Whether protecting exterior surfaces or enhancing interior spaces, products made from synthetic materials are improving landfill economics and the environment.

By Greg Northcutt

Faced with ever-rising land costs and stepped-up enforcement of stormwater management regulations, more and more landfill owners are turning to multi-purpose geosynthetics as a way to protect and even enhance profits.

Manufactured from common polymers or plastics, such as polyester, polypropylene, and high-density polyethylene, geosynthetics can be used in a variety of ways as more cost-effective alternatives to traditional construction materials–like rock and concrete—to reduce landfill construction costs.

"More and more people mean more and more waste," says David Snyder, CPESC. He's president of WEBTEC Inc. (Charlotte, NC), which markets a broad range of geosynthetic products. "All that waste plus increased scrutiny by regulators makes it more and more challenging to find a place to put it. This is pushing owners to make the most efficient use of every square foot of landfill area and every cubic foot of landfill volume. Geosynthetics can help them do that at less cost than using traditional construction materials."

On top of that, federal, state, and local government agencies are clamping down on owners and contractors who violate rules and regulations requiring them to control erosion and sediment on construction sites.

Frank Pace, P.E., CPESC, is a market development manager for geosynthetics manufacturer SI Geosolutions (Chattanooga, TN). "Over the past two or three years, we've been getting more and more calls from owners and contractors who've been fined for failure to prevent sediment from washing off a construction site," he says. "They want to solve their problem. Geosynthetics can help provide the solution."

Among the variety of geosynthetics used in landfill applications are geotextiles. For example, silt fences are designed to trap sediment on disturbed slopes before stormwater runoff can carry it into a stream. Geotextile filter bags, which filter sediment-laden runoff from pumped water, offer better sediment control than straw-bale structures or sediment basins. Other types of geotextiles can be used as a separation layer beneath the road base and the underlying native soil to reduce the amount and cost of aggregate when building haul roads.

In just the past few years, especially in more densely populated areas—where land for building and expanding landfills is becoming increasingly scarce and expensive—other types of geosynthetics have been attracting attention for their ability to improve the economics of landfill construction and operation in other ways.

These geosynthetics are used to control erosion on side-slope surfaces, swales used to slow flow velocities down a slope, and the downchutes used to drain the swales. Also, they're allowing engineers and contractors to enlarge the capacity of landfill cells by reducing the thickness of the landfill cover and steepening the side slopes. In doing so, these products offer a more cost-effective, attractive, and environmentally friendly alternative to rock and concrete for protecting soils and landfills from the impact of rain and runoff during and after construction and closure activities.

Here's a closer look at several of these types of geosynthetics and how they're helping landfill owners make the most efficient use of their land and money in three key areas.

Protecting the Landfill Cover

Geosynthetics can help protect the veneer of soil or final cover, which is placed over the liner system of a landfill, from erosion and sloughing.

Controlling Erosion

In many cases, grass or other vegetation is the best choice for protecting disturbed landfill slopes from both wind and water erosion. The plant roots hold the soil in place, while the leaves absorb the erosive impact of wind and raindrops. It's a natural and permanent option for controlling erosion, and it's also the least expensive.

Various types of mulches—such as straw, hydraulic fiber mulches, and bonded fiber-matrix products—can be used to protect seed and soil, and retain moisture to promote seed germination until the vegetation can grow and begin controlling erosion on shallower slopes.

However, as slopes get steeper and longer, stronger protection—like that provided by erosion control blankets—may be required. Depending on such factors as amount of rainfall and types of soils, these products are effective in controlling stormwater runoff on moderate slopes and channels with gradients as steep as about 3H:1V and with flow velocities of up to 6 feet per second. These rolled-on products are typically made of natural materials, such as straw, coconut fiber, excelsior, and jute. Their primary function is to control erosion for the short term until the vegetation becomes established. However, they also help maintain moisture and temperature to promote germination of the seed. Depending on components, these products have a life expectancy of about 10 months to three years and degrade naturally.

