocks hit the fence and the bottom is not restrained, allowing the rocks to continue to slide down the hill if they have enough energy," Muller explains.

PHOTO: PROFILE PRODUCTS

PHOTO: PROFILE PRODUCTS

When the April 2004 rockfall took out the previously installed fence, it was because the rocks sliced the wire ropes. The installation of metal sleeves over the wire ropes of the anchors is designed to prevent that by spinning.

The fences were installed over rockfall chutes. The Yenter Companies used anchor bolts—drilling them 6 feet deep for the columns—from the Williams Form Engineering Corp. The columns were anchored into the holes with concrete and the metal netting was then attached. In total, the project cost $400,000.

Roadway Construction in Ontario
In another slope stabilization road project in Ontario, Canada, a highway reconstruction effort of 15 to 18 kilometers along a stretch of Highway 26 was undertaken to level out hills in an area east of Owen Sound, ON. The erosion control project began in May 2005 and was expected to take place through the autumn months.

Rick Collins, president of InstaGreen, the Ontario-based company that performed the work, says the roadway construction was a multifaceted project with ditches and fairly steep slopes of 2:1, as well as poor-quality clay soil. "There also were places where the soil slopes terminate into a rock face at the top and bottom, so I tried to reason with the general contractors that straw blanket was not going to be practical nor the best-performing way to go because it was going to be so difficult to do it with that product," he says.

For erosion control and turf reestablishment, Flexterra from Profile Products was used. Flexterra is a flexible growth medium containing wood and synthetic fibers and additives, including tackifiers to help bond to the soil. There was a concern about using some types of materials near the area's many environmentally sensitive rivers, creeks, and bogs; unlike traditional erosion control blankets, Flexterra has no netting, staples, or mesh in which wildlife can get caught.

John Reynolds, president of Mulchit in Ontario, supplied the Flexterra and says it offers wind-erosion control and water-erosion control and has an "exceptional" ability to absorb moisture, and as such holds it longer to provide moisture for germination for turf establishment. It is also relatively easy to apply.

"It can be applied on a rough surface, so there's less critical grading. It is almost spray-painted, as opposed to the standard erosion control blankets where it is almost impossible to get 100% soil contact," he says.

Collins provides a point of comparison: At the same time InstaGreen was doing the roadway work, the company was involved in another job where the engineer specified straw blanket. "It was a far more cumbersome, messy job," he says. "We went into lawns, and [the engineer] couldn't understand that going into people's lawns was not a good idea."

For the highway work, Collins requested a substitution of Flexterra because of previous experience he had with the product. "I thought it would work better, and it is much more flexible when you are working up to the road edge and the top of the slopes where you meet the rock edge," he says.

PHOTO: ESSEX REGION CONSERVATION AUTHORITY

PHOTO: INTERNATIONAL EROSION CONTROL SYSTEMS

Collins also favors the environmental benefits of not having netting. "There were a few environmentally sensitive wetlands we were working up to and that was a concern for the waterfowl," he says.

The substitution was approved by the Ontario government, which had originally specified straw blanket. In previous decades, the slopes had been hydroseeded. The new slopes ended up a little steeper and longer, with the road being widened to three lanes.

Collins says there were a few challenges during the installation process, such as work stoppages with contractors and road traffic. Additionally, his company was "under the gun" to get the work done before heavy rains. And it was one of the largest jobs InstaGreen has ever had to do at 120,000 meters. His company's portion of the job cost $175,000.

Preparing for a 100-Year Storm
Near the US-Canadian border of Windsor, ON, and Detroit, MI, is the Little River, the site of an erosion control project involving the protection of two outside bend slopes on the river. Paul Mourad, a watershed engineer for the Essex Region Conservation Authority (ERCA) in Windsor, explains that a watershed draining into the Little River encompasses a large residential area on both sides of the river, which serves an important role in area drainage.

On each side of the river are dikes designed to convey runoff from a 100-year-storm event. ERCA needed to provide extra storage capacity on one side.

Mourad explains the project's intent: "It's basically parkland, a low-lying area adjacent to the dike. We wanted to protect that area in compensation for another low area that lies to the east. We wanted to cut down the dikes to a certain elevation, and we would design and construct weirs. We would have an inlet weir and an outlet weir, with the purpose of the inlet weir to allow flow to go over into that storage area. The outlet weir is to allow that storage—once it reached that 100-year level—to exit." By doing so, the project would increase the storage area of the Little River and increase the detention time or time of concentration.

