Our "soil" is boulders with dirt between them and we had to just pile rocks on the skirt wherever we couldn't dig. It was interesting. I noticed two dig attempts this year on the outside so I'm glad we did it even though it was a huge hassle at the time. And, of course, the chickens dig on the inside so there would be gaps for critters if we hadn't. The rock pile at one corner has resulted in habitat for lizards and centipedes (aka chicken treats) so it's all good.