a dozen screw jacks

I have inherited a cottage problem.
The posts on which it stands are starting to rot.
They are railway ties set into concrete tubes - badly. The tube form was
only a bit larger than the tie, and over time all the concrete has split off the sides, the posts are now rotting at the bottom.
My idea of how to fix it is to dig beside the existing posts one by one and construct another concrete footing beside it to support a screw jack; when that has hardened use the jack until it supports the house, and cut out the rotting post.
I'll need to do this a dozen times but I figure if I do them one at a time I can avoid a lot of cribbing, a lot of getting under, and risk of anything surprising happening. 10 of the 12 are on the outside edges, and it may be that I can leave the inside ones alone.
Can I leave the cottage on the jacks? This thing is about 20x30 feet, two- storey on one side (enclosed loft), built with local raw-dimension lumber; I have no idea how big a weight-rating jack is needed. Leaving it on the jacks saves me figuring out how to replace the jack with something else while keeping the cottage from moving.
Jacks cost money, and most of them are much longer than I'll need. Does it make sense to get the welder I know to make me a dozen? The bottom could be a plate with 4 stubs of rebar sticking down in the corners and a pipe centered on the top side; the top piece a plate with four corner holes (for big sucker screws) and a stub of threaded rod going down. (I saw this kind of thing on the interweb.) Could these be made to work and left in place? How big for the pipe/rod+nut and plates?
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I don't know about what the building code says but I have seen jacks that are meant to be inserted into pier blocks. When I built a deck for a hot tub I used these as it was the easiest way to get everything level and to get the load distributed correctly. But that was a deck. Homes now are fastened to the foundation around the perimeter so they don't move in an earthquake. If I had a cabin with your problem I would only use the jacks to support the house during the replacement of the posts. I would also try to make the replacements meet the building code. Eric
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