Geosynthetics for Erosion Control

Jan. 1, 2004
The winter of 1995 started out as usual for the Pacific Northwest—drizzly, wet, and rainy. But as 1995 slid into 1996, the rain came heavily and more steadily than usual. Soils had long passed their saturation levels, and tributaries of Columbia River flooded small towns. As the storm set one record after another, the uniform silt soils around Castle Rock, WA, had trouble holding onto the roots of vegetation and trees.In early February 1996, the rain-soaked soils dragged trees and debris down a steep slope and onto both railroad tracks of the Burlington Northern Santa Fe (BNSF) Railway in Castle Rock. The landslide closed the double-track mainline between Portland, OR, and Seattle, WA, for five days while work crews moved mud that was up to 10 ft. deep and another 5–10 ft. of root masses and uprooted trees off the tracks.
Castle Rock landslide area Stan Boyle, Ph.D., P.E., with Shannon & Wilson Inc., a geotechnical engineering firm in Seattle, remembers the five days preceding the slide as producing the “highest rain on record.” The storm also set the record for the highest rainfall in a consecutive period of four months. “The trees were maples and cottonwoods but primarily maples that had about 30 to 40 years of growth and a 2- to 3-foot-deep root system that spread out and matted together in the near-surface weathered soil,” Boyle explains. “Because very dense and hard native silt soils underlie the site, the trees were unable to form a taproot, which might [have helped] maintain stability.” The area of the landslide—around 60 ft. deep and 0.25–0.50 mi. long—was not a natural slope but rather a constructed slope cut in the 1930s so the railroad could be rerouted from downtown Castle Rock. The uniform silty soils at the site were deposited by glacial floods that eroded the silt from Idaho and eastern Washington. The soil particles are very fine, but because the soil does not contain clay, the particles do not stick together. They do, however, exhibit some apparent cohesion that might be associated with a weak cementing agent or have resulted from the structure of the deposit. When wet, they tend to weather and can lose their structure and cohesion, says Boyle. After the main landslide—commonly called a “skinslide”—in 1996, the area was impacted even more by another slide in 1997 and yet again by a skinslide in 1998. By the spring of 1999, BNSF Railway had hired a watchman who patrolled the tracks every hour after a storm. Shannon & Wilson was hired to develop methods to reduce the frequency of landslides at the Castle Rock site. Trent Hudak, manager of engineering with BNSF, says site constraints eliminated traditional methods of fixing the problem. The company was confronted with tight rights of way, extremely steep slopes, and surface soil weathering conditions. “The goal was to move loose debris down to the unweathered materials and to make it more weather resistant and be able to stop the skinslides,” Hudak explains. BNSF worked with the State of Washington to secure a $1 million Federal Railway Administration (FRA) grant for the project. The rail corridor between Seattle and Eugene, OR, is a federally designated high-speed rail corridor, which is being enhanced by the Washington State Department of Transportation to support the state’s intercity passenger rail program.“The funding from the FRA was established for testing new technology that hadn’t been tried before on railway facilities,” Hudak says. And that’s where the unusual solutions began. Boyle relates that his company considered several options, but each one had restrictions. For instance, the 45° angle and high slope didn’t lend itself well to the use of a retaining wall. The slope could not be flattened because of limited rights of way and residence boundaries. Debris fences weren’t an option because too much track occupancy would be needed to keep the track clear. Shannon & Wilson suggested that the problem be solved by a very nontraditional line of attack—that is, by decreasing the rate of weathering of the surface soils. The approach was to install a fully engineered, honeycomblike Geoweb system from Presto Products Company to protect the surface of the long, steep slopes along the railroad tracks and to fill the cells with a coarse aggregate. In other words, the idea was to protect the soils from vegetative roots and their destabilizing effects on the soil particles. This solution made sense to everyone involved. It was affordable, discouraged plant growth, decreased infiltration, and slowed the flow of rain down the slope. With the perforated geocells, the rate at which the water entered the drainage system would be slower than with other slope cover methods and would not overwhelm the retention system that was about a half mile away, says Boyle. The project contractor, Wilder Construction from Everett, WA, rose to the task of developing some alternative construction plans. Although clearing surface vegetation and debris sounds like a fairly typical task for a construction crew, the steepness and height of the slope had the workers head over heels. Mark Hillyard is the Wilder Construction project engineer who worked on the Castle Rock site. “The BNSF flagmen did an awesome job with scheduling while we were working there,” he says. “We had high lifts down by the railroad lifting crews up the slope, in addition to crews tethered from up above being lowered down the slope. And that’s with about 80 trains in any 24-hour time frame. They were so very well coordinated.” The considerable challenges the construction crews faced included not only the height and degree of steepness of the slopes but also access restrictions to the top and bottom of the slopes and the train schedules. In addition, the soft silt soils prevented the use of heavy excavators. Instead of larger excavators, crews used a spider excavator. Hillyard describes the machine as having six individual hydraulic pumps so the motor won’t lose oil on steep slopes. Cabled to a bulldozer at the top of the slope, a hydraulic winch was used to move the excavator across the face of the slope. “The