A.Jay Meunier, a contracting specialist at S.T. Griswold and Co., a ready-mix supplier in Williston, Vt., responds: We have seen too many garage slabs fail due to improper backfill and compaction. This applies to the overdig area as well as the raised sub-base underneath the slab. Each time a vehicle pulls into a garage and stops, that action creates a plate compactor effect. If the backfill under the slab has not been properly compacted, the structural fill will eventually settle, leaving voids. Since a slab-on-grade is not meant to be structural -- supporting its own weight plus any imposed loads -- the slab will certainly crack if the voids are large enough.

You are right to be concerned about the void created by the foundation overdig, which appears to extend under an adjacent living space supported by a 4-foot precast wall. You'll need to block off that extension of the overdig before backfilling; otherwise, the backfill material will migrate from under the slab into this sizable hole, leaving an...

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