When the walls come tumbling down

A wall is really boring … until it fails. A retaining wall is supposed to hold back soil to either support a structure or keep a space clear. When it fails, both of those roles are compromised. A retaining wall does not have to collapse to fail. In fact, failure is perhaps better defined as when the wall does not perform as expected.

A retaining wall is a structure designed to provide lateral support for soil or rock and vertical loads. For example, a basement wall supports both lateral soil loads and the vertical loads of the walls, floors and roof. Conventional retaining walls are gravity walls, counterfort walls, cantilevered walls and crib walls.

Gravity retaining walls are typically built of plain concrete or stone, and the self-weight is a mechanism to resist failure from overturning and sliding. Counterfort walls include a footing, wall stem and vertical ribs known as counterforts that tie the footing and wall stem together. Crib walls are made of interlocking concrete members that form cells that are then filled with compacted soil. There are also proprietary systems such as prefabricated concrete T-walls.

There are several types of cantilevered walls, with common features being a footing that supports a vertical wall stem. Cantilever wall shapes can be T-shaped, L-shaped, or reverse L-shaped. To prevent the buildup of hydrostatic water pressure on the retaining wall, clean granular material is good practice for backfill and a geofabric-wrapped pervious drainage system is included at the heel of the wall.

Common causes of failure

There are many different reasons why retaining walls fail, such as soil bearing, sliding, overturning, inadequate design, improper construction or unexpected loadings. Other causes of failure include the following:

1. A typical cause of failure is when the wall is backfilled with fine-grained soils such as silt or clay that have a low shear strength. They also have a low permeability. As a result, they can become saturated with heavy rainfalls and overload the wall. The clay backfill can also exert swelling pressures on the wall. The swelling pressures develop when water infiltrates the clay backfill that was originally compacted to a high dry density at a low moisture content.
2. To reduce construction costs, soil available on site is sometimes used for backfill. These soils have a lower strength and permeability than clean granular material. Using in-situ available soil, rather than importing granular material, is perhaps the most common reason for retaining wall failures.
3. Detailing errors, perhaps of the steel reinforcement, can lead to misinterpretation by the contractor.
4. Foundation problems can be prevented with a geotechnical investigation. A soils report will provide criteria for design such as the allowable soil bearing capacity, friction factor for sliding resistance, seismic, expansive soil or potential liquefaction. If a soils report is not provided, building codes provide minimum recommend values for bearing capacity, friction resistance and allowable passive pressure.

Whether the retaining wall is tilting out of plumb, cracking or collapsing altogether, the reasons for these types of failures may be the result of a combination of causes. A retaining wall expert can help provide a failure analysis in the event of a construction defect.

Carlos Zarraga with Warren Forensics has more than eight years of engineering experience in the structural field specializing in building design, building components and foundation design. He has designed and analyzed structures, supervised designers and drafters, prepared construction documents and provided on-site duties for field supervision and inspection of construction projects. Contact him at carlos@warrenforensics.com. This article has been reprinted with permission.

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