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Geosynthetics for Trails in Wet Areas: 2008 Edition


Geosynthetics—General Information

Geosynthetics have numerous uses in civil engineering. The basic functions of geosynthetics include:

  • Reinforcement—The geosynthetic acts as a reinforcing element in a soil mass or in combination with the soil to produce a composite that has improved strength and deformation properties. For example, geotextiles and geogrids are used to add tensile strength to a soil mass when there are vertical or near-vertical changes in grade (reinforced soil walls).

  • Separation—The geosynthetic acts to separate two layers of soil that have different particle size distributions. For example, geotextiles are used to prevent road base materials from penetrating into soft underlying subgrade soils, maintaining design thickness and roadway integrity. Separators also help to prevent finegrained subgrade soils from being pumped into permeable granular road bases.

  • Drainage—The geosynthetic acts as a drain to carry fluid flows through less permeable soils. For example, geotextiles are used to dissipate pore water pressure at the base of roadway embankments.

  • Filtration—The geosynthetic acts like a sand filter by allowing water to move through the soil while retaining the soil particles. For example, geotextiles are used to prevent soils from migrating into drainage aggregate or pipes while maintaining flow through the system. Geotextiles are also used below riprap and other armor materials in coastal and riverbank protection systems to prevent soil erosion.

  • Containment—The geosynthetic acts as a relatively impermeable barrier to fluids or gases. For example, geomembranes, thin film geotextile composites, geosynthetic clay liners (GCLs), and field-coated geotextiles are used as fluid barriers to impede the flow of liquids or gases.

  • Erosion Control—The geosynthetic acts to reduce soil erosion caused by rainfall impact and surface water runoff. For example, temporary geosynthetic blankets and permanent lightweight geosynthetic mats are placed over the otherwise exposed soil surface on slopes. Geotextile silt fences are used to remove suspended particles from sediment-laden runoff. Some erosion control mats are manufactured using biodegradable wood fibers.

Geosynthetic materials (figures 1 and 2) include geotextiles (construction fabrics), geonets, geogrids, and geocomposites, such as sheet drains and geocells. All these materials become a permanent part of the trail, but must be covered with soil or rock to prevent damage by ultraviolet light. Geosynthetic erosion control material also has important uses for slope and bank protection, but this report does not discuss those uses.

Manufacturers of erosion control geosynthetics are listed in the "Geosynthetic Product Information" section. Please contact the manufacturers for additional information. Geoblock, Lockgrid, EcoGrid and Grasspave2 are used for turf reinforcement and will be discussed. Because all these products are synthetic, their use in wilderness should be reviewed and approved before they are used.

Drawing of a trail fill diagram without geotextile. In the drawing the text reads, Aggregate cap, Shear force, Cross contamination leads to impacts from shear stress., Ground surface, Aggregate migration, Upward movement of soil, and Substandard soil base.
Figure 1—Trail fill material without geotextile. The
aggregate will lose strength as the fill material mixes
with the subbase.

Drawing of a trail fill with geotextile. In the drawing the text reads, Aggregate cap, Ground surface, Drainage, Seperation, Substandard soil base, and Geotextile layer.
Figure 2—Trail fill material with geotextile. The geotextile
layer enhances the trail performance by providing separation,
reinforcement, and drainage.

Geotextiles

Geotextiles (figure 3) are the most widely used geosynthetic. Geotextiles are often called construction fabrics. They are constructed from long-lasting synthetic fibers that form a fabric held together by weaving, heat bonding, or other means. Geotextiles are primarily used for separation and reinforcement over wet, unstable soils. They have the ability to support loads through tensile strength and can allow water, but not soil, to seep through. They can also be used in drainage applications where water flow is much greater than normal for wet areas. The physical requirements listed for all geotextiles in the "Geosynthetic Product Information" section are stringent enough that the products will work for properly designed high-flow drainage applications.

Photo of three different types of geotextile fabrics.
Figure 3—Geotextiles are made from woven and
nonwoven fabrics. Felt-like products are easier to
work with than slick products that are heat bonded,
woven, or made from slit film. Felt-like products are
easier to cut and their flexibility makes them easier
to place on curved trail sections.

Geonets

Geonets or geonet composites (figure 4) have a thin polyethylene drainage core that is covered on both sides by geotextile. Geonets are primarily used for drainage, but also may function as separation and reinforcement. Because geonets have a core plus two layers of geotextile, they provide more reinforcement than a single layer of geotextile.

Photo of a geonet that is between two layers of geotextile.
Figure 4—Geonets with the two layers of geotextile
shown are considered a geocomposite—the core
of geonet allows drainage to the sides that is normally
adequate for the seepage found under trails
in wet areas. The geotextile provides reinforcement
and separation.

Geogrids

Geogrids (figure 5) are made from polyethylene sheeting that is formed into very open gridlike configurations. Geogrids are good for reinforcement because they have high tensile strengths and because coarse aggregate can interlock into the grid structure.

Photo of a ruler next to a geogrid.
Figure 5—Geogrids are normally placed on top
of a layer of geotextile for separation from
saturated soils in wet areas.

Geocells

Geocells (figure 6) are usually made from polyethylene strips 50 to 200 millimeters (2 to 8 inches) high that have been bonded to form a honeycomb. The product is shipped collapsed so it is more compact. During installation, the material is pulled open and the honeycomb structure is staked to the ground surface. Each of the cells is filled and compacted. Compacting trail tread material within the cell increases the strength of the layer and reduces settlement into soft, saturated soils. Geocells are good for reinforcement and reduce the amount of fill material required.

Photo of a geocell.
Figure 6—Geocell usually has geotextile under it
for separation from wet, saturated soil. Normally,
the cells are filled with a soil that drains well.

Geocomposites—Sheet Drains

Sheet drains (figure 7) are a form of geocomposite material made with a drainage core and one or two layers of geotextile. The core of a sheet drain usually is made of a polyethylene sheet formed into the shape of an egg crate. The core provides an impermeable barrier unless it has been perforated by the manufacturer. Perforated cores are always covered with geotextile on both sides to prevent soil from clogging the drainage passages. Geotextile is bonded to one or both sides of the core to provide filtration and separation. When sheet drains are used under trail tread material, they provide separation, reinforcement, and drainage. Because sheet drains have greater bending strength than geotextiles or geonets, less tread fill may be needed above them. Sheet drains also can be installed vertically in covered trenches beside the trail to drain off subsurface water.

Photo of a ruler next to a geotextile on top of a cross section.
Figure 7—Geocomposites such as sheet drains have
a large cross section that allows drainage. If
geotextiles are placed under the trail tread, the
sheet drain should be oriented with the geotextile
on the bottom and the plastic core on top. This
orientation reduces the amount of fill needed.

Geo-Others—Turf Reinforcement

Other proprietary products used for reinforcement are considered geo-others. Typically, they are manufactured from recycled plastics to protect turf from rutting, erosion, and soil compaction. Geo-other products include Geoblock (figure 8), Lockgrid, EcoGrid, and Grasspave2 (figure 9). The Missoula Technology and Development Center (MTDC) report "Managing Degraded Off-Highway Vehicle Trails in Wet, Unstable, and Sensitive Areas" (Meyer 2002) has information on turf reinforcement materials and their installation.

Photo of a ruler next to a geoblock.
Figure 8—Geoblock, a very stiff material, is one of the many
products for turf reinforcement.

Photo of a ruler next to a Grasspave2.
Figure 9—Grasspave2 is another product for
turf reinforcement.