Case Study: Oklahoma Roads

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Case Study Overview

Project: Oklahoma Roads
Location: Oklahoma, United States
Project Type: Roads
Application: Roadway Support
Product Used: GEOTEX® Woven & GEOTEX® Nonwoven

Photo of Oklahoma rural road paving in progress.

Problem

In the United States today there exists approximately 1.5 million miles of unpaved roads surfaced with stone or a soil/stone mixture. Also, beneath most pavements there is a “hidden unbound aggregate road” in the form of a base or subbase. Therefore, most roads traveled today rely on the support function of crushed stone or gravel to allow traffic to traverse the subgrade soil. It has been shown (Jonesby and Hicks) that as little as 6% fines contamination into a unbound aggregate layer can dramatically reduce the structural support of layer and thereby destroy its ability to spread the traffic load over a subgrade. Therefore, the greatest challenge for road managers is to maintain the integrity of their unbound aggregate support layers by minimizing the potential for contamination form upward migration.

In 1987 to 1989 the US Federal Highway Administration, FHWA, in cooperation with the Oklahoma Center for Local Government Technology tested the effectiveness of separation/stabilization geotextiles in preventing the contamination of unbound aggregate roads form subgrade soils. Nineteen unpaved rural roads sites in six countries were chosen for testing across the state of Oklahoma (Figure 1.) The nineteen sites represented a wide variety of subgrade conditions, aggregate surfacing types, climates, and construction methodologies. A final report, FHWA-RT-89-050, was published as a Technology Sharing Report. The report, Geotextiles Selection and Installation Manual for Rural Unpaved Roads, is a genuine manual for the use of separation/stabilization geotextiles based on the results of the statewide testing of the concept. The geotextiles were found to be an extremely effective, low cost solution to the contamination problem. This is a brief summary of the testing which resulted in the recommendations of the report.

Solution

Both woven and nonwoven separation/stabilization geotextiles were tested. The procedure involved blading (grading) the existing roadbeds to a relatively smooth surface, crowned in the middle and slightly sloped to the road side for drainage purposes. Next the gotextiles were hand rolled out on the roadway with an 18 to 24 inch overlap at the road centerline (Figure 2). The outer edges of the fabric were laid into a graded shallow trench and nominally covered with road edge material. Fabric was rolled out just ahead of stone placement to minimize exposure to traffic, wind blowing, etc. In some locations where fabric was rolled well ahead of stone placement, temporary anchoring with soil or stone, or by nailing was necessary to hold the geotextile in place until the cover stone was placed. Woven and nonwoven geotextile sections were placed as well as control sections of road which received the stone treatment but had no fabric placed. The unbound aggregate surfacing material varied across the state from crusher run stone of varying hardness to naturally occurring pit run gravels. On most sections, only 4 inches of surfacing stone was placed over the geotextiles. In most locations, the surfacing stone was back dumped onto the geotextiles and then the stone was pushed out onto the geotextile using a dozer or a grader. In some cases, where the road was firm, stone was dumped from the belly dump trucks or end dump trucks as they were moving across the fabric. This is not recommended practice as the truck traffic may cause some damage to the installed geotextile. Figure 3 shows a section where the stone was back dumped as the truck backed over the geotextile. The referenced FHWA manual is an excellent resource for recommendations on installation methodologies due to various subgrades, aggregates and the equipment available to construct the roads.

Both the woven and nonwoven geotextiles used in the study were light weight styles with minimum weights of 4 ounces per square yard and required geotextile physical strength properties were based on projected vehicle loading for the roads. The AASHTO Guideline Geotextile Specification M 288 was published after this study and provides excellent guidance for geotextile selection. For most applications under paved roads, AASHTO Class 2 separation and stabilization geotextiles would be suitable. With field trials and proven performance, a Class 3 geotextile may be suitable for some conditions.

Photo of Oklahoma rural road paving in progress.
Photo of Oklahoma rural road paving in progress.

Performance

Immediately following the stone placement, it was not possible to distinguish the geotextile test sections from the control sections (Figure 4). The performance of the statewide geotextile test sections was monitored and dramatic results were evident after a single wet (winter) season. The control sections on several roads had completely lost the 4 inches of new surface stone due to intermixing with subgrade oil. With the loss of the surface stone due to soil contamination, the roads lost their structural strength and severe rutting occurred. Some road sections needed more stone applications just to make them passable. The sections with the separation/stabilization geotextile were still in the condition in which they were built (Figure 5). The 4 inch stone surfacing was still separated from the subgrade. As may be seen by the photos, the with-geotextile sections were almost like driving onto a raised, hard platform compared to the no-geotextile sections where intermixing of the subgrade and aggregate occurred (Figure 6). The study did indicate a difference in the performance of the woven and the nonwoven geotextiles. The FHWA manual recommends the nonwoven geotextile since the typical 4 inches of cover stone with some stone underneath causes a harsh, abrasive environment for the fabrics. Because of its ability to elongate to avoid puncture, the nonwoven survives better in this environment. The second benefit of a nonwoven is its higher coefficient of friction which tends to more effectively hold the unbound stone in place. Finally, the nonwoven geotextiles was preferred in any wet areas due to its ability to rapidly pass water while retaining the subgrade soil compared to the woven fabric used.


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Conclusions and Economic Benefits

The FHWA Manual “FHWA-RT-89-050” is available from the National Technical Information Service, NTIS. As the test sections indicated, the application and the performance of separation geotextiles does not require complicated design or installation.

The concept is very straight forward, place a geotextile between the subgrade soil and the aggregate support layer. This is just as applicable for the protection of pavement base or subbase layers as it is for unbound aggregate surfaces. The only other key to the success of the application is the proper selection of separation/stabilization geotextile. Guidelines for proper fabric selection are provided herein and in AASHTO Guide Specification M 288.

The value of the separation/stabilization geotextile is twofold. First, the separation benefit easily pays for the fabric since, as shown by the test sections, 4 inches of stone may be quickly lost due to subgrade contamination while a typical geotextile only costs as much as one or two inches aggregate. A simple nomograph is provided as Figure 1, which shows the relative low cost of the geotextiles compared to the aggregate materials. The secondary benefit is the road stabilization the geotextile provides. The presence of the geotextile provides lateral confinement to the area of subgrade soil being loaded by vehicles. This increases the effective strength of the subgrade by a factor of about 1.8. The presence of the geotextile also helps restrain stone movement at the bottom of the aggregate layer to improve the long term stability and increase the load carrying capacity of the unbound materials.

Geotextile Cost vs Aggregate Cost Nomograph

This nomograph shows the low cost of separation/stabilization geotextiles compared to aggregate used in unpaved roads. The use the nomograph simply identify your installed aggregate cost and a geotextile cost. A line connecting these two values will, on the middle line show the aggregate thickness which is equivalent to the cost of the geotextile. In the dotted line example shown, installed aggregate cost $12/cubic yard and the geotextile cost $.70/square yard. In this example, the geotextile cost about the same as 2” of aggregate in place. Since 2” to 4” aggregate can easily be lost in one year due to subgrade soil contamination, the fabric pays for itself in less than a year. Beyond these material cost savings, there are significant savings due to less required maintaining of a road over a geotextile.