One area that can be commonly overlooked, or at least underestimated, is the necessity to drain the backfill of rainwater and/or groundwater. 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. Weepholes should have a minimum diameter so as to permit free drainage. For large walls, 4 inch weepholes are common. Adequate spacing between weepholes allows uniform drainage from behind the wall. Weepholes should always have some kind of filter material between the wall and the backfill to prevent fines migration, weephole clogging, and loss of backfill and caving.

Drainage lines are often perforated and wrapped in geotextile or buried in a granular filter bed, and serve to carry water to the weepholes from areas deeper within the backfill.

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