hopefully mike or scott will jump in. moving that much dirt that deep , than pushing the sand back into the hole , about 10' of sand , then covering it with clay , they better pack the heck out of that base , i would be afraid of the water weight pushing the bottom down and separating from the sides ,causing cracks , with only 5" of clay, again i'm no expert. why not truck some clay in or dig it and seal with that benonite stuff. not best time year to be putting pond in north ohio , why not wait till summer when it drys up. i can't even take my tractor out of the barn , without screwing up the property with ruts , been very wet spring.