The groundhog here dug through my foundation - which is constructed of rock where he did it. He avoided the newer cinder block portion, but he did go 3 feet down to get into the crawlspace, more than once (we have filled holes with more rocks and dirt. Next step is cement with wadded up hardware cloth set in it.)
Not sure if it is just a psychotic fearless animal, or if it might be radioactive.
The skirting idea will probably work best. There are railroad ties in the back of the property we were going to pull out, because they make a bit of a terrace we really don't want. Going to rake the soil down into a slope. Can re-purpose those ties into construction materials for the run. Had nothing better to do with them, and they aren't rotted out.