Whether seen in the gradual downslope movement of individual grains of sand dragged toward the ocean with each wave from the sea, or a mammoth mountainside boulder, triggered by the weight of a snowpack into a tumbling avalanche, slope instability is a process of nature. Gravity, the force behind slope instability, operates mercilessly, whether it does us good or not, continually shaping and shifting the contours of the Earth. Because it occurs everywhere, all the time, it mostly goes unnoticed because there’s no one there to see it happen. But when slope instability occurs on a hillside that either houses or overlooks some valued asset, it becomes a matter of great concern.
One of the earliest methods discovered to prevent something from falling down was to lean something else against it, but that requires a backstop itself to lean against, and on sloping terrain that backstop is often not available. However, erosion control specialists can now do much more than merely choosing what objects to set in the path of a rockfall or landslide. An expanding palette of geogrid solutions can provide an integrated approach to geotechnical systems that can arrest instability at its root and preserve valuable assets while managing problematic slopes.
Rock in the Road
The Washington State Department of Transportation (WSDOT) has a system to evaluate the risks of rockslides and slope instability along its state highway network. According to Corie Henke, field engineer with WSDOT, the highway agency evaluates slopes along the roadsides using a rating system that assigns each stretch of road statewide a number designating its risk for slope instability.
“Part of the rating system is whether or not there have been incidents in the area,” says Henke. For example, there had been incidents of large pieces of rock and debris ending up in the middle of the road on sections of Highway 12, the only numbered highway to span the entire state, which eventually carries traffic back and forth as far to the east as Detroit, MI. Rockslides on Highway 12 near Clear Creek, WA, in the mountainous Cascades region of the state placed the stretch of road high on the priority list for slope remediation projects. The heavily trafficked scenic roadway cuts through the steeply sloped terrain of the Cascade Mountains ranges at elevations between 2,600 and 4,000 feet.
Henke has been field engineer on a multiphase project to keep Highway 12 clear of debris, safe, and passable by stabilizing the slopes of the hills into which it was carved. WSDOT recently completed a major slope stabilization project between White Pass, a town widely popular as a skiing attraction, and Clear Creek Falls with its idyllic lakeside vistas.
Although the primary goal of the project was keeping the road open and safe, another important goal was to preserve the natural look and aesthetics of the surrounding environment.
“The most dominant issue was large rock debris ending up in the road,” says Henke. In carrying out the project, the intent was “not to change the terrain of the rock face.” To meet these two almost contradictory goals, project designers planned to hold the stone-faced slopes in place with wire mesh.
Breaking Bad Rocks
Before wrapping the stones in the mesh, crews would need to go in and deal with loose rock, a job that Henke says demands considerable skill, not only to maintain one’s footing on precarious terrain, but also to identify where to apply the force needed to scale away treacherous shards of stone and where to leave nature to her own devices. “Our rock scalers are required to have 2,000 hours of experience before they are approved to work on highway projects,” says Henke.
Clambering over the rock faces wielding a steel rod 6 feet in length, the scalers must break loose all of the stone they might encounter that appears susceptible to spontaneous rock falls that could imperil the roadway or litter it with debris ranging from stones to branches and boulders. The job requires physical stamina to be sure, but also discretion. Henke says a good scaler also has to know when to stop. “Unless there is a large rock that our geotech department says needs to come down,” he says, a rule of thumb in scaling is to break off only those stones that one person can remove with the 6-foot scaling bar.
“If you can’t get it to break loose with that much force, it is probably not going to be a culprit in a typical rock fall,” he says.
At this stage of a stabilization project, says Henke, the biggest danger is overscaling: “If you take out too much of the rock at the toe of the rock face, the only fix is that you have to go up and remove the overhang above.” Henke believes the most efficient way to work is to avoid that trouble in the first place by establishing “a good relationship with the scaling personnel and making sure they don’t overwork the rock.”
Once the scaling had been completed on the White Pass project, the project team began installing chain link wire mesh to contain any rock that by force of nature might become disengaged and tumble down, instigating a potential cascade of rock fall onto the roadway below.
Hanging With the Highflyers
Although a number of contractors performed the installation of the wire mesh, Henke notes, “Most of the wire mesh used for slope protection was provided by Maccaferri.” And because the slopes were so high, “We hung most of it by helicopter,” he adds.
“There were two or three spotters on the ground communicating with the pilots by radio to direct them to the locations to lay the tether line,” he explains. After the helicopters laid down the mats, crews on the ground would go back in to install permanent anchors.
