A retaining wall is a wall that holds back earth. A concrete retaining wall can stabilize the soil and rock behind it from downward and out movement or erosion and provide support for vertical or near vertical grade changes. Retaining walls are generally made of timber, steel, masonry products, brick, stone or concrete. When designing a retaining wall or concrete retaining wall, one must consider the proper design and installation because the retained material is attempting to move downslope and forward due to gravity. The force of gravity creates a pressure behind the wall. This pressure is smallest at the top and is greatest at the bottom where it tries to push the wall forward. The pressure will push the wall forward or overturn it if not properly addressed. In addition, any ground water behind the concrete retaining wall that is not dispersed by a drainage system will cause an added hydraulic pressure on the wall.
There are three common types of retaining walls: gravity, cantilevered, and sheet pile walls.
- Sheet Pile Walls.
Sheet pile walls are often used in tight places. Sheet pile walls are often made of wood or sheet piles driven into the earth. Structural design methods for this type of wall exist but these methods are more complex than for a gravity wall. Usually, sheet pile walls are 1/3 above ground 2/3 below ground. Tall sheet pile walls usually require a tie-back anchor placed in the ground some distance behind the wall face. The anchor is then tied to the wall face behind a rod or cable. Anchors must be placed behind the potential failure plane in the soil.
- Reinforced-Soil Gravity Walls.
With reinforced-soil gravity walls, the soil reinforcement is placed in horizontal layers throughout the height of the wall. Common soil reinforcement materials include steel straps and a geogrid, a high-strength polymer mesh that provides tensile strength to hold the soil together. The wall face is usually precast, segmental concrete units that can withstand some differential movement. The reinforced soil's mass, along with the facing, becomes the gravity wall. The reinforced mass must be large enough to stand up to the pressure of the soil behind it. Gravity walls should be at least 55 to 65 percent as deep as the height of the retaining wall. The gravity wall may need to be larger due to the extra surcharge or slope pressures on the wall.
Prior to the introduction of modern reinforced-soil gravity walls, cantilevered walls were the most common type of taller retaining wall. Cantilevered walls are made from a relatively thin step of steel-reinforced, cast-in-place concrete or mortared masonry. This concrete mortared masonry is usually in the shape of an inverted T. These walls cantilever loads to a large, structural footing: converting horizontal pressures from behind the wall to vertical pressures on the ground below. Sometimes cantilevered walls are buttressed on the front, or include a counterfort on the back, to improve stability against high loads. Butresses are short wing walls at right angles to the main trend of the wall. These walls require rigid concrete footings below seasonal frost depth. This type of wall uses much less material than a traditional gravity wall.
- Gravity Walls.
Gravity walls are made from a large mass of stone, composite materials, or concrete. Gravity walls depend on the size and weight of the wall mass to resist pressures from the soil and water. Concrete retaining walls are a common example of gravity walls commonly designed today. Gravity walls often have a slight setback, or batter, to increase wall stability by leaning back into the retained soil. Low rising gravity walls are made from mortarless stone of segmental concrete units. Dry-laid gravity walls are more flexible and do not require a rigid footing below frost. Before the 20th century, tall retaining walls were often gravity walls made from large masses of stone or concrete. Today, taller retaining walls are more often built as composite gravity walls.
Concrete retaining walls are gravity walls that rely on their mass or weight for stabilty. The system consists of concrete masonry units which are placed without the use of mortar. This means that the concrete retaining walls are dry stacked on top of eachother. The concrete retaining walls rely on the combination of interlock and mass to prevent sliding or overturning. The concrete retaining wall units can also be used in combination with horizontal layers of soil reinforcement which extend into the backfill to increase the effective width and height of the gravity mass.