thanks for advise
trent is correct, there was no core built. it would have cost an extra $3k, so i decided to chance it. i can buy an epdm liner & underlayment for the cost of adding a core now. i believe the liner would be a better solution.

how smooth does the bottom need to be for a liner w/ underlayment? we have semi rocky soil. the damn & center of the pond are quite smooth, but the side walls are a little rugged. should i have a dozer resurface them or will underlayment & 45mil liner suffice? & what is generally used for the liner trench? trencher or backhoe? how wide should the trench be?

pond is about 70'x150', ~10k sq ft

thanks again
shane