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Am building a low retaining wall which will rise 7" above the soil line. I am using 8" x 4" extra paver bricks from a recently constructed walkway. The wall will measure 48" long with 24" sides

How deep of a crushed stone base should I lay as a foundation for this small wall? I plan to use construction adhesive to bond these bricks together

Should the bottom course of bricks be below the soil line?

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A good starting point would be to plan on burying one course on a minimum of four inches of compacted gravel. The buried blocks don't buy you much stability in that configuration but they will help keep the wall from sliding forward and prevent the ground washing out underneath the wall, which is unsightly.

The gravel base must provide enough drainage to prevent your wall from becoming a bathtub. Barring truly epic monsoons or snow melts a modest depth should suffice. The base, by the way, should be twice the thickness of the wall. I doubt that's an issue since a ditch narrower than 8" would be a real hassle to shovel or tamp down stone in.

That basic plan should be modified by the local frost line, soil composition, and common sense. If an extra couple inches will get you below the frost line that's good insurance against heaving. If going below the frost line means digging an extra two feet that's a good reason to stop early and have a beer. Remember, you never get perfect compaction even with a machine. An exceptionally deep base that's 90% compacted will still settle a couple of inches.

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The addition of a couple of drainage holes made of 3" or 4" PVC pipe at the base of the wall couldn't hurt, in case of massive snow melts or monsoon rains. You want to avoid water freezing inside and pushing outward on your structure.

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