Look into how to core a dam. I am about to start this today on a mainly excavated pond that I just started digging. The first step is to push soil behind the dam until you get down to good clay. Then good clay is added in 6" lifts, each of which is compacted with a sheep's-foot roller. The core is designed to keep water inside the pond. For me, I will also trench and pack a 2' wide core with a backhoe, packing the trench with the bucket as I go. We did this on our last pond and it holds water like a drum, but I was able to tie into good grey clay and the rest of the dam was red clay. My other 2 ponds were cored with a dozer, but that takes considerably more time (and of course is better). In your case, the old section of the dam may be fine, so you might be lucky find that an excavator could be used to "trench-core" the new section tying into the old core in the old section. I first saw "core-trenching" done on a wetland we had constructed a number of years back. This mixes the soil and hopefully disrupts and permeable lenses in the original soil, but it requires the soil to be mostly clay so the mixed soil is impermeable to water under the pressure generated by the depth of the water (deeper layers are most vulnerable to leaking due to greater water pressure.