Sprinkler box?

I have a set of 5 control values above ground with each zone's pipe about 1 1/2 feet under the ground. Due to a leak, I just dug up all of the dirt in between the pipes and replaced all of it.
So, the question is, is there any disadvantages to leaving these pipes alone and not putting the dirt back in? It took me a better part of the day to dig out the hard clay soil, rock, etc. and I don't want to go searching for a leak 5 to 10 years down the line.
As for an underground sprinkler box, the one's that I saw at Orchard Supply were way to small.
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Only thing I can think is it will freeze unless drained, quicker.
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