Geosynthetics for Trails in Wet Areas
Section 5: Identification of Unsuitable Tread Fill Material
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. Avoiding unsuitable tread fill is very important since poor materials will fail when wet, and costs for excavating and hauling are high. Poor materials can be identified by several methods:
Organic Soils: Identified by musty odor when damp, and dark in color.
Other Unsuitable Tread Fill Materials: Stability of tread fill material is primarily influenced by the amount of silt or clay present. If the percentage exceeds 20 percent, the materials will likely become very unstable when wet. Rough evaluations for suitability can be done by the following methods:
Method A-Field Comparison
Make comparisons between existing trail tread materials with borrow sources. Compare the proportions of gravel, sand, and fines. Individual "fine-size" material particles are actually 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 less fines exist in the borrow source, you can expect better performance.
Method B-Laboratory Test
Take a 5-kilogram (10-pound) sample of the proposed tread fill material to a materials testing laboratory, and have them perform a washed sieve analysis test to determine the percentage of minus No. 200 material. Since the minus No. 200 represents the amount of silt or clay, if the amount exceeds 20 percent, the material is not suitable. Typical cost for this test is between $35 and $50.
Method C-Geotextile Field Test
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), and should be saturated with water after placement to assimilate moisture contents that one would expect under the worst conditions. Evaluate the stability of the tread material by repeated load testing with your foot.
