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Despite the many types and uses of geosynthetics, they share at least one trait in common. To perform properly they have to be put in properly. Here's why it can pay to tune up your installation techniques, as well as some tips on how to do it. By Greg Northcutt |
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Developed in the past 30-40 years, geosynthetics are the latest refinement of a practice that dates back at least to the Roman Empire for improving the construction characteristics of soil. Back then, materials such as logs, rocks, and jute were used to increase the bearing capacity of dirt roads. Today, because of superior strength, durability, and other performance features, geosynthetics offer a number of advantages over natural materials as complementary components of soil. Woven and nonwoven geotextiles, usually made from polyester and polypropylene, are two types of geosynthetics. They can be used as a soil stabilizer to distribute weight over a wider area when building roads; as a separation layer to prevent one material, such as a native subgrade soil, from contaminating and weakening another, such as an aggregate base course; as filtering material to keep soil in place while allowing water to pass through to a subsurface drainage system; as an erosion control material performing a similar filtering function; and as a silt fence to trap sediment in stormwater runoff. Some geotextiles known as rolled erosion control products (RECPs) also provide temporary and permanent soil protection. Temporary erosion control blankets (ECBs) can be constructed of both natural and synthetic fibers and nettings that will either photodegrade or biodegrade over a specified period of time. Turf reinforcement mattings often are constructed of permanent three-dimensional structures made of such material as polypropylene or nylon. Synthetic or natural fibers may be incorporated into this permanent structure to improve immediate erosion control and mulching capabilities. RECPs provide temporary or permanent (when used in conjunction with vegetation) protection for soils on slopes, channels, and shorelines from the erosive forces of wind, waves, rain, and overland and stream flows. Multipurpose geogrids - made from such materials as integral polypropylene, integral high-density polyethylene (HDPE), or woven polyester yarns coated with polyvinyl chloride - can be used to stabilize road bases, reinforce slopes, and build retaining walls. Meanwhile, honeycomblike cellular confinement systems or geocells, made of HDPE, can improve the performance of infill materials to protect slopes and channels from erosion, support loads, and retain earth. Other geosynthetics - for example, ECBs constructed of both natural and synthetic fibers and nettings and turf reinforcement mats (TRMs) made of material such as polypropylene - can protect soil on slopes and in channels from the erosive forces of wind, rain, and overland and stream flows. The end result of these various applications of geosynthetics is often a more cost-effective project due to lower material and construction costs, reduced maintenance bills, and extended project life compared to many natural materials. Techniques Make a DifferenceNevertheless, despite their potential, geosynthetics aren't always utilized to their best advantage. "Many of us joke that the worst day in the life of a geosynthetic textile is the day it's installed," relates engineer Greg Richardson of G.N. Richardson & Associates Inc. in Raleigh, NC. "If a fabric for separating soil from aggregate is heavily torn or damaged during installation, it can't maintain its integrity and the quality of the clean stone. It's like putting up a screen with holes in it. Fines don't care if a fabric is 90% intact. They'll find the holes." You get an idea of how different installation techniques can affect the performance of a geosynthetic material, at least silt fence products, from an extensive field study conducted by Joel Sprague. He's a senior engineer for TRI Environmental Inc., a consulting firm in Austin, TX. Sprague measured the amount of water and sediment retained by 51 different test segments of silt fence installed using one of four methods. Three methods involved installing silt fence fabric in trenches while varying such things as the amount of backfill, the degree of compaction, and the spacing of posts using minimum, better, or best installation procedures. He also evaluated static slicing installation in which an implement with a blade inserted at least 10 in. into the ground opens the soil without vibrating or oscillating and pulls the fabric into this slit. The soil then is compacted mechanically. The tests also involved various types of soils and installation of posts before and after the soil was compacted. In general, he found that the more rigorous the installation efforts, the better the silt fence performed. In all cases, results with static slicing equaled or exceeded that of the best trench-based installation, which outperformed the better and the minimum trench-based installation methods. In fact, static slicing and the best trench installation retained half the initial water runoff (allowing sediment to collect behind the fence) while the other two trench-based techniques held back less than 20% of that runoff. The study also highlights a problem facing contractors when installing silt fence. "There is no such thing as a single standard procedure for trenching-based installations," states Sprague. "At best there is a minimum specification, such as ASTM D 6462, which reflects common practices. It also implies, but does not explicitly require, important installation details, such as complete backfilling of the trench and thorough compaction of backfill. This and other minimum specifications allow installation of posts prior to installing fabric and backfilling the trench. However, that interferes with efforts to compact the backfill thoroughly. Thus, current specifications may inadvertently encourage trenching-based silt fence installations that provide unsatisfactory performance." When Performance Suffers
