Hydrostatic pressure can cause or induce retaining wall failure, or at least damage.
Drainage of water as a result of rainfall or other wet conditions is very important to the stability of a retaining wall. Without proper drainage the backfill can become saturated, which has the dual impact of increasing the pressure on the wall and lessening the resistance of the backfill material to sliding. Granular backfill material offers the benefits of good drainage, easy compaction, and increased sliding resistance. Drainage systems usually utilize weepholes and drainage lines. Weepholes actually penetrate the retaining wall and drain the area immediately behind the wall.