operator of the hoe has total control over the winch to control movements on the slope and up and down the hill,” Hillyard explains. Working the individual legs, boom, and bucket of the spider excavator, the operator had to keep one step ahead of the geotextile crew blanketing the slope. After covering the slope with geotextile, it was time to start installing the Geoweb sections and tendons. Panels were measured, stapled together, and placed strategically at the top of the slope. After the tendons were threaded, they were attached to a deadman pipe. “The deadman pipe system is pretty basic,” says Boyle. “We dug a hole, tied the tendons to a pipe, and lowered the pipe into a 2-foot-wide trench at the top of the slope. Then we compacted the soil over it.” This got the Wilder Construction crew swinging. Tethered to safety harnesses, laborers moved the Geoweb structure slowly down the slope, expanding it as they went. They then fastened the tendons at the bottom of the slope. Working from hoists stationed at the base of the slope, the crew worked to drive the stake anchors, secure each Geoweb to the slope, and finally install the restraint clips in each tendon. Expanding the Geoweb system to accommodate the No. 4 and No. 5 aggregate was a feat in itself. The crew used a concrete conveyor truck equipped with 100 ft. of a 30-m conveyor boom. Hudak describes it as a telescoping conveyor system based on a truck with a 100-ft. reach. “We didn’t want it to free-fall, so they dropped it down a tube on the end of the conveyor. A man in a safety harness placed the rocks by positioning the tube. All of this [happened] on a very busy rail corridor.” The Geoweb cells that were used measured 3 in. deep, and as Boyle recollects, that caused a few challenges. “The slope is at a very steep angle, with some areas at 45°, and it was locally steeper. In these steeper areas, they couldn’t fill the cells all the way. The next time, I think I would use a tackifier to hold the stones in place better.” As part of the FRA grant, a 75-ft. test strip of grass was planted on the southern end of the project for future comparative monitoring purposes. Approximately a year and a half after the project was completed, the test strip was weathered to 24 in. Boyle reports that when he checked again in August 2003, the depth of weathering was pushing 2.5 ft. in the hydroseeded test strip. The soil protected with the Geoweb systems was moist and loose to a depth of about 1 in., and the next 2-3 in. of soil were moderately stiff. Still deeper, the soils were dry, firm, and unweathered.Installing the geocellular confinement system in southern New YorkRestructuring Ramps and Overpasses
The New York Department of Transportation (NYDOT) is converting some of its older roads to interstate standards. In the first project in the state to use geocells, a slope along Route 17 in the Southern Tier area of the state needed some creative reinforcement. Matthew Barendse, a civil engineer with the Geotechnical Engineering Bureau of NYDOT, says the department is removing many of the substandard constructed roads and putting in ramps and overpasses that will handle the additional traffic loads when the system is extended to Interstate 86.
Barendse describes the challenges of one particular area of the Route 50 West off-ramp: “We had to install a wall that would be steep—a vegetated wall. Most of it is [sloped] 1.5 to 1, and one area is 240 meters long with a maximum height of 9.5 meters. And it is all crossed by wetlands.” Considering the need for low costs, an easy installation, and the requirement to use native soils for infill materials, NYDOT chose the EnviroGrid geocellular confinement system. Six-inch x 8-ft. x 4-ft. perforated sections were installed horizontally and stacked. Additionally, 6-in. x 8-ft. x 8-ft. sections were used. The plan was to create a terraced wall system with green fascia strips to blend in with the vegetation and surrounding environment. “We stacked geocells with every fourth layer being laid with geogrid,” says Barendse. “There was a little learning curve with the geogrid installation, but we had the manufacturers helping us a lot.” Building the EnviroGrid wallThe finished productAt the beginning, Barendse says, the four-person crews were installing 90-100 units per eight-hour shift. But by the end they were doing about 50 2 a day or 140 units per eight-hour shift. All in all, he adds, it took about a month to install the entire 15,000-ft.2 wall, using a total of 121,920 ft.2 of geocell. B. Anthony Construction Company was contracted to install the EnviroGrid wall. Guy Rucki, the project supervisor, says the erosion and sediment control practices on the project could have been a full-time job by themselves. “The problem was that it was entirely encompassed by wetlands,” Rucki explains. “Staying on existing rights of way, you are still bordered by wetlands, and that poses a challenge. The state was very critical on erosion control, even more so than on a normal project.” NYDOT requires temporary seed and mulch cover to prevent erosion every time crews leave an area, even for a short time. Rucki says it makes the job very labor-intensive. In addition, crews were required to install silt fences at the top and bottom of the site to prevent sediment from entering or leaving the property. Outer cells of the wall were planted with native wildflowers; vines and other ground cover were used to present a natural-looking landscape. As the project was the first in New York to use a geocellular confinement wall system, Barendse says the department is excited to see how the wall looks in the spring when it is overgrown with vegetation and flowers. North Saskatchewan River and Yellowhead Highway
Old roads and new highway construction both are subject to severe erosion when the right climatic events occur. When the Yellowhead Highway in Alberta, Canada, was constructed, the contractors became concerned about the potential heavy rains and spring runoff eroding the newly created highway ditches. Because runoff from the ditch flows directly into North Saskatchewan River, sediment control was a very high priority.

