Trails in Wet Areas (continued)

Geosynthetics

Geosynthetics are synthetic materials (usually made from hydrocarbons) that are used with soil or rock in many types of construction. Geosynthetics can increase the effectiveness of construction methods and offer some additional alternatives to traditional trail construction practices.

Geosynthetics perform three major functions: separation, reinforcement, and drainage. Geosynthetic materials include geotextiles (construction fabrics), geonets, sheet drains, geogrids, and geocells. All these materials become a permanent part of the trail, but must be covered with soil or rock to prevent deterioration by ultraviolet light or damage by trail users.

Geotextiles (Figure 33) are the most widely used geosynthetic material. Sometimes they are called construction fabrics. They are made from long–lasting synthetic fibers bonded to form a fabric.

Figure 33—Felt–like geotextiles are easier
to work with than heat bonded, slit film, or woven
products that have a slick surface texture.

They are primarily used for separation and reinforcement over wet, unstable soils. They have the tensile strength needed to support loads and can allow water, but not soil, to seep through. Nonporous geotextiles can be used in drainage applications to intercept and divert ground water.

Geotextiles are often used in trail turnpike or causeway construction. They serve as a barrier between the silty, mucky soil beneath the fabric and the mineral, coarse–grained, or granular soil placed as tread material on top of the geotextile. The importance of separation cannot be overemphasized. It takes only about 20 percent silt or clay before mineral soil takes on the characteristics of mud—and mud is certainly not what you want for your tread surface. Most geotextiles commonly used in road construction work for trail turnpikes. The fabric should allow water to pass through it, but have openings of 0.3 mm or smaller to prevent silt from passing through.

Geotextile is sensitive to ultraviolet light. It readily decomposes when exposed to sunlight. Unexposed, it lasts indefinitely. Always store unused geotextile in its original wrapper.

Geonets or geonet composites (Figure 34) have a thin polyethylene drainage core that is covered on both sides with geotextile. They are used for all three functions—separation, reinforcement, and drainage. Since geonets have a core plus two layers of geo-textile, they provide more reinforcement than a single layer of geotextile.

Figure 34—The net–like core of geonet
allows sideways drainage that is normally adequate for the
amount of seepage found under trails in wet areas.

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