Types of Geotextiles
- Very strong
- Do not "elongate" or stretch very much when a force is applied
- Made of woven synthetic fabric (usually polypropylene or polyester) by weaving yarns together...just like your dress shirt or a linen tablecloth
- Stronger as thickness increases
- Highly permeable
- Able to stretch and take the shape of the adjacent surface
3 General Types of Woven Geotextile
1. A "slit tape" fabric has a flat tape-like strand produced by slitting and weaving a solid sheet of extruded film. These fabrics are the cheapest wovens and are typically used in road stabilization/separation applications. A general price range would be 10 to 20 cents per square foot.
2. A "monofilament" fabric has strands which are like individual fishing lines. It is much higher quality than the slit-tapes and is correspondingly more expensive (generally 20 to 30 cents per square foot). Monofilaments are typically used for erosion control and drainage purposes.
3. A "multifilament" consists of many fine continuous filaments that are held together by twisting or intermingling the strands. Generally, multifilament fabrics are not commonly used for routine projects.
2 General Types of Nonwoven
Nonwoven fabrics can range in thickness from a thin, lightweight material (4 oz./s.y.) to a fairly thick felt-type material (over 16 oz./s.y.). They are typically used for drainage purposes, such as in gravel underdrains. The thinner nonwovens generally cost 8 to 12 cents per square foot. The two most common nonwoven types are heat-bonded and needle-punched.
Primary Uses for Geotextiles
There are four primary uses of geotextiles in roadway construction and maintenance:
- Separation or Stabilization: The geotextile is used to permanently separate two distinct layers of soil in a roadway. The classic example is where a road is to be built across a poorly drained, fine-grained soil (clay or silt) and a geotextile is laid down prior to placing gravel. This keeps the soft, underlying soil from working its way up into the expensive gravel and it keeps the gravel form punching down into the soft soil. The full gravel thickness remains intact and provides full support for many years. Typically, woven and nonwoven geotextiles are used in this application. If a woven product is used, it should be at least 4 oz./sq. yd. and could be a "slit-tape" or monofilament" type for routine, non-critical situations. If a nonwoven product is used, it should be at least 8 oz./sq. yd. for survivability during construction.
- Drainage: The geotextile acts as a filter through which water passes while it restricts fine-grained soil from entering into coarse-grained soil (sand or gravel). An example is in an underdrain where gravel-filled trenches lined with a geotextile fabric are constructed along the edges of roads. The fabric allows water to drain into the trench, while it permanently separates the different soil materials. The gravel remains clean and cannot "plug up" with fine material. Not only can it be used in roadways, but also under parking lots, walls, athletic fields, lawns, tennis courts, and other areas. Normally, nonwoven fabrics are used because of their small pore size (opening size) and high flow capacity. They should be at least 4 oz./sq. yd. If installation stresses are more severe such as where sharp angular aggregate is in contact with the fabric, or a heavy degree of compaction is required, then a heavier nonwoven with a minimum of 8 oz./sq. yd. should be used. Woven fabrics can be used but they should be of the "monofilament" variety. "Slit-tape" wovens should NOT be used for drainage applications because of their poor capacity to pass water.
- Erosion Control: A layer of heavy stones or broken rocks (riprap) is commonly used to provide erosion protection for stream banks, culverts, ditches, stream channels, shorelines, and bridge structures. A geotextile placed between the rock layer and the underlying soil surface provides anchorage of the underlying soil and protects it from erosion and wave attack. Two key properties are important for proper erosion control. It must have sufficient capacity to pass water, especially if water is coming from behind the fabric. Second, the geotextile must be able to retain the finer soil particles under the fabric. Typical geotextiles used for erosion control are medium weight (8 oz./sq. yd.) nonwoven fabrics or "monofilament" woven fabrics. In some instances where the riprap is rounded or the fabric is protected by a this sand cushion before the riprap is placed, a lighter weight fabric (4 oz./sq. yd.) could be used, if care is exercised during riprap placement.
- Reinforcement: In some areas, construction is proposed in "soft" areas where the foundation soils are too weak to support a road or structure. Without sufficient reinforcement, the foundation cannot "hold up" the structure and it fails at considerable expense. When this condition exists, usually a soils engineer is needed to design the facility and the underlying geotextile and/or geogrid.
Selection will depend on the actual soil and hydraulic conditions, the following general considerations seem appropriate for the soil conditions given:
- Graded gravels and coarse sands - Very open monofilament or multifilament wovens may be required to permit high rates of flow and a low risk of blinding.
- Sands and gravels with less than 20% fines (very "dirty" or silty sand and gravel) - Open monofilament wovens and needlepunched nonwovens with large openings are preferable to reduce the risk of blinding. For thin heat-bonded nonwoven geotextiles and thick needlepunched nonwoven geotextiles, filtration tests should be performed.
- Soils with 20% to 60% fines (silt or silty sand) - Filtration tests should be performed on all fabric types.
- Soils with greater than 60% fines (silt or clayey silt) - Heavy weight needlepunched and heat-bonded nonwoven geotextiles tend to work best as fines will not pass. If blinding does occur, the permeability of the blinding cake would equal that of the soil.
- Gap graded cohesionless soils - Consider using a uniform sand filter with a woven monofilament as a filter for the sand.
- Silts with sand seams - Consider using a uniform sand filter over the soil with a woven geotextile to prevent movement of the filter sand; alternatively, consider using a heavy weight (thick) needlepunched nonwoven directly against soil as water can flow laterally through the geotextile should it become locally clogged.
These general observations are not meant as recommendations, but to provide some insight into the various considerations for selecting the optimum material. They are not intended to exclude other possible geotextiles that you may want to consider.
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