Why is my retaining wall leaning?

First, which way is it leaning?

Some lean is designed in. Allan Block calls the amount a wall leans into the hill its setback, and makes blocks with setbacks of about 6 and 12 degrees.^[1] A segmental block wall that steps back evenly toward the slope is doing what it should.

The warning signs are the opposite: the top of the wall tipping out toward the low side, a bulge in the middle of the face, courses pulling apart at the joints, the base kicking out, or cracks that open wider over the seasons. A poured or block wall that was plumb and is now out of line has moved.

The usual causes

Water behind the wall. Water is the cause to rule out first. The design and performance of most retaining walls depend on keeping the area behind the wall relatively dry,^[1] and wet soil adds pressure behind the wall. A wall with no drain, a drain with no outlet, or an outlet buried under mulch has nowhere to send that water. Allan Block notes that drains must be vented to daylight or connected to a storm sewer and protected from fine soil washing into them,^[1] and Fairfax County's typical details show a perforated drain pipe along the full length of the wall with crushed stone backfill.^[4]

Water can also come from outside the wall's own drainage. Anchor's guide asks the builder to plan where downspouts will expel water and whether there is an underground sprinkler,^[3] and Allan Block's design guide calls for check valves or similar provisions to stop flooding from broken irrigation lines or heads above walls.^[2] A downspout emptying just above a wall, or a leaking irrigation line behind it, can be enough to start a wall moving after years of standing still.

Clay or organic backfill. What fills the space behind the wall matters as much as the wall. Fairfax County's Marine Clay guide says Marine Clay should never be placed in the backfill behind foundation or retaining walls,^[5] and that the clay often goes through several yearly cycles of shrinking and swelling before the damage shows as cracked or failed walls.^[5] That is why a wall can look fine for years and then start to move. Anchor's guide says black or organic soils should not be used as backfill.^[3]

A weak base. A wall rotates or slides when its foundation gives. Allan Block's guide says the soil at the bottom of the base trench must be firm and solid,^[1] and where the soil under a footing is soft clay, Fairfax County's details call for 4 to 6 inches of crushed stone under it.^[4] Prince William County's list of problem soils includes ones with frost heave potential,^[6] which can move a shallow base.

Too tall for its design. A gravity wall relies on the weight and setback of its blocks, and each product has its own height limit.^[3] Past that limit, the wall needs soil reinforcement and an engineer. Allan Block's design guide says engineering should be required for walls of any height with poor soils, multiple terraces, steep slopes above or below, or roadway surcharges.^[2]

Weight added above. Patios, swimming pools and driveways are common residential surcharges,^[1] and a slope above the wall adds load too: Allan Block advises against slopes steeper than 3 to 1 above a wall without first consulting a local engineer.^[1] Construction traffic counts as well. Fairfax County's details require that heavy equipment stays back from the wall a distance equal to the wall's height.^[4] A new patio, a parked truck or a pile of soil near the top can be enough.

Materials wearing out. Timber walls depend on 6x6 lumber preservative-treated to category UC4A for ground contact, tied back into the slope with deadmen,^[4] and older timber walls built without either can rot or pull loose over time. For block walls, durability concerns occur where there are saturated conditions in repeated freezing and thawing.^[2] Roots and digging matter too: Allan Block's design guide says augers should never be used within the reinforced grid zone, and that an engineer should check that planting root balls will not disrupt the geogrid layers.^[2]

What to note before a pro looks at it

You can help whoever inspects the wall by writing down what you see, without digging or moving anything:

• Where it leans and roughly how far, and whether that has changed. A photo from the same spot every few weeks shows it.
• Whether the face is wet, stained or seeping, and whether any drain outlets at the base run after rain.
• What sits above it: a driveway, patio, pool, steep slope, downspout or sprinkler zone.
• When it was built, how tall it is at its tallest point, and whether there was a permit.

In the meantime, keep vehicles and heavy loads off the high side, for the same reason Fairfax County keeps equipment back from new walls. In Fairfax County, the Marine Clay guide notes that slides on those clays, shallow or deep, can damage underground utilities, especially rigid gas, water and sewer pipes.^[5]

What the fix usually involves

The repair follows the cause. For walls damaged by swelling clay backfill, Fairfax County's guide, written mainly about basement walls, says the most successful repair has been to remove the clay and replace it with soils that are mainly sand or gravel,^[5] and that the engineer or contractor must determine whether the wall can be repaired or must be totally replaced.^[5] The same guide advises owners to consult a qualified engineer experienced in soil and foundation investigations for design and inspection of difficult repairs.^[5]

A rebuild is held to the same rules as a new wall. Virginia's building code exempts from a permit only retaining walls supporting less than three feet of unbalanced fill with no surcharge beyond ordinary fill,^[7] and taking a wall down is digging: Virginia law says no one may begin any excavation or demolition without first submitting a locate request to the notification center.^[8] Do you need a permit for a retaining wall? has the local details.

When to call a licensed pro

Call one as soon as you see a wall tipping out, bulging, cracking wider or shedding blocks, and ask for an engineer as well if the wall is tall, holds up a driveway or building, or sits on clay. A leaning wall is structural work, and Fairfax County's Marine Clay guide warns that an incomplete or improper repair is likely to bring the problems back.^[5] Virginia's landscape service contracting specialty (LSC) includes construction of retaining walls for landscaping purposes,^[9] and masonry and concrete contractors build them too. Check the licence before you hire.

For what comes next, see how to keep a retaining wall from failing and what a retaining wall costs. If water is pooling at the house too, why water pools near a foundation covers the yard side.

Loclis lists DPOR-licensed businesses with their licence number and class. Browse retaining wall businesses, look up concrete and hardscaping contractors in Blacksburg, or see all concrete and hardscaping answers.

Describe the wall once, with photos, and licensed local businesses can quote an inspection or repair.