A different way to look at Retaining Walls
Building decorative garden walls is very rewarding and rewarding as well as good practice. We define “decorative” or “landscaped” walls as walls less than two feet high. Retaining walls for purposes such as retaining hills, shoring pavement or buildings, and controlling drainage are more serious projects. Let’s call them “Engineered Walls” because if the purpose of the wall is more functional, with long-term implications, some basic engineering is essential.
A functioning retaining wall looks different: Imagine the wall block as a “skin”, because a real wall actually consists of four other important components. Let’s call this footer, drainage, tieback, and backfill.
A wall footer begins with a trench that is at least twice as wide as the depth of the block from front to back. The bottom of the trench should drain to the lowest point, like a bathtub, with nowhere to hold water. The foot trench should always be compacted thoroughly. The trench should be filled with clean layers of gravel, which are compacted in “elevators” of no more than 10 cm at the same time. The higher (and heavier) the wall, the thicker this ballast foot should be. Since the bottom of the wall should be underground, a good rule of thumb is to make the foot trench twice the thickness of the block and fill it halfway with crushed stone.
Backfill is the material behind the wall, between the “skin” of the block and the slope you want to support. This should always be clean, crushed (square, not round) stone that is larger than an inch to allow the water to seep down behind the wall. Filling in with soil is a common mistake; wet floors expand and freeze, gradually pushing the wall from behind.
The drainage behind the wall is key; Improper use of water behind and under walls is the most common cause of walls collapsing. Perforated tiles at the bottom of the foot ditch that drain from the wall into daylight should be installed before adding gravel. Surface runoff behind the wall should be drained around it; Ideally, the wall should be high enough to allow a trough from one end to the other.
The wall block or the skin must be “tied back” into the hill behind it. Typically this is done by installing a strong geotextile fabric called a geogrid between the layers of block and extending back into the slope behind the wall. This net is held in place by the weight of the block and the backfill gravel and forms a kind of sandwich. For the wall to fail, the net would have to break or pull out from under tons of gravel.
Understanding these basics is a good place to start, but there are many more tips and tricks that come with training and experience. Every wall situation is different and there are many different types of segment wall systems, each with a cost-benefit ratio. It is important to know your own limits before you begin.
As a certified hardscape contractor, we are often called in hindsight when structural retaining walls fail for one reason or another. This is not a pleasant experience for us or the homeowner. It is much more expensive and time consuming to repair a poorly installed retaining wall than it is to install it first. Usually the cause of wall failure is built in and the only solution is to remove it and start over.
Steve Boehme is a landscape planner / installer who specializes in landscape makeovers. “Let’s Grow” appears weekly; Column archives can be found on the “Gardening Guide” page at www.goodseedfarm.com. For more information, visit www.goodseedfarm.com or call GoodSeed Farm Landscapes at (937) 587-7021.