How to Build a Retaining Wall
DEAR TIM: Can you tell me how to build a retaining wall? Are retaining-wall blocks a good choice when building a retaining wall? What mission-critical things do I need to know about these walls so that I do not have a collapse or other failure in the years to come? I have access to lots of large stone. Can I just stack them to make a retaining wall from stone? Janine W., Fort St. John, British Columbia, CA
DEAR JANINE: Retaining wall construction is an art that has been around for thousands of years. You can see them in just about any archaeological photo of ancient building sites. In almost all cases, these long-gone builders used stacked stone to build walls that still are in good shape today. All you have to do is understand what they perfected through trial and error.
Since you live in a cold climate, you have two primary forces of nature that try to make retaining walls fail. The one is gravity, and this affects all retaining walls. The second one is frost in the soil. Natural erosion is working at all retaining walls, but it can be very slow acting, so it is often not a factor.
The soil that you are trying to hold back is being pulled towards the center of the Earth, but there's also a down-slope component as well. You can readily demonstrate this with a small pile of dry sand. Build a small hill using dry sand, and if you take some away at the bottom or the middle the entire face of the hill moves down.
The retaining wall you construct has to be strong enough to offset the pushing force of the soil. Do not underestimate this force as a cubic foot of moist soil can often weigh up to 100 pounds.
Large stones that have lots of surface area touching one another can often do the job. The friction of the stones rubbing against one another works against the pushing force exerted by the soil.
Tall retaining walls are at the greatest risk of failure. As the retaining-wall height increases, the force trying to topple the wall increases by a large factor. For example, if you double the height of a wall, the tipping force may increase by a factor of three or four times!
My advice to you is to take a little road trip around your area to see if you can discover some old retaining walls made from stacked stone. See what kind of condition they are in, and study those that appear to be very old and in great condition. Measure the size of the stones, the height of the wall, what is above the wall and the construction method. Take lots of photographs as they will help you as you start to construct your wall.
Try to keep your retaining wall under 3-feet high. These small walls are fairly easy to construct, and the force of gravity against them is not too great. You may want to construct the wall so it leans back a bit and is not perfectly plumb. This technique offsets the center of gravity of the wall making it harder to tip over.
Frost damage to retaining walls can be sinister. As soil freezes it expands. On a normal flat area, this expansion force is normally in the up and down direction. Builders know this as frost heave, and water-works employees know that this pressure cracks water mains underground.
But the frost can push sideways when it is adjacent to a retaining wall. Frost will relieve its expansion pressure using the path of least resistance. Since the ground freezes from the top down, it pushes sideways against the top of the retaining wall where the leverage force against the wall is greatest. One way to minimize frost damage to retaining walls is to backfill the wall with a well-drained material. Rounded gravel is ideal. The soil itself needs to be well drained so that water can't build up behind the retaining wall.
Retaining walls can be constructed with many materials: wood, poured concrete, concrete block, decorative stackable concrete block, etc. The science and engineering of retaining walls is very complex, so do not try to do a tall wall without an engineered solution.
Loads on top of a retaining wall also must be considered. If you have a flat area adjacent to the top of a retaining wall and park a heavy vehicle on the surface, the weight of the vehicle also pushes against the wall.
Be sure the base of the retaining wall is set down in solid soil. Do not just install the first row of stone on the top soil. The organic material in the soil is not a sound foundation for the wall. Be aware of soil creep. The soil itself is being pulled down slope each year by gravity and frost.