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On a camp site only accessible on foot
(lemmy.ca)
Went to a camping in the Laurentians in Québec (Ste-Agathe) and there were those things near some camp sites. The sites are only accessible by footpaths. No car access. And if you bring a bike, you have to walk it up a hill, so I doubt they are bike racks.
Maybe it's a grate. Imagine having a concrete pipe extend to both directions, this could then act as a grate against larger objects into the pipe.