Question about Concrete (Pics)

Regarding a driveway i am building,

dimensions:

length: 5m
width: 2m
intended depth: 6 inches

i have a workshop/garage which has been built on a conrete slab/raft... this is 6 inches above the road level. i am basically preparing the area to its rear for a driveway. i have cut a straight edge into the tarmac road and have began removing all the old hardcore and tarmac to the relevant depth in order to fix some shuttering then have a cement truck come and pour the driveway for me,.

i have cleared most of the area and the shuttering is in place however i have not been able to maintain 6inches in depth because i found a huge conrete slab already in place ( this covers approx half the area i require ) but only leaves me 3 inches if i pour the new conrete straight ontop.

my plan is to prepare the ground all around this correctly and at the correct depth but leave this old massive bit of concrete in place and just pour the new stuff ontop...

Is this a problem? can i utilise the old concrete in place or would the new stuff i am pouring ontop not have enough depth?

thanks for any comments.

Probably won't matter, but it would be good practice to hire a big concrete breaker and break the lot up, spreading it about the whole area, before pouring onto it.

Added some images,,,

had a re-measure etc....

seems i have got between 4-5 inches from the big bit of concrete to where i require the top of my bay.

my plan is to have concrete poured, i have slope ok. My concern is the massive bit of concrete i have found,, i dont wanna mess with it becuase if you look closely you will see a bit of metal in the ground,, i believe this is the start of an old cesspit,,

Am guessing metal mesh would help. but please any thoughts would be great.

You definetly want to get some A142 mesh in there. Other than that crack on and good luck!

That ain't so bad after all, and a 'hear-hear' for the mesh.