Special Structures (continued)

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.

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