Geosynthetics for Trails in Wet Areas
Section 2: Basic Geosynthetic Design and Utilization Concepts for Trail Construction in Wet Areas
Geosynthetics provide a stable trail surface in wet areas. Unstable trail surfaces are usually caused by saturation from subsurface moisture and precipitation. Geosynthetics assist in obtaining stable surfaces by providing:
Separation- Geotextiles, geonets, and sheet drains keep saturated, weak native soils from contaminating stronger, load-bearing trail surface materials. They allow water, but not soil, to pass through.
Drainage-Geotextiles, geonets, and sheet drains improve subsurface drainage to avoid saturation and weakening of the trail tread.
Reinforcement and Load Distribution- All geosynthetics discussed in this paper provide some degree of tread reinforcement and load distribution. This decreases the amount of imported fill material required.
TrailMaster provides enough load distribution so it can be placed directly on the saturated soil and also serves as the trail tread with no additional fill needed.
Geosynthetics are relatively simple to use. Products that meet the physical requirements discussed in Section 4 are tough enough to be placed over small stumps that stick up from the ground surface after brushing. Cutting stumps and brush to within a few inches from the ground is usually all that is necessary. Where joints occur in geotextiles, geonets, or geogrids, the overlap should normally be at least 300 millimeters (12 inches). Pins or clips are used at joints for the other products. All geosynthetics must be stored in shipping wrappers until installation since they will gradually deteriorate when exposed to ultraviolet light.
Selecting good tread fill material is very important. Organic, silt, or clay soils should not be used as tread fill since they become muddy during wet weather. Use firm mineral soil, coarse-grained soils, granular material, or small well-graded angular rock instead. Soil from wet areas is normally not suitable for use as tread fill. Unsuitable organic soils are easily identified by a dark color and musty odor when damp. Many soils containing clays and silts are just as unstable, but identification is more difficult. Avoiding unsuitable tread fill is very important because poor materials will fail when wet and costs for excavation and haul are high. Poor materials can be identified in the field by several methods discussed in Section 5.
How much acceptable tread fill material you need over the geosynthetic selected depends on several site-specific factors:
| Factors Affecting Tread Thickness Needed | Maximum Thickness | Minimum Thickness |
|---|---|---|
| Trail fill quality | Mineral soil with little rock, less than 20% silt or clay | Granular, free-draining materials |
| Trail tread surface | Horse or motorcycle | Foot traffic |
| Tread surface moisture content during traffic | Predominantly high | Predominantly low |
| Amount of foundation settlement | Continuously wet areas over 2 feet deep | Intermittent soft, wet areas under 2 feet deep |
| Geosynthetic alternative selected | Single layer of geotextile (figure 1) | Geotextile with logs (figure 9) Geocell, (figure 13) |
| Trail surface crown maintenance | Less than annual | Annual |
In addition to the nine applications illustrated in Section 3, other combinations are possible and perhaps preferable, depending on mud hole conditions and natural building materials available. Once you understand the design/utilization concepts and product capabilities, try different applications.
