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SESK: Stuff Everyone Should Know · sesk.info/eng-retaining-wall-drainage

Why is drainage placed behind many retaining walls?

Drainage prevents a buildup of hydrostatic water pressure behind the wall.

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  1. To keep the soil moist so it holds
  2. To make the wall itself lighter
  3. To reduce water pressure behind the wall ✔

Why: Water in backfill can add substantial lateral pressure and reduce soil stability. Free-draining fill, filter layers, and drains provide a controlled path to an outlet.

Huh, didn't know that: A weep hole alone does not guarantee a fully drained wall if filters or drainage paths clog.

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The wall's drainage plan kept the pressure from building up.
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References

Answer: To reduce water pressure behind the wall. Water in backfill can add substantial lateral pressure and reduce soil stability. Free-draining fill, filter layers, and drains provide a controlled path to an outlet.

  1. FHWA LRFD Abutment and Wingwall Design Example www.fhwa.dot.gov/bridge/lrfd/us_ds7.cfm
    Hydrostatic water pressure should be avoided if possible in all abutment and retaining wall design cases through the design of an appropriate drainage system.
    Checked 2026-10-10.

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