GEOSYNTHETICS: Geotextiles for Foundations Engineering.

Geotextiles are the most widely used type of geosynthetic and they are often referred to as fabric. For example, common construction terminology for geotextiles includes geofabric, filter fabric, construction fabric, synthetic fabric, and road-reinforcing fabric. As shown in Figs. 17.2 and 17.3, geotextiles are usually categorized as being either woven or nonwoven depending on the type of manufacturing
process. Geotextiles are principally used as follows:

1. Soil reinforcement. Used for subgrade stabilization, slope reinforcement, and mechanically stabilized earth-retaining walls. Also used to strengthen the junction between the top of soft clays and overlying embankments.
2. Sediment control. Used as silt fences to trap sediment on-site.
3. Erosion control. Installed along channels, under riprap, and used for shore and beach protection.
4. Asphalt overlays. Used in asphalt overlays to reduce reflective cracking.
5. Separation. Used between two dissimilar materials, such as an open-graded base and a clay
subgrade, in order to prevent contamination.
6. Filtration and drainage. Used in place of a graded filter where the flow of water occurs across (perpendicular to) the plane of the geotextile. For drainage applications, the water flows within the geotextile.

Photograph of nonwoven geotextiles. The geotextile on the left has no ultravi- olet protection, while the geotextile on the right has ultraviolet protection.
FIGURE 17.2 Photograph of nonwoven geotextiles. The geotextile on the left has no ultravi-
olet protection, while the geotextile on the right has ultraviolet protection.

Photograph of a woven geotextile.
FIGURE 17.3 Photograph of a woven geotextile.

Probably the most common usage of geotextiles is for filtration (i.e., flow of water through the geotextile). For filtration, the geotextile should be at least 10 times more permeable than the soil. In addition, the geotextile must always be placed between a less permeable (i.e., the soil) and a more permeable (i.e., the open-graded gravel) material. An inappropriate use of a geotextile would be to place it around the drainage pipe, because then it would have more permeable material on both sides of the geotextile and it would tend to restrict flow.

Geotextiles to be used as filtration devices must have adequate hydraulic properties that allow the water to flow through them and they must also retain the soil particles. Important hydraulic properties are as follows:

1. Percent open area. Although geotextiles have been developed that limit the open area of filtration to 5 percent or less, it is best to have a larger open area to develop an adequate flow capacity.
2. Permittivity or flow rate. Manufactures typically provide the flow capacity of a geotextile in terms of its permittivity or flow rate. These hydraulic properties are often determined by using ASTM test procedures, such as ASTM D 4491-99, “Standard Test Methods for Water Permeability of Geotextiles by Permittivity Soil Retention Capability,” 2004.
3. Apparent opening size. The apparent opening size (AOS), also known as the effective opening size (EOS), determines the soil retention capability. The AOS is often expressed in terms of opening size (mm) or equivalent sieve size (e.g., AOS = 40–70 indicates openings equivalent to the No. 40 to No. 70 sieves). The test procedures in ASTM D 4751-99, “Standard Test Method for Determining Apparent Opening Size of a Geotextile,” can be used to determine the AOS. Obviously, if the geotextile openings are larger than the largest soil particle diameter, then all of the soil particles will migrate through the geotextile and clog the drainage system. A common recommendation is that the required AOS be less than or equal to D85
(grain size corresponding to 85 percent passing).

Some of the limitations of geotextiles are as follows:

1. Ultraviolet light. Geotextiles that have no ultraviolet light protection can rapidly deteriorate. For example, certain polypropylene geotextiles lost 100 percent of their strength after only 8 weeks of exposure (Raumann, 1982; Koerner, 1998). Manufacturers will often list the ultraviolet light resistance after 500 h of exposure in terms of the percentage of remaining tensile resistance based on the test procedures in ASTM D 4355-02 “Standard Test Method for Deterioration of Geotextiles by Exposure to Light, Moisture and Heat in a Xenon Arc Type Apparatus,” 2004.
2. Sealing of the geotextile. When used for filtration, an impermeable soil layer can develop adjacent to the geotextile if it has too low an open area or too small an AOS.
3. Construction problems. Some of the more common problems related to construction with geo- textiles are as follows (Richardson and Wyant, 1987):
   a. Fill placement or compaction techniques damage the geotextile.
   b. Installation loads are greater than design loads, leading to failure during construction.
   c. Construction environment leads to a significant reduction in assumed fabric properties, causing failure of the completed project.
   d. Field seaming or overlap of the geotextile fails to fully develop desired fabric mechanical properties.
   e. Instabilities during various construction phases may render a design inadequate even though the final product would have been stable.

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