Specification Guidelines to Cast an In-Place Concrete Retaining Wall

Considerations

• A retaining wall is built to withstand lateral pressure from the weight of the backfill being retained, and any surcharge -- additional weight -- from a slope, vehicle load or footings from a structure. The wall also has to resist hydrostatic pressure from the buildup of water in the soil. When the height of the retaining wall is over 4 feet, a building permit may be necessary. Consulting with an engineer is often a requirement for the building permit.

Types of Retaining Walls

• The structural design of retaining walls is based on the height required, to add strength while making cost-effective use of materials and labor. Gravity walls, the simplest type of retaining wall, are viable for heights up to 10 feet. Cantilever walls, where the walls have uniform thickness and put on a footing, are for heights from 10 feet to 25 feet. When supports are added at regular intervals to the backfill side of a cantilever wall, it becomes a counterfort wall, which is suitable for heights of 25 feet and up.

Joints

• Joints are planned breaks in concrete structures to contain cracks. Between two separate pours there is a construction joint, such as where the stem wall meets the footing. A key, or groove, is formed in the footing at this point to add to the wall's sliding resistance. Vertical contraction and expansion joints are placed to keep concrete shrinking from causing harm, and to allow for expansion due to temperature changes. Contraction joints are spaced at 30-foot intervals, and expansion joints every 90 feet.

Drainage

• It's essential that water drains away from the retaining wall to relieve hydrostatic pressure. The footing is placed on compacted, well-drained gravel, and a perforated pipe laid at the base of the footing, directing moisture away from the wall. Gravel is also used for backfill for a specified distance from the wall, put in 6-inch lifts, or layers, and compacted. Weep holes may be put in the walls, with pipes inserted to release water in the soil.