Keeping soil in place is hard enough on flat land. Engineers, contractors, and others who work with slopes have gravity to contend with as well as soil type and water.
And then there is whatever’s under the soil. The Millennium Pipeline project in New York restored environmentally sensitive agricultural land after a gas pipeline was installed. The project at the Police Evidence Storage Facility in Winston-Salem, NC, was defined not only by a debris field, but also by the discovery of an old utility pipe during construction of the parking lot. Projects are ongoing at the Williamson County Landfill in Franklin, TN, where water downstream that once was on Tennessee’s 303(d) list of impaired waters now has flourishing populations of fish and invertebrates.
These projects illustrate a number of methods to keep soil in place: geotextiles filled with mulch, soil, and limestone; blankets and turf reinforcement mats (TRMs); and steel mesh anchored to the slope. Whichever methods were used, they all allowed vegetation to take over in time.
The Millennium Pipeline
Regulations for restoring environmentally sensitive areas have been strict in New York state, ever since the controversial Iroquois Pipeline, an underground, natural-gas pipeline, was installed in the early 1990s, says Robert Hancock, president of Old Oak Environmental Restoration Inc., in Fort Plain, NY.
“It ended up with some very serious legal issues,” he says, “and as a result, New York has a very strict protocol for restoration, particularly of agricultural land.”
The Millennium Pipeline is a new underground natural-gas pipeline in the state, the centerpiece of a larger project that involves the expansion of three other pipelines, including the Iroquois. The Millennium Pipeline project extends 182 miles across the lower Hudson Valley and the Southern Tier, mostly along existing utility easements.
Old Oak Environmental Restoration restored 96 miles of the project, through four counties in western New York, most of which was in the environmentally sensitive Chesapeake Watershed. Most of the area is countryside, consisting of agricultural land, wetlands, and mountains with primarily 3:1 slopes and greater.
“It’s adjacent to the Susquehanna and Delaware rivers,” Hancock says. “There is a lot of environmentally sensitive farmland, which requires special treatment.”
The company used erosion control blankets made by East Coast Erosion Blankets in Bernville, PA, and bought from ACF Environmental. Jim Giumarra, the contact at ACF, was also a consultant on the project.
“They needed blankets that work well on slopes,” Giumarra says. “They also had to be biodegradable and have a netting that doesn’t entangle animals.” In addition to the blankets, Old Oak Environmental Restoration used native hay mulch and revegetated with a variety of seed mixes.
When Millennium Pipeline Co. dug the trenches for the pipeline, it separated the topsoil from the subsoil, as required by state regulations that protect environmentally sensitive agricultural areas. Crews installed 30-inch-diameter steel pipes, backfilled the trenches, and regraded the right of way, which is generally 75-100 feet wide.
Old Oak Environmental Restoration began erosion control work immediately, again as required by the state. Restoration crews followed within 1,000 feet of the construction crews, so the trenches had virtually no time to degrade. Work began in June 2008 and wrapped up for the winter in December. Old Oak Environmental Restoration went back and finished up the slopes and pastures in the summer of 2009, stabilizing the streambeds in November.
“We generally did the work six days a week, 10 to 12 hours a day,” Hancock says. “When you sign on to a project like this, you have to have the manpower and equipment.”
The company first used a subsoiler to decompact the soil to a depth of about 20 inches. The soil varies from clayey to rocky, and there was a lot of stone-picking to get rid of all the rock that made its way to the surface, Hancock says. Crews then returned the topsoil, added lime and fertilizer, and decompacted the soil again. From there, the project was separated into three areas, each of which was treated differently.
Upland areas included slopes, woods, brush, and rocky areas. Most of these were covered with East Coast Erosion Blankets’ ECS-2B biodegradable double-straw blankets to protect the soil from erosion and give the seeds optimal conditions to take root and grow. The blankets are made of 100% agricultural straw and covered by two jute nets sewn together with biodegradable thread. Their functional longevity is about 12 months, depending on the soil and climatic conditions. Most of the blankets were fastened with 6-inch pins. On some of the steepest slopes, where the soil was wet, they were secured with alder wood stakes spaced closely together.
The terrain and the weather were challenging. “There were a lot of very steep slopes,” Hancock says. “When you have rain and steep slopes, you have a lot of sliding, but there’s no stopping. The area has to be restored before you leave.”
Jared Van Benschoten, a partner in Old Oak Environmental Restoration and general superintendent of the project, says accessibility on the steepest slopes was so difficult that it was impossible to lay blankets.
“Any slope 3:1 or less got hay mulch,” he says, “and that had to be spread by hand.”
Crews seeded the upland areas with permanent seed, including Empire birdsfoot trefoil, white clover, and timothy, as well as some annual rye. The goal was to stabilize as much of the right of way as possible with permanent vegetation, and ultimately very little temporary erosion control was necessary, Hancock says. “Once the blankets were placed on the slopes, they pretty much stabilized everything.”
