If you look for a geosynthetic fabric in place on a construction site, about the only type you might see will be a black fence, or silt fence, stopping sediment-containing water long enough for the sediment to drop out while the water passes through the fabric.
But they’re there—in force. Silt fencing is just one of the hundreds of applications for geosynthetics that contractors are routinely using today to meet regulations and at the same time save money. Moreover, if we cross the fuzzy line between construction applications and erosion control applications, the numbers become far greater.
Because there are so many applications now, it would be impractical for any article to describe all of them. Hence, this article covers just a few of the more popular and/or interesting ones that are focused on construction applications, and it excludes erosion control applications.
Geotextile Blankets
Geotextile blankets represent the workhorses of the geosynthetics that are used for construction applications (and for erosion applications, as well). They represent by far the greatest use in the industry, and are offered by most dealers. According to Janet Lucas of CSI Geosynthetics in Vancouver, WA, there are three principal types of geotextiles in use today:
Woven geotextiles consist of two fibers (slit tape) woven together much like clothing fabric. This material offers high strength per pound (grab tensile) of material, and has minimal stretching (elongation). Woven fabrics are excellent for separation/stabilization applications for roads and construction entrances.
Nonwoven geotextiles are formed from fibers arranged in an oriented or random pattern into a planar structure. Generally, the fibers are bonded together by needle punching. Thousands of small barbed needles are set into a board, punched together through the loose fiber web, and withdrawn, leaving the fibers entangled. They are commonly used for pavement repair and drainage applications.
Spunbonded geotextiles are a sub-type of nonwovens, having specific properties separate from the needle-punched geotextiles. Thermally spun bonded fabrics have properties found in both woven and nonwoven needle-punched fabrics and hence can be used for a wide range of applications.
In addition, there are a variety of other geosynthetic products on the market for special applications, in the context of which these geosynthetic products will be described.
Road Construction
In any roadway construction project, subgrade preparation is critical to long-term stability and quality. According to Bill Hawkins, market manager for BBA Fiberweb’s Typar construction fabrics, when aggregate is placed directly upon the subgrade soils prior to paving, there is a high risk that over time the aggregate will mix with the underlying soils and lead to long-term road base deterioration. “On concrete highways, you can actually see the concrete slabs rocking,” he says. “When water gets in through the slab joints and goes down to the subgrade soil, cavities will result in the subgrade and continuous truck traffic can cause the water to spout, taking soil with it and deepening the cavity. The result is ‘center point cracking.’
“If you put down a geosynthetic fabric equivalent to our Typar, you add a permeable separator between the subgrade and the aggregate base. Typar prevents fine-grained subgrade soil from mixing with the base, which allows the base to retain its load-bearing capacity. Typar stops the base from sinking into the soft, wet soil and provides for improved compaction and reduced stone usage.
“Typar also provides a restraint member to the aggregate base and improves its resistance to deformation under repetitive loading. In fact, Typar permits the subgrade soil to improve itself after installation, by forming a barrier that allows excess water in the soil to be squeezed out through the fabric into the permeable crushed-stone base while restraining and compacting the soil particles below.”
Deron Austin, director of marketing for SI Geosolutions in Chattanooga, TN, agrees, adding that a strong fabric is needed to resist puncture by the aggregate. His company’s Geotex line has been successfully used in a number of paving projects, including secondary roads, airfields, and parking lots, as well as highways.
“A good example is the New Jersey DOT’s [Department of Transportation’s] program to reconstruct, reconfigure, and widen the roads that make up the interchange where US Route 1 and US Route 9 separate. The interchange is approximately 2 miles from the New Jersey coastline, and in coastal areas, soils tend to have a higher moisture level than inland areas. The project contractor had previously excavated the subgrade, and compaction procedures were under way when the exposed subgrade began to exhibit symptoms of having dangerously high moisture levels. During the operation of the smooth-drum vibratory roller, pumping and rolling of the surface under wheel loads had occurred and ruts had been created in the soft soil after multiple passes of a loaded tandem truck. With signs of structural breakdown occurring so soon in the construction process, there was no possible way that the condition surrounding this roadway would provide for safe driving in the years to come.
“Instead of using the usual DOT repair method of undercutting and replacing the subgrade with stone that was extremely expensive, they decided to use our Geotex blankets. They first compacted the subgrade and laid down our Geotex 801 fabric. They then added an 8-inch layer of sand to serve as a drainage layer, and placed our heavy-duty Geotex 4x4 on this sand layer, covered it with aggregate, and paved it. This use of geotextiles was so well received that NJDOT elected to use the same construction solution on the bridge approaches for the new bridge to carry US Route 1 North over Route 9 South. A subsequent project will also incorporate Geotex geotextiles, and will replace the first cloverleaf interchange ever built in the United States.”
