With the multitude of geosynthetic products in the market and the even greater number of purported improvement claims, there is confusion in the transportation construction industry regarding the benefits of geosynthetics to both the contractor and the owner.

By Ariel Soriano

Over the past 33 years, geosynthetics have evolved from new engineering materials with limited technical references to state-of-the-practice materials with numerous design and technical tools. No longer must the design professional rely on geosynthetics manufacturers for information on geosynthetic applications. There now are numerous generic references available and many ways to obtain them. These generic references allow the engineering community to properly design with and specify geosynthetics.

A tremendous amount of marketing and salesmanship blurred and confused the advantages and benefits of various geosynthetic products over the past decade, especially those of geotextiles and geogrids. Generally speaking, geotextiles are a family of industrial fabrics comprising polymeric (i.e., polypropylene, polyethylene, and polyester) fibers or yarns that may be needlepunched or woven, respectively, to produce an infinite variety of strong, dimensionally stable construction materials. Geogrids typically are extruded and drawn from polyolefins or woven from polyester, nylon, or fiberglass yarns to produce extremely strong construction materials.

Advantages

Common geotextile end uses include filtration, reinforcement, separation, and stabilization. Depending on site conditions, transportation projects may incorporate one or all of the uses named above. The most common transportation use of geotextiles, however, is in the construction of paved and unpaved roadways. Used this way, geotextiles provide several benefits through three primary functions: separation, reinforcement, and filtration.

The benefits derived from these three geotextile functions are most significant when subgrade soils are weak (California Bearing Ratio [CBR] <3). These benefits are well documented in the referenced literature. Long-term separation benefits, such as improved pavement performance over time in applications where the subgrade is competent (CBR >3), however, just now are beginning to surface. Additionally the National Cooperative Highway Research Program of the Transportation Research Board approved the funding of an extensive research program aimed at quantifying the benefits of geotextiles as separators in pavement systems. Although the benefits of geotextiles as separators are widely recognized, they have yet to be documented satisfactorily.

The obvious geogrid function is reinforcement, whether in soil slopes or mechanically stabilized earth (MSE) walls. Similar to geotextiles, geogrids possess very good environmental and installation-durability properties, as well as strength properties somewhere between nonwoven and high-strength woven geotextiles. Due to their aperture size range, however, hydraulic properties are not an issue and therefore are not tested.

MSE walls embed geosynthetics horizontally between backfill layers and integrate them to the fascia to produce very cost-effective structures, especially when space is at a premium. MSE walls are replacing conventional structural elements, such as bridge abutments, secondary waste- and water-containment systems, and earth-retaining systems. Among the most advantageous features of the MSE wall is its flexibility. Unlike conventional soil retaining structures, such as precast concrete walls and masonry walls, MSE walls can accommodate relatively large differential settlements that would damage conventional structures. An added benefit is the speed in which MSE walls can be constructed. Typical earthwork equipment is used, and unlike concrete structures, the system does not require a cure period.

Reinforced soil slopes also embed geotextiles between backfill layers but are characterized by having slopes of less than 70º. Again, cost-savings may be realized through space savings, avoiding costly remove-and-replace methods, and enabling the use of local and/or lower-quality backfill material.

Case Histories
Filtration, Separation, and Stabilization
Geotextiles Lay New Jersey Highways Out to Dry

Road construction is an ongoing project in most American cities. Although the inconvenience of closed lanes and detours creates a headache for commuters, the men and women who design and build our nation's roadways know the importance of deliberation and the careful planning that goes into construction. In other words, do it right the first time, or risk having to do it again.

Since September 2001, the New Jersey Department of Transportation (NJDOT) has been working in Woodbridge to reconstruct, reconfigure, and widen the roads that make up the interchange where US Route 1 and US Route 9 separate. US Route 1 runs from Maine to Key West, FL, and connects with the historic US Route 9, which travels from Canada to Maryland, entering New Jersey over the George Washington Bridge. From the George Washington Bridge to Woodbridge, the two major roadways are one.

Figure 1. Site subgrade

In March 2002, NJDOT became concerned with the subgrade conditions on the Route 9 South portion of the project. Woodbridge is approximately 2 mi. from the New Jersey coastline, and in coastal areas, soils tend to have a higher moisture level than inland areas. Project contractor E.E. Cruz Construction previously had excavated the subgrade, and compaction procedures were under way when the exposed subgrade began to exhibit symptoms of having dangerously high moisture levels (see Figure 1). During operation of the smooth drum vibratory roller, pumping and rolling of the surface under wheel loads had occurred, and ruts had been created in the soft soil after multiple passes of a loaded tandem truck. With signs of structural breakdown occurring so soon in the construction process, there was no possible way that the conditions surrounding this roadway would provide for safe driving in the years to come.