For example, WEBTEC offers the TerraGuard line of organic erosion control blankets. Each is stitched together using biodegradable thread and held together using photodegradable plastic netting. They range in durability and performance from the lightweight TerraGuard SS, a single-net straw blanket for protecting slopes and channels from moderate rainfall and runoff, to the heavier, slower degrading TerraGuard DC, a double-net coconut blanket designed for slope or channel protection in moderate to heavy rainfall and runoff.

Steeper slopes and higher channel velocities may call for the kind of long-term erosion control offered by turf reinforcement mats. They strengthen the natural ability of plant roots and stems to resist erosive stormwater runoff and channel flows. These nondegradable soft-armor products are typically made of mechanically bonded or melt-bonded synthetic materials—such as polymer nettings, monofilaments or fibers that are entangled to form a 3D reinforcement matrix. Most of this matrix is open space. Vegetation growing through these open areas is reinforced by the matrix, which is designed to provide permanent erosion control when flow velocities and shear stresses exceed what mature, natural vegetation can withstand. These TRM products also collect sediment from runoff while the vegetation grows in. Their performance increases as the vegetation becomes established and the root structure becomes entangled within the synthetic mat.

Manufacturers have also developed composite TRMs, which usually feature a biodegradable filler between a temporary layer of netting and a permanent layer of synthetic matrix. They withstand higher flow velocities and offer longer-term protection than an erosion control blanket.

Stabilizing the Veneer

Poor drainage can threaten the integrity of the landfill cap by allowing hydrostatic pressures to build up, causing the soil veneer to slide off. Three-dimensional, multi-layer geosynthetic products, featuring a polymeric core surrounded by one or more geotextiles, can replace less efficient and more expensive French drains and pipe drains to improve drainage. The geotextiles filter out soil particles and allow water to pass into the core, which directs the water to a perforated pipe, weep hole, or other discharge system. Typically, these geosynthetic drains transmit water and gases along two or three planes using tubes.

EnkaDrain, made by Colbond Inc. (Enka, NC), offers another variation. Instead of tubes, it consists of fused, entangled filaments. "This produces an omni-directional drainage path instead of a channelized flow," says Richard Goodrum, who manages sales and marketing of the company's civil engineering products. "As a result, if something impedes the flow water or gas in one direction, there's always a way around it."

When slopes are steeper than normal, the soil veneer can slide if the interface friction angle between the geomembrane or clay layer is lower than the soil-to-soil friction. In this case, another approach, using a cellular confinement system, may be a smart choice.

These engineered, polyethylene systems, which feature a honeycomb-like structure of open cells (geocells), can stabilize landfill covers by significantly improving the performance of typical infill materials. By structurally confining the vegetated infill material, the anchored cells prevent the materials from sliding downslope.

These systems are available in a variety of cell sizes, which typically measure about 8–20 inches in diameter and from about 3–12 inches high. They can be filled with a variety of common fill materials, such as topsoil, sand, gravel, or concrete. When filled with soil, the cells will also support establishment of vegetation.

These systems can be used to construct a stable structure that will flex to conform to differential settling, a common occurrence at landfills as solid waste degrades over time, reports Dan Senf, P.E., CPESC, director of business development for Presto Products Co. (Appleton, WI). A business unit of Alcoa, Presto Products produces the Geoweb line of cellular confinement systems.

"These systems dramatically change the engineering characteristics of how infill materials perform to solve some challenging hydrodynamic or load support problems in ways that no other technique can," he says.

The Geoweb system includes cellular confinement products that feature perforated cell walls. These openings promote parallel slope drainage of the infilled cells to reduce buildup of hydrostatic pressures and the potential for the system to slide downslope when the soil becomes saturated with runoff. Increased friction resistance between infill materials and the perforated cell wall also improves resistance to upward displacement caused by freeze-thaw cycles and hydraulic scour of aggregate infill. In vegetated systems, these perforations serve still another role. They provide a path for plant nutrients and earthworms to move between cells and for plant roots to extend into other cells to improve their ability to help stabilize and keep plants in place during short-term hydraulic events.

In addition to stabilizing landfill veneers to protect the landfill liner, cellular confinement systems can also be used to reduce excessive or concentrated loads on the cap. For example, by distributing live loads over a wide area, a geocell system installed above the cap can allow development of a park or roadway atop a landfill, Senf notes.

