By: Stephen N. Valero, P.E., Contributing Author
Rural roads make up 66% to 75% of all lane-miles in the U.S. Despite this fact, very little time has been devoted to “designing” these roads. Local officials, engineers or contractors often employ experience-based or codified standard sections that often include extra sacrificial aggregate to ensure performance. However, if one considers the fact that aggregate cost is constantly rising and that more than 6 million rural road lane-miles exist in the U.S., simple math shows that some design effort is warranted. A reduction of just 1 in. of aggregate thickness per existing rural lane-mile in the U.S. would have saved more than 2 billion tons of aggregate. At an average in-place cost of $15 per ton, this would have represented a saving of more than $30 billion today. Furthermore, this does not even consider the number of private driveways, parking areas, trails and other low-traffic areas that could benefit from some level of design effort.
This potential saving has not gone completely unnoticed. Over the last two decades, a few researchers have devised innovative design methods for low-volume roads. The most interesting of these incorporate geosynthetic materials into unpaved and paved low-volume road sections to reduce aggregate requirements. Most geosynthetic materials used in transportation applications consist of polymer sheets, known as geotextiles, polymer grids or geogrids. These materials are delivered to the project site in roll form and are easily deployed using simple hand labor.
At the time early design methods were developed, very little documented performance data from low-volume rural roads (especially those including geosynthetics) was available. Thorough calibration and validation was not feasible. Consequently, application of these design methods has been conservative.
In addition, the early design methods were not sufficiently robust to consider some of the more important material properties of geosynthetics and aggregates. In particular, a multitude of geogrid products are now commercially available, with each of these having significant differences in important properties. Nonetheless, the Federal Highway Administration recommends using the Steward method. The U.S. Army Corps of Engineers also has adopted this method and made limited upgrades to it based on empirical data.
Since the 1980s, geosynthetics have been widely used in low-volume road and subgrade improvement applications and studied in additional research programs. Thus, a much broader body of information exists today than when the classic design methods were developed. This new information facilitated development and validation of a revolutionary low-volume road design method by Giroud and Han. The Giroud-Han Design Method is significantly more robust in that it incorporates important properties such as the strength and modulus of the aggregate, variations of the stress distribution angles through the aggregate and stiffness (aperture stability modulus) of the selected geosynthetic reinforcement.
The Giroud-Han Method also considers all of the properties utilized in the classic design methods. This improvement allows designers to predict performance of geosynthetic reinforced or unreinforced low-volume roads leading to more efficient use of aggregate resources, construction equipment, labor and time.
Getting to know Giroud-Han
In simple terms, the Giroud-Han Method facilitates estimation of the aggregate thickness required to prevent a serviceability failure (defined by rutting) at the road surface. The method allows for comparison of geosynthetic reinforced and unreinforced sections. Through improved stress distribution and preservation of the aggregate’s ability to carry load over time, geosynthetic reinforcement can provide extensive savings in aggregate usage, undercut or fill requirements, construction cost and maintenance cost.
As with previous design methods, the Giroud-Han Method is based on a theoretical bearing capacity model. However, the researchers went on to calibrate the model using the results of a specially developed laboratory-testing program. This program incorporated a significant number of large-scale, cyclic plate load tests on unreinforced sections and sections incorporating Tensar biaxial (bx) geogrid reinforcement. The research provided data regarding the pressure induced on the subgrade and surface deformation as a function of the number of load cycles for multiple combinations of reinforcement and base thickness.
This data was used to estimate vertical stress transmission from repeated surface loading (i.e., traffic) to the subgrade. The research quantified the effects of aggregate reinforcement and thickness on both initial stress distribution angle and changes to the angle with continued load applications. It was observed that the initial load distribution angle was improved significantly by incorporation of geogrid reinforcement. Observations also suggest that the load distribution angle reduced significantly more rapidly in the unreinforced section than in reinforced sections.
Following calibration, the method was validated using results of other research programs and the available body of applicable field data. This validation in combination with consideration of many independent variables differentiates the Giroud-Han Method and classic design methods. Furthermore, it ensures that the method may be used with confidence to design the most efficient low-volume road sections possible without compromising performance.
