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

Trail Management--Responding to Trail Degradation (Continued)

Trail Hardening--Providing a Wear-and-Carry Surface Over Unstable Soils (Continued)

Porous Pavement Panels--Porous pavement panels (PPP) are three-dimensional, structural geotextiles designed to provide a durable wear surface and a load distribution system for driveways, parking areas, fire and utility access lanes, golf cart paths, and approaches to monuments, statues, and fountains. The panels are intended to be installed over a prepared subbase and filled with soil. They are designed to support grass growth and provide a reinforced turf surface for light or intermittent heavy traffic. In contrast to asphalt or concrete pavements, these porous pavement systems reduce surface runoff, increase infiltration, resist erosion, and enhance groundwater recharge.

The standard industrial installation technique is modified for hardening OHV trails. After surface leveling, the panels are installed directly over the existing trail surface. The grid cells are not backfilled unless fill material is readily available. After installation, the panel's surface provides a tread surface for vehicles, and the panel's structure distributes their weight. The open structure of the panels allows vegetation to grow through the panel after installation. On extremely muddy or boggy sites, a supplemental geotextile layer may be placed beneath the panels to increase flotation. Polynet PN3000, an open-grid drainage mat, has been used for that purpose in a number of test installations in Alaska.

One advantage of the panel system is the light weight of the panels (about 2 pounds per square foot). The panels do not add any significant weight load to wetland surfaces and have little impact on surface hydrology. Their use can dramatically reduce the need for culverts or other water transfer structures along the trail.

Two porous pavement panel products have been the subjects of extensive field testing in Alaska. They are GeoBlock (figure 21) and SolGrid (figure 22).

Figure 21--A GeoBlock panel. Note the edge
tabs used to connect the panels and
transfer loads between them.

Figure 22--Two SolGrid panels. Note the U-shaped
flex connectors between panel subsections.

GeoBlock is a commercially developed porous pavement system manufactured by Presto Products of Appleton, WI. GeoBlock has been on the market, in one form or another, since the early 1990s and was specifically tested in two earlier configurations by the Park Service in the Wrangell-St. Elias National Park and Preserve. Its primary industrial applications are emergency vehicle lanes, light service roads, and auxiliary parking areas.

SolGrid is a commercial porous pavement system developed by SolPlastics, of Montreal, Canada. SolGrid is a newer product. It has a unique configuration that makes it suitable for irregular terrain and sloped areas. Its primary industrial applications are walkways, bikeways, golf cart paths, and driveways.

Both products are partially recycled polyethylene plastic panels about 39 inches long, 19 inches wide, and 2 inches thick. Geo-Block has also been manufactured as a 1 1/4-inch-thick panel. The panels are stabilized with carbon black to help them resist degradation by ultraviolet light. Both GeoBlock and SolGrid are constructed with an open grid surface and interlocking edges. The GeoBlock products have a 3- by 3-inch-tall vertical grid reinforced by a base sheet perforated with 2 1/4-inch-diameter holes on a 3 3/4-inch spacing. About 44 percent of the base is open. The GeoBlock products form a rigid panel with good weight transfer between panels.

The SolGrid product has a 2 1/2- by 2 1/2-inch-tall vertical grid pattern with an 8- by 8-inch subpanel. When assembled, sub-panels form 16- by 16-inch weight-transfer panels. Flexible U-shaped connectors join the subpanels. There is no base sheet. About 85 percent of the grid surface is open for vegetation regrowth. Weight transfer between panels is poor because of the integrated flexible connectors. This can be mitigated somewhat by the use of supplemental geosynthetics underneath the panel. Polynet-PN3000 has been used for that purpose in Alaska and has demonstrated some benefit. The flexibility of the SolGrid panels increases their utility on irregular surfaces and on slopes. It also provides an integrated buffer for thermal expansion and contraction.

In 1996, earlier configurations of GeoBlock 1 1/4- and 2-inch panels were tested by the Park Service in the Wrangell-St. Elias trail mitigation study. The test demonstrated that the panels perform very well as trail-hardening materials. They provided a suitable wear surface for foot and OHV use and were easy to install. In addition, they readily facilitate vegetation regrowth.

