Whether they are new roads or the rehabilitation of a portion of an aging transportation system, the use of geosynthetics is playing a pivotal role in those projects.
There are two main functions of a geosynthetic product in road construction, says Brittany Garner, sales and technical manager for the geotextiles division for Agru America.
“One is the initial project cost by eliminating some of the aggregate that’s used to build the roads,” she says. “It’s also important for extending the life of the roadway and letting the other solid materials do their job, which would be that aggregate base not migrating into your sub-base. That may be a softer soil material.”
There are a number of ways in which geosynthetics can be used to improve the performance and durability of roads, resulting in lower construction and life cycle costs, points out Bryan Gee, P.E., product manager for Tensar International.
A US Government Accounting Office study pointed out that, although the nation has more than four million miles of roads that are “highly durable and can last for decades, they deteriorate from traffic wear and tear, inadequate drainage, construction deficiencies, and weather.”
According to 2012 Congressional Budget Office estimates, to maintain current spending levels and account for inflation from 2013 to 2022, the Highway Trust Fund will require more than $125 billion over what it is expected to take in during that period.
The GAO points out that state highway agencies will need to develop strategies for doing so at reduced costs, with one potential strategy being the use of more cost-effective materials and practices.
The use of geosynthetics is listed as one of 40 materials and practices the GAO identified that contribute to improving the performance of pavements, extending service life, and reducing life cycle costs.
“These materials and practices cover a range of uses and applications across the stages of a pavement’s life cycle, from initial design and construction through maintenance and preservation cycles, and at the time of reconstruction,” the GAO points out, adding that there are, however, some challenges.
One primary challenge is that some materials and practices may not be applicable or beneficial to all states. Another is that a decentralized and segmented highway industry may impede change, and yet another is that resource constraints and procurement methods may limit states in implementing new approaches to building and maintaining their highways.
States are developing programs that address these challenges through research, training, and information and outreach programs.
In citing the use of geosynthetics, the GAO points out the materials are used for a variety of purposes in pavement structures, such as reinforcement, separation, and drainage.
Synthetic polymeric materials include geotextiles, geomembranes, geogrids, geocells, and erosion control products. The GAO study found that geosynthetics have been used in a multitude of states in the base or sub-base layers of a pavement structure.
“Used in this way, the materials can improve the stability and strength of those layers,” the GAO points out. “Geosynthetics can also be used in the surface layers of asphalt pavements where it may keep water from penetrating to the lower pavement layers and may reduce the transfer of cracks from an old pavement to a new pavement overlay.”
The study pointed out that officials from two states expressed concerns that using geosynthetics in this fashion could create challenges for future pavement repair.
Case in point: one pavement preservation practice involves milling, or removing the surface layer of an existing asphalt pavement, and replacing it with a new layer of asphalt.
One official expressed concern that geosynthetics used in pavement surface layers might interfere milling equipment operations, leading to additional efforts to separate geosynthetic material from the asphalt millings before they are reclaimed.
A geosynthetic material allows a contractor to build in areas that in other cases may not be able to be built based on California Bearing Ratio (CBR), a penetration evaluation of the mechanical strength of road subgrades and base courses.
“In some areas where roadways may not be able to be built or where there is an extreme cost of excavating some of its really soft material, you can use a geosynthetic like a geotextile or a geogrid material to improve the stability of that base and therefore be able to build a solid road on top of it at a much reduced cost than before,” says Garner.
Agru America manufactures geotextiles in weights ranging from a 3-ounce-per-square-yard material typically used in less-traveled, light-traffic roadway conditions such as driveways to a 32-ounce, a heavy fabric used in applications such as mining haul roads utilized by large construction equipment on a regular basis.
Garner’s advice to contractors in choosing the appropriate road materials is to consult the manufacturers’ experts.
“Every manufacturer, including Agru America, has technical experts for the products they offer, and leaning on them is going to be a huge help for a contractor,” she says. “What they want to look at initially is the traffic expectation on the roadway or the haul road: What is the load bearing they’re going to see, the size of the equipment, number of passes, and other factors?”
Contractors also want to consider onsite materials, the type of sub base, how soft it is, what kind of aggregate they are using to build the solid base, and whether the road will be paved or unpaved.
“They’re going to look at some differences in materials versus something that’s going to be unpaved, such as a haul road, or you’re going into a paved artery of a main highway or an off ramp,” says Garner. “All of these things can be taken into account.”
Garner points out that for the most part, the only time a geosynthetic will incur stress is during the installation process.
“Do you want something strong enough to withstand back dumping from dump truck, such as large aggregate? The installation method also is something you want to look at,” she says.
Garner says there are not as many geosynthetics used in road building as there should be. She points to a study by the US Government Accounting Office that outlines the benefits of using geosynthetics on roadways and the savings that can be derived on rehabilitation and new construction.
“We do have a very aging infrastructure,” Garner says. “The key point with design is in having the engineers look at this as an option and specify this from the beginning. The studies are out there, but the acceptance is needed by more of the engineering community to get these into their original specifications.”
Garner, who herself is an engineer, says she believes professional engineers are slow in adopting change.
“They’ve been doing it one way for a very long time,” she says. “There is a big force and big dollars behind aggregate, rock quarries, paving manufacturers, and asphalt manufacturers that also have a very big push in certain areas.
“People are more willing to put in more aggregate, more asphalt and use some of these materials but at the end of the day, looking at the life cycle costs, there is savings in using any sort of geosynthetic in a roadway or infrastructure project.”
Tensar offers several geosynthetic products for road building that offer improved performance and cost effectiveness compared with conventional construction practices.
For example, Tensar TriAx Geogrid is designed to enhance the performance of roads by providing stabilization of granular materials used in aggregate base courses and sub bases.
A Mechanically stabilized layer constructed using TriAx Geogrid can be used to address unsuitable subgrade conditions or to optimize pavement performance by maximizing the structural contribution of the aggregate base course to the pavement section.
GlasGrid System and GlasPave asphalt interlayers are designed to help extend the life of asphalt overlays by reducing reflective cracking and the intrusion of water into the pavement section.
RoaDrain System drainage composite is designed to improve the drainage capacity of the pavement section, which prevents the dramatic reductions in traffic capacity that can result from excess water intrusion and retention.
It can also be used to provide a capillary break layer over frost-susceptible soils to prevent heaving.
Mechanically stabilized earth structures, such as retaining walls, reinforced slopes, and reinforced embankments over soft soils, provide significant savings of time and cost compared with conventional structures, says Gee.
“They are used in applications such as bridge abutments and grade changes and can also minimize right-of-way requirements by allowing the construction of steeper slopes,” Gee says.
Gee points out that the fundamental principles for maximizing the performance of a road are to build on a firm foundation, ensure good drainage, optimize pavement design to traffic and budget requirements, and extend the lifespan and reduce the life cycle cost of the road by minimizing the deterioration of the riding surface due to cracking, fatigue, and water intrusion.
“Geosynthetics provide a cost-effective way of improving road construction in each of these important areas when they are incorporated using accepted design methods,” says Gee. “It is critical that both the design engineer and the contractor make the effort to understand the design method used, the supporting research and performance validation for the geosynthetic chosen, the intended benefit of the geosynthetic, and proper construction techniques in order to ensure a successful product.”
There are numerous geosynthetic materials which can be used in dozens of different applications and they are not interchangeable, Gee points out.
“Particularly in the area of optimizing flexible pavement design, it is vitally important to understand that the benefits of using geogrid in an MSL must be supported by full-scale accelerated pavement testing,” Gee says. “In more than 30 years of research on geosynthetics, no correlation has been found between any material property-such as tensile strength-and roadway performance.
“This can best be seen by noting the fact that the accepted design methods, such as AASHTO and Giroud-Han, do not include material properties as input parameters.”
As the costs of conventional road building materials continue to escalate, the benefits of using geosynthetics to support the construction of solid roads will continue to increase over time, Gee says.
“With infrastructure budget restrictions continuing for the foreseeable future, our roads depend on public and private owners taking the opportunity to obtain improved performance at reduced cost,” says Gee.
The use of geosynthetics in road building can offer many opportunities for contractors to add value to projects, but proper application is essential, Gee says.
“Comprehensive technical support for each specific application is a must,” Gee points out. “While contractors can sometimes get acceptable results using a rule-of-thumb approach in non-critical applications, capturing maximum value requires design expertise.
“Whether value engineering a conventional design or addressing unexpected conditions on a project, contractors can minimize schedule impacts and maximize profitability by using the best available technology.”