The majority of the agricultural fields required no blankets or mulch, Van Benschoten says, and the company revegetated them according to what the farmers requested. Because most were pasture, crews usually used a mix that included perennial rye, medium red clover, and birdsfoot trefoil. It was too late in the season to plant seeds in fields where cash crops were grown. Some farmers requested winter rye, and others asked that the fields be left unseeded.
If water was channelizing in the pastures, the company repaired the areas and put down blankets, fastening them with East Coast Erosion Blankets’ biodegradable stakes. “Cows will eat the metal pins,” Van Benschoten says.
On the wetland areas, crews planted live stakes of species that root easily, including willows and red-twig dogwood. They also used annual rye for temporary erosion control. “The native plants will come back,” he says.
On flat areas that weren’t agricultural or wetland, crews used native hay mulch, about three tons per acre, which they ran over with a crimper to improve contact with the ground. Straw isn’t readily available in the Northeast, and the hay used makes better mulch anyway, Hancock says. It isn’t weedy, and there’s always a certain amount of germination of hayseed, which is the reason he likes to use local hay.
The project was done with union labor. Because of the pipeline construction schedule, 80% of the restoration had to be done out of season, Van Benschoten says. “A lot of seeding was done in midsummer and late October. That typically doesn’t do very well, and we had to reseed bare spots in the spring. But you can’t build a construction job around the seeding.”
It isn’t ideal, Hancock agrees, but ultimately, there was very little erosion. “In terms of restoration, it was very successful.”
Police Evidence Storage Facility
Construction of the parking lot of the newly built police evidence storage facility in Winston-Salem, NC, was almost complete when the contractor, Davie Construction, made a couple of unwelcome discoveries.
“As they were developing the parking area behind the building, they determined that it was a debris field, which included an old chimney stack that had been blown up and left there,” explains Jim DeSpain, P.E., president of Uretek ICR Mid-Atlantic in Kernersville, NC, which specializes in repairing unstable soil. “Then, when they got to the edge, they discovered a utility pipe. They had to cut in a road and uncover the pipe so it could be maintained.”
The time to make the unexpected repairs and the cost were both factors in choosing slope stabilization products, since neither had been included in the budget, DeSpain says. Davie considered shotcrete and a retaining wall, but ultimately chose a combination of three products that also allowed more natural drainage and revegetation.
Uretek anchored the slope with soil nails and covered most of it with Geobrugg’s Tecco hexagonal mesh system, which is made of high-strength steel wire, and Tenax Corp.’s Multimat, a three-dimensional polypropylene TRM. For the highest erosion areas, below water discharge points, Uretek used Donnelly Fabricators’ Texicon, a woven, double-layer fabric matting filled with structural grout.
The project began in January 2009 and finished the following month. The slope was about 60 to 70 degrees and 25 feet long, covered with large pieces of debris.
Uretek installed the nails first, using a Con-Tech Titan Bar system and an excavator-mounted rock-drill unit to drive them 15 feet into the slope. The system allows for faster placement of the nails in material that won’t remain open after drilling. As the drill bit rotates, it breaks up what’s in front and shoots out grout at the same time. The grout flushes out all the debris created by the drill bit.
“Because of the shallow nature of the large pieces of debris, the concern was how far back to take the nails,” DeSpain says. “And because there was so much debris, we used a lot of grout. It really makes a good bond with the soil and can shorten the required depth.”
Crews arranged the nails in a grid. Wherever it was possible, they left the existing vegetation to facilitate plant regrowth. When the grout had reached its curing strength, they rolled the TRM down the slope, leaving an 8-inch overlap between the rolls.
According to Tenax, Multimat is designed for slopes that already have a suitable vegetative substrate soil, so existing vegetation easily grows through it. It can be seeded and filled with soil, and if there are exposed areas, additional seeds can be applied without cutting into it.
Uretek crews rolled the Tecco mesh over the TRM. The mesh comes in rolls 11.5 feet wide and 98.4 feet long and has an open structure, which eliminates the need for drainage and facilitates revegetation. Crews lifted both the TRM and the mesh over the nails, overlapping two diamonds of the mesh. They then secured the mesh panels together with steel compression claws from Geobrugg and trimmed them at the bottom of the slope.
They then placed galvanized spike plates over the nails and tightened them with bolts. A crucial aspect of the Tecco system is to fasten the mesh as tightly to the slope surface as possible, DeSpain says. This transfers the forces from the mesh to the anchors and improves the connection to the subsoil. “You dig a depression about one-and-a-half to 2 feet deep around the nails, put down a plate, and tighten it,” he explains. “As you tighten it, you stretch the mesh into the hole.”
Afterward, the company made sure that all the spike plates had the proper torque and checked for loose mesh on indentations on the slope. Where they found loose areas, it was easy to drill through both the mat and the mesh and tighten them. When the work was finished, they cut the nails within 3 inches of the bolts.
One area, where runoff from the roof of the building and the parking lot pours out a drain at the top of the slope, required a different solution.
“Rather than use riprap, which can move over time, we used a filter fabric filled with very lightweight concrete,” DeSpain says. “It looks like an air mattress.” The concrete material is Terra-Crete, a proprietary product that the company produces.
