Retaining walls are usually strong and can resist anything, but sometimes they can show signs and indications that they might brake them. A retaining walls may slide, topple, overturn and very rare they can totally collapse but all of that can be rehabilitated. Rebuilding the wall is often the best and only solution for these failures. Very often, most of retaining walls could be rehabilitated and saved after when we are evaluating and specifying the reasons of deficiencies.
The most common causes for failures of retaining walls are:

Improper reinforcement placement
Saturated backfill
Weep holes that do not weep
Design error
Calculation errors
Unanticipated loads
Mistakes in utilizing software
Detailing errors
Foundation issues
Inadequate specifications and notes
Shoddy construction
Retaining wall age

Source.

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