Vegetation cover along two hardened trail segments increased on average from 70 to 90 percent and from 48.5 to 77.5 percent respectively, within the 4-year study period.

In 1996, the total installation costs for 100 feet of 6-foot-wide trail were:

1 1/4-inch Geoblock	2-inch Geoblock
$6.67 per quare foot	$8 per square foot
Installation, 32 hours	Installation, 36 hours
Panel costs with shipping, $3.14 per square foot	Panel costs with shipping, $4.50 per square foot

Since 1996, GeoBlock panel costs have fallen to about $2.15 per square foot for the 2-inch panel, depending on volume. Presto no longer manufactures the 1 1/4-inch GeoBlock.

SolGrid costs about $1.60 per square foot, depending on volume. In some areas, SolGrid would require the use of a supplemental geotextile, such as Polynet PN-3000. This would add 15 to 25 cents per square foot to installation costs.

Both products have been tested in Alaska on OHV trails during 2000, 2001, and 2002. In 2000, two 100-foot test sections were installed on a dedicated recreation OHV trail in the Forest Service Starrigaven Recreation Area near Sitka. Several test sections of GeoBlock and one section of SolGrid were installed in cooperation with the Alaska Department of Fish and Game at the Palmer Hay Flats State Game Refuge near Palmer (figures 23 and 24). The Forest Service reports that the installations were more economical than the standard gravel cap placement and may be applied more widely in the future (LaPalme 2001).

Figure 23--Test installation of 2-inch GeoBlock
at the Palmer Hay Flats State Game Refuge
in Alaska. This configuration of panels provided
a 4.8-foot-wide trail. Note the interlocking tabs
along the panel edges. These tabs transfer weight
between panels. In this test, GeoBlock was installed
over Polynet PN-3000.

Figure 24--Test installation of SolGrid at the
Palmer Hay Flats State Game Refuge in Alaska.
Note the U-shaped flex joints between sub-panel
sections. The SolGrid was tested without Polynet
PN-3000 to test the characteristics of the product
when installed on a soft, silty substrate.

The 2000 test project on the Palmer Hay Flats Game Refuge was successful enough for the department to install an 800-foot section in 2001. That installation included a 600-foot-long shallow underwater section that was supported by a base layer of geogrid and a gravel cap infill to ballast the installation to the pond floor. Also in 2001, the Bureau of Land Management sponsored test installations in the White Mountains National Recreation Area north of Fairbanks, AK, and the Tangle Lakes Archeological District west of Paxson, AK. More than 400 feet of hardened trail was installed at those two sites. In addition, a 300-foot test section was installed in the Caribou Lakes area on the lower Kenai Peninsula in Alaska. Average material costs ranged from $3 to $3.50 per square foot. Among the four sites, trail surfaces were constructed in 4.8-, 6.5-, and 8-foot-wide configurations. Labor requirements varied from 6.5 to 14 hours per 100 square feet, depending on site conditions, logistics, and layout configurations.

In the contiguous 48 States, GeoBlock was tested on the Wambaw Cycle Trail in the Francis Marion National Forest near Charleston, SC. Sections of the trail had extensive trail braiding due to wet soils. Fifty-five feet of GeoBlock was installed with a clay-sand fill and a 2-inch cap over a geofabric layer. The installation completely stabilized the soils at the site. More than 3,500 passes had been made over the installation by endurotype motorcycles within the first 3 months of installation. According to the project manager, not one vehicle has ventured off of the hardened trail to further impact the wetland site. Except for minor rutting of the surface cap, there has been no noticeable wear to the surface of the GeoBlock panels (Parrish 2001).

Appendix A provides an evaluation of the two products. GeoBlock is highly suitable for use as a trail-hardening material. The 1 1/4-inch product, if available, would be suitable for most installation sites, while the 2-inch product could be used for extremely degraded segments, for crossing large ponded areas, and possibly for shallow water fords. SolGrid, which is still undergoing field tests, is suitable for irregular terrain and sloped areas, but is not as suitable for extreme conditions because of its limited ability to transfer lateral loads.

