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.
Retaining walls hold soil upright, and soil can hold moisture. Moisture and storm water can tear down substantial wall structures with ease, so it is important to incorporate weep holes, drainage pipe, and gravel backfill to deal with the extra pressure moisture brings to the back side of every retaining wall. Stone walls without mortar allow moisture to pass through. Freezing ice can shift the stones up and down with changing weather patterns, and built-in flexibility can prevent damage. Concrete and mortared walls are rigid and need special drainage structures behind the walls to deal with the shifting due to moisture and pressure forces.
Do not trust retaining wall construction to amateurs! Here’s a photo of what can happen if a wall is not built properly. A six-inch-wide ribbon of porous backfill material lying flat along the entire length and height of the back of a retaining wall should be installed. It needs a drain running along the bottom that is tilted down to an outlet beyond the wall. If excess moisture has nowhere to go, it will push against the wall and cause it to fail.
Most retaining walls are built with a level backslope at the top of the wall. If the ground continues to slope behind the wall, it can cause tremendous pressure against the wall in the same way trapped moisture does. Designers attempt to deal with steep grade changes by creating a series of small, terraced retaining walls, rather than a single, tall wall with sloping back slopes. This can become a dangerous situation, too, unless special reinforcements are added to the design. Call a civil engineer or manufacturer’s representative when dealing with excessive slopes. Too many grade contours may mean the siting of the structures in the project are wrong. Forcing a building site on to a steep slope, especially a steep fill slope, is not wise.