| Mike,
You need to check if a pond over x depth or x gallons is under swimming pool regulations where the pond will be built. Neither you nor the owner will want a surprize from the local authorities. Since koi ponds are usually in the 4-8' deep range, you might have soil conditions (or local regulations) that will force you to use a concrete floor with block walls, gunite or shotcrete, or concrete floor with poured concrete walls. If you have to do poured concrete, at least you can put additives in the concrete to help with water proofing.
You can increase the deep, and reduce the excavation by doing a semiraised koi pond. It is very nice to have a pond wall that is 18"-24" above grade. It gives you a nice place to sit and feed the fish. It also helps keep our windblown debris, small kids, and other pets.
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Too much sanity may be madness and maddest of all is to see life as it is and not as it should be.
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