Mirafi Construction Products of Pendergrass, GA, also offers a product that can be used for soil stabilization for road construction. According to information supplied by Janet Lucas, Mirafi’s Geolon HP-Series products are woven geotextiles composed of high-tenacity-polypropylene yarns. HP-Series Woven Polypropylene Geotextiles yield ultimate tensile strengths up to 20,000 pounds per foot per ASTM D 4595. Geolon HP-Series products combine the properties of high-tensile strength and modulus and high confinement with their ability to act as a filter and separator. Lucas cites situations where roadways with soft soil subgrade conditions could use Geolon HP-Series products directly placed on this subgrade, precluding the need for over-excavating and filling to create a stable subgrade.
Road Repair
One of the prevalent problems leading to road repairs is reflecting cracking. Mirafi describes it as follows: “Cracked pavements allow surface water to permeate to the subgrade soils, which then saturate and weaken the subgrade. Normal maintenance procedure is to repave with an asphalt overlay. This resurfacing procedure can extend the useful life of a pavement section at a lower cost than a full replacement. However, the extended useful service life of an overlay can be reduced by subgrade saturation/weakening, which causes fatigue cracking and propagation of cracks from the original pavement through the existing overlay [reflecting cracking].”
Geosynthetics are also used for the repair of roads and highways, but different geosynthetics are used for this application. According to Lucas, Mirafi makes two geosynthetics for this application: Mirapave and Pavegrid. Mirapave is a needle-punched heat-set polypropylene nonwoven fabric with a high-asphalt-absorption capability. When installed with a proper surface preparation and tack coat, Mirapave forms an impermeable membrane that prevents the penetration of surface water through the pavement and also provides a stress-relieving layer that inhibits reflective crack growth.
Pavegrid is composed of high-modulus fiberglass grids bonded to a lightweight scrim. The grids, which are constructed with fiberglass yarns aligned perpendicularly, interlock with the hot asphalt mix to reinforce the new pavement layer. The strength of the grid helps contain cracks before they create reflective cracking in the new overlay.
Owens Corning has developed a paving mat called TruPave that also delays reflective cracking and provides a moisture barrier, but it has other advantages as well. According to John Kochilla, “TruPave is a unique interlayment made of nonwoven fiberglass/polyester fabric engineered specifically for hot-mix asphalt pavements. It will not melt, shrink, or stretch, even at temperatures approaching 500 degrees Fahrenheit. Beam fatigue testing indicates that glass/polyester mat-protected pavement has four times the fatigue life of untreated control pavements.
“What’s more, TruPave is millable and recyclable. When milled, the mat is reduced to small pieces and goes to the hot-mix plant with the milled HMAC [hot-mix asphalt concrete]. Unlike other paving fabrics, these pieces will not clog the milling equipment or be rejected by the hot-mix plant. Thus, not only does TruPave extend the life of the asphalt pavement by 50%; it enables this asphalt to be readily recyclable, thereby providing further savings and credits against recycling goals.”
Marshall Hughes, president of California Paving Fabrics, called our attention to GlasGrid, a pavement reinforcement mesh that is even stronger. According to information he supplied, GlasGrid is a knitted grid of glass fiber strands coated with an elastomeric polymer.
“Each strand has a remarkably high tensile strength and high modulus of elasticity at low elongation. This combination of outstanding tensile strength and high modulus makes GlasGrid actually stronger than steel. When GlasGrid is sandwiched between the prepared asphalt surface and the new asphalt overlay, it is the hidden strength in the road, designed to turn crack stresses horizontally and reduce the stress in the pavement. While application results vary for individual applications, independent laboratory tests indicate that GlasGrid extends pavement life by approximately 84%.”
Sinkhole Repairs
Sometimes, of course, damage to roads goes well beyond just reflective cracking. One of the more serious damages is the sinkhole. A major difficulty with sinkholes is the risk that one will sink deeper and/or recur when it has been filled, and after road pavement has been reinstalled on this fill.