While improper use and installation of geosynthetic materials is the overwhelming exception, such incidents are still too common to suit some experts. One of them is Samuel Randolph, GeoSystems division manager of Soil Stabilization Products Company Inc., a Merced, CA, firm that provides design support services and supplies materials for soil stabilization projects. "I'm amazed at some of the things I see at construction sites," remarks Randolph, a Certified Professional in Erosion and Sediment Control (CPESC). "Some guys approach the use of geosynthetics with humility and read the manufacturers' manuals and instructions. Others either don't understand how to use them properly or think they know, but don't." One time, he says, a designer took information from a geosynthetic manufacturer's Web site and pasted that into the project specifications. "In doing so, the designer unintentionally gave the contractor a choice between either of three options. Unfortunately, the contractor chose an inappropriate alternative for this project and it failed." Another time, success of plans to use a cellular confinement system for stabilizing a channel was threatened by hydraulic uplift pressure. This pressure was causing the channel bottom and sideslopes to heave about 1 ft. both horizontally and vertically. "It was like the channel was alive, but the project engineer didn't think there would be any problems installing the cellular confinement system," Randolph says. At his recommendation, the problems presented by the unstable soil conditions were solved by lining the channel surface with a geotextile to separate the poor-quality, saturated, onsite soils from the imported materials and the cellular confinement system. After that, a 1-ft.-thick layer of compacted stone was placed over the geotextile to provide a stabilized base for the cellular confinement system. David Snyder, CPESC, with Webtec Inc., a geosynthetics company, recalls a channel stabilization project where geocells were installed. As recommended, the system was anchored in place with rebar stakes prior to filling the cells with aggregate. Once installation was complete, however, the stakes were removed - the wrong thing to do. "Before long, the geocell system and fill material slid down the channel," he describes. A case in Tucson, AZ, illustrates the economic costs of installing geosynthetics incorrectly. About 1 mi. of a four-lane arterial was constructed through part of Davis Monthan Air Force Base. This roadway segment was depressed about 24 ft. below grade with excavated slopes of 2:1. In 1993, a project was undertaken to stabilize and beautify the cut slopes. To control erosion on the slopes, a cellular confinement system was installed and backfilled with a gold-colored decomposed granite. Short walls of varying lengths were installed at various locations on the slope to add an aesthetic element and to serve as planters for vegetation to break up the stark appearance of the slope. Contrary to the manufacturer's recommendations, the cellular confinement system was not anchored at the top of the slope and a geotextile was not installed underneath the cells of the confinement system to protect the subgrade from the erosive action of sheet flow running down the slope. Over time, concentrated stormwater flows, which collected at the top of the embankment, began coming down the slope, undermining the subsoil beneath the cellular confinement system. In addition, the cellular confinement system was not anchored adequately to the slope, nor were the adjacent panels adequately tied together in accordance with the manufacturer's recommended installation practices. As a result, the installation was failing. "The subsoil and decomposed granite were washing down the slope and ending up in the street where they clogged storm drains and created a hazard for bicyclists next to the curb," says John Lynch. A principal and director of civil engineering for GLHN Architects and Engineers in Tucson, he was retained in 1998 to study the problem and propose solutions to prevent total failure of the slopes and the attendant walls. "Correcting the situation and salvaging the project's aesthetic elements would have cost as much as $3 million," he points out. "That's almost twice what it cost originally to stabilize and beautify the slopes. Now plans are to remove most of the walls because they are starting to fail and potentially slide down the slopes, while funds are being sought for a long-term correction of the problem." Recommendations and Reasons