GeoBlock is available from Presto Plastics, Inc., P.O. Box 2399, Appleton, WI 54913. Phone: 800-548-3424; Web site: http://www.prestogeo.com

SolGrid is available from SolPlastics, 1501 des Futailles St., Montreal, PQ, Canada H1N3P. Phone: 888-765-7527; Web site: http://www.solplastics.com

Appendix B provides an installation guide for these products.

Matting--Matting is another method of wear-and-carry trail hardening. Metal matting was used extensively during World War II to reinforce soft soils during airport construction on tropical islands and at remote sites in Alaska. Some of those installations are still in place. The military stopped stocking metal matting in the 1950s and it is no longer manufactured. Its availability as a surplus material is very poor; therefore, it is not considered a viable material for trail hardening.

Matting available in the commercial market today is typically plastic decking or industrial antifatigue matting made from PVC or rubber. Plastic decking costs too much for trail hardening and is not discussed further. Rubber and PVC matting are somewhat more cost effective and are readily available. In contrast to the rigid porous pavement systems, matting is generally thinner and more flexible. It drapes across the terrain and provides an excellent wear surface, but has a limited ability to transfer lateral loads.

PVC Matting--Safety Deck was a commercially available PVC mat tested in the Wrangell-St. Elias mitigation study. Safety Deck is a high-density, semirigid, open-grid PVC mat that is 3/4-inch thick. It was supplied in 20-inch-square tiles that were laced together with parachute cord (figure 25). Safety Deck was installed on moderately impacted trail surfaces so the need to transfer lateral loads wasn't too extreme. In this less demanding condition, Safety Deck provided an excellent surface for all forms of use. However, it was expensive to procure and time consuming to install. Safety Deck had good vegetation regrowth values with an increase from 69 to 91 percent mean cover over the 4-year study period.

Figure 25--Safety Deck installed across a tundra
surface in Wrangell-St. Elias National Park and
Preserve in Alaska. Individual 20- by 20-inch tiles
were lashed together with parachute cord and
fishnet line. This time-consuming task drove up
installation costs. The material performed very well
on moderately degraded trails and provided an
excellent surface for most uses.

Safety Deck was the most expensive material tested in the Wrangell-St. Elias study. Costs were $7.50 per square foot, including shipping. Forty-three labor hours were required to install a 100-foot-long, 6-foot-wide section of trail, for a total cost of $5,274 per 100 feet.

Appendix A shows the positive attributes of PVC Safety Deck. Although Safety Deck is a strong performer for moderately impacted sites, its high cost limits its use for most OHV trail-hardening applications. It may have excellent application on foot or horse trails where the volume of material is much reduced, or in providing a surface for accessible trails where the costs might be better justified.

Safety Deck is no longer commercially available. It was originally purchased from The Mat Factory, Inc., of Costa Mesa, CA, phone: 800-628-7626. The company carries a similar product called Dundee Grass Retention & Erosion Control Mat. That product sells for about $5.33 per square foot. Other PVC matting products may also be available.

Rubber Matting--Rubber antifatigue matting is commonly available in discount and hardware supply stores. Rubber mats are typically available in 3- by 3-foot panels, are 3/4-inch thick and have an interlocking system along their edge (figure 26).

Figure 26--A section of rubber antifatigue mat
undergoing preliminary field trials at the Palmer
Hay Flats State Game Refuge in Alaska. The large
panels install quickly, but the rubber's flexibility
limits the panels' ability to transfer lateral loads.

Omni Grease-Proof Anti-fatigue Mat (manufactured by Akro Corp. of Canton, OH) and Anti-fatigue Mat (manufactured by Royal Floor Mats of South Gate, CA) were tested in a preliminary field trial in the spring of 2000 by the National Park Service and the Alaska Department of Fish and Game at the Palmer Hay Flats State Game Refuge. The mats protected the soil surface and conformed well to surface terrain, but provided no lateral load transfer. The wheel track was noticeably lower after 10 passes by an OHV on a silty substrate. The low rigidity of the rubber products and their inability to transfer load across the mat's surface limit their application for all but the lightest of impact areas.

