A. Brent Anderson responds: There are many ways to support a garage slab next to a basement wall. The best approach is to provide compacted backfill to 95% Proctor density from the footing to the sub-base (see "Soil Compaction Basics," 3/94). The sub-base should consist of 4 to 6 inches of coarse aggregate (illustration A below).
If the job schedule or budget does not allow for careful soil compaction around the basement wall, then any area of overexcavation should be backfilled with minimum 1 1/2-inch-diameter clean gravel, which will self-compact evenly under the weight of the slab and any future loads.
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