that can be a tough one okie. i believe there are some folks here who've had success trenching cross-wise the lower portion of outside dam face, and installing a "core" of clay, compacting and bringing back up to grade.
if this doesnt work, the only other solutions i can think of need to be done on either the inside face of dam of the base of the dam itself (obviously w/ no water present).
lets wait to see who else chimes in on this one....great question.
