The Sandmines trail is found on the west side of Royal Blue and is one of the best off road trails in the area outside of Windrock. Due to the steep rocky climbs and rutted sections I'd rate this trail quite hard. It's doable on 31's and open differentials when dry but be prepared to bottom out on a lot of obstacles and if it's wet be ready to winch.
There are 2 main trails into the Sandmines. The more difficult trail is located just past red ash on Old Highway 63 north out of Caryville, TN. Just past the footbridge over the creek is a barely noticable turn off to the right that drops down into the creek. If you cross here and head downstream to your right just a bit you'll see the trail leading out of the creekbed. Warning: Don't try this route after heavy rains as the creek becomes very treacherous and can easily sweep a Jeep downstream.
The easier way in is from the next exit to the north on I-75, the Huntsville/Oneida exit. If you make a left onto Highway 63 and go past the Pilot the next road on your left will take you into the sandmines trail.