What's your soil like? I'm guessing not a huge percentage of clay. Sep Holzer uses a technique when he digs a pond of picking the soil off the bottom (when there's some water in it) and dropping it, stomping around on it (with a machine/bucket). I think this allows the larger and coarser chunks to go deeper, and the finest clay stuff to sift down last and form a sealing layer, as opposed to the clay being evenly disbursed through the soil.
The harder I work, the luckier I get. -Sam Goldwyn So tiny. - this ad:
Getting ready for the Better World Book kickstarter - February 2019https://permies.com/t/99513/ready-World-Book-kickstarter-February