How Retaining Walls Work

Retaining Walls

Retaining walls are used to retain soil to create a level surface in a sloped area to allow for construction. The volume of soil that has to be retained exerts lateral pressure on the wall. A load imposed in addition to the retained soil, called the surcharge, increases the pressure. The surcharge may result from sloping retained soil, structural footings supported by the retained soil, or a traffic load. Retaining walls usually fail from sliding, overturning, or foundation failure.

Gravity walls resist overturning from lateral pressure with the weight of its structure and batter -- angling back into the slope. These walls are thicker at the base than the top, and, as they rise, they become thinner. Gravity retaining walls are suitable for heights up to 10 feet. Cantilever walls are used for heights from 10 to 25 feet and is a wall of uniform thickness connected to a footing. The footing resists overturning and sliding forces caused by the weight of the earth.

A counterfort wall is a cantilever design with the addition of triangular supports -- counterforts -- attached to the top of the wall and the footing at regular intervals on the slope side of the wall. The concrete footing, wall, and counterforts are tied together with rebar and poured as a single structure. This construction method increases the resistance to high lateral loads. Counterfort walls are more economical than cantilever walls once the required height is greater than 25 feet.

Design for the type and sizing of retaining walls includes a number of technical calculations to check the stability of the wall design against possible failure. Moisture in the soil increases the pressure on the wall; allowing for drainage with weep holes and drainage tile at the base are an important part of the construction for retaining walls. Apart from the structural components, the ground has to be compacted to prevent settling that might cause the wall to rotate.