IMO, the spell description clarifies that this spell only moves 125 cubic feet per round. Multiply rounds by the level of the caster and you are done. In other words, an 8th level M-U can dig a 10'x10'x10' pit and it takes her 8 rounds to do it. The spell is clearly slow at moving earth so the stuff about falling in is just nonsense. Any fool would just walk away while it's happening. Collapse would therefore likely only come into play if the MU dug a 5'x5' hole straight down and some creature then entered of their own volition. Upon entering the shaft or approaching the edge of the shaft, the interloper would get diced for. If a collapse happens, you are on your own. DM fiat. Generally, my rules for cave-ins avalanches rockslides and so on go like this: When the event happens, the PC takes 2d20 dmg. If they take >10 dmg, they are buried. A buried character must be rescued in 1d6+8 rounds or they die. A random buried character can be found and rescued by one digger in 3d10 rounds minus ½ the digger's WIS (rounded up). For each digger assisting the digger with the highest WIS, the time of finding and rescuing that buried character is reduced by 2 rounds. It will always take at least 1 round to find and rescue a buried character. Example: Pinhead and Black Jake are both buried in snow and the roll indicates Pinhead will suffocate in 10 rounds while Black Jake will suffocate in 12. Four friends are searching for them. The highest WIS among the searchers is 13. A roll on 3d10 indicates 22 rounds minus 7 from WIS, minus 6 from the three additional searchers = Pinhead's friends will find and rescue one of the buried characters on round 9. Whether it is Pinhead or Jake is determined by coin flip. Rolling 3d10 again indicates 15-7-6=2. Therefore 2 rounds later they will pull the other character out. If the second character is Jake, both are saved. If the second character is Pinhead, he will be dead.