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The Benefits of Utilizing a Continuous Moisture Management (Wicking) Geotextile Horizontally Within a Pavement Section

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Course Information

There are five key performance functions associated with typical geosynthetics (geotextile or geogrid) that determine performance in unpaved and paved roadway applications. In addition to these five key functions, geotextiles with wicking technology that offers continuous moisture management provide enhanced lateral drainage and continuous water management.

Key Performance Functions

1. Separation: Prevention of subgrade soil intruding into the aggregate base (or subbase) and prevention of aggregate base (or subbase) from migrating into the subgrade. When effective separation is provided, the result is a perpetual aggregate base. Typically, when fine-grained and/or soft soils are present, a separation geotextile is required to maintain the integrity of the base course aggregate. Geogrids do not provide separation on their own due to the open apertures. Therefore, to prevent subgrade materials from piping up through the geogrid apertures into the aggregate, a filtration layer (typically sand) is required or a separation geotextile (i.e., fabric) is used to provide filtration.

2. Filtration: Restricting the movement of soil particles while allowing water to move perpendicularly through the geosynthetic from the filtered soil to the coarser soil adjacent to it during the performance life of the structure. Civil engineers typically rely on the AOS (O95) value of a geotextile when selecting a product for soil filtration. The AOS value can be misleading, as this primarily identifies only the largest opening(s) in the geotextile and not the range of opening sizes (i.e., pore size distribution (PSD)), which is an essential factor in determining filtration capability of the geotextile. PSD of the geotextile can’t be obtained using ASTM D 4751, but PSD information obtained using ASTM D6767 (porometer test) provides information regarding configuration of the openings, hydraulic flow capabilities and the open area of the geotextile. The O95 and O50 information obtained provides critical opening size data for comparison to the soil particle size distribution. This enables a much more accurate determination of the geotextile’s filtration capabilities.

3. Drainage (i.e., transmissivity or in-plane permeability): Flow of water within the plane of the geotextile. Typically, a geotextile installed at the subgrade interface with a gradient shall enhance drainage below the paved or unpaved roadway. This is totally dependent upon gravity and the amount of pore space within the geosynthetic.

4. Reinforcement/Confinement: In the early development of geosynthetics, the “tensioned membrane” effect was believed to be the primary mechanism for geosynthetic reinforcement; the concept of an improved vertical stress distribution results from tensile stress in a deformed membrane, which results in the addition of structural load-carrying capacity to a paved or unpaved roadway system by the transfer of load to the geosynthetic material. However, this requires deformation of the geosynthetic as well as the subgrade and base aggregate, so immediate reinforcement effects are unlikely, as reinforcement would occur after rutting.

Author

Rich Sack, PE, Senior Engineer/Technical Filtration & Drainage Solmax Americas

Learning Objectives

At the conclusion of this article, the reader should be able to understand/perform the following:

• The five key performance functions associated with typical geosynthetics that determine performance in unpaved and paved roadway applications.

• How geotextiles with wicking technology can provide enhanced lateral drainage and continuous water management.

• How changes in moisture content in soils and aggregates directly affects shear strength.

• How to determine the hydraulic improvement factor.

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