Special Structures (continued)

The batter should range from 2:1 to 4:1 (Figure 64). Factors determining this angle include the size and regularity of the rock, the depth of header stones, and the steepness and stability of the slope. At batter angles steeper than 4:1 or so, cement, or internal anchors (or both) may be needed for stability.

Figure 64—Terms used to describe crib walls.

On short walls, it may be possible to construct the entire structure starting upon a single keystone. The keystone is laid into the footing and successive tiers are laid. Each tier's face stones overlap the gaps between stones in the next lower tier. Each face tier includes tie or header stones that overlap the gaps between face stones and those deeper in the wall. The foundation tier (or the keystone) should be insloped slightly and rest on the excavated surface, not on fill. Each successive face tier should be staggered slightly into the hill to create the desired amount of batter. Header stones should also be used to tie deeper stones to those closer to the face. This is particularly important if the wall widens in cross section as it gains height.

Stones in each successive tier should be set so they have at least three points of good contact with the stones below. Good contact is defined as no wobble or shifting under a load without relying on shims (or chinking) to eliminate rocking. Shims are prone to shifting and should not be used to establish contact, especially on the face of the wall, where they can fall out. Backfill and tamp as you build.

Other forms of retaining walls include gabion and variations of wet masonry. Gabion is a series of wire baskets filled with rock. The baskets are wired together in tiers and can be effective where no suitable source of crib stone is available. Gabion is more artificial looking than crib (in the eyes of traditionalists at any rate), and may have a shorter 'lifespan,' depending on the type of wire used and the climate.

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