Retaining walls fail slowly and then quickly. A wall leans a little more each winter for years, and the owner gets used to the look of it. Then one wet February the whole face moves and the yard behind it drops.
The useful thing to know is that the blocks are almost never what failed. Concrete block and quarried rock are stronger than the soil they hold. What fails is the water behind the wall, or the ground under it.
What actually pushes a wall over
A retaining wall is holding back soil, and wet soil weighs far more than dry soil. Every hour of rain that soaks into the ground behind the wall adds load. If that water has no way out, it builds pressure against the back of the wall and pushes.
That is why walls around here move in the wet months and sit still all summer. It is also why two walls built the same way, one with drainage behind it and one without, do not age the same at all.
Reading the warning signs
Leaning outward at the top. The most common sign, and the one people ignore longest. A wall that tilts a little more each year is being pushed by water pressure. It will not correct itself and it does not stop.
A bulge in the middle of the face. More serious than a lean. A bulge means the wall is failing in one section while the rest holds, usually where water is concentrating. Bulges tend to be the last stage before a section comes out.
Stair-step cracks through block or mortar. Cracks that run diagonally along the joints mean the wall is being pulled apart as parts of it move at different rates. Worth looking at soon.
Gaps opening at the joints. Blocks that were tight when built and now show daylight between them have shifted. Check whether the gaps are wider at the top than the bottom, which points to a lean, or widest in the middle, which points to a bulge.
Ground sinking behind the wall. If the soil behind has dropped, material is washing out through or under the wall. The wall may look fine from the front while it is being hollowed out from behind.
Water seeping through the face after rain. This one is mixed news. Water coming through means pressure is escaping, which is better than it building. It also means water is running where it was not meant to, and it is carrying fines out with it.
What is usually behind it
Most failed walls we are called to look at come down to one of these.
No drainage behind the wall. The wall was built straight against the dirt with no clear crush and no pipe, so water has nowhere to go but against the back of the blocks.
A base that was too thin. If the wall was set on topsoil or a shallow layer of gravel, it settles unevenly and the courses go out of level. Everything above that follows.
Backfilled with the material that came out of the hole. Native clay holds water and swells. It belongs in the yard, not directly behind a wall.
Built too tall for the way it was built. Height changes the engineering. In BC, walls over about 1.2 metres generally need engineering, and taller walls often need geogrid tying the wall back into the slope behind it. A tall wall stacked without that is holding on by weight alone.
Can it be repaired, or does it need rebuilding?
It depends where the movement is. A wall that has shifted only in the top course or two can sometimes be taken down to a sound course and rebuilt from there, with drainage added while it is open.
Once a wall is leaning through its full height, or bulging in the middle, rebuilding is usually the honest answer. The problem is behind the wall, and you cannot fix that without taking the wall apart. Pinning or facing over a wall that is moving buys a season and costs you the price twice.
The distinction matters for cost, so it is worth having someone look before you assume the worst.
What to do about it now
If you have a wall doing any of this, take photos from the same spot a few weeks apart. Movement is hard to see day to day and obvious in two pictures side by side. If the second photo is worse, stop waiting for spring.
We build retaining walls across the Lower Mainland and the Fraser Valley, and we dig the drainage side of the job ourselves, so we can tell you whether a wall is worth saving or not. You can see recent walls we have built, including a before and after of a slope rebuilt from dirt.
Send a photo and rough measurements and we will come look. The quote is free.