Geosynthetics for Trails in Wet Areas
Introduction
Trails in soft, water-saturated soils present special challenges to trail managers. Muddy trails are anathema to livestock and hikers, which tend to skirt the edges of mud holes. This increases the area of damage to sensitive soils. Incorrectly constructed trails in wet areas lead to soil compaction, sedimentation, multiple trails, and unhappy trail users. Traditional trail construction methods for wet areas include turnpike or puncheon. These have worked well where rock or wood materials are readily available. However, geosynthetics can increase the effectiveness of construction methods and offer additional alternatives.
Geosynthetics are synthetic materials (usually made from hydrocarbons) that are used with soil or rock in many types of construction. Their use has grown significantly in road construction over the past 20 years, and in trail construction for the past 10 years.
Guidelines on how to use geosynthetics in trail construction have not been readily available to trail managers. The information presented here applies some roads technology to trail design and construction in five categories:
- General information on geosynthetic products.
- Basic geosynthetic design and utilization concepts.
- Specific design diagrams for trail construction over wet saturated soils.
- A list of product manufacturers, price ranges, and physical properties.
- Identification of unsuitable tread fill materials.
Section 1: Geosynthetics General Information
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 early deterioration by ultraviolet light. TrailMaster, a proprietary product, is also discussed. TrailMaster can be used as trail tread, and is placed directly over wet areas. Since all these products are synthetic, their use in wilderness should be reviewed and approved prior to use.
Geotextiles
Geotextiles (Figure 1) are the most widely used geosynthetic. Geotextiles are often called construction fabrics. They are constructed from long lasting synthetic fibers that are bonded to form a fabric held together by weaving, heat bonding, or other means. They 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 normally exists in wet areas. The physical requirements listed for all geotextiles in Section 4 are stringent enough so the products will also work for properly designed high-flow drainage applications. These representative products are low cost, readily available, and easy to use. There are many other products on the market.
Figure 1-Geotextiles. Felt-like products are easier to work
with than heat bonded, slit film, or woven products that have a slick surface
texture. They are easier to cut and their flexibility makes them easier
to place on curved trail sections.
Geonets
Geonets or geonet composites (Figure 2) 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 2-The net-like core of geonet allows sideways drainage
that is normally adequate for the amount of seepage found under trails in wet areas.
Sheet Drains
Sheet drains (Figure 3) 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 that is formed into an egg-crate shape. The core provides an impermeable barrier unless perforated by the manufacturer. Perforated cores are always covered with geotextile on both sides to prevent soil clogging the drainage passages. Geotextile is bonded to one or both sides of the core to provide filtration and separation. 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. They can also be used vertically in covered trenches beside the trail to drain off subsurface water.
Figure 3-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. This orientation reduces the amount of fill needed.
Geogrids
Geogrids (Figure 4) 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.
Figure 4-Geogrids are normally placed on top of a
layer of geotextile to obtain separation from saturated soils in wet areas.
Geocells
Geocells (Figure 5) are usually made from polyethylene strips from 50 to 200 millimeters (2 to 8 inches) high that are bonded at intermediate points to form a honeycomb structure. The product is shipped in a collapsed and compact form. During installation, the material is pulled open and the honeycomb structure is staked to the ground surface. Each of the cells is filled with select backfill and compacted. Compacting trail tread material within the cell increases the bending strength of the layer, which reduces settlement into soft saturated soils. Geocells are good for reinforcement and reduce the amount of fill material required.
Figure 5-Geocell usually has geotextile under it to
provide separation from wet saturated soil.
Proprietary Products
TrailMaster (Figure 6) is made from 3-millimeter (one-eighth-inch) thick polypropylene that has 13-millimeter (½-inch) diameter extruded holes about 32 millimeters (1¼-inches) on center. The extrusions extend 16 millimeters (one-eighth-inch) below the top of the sheet. This product was originally marketed under the Gripmaster name and weighs about 2.8 kilograms per square meter (0.57 pounds per square foot). Bending strength is high enough to distribute loads from pack animals over a large enough area so that the material essentially floats on top of saturated soils. TrailMaster is excellent for reinforcement because no fill material is required.
Figure 6-TrailMaster is a very stiff material,
but it can be rolled up for relatively easy transport.
