Thanks for the advice.

I've been quoted $325 for a roll-off if i'm filling it with concrete -
i might be able to find cheaper since i'm only about a mile from a
landfill.

It appears to be an on grade slab. I dont think it has any footings,
i'll do some digging this weekend and confirm that.

I'm just glad it's not my front porch, that thing is about 12'x6' and
about 5' thick, it is structural and has footings below that!

I suspect the grade around the house has a lot to do with how thick
these things are.

I will need to backfill around the house to meet the engineered
drainage requirements. Is there anything stopping me using concrete
rubble as part of the fill? It seems very excessive to have to
wheelbarrow the concrete out and then move fill dirt in to replace it.

Graham