A variety of missteps when installing a geosynthetic can bedevil its later performance. To help prevent that, manufacturers provide instruction materials for using their products. While specific details vary from one product or application to another, good installation techniques usually include some general principles: Preparing the Subgrade If the subgrade isn't smooth, protruding rocks or other objects could punch through the geotextile, threatening its integrity and functions, notes Bruce Lacina, a senior engineer for technical services with Ten Cate Nicolon, a geosynthetics manufacturer in Pendergrass, GA. Protecting the Material Proper installation of geosynthetics also means protecting the material from damage both before and during actual installation. For example, dumping aggregate directly onto a geotextile can puncture it. Driving on a geotextile before it's covered with the required amount of aggregate can damage the material, as can turning heavy equipment too sharply or applying braking too quickly. "Placing aggregate on top of a stabilization or separation geotextile that's loose, wrinkled, and bunched up can also lead to trouble," warns Lacina. "Once traffic loading starts, it could cause the layers of geotextile to rub against each other and any rock trapped in between the folds. That could lead to abrasion of the geotextile and excess displacement of the aggregate." Also, leaving a geosynthetic that is not UV-stabilized exposed to sunlight after installation can cause it to deteriorate. Overlapping Geotextiles When installing more than one width of geotextile, the amount of overlap must be correct. Too much overlap can create a slip surface between the two materials. Lacina says this could cause a failure on a slope or threaten the integrity of the material on a horizontal pavement application in an area where trucks brake. On the other hand, too little overlap risks creating voids in the geotextiles if the ground settles later. Placing the Materials In most cases, the way in which the product is placed during installation is critical to success. For example, states Webtec's Snyder, manufacturers of ECBs and TRMs recommend installing them in a shingle fashion so that the uphill or upstream blanket or mat overlaps the adjacent one below it, rather than the other way around, or butting the two pieces together. Otherwise, the water flowing off the upper blanket or mat could work its way underneath the lower one and cause erosion. Unlike biaxial geogrids used for stabilization purposes, geogrids that are used to reinforce slopes or a mass of soil in building retaining walls are uniaxial, meaning they're stronger in one direction than the other. Usually that's along the length of the material when it is unrolled "If you install a uniaxial geogrid in the wrong direction [most of the time that would mean laying the strength parallel to the face of the slope], you may get only a fraction of the geogrid's strength," Snyder cautions. "I've known of retaining walls that failed because of this." At the same time, he adds, a geogrid must be installed in one continuous piece to be most effective. "In a retaining wall or reinforced slope application, the geogrid and the fill material create a reinforced zone. The geogrid, which runs horizontally through the reinforced zone, must be one continuous piece. You can't overlap the grid in this direction." Securing the Products
Improper installation of RECPs can reduce their performance in two ways, points out Roy Nelsen, manager of technical services for manufacturer North American Green, Evansville, IN. "For example, by not anchoring the blankets or mattings in trenches as required, water flow can undermine the products to erode the soil," he explains. "Also, if the blanket is pulled too tight and not allowed to contour to the soil surface, or less than the recommended number of staples are used, or the proper staple pattern isn't followed, grass and other vegetation may push the blanket up without emerging through it, as designed, and the plants will eventually die. This would then limit the permanent soil-protecting benefits of the plants." Skimping on staples may be a way to cut material and labor costs when installing these materials, Nelsen adds. "However, manufacturer-recommended staple patterns and staple lengths are designed to keep erosion control blankets and turf reinforcement mats in intimate contact with the soil to prevent erosion and ensure the best possible vegetation establishment." "On a good day, you can roll out a geotextile, place rock on it, and you're done," says Lacina. "But that's the exception." On a windy day, the edges of the fabric probably will have to be secured, he notes. On a sunny, hot day, the heated geotextile might relax, leading to billowing and folding, unless it is secured properly. If a geotextile is folded to fit around a curve, it too must be secured properly before placing aggregate on top of it. "If a geotextile isn't properly secured before you start placing the overlying aggregate, the geotextile is likely to move up and down once heavy equipment starts pushing the aggregate around," he cautions. "I've seen a lot of grader blades dig up geotextile by accident because it wasn't secured properly." How to Make the Most of Geosynthetics"Installing geosynthetics properly isn't rocket science," remarks technical representative Lacina. "Much of the success in preventing mistakes goes back to using common sense and knowing when you need help and where to get it." Here are some tips to help contractors as well as designers use geosynthetics to full advantage: Start With the Basics One excellent reference is the American Association of State Highway and Transportation Officials's (AASHTO) document M-288-00: Standard Specifications for Geotextile Specification for Highway Applications (available from AASHTO, Washington, DC, www.transportation.org). It details materials specifications of geotextiles used in subsurface drainage, separation, stabilization, erosion control, temporary silt fence, and paving applications. The publication also provides construction installation guidelines for these materials. "This document is based on decades of field experience," notes engineer Ryan Berg of Ryan R. Berg & Associates Inc. in Woodbury, MN. "There are very well-established guidelines for installing geosynthetics properly. If you ignore them, you run the risk of damaging or destroying the material during installation and the geosynthetics won't function as designed." He also suggests three other publications:
When it comes to RECPs, manufacturer representatives or technical service departments can address site-specific installation questions. Another good source of general information on installing RECPs is the Erosion Control Technology Council (www.ectc.org). Understand the Causes of Failure "As the complexity of a geosynthetics system increases, so do the number of ways it can fail," observes Randolph of Soil Stabilization Products. That's why he recommends anticipating where things can go wrong and taking steps to prevent missteps. His list of potential problem areas includes the manufacturing process, the project design phase, order fulfillment, and contractor errors, not to mention attitude and lack of knowledge anywhere along the way. "Whatever your role in the project, learn to look at possible failure modes of any technology before you install it," he suggests. "Try to identify possible problem areas and communicate them to the people you're working with." Know the Products One reason geosynthetics sometimes are used and installed improperly is simple ignorance, Randolph contends. "Some people seem to think that if they've worked with one type of geosynthetic, they know how to work with others. Geosynthetics are not commodities. There's a direct link between what is designed and what gets installed. A contractor can't remedy what a designer doesn't know." As he and others in the business point out, features, performance, and requirements of a given type of geosynthetic can vary from one brand to the next. "There are hundreds of different geosynthetic products and dozens of different applications for some materials," explains Lacina. "Similar products can differ in a number of physical and mechanical properties, such as weight, strength, thickness, aperture opening size, tensile strength, [and] puncture resistance." Contractors should get as much information about the geosynthetics specified in a project before bidding on the job, advises Berg. "Get all your questions answered before you start construction. It's important that the geosynthetics you use meet all the specifications. For example, if you select a geotextile that's less permeable than specified, you may pay a lower price. But you could end up trapping water below the geotextile to the detriment of the project." Know the Project A thorough understanding of site conditions is another important key to proper selection and installation of geosynthetics. One size, one product, or one installation approach doesn't necessarily fit all situations. That's especially true in the case of cellular confinement systems, notes Daniel Senf, chief engineer and global sales and marketing manager with manufacturer Presto Products Company in Appleton, WI. "Because most applications of cellular confinement systems are engineered to solve a problem at a particular site, some construction details will vary from one project to the next," he points out. "It's not simply a matter of rolling out a cellular confinement product and securing it to the ground, because the system you're creating - not the product itself - is the solution. The details of the problem define the details of the solution and how it is installed. For example, the size and type of cells and appropriate anchoring method vary from one type of soil and slope angle to another. As a result, what worked in one situation may not work in another. Unless you have a good understanding of these systems, you may overlook some of these important details." Follow Manufacturers' Recommendations Manufacturers also publish installation manuals, software, and other instructions that describe in detail how to install their products properly. For example, installation instructions provided by Webtec for its geotextiles used in stabilization or separation applications cover installation procedures and considerations similar to those in M-288-00. They include:
Communicate Clearly "When I conduct a forensic analysis of a failed geosynthetic project, I often find the designer, not necessarily the installer, responsible for installation mistakes," states J.P. Giroud. Founder and former chairman of GeoSyntec Consultants and past president of the International Geosynthetics Society, he now operates JP Giroud Inc., an independent consulting firm in Ocean Ridge, FL. Giroud isn't necessarily referring to a poor design or poor specifications but, rather, poor communications between designer and installer. "During more than 30 years of experience with geosynthetics in the field, I've seen a number of problems caused by a contractor either not understanding or trying to improve a design," he relates. "A good design engineer will not only design a proper geosynthetics project but will anticipate possible