Gene Raisley, P.E., resident engineer for NJDOT, called Bill Ragen of Ragen Associates, specialists in geotextile materials and applications, to appraise the situation and offer solutions. The usual DOT repair method of undercutting and replacing the subgrade with stone was extremely expensive and time-consuming, and Raisley wanted to review his options before making a decision. After careful consideration of the project and its problems, Ragen recommended the usage of SI Geosolutions' Geotex 4x4 and Geotex 801 to prevent the road deterioration that results from excessive moisture.

Aggregate base contamination by subgrade soils is the leading cause of pavement failure in the construction industry, and highway engineers typically thicken base and sub-base layers using sacrificial aggregate to offset the expected losses. The latest alternative to superfluous aggregate is the use of geotextiles integrated with the subgrade, sand, and/or aggregate. Geotextiles have a variety of applications that range from reinforcement on weak subgrades to separation on firm foundation soils. With the proper use of geotextiles, the subgrade soil is permanently separated from the aggregate layer, which contributes to the individual layers maintaining their original strength and structural integrity. Ragen's suggestion of using two different geotextile layers would not only provide for superior drainage and filtration but also would prolong the life of the road, minimizing rutting and contributing to better distribution of heavy loads placed on the pavement surface to the subgrade.

Figure 2. Geotex 4x4 installed over the subgrade layer

At Ragen's recommendation, NJDOT made plans to roll out a layer of Geotex 4x4, a high-strength woven geotextile, over the subgrade layer, separating the subgrade from the layer of sand (see Figure 2). "I felt that Geotex 4x4 would be strong enough to survive the installation," Ragen recalls. This heavy-duty geotextile is made up of dense monofilament and fibrillated yarns that are woven together to form a unique twill pattern. Geotex 4x4 is ideal for the construction of embankments over soft soils, steepened slopes, and modular block and wrapped-face retaining walls. It is resistant to ultraviolet degradation and to the biological and chemical environments normally found in soil. Woven geotextiles also allow aggregate layers to maintain their original design thickness so the strength and the durability of the road are not compromised.

Figure 3. Geotex 801 beneath dense graded aggregate

After placement of the required sub-base material, the second geotextile, Geotex 801, a nonwoven separation/subsurface drainage geotextile, was placed over the layer of sand and under the dense graded aggregate (DGA) (see Figure 3). This geotextile, DGA, and a sand sub-base system prevent intrusion of the subgrade into aggregate layers and improve roadway subsurface drainage. "Because they were putting two different materials down - the sand and DGA - I felt that we needed a separator between the two," Ragen continues. "The plans called for only an 8-inch layer underneath, and I wanted to have just a little more strength in the material to help the whole system survive, which the Geotex layers provided successfully."

Geotex 801 is a polypropylene staple fiber, needlepunched geotextile. The Geotex 801 fibers form a stable network that retains dimensional stability. As with Geotex 4x4, Geotex 801 resists ultraviolet breakdown and biological and chemical degradation, and both geotextiles meet or exceed the American Association of State Highway and Transportation Officials' (AASHTO) M288-96 guidelines.

Construction to repair the site began in mid-April. Ragen prepared installation instructions with help from SI Geosolutions. The construction team used a smooth-drum static roller to smooth and compact the subgrade without disturbing the water beneath. "Vibratory rollers tend to liquefy the soil underneath the fabric, and it makes it very difficult to stabilize the project," explains Ragen. Geotex has equal strength whether rolled parallel or perpendicular to machine direction, so NJDOT decided to roll the Geotex 4x4 parallel to machine direction to minimize overlap, keeping in mind that AASHTO recommends a 1-m minimum overlap length. Eight inches of sand were placed to serve as a drainage layer. The workers were careful to keep heavy machinery off the roadway until the DGA layer was placed. If any rutting occurred, they filled in the ruts with additional sub-base material instead of cutting off the peaks, which allowed them to achieve proper grade. The first portion of this massive project was finished in mid-June.

The remaining portion of Route 9 South still is under construction. Approximately 23,000 yd.² of Geotex 4x4 and 23,000 yd.² of Geotex 801 have been used in this project to date, and Ragen expects that upon completion the project will use an additional 23,000 yd.² of each geotextile. According to Ragen, the project thus far has been very successful, and everyone seems happy with the results. "There is discussion in the NJDOT about putting Geotex in their standard contracts. If this occurs, Geotex would be used in all applicable NJDOT projects."

