Grant,
Is the gravel in your hardpan clay layered in veins, or is it just some gravel particles mixed in with the gravel. If it's veins of gravel, that could be bad news, and you'll want to make sure you cut through those layers completely until you have solid clay at the base of the core.
I'd also suggest that if you haven't already, check the soil conditions along the length of the dam, especially where it crosses the existing channel or drainageway. Often times the substratum changes significanly where the water flow typically is.
Compaction is critical on the core trench, as as you'll read on this site, fill in 6 inch lifts, and make sure the fill material has sufficient moisture to adequately compact. You'll want to pull a sheepfoot roller over the fill, as the tracks of the dozer won't provide the pressure needed.
