Building retaining walls is more that just pouring a pile of concrete—-or stacking a bunch of stones together and hoping for the best. Retaining walls are “systems” that require proper footings, proper construction, proper drainage and proper back-fill—-depending on the types of walls, height etc. Many homes have retaining walls installed along the driveways so that access is possible to the garage or basement. These walls are often poorly constructed, usually have inadequate drainage systems, and have no barriers to prevent falls—-especially in older homes.
Of all the components of the retaining wall (putting personal safety aside for a moment), its drainage system is perhaps the most important. You can build walls like the this brick retaining wall and if it is drained properly it will do its job. But, if you build a nice concrete wall and backfill it with non-draining fill, the result will be leaning and possible collapse of the wall—-sooner or later. Here are a couple of pictures of poorly drained retaining walls that are nearing the point of collapse.
As a general rule, if you run an imaginary plum line down from the top of the wall, and the distance from that line to the base of wall is greater than ½ the thickness of the wall—-collapse is possible due to gravity—-regardless of pressures against it. (As a side note—-concrete is heavy, and tipping over concrete is not a good idea.)
Loose stone walls can sometimes be more forgiving because water that gets behind the wall can drain out though the many openings. Solid concrete and brick walls need weep holes along the base of the wall to relieve hydrostatic pressure. All types of retaining walls should have drainable fill behind them so that hydrostatic pressure is not an issue.
Some loose stone walls employ TROLLS to keep an eye on things.
Charles Buell, Seattle Home Inspector
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