Why the haulers got stuck

When an ant digs a cell it has to carry the dirt out of the nest and put it down somewhere, and for a while they would get about halfway up the shaft and just stop there holding it. Enough of them did it that the shaft jammed and nothing got dug.

The cause was how I decided what "outside" means. I was treating any cell with open sky above it as surface, and a straight shaft has open sky above every cell in it all the way down. So an ant in the shaft thought it had already arrived and looked for a place to drop its load, and there isn't one on the wall of a shaft.

Now there's a separate idea of ground. The floor of a shaft or a pit doesn't count, and a patch under open sky has to be at least 24 cells across before an ant will treat it as a way out. Haulers follow their own route up to real ground and drop the dirt there. I also added a rule that an ant which hasn't made progress in a while gives up and picks its job again, since I'm sure there are other ways for them to get stuck that I haven't found.