Should I ?
Answers:
I believe CR6 is crushed stone used as a base. Since it is stone, it can certainly be re-used if you wish.
As far as drainage under the deck you are enlarging, you want to be sure water drains away from the foundation of the house.
Other answers:
Since I don't know what CR6 actually IS, I'm going to assume it's plywood with a chemical preservative. If that is so, remove it completely. I would think that it would interfere with drainage, and most certainly it wouldn't be a good idea to cover it with soil.
Since I don't know what CR6 actually IS, I'm going to assume it's plywood with a chemical preservative. If that is so, remove it completely. I would think that it would interfere with drainage, and most certainly it wouldn't be a good idea to cover it with soil.