EMPLOYEE OPERATING LOADER WAS MOVING STOCKPILED MATERIAL FROM ONE AREA AND THEN DUMPING IT OVER THE EDGE ON AN ESTABLISHED STOCKPILE. BEFORE BE-GINNING TO DUMP, THE OPERATOR PUSHED OFF THE EDGE AND THEN BEGAN DUMPING MATERIAL OVER THE EDGE(PART ON TOP AND THE REST GOING OVER THE SIDE). THE BANK UNDER THE RIGHT TIRE GAVE WAY. THE LOADER SANK AND THEN ROLLED OVER. NO INJURIES.