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I also have highly reactive blacksoil/clay at my place and my obs is sited on just that sort of soil. SO I am not unfamiliar with it. The thing about black soil/clay is that it will shift when it gets wet but it will also shift when it dries out again. If you can't get your pier deep enough into bedrock to make it stable, you have to engineer it to accommodate the grim reality that it will move wet and dry and the answer, as has been mentioned here, is to ensure you have a reasonably accessible adjustment plate on your pier.
Peter
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