How to remove metal pipe stuck in ground that is filled with concrete at bottom
Look up "how to cut a lally column" for illustrations, as that's exactly what they are (steel tube full of concrete). Dig a hole, cut the steel with whatever tools you have (hacksaw, grinder, sawzall) hit the top with a hammer and it will snap easily. If you happen to have a large pipe cutter for steel pipe you can use that, but odds are you don't. There are also specialized tools for lally columns - again, you probably don't have convenient access to one.

The essence remains that once the steel is cut the concrete will snap easily.

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