Crib Walls for Mountain Bike Trails
The use of cribbing (a timber wall built with 4x4's) reduces erosion at switchbacks.
By MaryAlice Stoner
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.
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.
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
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Updated January 20, 2009