The company installed the mat under the piping. It consists of a woven double layer of fabric, so it’s flexible and provides intimate contact with contours, corners, and curves for both erosion control and scour protection. A high-strength, fine-aggregate concrete (structural grout) was pumped into mat. Both the fabric and the filling are pervious, so water coming out of the pipe can infiltrate slowly.
Uretek planted rye grass to get the vegetation going in the winter, DeSpain says, and Davie Construction sprayed seed the end of March. New vegetation was already growing through the mat and the mesh in April 2009.
Williamson County Landfill
Few things are more discouraging to stormwater managers than watching silt-laden stormwater break through a carefully constructed barrier.
“We’d put up mulch berms and started construction,” says Lewis Bumpus, director of solid waste management for Williamson County Landfill in Franklin, TN. “Then high-velocity water hit and washed it out. I needed a quick fix for an emergency energy dissipator.”
The 400-plus-acre landfill is at the pinnacle of the Tennessee Divide, surrounded by 4,500 acres of Nature Conservancy land. Stormwater runs down to two major rivers in the state, the Tennessee and the Harpeth, by way of Arkansas Creek.
By 1998, when Bumpus was hired to manage the landfill, there was no aquatic life in Arkansas Creek because of operations and runoff from the landfill. It was on Tennessee’s 303(d) list of impaired waters for siltation, habitat alterations, inorganics, organic enrichment, and low dissolved oxygen.
About three years later, it was back to a natural, pristine stream, he says. Turbidity and sediment had disappeared, and the creek was taken off the 303(d) list. The Tennessee Department of Environment and Conservation presented Bumpus with the Aquatic Resource Preservation Award for his work in mitigating the damage done by the landfill.
“We have to be very conscious of how we manage our stormwater and groundwater,” he says. “There’s a lot of elevation on our site. Rainfall is between 48 and 53 inches per year, pretty well year-round. The soil is mainly clay. And the landfill is a constant construction project. We’re dealing with dirt all year long.”
These days, 23 species of fish and 57 species of invertebrates, including four species of flies that are intolerant to silt, flourish in the creek. The landfill has eight wetlands, with plantings that include cattails, because the local soil is high in iron and cattails store it. Bumpus uses a soil-moisture monitoring system to measure the amount of water that enters the soil and to check how much runs off and where it goes.
The landfill also has more than 30 check dams, constructed using Typar Matrix 3D Cellular Confinement System for Earth and Water by Fiberweb.
“If you keep silt onsite, you’re not sending it to your neighbors,” he says. “It’s cheaper to keep it onsite than to go and get it.” Before landfill crews begin construction or demolition, they put in liners to protect the groundwater. “We go the extra mile. I look at it as an insurance policy. I’d rather put up the money up front to reduce the possibility of having issues later.”
Mulch berms are the next line of defense. Much of the stormwater program involves bioengineering, but unlike many other stormwater managers, Bumpus doesn’t revegetate.
“Tennessee is a very, very green state. We found out a long time ago that Mother Nature will revegetate in a heartbeat.” One 6-acre area was covered with mulch but no seeds, and 98 species of plants have grown up.
The berms can withstand a certain amount of stormwater, but once in a while, a strong enough rainfall can wash them out, Bumpus says, and usually there’s little notice before a storm hits. “That makes it real expedient to build a check dam to slow down the force of water. I wanted something effective, quick to establish, and affordable.”
Traditional gabion baskets take a fair amount of time to build, because the equipment has to be put in place and the baskets have to be set at the same elevation. Bumpus uses Typar because the installation doesn’t have to be as precise, so it doesn’t take as long to prepare.
“Typar lets us get in fast, use less material, and get the job done,” he says. Each cell is a three-dimensional honeycomb shape made of a geotextile, open at the top and the bottom. The metal frames that support the cells until they’re filled can be configured to each site. The cells interlock and adapt to the terrain and to each other. The cells also can be stacked on top of each other once the lower cell has been filled.
Bumpus excavates upstream of the mulch berm, installs the Typar Matrix, and fills it with limestone from a nearby rock quarry. Because the cell walls are straight up and down, it takes about one-fourth to one-fifth less rock to cover the same amount of ground as it does for a rock check dam, which needs a broad base. An additional benefit is that when it’s time to remove the cells, there’s less rock to take out.
Limestone is a good material to use because its sides are flat, so it doesn’t roll around inside the cells, he says. Crews use rock 5-7 inches in diameter and always keep some onsite, so they can set up in a couple of hours.
For landfill projects that require a diversion berm, Bumpus fills the cells with dirt instead of rock. “If a spot blows out, you can take some baskets, line them up, put dirt inside, and you have your temporary berm.”
Since Typar is UV-stabilized for a minimum of two years-longer if it’s covered-he’s been experimenting with it as a more permanent erosion control solution. In a few small areas, he has filled cells with mulch to test its effectiveness at filtering silt. It does a good job of taking out silt and turbidity alike, he says, and some test areas are still working well after a year.
“I try to work with Mother Nature and follow her needs,” he says. “It makes the program work a lot smoother.”
