A retaining wall is the only thing most homeowners will ever build that is holding back several tons of moving material, and it is also the one where almost all the engineering is invisible by the time the job is finished. That combination is why bad walls get built. On the day the last cap goes on, a wall built properly and a wall built badly look exactly the same.
Here is the thing people get wrong from the start. A retaining wall does not fail because the soil is too heavy for it. It fails because of water. When soil behind a wall saturates in a wet winter, the water in it cannot drain away and it exerts pressure sideways against the back of the wall — hydrostatic pressure. Saturated soil pushes several times harder than the same soil dry. Almost every leaning, bulging, cracked wall you have ever walked past was pushed over by water that had nowhere to go.
Which means the most important parts of a retaining wall are the ones nobody ever sees: the footing under it, the drainage rock behind it, the perforated pipe running through that rock, and where that pipe actually comes out. Get those four right and the face on the front is a styling choice. Get them wrong and no amount of expensive block will save it.
The footing comes first. It needs to be excavated into undisturbed ground — not built on fill, and not built on the loose material that came out of the trench — and it needs to be dead level, because every course above it inherits whatever error is in the base. A base course set half an inch out will be several inches out by the top of the wall, and you cannot correct it on the way up without it showing.
Then the drainage. Behind the wall goes free-draining aggregate — clean rock, not the soil that came out — running the full height, so water reaching it drops straight down instead of soaking into the fill and building pressure. At the bottom of that rock sits a perforated pipe, wrapped or sleeved so it does not silt up, and that pipe has to actually go somewhere. This is the step that gets skipped most often: a drain line that stops halfway along the wall and outlets into more soil is not a drain, it is a decoration.
Backfill is the last piece, and it goes in in lifts. Filling a wall in one go and letting it settle on its own leaves voids, and voids mean the ground behind the wall drops and the wall takes load unevenly. Compacted in eight- or twelve-inch layers, it stays where it was put.
There is one more decision worth understanding, because it changes both the cost and the result: terracing. Two shorter walls with a planted step between them handle a slope far better than one tall wall, because each is retaining much less material. They also usually look better, and they are less likely to need engineering sign-off. Where the grade allows it we will nearly always recommend the terrace over the single tall wall, and it is worth asking any contractor who does not raise the option why not.
If you have a wall already showing signs — a lean, a bulge in the middle courses, cracking that steps diagonally, or soil pushing through the joints — those are not cosmetic. They are the wall telling you the drainage behind it is not working. It is a great deal cheaper to deal with at that stage than after it comes down.
Millan's landscaping builds retaining walls across Glendale, Pasadena, La Cañada Flintridge, Monterey Park, Sierra Madre and the rest of Los Angeles County. Call (323) 697-8198 for a free on-site estimate.