Terraced Retaining Walls: When Two Short Walls Beat One Tall One

Splitting a tall slope into two or three short walls (terraces) is popular because each wall can stay under the 4-ft engineering line and look better. But there's a catch.

The surcharge trap

If the terraces are too close, the upper wall acts as a surcharge on the lower one, and now the lower wall is designed for far more than its own height. A common engineering guideline is that walls behave independently only when the horizontal offset between them is at least twice the height of the lower wall (some codes use the sum of the heights). Closer than that, treat them as one tall wall and design accordingly.

Practical takeaways

• Wide, well-separated terraces = two genuinely independent short walls (often no engineer).
• Stacked, close terraces = one tall wall in disguise (engineer + permit).
• Each terrace still needs its own drainage; water from the upper bench must not dump behind the lower wall.

Design the lower wall in the calculator with the upper wall's load entered as a surcharge to see the difference for yourself.