Our garden arbor's underground legs are completely rotted and the arbor has fallen. I wonder what's the best way to fix it. I am thinking in extending the above-ground part of the legs (which are in good state) with new wood, attach it with steel plates, and bury it again. Any other idea? There is no concrete as in fence posts, just the bare wooden legs into the ground, so I am not sure fence post menders could help. The four legs are 3-1/4in x 1-1/2in each. It is a relatively light arbor. Can be lifted by one person.
It sounds like there isn't much weight to be dealt with, although appearance is important. How about setting the arbor on pressure-treated sections of 2x4? Specifically:
Measure the existing rotten ends of the arbor, keeping track of where the ground level was in relation to the arbor
Cut off the rotten ends of the arbor to make the ends clean
Take sections of pressure-treated 2x4 and embed them vertically into the ground where the arbor will sit, so that they're placed right where the original 2x4 legs were. Set them so that the newly-clean ends of the arbor legs will sit on them, with the PT 2x4s extending the length of the arbor legs and with the arbor at the correct height (as measured above).
Fasten the arbor legs to the PT 2x4 sections. Possibilities include mending plates, 1' sections of rebar set into holes drilled lengthwise into the center of the PT wood and arbor legs, or tying them together with (rot-proof) string between holes in the arbor legs and the PT 2x4s. Your biggest concern will be wind blowing the arbor over, so plan the connection strength accordingly. Or, just be ready to stand the arbor back up if it ever happens.