A typical rubber mat sells for about $3.20 per square foot. Estimated installation time would be about 14 hours per 100 linear feet of 6-foot-wide trail.

Appendix A identifies a number of positive attributes of the rubber matting. Rubber matting is not suitable for trail-hardening applications on degraded trails because of its extremely low ability to transfer lateral load. Rubber matting would not prevent shear impacts on wet, finely textured soils. The material may have some limited applications before sites become degraded or could provide a temporary wear surface for special events.

Cost Comparisons for Trail-Hardening Techniques--Table 7 compares installation materials and labor costs for the trail-hardening methods discussed. The costs and hours of labor were developed for data in the Wrangell-St. Elias OHV mitigation study and other Park Service projects. These figures are rough estimates to assist in project scoping. Actual cost and the hours of labor depend on site conditions, logistics, and project design.

The test installations were small scale. Larger projects would benefit from volume discounts on materials and labor efficiencies. This is especially true when considering shipping costs of raw materials. The unit cost of shipping large quantities of bulky materials, such as the porous pavement products, can be much less than the cost of shipping small quantities.

Another important consideration is the cost of labor. The figures presented use an $18 per hour labor rate. This represents the cost of a typical government wage-grade seasonal maintenance worker in Alaska. The installation of trail-hardening materials is well suited for summer field crews, such as fire crews, Student Conservation Association crews, or volunteer crews. The work is relatively simple and doesn't require extensive use of power equipment. Fire crews filling in between fire calls or seeking early season training would be excellent sources of labor. The availability of cheaper labor could significantly reduce installation costs.

Table 7--Materials and labor costs of different trail-hardening methods.

Material	Cost per square foot ($)	Cost per 100 linear feet¹ ($)	Hours to install per 100 linear feet¹	Labor costs per 100 linear feet¹ at $18 per hour	Installation cost per square foot ($)	Installation cost per 100 linear feet¹ ($)
Corduroy	1.00	600	25	450	1.75	1,050
Wood matrix	0.80	480	70	1,260	2.90	1,740
Onsite puncheon	0.83	500	24	432	1.55	932
Gravel/geotextile	2.25+²	1,350+²	45	810	3.60+²	2,160+²
GeoBlock, 1 1/4 inch	2.75	1,650	38	684	3.89	2,334
GeoBlock, 2 inch	3.50	2,100	40	720	4.70	2,820
SolGrid	2.25	1,350	40	720	3.45	2,070
PVC matting	7.50	4,500	43	774	8.79	5,274
Rubber matting	3.50	2,100	14	252	3.92	2,352
¹ Trails are 6 feet wide. ² Depends on gravel source and haul distance.

Trail Closure

The final management option to be discussed is trail closure. As a last resort, resource managers may close a trail to protect threatened resources. This would halt direct trail impacts, but might not halt secondary impacts, such as erosion and sedimentation. A trail identified for closure needs to be assessed and stabilized or reclaimed as necessary.

Closing a trail is seldom popular with trail users. Before the action is taken, the proposed closure should be discussed at a public forum. Alternatives to the closure--such as reroute options, seasonal or type-of-use restrictions, controlled use, trail hardening, or other surface improvements--should be addressed and evaluated. Agency budgetary and workforce limitations that may restrict implementation of alternatives should be discussed. User groups may offer to accept some of the responsibility of maintaining or implementing necessary trail improvements to avoid losing access.

In the contiguous 48 States, user advocacy groups such as the American Motorcyclist Association and National Off-Highway Vehicle Conservation Council have often been able to help facilitate projects that protect trail access while assuring resource protection. These on-the-ground projects have rallied a large response from volunteer groups and individuals who develop a certain "ownership" of the trail resources they are working to protect. Often the energy generated by a resource conflict has been harnessed by land management agencies to generate support for work that has prevented trail closure.