This issue of keeping underground conduits dry comes up on occasion. I'll concur with the others who have said this is a pointless exercise. The NEC knows and understands that these are wet locations and requires them to be wired as such. That being the case, if they are required to be treated as wet locations anyway, why go to the trouble to keep them dry? It doesn't make sense to me.
Then why have dry conduit requirements above grade? It seems like a circular discussion to me. This would mean all conductors pulled above and below grade outdoors should be rated for wet locations. What would be the purpose of RT fittings?