I recently had a fiberglass pool installed. On the shallow end, when the children jump i can feel a vibrating sensation. It is along the step and each side of the shallow end. My builder dug up the pavers and "sand blasted" more peeble under pool. The sensation on step is less, but still there. The sides are unchanged. Is it acceptable to feel any vibration? Will this cause any cracks? My builder is not interested in fixing the problem. He says it is normal and will not cause harm..