Recommended depth of plumbing

[msm stl]

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?

 

[gwegan]

It sounds like your concrete guy knows how to do it right.

Generally you want all lines at least 1 and 1/2 foot below grade. You comply with code, you get them out of the way of small children and ambitious gardeners and you allow for adequate compaction of the subgrade of the concrete above. Your wall is a different matter. It is important to be sure that the wall does not rest on the plumbing or electrical lines and that their is allowance for everything to move a bit. So the deeper the better. Depending on the wall your building official may require some sort of engineered drawing. But even if he doesn't you need to be sure that the plumbing is at least a foot below the footing for the wall.

You want to be able to compact the trench fill material so that the concrete has a uniform level compact base and your underground lines are, to the greatest extent possible well bellow that compacted base.

 

[msgtdan]

Also if in the future you have to cut/remove some of the concrete you don't want to damage the water lines.