They have very detailed physics-based models of terrain-wheel interaction ("terramechanics" is the general term). The tread depth must be one of the design parameters.

Slip is something they watch closely, and 100% slip is something they worry about a lot. I worked on a project using a small prototype in that same yard (metal wheels with metal tread) and was really f-ing hard to get the thing to slip with anything like a reasonable load and slope.