Today I saw the town maintenance man repair the elbow in a 2" plastic
water pipe, two feet deep in sandy soil.

He said water pressure had separated the joint. His solution was a
steel stake on either side of the joint. That's what he had before, and
it failed.

It seems to me that enough pressure to separate a joint could also, in
time, move a stake in wet, sandy soil. Is that the best way to brace an
elbow undeground?