We're under construction in St Louis. The guy who will be doing our concrete flatwork (walkway & patios around pool) said to make sure that the plumber buries his lines deep - so that they're under the gravel bed that the concrete is poured on. Is that so important? These are lines that should be blown empty in the winter, so going below the frost line isn't our goal. I just want to make sure that both the plumbing & concrete both have the best chance to survive!
Also, after running under the concrete, the lines will pass down deeper, to go under a short retaining wall (2 feet high, to be constructed out of typical versa lock type blocks). How deep under this wall will the plumbing need to be?