The use of SI Geosolutions's geotextiles was so well received that NJDOT elected to use the same construction solution on the bridge approaches for the new three-lane and shoulders bridge to carry US Route 1 North over Route 9 South. Geotex geotextiles also will be used on the next project immediately north where Routes 1 and 9 cross Route 35, which will replace the first cloverleaf interchange ever built in the United States.

Reinforcement, Separation, Stabilization, and Filtration
Geotextiles Help Prevent Erosion and Preserve Billions of Gallons of Pristine Colorado Water

Colorado's Snowmass Canyon is home to some of the most spectacular scenery and the purest water in the world, courtesy of the Roaring Fork River. The river is a 70-mi.-long ribbon of pristine, scenic water that starts as a trickle at 12,900 ft. before pouring approximately 325 billion gal. of water into the Colorado River each year.

When it became necessary to expand the Snowmass Canyon portion of Colorado State Highway 82 (SH82), environmental and aesthetic concerns were at the top of the list. Environmental priorities included tree preservation and effective management of wildlife migration. In addition, there was the concern of rerouting motorists during the construction, since SH82 carries a large amount of daily traffic. The Snowmass Canyon portion of SH82, which runs parallel to the Roaring Fork River, is the final project to widen the highway that connects Glenwood Springs to Aspen. This portion of roadway is very shaded during the winter months and at times becomes quite treacherous to drivers who have to negotiate the canyon.

Figure 4. Left: Before expansion. Right: Digital rendering of expansion

Snowmass Canyon's section of SH82 will have 3.7 mi. of new, four-lane, divided highway to replace the existing two-lane road (see Figure 4). The projected approximate cost of the project is $105 million, and the construction will involve moving nearly 900,000 yd.³ of earth and rock and installing six bridges and 37 retaining walls between October 2000 and the fall of 2005.

The Colorado Department of Transportation (CDOT) planned the Snowmass Canyon SH82 expansion to resemble Interstate 70 through Glenwood Canyon, which also travels through many steep and environmentally sensitive areas. Snowmass Canyon is a complex geologic area with slopes in excess of 500 ft. Engineers Yeh and Associates provided a geotechnical investigation using helicopter-transported drilling rigs, a slope-stability evaluation, and a retaining wall design. The design included blasting and excavating the sides of mountains adjacent to the highway to make room for the new road system, as well as an excavation with an MSE wall of up to 20 ft. Precast concrete panels up to 40 ft. high also will be placed to support the new road system, which is very different from traditional designs. The SH82 panels will be created using an aesthetically pleasing, textured concrete design and then tinted with an earth-tone dye.

Figure 5. Geotex geogrid

CDOT selected Ames Construction as the general contractor, and the two partners began working together to develop environmentally sound and cost-effective construction methods. "It's called value engineering," describes Joe Elsen, CDOT resident engineer. "When we do that, we achieve the same or better quality and the same level of service, but we may use some different materials or some different techniques. There's always a different way to skin a cat." CDOT and Ames used more than 250,000 yd.² of SI Geosolutions' Geotex geogrids to construct the embankment on which the concrete panels are positioned. Geotex geogrids reinforce soils in the construction of retaining walls, steepened slopes, and embankments over soft soils (see Figure 5). The performance of these high-strength geogrids has been proven in countless applications around the US.

In addition to their performance capabilities, CDOT and Ames selected SI Geosolutions' geogrids because they could deliver varying strengths in a timely manner. This gave the project flexibility and convenience, which allowed the state and the contractor to maximize the cost-effectiveness of the design of the MSE wall beneath the tilt-up system.

Figure 6. Mechanically stabilized earth embankment construction

Ames installed the Geotex geogrids after overexcavating the subgrade 12-24 in., which then was used to build up the MSE embankment supporting the precast panel walls (see Figure 6).

In addition to the geogrids, approximately 25,000 yd.² of Geotex 4x4 medium-strength geotextile were incorporated into the MSE bridge abutments and temporary wrapped-face walls. Also used to extend the life of the roadway were 170,000 yd.² of Geotex 701, which is a nonwoven polypropylene staple fiber needlepunched geotextile. The Geotex 701 was used to provide separation and prevent the intermixing of the dissimilar aggregate road base and the underlying soil substrate. The mixing of these two materials is a leading cause of roadway failure. As with Geotex 4x4, Geotex 701 resists ultraviolet breakdown and biological and chemical contamination. Both geotextiles exceed AASHTO guidelines.

