So far I've found live colonies of carpenter ants, some kind of wasp (possibly just hiberating in a cavity), and powder post beetles. Some treatment will be necessary in areas that I don't replace completely and to prevent reinfestation.
Here is the same post after getting it onto a solid base:
After much hemming and hawing I decided to use sandwiched pressure treated lumber rather than purchasing new 8x8 beams from a local sawmill. I can move the smaller boards into place by myself with out hiring help.
Note the staggered boards in the portion of sill beneath the post. That part went in first and gives me a good way to patch in the rest of the sill as I move along. Luckily, however, not all the posts are this bad. Here's the next one, partially completed (two more layers of sill will be added on the right, and four layers on the left):
The general technique is: (1) Jack up the area around the post from inside the barn, (2) cut off the rotten part and remove a section of sill around the post, (3) build a new solid foundation (for the first one above, I poured some concrete and also used field stones), (4) build up the sill by the post and any extension for the post to rest on, and (5) then cut out and replace the sill between posts.
I've raised up the sill somewhat to account for removal of rotten wood on the verticals and so that it doesn't rot out again. As shown below, the problem is that the foundation sloped towards the sill:
By building up a bit of a field stone wall for the new sill to rest on, it should stay dry and last a long time. Here is the job on one side, half way done (total time so far, 14.5 hours including removal of the lower siding all around and planning):
Of course I made some archaeological discoveries
And here's my nearly limitless source of additional "field" stones for building up the foundation:
