Geogrids are a practical solution for subgrade reinforcement while reducing aggregate base course thickness
Road construction remains one of life's great mysteries to the vast majority of us who use roads. We just drive on them.
Sometimes, maybe often, we complain about them.
Road design belongs to the "out of sight, out of mind" phenomena because the average driver seldom sees or thinks about the underlying structure of a road. Construction time and taxpayer responsibility rest in the forefront of public thought.
Nevertheless, it is the structure that determines much of the longevity and cost-effectiveness of a road.
On the design side, the almost obsessive requirement for low bid rather than best bid can be nettling. Cost is, undeniably, a major factor in road approval. And funding is often in the hands of elected officials who feel great pressure to press upon price more than design for all aspects and parties involved in a job, not just in regard to materials.
But low bid and best bid do not have to be antonymous concepts. In many situations, the construction and maintenance costs of a road may be reduced and its life extended when geogrids are selected as one of the construction materials.
Out of sight, out of mind As useful and cost effective as they can be, one of the truly curious things about geosynthetics—the umbrella class of materials to which geogrids belong—is their widespread use but fairly low profile.
Not long ago, Donald Trump paid nearly $60 million to rebuild the 18th hole of his coastal California golf course, Trump National Golf Club Los Angeles, because the hole experienced a landslide toward the ocean. For all the press this massive rebuild project generated in engineering and golf publications, in newspapers, and on television, geogrids rarely were mentioned as one of the reinforcing materials, nor did they appear in cross-sections detailing the new hole's structure. Sod and hydroseeding were given greater attention.
The press commonly makes significant oversights, beyond the application of geosynthetics, when covering engineering projects. But if more public representatives understood the role played by geosynthetic materials in projects and, subsequently, approved their use, engineers would have increased design options for economy and long-term performance. This would be especially beneficial for civil engineering projects, which are subject to the volatility of public-expense decisions.
What Donald Trump spends on a project is of interest to his accountants, and perhaps his family and investors. What a county engineer or road commissioner greenlights affects many elected offices.
And they affect your work.
Geogrids are polymer grid structures for use in soil reinforcement. Some construction professionals compare them to rebar in concrete, since they provide interior strength. They are manufactured to provide either uniaxial or biaxial reinforcement, meaning they are designed to accept and distribute strain in one or two directions (referred to as longitudinal and tranverse). When soil reinforcement is needed to support strain in one direction, such as behind a retaining wall, uniaxial geogrids often are applied. For roads, the strain is more diffuse, so biaxial geogrids are generally applicable.
Placed within an aggregate layer, the grid provides a surface against which the aggregate grabs while helping distribute the load of the traffic above. This interaction greatly reduces migration within the aggregate layer; migration is a chief cause of road failures. When coarse aggregates mix with fine aggregates, small voids and weak spots develop beneath the road, which in turn causes cracking, rutting, and pot holes.
Geogrids minimize the risk of intermixing, improve the strength of the base aggregate layer, and minimize the amount of aggregate needed.
Many designers add more base course than the basic design calls for in an effort to buy a little extra time before the road support declines to the point of failure. A tremendous amount of money goes into adding this extra inch or two of base course, representing billions of dollars and, literally, tons of aggregate in construction.
A better option is using geogrids for both paved and unpaved roads, which reduces the need for base course aggregate—a major cost, even in aggregate mining regions. And the structural role played by geogrids within the aggregate layer reduces the wear and tear on the overall road system, thus extending its life.
Regarding the longitudinal and transverse "ribs" of the geogrid, the space between them is referred to as the aperture, which provides the locking strength. The coarser the aggregate, the larger the aperture may be between the ribs. Essentially, this means less grid material, which may mean lower cost. But it is important, of course, to select the geogrid for the aggregate and traffic volume on which you base your road design. You cannot cut corners, as there's no economic advantage to poor design. Too large of an aperture reduces the effectiveness of the geogrid and may ultimately lead to the same road performance problems that one hopes to avoid in using geogrids.
Just as design software does not relieve the designer of the responsibility to understand the calculations, so too it is with geogrids (and aggregate, drainage, paving materials, et cetera). Know what you are using and why, and seek assistance.
Education as a design tool
A number of software aids are freely available from the manufacturers of geogrid products. Some potential users feel a knee-jerk reaction against proprietary software, for these programs may recommend only one company's brand of products.
But it is important to note that these programs are authored with stringent engineering concerns for soil stability, aggregate thickness, traffic volume, and the proper role of a polymeric grid within that system.
Much can be learned from these programs.
