Landscape Retaining Walls
by Joelle Steele
One early day in spring, a woman called and asked me to look at re-doing her landscape after a mini-landslide that had occurred when the retaining wall that bordered her yard and a neighbor's gave way following heavy rains. It was a 6' tall wall that her husband and the neighbor had built of hollow cinder blocks and mortar about 25 years earlier. It was holding back four pine trees and a handful of large shrubs. The wall was too high and not built correctly, was supporting the wrong kind of planting, and as a result could not hold back the soil when it became overly soggy from the rain. The rest of her small yard was ruined in the process. She said that the wall had been cracked in two places for at least ten years.
WHAT RETAINING WALLS DO
Most people don't realize how much work a retaining wall is really doing and how easily it can be destroyed by the pressure of whatever it is holding back. They hire companies that provide the cheapest bid (usually because they are not sufficiently competent to install a proper retaining wall). Or, some people decide to do it themselves – not a task for most amateurs.
Retaining walls have to be engineered correctly in order to withstand the enormous pressure of all the soil (and sometimes a building on top of that soil) that they are holding back. An average retaining wall built to no more than about 4 feet in height can be expected to hold back anywhere from 20 to 40 tons of soil. If that soil becomes very wet, that tonnage could almost double, depending on the porosity of the soil (e.g., sand drains better but allows more water into the soil; loams and clays let less water in but they retain what water they do hold over a longer period of time). Some walls lack drainage, making them even more likely to deteriorate with time.
In general, a retaining wall should not exceed 3-4 feet in height unless it is specifically engineered and built to hold back a substantially larger load. The reason? Because the taller the wall, the less sturdy it is and the more likely it is to fail. Taller walls that are engineered correctly usually have a 45-degree slope to them. In addition, most larger walls require permits to build them. A better option for holding back a hillside that is taller than 4 feet, is to terrace the hillside with a series of lower walls whenever space allows.
BUILDING RETAINING WALLS
Before you start building a retaining wall, keep in mind that it should never be used to hold back soil that is planted with big trees or large shrubs. Instead, plant that soil with low-growing, spreading plants with shallow root systems. Roots are the number one cause of damage to retaining walls, and even a large tree (like a 90-foot pine) that is 30-40 feet away could damage the wall with its wide-spreading roots.
Medium height retaining walls of 3-4 feet in height can be easily built with stackable or interlocking stones that you buy at home improvement centers. When you build your own wall at this height, start the first course of stones below ground level. To divert any water runoff, run a perforated drain pipe behind the wall before you backfill.
Low retaining walls of 2 feet in height can be built of loose, stacked, field stone. To do this, you don't usually backfill. Instead, you make the bottom of your wall about twice as wide at the bottom and let it lean against the slope of the land you want to hold back. As with interlocking stones, start the first course of stones below ground level. Walls of loose stone built at this height should be at least 1-foot wide at the top and 2-feet wide at the bottom.
It is advisable to avoid building any retaining walls that are more than 4-feet tall or that are constructed of hollow blocks unless the latter is reinforced with rebar and a concrete pour – a job for professionals. While mortar alone is fine for building a regular wall, it is not sufficient for a retaining wall that is holding back either soil alone or structures on top of that soil.
If you buy a property with a retaining wall on it, you should inspect that wall or have it inspected very carefully before you buy so that you don't end up having to bear the cost of replacing it. Damaged walls usually have to be replaced in their entirety, and it's very expensive. If you have a newly-built wall, you should inspect it regularly. A retaining wall that is holding back landscape only can be inspected every six months. If there's a building sitting on the soil that is being held back by the wall, the wall should be closely examined monthly (even if the wall is brand new) for even the slightest signs of leaking (puddling along the wall or water leaking out of the wall itself), cracks (even tiny hairline ones), bulging (even a small isolated bulge), or sinking (the wall is no longer level all the way across). These are all indicators that the wall is failing and is on its way to eventual collapse. It will happen -- sooner or later. An engineer should be called in immediately if any of these things are detected so that you can save your property from being ruined.
This article last updated: 09/07/2016.