We have old railroad ties that my husband is thinking would be perfect for the base of the run...we are doing a 20x20 run and it will have to be really sturdy...his plan is this (give or take ): use railroad ties for the base. take corrugated sheet metal and dig that down a foot on the outside of the base to keep things from digging under. attaching the actual frame of the run to the railroad ties and building up from there. I am thinking about burying the railroad ties slightly and then covering them with dirt on the inside part as well. Does anyone have any experience using these for the base? they are nice and heavy....they don't have any of the sticky gunk on them or anything. thanks!