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Crib Walls for Mountain Bike Trails

The use of cribbing (a timber wall built with 4x4's) reduces erosion at switchbacks.

By MaryAlice Stoner
USFS Missoula Technology and Development Center

Trail use, especially mountain bike use, has increased dramatically over the past decade. As a result, there has been an increased impact on trails, especially at switchback turns. The use of cribbing (a timber wall built with 4x4's) reduces erosion at switchbacks by stabilizing the soil and preventing trail users from cutting the turn.

MATERIALS

The 4x4's should be pressure treated for below-ground use (0.6 PCS) and all cuts should be field dressed with a water-borne preservative to insure longevity. The length of the crib wall is determined by the extent of erosion on the switchback. Materials needed for a crib wall are: 8-foot 4x4's (shorter ones may be used for a shorter cribbing); 12-inch spacers (cut from the unused portions of 4x4's); 6-foot, 4-inch metal fence posts; 3-foot, 3/4-inch dia. pipe/#6 rebar anchors; 3/8-inch x 6-inch galvanized lag bolts with washers; and heavy galvanized fence wire. The quantities of each depend on the wall size.

CRIB CONSTRUCTION

Begin construction of the crib wall on the upper trail of the switchback. Lay the first 4x4 down at least one foot up trail from the start of the resource impacts on the outside edge. Overlapping the ends at least 12 inches, continue to lay down as many 4x4's as needed. The end 4x4 should allow a 6-foot turning radius. It is better to construct a crib wall that is too long rather than too short. It can be trimmed later if necessary. Set the metal fence posts 4 to 6 inches from the ends of each 4x4, except the end nearest the turn. Pound the fence posts in far enough to secure them, but be sure they can be easily removed. The post nearest the turn will be secured later.

After determining crib wall length, join the 4x4's with shiplap joints. Mark where each 4x4 overlaps, about every 12 inches, as specified above. Use a chainsaw to make the shiplap joint cuts. Be careful not to cut too much. After the shiplap joints are cut, fit the 4x4's together and mark holes to be drilled at 2-inches and 10-inches from the joint's edge. Countersink 1-3/8-inch holes 1/2 inch deep from a 1/4-inch pilot hole.

Bolt the foundation pieces together and place it on its side next to the pre-set fence post so that the lag bolts lie horizontally on either side of the fence post. This piece must lie flat along the ground. Some digging may be necessary. The rest of the wall pieces will be bolted together later. Either a stair-step design for steep slopes or a masonry design for flat or nearly flat ground may be used.

SECURING THE FOUNDATION

Stack the crib wall to the desired height and pound the fence posts into the ground so that the tops are 2 to 3 inches below the top of the wall. The fence posts can also be flush with the top of the wall. If the ground is too hard to pound the post past a certain point, the post tops can be cut off with a torch or hacksaw. Make sure the fence posts are pounded far enough into the ground to secure the structure. (Note: When building the crib wall in solid rock, drill a pilot hole with a rock drill and put rebar in the hole so that six inches stick out above ground. Let the fence post rest against the rebar. Do not use sharp-pointed fence posts in this case). Unstack the wall, leaving the foundation 4x4's in place for the next step.

FINISHING THE CRIB WALL

The remaining pieces of wall are constructed the same way as the foundation piece. Cut all shiplap joints, countersink holes for lag bolts, and stack the pieces with the lag bolts in the vertical position this time. If the lag bolts are countersunk 1/2-inch to 3/4-inch, they will easily reach into the 4x4 below. Extra lag bolts can be added between the shiplap joints for strength. Add a lag bolt 2 to 3 inches in from the uphill end of the 4x4 to secure the structure. Drill a hole completely through the downhill side of the bottom 4x4. Pound an anchor through this hole into the ground and make sure the top is flush with the 4x4. Backfill the trenches and smooth out the tread.

CONSTRUCTING THE LOWER CRIB WALL

The lower crib wall is constructed in the same manner as the upper wall. Maintain the turning radius of 6 feet so that horses can easily negotiate the switchback. The length of wall depends on the extent of erosion. The height and design are determined by the pitch of the slope.

For more information on crib wall construction, contact MaryAlice Stoner, Missoula Technology & Development Center, BLDG 1, Fort Missoula, Missoula MT 59801 (406) 329-3935; FAX (406) 329-3719

RESOURCES

Natural Resource Managers In International Tourism and Rural Development; By Arthur W. Magill

Establishment of the relationships needed for effective communication among the variety of people involved with the tourism industry and natural resource management may rest with the professionals who have had the greatest difficulty interacting with the public: natural resource managers. They have a specific responsibility to support government programs that encourage the economic and social development of rural communities. Though less obvious to them, they also should contribute to efforts to reduce the nation's trade deficit. Both goals may be served through increased visits by international tourists to our public lands.

The tourism industry is unlikely to feel any responsibility to serve either goal. They are after all, in business to earn a profit, and fostering rural development or trade deficit reduction are unlikely endeavors, without external encouragement. Therefore, natural resource managers may need to set aside their reluctance to public interactions and establish a leadership role in garnering support for a tourism strategy for our wildlands.

This 10-page report, with bibliography, was presented at the 1992 Region Five Landscape Architects Workshop, Yosemite National Park, CA, March 24-26, 1992. Send your request for a free copy to the State Trails Program or call (303) 866-3203 Ext. 336.

Maintaining Desert Trails; Arizona Mountaineering Club

Specialized information on erosion, repair, and maintenance work on desert trails;. 25 pgs.

Available from Arizona Mountaineering Club, P O Box 1695, Phoenix AZ 85001-1695 (602) 256-0052.

CARIBOU PASS TRAIL REPAIR

By Jack Placchi

The Caribou Pass Trail is a main route that crosses over the Rocky Mountain's Continental Divide and is also an historic wagon toll road. The trail, which is in the Indian Peaks Wilderness of the Arapaho and Roosevelt National Forest, traversed across a very steep slope with several chutes. In a couple of these chutes, the tread was reduced to a hazardous condition, with less than a foot width and a drop off of about 800 feet to Caribou Lake.

Initially, the Forest considered constructing a hanging bridge that would suspend over the two worst chutes, but these would have required drilling anchor holes with motorized rock drills as well as utilizing materials that would have looked out of character in a wilderness setting and on an historical wagon road. A decision was made to maintain the character of the wilderness, adhere to primitive methods of construction, and build a rock wall. Rock walls would also fit in with the character of the trail since similar walls were used when the old wagon road was built and several of the old walls were still standing and in good shape.

A trail crew headed by Mark Manda, out of Bridgeport, California, specializing in primitive skills, was contacted for a two week detail to Colorado to build the rock walls through the steep rocky sections of trail. Mark and his crew constructed two large-dimension rock walls across the hazardous chutes, and reconstructed approximately 300' feet of trail. While the trail was being repaired to a safe condition, several people from the Arapaho and Roosevelt Forest received valuable training from the California crew and learned how to perform the work using only primitive skills and tools.

Also, the entire crew camped approximately 2 miles from the project site and hiked in daily to mitigate impacts to sensitive areas. They utilized a horse pack string and llamas to carry equipment and gear. With the project location at 12,000 feet, the crew was subjected to extreme weather conditions, including rain and a summer snow storm which dumped eight inches of snow on the project site. Mark and his crew hung in there and still completed the project within their two week detail.

The total cost to complete the project was approximately $9,000, including travel and per diem to Colorado. This was was $3,000 less than was originally estimated for the hanging bridge.

We encourage hikers to head out to take a look at the finished project! If you've crossed this section of trail before, you'll be very impressed with the ease of your next trek.

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