Stabilizing Channels

Geosynthetics can also be used to replace riprap or concrete-lined drainage swales and downchutes at a landfill with a less expensive approach for managing stormwater runoff to control erosion.

Green Drainage Ways

The selection and use of soft armor materials—such as turf reinforcement mats that protect stormwater channels from the erosive force of concentrated flows using vegetation—continues to grow, reports Pace, whose company, SI Geosolutions, makes both the Landlok and the Pyramat line of TRMs.

"Many engineers, regulators, and owners want to get away from hard armor," Pace says. "As they gain experience with turf reinforcement mats, they're becoming more comfortable with them. They're using these vegetated TRM systems to replace as much as 24 inches of riprap or a 4-inch-thick slab of concrete."

One reason is their effectiveness in keeping sediment onsite. "Research shows that vegetated channels can capture as much as 700% more sediment than channels lined with hard armor," says Pace.

Permissible shear value measures the ability of a material, such as grass, sand or rock, to withstand the forces imposed by water flowing over it. Goodrum, with Colbond, which produces the Enkamat and Enkamat II brand of TRMs, notes that unreinforced grass can withstand shear values in the 1-pound to 2-pound per-square-foot range without being torn out of the soil, while shear values for well-established sod may approach 3 pounds to 3.5 pounds. Those figures compare to about a 2- to 5-pound permissible shear value for 6-inch riprap.

"Depending on the product, the permissible shear values of TRMs can range from about 2 pounds per square foot up to 10 or even 12 pounds per square foot, which is equivalent to a flow velocity of about 20 feet per second, depending on channel geometry" he says. "The shear values of most stormwater channels seldom exceed 10 pounds per square foot."

WEBTEC markets the TerraGuard line of polypropylene TRMs. It includes one mat that is designed to withstand a sustained channel flow velocity of 8 feet per second during a 48-hour period—without vegetation. Another mat, according to the company, can protect channels from flow velocities of 14 feet per second and a shear stress of 5 pounds per square foot after a 24-hour saturation period. (Table 1 compares the performance of various erosion-control systems.)

Lower construction costs are also spurring interest in TRMs. The installed cost of these products is around $5 to $20 per square yard, according to Pace. By contrast, the installed cost per square yard of riprap ranges from about $25 to $60, and for concrete this cost can total $40 to $80.

Where shear values of stormwater flows exceed the ability of TRMs to control channel erosion, the flexible nature of a cellular confinement system can offer a more attractive alternative to a conventional poured-concrete lining. This approach offers the advantages of a semi-rigid channel lining and faster construction at a lower cost.

Differential settlement, like that found in landfills, can cause a rigid concrete slab channel to crack and fail. When infilled with concrete, a cellular confinement system becomes a flexible concrete mat with built-in expansion joints. "This system can handle a wide range of structural requirements and severe hydraulic stresses, while conforming to significant subgrade movements to maintain integrity of the channel lining over the life of the landfill," says Senf.

Senf notes other money-saving features of a cellular confinement system.

• It can eliminate the need for complicated structural elements, like reinforced steel or expansion joints and expensive, time-consuming construction techniques, like building forms.
• A defined, uniform thickness of the cells offers much more accurate control of concrete quantities and costs.
• The quality, surface finish, and thickness of the concrete can be selected to meet specific design needs.
• It eliminates the need for special, compact, granular bedding materials required with conventional poured concrete slabs.
• It prevents uncontrolled cracking of the concrete and reduces the chances of piping or undermining. Hydrostatic pressures below the cellular confinement layer are relieved by installing an underlying geotextile and/or groundwater outlet ports where needed.

What's more, when stacked in stepped-back layers, a cellular confinement system can be used to provide stable, green-channel side slopes that can withstand differential settlement. In this case, the outer cells are filled with an aggregate/topsoil mixture that supports vegetation. "This gives the facia a natural appearance and the ability to withstand high flows for short durations," says Senf. "You can also choose colored face panels so that the system blends naturally with the environment."

Increasing Landfill Capacity

Ever-rising land costs and the increasing difficulty of getting permits to build landfills in some areas is pressuring landfill owners to create as much storage space as possible within a permitted landfill's footprint. At the same time, more airspace within the landfill also helps owners buy more time to obtain permits for future cells. Geosynthetics offer several options for gaining this extra capacity.

