I only know of one but I don't think it's illegal because I've seen 15 people back there at one time before, its in Barclay neighborhood over by the DMV.
it is a half mile down WhiteHall Road. the main trail leads to the gas line but I ride on the secondary trails which are more like rock crawling.
that land is owned by Springs North Augusta LLC, it looks to be a developer out of greenwood.