Surprisingly, geosynthetics have an answer to this problem too in the form of ParaLink, a product manufactured by Linear Composites USA, headquartered in Chattanooga. According to Robert Lozano, “ParaLink and ParaGrid are manufactured from high-tenacity, multifilament polyester yarns placed in tension, then co-extruded with polyethylene to form polymeric strips. The polymeric strips are laid flat in the machine direction and a secondary member is laid and welded across the full width in the cross direction. The process generates a stable and strong geogrid. Although polyester is the load-bearing element, maintaining minimal deformation, it is the polyethylene sheathing that both maintains the integrity of the product and encases the yarns, protecting them from aggressive environments [such as high/low pH] and harsh installation conditions. ParaLink and ParaGrid are ideal for applications where reinforcement of soils is essential, such as MSE [mechanically stabilized earth] walls, embankments over soft soil, steepened slopes, lagoon closures, load transfer platforms, basal foundations, and any other geotechnical application in which soils require enhancement.”
Linear Composites Inc., parent company in the United Kingdom, encountered the problem about 10 years ago in the city of Ripon in northern England. Underground caves had caused large voids to progressively migrate upward, eventually leading to sinkholes, sometimes up to 80 meters in diameter at the ground surface. This was the first instance of ParaLink geotextiles successfully mitigating the sinkhole problem.
Today, Linear Composites routinely fills the sinkhole partway to the surface and tops the fill with a ParaLink blanket covering the mouth of the sinkhole and extending at least 4 feet beyond, where it is anchored to stable ground. Then the company adds fill and aggregate, forming a cap over the sinkhole, which can then be paved, if this is a roadway over the sinkhole.
Subsequently, if the sinkhole again subsides so that the fill under the cap collapses, the cap, including its fill, aggregate, and pavement, will remain in place. The cap essentially becomes a bridge upon which traffic continues to travel.
Drainage Protection
According to Bill Hawkins, subsurface drains are used for a wide variety of purposes, including edge-of-pavement drains, interceptor drains, subsurface structure drains, and blanket drains. Whatever the exact configuration, subsurface drains are used in highways, airfields, railroads, parking lots, buildings, storage yards, retaining structures, mines, dams, and levees. In all of these applications, adequate drainage is essential for good performance.
Subsurface drains require a properly designed protective filter that prevents the piping of the native soil into the drain and the possible clogging of the drain. Graded aggregate filters have traditionally been used as protective filters to prevent piping and clogging. Unfortunately, graded aggregate filters are expensive and very difficult to install properly.
Geotextiles offer a proven alternative to graded aggregate filters. They have been used in thousands of subsurface drainage projects. In its simplest configuration, a subsurface drain is constructed by first digging a trench and then lining it with a geosynthetic textile such as BBA Fiberweb’s Typar, Webtec’s Terratex, or Mirafi’s N-Series nonwoven polypropylene geotextiles, in a U-shaped configuration covering the bottom and the sides of the trench with ample overlap. Then, bedding stones are placed in the bottom, and (optionally) a drainage pipe is set in place. The trench is next filled with larger stones or riprap, and finally the overlapping fabric is wrapped over the top of the trench in an action Lucas describes as “burritoing.”
Typically, these drains are located beside roadways so that runoff water will drain through the permeable geotextile that prevents the movement of soil into the drain that would cause clogging of the drainage system. Sometimes, Hawkins says, transverse drains of this design are located under the road so that runoff water will go under and drain off without flooding it.
Slope Stabilization
“In a slope stabilization application, for any given soil at a certain moisture content, there is an angle beyond which the soil will not stand without some kind of support,” explains Dave Snyder of Charlotte, NC–based Webtec Inc. “Whenever the steepness of the slope exceeds this failure angle, gravitational forces act on the soil mass to create shear stresses, causing the slope to fail through sliding or sloughing off. The process ends when a new equilibrium is achieved at an angle less steep than the initial one.” In many situations, it is desirable to have a slope steeper than the shear strength that the soil will allow. One method employed to achieve this objective is to provide reinforcement at specified increments back into the slope. The resulting reinforcement increases the effective failure angle, preserving the stability of the slope.
In most cases, this reinforcement requires higher tensile strength and resistance to elongation than is possible with most geotextile blankets. The needed capability can be supplied by geosynthetic structures such as grids and cellular containment structures, or geocells.
Typically, synthetic fabric grids are constructed of high-strength, high-tenacity-polyester yarns woven and coated in a polyvinyl chloride paste. The two-dimensional, open-mesh structure allows for superior interlocking with the surrounding soil. Their high resistance to elongation makes geogrids ideal materials for stabilizing slopes, bases, retaining walls, and other applications where long-term design strength is necessary for the stability of the structure. Among the geogrids on the market are TerraGrid from Webtec Inc., ParaGrid from Linear Composites USA, and MiraGrid XT geogrids.
