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Trail Construction and Maintenance Notebook

Special Structures (continued)

Crib Walls and Other Retaining Structures

Retaining structures are designed to keep dirt and rock in place. The crib wall keeps fill from following the call of gravity and taking the tread with it. Retaining structures are useful for keeping scree slopes from sliding down and obliterating the tread, for keeping streams from eroding abutments, and for blocking traffic from going places it shouldn’t.

The most common retaining structure is the crib wall. “Crib” is used primarily to keep compacted fill in place. Well-built crib is the most stable kind of uncemented retaining structure (except perhaps wire gabion).

Construct wood crib by interlocking logs or beams, pinned or notched (if logs) at the joints. Lay sill logs at right angles to the direction of travel and alternate tiers of face logs and header logs (Figure 62). Each successive tier is set to provide enough batter to resist creep pressure from the slope and to reduce pressure on the face logs from the fill. The ends of the header logs are seated against the backslope of the excavation for stability. As fill is tamped in place, filler logs are placed inside the structure to plug the spaces between the face logs, and are held in place by the fill. Outslope the tread to keep water from saturating the fill and excavation. Use guide structures to keep traffic off the edge.

Figure 62—The characteristics of a crib wall. Treated
logs are recommended.

Wood crib is also used to construct piers for bridges and to hold rock fill for abutments. Wood crib is easier to build than rock cribbing, but is less durable, especially in environments visited by rot or fire. Be sure to select rot-resistant logs if using native materials.

Rock or crib retaining walls are used when a sturdy wall is needed to contain compacted fill or to hold an excavation wall in place. Rock retaining walls are also called “dry masonry” because no mortar is used between stones. Rock, when available on site, is preferred over logs.

To build a rock wall, excavate a footing in soil or to solid rock. The footing should be insloped to match the designed batter angle and deep enough to support the foundation tier of stones (these are usually the largest stones in the wall) for the full width of the tread. Ideally, the footing is dug so that the foundation tier is embedded for the full thickness of the stones (Figure 63).

Figure 63—Rock placement and batter is critical on
the first courses.

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