Geotextiles

Geosynthetics

Geosynthetics are polymeric products which are widely used in many geotechnical and environmental applications related to groundwater quality and control. It can be defined as planar products manufactured from polymeric material, which are used with soil, rock, or other geotechnical engineering-related material as an integral part of a man-made project, structure, or system (ASTM, 1995).

Among the different geosynthetic products, geotextiles are the ones that present the widest range of properties. They can be used to fulfill the different functions such as Separation, Reinforcement, Filtration, Drainage Infiltration barrier and protection for many different geotechnical, environmental, and groundwater applications. For example, Figure 1. shows the construction of a reinforced slope in which geotextiles were selected as multipurpose inclusions within the fill, because they can provide not only the required tensile strength (reinforcement function), but also the required transmissivity (drainage function) needed for that particular project (Zornberg et al., 1996).

FIGURE 1. Placement of a high-strength nonwoven geotextile to perform a dual function of reinforcement and in-plane drainage in a reinforced slope.

Geotextiles are manufactured from polymer fibers or filaments which are later formed to develop the final product. Approximately 75% of the geotextiles used today are based on polypropylene resin. An additional 20% are polyester, and the remaining 5% is a range of polymers including polyethylene, nylon, and other resins used for specialty purposes. As with all geosynthetics, however, the base resin has various additives, such as for ultraviolet light protection.

The most common types of fibers used in the manufacture of geotextiles are monofilament, staple, and slit-film. If fibers are twisted or spun together, they are known as a yarn. Monofilament fibers are created by extruding the molten polymer through an apparatus containing small-diameter holes. The extruded polymer strings are then cooled and stretched to give the fiber increased strength. Staple fibers are also manufactured by extruding the molten polymer; however, the extruded strings are cut into 25- to 100-mm portions. The staple fibers may then be spun into longer fibers known as staple yarns. Slitfilm fibers are manufactured by either extruding or blowing a film of a continuous sheet of polymer and cutting it into fibers by knives or lanced air jets. Slit-film fibers have a flat, rectangular cross-section instead of the circular cross-section shown by the monofilament and staple fibers.

The fibers or yarns are formed into geotextiles using either woven or nonwoven methods. Figure 2. shows a number of typical woven and nonwoven geotextiles. Woven geotextiles are manufactured using traditional weaving methods and a variety of weave types. Nonwoven geotextiles are manufactured by placing and orienting the fabrics on a conveyor belt and subsequently bonding them by needle punching or melt bonding. The needle-punching process consists of pushing numerous barbed needles through the fiber web. The fibers are thus mechanically interlocked into a stable configuration. As the name implies, the heat (or melt) bonding process consists of melting and pressurizing the fibers together.

Common terminology associated with geotextiles includes machine direction, cross machine direction, and selvage. Machine direction refers to the direction in the plane of fabric in line with the direction of manufacture. Conversely, cross machine direction refers to the direction in the plane of fabric perpendicular to the direction of manufacture. The selvage is the finished area on the sides

FIGURE 2. Typical woven and nonwoven geotextiles.

of the geotextile width that prevents the yarns from unraveling. Adjacent rolls of geotextiles are seamed in the field by either overlapping or sewing. Sewing is generally the case for geotextiles used as filters in landfill applications but may be waived for geotextiles used in separation. Heat bonding may also be used for joining geotextiles in filtration and separation applications.

Numerous tests have been developed to evaluate the properties of geotextiles. In developing geotextile specifications, it is important that the designer understands the material tests and that he or she specifies material properties important for the geotextiles’ intended use.