how to form new foundation for wall in existing slab

[browfish]

I am putting in a new foundation to extend a wall that is load bearing

The slab is only around 4-6" thick and this is laid on flint which is about 12" thick. There is no DPC bu i guess the moisture can not rise throught the flint as there are no damp problems

I have dug down to solid ground, around 500mm below floor level. If i lay a dpc sheet and fill with concrete i fear this will get pinholed by the flint and i cant really sand blinde teh side as the sand will disapear into the gaps in the flint. If i cast the slab in put a dpc on top then i feat the damp will rise up past the inpenatrable flint course.

What is teh best way to do this?

 

[^woody^]

Fill it all with concrete to floor level

 

[browfish]

How will that stop the damp rising? Moisture will rise up the new concrete creating a damp surface and also soak into thte adjacent old slab making that damp will it not?