I would imagine that one needs a concentration check to cast while buried, and that one can't cast spells with somatic components (like while grappled) and it should include 1/2 the damage dealt while casting but what should the specific dc be to cast when you basically cant move?
I thought I remembered that cave-ins used the grappled condition, but after looking it up I can't find any such reference. The DC for concentration checks is not affected by mobility, though, only particular conditions such as entangled or grappled. Just use the usual DC along with any adjustments for per-round damage. 10 + 1/2 damage dealt + spell level.
If you can find a reason to apply the grappled or pinned condition, use that formula if you like. You'll have to make up something for the CMB of the thing(s) burying you. Good luck.
That's the problem how do you calculate the CMB of a cave in?
It should be the same as the DC to wriggle free using the Escape Artist skill. If the cave-in is severe enough that one can't wriggle out from under it with Escape Artist, then that should also prevent spellcasting while in it.
What is the crushing damage?