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Q.I need to pour a thickened-edge slab foundation for a two-story garage apartment across a recently dug 2-foot-wide utility trench. The backfill over the trench has subsided, and I'm concerned that the foundation may sag and crack at that point. Can I add rebar to the slab to span this area, or is it better to mechanically compact the trench area?

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.