SUBJECT WAS RECLAIMING A SLOPE AND WENT TOO FAR OUT ON THE EDGE OF THE FILL. THE DOZER WAS ALLOWED TO GAIN TOO MUCH SPEED, DUE TO OPERATOR ERROR. OPERATOR DID NOT TRY TO SLOW THE DOZER BY LOWE RING THE BLADE. EXCESS SPEED CAUSED THE DOZER TO HIT SOME LARGER ROCKS. THE OPERATORS SEAT BELT OVER TIGHTENED AND JERKED HIS BODY WHICH SPRAINED HIS BACK.