Engineering and Construction

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Geosynthetic clay liners are frequently used as liners for municipal landfills. The geosynthetic clay liner typically consists of dry bentonite sandwiched between two geosynthetics. When moisture infiltrates the geosynthetic clay liner, the bentonite swells and creates a soil layer having a very low hydraulic conductivity, transforming it into an effective barrier to moisture migration.

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Geonets are three-dimensional netlike polymeric materials used for drainage (i.e., flow of water within the geosynthetic). Figure 17.5 shows a photograph of a geonet. Geonets are usually used in conjunction with a geotextile and/or geomembrane, hence geonets are technically a geocomposite.

Depending on the particular project requirements, different types of geosynthetics can be combined together to form a geocomposite. For example, a geocomposite consisting of a geotextile and a geomembrane provides for a barrier that has increased tensile strength and resistance to punching and tearing. Figure 17.6 shows a photograph of a geocomposite consisting of a textured geomembrane, geonet, and geotextile (filter fabric).

FIGURE 17.5 Photograph of a geonet.

FIGURE 17.6 Photograph of a geocomposite. The geocomposite consists of a geonet having a textured geomembrane on top, and a filter fabric (geotextile) on the bottom.

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Common construction terminology for geomembranes includes liners, membranes, visqueen, plastic sheets, and impermeable sheets. Geomembranes are most often used as barriers to reduce water or vapor migration through soil (see Fig. 17.4). For example, Fig. 12.20 shows design specifications for a below foundation moisture barrier that includes a 6-mil visqueen vapor barrier (i.e., a geomembrane). In the United States, 1 mil is one-thousandth of an inch. Another common usage for geomembranes is for the lining and capping systems in municipal landfills. For liners in municipal landfills, the thickness of the geomembrane is usually at least 80 mil. The surface of the geomembrane can be textured in order to provide more frictional resistance between the soil and geomembrane surface.

FIGURE 17.4 Photograph of a geomembrane that has a surface texture for added friction.

Some of the limitations of geomembranes are as follows:

1. Puncture resistance. The geomembrane must be thick enough so that it is not punctured during installation and subsequent usage. The puncture strength of a geomembrane can be determined by using the test procedures outlined in ASTM D 4833-00 “Standard Test Method for Index Puncture Resistance of Geotextiles, Geomembranes, and Related Products,” 2004.
2. Slide resistance. Slope failures have developed in municipal liners because of the smooth and low frictional resistance between the geomembrane and overlying or underlying soil. Textured geomembranes (such as shown in Fig. 17.4) have been developed to increase the frictional resistance of the geomembrane surface.
3. Sealing of seams. A common cause of leakage through geomembranes is due to inadequate sealing of seams. The following are different methods commonly used to seal geomembrane seams (Rollings and Rollings, 1996):

      a. Extrusion welding: Suitable for all polyethylenes. A ribbon of molten polymer is extruded over the edge (filet weld) or between the geomembrane sheets (flat weld). This melts the adjacent surfaces that are then fused together upon cooling.

      b. Thermal fusion: Suitable for thermoplastics. Adjacent surfaces are melted and then pressed together. Commercial equipment is available that uses a heated wedge (most common) or hot air to melt the materials. Also, ultrasonic energy can be used for melting rather than heat.

      c. Solvent-based systems: Suitable for materials that are compatible with the solvent. A solvent is used with pressure to join adjacent surfaces. Heating may be used to accelerate the curing.

The solvent may contain some of the geomembrane polymer already dissolved in the solvent liquid (bodied solvent) or an adhesive to improve the seam quality.

      d. Contact adhesive: Primarily suitable for thermosets. Solution is brushed onto surfaces to be joined, and pressure is applied to ensure good contact. Upon curing, the adhesive bonds the surfaces together.

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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.

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.

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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Figure 17.1 shows a photograph of a geogrid, which contains relatively high-strength polymer grids consisting of longitudinal and transverse ribs connected at their intersections. Geogrids have a large and open structure and the openings (i.e., apertures) are usually 0.5 to 4 in. (1.3 to 10 cm) in length and/or width. Geogrids can be either biaxial or uniaxial depending on the size of the apertures and shape of the interconnecting ribs. Geogrids are principally used as follows:

1. Soil reinforcement. Used for subgrade stabilization, slope reinforcement, erosion control (reinforcement), and mechanically stabilized earth-retaining walls. Also used to strengthen the junction between the top of soft clays and overlying embankments.

2. Asphalt overlays. Used in asphalt overlays to reduce reflective cracking.
The most common usage of geogrids is as soil reinforcement. Compacted soil tends to be strong in compression but weak in tension. The geogrid is just the opposite, strong in tension but weak in compression. Thus, layers of compacted soil and geogrid tend to compliment each other and produce a soil mass having both high compressive and tensile strength. The open structure of the geogrid (see Fig. 17.1) allows the compacted soil to bond in the open geogrid spaces. Geogrids provide soil reinforcement by transferring local tensile stresses in the soil to the geogrid. Because geogrids are continuous, they also tend to transfer and redistribute stresses away from areas of high stress concentrations (such as beneath a wheel load). "Figure 11.10 Two views of the installation of the geogrid for a mechanically stabilized earthretaining wall. The large arrows indicate the width of the mechanically stabilized zone and thesmall arrow points to a geogrid splice."  shows geogrids being used as soil reinforcement for a mechanically stabilized earth retaining wall.

FIGURE 17.1 Photograph of a geogrid. (From Rollings and Rollings 1996; reprinted with permission of McGraw-Hill, Inc.)

Similar to other geosynthetics, geogrids are transported to the site in 3 ft (0.9 m) to 12 ft (3.7 m) wide rolls. It is generally not feasible to connect the ends of the geogrid, and it is typically over- lapped at joints. Typical design methods for using geogrids are summarized by Koerner (1998).

Some of the limitations of geogrids are as follows:

1. Ultraviolet light. Even geogrids produced of carbon black (i.e., ultraviolet stabilized geogrids) can degrade when exposed to long-term ultraviolet light. It is important to protect the geogrid from sunlight and cover the geogrid with fill as soon as possible.

2. Nonuniform tensile strength. Geogrids often have different tensile strengths in different directions as a result of the manufacturing process. For example, a Tensar SS-2 (BX1200) biaxial geogrid has an ultimate tensile strength of 2100 lb/ft in the main direction and only 1170 lb/ft in the minor (perpendicular) direction. It is essential that the engineer always check the manufacturer’s specification and determine the tensile strength in the main and minor directions.

3. Creep. Polymer material can be susceptible to creep. Thus, it is important to use an allowable tensile strength that does allow for creep of the geosynthetic. Tensile strengths are often determined by using ASTM test procedures, such as ASTM D 6637-01 (“Standard Test Method for Determining Tensile Properties of Geogrids by the Single or Multi-Rib Tensile Method,” 2004) and ASTM D 5262-04 (“Standard Test Method for Evaluating the Unconfined Tension Creep Behavior of Geosynthetics,” 2004).

Many manufacturers will provide their recommended long-term design tensile strength for a specific type of geogrid. This recommended long-term design tensile strength from the manufacturer is usually much less than the ultimate strength of the geogrid. For example, for a Tensar SS-2 (BX1200) biaxial geogrid, the manufacturer’s recommended long-term design tensile strength is about 300 lb/ft, which is only one-seventh the ultimate tensile strength (2100 lb/ft). The engineer should never apply an arbitrary factor of safety to the ultimate tensile strength, but rather obtain the recommended long-term design tensile strength from the manufacturer.