I used to do that sort of thing for a living. Hope 18" is deep enough, should be, sitting on clay wouldn't help either, but, should be okay if it doesn't get wicked cold for too long. Worse case, let it drip slightly, to let the expanding mass have somewhere to go. Myself I would've duct taped a wire to it, this way the line could be traced, if you need any other digging done.. but the pics will help, looks like you've got a decent sized spread.
Couplings (compression likely) went on ok? Leery of plastic ones, sometimes a bit of sand in a coupling's thread can worm it's way through plastic (when the line runs, the debris can cyclone around, pressure forcing deeper), had it happen once on a job, had to come back almost a year later to fix it.
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