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,
Water. Wet the sand and tamp it in. flow more sand in and wet it. Pack
it and let it dry. If it settles, put some more "slurry" in and let it
settle. Then pour your concrete over it. The watering will wash sand
down into the gravel below too, so it won't sift down later.
Use a low strength concrete wet mix - about a third to a quarter as much
cement powder as when you want a proper strong mix. Pour it or shovel it
in. It won't shrink at all but it can settle if you don't work it a bit
into the cracks and crevices or prevent it from sloughing out the front
Concrete yards know this as a "six sack mix". You can get it with
sand or fine gravel aggregate. I've used it often to fill undermined
slabs even in structural settings (even driveway ramps in gas
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