Engineering and Construction

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.