The guard on this bridge 20 miles northeast of Knoxville probably is Union but the photo was taken soon after Dec. 3, about the time when Longstreet’s retreating troops passed this way, so who knows?
Biggerize the photo with a click and you’ll notice the guard is barefoot. Unusual for Union, but then quite common for Rebels. Biggerizing will also give you a better look at the dirt fort in the right top. It is not Fort Sanders but it could be, from all we know of the latter, at least from this angle. See here for more details.