Why Is My Retaining Wall Leaning? Almost Always This

Because the soil behind it is pushing harder than the wall was built to resist, and in almost every case the extra push is water. A retaining wall holds back a wedge of soil that wants to slump. Dry, that wedge has a certain weight. Saturated, it is heavier, it loses internal friction, and water pressure builds against the back of the wall on top of it.

A wall built without drainage stone, without a perforated pipe and without an outlet is effectively a dam. Every wet spring tests it a little harder, and each season it gives up a little more.

The industry design guidance for segmental block walls is explicit about this. The drainage aggregate behind the units is there to let incidental water drain, relieving hydrostatic pressure, and to stop retained soil washing out through the face.

1. No drainage. By far the most common. Look for an outlet pipe or weep points near the base. If there is none, and the wall is holding back clay, that is probably your answer.

2. A poor footing. No levelling pad, or a pad on soft, uncompacted ground. The wall settles unevenly, goes out of plane, and a block wall out of plane cannot be pushed back.

3. No reinforcement on a tall wall. Beyond a modest height a segmental wall needs geogrid layers tied back into compacted fill, so that the soil mass becomes part of the wall. Without it, a gravity wall is fighting the whole slope on its own weight.

4. Backfilled with the clay that came out. Free-draining stone behind a wall exists so water reaches the pipe instead of sitting in the soil. Putting the excavated clay back is cheaper on the day and expensive later.

Timber and railroad-tie walls have a fifth cause: they rot. A timber wall that is leaning after years in wet ground has usually simply reached the end of its life.

The ground makes water behind a wall likely. On the Springfield Plateau, around Bentonville, Rogers, Bella Vista and Cave Springs, the surface is red clay full of chert fragments weathered from the Boone Formation. It holds water, and it drains slowly.

Around Fayetteville the problem compounds. The Fayetteville shale swells as it takes on water and shrinks as it dries, so the soil behind a wall does not just get heavy, it pushes. That is why walls over shale need more drainage and better backfill than the same wall on the plateau.

Add freeze-thaw. Water trapped in the top of a wall or in the soil behind it freezes, expands by about nine percent, and pushes on the courses. Over enough winters, that alone will displace a cap and the courses under it.

It depends on where you live, and the local lines are lower than people assume.

Those rules are checked in our town permit guides, and they change, so confirm with the city. Separately from the rules, a wall that holds back a slope above a house, a drive or a neighbour deserves engineering whatever its height.

Sometimes. If only the top courses have moved and the base is straight and sound, resetting them and fixing the surface water can work. That is worth assessing properly rather than assuming either way.

If the wall has rotated, the base has moved, or the face is bulging, rebuilding is the honest answer. A block wall out of plane cannot be pushed back, because the courses no longer bear on each other the way they were designed to.

Sometimes the best answer is not the same wall again. Replacing one tall wall with two properly drained terraces can solve the engineering problem, drop each wall under a permit threshold, and make a better garden at the same time.

• Excavation to firm ground and a compacted levelling pad.
• A zone of free-draining aggregate directly behind the units. Industry design guidance calls for at least twelve inches of it behind segmental retaining wall units.
• A perforated pipe at the base, wrapped or filtered, discharging to an outlet that actually drains.
• Geogrid where the height and loading need it, laid at the designed levels.
• Backfill placed and compacted in lifts, not dumped.
• Surface water above the wall directed away from the top of it.

If a quote for a new wall does not mention stone, pipe and an outlet, it is pricing a dam.

Most garden walls here are one of two types. A gravity wall holds the soil back by its own weight and a slight backward lean, called batter. Stacked stone, most segmental block walls at modest heights and many older walls are gravity walls. They work well within their limits, and those limits are set by height, the slope above, what sits on top, and how wet the soil gets.

A reinforced wall ties layers of geogrid back from the wall into compacted fill, so the wall and a block of soil behind it act together. That is how segmental walls get taller safely. It also means the excavation behind the wall is much bigger than people expect, which matters on a tight lot or near a boundary.

A wall that was built as a gravity wall but asked to do a reinforced wall's job, too tall, too loaded, too wet, is one of the most common leaning walls there is.

Splitting the height between two walls reduces the load on each and can keep each below a permit or engineering threshold. It also gives you a planting bed between them, which softens a slope far better than one blank face.

The catch is spacing. Put the upper wall too close to the lower one and the upper wall's weight bears on the lower, so the pair behaves like a single tall wall. Bella Vista's retaining wall guidance, for example, treats terraces closer than twice the lower wall's height as one wall for design. Terraces have to be designed as a system.

"Weep holes are enough." A few gaps in the face let some water out, but without drainage stone and a pipe behind them, clay just seals them up.

"Bigger blocks will hold it." Weight helps a gravity wall, up to a point. It does not fix water pressure, a soft footing or a wall that needed reinforcement.

"Landscape fabric behind it does the drainage." Fabric is a filter that stops soil migrating into the drainage stone. It is not the drainage itself.

"It has been fine for years, so it is fine." Walls often lean slowly and then quickly. One very wet season can do what ten ordinary ones did not.

Dry-laid natural stone walls, common on older Northwest Arkansas properties, are gravity walls and drain through their own joints, which helps. They lean when the base stones settle, when roots get behind them, or when the soil behind turns to saturated clay. Often a section can be taken down and relaid on a better footing with stone behind it, keeping the character of the original.

Mortared stone walls are different: they are rigid and do not drain through the face, so they depend entirely on drainage behind and through them. A mortared wall with cracks stepping through the joints is telling you it is moving.

• What goes behind the wall, how thick, and where does the pipe discharge?
• What is the footing or levelling pad, and how deep?
• Does this height need geogrid, and at what levels?
• Does it need a permit or an engineer in my town?
• What happens to the clay that comes out?

A perforated pipe behind a wall is only useful if the water in it can leave. Pipes that run to nowhere, or that end buried in the soil at the wall's end, fill up and do nothing. The outlet has to daylight somewhere lower, into a swale, a storm system where that is permitted, or a slope that can take it, and it has to stay clear of leaves and soil.

Take the load off. Keep vehicles, stacked materials and hot tubs away from the top of the wall. Redirect any downspout or surface water heading toward the top of it. Keep people, especially children, away from the face if it is significantly out of plane.

Then get it looked at. A leaning wall does not stabilise, and waiting for a wet spring to decide for you is the most expensive way to find out.