Retaining wall inspections

A retaining wall holds back soil, and the force it resists grows sharply when the soil behind it is wet or frozen. Most retaining wall problems come from poor drainage, inadequate design or construction for the height and load, or deterioration of the wall materials. Leaning, bulging, cracking and water problems are the main warning signs, and taller walls or walls near buildings, parking and walkways deserve regular inspection.

How retaining walls work and fail

Soil pushes sideways on a retaining wall. The wall resists by its own weight (gravity walls such as stone or large block), by a footing and stem acting as a cantilever (reinforced concrete walls), or by reinforcement layers extending back into the soil (geogrid-reinforced segmental block walls).

The pressure behind the wall goes up when:

• Water builds up in the soil because drainage is missing, blocked or overwhelmed.
• Soil freezes and expands behind the wall.
• Extra loads are added near the top, such as parked vehicles, driveways, buildings, snow piles or more soil.
• The slope above the wall is steeper than the design assumed.

Walls fail by tipping, sliding, bulging, settling, or by the wall material itself cracking or rotting. In cold climates, frost heave and freeze-thaw damage to concrete and masonry add to the stresses. See freeze-thaw damage to concrete and masonry.

Warning signs

• Leaning or tilting away from the soil it retains.
• Bulging or bowing in the face of the wall.
• Cracks, especially horizontal cracks, widening cracks, or cracks with offset faces.
• Gaps opening between the wall and adjacent structures, or between wall sections.
• Soil or water coming through joints, or wet areas at the base.
• Blocked, missing or non-flowing weep holes and drain outlets.
• Settlement, sinkholes or cracks in the ground or pavement above the wall.
• Displaced or missing blocks or caps.
• Rot, insect damage or bowing in timber walls.
• Exposed or corroded reinforcing steel, spalling or efflorescence in concrete walls.
• Trees growing in or close to the wall.

What an inspection covers

A retaining wall inspection typically reviews:

1. History and design. Drawings, the height and type of wall, whether it was engineered, and any past repairs.
2. Alignment. Checking for lean, bulging and settlement, often with a level, string line or survey points that can be measured again later.
3. Wall materials. Condition of concrete, block, stone or timber, and of any visible reinforcement or connections.
4. Drainage. Weep holes, drain outlets, surface drainage above and below the wall, and downspouts or catch basins that discharge near it.
5. Loads above. Parking, driveways, structures, slopes and snow storage near the top of the wall.
6. Surroundings. Cracking in pavement, slopes, and neighbouring structures that may show movement.

Dated photos and measurements make each inspection more useful, since the rate of movement matters as much as the amount.

Setting up simple monitoring

Because the rate of movement matters so much, a wall that shows any lean or cracking is worth monitoring in a way that can be repeated by anyone. A simple approach:

Readings taken at the same points each spring and fall show whether movement is seasonal, steady or speeding up, which is exactly what an engineer needs to know.

A typical case

A commercial property has a segmental block retaining wall along the edge of its parking lot, roughly the height of a person. The property manager notices a slight bulge near the middle and a crack in the asphalt a short distance back from the top. An inspection finds that the weep outlets at that section are buried under landscaping, the asphalt above slopes toward the wall, and snow from the lot has been piled along the top all winter. The bulge is measured and photographed, temporary barriers keep vehicles back from the edge, and a geotechnical engineer is brought in because the wall supports parking. The engineer's review looks at the design, the reinforcement behind the wall and the drainage, and recommends repairs. The site plan is also changed so snow is stored elsewhere. Without the inspection, the bulge might have been treated as a cosmetic issue until the wall moved further.

When to involve an engineer

Many municipalities require walls above a certain height, or walls supporting loads such as driveways or buildings, to be designed by a professional engineer and to have a permit. The thresholds vary, so check with your municipality. Bring in a geotechnical or structural engineer when:

• A wall shows active movement, significant lean, bulging or structural cracking.
• A wall supports a building, parking, a road or a public walkway.
• Repairs or reconstruction are being planned.
• Loads above the wall are going to change.

Maintenance that helps

• Keep weep holes and drain outlets clear.
• Direct downspouts and surface runoff away from the top of the wall.
• Avoid piling snow or storing heavy materials above the wall.
• Control tree and shrub growth near the wall.
• Repair minor concrete and masonry damage before water gets in.

Concrete cracking in walls follows many of the same rules as other concrete. See concrete cracks: when to worry.

How ehZee can help

ehZee includes site features such as retaining walls in our commercial building inspections, documenting condition and warning signs and recommending engineering review where movement or structural damage is found.

Common questions