GeoInsighter Summer 2002 Newsletter

Geotextiles - High Tech Blankets

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An article in a previous GeoInsighter newsletter provided an introduction to geosynthetic materials and their common uses. This article is the beginning of a series that will describe more in-depth applications for certain geosynthetics and, hopefully, increase your knowledge and use of these materials.

Geotextiles is the technical term applied to synthetic blankets, commonly referred to as filter fabric, that are used to separate two materials, act as a drainage filter between two materials, or provide tensile reinforcement within a material. The fabrics industry unknowingly created these original geosynthetics when experiments in the construction world identified the benefits of laying a layer of cloth to separate two different types of soil or rock material or to retain soil while allowing water to pass through. Because these original applications used materials such as cotton, which is biodegradable, synthetic materials were eventually used to create the materials, greatly increasing strength and durability. Most geotextiles in use today are manufactured of polyethylene, polypropylene, polyester, or nylon, materials that allow manufacturers to carefully control the desired properties of a final product.

Geotextiles are generally characterized as woven or non-woven fabrics and come in a very wide variety of thicknesses, strengths, colors, and textures. Woven fabrics are manufactured in a process similar to cloth and consist of perpendicular single strands or multiple strands of materials with a tight weave or small openings at the intersections of the strands. Non-woven fabrics consist of a felt-like material where randomly oriented strands are bonded together by needle punching (which more or less tangles all of the strands together) or by heat bonding (which melts enough of the strands together to hold the fabric together as a blanket of a relatively uniform thickness).

Common uses for geotextiles include:

• Separation - used between dissimilar soil and/or aggregate materials to prevent smaller-sized particles from moving into a zone of larger-sized particles (such as behind a rip-rap stone facing along a beach front slope) or to prevent larger-sized particles from being pushed into a finer base (such as between a stone walkway and a sandy subgrade).

• Filtration - used to allow transmission of water or other fluids while retaining particles (such as between a wet soil and a stone drainage trench, as a wrap around a perforated drain pipe buried below the water table, or as a flexible form for concrete). Perimeter silt fence commonly seen around construction areas is a good example of filter fabric in use to retain soil particles while allowing water to pass through.

• Erosion control - when laid out onto the ground surface, filter fabrics dissipate the energy of falling precipitation; and when weighted by a covering of surface stone to hold the fabric in place, it can be used to press down upon exposed/erodible soils to help keep them in place.

• Water or Air Transmission - significant flow of water or gas through a fabric is possible in the same plane as the fabric (as opposed to perpendicular to the plane of the fabric for filtration applications) and thicker non-woven fabrics can provide preferential pathways to help control water or gas flow (such as when used against the outside of foundation walls or below basement slabs). 

• Reinforcement - fabrics with high strengths are installed within constructed layers of soil and aggregate to increase bearing stability over weak layers (such as when used between a soft subgrade and pavement), placed in between layers of a backfilled embankment to reduce the chance of internal failure of the soil, placed in between backfilled layers of an embankment to connect retaining wall facing material to the backfilled soil, and embedded in layers of soil and then anchored at the top of a slope to help hold a layer in place (such as in a landfill cap).

Geotextiles are also used as a cushion to protect sensitive surfaces, such as geomembrane liners in ponds and landfills, from being punctured by sharp objects and are a major component in composite geosynthetic systems. The design of a project that requires the use of a geotextile will consider the effective opening size of the material and how easily it might clog or how well it will separate materials; the tearing, puncture, and overall tensile strength requirements for fabric's application; whether it will be damaged during installation; whether it will creep or stretch over time; potential exposure to sunlight or other chemicals that might cause degradation or strength loss; and the life expectancy of the design. A well-conceived design will allow a single geotextile to meet multiple functional criteria (i.e., separation, filtration, and water transmission).

Most manufacturers of filter fabrics have very informative websites that provide case studies and material specifications to assist in the identification of appropriate materials and project design. GeoInsight can provide you with additional information regarding geotextiles upon request.

Michael C. Penney, P.E., L.S.P.
mcpenney@geoinc.com

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