• Identification of Unsuitable Tread Fill Material
  • Method A—Field Comparison
  • Method B—Laboratory Test
  • Method C—Geotextile Field Test

Soils from wet areas are normally not suitable for use as tread fill because they are too moisture sensitive and lose strength easily when they become wet. It's important to avoid spending scarce dollars to excavate and haul fill that will fail when wet. Poor materials can be identified by several methods.

Organic Soils: Identified by musty odor when they are damp, and they are dark in color.

Other Unsuitable Tread Fill Materials: The stability of tread fill material is influenced primarily by the amount of silt or clay. If the fill is more than 20 percent silt and clay, the fill will probably become unstable when wet. Rough evaluations for suitability can be done by the following methods.

Compare proportions of gravel, sand, and fines in existing trail tread materials with the proportions in borrow sources. Individual "fine-size" material particles are not visible to the naked eye and are classified as silt or clay. If the proportions of gravel, sand, and fines are similar, you can expect the borrow materials to perform as well as the existing trail tread materials. If the borrow source has a lower proportion of fines, you can expect better performance.

Take a 5-kilogram (10-pound) sample of the proposed tread fill material to a materials testing laboratory for a washed sieve analysis to determine the percentage of minus No. 200 material. The minus No. 200 material represents the amount of silt or clay. If the sample has more than 20 percent minus No. 200 material, it is not suitable for fill. A washed sieve analysis typically costs $50 to $100.

Build a short section of a small-scale trail over a wet area with a 2-meter (6-foot) square piece of geotextile and the proposed tread fill material. The depth of tread fill should be at least 150 millimeters (6 inches). Saturate the section with as much water as would be expected under the worst conditions. Evaluate the stability of the tread material by stepping onto the tread repeatedly, mimicking traffic.