Thinner Landfill Caps

Typically, landfill regulations require placing a minimum depth of soil, say 3 feet, over the landfill's liner system to protect it from frost damage or erosion. In climates where soils don't freeze, TRMs can be used to reduce the thickness of this soil cover, freeing up valuable space within the landfill while controlling erosion.

"We point out that, if approved by regulators, you may be able to substitute as much as about 2 feet of soil cover with a TRM to provide the necessary erosion control where frost isn't a concern," says Pace. "When spread over an entire landfill, an extra foot or so of airspace represents a lot of money."

In a similar manner, a cellular confinement system, can replace a much thicker layer of aggregate, riprap, or concrete cover in a landfill to gain airspace within the landfill cell.

Steeper Slopes

Typically, landfill covers are constructed with side-slope gradients between about 4H:1V, or 14 degrees; and 2:1, or 26 degrees. Increasing the angle of these side slopes, can increase the landfill's storage capacity by adding a wedge of extra airspace at the perimeter of the landfill without extending the outer limits of the landfill's base.

"Ten years ago, steeper side slopes on landfills were the exception," says Terry Sheridan, environmental manager for Tensar Earth Technologies Inc., Atlanta, GA. "Today, they're much more common. A number of reinforced perimeter berms have been constructed in the last few years. Generally, in areas where land is scarce and tipping fees are high, like in metropolitan areas, more landfill owners are choosing to steepen the outboard slopes of new cells at existing landfills rather than when building new landfills in these areas."

In some cases, TRMs offer a cost-effective way to add extra storage capacity. For example, Pace notes that landfill cover side slopes typically require construction of tack-on berms to slow runoff flow and capture sediment. However, in some cases, these tack-on berms can be eliminated with the use of a TRM to control erosion. This steepens the slope of the landfill cover sections, increasing the amount of available airspace.

Soil reinforcing geogrids make it possible to build landfills with a much steeper outboard slope. These products are usually made from extruding and stretching polypropylene or high-density polypropylene or weaving or knitting and coating high tenacity polyester yarns to form a grid structure with relatively large openings. When placed horizontally and covered with soil, this grid interacts mechanically with the soil, increasing the soil's tensile strength and creating a reinforced soil mass.

Tensar produces the ADD3 geogrid system. In landfill applications, it is normally used to build slopes as steep as about 1:3, or 71 degrees, Sheridan reports. However, he's worked on landfill projects that have involved vegetated side slopes of 1:6, or about 80 degrees.

To appreciate the value of steeper slopes in adding to landfill capacity, he offers this example: Consider a 15-foot high berm built with a 3:1 slope. The crest of that berm would be 45 horizontal feet back from the base of the slope.

By contrast, building that same 15-foot high berm with a 1:3 slope would put the crest just 5 feet back, horizontally, from the berm's base—a difference of 40 feet.

"If the vertical limit of your landfill is 100 to 200 feet above the ground elevation, that extra 40 feet adds a significant amount of storage capacity inboard of the reinforced perimeter berm," Sheridan says.

In constructing these mechanically stabilized earth walls, the slopes are built in lifts separated by the geogrids. "Generally, earthmoving work with our system proceeds at the same rate as with conventional construction practices," Sheridan explains. "However, it requires enough laborers to place the geogrids and construct the slope face in order to keep pace with the earthmoving activities."

Long-term stability at the face of the ADD3 system requires UV-stabilized geogrids wrapped between soil lifts, along with a healthy stand of vegetation, Sheridan notes. "The seed mix that vegetates during construction is usually overtaken by the local plants by the following spring," he says. "The root balls within the topsoil at the front of the system are confined behind the UV-stabilized geogrid that wraps around the face of the slope. Local rainfall has been sufficient to support the vegetation without the need for irrigation on projects built to date."

These examples illustrate how enterprising owners are harnessing geosynthetic technology to construct and operate landfills more efficiently. It's paying off for them as well as the environment.

Greg Northcutt writes frequently on construction and business issues.

MSW - January/February 2006