United States Department of Agriculture Forest Service Technology & Development Program
		August 1992	9223-2336–MTDC
		2300

Mountain bike use on National Forest trails has risen dramatically over the past decade. As a result, there has been an increased impact on resources, especially at switchback turns. The use of cribbing (a timber wall built with 4 x 4s) reduces resource impacts on switchbacks by stabilizing the soil and preventing bicyclists from cutting corners.

The technique for building a crib wall described in this report was developed by the Los Padres National Forest in the Pacific Southwest Region.

Materials

The 4X4s should be pressure treated for below ground use (.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 14-foot by 3-foot high crib wall are:

• (10) 8-foot 4x4s (shorter ones may be used for a shorter cribbing)
• (9) 12-inch spacers (cut from the unused portions of 4x4s)
• (3) 6-foot, 4-inch metal fence posts
• (6) 3-foot, 3/4-inch dia. pipe/#6 rebar anchors
• (25) 3/8-inch x 6-inch galvanized lag bolts with washers
• 60 feet of heavy galvanized fence wire

Tools

• Marking pencil
• Measuring tape
• Level
• Shovel
• Hammer
• Socket wrench
• Fencing pliers
• Sledge hammer
• Heavy duty fence post pounder
• Chainsaw
• Portable drill or wood auger brace
• Acetylene torch or hacksaw (not
  always necessary)

Crib Construction

Begin construction of the crib wall on the upper trail of the switchback. Lay the first 4x4 down at least 1 foot up trail from the start of the resource impacts on the outside edge (Figure 1). Overlapping the ends at least 12 inches, continue to lay down as many 4x4s 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 easily be removed. The post nearest the turn will be secured later.

Figure 1.-Crib walls are constructed on the outside edge of the trail.

After determining the length of the crib wall, join the 4x4s with shiplap joints (Fig. 2a). Mark where each 44 overlaps, about every 12 inches, as specified above. A chainsaw can be used to make the shiplap joint cuts. Be careful not to cut too much. After the shiplap joints are cut, fit the 4x4s together and mark holes to be drilled at 2 inches and 10 inches from the edge of the joint. Countersink 1-3/8-inch holes 1/2-inch deep from a 1/4-inch pilot hole (Fig. 2b).

Figure 2a-Join the 4x4s with shiplap joints.

Figure 2b-Countersink bolts.

Bolt the foundation piece 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 (Fig. 3). 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 (Fig. 4).

Figure 4-Stair-steps are used on steep slopes. A masonry design is used on more level ground.

Figure 3-Lag bolts on either side of fence post.

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 6 inches stick out above the 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 4x4s in place for the next step.

Dig a trench for the anchor bars (rebar) at least 2-feet below the tread of the trail and perpendicular to each fence post (Fig. 5). Pound an anchor bar into each trench at a 45-degree angle. Make sure the anchors are in a straight line from their respective fence posts. Anchors should be pounded in so that the tops rest 15 inches below the trail bed when covered up.

Figure 5-Trench for anchor bars.

Drill holes into the 4x4s on each side of each fence post. The holes should be in between each lag bolt (Figs. 6a & 6b). Run fence wire through the holes to tie the anchors, foundation, and fence post together. Twist the wire ends together and tighten them using a turnbuckle-style tensioner to take out any slack. This is typical of an H-brace (Fig. 5). Make sure the anchors are solidly planted in the ground, that the wire is taut but not pulling so hard that the anchors will be pulled out, and that the foundation 4x4s are secure and flush with the ground. This is the last chance to make sure the foundation is well constructed and secure before backfilling the trenches.

Figure 6a-Secure fence post to 4x4.

Figure 6b-Cross-section of 44 secured to fence post.

Finishing the Crib Wall

The remaining pieces of the wall are constructed in 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 (Fig. 7). 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 44. Backfill the trenches and smooth out the tread.

Figure 7-Finishing the crib wall.

Constructing the Lower Crib Wall

Figure 8-Completed crib wall.

The crib wall on the lower part of the switchback turn is constructed in the same manner as the upper crib wall (Fig. 8). The turning radius should be maintained at 6 feet so that horses can safely and easily negotiate the switchback. The length of the crib wall depends on the extent of erosion. The height and design are determined by the pitch of the slope.

Substitutions

Although they are not as strong, nails and spikes can be substituted for lag bolts to save time and money. Wood posts can be substituted for metal fence posts, but it may be difficult to dig post holes on a cut bank.

This page last modified March 22, 2001