When evaluating options, consider the total cost of the solution, not just the unit cost of the geosynthetic, says Gee.
“Paying a slightly higher price per square yard for a product that will significantly reduce the quantities of other materials on the project is often the most cost-effective way to go,” Gee adds.
Strata Systems’ StrataBase is a rigid biaxial geogrid consisting of extruded polypropylene and designed to provide structural reinforcement for paved and unpaved roads through superior stiffness, aperture stability and interlock capabilities, says David Butchart, business development director.
“For 30 years, biaxial geogrids have been utilized in pavement construction and soil stabilization projects throughout the US and worldwide,” he says. “StrataBase stabilizes aggregate and soil through confinement and introduces tensile elements, thereby increasing bearing capacity and load support capacity.”
Lateral spreading of base course aggregate or sub-base material is the most critical and common failure in pavement structures, says Butchart, adding that StrataBase “effectively reduces lateral spreading, resulting in increased structural performance and pavement life.”
Lance Carter, technical director for Strata Systems, points out that geosynthetics provide many benefits to the roadway building industry by shortening the construction timeline or serving as proven, cost-effective alternatives to historical construction practices.
“Strata supports the roadway construction industry with a range of beneficial products,” Carter says. “Geotextiles such as StrataTex nonwoven and woven products are effective products used as separation, filtration, and in some case stabilization of sub-base materials.”
Geogrids, such as StrataBase extruded polypropylene biaxial products, provide confinement to unbound roadway aggregates to provide effective sub-base stabilization or base course reinforcement, thereby improving performance or reducing material volumes, says Carter.
Composite drains such as StrataDrain work well to remove surface and subsurface water infiltration that can be detrimental to long-term roadway performance, Carter adds.
Structural geogrid products such as Stratagrid uniaxial geogrids are effective products for the construction of roadway embankments incorporating retaining walls or steep slopes, Carter says.
There are many pavement applications where geosynthetics have proved to increase pavement life, says Butchart.
Geotextiles are used for filtration, separation, and stabilization, and geogrids are commonly used for stabilization or reinforcement.
“Whether extending pavement life or reducing the cost of initial construction, the life cycle benefits of using geosynthetics always outweigh the cost of the material,” Butchart points out.
In choosing the right material for the application, “one needs to have knowledge of the subsurface soil conditions-the geotechnical information, the loading requirements, and the expected or required life of the road,” says Butchart.
“These factors can be incorporated into design models to establish the most practical design incorporating geosynthetics,” Butchart adds.
Woven and nonwoven geosynthetic layers serve different functions, points out Scott Falkenberg, Layfield Geosynthetic’s business development manager for western Canada.
“A nonwoven geotextile allows for good separation and good filtration, whereas a woven geotextile is very good for reinforcement and separation,” he says. “The woven geotextile is used for strength and the nonwoven for separation, so if you’re in an application with soft ground and you wanted to use a nonwoven, you would probably have to use a geogrid as well to add strength to the system.”
Based on CBR information, “the softer the ground, the higher-strength geotextile you’ll need and a combination of nonwoven geotextiles and biaxial geogrid.”
The products are not only used for making stronger roads, but also for making roads more inexpensively, using less gravel, and lowering the carbon footprint, Falkenberg points out.
The Layfield Group offers a full range of geosynthetics for road building.
The company can slit full rolls of geotextiles to different sizes using a band saw and can sew panels of geotextile to large sizes, such as double-wide and triple-wide panels of fabrics for road and embankment construction.
Woven monofilament geotextiles are available for erosion control applications and high-strength wovens are available for embankment reinforcements.
Layfield products include nonwoven geotextiles, woven geotextiles, wick drains, Typar spun-bonded nonwoven geotextiles, high-strength geotextiles, monofilament geotextiles, and filter socks.
The company also provides installation and construction services.
The GeoDetect Strain Monitoring System is a sensor-enabled geotextile system that includes geocomposite, fiber optics, instrumentation, and software to provide measurement of strain and temperatures for health monitoring of soil structures.
The RSi Series high-efficiency geotextiles are advanced integrated woven geotextiles designed with higher-tensile modulus and water flow properties for efficiencies in base course reinforcement and subgrade stabilization for roadways, haul roads, and other applications.
Layfield also manufactures a range of geogrids for soil reinforcement applications.
