Trails in wet areas often are unstable because they are saturated by subsurface moisture and precipitation. Geosynthetics help create stable trail surfaces by providing:

• Separation—Geotextiles, geonets, and geocomposites (sheet drains) keep saturated, weak native soils from contaminating stronger, load-bearing trail surface materials. These materials allow water, but not soil, to pass through them.
  
  
• Drainage—Geotextiles, geonets, and geocomposites (sheet drains) improve subsurface drainage to avoid saturation and weakening of the trail tread.
  
  
• Reinforcement and Load Distribution—All geosynthetics provide some degree of tread reinforcement and load distribution. This may decrease the amount of imported fill material needed for trail surfacing.

Geosynthetics are relatively simple to use. Products that meet the physical requirements discussed in the "Geosynthetic Product Information" section are tough enough to be placed over small stumps that stick up from the ground surface after brush has been cleared for trail construction. Cutting stumps and brush to within a few inches of the ground usually is all that is necessary. Normally, joints in geotextiles, geonets, or geogrids should overlap at least 300 millimeters (12 inches). Sometimes sections of material are joined with pins or clips rather than being overlapped. All geosynthetics must be stored in their shipping wrappers until installation because they will deteriorate gradually when exposed to ultraviolet light.

Selecting good material for tread fill is very important. Organic, silt, or clay soils should not be used as tread fill because they become muddy when wet. Use firm mineral soil, coarse-grained soils, granular material, or small wellgraded 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 such soils are more difficult to identify. The "Identification of Unsuitable Tread Fill Material" section discusses several methods for identifying unsuitable soils.

The amount of acceptable tread fill material you need over the geosynthetic depends on several site-specific factors (table 1).

In addition to the applications illustrated in the "Specific Design Applications" section, other combinations of geosynthetic materials are possible and perhaps preferable, depending on conditions at the site and the native building materials available there. Once you understand the function of the different types of geosynthetics and product capabilities, you may be able to identify many other applications.