How to Properly Build a Retaining Wall: The Importance of Drainage Retaining walls do not fail because of weak materials, but because of an invisible and constant enemy: water. The image clearly illustrates one of the most common causes of structural collapse: hydrostatic pressure building up behind the wall when there is no proper drainage system. The Problem: Hydrostatic Pressure When the soil behind a wall becomes saturated due to rain or irrigation, water becomes trapped. Unlike dry soil, wa