Many times , major problems exist below the slab with little evidence on the surface.  Voids or hollows can be present even though the slabs haven't broken or settled yet. Animals seen burrowing nearby or sand at the bottom of the drive after heavy rain, are good indications of this type of problem.

Before

After

These voids can be filled by the same process as well.  We simply pump our material in until the void is filled and full contact with the bottom of the slab is restored.