To address the needs of the project, ERCA used cable concrete mats from International Erosion Control Systems on the slopes. The cable concrete system integrates flexible stainless steel cable into high-strength concrete, paired with a polyester geotextile base cloth. The needle-punched geotextile allows moisture in the subsoil to drain, preventing buildup of hydraulic pressure beneath the protection of the concrete mat. Subgrade material is held intact by the weight of the concrete and separating ability of the geotextile. The system also protects the subgrade material from high water velocities and wave action.

Mourad explains, "The purpose of the cable concrete mat was to basically provide slope stabilization and erosion control for the weirs, because the flow over the weirs was 19.4 cubic meters per second with a velocity of about a meter a second. We wanted something that could convey the water and provide that stability and erosion control."

Working with a consulting firm, ERCA chose concrete cable after obtaining positive feedback about it from such sources as a professor at the University of Windsor who had done extensive testing on concrete cable mats, as well as two institutions in the United States that have conducted extensive hydrodynamic testing.

"We wanted to go with something more durable," Mourad says. "We have existing riprap along the lower reach of the dikes, going along their length, but for the weir section—because you have that constriction in the weir, and the flow going over it—we wanted something that would withstand those velocities and could be tied in on each side. These cable concrete mats seemed to be a good choice for that." Another alternative considered was the installation of a geotextile fabric, but the cable concrete mats seemed more durable, Mourad adds.

Mourad says ERCA really hasn't had an opportunity to weigh in on the cable concrete's performance. "We haven't had any drastic rain events—certainly not 100-year-flood events—so from our own experience, we can't say how well they perform," he says. "So far, we have been pleased at the appearance of them and the ability to fill the voids with topsoil. We've already got grass growing there. We feel it is going to provide a nice finish to our Little River dikes, and so far we are pretty impressed with it. And the pricing was good."

The project cost $400,000 and was conducted over 35 working days during the summer of 2005. The mats were installed into the existing dike and covered with topsoil. The soil type in the area is predominantly clay, and the mats were laid on a 3:1 slope.

Mourad notes no special challenges, saying the project proceeded smoothly. But there was extra work involved: "We decided to use the mat for a couple of other areas on adjacent sides of a bridge," he says. "We had noticed that there was some scouring on each side of the Little River Bridge, and we had some riprap on either side and that we grouted in place." The authority didn't like its appearance and, because of the scouring, decided to replace the riprap with the cable concrete mat on each side of the bridge.

Grouting a Bluff in Place
In another water-related erosion control project, an unattended irrigation system in southern California led to the failure of a 12-meter-thick sandy marine terrace deposit, undermining a large portion of a residential backyard and threatening an adjacent home. Moore & Taber in southern California stepped up to the plate to stabilize the bluff following the slope failure.

PHOTO: MOORE & TABER

To mitigate the situation, the company stabilized running sand with permeation grouting techniques to permit the steep, near-vertical back cuts required for remedial grading. Permeation grouting is a ground treatment method in which grout is injected into a porous medium without disturbing its original structure. Pores and joints in soil or rock deposits are filled to change the geotechnical properties.

Moore & Taber used more than 1 million liters of two distinct grout mixtures, sodium silicate and ultrafine cement, to address the problem. The cement aided in developing greater strengths while the sodium silicate was used within the outermost part of the slope face.

The chemical grout consisted of a Minova Terraset System of 40-degree Baume sodium silicate, Terraset hardener, and water. The Nittetsu Cement Co. SuperFine cement came from SureCrete.

The crews did all work by hand, using limited-access techniques in installing more than 600 sleeve-port grout injection pipes. The grouted material was excavated with digging spades. During the excavation of the back cut and in-grading benching, the grouted terrace sands performed well, and the "running" sands took on the character of sandstone. Soil fill was used to mitigate the failure scar in the slope face reconstruction. A horizontal drain from the heel of the back cut through the grouted terrace was drilled to address seepage.

As the slope was reconstructed, tree wells were periodically installed and each was fitted with its own drain. The planters were stocked with drought-resistant native scrub not only to maintain coastal aesthetics but also to avoid the need for an irrigation system.

Carol Brzozowski is a journalist in Coral Springs, FL.

EC - January/February 2006