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A Wall is Really Boring Until it Fails

Authored by Ed Davidson and originally posted on LinkedIn:

Not good...

When shoring walls collapse, nothing good comes from it!

Nobody was injured when a shoring wall collapsed into the foundation excavation of a condo construction site in Coquitlam this afternoon on 30Nov2023.

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, a failure is perhaps better defined as when the wall does not perform as expected.

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 all together, the reasons for these types of failures may be the result of a combination of causes.

Stay safe y'all, and if you want to help with my grandsons situation it would be greatly appreciated. I have included the gofundme link here: https://gofund.me/f976d125