OPERATOR WAS DOZING GRAVEL WHEN HE DROVE OVER FROZEN CHUNKS OF GRAVEL CAUSING DOZER TO JAR ANDOPERATOR SLIPPED TO THE SIDE, INJURING HIS RIB CAGE. OPERATOR ADMITTED THAT HE FORGOT TO BUCKLE HIS SEAT BELT. OPERATOR IS A WELL SKILLED EE & NORMALLY AWARE OF SAFETY PROCEDURES.