Henke says safety has to be another number one goal for any project on steep, sloped terrain. “For all of our projects, safety is a big issue. I strive for zero safety incidents,” he says. By “creating an awareness on a daily basis” with pre-activity safety meetings every morning, strict adherence to eyewear, hearing protection, and headgear, and other safety measures, says Henke, he met a goal of zero injuries for the entire project. “All of the contractors were highly motivated and hanging the netting by hand.”
However, slope repair does not always go as planned. Henke says one of the biggest challenges on the Highway 12 stabilization project was the fact that nature doesn’t take breaks. As the project progressed, a storm arose, dousing the project area with 2 inches of rainfall in less than an hour, causing a landslide that washed out a section of the nearly mile-high roadway and requiring an emergency repair.
“I went up and called a contractor on the phone and told him he had 30 days to repair the road,” says Henke, including stabilizing the slope and installing a drainage system to make sure the incident would not be repeated.
The $2.2 million emergency repair required treating the area to ensure adequate drainage in the future. The solution was a soil nail wall backed by TenCate’s Mirafi drain. Contractors installed the Mirafi drain on top of the natural soil to create an internal drain behind the planned nail wall. They then built a 700-foot soil nail wall and used 20,000 cubic feet of dirt from existing fill to backfill the landslide area.
“With soil nails, the biggest issue is choosing the right length of nail for the soil conditions,” says Henke. “The length of soil nail is dictated by the soil type and the friction you can create.” Geotech specialists from WSDOT headquarters came out to do soil bore analysis to determine the soil’s inherent characteristics. This analysis indicated the need for soil nails ranging from 18–78 feet, depending on the location, to secure the soil nail wall against a subsurface comprising a variety of different types of rock. Henke explains: “Years ago when the road was built, they were just blasting the rock and laying the road, so the type of material is variable.”
After installing the soil nails and injecting the grout to hold them in place, technicians bolted the wall plates and tightened them to the rock bank with bolts 7 feet by 7 feet on center. With a rebar mat incorporated into the bank structure and the bolt plates fastened down to 100,000 psi against the rock bank, says Henke, the installation holds together as a single unit.
With a spray of shotcrete with “aggregate the size of pea gravel” and coat of naturalistic paint to match the original rock face colors, says Henke, the site of the emergency repair approximates the natural landscape in appearance, and the emergency project was carried out within WSDOT’s 30-day deadline.
A Bike Path of Least Resistance
Where infrastructure rests in proximity to a slope failure, there are often practical constraints to the types of measures that can be taken to secure the site against future erosion. Chris Nardi, principal geotechnical engineer with Kleinfelder, says the city of Fairfield, CA, ran up against this obstacle in its efforts to revitalize the slope supporting McGary Road, a secondary road that mirrors the newer Interstate 80 as it passes through Fairfield.
There had been a campaign to integrate the 2-mile section of McGary Road within the city of Fairfield into a regional bike trail system linking the town by bike route to Vallejo. There had been talk that McGary Road could serve as an ideal temporary bypass in case an emergency blocked traffic on the parallel stretches of Interstate 80, and Solano County, which had jurisdiction over the majority of McGary Road that lay in unincorporated parts of the county, had been contemplating annexing the road and taking over its maintenance and administration, enhancing mobility for drivers and cyclists. The only problem was that the road had mobility issues of its own, and parts of it had taken off downhill. A landslide had taken out the road and ruined much of the supporting bed.
Although most of the McGary Road right of way was already under Solano County jurisdiction, officials declined to go forward with the annexation of the 2-mile section passing through Fairfield until the city addressed the landslide-induced road collapse issues that had resulted in slides in the road up to 150 feet in length by 35 feet wide with a depth of up to 35 feet. (See a related article on the landslide in the May 2011 issue of Erosion Control.)
Nardi says landslides are a common geological feature of the region. “The route that Interstate 80 takes through the area just happens across a massive old and active landslide.” These areal slides, he says, in some spots could range “down to 100 or 200 feet deep.”
According to Nardi, a common practice engineers use to combat slope failure is just to get rid of the slope. But grading and leveling the roadbed and extending and moderating the slope was just not feasible in the case of McGary Road. Stretching out the slope to the extent needed to achieve stability would have required encroaching upon Interstate 80’s parallel rights of way just a few hundred feet away. There was no question of building a retaining wall to hold the earth back, either; the slope was just too massive. “There is nothing to provide the resistance,” says Nardi, who has had considerable experience dealing with unstable slopes in the region. He adds, as a general maxim, “Retaining walls and landslides don’t get along.”
