Thanks, I cleared most of the muck out. Turns out supports were built on top of crude brick columns on top of loose hard-core. Terrible.i am removing them and replacing with struts that go all the way to concrete base. Pics attached.probably OK but at first only remove the plywood covering that you've already cut access pockets in - and the clear out and vacuum the interior.
then post more pics.
what youve got to be careful of working under stairs is that you dont damage or remove any supporting framing.