EE WAS OPERATING A ROCK TRK THAT WAS LOADED WITHSILT. TRUCK WAS TRAVELING UP A STEEP GRADE WHEN THE LOAD SHIFTED AGAINST THE TAILGATE CAUSING THE TRUCK TO TIP STRAIGHT ONTO THE TAILGATE. AFT ER THE LOAD EMPTIED ONTO THE ROAD THE TRUCK SLAMMED TO THE GROUND UPRIGHT ON ITS TIRES