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Crib walls versus segmental block walls

Crib block wins on moving ground, curves and drainage; segmental block wins on finish. A crib wall is a cellular gravity structure of interlocking precast units filled with rock: it flexes with the ground, follows curves without special units, drains through its face and can be planted. A segmental wall is a stack of solid or hollow blocks with a finished masonry face, reinforced with geogrid above about a metre: it gives a clean architectural surface for estates, schools and public sites. On a rail corridor or a reactive clay batter we lean to crib. On an estate frontage or a campus we lean to Keystone or Allan Block. Where a wall is both visible and on difficult ground we often combine the two, as we did beside Sefton station on the Southern Sydney Freight Line, and design the pair as one structure.

Structure

Crib is a gravity wall: the assembled mass of units and rock fill resists the earth. Segmental is a facing system: below a metre it stands on block weight and friction, above that geogrid in the backfill does the work. Crib needs no geogrid and no deep excavation behind it; segmental at height needs both.

Ground and geometry

Crib tolerates settlement, vibration and reactive clay because nothing in it is rigid. It curves freely. Segmental is more sensitive to base preparation and prefers gentle curves; Allan Block handles tight curves better than pinned systems.

Finish and cost

A crib face is a pattern of open cells; planted, it disappears into landscape. A segmental face is finished masonry in a chosen colour and texture. On cost, crib is usually more economical at height on difficult ground, and segmental is often more economical where a finished face is required and the ground is straightforward.

Questions we are asked

Which lasts longer?

Both are precast concrete and both are long-lived when built properly. Crib's tolerance for movement gives it the edge on ground that shifts.

Can a crib wall have a finished face?

A closed-face crib unit gives a solid appearance, and a planted open face is its own finish. For a masonry look, a segmental system is the better choice.

Crib block Keystone Allan Block Segmental block

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