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Hi, I'm Mike G and I'll be assisting you. With PVC conduit from the box to 18inch below grade you'll need to have an expansion coupling on the conduit. In the trench a 2 ft loop of wire is required for ground movement.
Any home center or electrical supply house will have them. All it is, is a slip fitting that allows the stub up movement preventing pulling the junction off its mounting.
Sealing isn't required. Your other installations may have been done before expansion coupling were required.