What is a Retaining Wall?

A retaining wall is a structure that holds back soil so that the ground on one side is higher than on the other. FHWA names three uses. Engineers build retaining structures as bridge abutments and wing walls, for slope stabilization, and to reduce the right-of-way that an embankment needs.

Wall types

For many years, engineers made retaining walls of reinforced concrete and designed them as gravity or cantilever walls. These walls are rigid. They cannot accept large differences in settlement unless they stand on deep foundations. A modular block wall is a gravity wall. It resists soil forces with the mass of the wall, and sometimes with the soil in the voids of the blocks.

A mechanically stabilized earth (MSE) wall has three components: reinforcing elements, a facing system and reinforced fill. The reinforcement is steel strips, geosynthetic sheets, grids or similar inclusions in layers in the fill. The facing keeps the soil from raveling out between the rows of reinforcement.

Water behind the wall

FHWA calls good drainage essential to MSE wall performance, and it applies the same rule to any other wall type. Let the water flow through the wall, or collect and remove it before it enters the zone of influence of the wall. This prevents three problems:

• hydrostatic forces that raise the lateral pressure on the wall
• piping, which is erosion of one soil into another and creates paths for more water
• erosion at the toe, around the edges and at the top of the wall

Drainage features need maintenance. FHWA gives two examples. Seal the cracks in the pavement above the wall. Install and maintain screens on drain outlets so that animals do not clog the pipe.

Retaining walls on airfields

FAA AC 150/5370-10H, section 10, defines “structures” as airport facilities that a project may meet in the work. The list includes bridges, culverts, catch basins, inlets, retaining walls and cribbing. The circular gives no wall design or inspection rules.