When building a wall, whether it is a basement wall or an outdoor retaining wall, it is necessary to make it strong enough to resist the pressure differential from the soil side to the open side. This pressure will consist of two elements:

1. Soil Pressure, which is a function of the soil depth and type
2. Hydrostatic pressure, which will be simply the depth of the wall times the density of water

The designer has two choices on what to do about hydrostatic pressure:

• Hydrostatic pressure can be factored in to the wall design, requiring a wall at least twice as strong as would be required from soil pressure alone
• Hydrostatic pressure can be eliminated by providing a drainage path for moisture adjacent to the wall.

It is much more cost-effective in most cases to provide a hydrostatic pressure relief system. Today there are two basic types:

• Crushed Gravel: A clean gravel is placed against the wall as backfilling takes place. This is a very slow labor intensive job with unpredictable results. Furthermore, waterproofing membranes applied to the outside of the wall can be damaged in this process.
• Synthetic Sheet Drain System: Some type of sheet drain is used