vapor barrier on rock hard dirt

Hi, I was planning on using 12 mil plastic as a vapor barrier in my crawlspace. Upon closer inspection, a lot of the dirt down there is rock hard and somewhat jagged in places. Not a lot of room to move around so I don't think I could really break it up without having it take weeks. So instead I though maybe I will use a 20mil barrier instead. Would 20mil be sufficient or is there maybe something else I should do? thanks.

vapor barrier on rock hard dirt