A. Jay Meunier responds: If you're in doubt but don't plan to have an engineer's compaction test, you should probably do both. Run over the trench area with a plate compactor, preferably, or at minimum a jumping jack, and top up any depression that's left with the clean stone that you spread under the slab. Then, for added insurance, add rebar to the slab where it crosses the trench area. Use 1/2-inch or 5/8-inch rod, placed perpendicular across the trench and spaced 12 inches on-center. Cut the rebar so you have at least 1 to 2 feet of length over the undisturbed soil on both sides of the utility trench.
Jay Meunier was formerly owner and operator of a residential concrete business. He is now an estimator for Pizzagalli Construction in Burlington, Vt.
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