Signs a Retaining Wall Is Failing, and What Each One Means

When should you inspect a retaining wall?

Once a year, in spring. A segmental wall manufacturer's operation and maintenance guide (Belgard) recommends an annual inspection in the spring after snow events have subsided, which suits western Montana: by then melt water has done whatever it was going to do, and any movement from the winter is visible. The same guide recommends looking again after an extreme event such as a major storm or flood.

Take photos from the same spots each year, including one sighting along the top of the wall. Comparing pictures is the easiest way to see slow movement you would never notice day to day.

What does a leaning wall mean?

Every well built segmental wall leans back slightly into the soil it retains; that setback is called batter and is part of the design. What matters is a wall that has rotated forward. The Belgard guide calls this negative batter and gives a simple test: hang a plumb line from the top of the wall. If it falls freely without touching the face, the wall is leaning outward, which it says is not possible in a properly constructed wall with batter. Isolated negative batter may not be a concern unless it keeps growing, but the guide says a qualified engineer should inspect the wall and make recommendations.

Forward lean usually means the wall is carrying more load than it was built for: saturated backfill, a new load above the wall, or a base that has softened or eroded at the toe. Our retaining wall materials comparison explains how water pressure behind a wall builds up.

What does a bulge in the middle mean?

A bulge is a section of the face pushed out further than the courses above and below it. Indiana's Department of Transportation retaining wall inspection checklist lists bulging among the defects with a higher likelihood on masonry, reinforced concrete and timber walls. On a homeowner's wall it usually means pressure concentrated behind that section, most often from water that cannot drain or from backfill that was never properly compacted.

A bulge is a structural sign, not a cosmetic one. Watch it through a season if it is small and the wall is short, but get it looked at if it grows, if it sits under a driveway or near a house, or if the wall is tall.

Are cracked blocks a problem?

Often not, by themselves. The Belgard guide says cracked facing units are normal and to be expected in a flexible system intended to withstand some settlement, and that occasional thin cracks with no displacement are not typically a concern. It says cracks that represent more than 5 percent of the units, follow a pattern, or show significant displacement should be carefully assessed by a qualified segmental retaining wall engineer.

On mortared stone or poured concrete walls, cracks mean more, because those walls are rigid and are not designed to move. The Indiana checklist lists cracking, mortar breakdown and masonry displacement as the higher-likelihood defects on masonry walls. A stair-step crack through mortar joints or a crack that is widening deserves attention.

What do gaps between blocks mean?

Gaps open when courses shift relative to each other. The Belgard guide notes that small movements are not necessarily a sign of imminent failure, especially if horizontal and vertical alignment is maintained, but that opening between units can let drainage aggregate escape through the face. It says gaps over half an inch should typically be monitored and corrected to prevent loss of drainage stone. Gravel trickling out of the face is a sign the drainage zone behind the wall is emptying.

Separation also shows at the ends of walls, where a wall meets a step, a house or another wall. The Indiana checklist lists separation as a higher-likelihood defect on masonry walls.

What does settlement or a sinkhole behind the wall mean?

Soil sinking behind a wall, or a depression along the top, usually means material is leaving: washing through gaps in the face, out a broken drain, or down into a void. The Belgard guide's annual checklist includes verifying that drainage is working, that erosion has not occurred along the top, ends or bottom of the wall, that drainage aggregate is not being lost at the face or ends, and that there is no unacceptable settlement above or below the wall. It also notes that poor compaction in the reinforced or foundation zones can play a big part in settlement.

Settlement at the base, where the bottom course has dropped or tilted, is more serious than settlement behind the top course, because the base carries the whole wall.

Is water coming through the wall a bad sign?

Some water should come out of a wall, through its drain outlet or weep openings. That is the drainage working. What you do not want is water that cannot get out: a wall face that stays wet long after the yard dries, water pooling along the base, or no flow ever coming from a drain outlet that should be carrying melt water in spring. The Belgard guide says all water must be diverted away from the base of a wall to avoid erosion and undermining, and that watering and drainage above the wall should avoid standing water, water cascading over the wall and saturation of the reinforced zone.

Find the drain outlet and check it every spring. If it is buried, crushed or plugged with roots, fix that before anything else. Our guide to French drains and erosion control on slopes cover the water side.

What about white stains, moss and plants on the face?

White powdery deposits are usually efflorescence, soluble salts carried to the surface by water. The Belgard guide calls efflorescence strictly an aesthetic issue that will not affect structural performance. It says the same of mold on a wall face kept damp by seepage or sprinklers. Both do tell you water is moving through the wall, which is worth knowing.

Vegetation on the face is not harmful in itself, according to the same guide. Roots from trees or large shrubs behind the wall are a different matter, and the guide suggests an arborist for those. If a tree is growing in or against a wall, our tree health and risk assessment looks at both the tree and what it is doing to the structure.

What does failure look like on a timber wall?

Timber walls fail by rot and by fasteners letting go. The Indiana checklist lists decay and section loss, splits and corrosion as higher-likelihood defects on timber walls, along with bulging. Probe the timbers at ground level and at the back where you can reach: wood that is soft, spongy or crumbles under a screwdriver has lost strength. Rot usually starts where the timber touches wet soil, so the visible face can look sound while the buried side is gone.

Timber walls are usually rebuilt in a more durable material once they start going, rather than patched.

When should you call an engineer instead of a landscaper?

Call an engineer when a wall is tall, when it holds up a driveway, building or steep slope, when movement is growing year over year, or when a wall that was engineered originally is showing any of the structural signs above. The Belgard guide repeatedly points to a qualified engineer for negative batter, differential settlement and patterned cracking. Montana's residential code requires engineered design for walls retaining more than 48 inches of unbalanced fill, and our guide to retaining wall permits explains that rule and who enforces it.

For short garden walls with clear causes, such as a blocked drain, a downspout dumping on the wall, or a section of timber that rotted out, a hardscape contractor can usually diagnose and rebuild. Do not dig within a few feet of the base of a moving wall or add load above it while you wait for advice. The Belgard guide says no digging should be done within 3 feet horizontally of the bottom face of its walls.

Rebuilding a failing wall with HJ

We rebuild and replace walls through our retaining wall installation service, starting from drainage: base, clean backfill, drain rock and a drain with a real outlet. When a wall needs an engineer's design we will say so. Walls are planned with grading and planting through our landscaping and design work, and our hardscaping page covers the rest of the hardscape. This guide is part of the Montana Landscaping, Lawn & Irrigation Guide; for budgeting, see retaining wall cost factors.

Call HJ Property Care & Tree Service at (406) 215-7319 or request a free estimate and we will walk the site with you.

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