Soil Stabilizers On Universally Accessible Trails
Comparing stabilization products to determine their benefit in providing firm and stable trail surfaces. Download the pdf (271 kb) of this document.
By Roger Bergmann, Mechanical Engineering Technician
The Americans with Disabilities Act Accessibility Guidelines (ADAAG) state that ground and floor surfaces should be firm, stable, and slip-resistant. These three terms are not well defined and may be interpreted differently by many people. Not all ground surfaces need to be constructed for universal access, however when constructing a universally accessible trail, keep in mind that these three terms—firm, stable, and slip-resistant—are being used to define a trail surface that is easily traveled by individuals using canes, walkers, crutches, wheelchairs, or other mobility aids.
For the past several years, national forests around the country have been looking for ways to make areas more universally accessible, while maintaining a natural appearance that is not as distracting as concrete, asphalt, boardwalks, and other obviously manmade pathways. There are a number of products available on the market that claim to stabilize native materials used for trail surfacing without impacting the visual appearance of the surrounding area. Many of these products have been used all across the country with results that varied from extremely poor to very satisfactory stabilization. This report presents the results of the multi-year study, including the products and construction methods used.
In the summer of 1994, San Dimas Technology and Development Center (SDTDC) began the process of comparing stabilization products to determine their benefit under similar conditions. The Wood River Accessible Fishing Site and Day Use Area on the Winema National Forest was selected as the initial study site.
The coarse and the fine aggregate control sections located in the sun both failed in a matter of days as the water used for compaction evaporated. These two sections never improved. Both the coarse and fine aggregate control sections in the shaded area failed when the water initially evaporated from the compacted surface. Three years after construction it was still possible to use the edge of a boot to easily scrape down 2 inches (50.8 millimeters) through the surface. However, John Parker's evaluation of these two sections indicated that they were very good from the perspective of a wheelchair user. The location of these sections under a canopy of aspen trees provided a mat of leaves over the trail which made it easier to travel across the surface, and the leaves also helped to retain moisture in the aggregate materials.
Three percent bentonite clay was not an effective stabilizer on any of the trail sections. Those sections that were in the shade were under a canopy of pine trees and did not get the benefit of fallen leaves to help hold in moisture.
The trail sections constructed with enzyme product EMC2 and sulfuric acid product Roadbond EN-1 did not noticeably stabilize the aggregate materials at all. Both of these types of product claim to work very well when mixed in with clay. They did not work with 3 percent clay at the Wood River project and they did not work with greater than 50 percent clay at the Bell Rock Pathway. At the end of the first winter snowmelt, a wheelchair had left a 1-inch (25.4- millimeters) deep rut as it traveled across both of these sections at Wood River.
Flyash did not perform any better than the water/compaction treatment the control sections received. The 3 and 5 percent flyash sections were under an aspen canopy that helped improve the trail surface with a mat of fallen leaves. The 8 and 10 percent flyash sections were in the sun and had a very soft surface that was difficult to push a wheelchair over.
The product Stabilizer, which is made from ground seed hulls, stabilized marginally better than the water/compaction method for the control sections. This product allows water to permeate through the surface when it gets wet. The aggregate material will get firmer again as the material dries out. This product has worked well in very dry climates, such as Arizona and New Mexico, but it did not work well at the Wood River Day Use Area in Oregon.
The latex polymer product Soil Sement was applied in September 1995, and was considered to be the second best surface by John Parker in his wheelchair evaluation in August 1998. If this surface was to receive regular maintenance coatings it may hold up for a number of years.
The pine tree resin emulsion product Road Oyl was the most expensive and the most difficult to apply, but it provided the best surface for universal accessibility.
The macadam construction method did provide a firm surface though there was a considerable amount of surface raveling of the pea gravel. Regular maintenance of the blotter sand top surface may solve the problems of raveling, noise, and bumpy ride.
Reduced budgets for construction and maintenance of trails is an ongoing concern for all public agencies. Soil stabilizer products are not magic potions that will solve budget concerns. Do not think that the use of soil stabilizers will relieve the responsibility of doing a thorough engineering design. In October 1995, SDTDC published an initial report on the subject "Soil Stabilizer For Use on Universally Accessible Trails." That report discussed the importance of following good construction practices. The necessity of this cannot be over emphasized! A properly designed trail is essential, and is especially important regarding issues of water. Water can cause a great deal of damage to a trail in a number of ways: erosion across the trail or down the length of the trail; subsurface water causing the trail to fail from underneath; and surface water penetrating voids or cracks in the trail surface that will allow vegetation growth or may cause freeze/thaw damage.
Many people believe that the use of a soil stabilizer product will allow them to apply the product directly to the in-place native material; compact; and the trail is finished. This can be done, but it is NOT recommended for a number of reasons:
To achieve a firm and stable surface it is recommended that one use fine aggregate (3/8-inch minus; 9.53-millimeters minus), rather than coarser aggregate that can cause problems for individuals using mobility aids. If a surface with coarse materials has any raveling, the loose surface rocks can make it very difficult to turn the wheels on a chair or walker. Loose surface rocks can also make placement of canes or crutches very difficult. (See figure 7.) Two types of fine aggregate material that are not suitable for accessible trails are sand and pea-size gravel. Both of these materials create surfaceJune 28, 2007ross. Sand does not have any large particles (1/2-inch, 12.7-millimeters) to help keep the wheels from sinking into the soft surface, and pea-size gravel does not have enough interlocking surfaces or any fine material to give the pea-size gravel stability. A well graded mixture from 3/8-inch (9.53- millimeters) down through the very fine material is what is needed.
For more information:
San Dimas Technology and Development Center, 444 E. Bonita Avenue San Dimas, CA 91773 Phone: (909) 599-1267 x201 - Fax: (909) 592-2309
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Updated June 28, 2007