Q.: We generally use a No. 57 crushed stone (1 inch to No. 4) for backfilling plumbing trenches beneath a concrete slab on grade. Is it acceptable to dump the stone into the trench without compacting it? If not, what's the best method for compaction?

A.:Concrete Floors on Ground, published by the Portland Cement Association, recommends backfilling underfloor utility trenches with soil like that surrounding the trench, and compacting the soil in layers to duplicate moisture and density conditions in the adjacent soil. The book emphasizes restoring original subgrade uniformity so floor slabs don't crack as a result of bridging over soft spots or riding on hard spots.

If you backfill with crushed stone and don't compact it, it's almost certain to settle and create a void beneath the floor. Then the slab may crack where it bridges the void. Vibratory rollers or flat-plate vibrators are the best choice for compacting granular materials. For deep trenches, compact the stone in layers no thicker than 6 inches.