This 6+ minute video describes in detail, but the bottom line is this. The Liberty ships that carried food and other aid to Britain in WW2 needed a certain amount of ballast to return back to the US for the next trip. Rubble from the German bombing of the British port city of Bristol, where the ships delivered their cargo, was used to provide that ballast. When it reached NYC it was off loaded and used to support the East Side highway that was being constructed at the time.(Later to be renamed FDR Drive)