I have an old house (from 1893) and want to redo the basement floor. It is a classic New England 3 family with a brick on stone foundation. The floor is concrete (and use that word loosely) on dirt. The concrete wicks moisture and is always damp. I want to tear it up and put in a new floor. I know how to do the floor. I will dig down 10-12 inches put in an internal perimeter drain (geotextile, 3/4, crushed stone, 4" perf drain pipe connected to a sump ) . The floor itself will be: first 4" of crushed stone, then 2" rigid insulation, then 10 mil poly with 4" of steel mesh reinforced concrete poured on top - expansion joints around all perimeter and columns.

The big question...because I'm going so deep I want to first put in a concrete curb around the internal perimeter so the foundation will not push in. I figured 12" wide by 16" deep. I will do this in 6 foot increments with rebar protruding from each section to tie it into the next section. This way I can dig down for the drain and floor as noted above without worry of the foundation caving in or getting undermined by water.

Should I put crushed stone under this curb? I'm torn..I don't want to go any deeper than I need to near the foundation, but the soil is clay and I'm afraid that if I just pour concrete on it it will get pushed up by hydrostatic pressure. I also thought having some stone under there would permit any water pushing in to pass more directly to the perimeter drain and into the sump.

Any thoughts?