HAUL TRUCK DRIVER WAS HAULING WASH SAND TO STOPCKPILE. THERE WAS ADEQUATE BERMING, DRIVER FAILED TO INSPECT FRONT OF STOCKPILE WHERE IT HAD BEEN UNDER MINED. DRIVED BACKED UP TO BERM, THE STOC KPILE GAVE WAY. THE TRUCK WENT HALFWAY DOWN PILE THEN THE TRUCK FLIPPED OVER ONTO ITS ROOF. DRIVER WAS THROWN HALFWAY OUT THE DRIVER DOOR. IT BELIEVE HE WAS NOT WEARING HIS SEATBELT.