It's not the moisture so much as what's in the fill - what comes in contact with the metal - anything far off from neutral will cause problems eventually, especially base/alkaline soils. Culvert pipes are in contact with the soil for many, many decades. While I technically don't need it i put a rubberized asphaltic coating on my bins below grade and if the soil has sizable rocks I might add 1" of rigid insulation - belt and suspenders means no problems for at least a hundred years. I use the insulation (R-7/in) as structure. Arches/Quonset huts are very strong if loaded evenly - you'll know it wasn't when it fails. Take a lot of time to figure this out - it is resolvable - haste makes waste and probably corpses in this case. You can come up with a lot of "greener" solutions (they are not green if the project fails) but in the end the structure foam is the ideal material in the big, long-term picture (gosh knows I've looked at alternatives) - high R value - adds structure, completely resistant to moisture, insects rodents etc. It is very conducive to decommissioning (or repurposing) - the foam burns clean in a blast furnace. Do it that way and it is something you will never have to worry about again - or your offspring for that matter.