From Mike Otto's book "Just Add Water" on building dams.

Also called a "keyway" a trench dug below ground level through layers of porous soils to a solid substrate and then backfilled with compacted soils to create a solid, waterproof, underground structure along the middle of the dam. There are illustrations in the book.

My addition: It should be compacted with a sheepsfoot roller in 6 to 12 inch lifts.

Buy Mikes book from the Pond Boss website. It will save you much more than the cost and help you sleep nights.

If the pond builder doesn't know these things, he's just a dozer driver.

It's not about the fish. It's about the pond. Take care of the pond and the fish will be fine. PB subscriber since before it was in color.

Without a sense of urgency, Nothing ever gets done.

Boy, if I say "sic em", you'd better look for something to bite. Sam Shelley Rancher and Farmer Muleshoe Texas 1892-1985 RIP