Why Do Retaining Walls Collapse?

The YouTube video  https://www.youtube.com/watch?v=MrNluXrrHKY of the recent retaining wall failure in Baltimore after several days of heavy rains highlighted to many how devastating a retaining wall collapse can be. While we were not involved with this situation, we are currently working on several retaining wall replacement projects that resulted from collapses.

There are typically two causes for a retaining wall failure.  The first is old age and the associated general deterioration of the structural integrity of the wall.  As the wall ages and decays it gets to the point that it is not strong enough to support the weight of the soil behind the wall , so it becomes overloaded, and fails.  The second is saturated soils, usually from rain, due to a lack of a proper drainage system behind the wall.

A drainage system usually includes a layer of gravel, drain pipes, and weep holes through the wall that allow water that tends to collect behind the wall a way to exit.  Retaining walls are not typically designed to resist the weight of soil and water.  So when hydrostatic pressure builds up behind a wall, it can become overloaded and fail.