Diagnosis

Retaining Wall Failure Signs: Lean, Bulge, Separation and Drainage

A retaining wall gives you months of warning before it goes. Four signs to read, in the order they usually show up on Middle Tennessee clay.

The wall is working every second of every day

A retaining wall has one job and it never gets a break. Every hour it is holding back a slope that would rather be flat, and after a heavy spring rain in Rutherford County it is holding back that slope plus the weight of the water soaked into it.

The good news is that walls almost never fail without warning. They tell you for months, sometimes years. The problem is that the warnings are slow enough that you stop seeing them, because you walk past that wall every day and today looks exactly like yesterday. Four things are worth learning to spot: lean, bulge, separation, and drainage that has quit.

Lean, and how to measure it without guessing

Some walls are built with a deliberate backward tilt into the hill, called batter, and on segmental block walls it comes from the way the units set back on each other. So a wall that tips slightly back toward the slope may be doing exactly what it was designed to do. A wall tipping out toward you is not.

Measure it instead of squinting at it. Hold a four foot level vertically against the face and see how far the bottom has to come out for the bubble to center. Do that at three or four points along the run and write the numbers down with a date. Repeat after the next wet stretch. A number that holds steady across seasons is history. A number that grows is a live problem, and the rate matters more than the reading.

A bulge in the middle of the run

A bulge is worse news than a lean, and homeowners usually notice it later. It shows up as a belly in the middle of the wall, both in height and along its length, while the ends stay put.

Stretch a string line tight between the two ends of the wall and stand back. Anything pushing past that line is deflection, and deflection means the wall is bending under a load it was not built to carry. On mortared block and stone you will often see it before you feel it, because bending opens cracks in the joints along the belly. On dry laid or segmental block walls the units start to look out of line and gaps open at the vertical seams. A bulging wall is closer to failure than a leaning wall of the same age, because the middle has nothing holding it and the ends are already doing all the work.

Things pulling apart at the ends and corners

Look at where the wall meets something else. Corners, returns into the bank, step ups along a sloped run, the joint against a set of stairs, the point where the wall dies into a driveway or a house corner. Separation shows up at those transitions first because that is where two things with different amounts of soil behind them meet.

A vertical crack opening at a corner, a step that has dropped out of line with its neighbor, a cap stone that has slid or now sits proud of the one beside it, or a gap you can slide a finger into where the wall meets a stair cheek. Those are all the same message. One part of the wall is moving and the other part is not going with it.

• Vertical cracks widening at corners and returns
• Cap stones sliding, tipping or opening gaps between them
• Steps in a tiered wall dropping out of alignment
• A finger width gap where the wall meets stairs, a drive or a house corner
• Mortar joints crushed or spalling at the bottom courses

The drainage has quit, which is usually the actual cause

Nearly every failed wall we look at around Murfreesboro failed for the same reason. Water got behind it and had no way out. Middle Tennessee clay swells when it takes on water and it does not drain freely, so a wall with a blocked or missing drain is holding back saturated soil that weighs far more than the dirt the builder had in mind.

Go look at the wall during a rain, not after it. Weeps or a drain outlet that stay dry while the ground above is soaked mean the water is going somewhere else, and behind the wall is the only place left. Water seeping through the face rather than out the bottom says the same thing. Dark staining, white efflorescence bleeding down the front, moss on the lower courses and mud washing out through the joints are all evidence that water is moving through the wall instead of around it.

Read the ground, not just the masonry

Half the diagnosis is above the wall and behind it. If the top of the wall has moved out, the soil it was holding has to have moved with it, and that shows on the surface.

Walk the ground above the run and look for a crack in the lawn or the mulch bed roughly parallel to the wall, a fence line that has started tipping, a patio or walkway settling at the edge nearest the drop, or a soft depression that was flat last year. Downspouts discharging straight onto the backfill, an irrigation line running along the top, or a neighbor's regraded yard now sending runoff your way are all loads nobody accounted for. Fixing where the water lands is often cheaper than anything done to the masonry.

What to watch and when to stay off it

Plenty of walls can be monitored honestly. A low garden wall under about three feet with a stable lean, no bulge and dry drainage is usually a photograph and a date, checked twice a year. Say that out loud to any contractor who tells you every crack is an emergency.

Stop using the area and get eyes on it when a wall over about four feet is leaning and the lean is growing, when there is a visible belly, when soil or mud is coming through the face, when the ground above has dropped, or when the wall holds up a driveway, a pool deck, a structure or anything people stand on. Walls of that size generally need engineering, and the height that triggers it is set by the county, so confirm the current requirement with the Rutherford County building department rather than trusting a rule of thumb from a forum.

Related services: Brick Masonry · Stone Masonry · Outdoor Living Masonry