By using innovative geosynthetic solutions in the construction of SH82, CDOT and Ames have provided the project with more than just monetary savings and an aesthetically pleasing appearance. Installation of the geosynthetics is ongoing and has been perceived by the contractor, engineer, and CDOT as successful to this stage. SI's Landlok erosion control blankets and Geotex geogrids and geotextiles have offered the state and the project's contractor a cost-effective and flexible method of reinforcing the subgrade and the MSE subembankment system beneath the very heavy and very expensive precast panel system. Perhaps more importantly, they will offer long-term environmental benefits that Snowmass Canyon's residents and tourists will appreciate for years to come.

Economics

Obviously, if a construction alternative does not make or save money for the contractor compared to conventional methods, it is not worth their attention. The following are some thoughts that contractors, designers, and specifiers should consider when evaluating the possible use of geosynthetics on transportation construction projects:

Labor and Time

When working on soft-soil sites, there is nothing more frustrating than watching the contractor "farm" the soil to get it dry enough to establish a stable work platform - especially on fast-track construction projects. These methods are tried and true and effective but are not the best use of time, especially if the weather is not favorable. Depending on site conditions, the use of a geotextile or a geogrid in the design can help establish the work platform and ensure long-term performance. As previously noted, plenty of reference materials are available to aid in designing and constructing through weak subgrade areas. Thus, there are potential savings not only in time and labor but also in the materials saved by not removing and replacing poor soils with select fill or chemical stabilization.

Materials

Material costs and many other issues affect the bottom-line life cycle cost of pavement structures. Again, many references, including technical memoranda from various manufacturers, describe the use and benefits of geotextiles and make recommendations for their incorporation in pavement structures. None of these documents indicates direct life cycle cost effects, however.

The attached chart (see Figure 7) demonstrates the benefits of using geosynthetics as only a separation layer between the aggregate base and the subgrade soil without regard to additional drainage and reinforcement benefits. In soft-subgrade soils with a resilient modulus (M_R) less than or equal to 3,300 psi (CBR ¾3), pavement technologists accept that approximately the bottom 10% of the aggregate base thickness is contaminated by the subgrade soil, thus reducing the overall pavement structure.

Figure 7. 18-Kip Equilavent Singel Axel Loads vs. Structural Number for Subgrade Resilient Modulus 3,00 psi (CBR 3)

The top line on the chart illustrates the number of 18,000-lb. equivalent single-axle load (ESAL) applications that the roadway can accommodate during the design life if the full structure actually is placed. The bottom line shows the corresponding ESAL application reduction for a 10% structure loss due to construction, such as the aggregate being forced into the subgrade. As can be seen, the effect is not linear and worsens at greater loading or load applications. This strength reduction is not a big factor in conventional construction projects where the contract is paid based on quantities and/or layer thicknesses and density requirements because there are no direct correlations between these field measures and long-term pavement performance.

On warranty projects and/or projects where strength testing is used for quality control, however, this strength reduction is significant. On warranty projects, the contractor has three options:

1. Construct the roadway based on design quantities and layer thicknesses knowing that the subgrade soils likely are contaminating the bottom of the aggregate base layer. In this scenario, the contractor is taking the risk that premature pavement distress might occur during the warranty period due to a reduced pavement structure. The result is expensive pavement repairs that are paid for by reducing profits.
2. Construct the roadway based on a 10% increase in design quantities and layer thicknesses knowing that the aggregate base layer contamination is taken into account. In this scenario, the contractor takes an immediate hit by paying for additional materials up front. The result of this method is long-term pavement performance ensured by incorporating additional material that was paid for by reducing profits.
3. Construct the roadway according to the design, and maybe nothing detrimental will happen. The result is sleepless nights for the contractor during the warranty period.

On projects where insitu strength testing is used for quality control (i.e., falling weight deflectometers, road raters, and dynamic cone penetrometers), it will be apparent relatively quickly if the strength associated with an aggregate base thickness is not achieved. If this occurs, placing additional material is no longer an option because the as-built elevations will be incorrect. Therefore the remedies most likely will be remove-and-replace and/or some other type of strength remediation, such as chemical grouting/injection.

References

A tremendous amount of information is available regarding the use of geosynthetics. Recognizing the appetite the design and specifying community has for generic information on geosynthetics, the following reference list was compiled to aid in the promotion of geotextiles and geogrids specifically for use in transportation constructs.

Author Ariel Soriano, P.E., is a senior regional engineer with SI Geosolutions.

GEC - March/April 2004

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