From a post on the architecture of burrows:
Rocks, roots and barbed wire at the entrance:
This is so common that it is clearly a planned design feature. It is not uncommon to find dens exiting inside a stump or hollow tree, or to have a strand or two of barbed wire running along the lip. Groundhog dens frequently start on one side of a barbed wire fence and exit out another. If there is an abandoned vehicle on the edge of a farm, a groundhog will invariably makes its home under the chassis -- a nice shelter from the rain, but also from predators who will have to slow down to a crawl to avoid beaning themselves on the I-beam and suspension. Hard structures not only makes digging out more difficult, it makes a mad dash at the den hole a dangerous and maladaptive strategy for large predators. In addition to hard structures at den entrances and exits, there are "soft structures" such as thickets of multi-flora rose, bramble, poison ivy, and thick wild grape vine.