When a geogrid is used to reinforce a slope, the grid layers are placed such that they interrupt the potential failure surfaces. The tensile strength and resistance to elongation of geogrids prevent failure of the slope along these surfaces. Slopes can be built more steeply than without geogrid reinforcement so the engineer or designer can make the most of the land available on the site.
While geogrids are quite expensive, Snyder points out that the use of TerraGrid can result in economic savings in several ways. In cut situations where the foot of the slope is fixed, the use of TerraGrid can greatly reduce the amount of soil that would normally have to be removed. In fill situations, the use of TerraGrid can greatly reduce the amount of fill that must be imported and placed. In developing a commercial, residential, or industrial site, the amount of usable land can be increased by creating steeper slopes with the use of TerraGrid. By making the slopes steeper, TerraGrid can help the engineer or designer economically use his construction site.
Cellular containment systems utilize geocells, innovative three-dimensional geosynthetic products that can confine native or select fill materials. According to Snyder, geocells are constructed from a lightweight, flexible mat made of high-density-polyethylene strips. These strips are ultrasonically bonded together to form an extremely strong, honeycomb configuration. A variety of fill materials can be placed within the geocell system: soil, sand, aggregate, concrete, etc. The use of geocells and the appropriate fill material creates numerous opportunities for versatile and economical solutions for many applications, including ground stabilization, slope erosion control, retaining walls, stream crossings, channel erosion control, and embankments.
“To satisfy the application, our TerraCell geocell is available in various heights and three different cell sizes: standard, mid cell, and large cell,” Snyder says. “It can be supplied in solid wall or perforated, to allow flow between the cells. Holes can be made in TerraCell to facilitate a tendoning system for some applications.”
DuPont Presto manufactures a similar geosystem. Based on information supplied by one of its distributors, Tray Hightower of Jen Hill in Henderson, TN, Presto’s perforated Geoweb earth retention system can form “a very steep front surface while maintaining structural stability under its self-weight and known externally imposed loads even when subgrades are composed of compressible, unstable soils.” Again, geocells are quite expensive, but the costs are somewhat defrayed by the fact that local fill materials can be used in lieu of imported riprap or other such materials.
Biobarriers
In the final grading process of a construction project, care must be taken to ensure that tree roots do not impinge upon structures and conveniences. Tree roots grow laterally in the soil as they search for nutrients and water. As tree roots grow in length and diameter, they take valuable nutrients and water from grasses and gardens. Worse, they can damage adjacent structures, clogging clay sewer drains and literally uprooting streets, foundations, sidewalls, greens, and cart paths.
“Conventional methods of herbicide application include spraying liquids, distributing granules, and dusting,” reports Hawkins. “Most require repeated seasonal application and depend on the environment and the applicator to minimize detrimental effects to surrounding vegetation and personnel. Biobarrier, developed and marketed by BBA Fiberweb, is a vegetation root control system consisting of a unique 80% polymeric product enabling total placement control. Depending on use, it is effective for a minimum of 15 years with a maximum life approaching 100 years.
As described by BBA Fiberweb, Biobarrier is a polypropylene spun-bonded fabric with a pattern of permanently attached herbicide-impregnated polyethylene composite nodules. Each nodule serves as a protective reservoir and functions as a long-term controlled-release mechanism. Strategically positioned in soil, Biobarrier creates a root inhibition zone of herbicide. Since it is a fabric, it can be contoured to protect virtually any shape structure from roots, and since the fabric is permeable, it does not alter the soil hydraulics.
The nodules are actually a trifluralin-impregnated polyethylene-based composite. This material is injection-molded through the Typar polypropylene fabric to ensure maintenance of proper nodule spacing and permanent attachment. The nodules release herbicide vapor at a controlled low rate established predominantly by nodule molecular structure, surface area, and ambient temperature. The migrating vapors adsorb onto the surrounding soil, building a diminishing-with-distance herbicide concentration profile.
With adjacent nodule support, a contiguous zone is established enveloping the barrier. As the exposed herbicide degrades, zone concentration is maintained by vapor emission from the nodules. A 10-ppm trifluralin concentration stops most species’ root growth by preventing root tip cell division. This concentration typically occurs about three-quarters of an inch from the barrier surface. Roots entering the zone swell and cease growing. Since trifluralin is not systemic, other roots in the system continue to develop normally but are diverted away from the zone.
