Characteristics of Counterfort Retaining Walls

Retaining Wall Types

• Retaining wall structures are classified into four types. Gravity walls depend on weight and batter (the wall is angled toward the slope) to resist pressure exerted on them. Cantilever walls have a footing; a basement wall is basically a cantilever wall. Counterfort walls are similar to cantilever walls, with the addition of supports spaced evenly on the back of the wall for increased resistance to lateral forces. Buttressed walls are the same construction as counterfort, but the supports are on the exposed face of the wall.

Structural Characteristics of Counterfort Walls

• The supports -- counterforts -- are buried in the earth. This leaves the exposed face of the wall clean and creates more functional space in front of the wall. The counterforts are tied to the footing and wall with reinforced steel; when concrete is poured, the structures become a single unit. It's almost impossible for the wall to become detached from the footing. The ability of concrete to combine the structural elements as one is an advantage of the material.

Retaining Wall Design

• Retaining walls fail because of overturning, footing failure, or sliding caused by pressure from the soil or backfill behind the wall. Cantilever and counterfort walls are used to retain high volumes of soil or backfill where the weight of a gravity wall isn't sufficiently strong. The footing for the walls resists tipping or sliding. Counterforts reduce shear forces and bending moments caused by the soil, and the additional weight of the concrete supports also adds strength.

Engineering

• Walls higher than 4 feet generally need a building permit, and professional engineer design is recommended if not mandatory. The thickness of the wall, the dimensions of the footing and the extent of steel reinforcement are critical decisions for the design of counterfort walls. Drainage issues, and in some regions seismic activity, also have to be addressed. Knowledge of concrete properties is also essential for the construction of counterfort walls. Failure of retaining walls is costly to repair.