A gravity wall uses the weight of the wall to hold back the ground. Sometimes the top of this wall is set back to resist the top pressures of the ground. In the beginning most of these walls were dry-stacked, flat rock placed on another. This allowed the water to drain out with damming up and creating more pressure. Modern stone and mortar retaining walls use small pipes through the face to drain out excess water. These are good for levels under 4 feet but the forces at higher levels can produce warping and cracking. Thick, reinforced-concrete gravity walls are used for taller walls.
When a post is pounded into the ground the forces of the ground around it hold it upright. This is called a piling force and can be applied to retaining walls. How it works is that the reinforced-concrete wall is buried underground and then backfilled in the same way as a gravity system. So there are forces making it work: the ground holding the piling at the base and the weight and strength of the reinforced concrete.
One of the oldest form of retaining walls dating back to the earthen works of the Roman Empire th is system uses an upright, gravity wall attached to a “foot” which extends back underneath the embankment to form and “L” turned backwards. This ingenious form converts the horizontal pressure of the embankment to a downward force on the foot below which takes much of the pressure off the vertical wall. In highway construction some cantilevered designs include cross-pieces which boxes behind the wall. When these boxes are filled with gravel the whole wall exerts a downward force and so the gravity wall in front just becomes part of the system instead of taking much of the force.
These are another version of the “boxed cantilever” system. Heavy-wire, square cages are placed on the ground in front of the earthworks and wired together until they are on unit. Then the cages are filled with rocks until full whereby covers are wired down over the rocks. This requires no gravel backfill as the rocks, themselves, provide an escape for the water.
Anchor Wall System:
The anchored wall is a gravity wall that has a piece of treated wood or a long piece of reinforced concrete buried behind the wall. This is attached by a piece of steel or a steel cable to an object inside the reinforced concrete wall. The anchor becomes like a man holding onto a rope and digging his heels in the ground to prevent the horizontal force of the earth.
Concrete Block Walls:
Perhaps the greatest innovation in retaining wall architecture is the invention of the interlocking concrete block. These are basically hollow cinder blocks that are stacked and the holes filled with gravel. Each level moves back 1” so that it is a gravity-type wall that angles back. These are only good to around 6 feet high but the landscapers can build 2 or 3 tiers to achieve the height they want. The great thing about these blocks is that they come in many front shapes and colors to mimic rock so that the wall becomes an aesthetic feature for the yard.