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 unsupported void. Build
a small containment wall with masonry block, concrete rip-rap,
or a similar permanent material.
Once that area is blocked off, use a clean structural fill
such as crushed stone or gravel. If you use stone, you can
typically avoid compacting in place, as it is considered a
self-compacting material. If you use gravel, you will need to
compact in lifts of 8 to 12 inches.
An alternative is to use a cementitious product such as
flowable fill. Flowable fill is more expensive per cubic yard
than structural fill (stone or gravel) but provides several
advantages. It can be placed directly from a ready-mix truck,
it requires no compaction and little labor, and its strength
can be adjusted downward so it can be excavated later on if
changes are made to the house.
If you spend the time and money to properly prepare the
backfill and sub-base, you'll prevent slab failure later.