Trails in Wet Areas (continued)
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. 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.
Figure 33--Felt-like geotextiles are easier to work with
than heat-bonded, slit-film, or woven products
that have a slick texture.
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 geotextile, they provide more reinforcement than a single layer of geotextile.
Figure 34--The net-like core of geonet allows drainage.
Sheet drains (Figure 35) are another form of composite made with a drainage core and one or two layers of geotextile. The core is usually made of a polyethylene sheet shaped like a thin egg crate. The core provides an impermeable barrier unless it has been perforated by the manufacturer. When used under the trail tread material, sheet drains provide separation, reinforcement, and drainage. Since they have greater bending strength than geotextiles or geonets, less tread fill is often needed.
Figure 35--Sheet drains have a large cross section that
provides significant drainage capacity. If placed under
the trail tread, orient the sheet drain with the geotextile
side on the bottom and the plastic core on top.
Sheet drains or geonets can be used as drainage cutoff walls (Figure 36). If the trail section is on a side slope where subsurface water saturates the uphill side of the trail, a cutoff wall can be constructed to intercept surface and subsurface moisture, helping to drain and stabilize the trail section.
Drainage Cut-Off Walls
Figure 36--Sheet drain or goenet used to intercept seepage.
Geogrids (Figure 37) are made from polyethylene sheeting that is formed into very open grid-like configurations. Geogrids are good for reinforcement because they have high tensile strengths, and coarse aggregate can interlock into the grid structure. Geogrids are normally placed on top of a layer of geotextile to obtain separation from saturated soils in wet areas.
Figure 37--Geogrids are normally placed on top of
a layer of geotextile to obtain separation from
Concrete grid blocks are another technique for armoring switchback turns or steeper slopes, especially on trails designed for motorized traffic.
Geocells (Figure 38) are usually made from polyethylene strips bonded to form a honeycomb structure. Each of the cells is filled with backfill and compacted. Geocells are good for reinforcement, reduce the amount of fill material required, and help hold the fill in place. Geocell usually has geotextile under it to provide separation from saturated soils. The grids need to be covered with soil so they will never be exposed. Exposed geocells present a substantial hazard to vehicles due to loss of traction.
Figure 38--Geocell usually has geotextile under it to
provide separation from saturated soils.
Multiple layers of filled geocells, each level offset to provide sufficient batter, are used as retaining walls. Vegetation grows in the "flower pot" cells along the face of the wall, providing attractive camouflage (Figure 39).
Figure 39--"Flower pot" retaining wall of geocells.