Managing Degraded Off-Highway Vehicle Trails in Wet, Unstable, and Sensitive Environments

Trail Management--Responding to Trail Degradation (Continued)

Seasonal or Type-of-Use Restrictions

Seasonal-use restrictions are another option for responding to trail degradation. Because soils are most sensitive to impact when they are wet, restricting use of sensitive trails during spring breakup or periods of high rainfall may significantly reduce trail degradation. Also designating winter-season trails that cross wetlands as 'WINTER ROUTES ONLY' would significantly reduce impacts on sites that are extremely sensitive to impact by motorized vehicles during summer months.

Type-of-use restrictions limit the kind of equipment allowed on trails. For example, restricting gross vehicle weight to less than 1,500 pounds could significantly reduce the size of equipment operating on a trail. This would allow managers to build trails to a much lower design specification than when weight is unlimited.

In general, the potential of trail activities to create impacts ranges from slight to heavy as shown in table 6.

Table 6--Activities that have the potential to impact trails.

Impact	Use
Generally slight	WINTER, WTH FROZEN GROUND AND ADEQUATE SNOW COVER:
	Nonmotorized	Motorized
	(Minimum snow cover 6 inches)	(Minimum snow cover 12 inches)
	Skiiing/snowshoeing	Snowmobiling
	Dog sledding
to	SUMMER, WELL-THAWED GROUND:
	Nonmotorized	Motorized
	Hiking	Light, tracked vehicles
	Mountain biking	Motorcycle riding
	Horseback riding	OHV riding (less than 1,500 pounds gross vehicle weight)
Potentially heavy		Unlimited off-higway vehicle use

Reducing the types of use would lessen the potential for impact. Successfully restricting trail use requires user cooperation and enforcement. Signs, gates, and barriers aren't enough to discourage some users, so public education and development of alternative routes on more resilient trails are needed to encourage compliance.

Controlled Use (Traffic Volume Restrictions)

Controlled use is another management option when responding to trail degradation. Trail degradation occurs when use exceeds the ability of the trail surface to resist impact. Controlling the level of use can be a powerful tool in reducing impacts. Determining the appropriate level of use can be difficult, especially since a trail's resistance to impact can change with weather and type of use. Good decisions require knowledge of existing trail conditions, patterns and levels of use, and trail condition trends. If trail conditions are stable under existing loads, no volume restrictions may be necessary. If trail conditions are deteriorating, traffic volume may have to be decreased or trail surfaces may need to be modified to support the increased use.

Managing trails through controlled use is complicated because there may not be a linear relationship between use levels and impact. Typically, after a certain level of impact is reached, trails will continue to degrade without any further use. This is clearly the case when vegetation stripping exposes soils to erosion. Finding the balance between appropriate levels of use and acceptable impacts is a resource management art form, ideally backed up with good monitoring of the level of use and resource damage.

Controlled use also requires an authorized and determined enforcement presence. This may not be readily available. But where it is, monitoring impact and setting the allowable use may be a good management approach to controlling degradation problems.

Trail Hardening

Another management option is trail hardening. Trail hardening is a technique of modifying trail surfaces so they will support use without unacceptable environmental impacts to vegetation, soils, hydrology, habitat, or other resource values. Trail hardening should be considered under the following conditions:

Existing trail impacts are causing or are projected to cause unacceptable onsite or offsite impacts, and
More suitable alternative trail locations are not available, or
Alternative trail locations are not environmentally acceptable or economically feasible.

Trail hardening provides the following benefits:

Defines a single trail alignment for vehicle travel.
Stabilizes surface soil conditions along the hardened trail section.
Provides a stable, durable trail surface for OHV traffic.
Halts trail widening and the development of braided trail sections.
Allows formerly used trail alignments to naturally stabilize and revegetate.
May provide for vegetation growth (or regrowth) within the hardened trail surface that helps to reduce visual impacts, maximize site stability and increase site productivity.

Trail hardening seeks to improve trail surfaces by one of three methods:

The goal of trail hardening is to reinforce soils so they will support a specified level of use under all environmental conditions. Because of the range of trail-hardening methods available, a trail manager must select a method that provides maximum utility for the investment in time, labor, and cost. Utility includes site stabilization, resource protection, and suitability for use as a surface for OHV traffic (figure 8).

Aerial photo of a hardened OHV trail.
Figure 8--This aerial image shows a recently installed
section of hardened trail crossing a wetland in southcentral
Alaska. The new trail alignment defines a single route of travel
that will prevent the continued development of braided trails.

The following section introduces a number of trail-hardening techniques.

Trail Hardening--Replacing or Capping Unsuitable Soils

Replacing unsuitable soils is the most intensive, trail-hardening technique. Problem soils are excavated and removed until a subbase of competent subsoils or gravel has been exposed. High-quality material is placed over the subbase to bring the trail surface up to the trail's original level. This process is appropriate for trails with a suitable subbase close to the surface and a convenient source of high-quality fill. The work generally requires heavy equipment. It is most appropriate near trailheads and along highways where heavy equipment can be used to good advantage.