Also, short courses are taught commonly by transportation agencies, material manufacturers, institutes (such as the Philadelphia-based Geosynthetic Institute), and universities.
Furthermore, geogrid manufacturers post openly on company websites and in literature one of the most useful design reference tools: product data. This is generally empirical data arrived at through independent testing. These are specially formulated products, so the posted data, such as a product's shear strength, is quite dependable. It's important to verify with the manufacturers, however, that the posted data is the most recent.
The following companies offer such resources, including case histories and discussion of design methodology:
- Colbond Geosynthetics, www.colbond-usa.com;
- Huesker Inc., www.hueskerinc.com;
- Naue GmbH, www.naue.com; and
- Tensar Corporation, www.tensarcorp.com.
Look to other engineers
Geogrids are well accepted in road construction, but general awareness and user knowledge of them is just now filtering out as more state, county, and municipal departments of transportation and road commissioners include the materials in their formal process considerations and manuals. The Internet is a boon, in this regard, for it now enables engineers to access the resources created by other departments, agencies, and consultants—the very people you may one day work with on a project.
For example, the Fort Wayne, Ind., Department of Public Works website includes a section on geogrid selection and installation. Its requirements note all the proper design concerns, such as maximum tensile strain and aperture size, and information for contractors regarding storage and site conditions for the materials.
The city of Fort Worth, Texas, has released a short manual on proper drainage and reinforcement design, including graphs of California Bearing Ratio performance for unreinforced roads, roads with a geotextile separation layer, and roads with geogrid and geotextile layers.
It makes the world go 'round However, information is not the entire solution. Money remains a sticking point in many civil engineering roadway projects.
Architects such as Rem Koolhaas and Italo Calatravva can build the libraries and art museums of tomorrow, but let's see them try to win the same sort of "golden" funding for something as utilitarian as a road. They won't get it.
But when billions of dollars are spent on extra aggregate for roads in anticipation of failure, engineers can do a great deal to resolve some of that public dollar concern by capitalizing on a simple design principle: Keep the base course intact. Properly applied, geogrids will do that and, consequently, save considerable public money.
Christopher Kelsey is an editorial consultant with geosynthetica.net, West Palm Beach, Fla., a free, online resource for geosynthetic design, bid assistance, and material data information.
Sidebar: The loneliness of the unpaved road
The United States has millions of miles of unpaved roads. Maintenance on these roads is not easy, especially with tight funding. Far too often unpaved roads are ignored until dire road failures or accidents occur.Lesser use is seen as less worthy of attention, it seems. As such, many rural, unpaved lanes are built in the style of temporary roads: less-stringent subgrade compaction and preparation, placement of base aggregate directly above the subgrade, heavy use of immediate soils (as if they were naturally appropriate for roadway drainage), et cetera. But the rutting and pitting that develops on unpaved surfaces is a huge safety concern.
The transportation research group TRIP (www.tripnet.org) estimates that more than 22,000 fatalities—roughly 50 percent of all roadway fatalities in the United States—occur on rural roads each year.Many of them are unpaved, and even the paved ones share with the unpaved ones many of the underplanned design characteristics.
Unpaved roads are also an environmental hazard, something all agencies are putting a sharper eye to now that fines are mounting over poor erosion and sediment control practices.
Sediments from worn roads along logging routes, farms, construction sites, and other active zones contribute significantly to water pollution in many states. For example, the Pennsylvania State Conservation Commission even considers unpaved road sediment its largest source of water pollution.As such, the commission has established a Dirt and Gravel Road Maintenance Program to monitor the problem and educate those responsible for repairs.The annual outreach budget alone is $4 million.
The stress in Pennsylvania's program is on long-term performance, not short-term corrections. While dirt and gravel roads are far cheaper to construct, their cost advantage changes when evaluated on a per vehicle basis, in which case rural roads are far more expensive to maintain. Longterm solutions begin at design.
Road reinforcement is something that one might consider "unsexy," as professional interests go. But this segment of civil engineering design continues to evolve with new and very worthy contributions.
From 2004 to 2006, J.P.Giroud,Ph.D., and Jie Han,Ph.D., published a new calculation design for unpaved roads to help agencies, road designers,and builders properly incorporate geosynthetics into subgrade reinforcement.
The three-part technical paper,"Design Method for Geogrid-Reinforced Unpaved Roads," is available from its publisher,the American Society of Civil Engineers. See also an April 2006 discussion of the series contributed by Michael R. Simac, David J. Elton, and Stephen M. Gale in the Journal of Geotechnical and Geoenvironmental Engineering.