Safety is the Key Design Element
Retaining walls must hold steep slopes. The only reason you don’t see them used more often as a feature is because they are so expensive.
Retaining walls can be both a beautiful and functional part of a landscape. Gabion walls are boulders contained in wire grids. They can be used in inexpensive ribbons or mattress shapes to provide slope stability in difficult situations. They can be made more attractive with live stakes or other vegetation inserted between the openings in the walls. Concrete retaining walls are massive slabs of gray, but they can be made more attractive by incorporating attachment ties along the façade for adding a decorative face. Textured forms can build in interesting surfaces on concrete wall surfaces. Modular units are often used to build attractive and easy-to-construct terraces.
Retaining walls can be dangerous. If a wall is over four feet tall, a landscape architect must call in a civil engineer for design assistance. Walls can fail, and when they do, they can fail in a large way, posing a hazard to people and property.
Gravity walls are massive structures that use large stone units, battered back about one-half inch each top course to form a slanted wall. They hold back soil through sheer weight. Most gravity walls are low. Because it is difficult to gauge how well they can do their job, the chance of damage during unusual storm events is high. A “dead man” can be added to hold the wall in place, by keying perpendicularly to the hillside. Railroad tie walls use T’s made from additional ties and secured to the wall to do this.
Larger retaining walls are reinforced concrete with rods or interlocking keys. Additional geosynthetic fabric is often used every few feet, woven into the layers of backfill to add strength. Modular units can vary in quality, so shop carefully and choose wisely. Good pieces typically weigh over fifty-five pounds each.