I would forget about lining the pond with clay and just purchase a
Rubber EPDM Pond Liner and underlayment and be done with it. If I were
you I would cut out shelves on the edges of the hole and use that
material to make the pond shallower. 8 ft. deep is very, very deep for
a koi/plant pond and will be very hard to keep clean.

Paul
www.pondkoi.com