Well as I've said they would the ants have used some of the wood chips to improve their nest. In this case they've simply narrowed the entrance. No sense in being invaded by swarms of ants after all. Creating a chock point is as good a strategy as any. If they ever produce a major caste blocking the entrance up will be simple. Of course the only thing this colony has to fear is a pest species invading my house.
I'm lucky enough that it's only been Tapinoma sessile, which isn't that invasive a species but they do harasser other colonies of ants for some reason or other. In other parts of the country though ant keepers might have to deal with Fire Ants, Solenopsis invicta, or worse, the Argentine Ant, Linepithema humile. These two invasives will forage in your home happily. It's one thing for them to find a food source but quite another to stumble on a cash of rival queens in test tubes trying to start colonies in what they consider to be their territory. You'll have a line of invaders all swarming to get in your ant setups and there's little you can do about it. Surrounding them with water doesn't help either. Argentine ants have been known to wonder in large numbers right into fish tanks, in an attempt to get a drink. They don't break the surface of the water either. Fire ants might be better detoured by water though despite their behavior to flooding. I still don't see either being a good solution though and I'm not sure there is one. :(