It’s tragic to spend the time and money to build a retaining wall only to have it fail. Unstable walls can be dangerous; at the very least they are a discouraging waste of resources. Over the years we’ve seen far too many of these “hardscape fails”, and they have certain things in common. Here’s how to avoid some of the most frequent mistakes. First, let’s examine a few examples of typical wall failures, shown in the photo:
1. Segmental concrete wall: The wall in this photo was held together with plastic “pins”. Nothing was done to tie the wall back into the hill. Water runoff was allowed to soak in behind it. Tons of mud put so much pressure on the wall that it just collapsed, only a few days after it was built.
2. Dry-laid stone wall: This wall also failed because of backfill pressure. It was laid “plumb”, with no tie-backs behind it. In addition, fence posts behind the wall exerted pressure.
3. Interlocking block wall: These blocks weigh a ton each, so imagine all the weight bearing down on the wall footer! Again, runoff was allowed to flow behind the wall. It was built “plumb” (straight up and down) instead of leaning back toward the hill, and nothing was done to tie into the hill. Obviously the footer wasn’t substantial enough to hold the wall’s tremendous weight!
4. Railroad tie wall: We like to say that “anything wood in contact with the ground is temporary”. Again, the wall is absolutely vertical, with nothing but gravity holding it together. Soil expansion from moisture and freezing pulled it apart an inch each winter.
No matter how heavy the individual wall rocks may be, physics will overpower your wall if it isn’t designed properly. Even reinforced poured concrete walls will fall over if they are pushed sideways by the weight of backfill behind them, or heavy objects like cars parked above them. There are some basic rules of wall building that you need to understand before building a retaining wall, regardless of what material you are building with.
The first and perhaps most important is “batter”. In segmental retaining wall building, “batter” means that the wall should lean toward whatever it’s holding up. The center of gravity of the wall should be BEHIND the wall foundation, or footer. This way the weight of the wall actually helps hold the backfill in place.
Walls should be tied back into the hill, so that the weight of the backfill actually prevents the wall from moving. The face of the wall is the “skin”, but the wall gets its strength from the weight of the backfill on the tie-back. Geotextiles are a typical method; there should be a layer of “geogrid” back into the hill every two courses on taller walls.
The wall footer itself should be below ground. The taller the wall, the more it should start below ground, because the point of most pressure is actually at the BASE of the wall, not the top. The heavier the wall, the more substantial the footer needs to be.
There needs to be a way for water to freely escape from the footer trench, and from behind the wall, by gravity. Footer trenches need to drain like a bathtub, from their lowest point, to daylight.
Backfill behind the wall should be clean crushed (not round) stone, to allow water to easily drain. Backfilling with soil will cause the freeze-thaw cycle to push the wall outward, eventually collapsing it. Gravel backfill also prevents muddy water from seeping out and discoloring the face of the wall.
Water runoff behind the wall should always be diverted, never allowed to run into the backfill behind the wall. This may mean making the wall higher, and re-grading above the wall to guide runoff away from the wall.
Curved walls, or walls with multiple corners, are vastly stronger than long straight walls. This is simple physics. Try standing a business card on its edge; it will immediately fall. Simply curving or folding it allows it to stand on its own. Walls are no different.
Hardscapes should never be built on fill, or on ground that stays wet because of improper drainage. The footprint of a retaining wall concentrates the entire weight of the wall on a very small area, so the footer needs to be strong and solid, carefully and patiently compacted in “lifts” of two or three inches at a time so it won’t settle.
Wall building is hard work, and it can be expensive. It makes sense to understand the physics, and engineer the wall properly before you buy your materials. We like to say that “anything worth doing is worth doing twice”, however I’m not sure I like this philosophy when it comes to wall-building!
Steve Boehme is a landscape designer/installer specializing in landscape “makeovers”. “Let’s Grow” is published weekly; column archives are online at www.goodseedfarm.com. For more information call GoodSeed Farm Landscapes at (937) 587-7021.