After considering the options, the contractor decided to install Nilex GeoRidge berms and erosion blankets. Prior to laying the erosion blankets, grass seed was broadcast over the area after soil grading. After the blankets were stapled down using specs issued by North American Green’s DOT System, the GeoRidge berms were installed perpendicular to the flow in the ditch.
The permeable plastic berms controlled the velocity of the stormwater in the channel and prevented erosion of the soil and seed under the blankets. GeoRidge berms reduce the velocity and energy of the water and slow the water movement over the erosion control blanket, allowing sediment to build up over the blankets. This aids in reducing the velocities further and allowing vegetation to establish. Eventually the vegetation in the Yellowhead Highway ditches became established, creating a more permanent system. The 1.5:1 slope of the mini-landfill (above) and geosynthetic placement (below) at the Northeast Philadelphia airport siteGeosynthetics Arrive at Philadelphia Airport
Road construction is only one of many uses for geosynthetics. In developing 100 ac. at the privately owned Northeast Philadelphia Airport, the Philadelphia Industrial Development Commission ran into problems when an old dump area, used for sewage dumping some 35 years previously, was discovered. Options were considered, but hauling all of the debris off-site was totally cost-prohibitive, says Archie Filshill, manager of InterGeo Services.
“So they had to excavate it and basically build their own landfill and contain it in order to develop the site,” he says. “The area had two sides with a three-to-one slope. But the other side was near vertical at about a one-and-a-half-to-one slope.” Filshill explains that high-strength fabric, SI Geosolutions Geotex 4X4 and Tensar BX1100 Geogrid, was used as a face wrap. InterGeo Services is primarily a geosynthetics installer and in certain areas of high erosion opted for ACF Environmental Landlok 450 turf reinforcement mats as a long-term solution. When finished, the mini-landfill site measured 20 ft. high and 600 ft. long and was fully vegetated.“On the three-to-one slope, it would have reduced their area. But by pushing it up to the property line, there was a savings in area to develop,” explains Filshill. “That gave them about 4 acres that could be developed for a parking lot for employees.” A 1 million-ft.2 retail storage warehouse now stands on what was once only a weed-infested lot.Stabilizing a Vertical Slope
High-strength geogrid and geocellular confinement systems have made it possible to develop in high-priced areas as never before. These systems are exactly what the Irvine Company discovered would help develop a pricey southern California property known historically for landslides, steep slopes, and poor soils. Geogrids made it possible for the land development without compromising views of the renowned Pelican Hills Golf Course or the Pacific Ocean.
A more-than-200-ft.-high mechanically stabilized earth slope required in excess of 400,000 yd.2 of Mirafi Miragrid to withstand a large displacement of adjacent landslide soils sprawling along the property lines. The majestic-looking reinforced slope measures more than 500 ft. wide. After receiving the master rolls of geogrid, the contractor put the rolls on a large spool and unrolled the 3,000 lb. of material over the face of the slope. After stabilizing the area, he moved the spool to the next area and unrolled for the next cut.“Surprising” was how the contractor with Sukut Construction described the process and the ease of installation. Crews installed more than 5,000 yd.2 a day to complete the project. Lots once described as “unbuildable” are now vegetated and look natural enough to blend in with the rolling hills of the Newport Beach coast. How Old Is a Road Before It’s Not a Legal Road?
As communities spread out—whether to expand to the suburbs or in search of open space for recreational use—development of the transportation infrastructure seems imminent. Old roads are being scrutinized by transportation departments and the courts and are being distinguished based on how they were created. Some historic roads are even being questioned in court regarding issues of expansion, intensification of use, or abandonment; debates take place over whether these should be treated as new construction or reconstruction projects.
