shr,

When we talk of settlement being one-off, it usually refers to differential settlement between piled and slab formed foundations such as a vessel (piled) and a pump base for instance. There are circumstances such as sttlemnt of large storage tanks during filling / emptying which is not one-off, but cyclic due to the massive load change and the underlying soil spring rate. In addition there may be tank bulging. A double tied bellows expansion joint is in my opinion an elegant solution here. That said, I would not use flex joints or springs for one-time settlement.

Another point ... 'Design' settlement of for instance, a slab foundation, may well occur almost fully before any piping is hooked up.