Fixing porch-supporting pier that settled because of bad drainage
I would endeavor to keep the pier but "fix" it.

First would be to get the pier block back into its original, ideal position.  Probably this can be done DIY with a car jack, a bunch of scrap lumber, shovel, and a helper to muscle the block out of the way, dig/fill appropriately, then replace the block.

Then drive some mini-pilings in front to help hold its position even as the ground wanders away.  If you can rent a jackhammer (or vibratory hammer or pneumatic hammer), you can probably install the mini-pilings yourself.


Rig up a platform for the jack so it can relieve all weight from the pier block and restore the deck position.  It is probably expedient to raise the deck another inch or two above ideal position to provide adequate working space re-inserting the pier.
With all weight off the pier block, move it out of the way.
Rework the underlying soil so it will support the pier block as it was originally.  Probably you'll have to remove the sagged material and add gravel to rebuild its support.
Put the pier block back into position and test it by temporarily jacking down the deck, let the block bear the weight, jump on the deck a few times, and inspect.  Repeat until satisfactory.
Remove the jack and its platform, and install mini-pilings according to instructions.  Note that it is very difficult to pull out a piling if it is not positioned correctly.  Set up ABS or PVC pipe with braces as a guide to starting each.




Having all the foundation be the same design is superior because as the earth moves is the best chance of the deck moving in the same direction at the same time.  Using different foundation technologies will cause the deck to travel in different directions.