Joe, sorry to hear about your leaky pond.

Your pictures are showing up for me.

However, the pictures don't give me a clear view of the actual situation on your property.

I would want the core trench to go about four feet below the level of the pond BOTTOM, not the normal pond water level. Or is there still a little water in your "drained" pond and they went four feet below that?

Clay does need to be moist for proper compaction. However, if the clay is water saturated, that is too much moisture and the clay will flow rather than be compacted.

Also, I can see some pebble/gravel size material in your first picture. If there is too much gravel, you cannot compact the material enough to form a seal against water losses. If the gravel pieces touch enough other pieces of gravel (point contacts), then they create a rigid structure such that the clay particles in between cannot be compacted.

I know I did not give you any answers. Hopefully, I provided a little clarification so you can ask more questions of your contractor, or more specific questions on the forum.

Good luck on your dam repair project.