I added about 4" deep of ~1" rock over my anti-dig skirt and it was kicked around almost immediately by the chickens once they got access to that area; it's all in the grass and where I didn't want it. Have since buried it with dirt from the run and wood mulch over the top. I've also had many chickens who are able to kick around 4-6" cobble stones and chunks of firewood quite easily.
As for the comment about pop doors - I don't even have a door between my coop and attached run, because the whole structure is fully secured. I added a 2nd run like you have done for more daily space - that's where I added an auto door because opening/closing the man-door got old real quick; also we had a great horn owl fly through the man-door and get stuck inside the coop.
As for the comment about pop doors - I don't even have a door between my coop and attached run, because the whole structure is fully secured. I added a 2nd run like you have done for more daily space - that's where I added an auto door because opening/closing the man-door got old real quick; also we had a great horn owl fly through the man-door and get stuck inside the coop.