I've seen Camponotus novaeboracensis nest in old rail road ties around a garden in our yard. The wood is almost buried at this point, it has been in the ground being watered over and over for a few decades now. They seemed to be living in both the wood and the soil around it. Maybe try a bit of both? My Dad's a messy gardener who seems addicted to tomato bushes and lettuce but the only plant they mess with is the old rhubarb no one bothers to do anything with growing in the corner.
What about finding something that's soft and rotten enough you can break it into many smaller pieces? Layer pieces of that over some soil so they have lots of shaded, hidden spots plus the dirt to play with. Would give them extra space to wander around also.
I'm just not sure how dry the soil should be. My dad really likes to water back there but I haven't heard of them liking high humidity.