Searching for Solid Ground
The challenge was to find a way to prevent future slides while working within the limited space constraints of the right of way. The city of Fairfield would have to dig deep for a solution, and that’s just what it did.
“Before we could protect the slope, we had to take out 40 feet of it,” says Nardi.
Troy Simning of Ghilotti Brothers, the firm contracted to construct the McGary Road slope stabilization project, says that when attending to a slope failure, “You have to excavate back to where you have cohesion, back to a strong point, and that’s where the Strata product comes in.”
Stratagrid is a geogrid reinforcement product for soil made with high-tenacity polyester yarns. Manufactured to be both mechanically and chemically durable, the polyester geogrid can withstand both the harsh construction installation phase and aggressive soil environments. On the McGary Road project, contractors used both Stratagrid 200 to provide internal cohesion for the rebuilt slope and Strata microgrid to encapsulate the slope once it had been reconstructed.
As Nardi explains, “Ghilotti excavated to a depth of 25 to 40 feet.” After reaching stable ground at a level below the slide, crews began laying Strata geogrid in horizontal layers, each layer interleaved with 18 inches of dirt, until the surface was brought back to grade and the slope restored, but with the added strength of the Strata geogrid.
Installers wrapped Strata microgrid around the face of the slope to envelope the surface soil, relying on Strata Microgrid’s small 0.10- by 0.25-inch grid apertures and its 871-pound-per-square-inch long-term design strength (LTDS) to be able to retain the soil and to tie the slope surface together as a unit, says Nardi.
Elizabeth Nicholson of Strata Systems endorses the strategy. “The biggest misconception that most people have about slope failure is that they see it as a surficial need and they are not thinking about global stability. They might say, ‘We’ll just face it,’ but in reality they might need to excavate out the slope and rebuild it again.”
Nardi agrees, but adds that vegetation can add an additional essential degree of strength to a completed slope stabilization project. “When working on slopes, don’t forget the finished surface,” he advises. “You can’t just leave bare soil on the slope when you’re finished. You need to do something to protect it, because it’s going to get wet before everything is established, and if that happens you could lose some of the good work you just did. Whenever you can get good vegetation established the roots always help give you a little more natural reinforcement.”
After an application of hydroseeding on the slope, McGary Road reopened in 2011 and now supports bicycle and vehicular traffic. A major celebration among the cycling community welcomed the road into the San Francisco Bay Area’s biking network. According to the Times Herald News, the total cost of the project was about $2.53 million. Although local papers report the road is not quite ice rink smooth, Tom Martian, Fairfield city engineer, says the stabilization project has proven a success and no further slides have been reported. Martian told the Daily Republic in Fairfield in July 2014 that geotechnical analysis had determined that the few bumps that remained in the roadbed were the result of settlement of construction materials and were unrelated to the history of sliding. He told the publication the bumps could be readily smoothed out by repeating the compaction over the work site. That fix would cost less than $150,000, according to Martian.
Piping Up in the Piedmont
When Eric Snyder got involved in a project to stabilize the slopes along a 13-mile stretch of natural gas pipeline south of Nashville, TN, much of the concept and design work had already been completed. “They knew they wanted to use Geobrugg and soil nails, and after that we did a design-build.”
The project consisted of installing a 13-mile-long, 20-inch-diameter natural gas transmission line. The pipeline traversed very steep terrain that ranged from a 2:1 to a 1:1 slope ratio. Snyder says the soils were of such a nature that the very disturbance involved in installing the pipeline could render the slopes unstable, and besides that, one of the first things the owner had to do to prepare the right of way was to remove all of the vegetation.
A portion of the steep terrain is within the Fort Payne geologic formation, a feature that includes colluvial soils. According to TEI Rock Drills, a vendor to the project providing geotechnical drilling equipment, any disturbance of these soils, such as removal of stabilizing vegetation or increased water saturation, can cause a loss in stability and produce slope failures.
The soils on the site were “of colluvial materials with very little cohesion,” says Snyder, and the owners were concerned that they should be secured lest the pipeline become subject to the stress of landslides and shifting earth.
In addition to making sure the buried pipeline would not be susceptible to movement, the client also required a solution that permitted ongoing accessibility to the pipeline.
A key component to ensuring the pipeline integrity would be to provide an artificial matrix to hold the soil in place and anchored.
