LUMP OF DIRT HUNG UP IN DUMP BOX CAUSING LOAD TO SHIFT TO BACK OF BOX. THE LOAD SHIFTED LIFTING FRONT OF TRUCK OFF GROUND. ATRICULATED CAB SWUNG, CAUSING DRIVER TO BE THROWN ONTO RIGHT SHOULDE R OR CAUSING SHOULDER INJURY. DRIVER VIOLATED SAFTEY RULES, WAS NOT WEARING SEAT BELTS.