Unless you live in Saskatchewan or you home is in one of the many large river valleys there is a good chance that your house is built on a slope. In cases like this the developer would have carved the lot out of the side of the hill and bulldozed this fill to other side to level off the site. To hold back the bank and to hold up the lower end either the slopes were smoothed and grassed over or a retaining wall was built.
In its simplest terms a retaining wall is an earth dam, constructed to hold up a bank of earth or rock in order to maintain the consistency of the land above and below. They can be as low as a metre high for a small garden or towering several stories to make the grade approachable for a highway interchange. Once erected, their job is only in the beginning stages because over the lifespan of the structure it must hold back the natural flow of the earth due to gravity. There are many types of retaining walls depending on the usage and every type can be made from a variety of materials.
The original retaining wall for yards and gardens was creosote wood planks and longs that resisted water for a long time but were unsightly. Environmentalists hated them because they slowly leeched poison into the ground that could get into the aquifer.
Ground water is the enemy of the retaining wall because it can be absorbed into the area behind the wall both from the surface and from aquifers. It has to be diverted or else the pressure might exceed the tolerances of the wall design. In most cases a perforated pipe is placed at the bottom of the wall and the backfill is 1” gravel which will allow the water to drain down into the pipe and be led out to the storm drain. To protect the gravel from getting clogged with silt over time landscapers use a fabric between the soil and the gravel backfill.
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.
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.
Retaining walls, be they large or small, are also great for forming gardens and making water features in your backyard.