The Natural Philosopher wrote:

However in the context of using random hardcore to make a subfloor base,
I think the relevant issues are that almost any stuff will do, provided
its graded right..i.e. big lumps with concrete poured over are not as
good as smashed up stuff with a good amount of particle size variation,
with added sand and ballast to fill in the sizes that smashing wont deliver.

Plus the fact that if you have e.g. old footings and bricks lying about,
its cheaper to use them - mixed with MOT or some other substance, and
well whacked with a sledge hammer - than paying to bring in new MOT and
paying to take away the scrap.

And if a load bearing wall is to be inserted, its advisable to use a
proper strip foundation under it as well. i.e a proper possibly
reinforced, cocrete filled trench of at least 600mm deep, and maybe a
lot more in tree root filled soil.


I don't disagree with any of those general principles, however my
reply was intended to address the OP's specific question.