I have redone plumbing under the concrete slab and in some cases I had to
dig up soil (sand mostly) under the slab as the plumbing went under it. I
had hollowed out a cavity three feet deep and about 18" under the slab (the
overhang is 18" in addition to the actual trench which is 24").

When I put sand back in to backfill I can compact the sand in the open
trench by jumping up and down on it. However on the overhang area I can
step on it until the soil is built up till about 8" below the slab. Any
tricks in getting the soil compacted sideways into this overhang to get it
as tight as possible?

Thanks in advance,

MC