Concrete blocks(8x8x16) three high with steel rebar yes,but you should drill
into the oldslab because it won't bond. Also a 6 inch cove sand around the
inside
before the liner goes in would support some of the weight.That would be my
plan,

I agree....you NEED rebar. I don't think it absolutely necessary to drill into
the base...but it would be better to do it. I think the sand inside is a good
idea to cushion the bottom of the liner, especially where the block meets the
bottom base. I assume you will then line it, and put more blocks on the top of
liner to hold it in place.....Jerri

http://www.fringeweb.com/Ponds/JerrisPond