responds: There are many ways to support a garage slab next
to a basement wall. The best approach is to provide compacted
backfill to 95% Proctor density from the footing to the
sub-base (see "Soil Compaction Basics," 3/94). The sub-base
should consist of 4 to 6 inches of coarse aggregate
(illustration A below).
If the job schedule or budget does not allow for careful
soil compaction around the basement wall, then any area of
overexcavation should be backfilled with minimum 1
1/2-inch-diameter clean gravel, which will self-compact evenly
under the weight of the slab and any future loads.
Another option is to use a shelf angle to support the edge
of the garage slab (B). We usually recommend a continuous
3x3x3/8-inch steel angle, bolted to the concrete foundation
wall every 16 inches with 1/2-inch-diameter bolts. However,
this will support only the slab edge, and will not prevent
distress cracking that may occur as the soil under the slab
consolidates, leaving voids where the slab is not supported. A
preventive measure is to use #3 or #4 reinforcing steel placed
12 inches on-center both directions in the slab. This will
enhance the slab’s ability to span the voids as soil
Slabs can also be connected to the basement wall with rebar
pins (C). This is essentially the same concept as a shelf
angle. However, because the steel reinforcement restricts slab
edge movement, there may be more cracking on the garage
Another technique is to support the slab on wall pilasters
or concrete piers (D) bearing either on the footing below or
wherever areas of soil subsidence are suspected.
Brent Anderson, P.E., is
president of BA Associates, specialty contractors and
consulting engineers in Fridley, Minn.