One thing I’ve been trying to understand: are there lifts in a dug pond? Isn’t that only for a dam that is built up in layers?
If the basin of the pond is excavated into a clay or clay loam bed, then it usually can be sealed by deeply scarifying (tilling/plowing/etc.) to break up the natural layers and then compacting with the sheepsfoot to make the seal.
However, if you have even a single layer of sand, gravel, sandy loam, etc. that can transmit water, then that needs to be well sealed.
If that is only at the 4' deep level, then it might be sealed with a single 6" lift of clay material since the hydrostatic pressure is quite low at that depth. Unfortunately, at shallower depths it is possible for living things to puncture a 6" seal and cause the pond to leak. Muskrats and otters might dig through. Also, various plants can have roots penetrate the seal and then cause a leak when the plant dies and the roots decay.
If you do reach a maximum depth of 18' and bottom out in a sand layer, then you are really going to need a good seal! That is a LOT of hydrostatic pressure trying to push water out of your pond through any tiny available path. For a depth of 20' the recommendation is usually a seal of 2' thickness in four 6" lifts.
The best "general" advice is 1' of seal down to a pond depth of 10', and then increasing the seal thickness with depth such that you reach 2' of seal by 20' depth. All seals should be constructed in lifts of 6" thickness with properly wetted material. (One good source recommends an additional 2" of seal for each foot of pond depth below 10' to really be safe.)
Your contractor will probably claim that is overkill (depending upon how you are paying him). And he would probably be correct in many circumstances. I suspect many ponds have been sealed with less than the recommended seal thickness. However, I have never read a single report on Pond Boss where someone complained they spent too much money on sealing their pond the right way the first time. Also, if the pond does leak, it is much more expensive to come back and fix it compared to doing it right during the initial construction.
Cody note - For the contractor to rent a double barrel sheepsfoot roller or powered vibratory sheepsfoot is cheap leak insurance IMO when compared to the price of building a pond.
A lot of your money spent on the construction equipment goes to getting the equipment on your property. Another small chunk goes into getting the operator's butt in the seat at the start of the day. Another big chunk goes into shaping the pond and moving the spoils piles into the proper locations. After all of that money has been spent, spreading and compacting an extra lift or two from your clay material pile back into the pond does not add than many more hours of equipment time and diesel expense.
I hope that helps your thinking a bit.
Good luck on your new pond!