The Giroud-Han Method is applicable for the design of unpaved low-volume roads, parking areas and trails. The method may be used to design temporary or permanent construction equipment access over soft, wet or otherwise problematic soils.
Furthermore, the method is useful in subgrade improvement or stabilization applications to speed construction of paved roads, parking areas and other trafficked surfaces. The method facilitates design of unreinforced aggregate sections as well as those incorporating geogrids or geotextiles. Validation against other research and observed field performance indicates that the method accurately predicts the performance of unreinforced sections, sections including geotextiles and sections including geogrids having property values falling within the range of those used for calibration of the method.
Figure 1 (in magazine) illustrates the predicted performance of several sections using the method. Over relatively weak subgrade soils, certain geogrids provide the most benefit (i.e., require the least aggregate). Essentially any geotextile that survives installation will provide some degree of benefit compared to the unreinforced section. The difference in required aggregate thickness for geogrid reinforced sections is due to inherent property differences in available geogrid products. Therefore, a degree of conservatism must be applied to results obtained through application of the method to geogrid products with property values outside the limits of calibration.
The effectiveness of a geogrid in reducing aggregate requirements is dependent on several factors. The geogrid must be able to effectively (i.e., broadly and evenly) distribute load over the subgrade to reduce induced vertical stress. This involves a complex interaction of geogrid, soil and aggregate.
Success in numbers
Since development in early 2003, the Giroud-Han Design Method has been successfully used to design many low-volume rural road and subgrade stabilization (i.e., working platform) applications.
The method was used (and continues to be used) to design construction access roads and temporary parking areas at the Woodrow Wilson Bridge Project near Washington, D.C. In this application, geogrids were used to reduce the amount of aggregate required and eliminate undercut and removal of poor subgrade soils. In this manner, disturbance to the surrounding areas was minimized. Construction time and costs were reduced substantially.
The Giroud-Han Design Method also was used to design subgrade stabilization for the Santa Monica Boulevard Transit Parkway in Los Angeles. In this case, geogrids were used in conjunction with sub-base aggregate to reduce the required undercut by 50%. This resulted in significant savings in aggregate costs and avoided costly relocation of near-surface utilities.
At the Port of Greater Baton Rouge, La., the Giroud-Han Method was used to design an improved subgrade under the main cargo transport road between docking areas and rail/truck transfer facilities. This road carries more than 8.3 million tons of cargo each year. Geogrid was used with a layer of aggregate to enhance the subgrade in the area of the road. This additional support was used to economize design and reduce maintenance requirements of the permanent pavement.
A major residential developer recently embraced the design method to improve subgrade conditions at a site near Austin, Texas. This 680-home site was underlain by expansive clay soils. Expansive clays change volume rapidly with varying moisture conditions and can severely reduce the longevity of pavements.
Lime stabilization has been the traditional solution to this problem in Texas. However, lime does not always work effectively and is very sensitive to environmental conditions during installation.
To save time and ensure success, the developer’s engineer used Giroud-Han design methodology to design a subgrade stabilization platform incorporating geogrid. This approach was approved by the city of Austin (eventual owner of the subdivision roads). Overall construction time and cost was reduced significantly over the lime stabilization alternative. Moreover, the developer has used this approach on two additional subdivisions since the success near Austin.
Additionally, the method is being used regularly by project engineers, contractors and public agencies to efficiently reduce aggregate and undercut requirements in roads, parking lots, construction areas and other trafficked surfaces. Although the method may be applied using hand calculations, this approach can be cumbersome since the empirical nature of the method requires an iterative solution. However, the method has been incorporated into the SpectraPave2 pavement design software for office use and into slide rule and pocket card form for quick and simple estimation of aggregate requirements and savings in the field. These tools are available free of charge from Tensar Earth Technologies Inc. (www.tensarcorp.com).
The Giroud-Han design method represents the most significant advancement in low-volume road design in the last 20 years. Given the decreasing availability and increasing cost of aggregate roadbuilding materials, it is in the best interest of anyone considering building a low-volume road to spend time designing the structure with the Giroud-Han Method. Regardless of whether the application is a driveway or a construction platform for a superhighway, the potential rewards are significant.
About The Author: Valero is vice president of technology at Tensar Earth Technologies, Atlanta. For more information, call 800/TENSAR-1 or e-mail [email protected].