Where a suitable subbase is not close to the surface or excavation work needs to be minimized, geotextile fabrics may be used to provide a base for surface capping. The use of geotextile materials extends the application of capping to many areas where removal of substandard surface soils is impractical.

Geotextiles, also known as construction fabrics, are widely used in roadways, drains, embankments, and landfills. They are constructed of long-lasting synthetic fibers bonded by weaving, heat, extrusion, or molding. They come in a wide variety of types including fabrics, sheets, or three-dimensional materials. They can be pervious or impervious to water passage.

Geotextiles provide four important functions in road and trail surface construction:

Separation
Stabilization
Reinforcement
Drainage

These functions are illustrated in figures 9a, 9b, and 9c. Geo-textiles work as separation fabrics when they are placed between gravel caps and underlying soils to prevent the materials from mixing. The geotextile serves to maintain the original thickness and function of the gravel cap as a load-bearing layer. Geotextiles increase soil stabilization by maintaining the load transfer capability of the gravel cap. This increases effective bearing capacity and prevents subsoil pumping. Geotextiles reinforce soils by providing a structure to bond the gravel cap and underlying soils. The geotextile fabric locks the two materials together and allows the soil to receive a load across a broader footprint. Geotextiles also help maintain the drainage characteristics of the gravel cap. In addition to use in trail tread, geotextiles can have important applications in erosion control, drainage interception (sheet drains), and ditch liners.

Cross-sectional drawing of the directional forces from a tire on a gravel surface without geotextile.
Figure 9a--Gravel cap without geotextile. The
aggregate cap will lose strength as the gravel is
contaminated by the subbase.

Cross-sectional drawing of the components of a tire on a gravel surface with geotextile.
Figure 9b--Gravel cap with geotextile. The
geotextile layer prevents the migration and
contamination of the surface gravel cap by
underlying poor-quality soils.

Close-up cross-sectional drawing of the directional forces from a tire on a gravel surface with geotextile.
Figure 9c--Gravel cap with geotextile. Using a
geotextile enhances trail performance through
separation, stabilization, reinforcement, and drainage.

Site conditions such as soil texture, moisture, depth to foundation materials, and the type of use indicate when a geotextile fabric should be used. Because gravel is difficult and expensive to deliver onsite, the use of a separation fabric makes good economic sense to protect the function of the gravel cap. The use of a geotextile fabric requires adequate capping (a minimum of 6 inches) and regular maintenance to maintain the cap. Regular maintenance prevents the geotextile fabric from being exposed at the surface.

The National Park Service experimented with the use of geo-textile and gravel placement during the summer of 1999 on degraded trail segments of a former mining road connecting two administrative sites in the Yukon-Charley Rivers National Preserve (Meyer 1999a). About 678 feet of geotextile with a 4- to 6-inch gravel cap was installed over soils in areas that crossed melted permafrost soils. Using this technique, the road alignment was reclaimed as an OHV trail.

Geotextile and gravel placement is relatively simple. The Yukon-Charley approach was adapted from Forest Service methods (Monlux and Vachowski 1995, figure 10). This technique provides a rim structure to minimize the loss of cap material (figures 11 and 12). A local source of suitable gravel was identified. One-half-cubic-yard belly dump trailers, loaded by a skid-steer loader and towed by 4x4 OHVs, transported gravel to trail construction sites.

Cross-sectional drawing of a layer of gravel fill over a layer of geotextile spaced 6 feet apart and bordered by two rim poles.
Figure 10--Adapted geotextile installation design.

Photo of the author placing geotextile around a rim pole at an installation.
Figure 11--The author installing woven geotextile
around a rim log in the geotextile gravel-capping test
installation at the Yukon-Charley Rivers National Preserve
in Alaska. The rim log held the gravel cap on the installation.

Photo of the gravel fill being spread over an insallation of geotextile.
Figure 12--A geotextile and gravel-cap installation
over permafrost-degraded soils in Yukon-Charley
Rivers National Preserve in Alaska. Use of geotextile
and a gravel cap on this trail allowed the National Park
Service to construct a 6-foot-wide OHV trail over a 3- to
4-foot-deep muck hole.

About 45 labor days and 80 cubic yards of gravel were required to construct 678 linear feet of 6-foot-wide trail, roughly 45 work hours per 100 feet of hardened trail. Construction efficiency dropped considerably when construction sites were more than one-quarter mile from the gravel source because of the small size of the transport vehicles and the round-trip travel time. The loaded trailers, weighing about 2,000 pounds gross vehicle weight, also seriously degraded marginal trail segments along the haul route. Future operations at the site will use larger haul vehicles operating over frozen soils during the winter months.

The geotextile used on the project was AMOCO 2000, a light-grade woven synthetic fabric. The material cost about 5 cents per square foot, quite inexpensive, considering all other costs. Overall construction costs for the project were estimated at $3.60 per square foot using a labor rate of $18 per hour.