A second product, Biobarrier II, can be used for weed control. It too consists of a permeable geotextile fabric with permanently bonded nodules of time-release trifluralin herbicide. When properly used, Hawkins says, it prevents weed encroachment for at least 10 years. He explains how:
“Placed on the ground like a landscape fabric and covered with up to 2 inches of stone, mulch, or other cover material, the trifluralin is slowly removed from the nodules. It works because root tip cells cannot divide in the presence of trifluralin. The constant release of trifluralin creates a narrow [approximately 2-inch] zone in the cover material on either side of the fabric. Since weeds are unable to develop a strong root system in the presence of trifluralin, they are unable to survive when their roots reach the zone. Trees and shrubs, which have roots below the trifluralin zone, can continue to grow. Because Biobarrier II is porous, water, air, and nutrients can flow through it and not disturb the plants’ hydrology. Hence, Biobarrier II is more effective long-term than any landscape fabric available.”
Sediment Containment and Filtration Using BMPs
During the construction process in a populated area, large amounts of sediment can be washed off the denuded site and can enter storm sewers or other stormwater collection systems. This can result in violation of the Storm Water Phase II Rule as well as cause clogged drains and sediment collection tanks. This problem can be greatly eased by a family of BMP devices.
In this context, a BMP, which is short for “best management practice,” is defined as “any measure or practice used to reduce the amount of pollutants entering surface waters, air, land or ground waters. These measures are used depending on a given set of conditions to manage the quantity and improve the quality of storm water runoff in the most cost-effective manner.” Here again, geosynthetic-based products are effectively used for both sediment containment and filtration. And The BMP Store, a division of ACF Environmental of Richmond, VA, handles a line of this type of BMP products.
“Sediment containment is an ongoing problem,” says David Kelley of The BMP Store, “even though there are complete sediment containment systems that allow soil particles held in suspension to settle out of stormwater runoff. The problem is that it is not always possible to retain all runoff waters from a construction site where there is a large containment area and large volumes of water. Also, evaporation and infiltration might not be sufficient to drain the system before the next storm, thereby risking flooding problems. In these cases, sediment can be controlled by temporary BMPs that can be deployed on construction sites.”
Perhaps the best known of these are the Gutterbuddy curb inlet and ditch pavement filters and the GutterGator slim curb inlet and ditch pavement filters. Both are made from recycled synthetic fibers that filter out sediment and debris, and both are designed to rest against curb inlets. “Their filtering action lets water freely flow through the fibrous material while stopping sediment and debris,” Kelley explains. “Built-in overflows drain water even more quickly during extreme events.
“Gutterbuddies are 9 inches in diameter and are available in 2-foot increments from 4 feet to 16 feet long as needed to match the size of the curb inlets. Since they are made from recycled materials, they can be washed and reused, and they are flexible enough to conform to any curb radius, so they can be quickly and easily installed.”
The GutterGator is the newest curb filter from The BMP Store. Designed for curb outlets without grates, the new product is the same size as the Gutterbuddy, but it is able to handle higher volumes of water during wet-weather flow. “Like the Gutterbuddy, the GutterGator can handle extreme sediment loadings during the construction phase,” Kelley explains. “But it is designed with a three-dimensional, high-flow outer layer that encloses a multidimensional inner layer that allows high flow volumes to continue through the filter. And it has attached weight pockets that can secure the unit tight to the curb opening.”
BMPs that are designed to provide filtration include geotextile filter bags called Dirtbags. These products allow pumped water to flow out through them while filtering and capturing sediment. Appropriate uses of Dirtbags would be the dewatering of detention/retention ponds or dredging of construction and/or industrial spoils. The BMP Store’s Dirtbags are made of a nonwoven geotextile fabric manufactured by SI Geosolutions. Available in various sizes up to 15-feet by 15-feet, each Dirtbag has a fill spout large enough to accommodate a 4-inch discharge hose from the pump being used to dewater the site. Straps are attached to secure the hose and prevent pumped water from escaping without being filtered. The bag collects the silt carried by the incoming dirty water and enables the cleaned water to gently filter out from all sides. Although flow rates will vary, Kelley says, under most circumstances Dirtbag will accommodate flow rates of 1,100 gallons per minute.
“Man-made geosynthetic filters have numerous advantages over traditional sediment control practices derived from natural materials,” Kelley concludes. “They are easy to transport, install, and maintain, and manufacturing and fabrication consistencies enable their performance to be more predictable and generally superior to natural materials.”
That conclusion could well be extended to cover all of the geosynthetics in use in construction applications. They provide flexibility and performance that often outstrips the capabilities of traditional building products like concrete and riprap; they offer significant environmental advantages; and they almost always offer significant cost advantages, particularly on a total-project-cost basis. All in all, it is hardly surprising how many of these useful products are on the market today and how many contractors are taking advantage of them.
Charles D. Bader is with Dateline II Communications in Los Angeles, CA
GEC
- January/February 2006
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