Quote:
Originally Posted by MindBomber
It's likely a void beneath the slab, which typically occurs when they dig too deep during excavation and back fill to intended level. One solution is to inject foam under the slab to eliminate the empty space.
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you always over excavate...but it's normally only by about 5-6" to put down a layer of 3/4" rock (and poly) prior to pouring the slab
but as you say, if it was over excavated by a decent amount then back filled and not packed it theoretically could have settle but generally the slab would have settled a little with it (slab on grades are generally not reinforced or just have wire mesh) and you'd see a lot of cracks
but anything is possible