Guidelines for a Poured-in-Place Concrete Retaining Wall

Retaining Wall Design

• Concrete retaining walls are usually constructed either as gravity walls or cantilever walls. Gravity walls use weight and an angle (batter) back into the slope being retained for strength; the gravity wall is wider at the base than the top. Cantilever walls have a footing, with a wall of uniform thickness placed on it. Gravity walls are recommended for heights up to 10 feet, and cantilever walls for heights 10 to 25 feet, but cantilever wall design is often used for lower walls.

Site Preparation

• The base or footing should be placed on well-drained, compacted, granular material. Organics and soft soil should be removed and replaced with at least 6 inches of gravel. Typically the footing is 6 inches thick and extends 4 inches past the wall on each side; it extends farther from the wall on the low side than the backfill side for high walls. The footing is installed below the frost line in cold climates, and 6 to 12 inches below grade otherwise.

Reinforced Steel and Control Joints

• Concrete has strong resistance against compressive forces -- weight pressing on it -- but less tensile strength -- forces pulling it apart, such as movement created by freezing and thawing action. Reinforced steel increases its tensile strength. Reinforced steel bars are installed on the side of the retaining wall that's backfilled. They're also used to dowel the footing to the wall. Control joints to manage cracks in the concrete are installed at least every 20 feet along the wall.

Concrete and Backfill

• The concrete mix for retaining walls is usually designed to bear a load weighing 3,500 pounds per square inch (psi) 28 days after it's poured. Monolithic pours, where the footing and wall are poured at the same time, eliminates a potential weakness where the two connect when they're poured separately. The wall isn't backfilled until 28 days after it's poured, to allow it to gain strength. Granular backfil is used against the wall to 12 inches from the top of the wall.