The State of Utah’s Department of Natural Resources found the issue of roads and public rights of way important enough to publish a Web site that categorizes roads according to their creation and their use ( When a 1995 flood washed out a public road in the small town of Jarbidge, NV, it sparked a court battle that has lasted for more than eight years. After the washout, Elko County attempted to repair the road because historically it was used by locals and tourists to reach public forestlands and some private land in Elko County. In 1999, the United States Forest Service placed a moratorium on road construction, allowing 18 months to draft a new road-management policy—and time to let the dust settle on South Canyon Road. With 380,000 mi. of road to manage, the US Forest Service currently has eight times more miles of roads in the national forest system than in the whole interstate highway system. Public lands need public roads, and in April 2003, Humboldt National Forest Supervisor Robert Vaught announced that the Jarbidge Canyon draft environmental impact statement was available for public comment. The draft includes alternatives from Elko County to reestablish South Canyon Road to preflood configuration. Other alternatives include building a trail instead of opening the road and a plan that would relocate the road to a place outside the canyon bottom. As of September 2003, Doug Clark, who is the Humboldt National Forest Service planner for the Jarbidge area, says no decision has yet been made, but he says any roadwork that does take place will have to involve the use of geosynthetics to stabilize the historic road. South Canyon Road dates back to the 1800s when sheepherders moved their stock from Elko to Jarbidge and over the passes toward Boise, ID.Geotextiles and Other Geosynthetics Applications
According to the Geosynthetic Research Institute (GRI), geotextiles now form one of the largest groups of geosynthetics. Unlike traditional textiles, geotextiles are made of synthetic fibers instead of natural ones, and this lessens the importance of biodegradation. Geotextiles are permeable to water flowing across their plane and within it, and they always perform at least one of five functions—separation, reinforcement, filtration, drainage (when impregnated), and barricading moisture—according to GRI.
Although geotextile uses are constantly growing, listed below is a selection from the GRI Web site, of Dissimilar MaterialsBetween subgrade and stone base in unpaved roads and airfieldsBetween subgrade and stone base in paved roads and airfieldsBetween subgrade and ballast in railroadsBetween landfills and stone base coursesBetween foundation and embankment soils for surcharge loadsBetween foundation and embankment soils for roadway fillsBetween foundation and soils and flexible retaining wallsBetween slopes and downstream stability bermsBeneath precast block and panels for aesthetics pavingBetween drainage layers in poorly graded filter blanketsReinforcement of Weak Soils and Other MaterialsOver soft soils for unpaved roadsOver soft soils for airfieldsOver soft soils for railroadsOver soft soils for landfillsOver unstable landfills as closure systemsFor lateral containment for railroad ballastTo wrap soils in encapsulated fabric systemsTo aid in construction of steep slopesTo stabilize slopes temporarilyTo halt or diminish creep in soil slopesTo anchor facing panels in mechanically stabilized earth wallsTo create a more stable sideslope due to high frictional resistanceFor use in insitu compassion and consolidation of marginal soilsFiltration (Cross-Plane Flow)In place of granular soil filtersBeneath stone base for unpaved roads and airfieldsBeneath stone base for paved road and airfieldsAround crushed stone surrounding underdrainsAround crushed stone without underdrains (i.e., without French drains)Beneath landfills that generate leachateAs a silt fenceAs a silt curtain As a flexible form of containing sand, grout, or concrete in erosion control systemsAs a flexible form of restoring scoured bridge-pier bearing capacityBetween backfill soil and void in retaining wallsBetween backfill and gabionsAgainst geonets to prevent soil intrusionAgainst geocomposites to prevent soil intrusionAs a filter beneath stone riprapDrainage (In-Plane Flow)As a drainage interceptor for horizontal flowAs a drainage blanket beneath a surcharge fillAs a drain behind a retaining wallAs a drain beneath railroad ballastAs water drain beneath geomembranesAs an air drain beneath geomembranesAs a capillary break in frost-sensitive areasTo dissipate seepage water from exposed soil or rock surfacesGeogrids Function as Reinforcement Materials
They are plastic and formed with an open gridlike configuration and have rather large openings that are frequently filled with aggregate. They are high in strength and resist creeping. GRI reports that geogrids can be used as follows:
Beneath aggregate in unpaved roadsBeneath surcharge fills or temporary construction sitesTo reinforce embankment fills or earth damsTo repair slope failures and landslidesTo construct of mattresses for fill over soft soilsAs sheet anchors and facing panels to form entire retaining wallsTo reinforce disjointed rock sectionsAs inserts between geotextilesTo reinforce landfills to allow for vertical expansionTo reinforce landfills to allow for lateral expansionTo stabilize landfill cover soil as veneer reinforcementAs three-dimensional mattresses for embankments over soft soils