EE WAS OPERATING THE WATER TRUCK IN THE PIT. EE CLAIMED THAT THE WATER VALVE CONTROL BOX BROKE LOOSE & FELL ON THE FLOOR. AS HE ATTEMPTED TO MOVE THE BOX, HE TOOK HIS EYES OFF THE ROAD. THE TRUCK VEERED SHARPLY TO THE LEFT, AND STRUCK THE BERM. THE TRUCK PROCEEDED THROUGH THE BERM AND OVER THE WALL, APPROX. 20', COMING TO REST UPSIDE DOWN.