Retaining walls reduce the angle of a slope so it is easier to plant, walk on or use as an accesible landscape. Retaining Walls are typically made by 'cutting and filling'; a practise of excavating a portion of the slope and then using the cut soil to backfill behind the retaining wall creating a relatively flat area. A single retaining wall may be enough to support a short stretch of slope, while a series of parallel retaining walls is often used to support larger slopes.
The taller the retaining walls are and the closer they are together, the flatter the slope is above them. Terraces are created when a retaining wall is tall enough to create a level surface above it. Terraces are necessary for pathways, lawns, patios, vegetable gardens and other areas where people need to move around comfortably. But if the goal is only to plant the landscape with ornamentals, the retaining walls can be shorter -- most groundcovers will thrive on slopes as steep 45 degrees, but getting the slope down to about 20 degrees opens up more possibilities for landscaping.
Retaining Walls Material and Construction
Retaining walls are built with natural stone, concrete blocks, railroad ties and a variety of other materials. Seattle Rockeries uses material that match the look of your house and fit with the overall style of your landscape. Sticking with the same material for all the walls and stairs is a wise choice because it gives cohesiveness to the overall design.
Retaining walls over three feet tall generally require a building permit and should be built by a professional contractor. Retaining walls of any size need to be built on compacted subsoil to prevent them from settling over time and need a perforated drain pipe and a layer of drainage gravel behind them so water can flow away rather than build up behind the wall. Walls may be constructed with a perfectly vertical profile or angled slightly into the hill which reduces the complexity of building them.