installation mistakes and communicate properly with the contractor to try to prevent them." For example, many designers specify a typical overlap of adjacent rolls of geotextiles of, say, 1 ft., Giroud explains. Rarely does he see a drawing that specifies a minimum overlap of 1 ft. and a maximum of, perhaps, 2 ft. Such limits could help prevent installers from making overlaps that are too wide. At the same time, Giroud has seen cases where contractors have taken it upon themselves to alter a design they considered inappropriate or not constructible without discussing such changes with the designer. He recalls one project that specified a perforated pipe. The contractor installed a perforated pipe that included a geotextile filter for the same price as the required perforated pipe without a filter. "The contractor thought he had improved the design by increasing filtration," Giroud says. "However, he didn't realize that at this site the geotextile filter would clog and actually reduce filtration. It's certainly appropriate for a contractor to comment on or question a design. But in this case he should have talked with the designer about adding the geotextile filter and left it to the designer to approve or disapprove its use." Keep Current Manufacturers continue to improve existing products and introduce new ones. To help ensure that the proper staple pattern is used and to increase installation rates when installing RECPs, one maker introduced a color-coded staple pattern two years ago that was applied directly to its products. This system shows where to place staples for various staple pattern rates based on the types of applications. In addition, manual staple guns are available to make fastening the materials to the soil surface easier and faster. Sometimes installation practices and materials change too. That's the case with TRMs. At one time, most manufactures recommended installing the mats and then filling them with soil. Now, observes North American Green's Nelsen, many recommend installing the mats after the site has been seeded. "Many companies now specify this method. Surface application is more economical since it eliminates the labor-intensive task of soil infilling the mats. Even more importantly, surface application removes the potential risks of damaging the matting from the use of wheeled or tracked machinery during the infilling process. Also, some mattings have shown improved performance from surface application. By placing the mats over the seed, the product can reinforce the stems and roots of the grass or other vegetation compared to just the roots of soil-filled mats. Also, a surface-applied matting provides erosion protection immediately after installation." Still, that doesn't mean it makes sense to surface-apply a matting that previously was designed for soil infilling, Nelsen explains. Surface-applied TRMs have been designed specifically for this application through the use of randomly oriented organic or synthetic fibers incorporated into a permanent three-dimensional structure. "It's important to read, understand, and follow the manufacturer's and designer's installation instructions for each individual product to maximize its performance," he stresses. Meet Any Vegetation Requirements When controlling erosion, success requires more than proper installation of ECBs or TRMs. It also requires providing suitable conditions for vegetation to germinate and grow. "When vegetation doesn't grow at a site, we often find that basic plant growth requirements weren't considered," relates Nelsen. "Maybe no soil tests were done to determine any need for fertilizer or other soil amendments, or the soil was excessively compacted." Fix the Right Problem the Right Way It's also important to understand and appreciate the difference between a failure of a geosynthetic and poor site conditions, notes Berg. He points out, for example, that rutting above a geotextile or a geogrid doesn't necessarily mean the material is failing. It could mean that the subgrade is weaker than assumed for the project design. If so, regrading would not be the proper solution to the problem. "Once the surface of the site is rutted, the geosynthetic underneath has become wavy," Berg states. "Regrading could rip the material at the top of the waves and destroy it." The appropriate repair would depend on the type of project, he explains. "If the geosynthetic is being used for only the short-term, you may be able to simply install more stone to get the right subgrade. However, if it's a long-term project, you may have to pull out the material, fix the problem with additional excavation, and reinstall the geosynthetic." Ask for Help In addition to installation instructions, information on Web sites, and training aids such as videos, many manufacturers provide onsite technical assistance if desired. Lacina says contractors and engineers call him daily with questions about proper installation procedures for various applications. Such prudence can save a lot of headaches later. "If you have a shadow of a doubt about how to install a geosynthetic product, call the manufacturer," he advises. "Getting a faxed set of guidelines may be all you need. Even if it takes half an hour on the phone to resolve your problem, you're still money ahead if it saves you six hours of removing and replacing the material." Greg Northcutt is a frequent contributor to Forester Communications publications.
GEC - March/April 2003
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