EE WAS HAULING COAL TO THE ""D"" STOCKPILE IN A UNIT RIG 300 TON COAL HAULER. WHEN HE APPROACHED THE DUMP AREA, HE HIT A DEPRESSION IN THE COAL, WHICH CAUSED HIM TO BOUNCE FROM THE OPERATOR'S SE AT, AND STRIKE HIS LOWER RIGHT SIDE AND BACK ON THE ARMREST OF THE OPERATOR'S SEAT AS HE CAME DOWN. OPERATOR CLAIMED HE WAS WEARING HIS SEAT BELT AT THE TIME, BUT STATED IT ""SLIPPED"" AND DIDN