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?