Adding to the complexity of the task, the fix had to be aesthetically pleasing, Snyder notes. “The entire length of the project was in public view from a nearby interstate,” he says, and a part of it was directly adjacent to a popular golf course. The owners, in the interest of being good neighbors, ranked a natural look to the finished product high among the list of priorities. For all of these reasons, the Geobrugg Tecco 3-mm chain link system was a much better solution than stabilization products using heavier technologies such as concrete.
However, with combined 25 years experience as a geotechnical engineer and now as president of the specialty geotech engineering firm Geofirma LLC, Snyder concedes the biggest challenge in any project is putting it in place. “They can look good on paper, but if you can’t get it in safely, it doesn’t do any good,” he says.
Describing conditions at the site, he says, “Footing can be an issue working on those slopes. The soils are unconsolidated; in the winter they become overly saturated and heavy and weakened at the same time.” With the vegetation removed for construction, the site could be expected to display even less integrity than usual. To top off the challenge, to meet the construction schedule the stabilization work would need to proceed during the fall and winter, typically the wettest months of the year.
However, says Snyder, Geobrugg’s products come with built-in efficiencies to minimize time spent on precarious slopes. “Being a chain link fence, it is easily molded around any obstacle, and it comes with instructions on how to weave around trees and mold it back together.
“You can join pieces together with the clipping system. You put one clip per aperture opening in the chain link mesh.” Furthermore, he says, GeoBrugg provides tremendous product support.
Although the mesh weighs more than 300 pounds per 96-foot by 11-foot roll and his crews used bulldozers to get it to the top of the slopes to roll out, he says, “It’s not so heavy that the same job could not have been done by manpower.”
Stronger Than Dirt
However, what could not be done by manpower was digging the holes for the soil nails to which the Tecco System3 wire mesh would be anchored. Anchoring the system to cover the 1,400-linear-foot section of the 50-foot-wide pipeline alignment would require 2,000 holes, each 4 inches in diameter, drilled to a depth of between 10–20 feet. Each of the holes needed to be arranged in a 6-foot by 6-foot grid across the 50-foot right of way, and each hole would receive a solid bar steel element with a tremie tube before being grouted to serve as anchors.
That job required a special piece of gear that Snyder was able to obtain from TEI Rock Drills. Snyder says the drilling of the nails “was the biggest part of the project, and TEI made it quick.” According to TEI, the drilling for the shallow slopes (less than 2H/1V) was primarily accomplished with the TEI HEM550 excavator mounted drill. For the steeper slopes, Snyder contemplated having a custom-built wagon outfitted to one of his company’s existing skid-steers, but technical issues such as how to wrangle long spans of hydraulic line during drilling, in addition to the technical difficulties of mounting a diesel engine to power the drill hydraulics on the wagon and routinely changing the mounting angle of the engine to match the slope angle, made a custom-built wagon a less than optimal solution. Instead, he opted to use the new TEI MT100 Mountain Drill to create the holes. This drill uses an onboard electric motor to turn the hydraulic pump and also features a variable-volume hydraulic reservoir that allows it to drill at any angle. It comes equipped with a hydraulic winch and rear steer wheels that Snyder says made it easy to move the drill around the job site. With just one power cord and one air hose to deal with, rather than a tangle of hydraulic lines, says Snyder, TEI’s drill made for huge gains in efficiency of operations.
While Piedmont Natural Gas says the overall cost of the pipeline project amounted to a $60 million investment, Snyder says the cost of stabilizing the 1,400 feet of sloping terrain for the South Nashville alignment amounted to $1 million. However, he adds, “Without the ability to drill the slope efficiently the cost would have been significantly higher.”
He also credits Geobrugg for the favorable economics of the project. The product is cost efficient and effective, he says, pointing out that the wire is lightweight, installation is simple, and it performs with the tensile strength of 25,000 psi.
As a finishing touch to the project, says Snyder, the design team advised against introducing the burden of fresh topsoil on the newly stabilized slopes; instead, a dense application of hydroseeding was applied to get the revegetation cycle started with minimal disturbance of the surface. Once the vegetation started growing in, says Snyder, it completely hid the mesh grid so that it was impossible to detect that such a massive stabilization project had taken place. During the following summer, pipe installers for the South Nashville pipeline alignment project arrived and were able to lay the 20-inch natural gas pipeline, as planned, without a hitch.
Moving through into second decade of the 21st century, science has yet to come up with a technology that can counter the force of gravity. As long as we have gravity and rain, slopes will be in motion. But advancing techniques in geotechnical engineering and geogrid products will also allow engineers, planners, designers, and installers working together ever greater capability to protect hillside assets.