Spongy sub-base

[Ruatha]

Hi
I have started backfilling my porch foundations which are 3 courses of blockwork high.
I have backfilled the area around the blocks to the same level as the stable ground with the original gravel/sandstone (no soil)and given it a good run over with the wacker.
Now I have added 150mm of MOT Type 1 and given that a good wacking.
I now have some "spongy" areas which bounce when you walk on them.
The next layer is the DPM, Mesh rebar and 100mm concrete slab.
I am not happy putting the slab on top of something that is already spongy.

I guess I have 2 options - dig out to solid ground and then backfill and wack it again OR keep adding the hard core in the spongy areas and wack until it stops being spongy.

What would be your advice?

Thanks

[dig dug dan]

you need to remove the subbase back to soil and add a layer of terram (which is a fabric membrane), then add the subbase.
This should eliminate the problem

[Dave_L]

I'd remove the original gravel/sandstone too if there was any doubt as to its suitability/condition.

[Ruatha]

Thanks for the help.
I decided to dig it out and refill with hard core.
Hopefully it will be more stable when I have it finished.