What Is the Purpose of Counterfort Retaining Walls?

Retaining Wall Types

• Retaining walls are constructed where high volumes of earth need to be held back, usually for the purpose of creating useable space for construction. Retaining walls are classified in three types: gravity, cantilever and counterfort, in order of height and the associated increase in strength required. Gravity walls are suitable to a maximum of 10 feet, where weight and batter (angling into the slope) are sufficient to resist lateral pressure from the weight of the soil. Cantilever and counterfort walls are designed to resist increased pressure.

Cantilever and Counterfort Walls

• Retaining walls may fail by either overturning, footing failure, or sliding. Cantilever retaining walls are walls of uniform thickness, attached to a footing to protect the wall from tipping or sliding forces. Counterfort walls are similar to cantilever with the addition of thin triangular supports, called counterforts, that anchor the back of the wall -- the slope side -- to the footing. The footer, wall and supports are tied together with reinforced steel, and often poured as one unit.

Counterfort Design

• The counterforts provide added strength to the wall to resist the shear forces and bending moments that the weight of the soil brings. The support side of the wall is backfilled with the gravel or earth of the slope. It's impossible for the footing to be detached from the wall. A variation on this design is the buttress wall, where the supports are on the level side of the wall for stabilization. Counterfort walls are more common than buttress walls.

Choosing Counterfort Walls

• Once a wall has to be higher than 25 feet, counterfort walls are more economical than cantilever walls, requiring less material. Otherwise, the footing and wall of the cantilever design has to be made thicker and wider to achieve the same strength as the counterforts. The clean exposed face of the counterfort wall leaves more room for development since the space isn't shared with structural members. Ultimately, deciding on the type of retaining wall for a given situation requires balancing technical, environmental and economic factors.