EMPLOYEE WAS OPERATING THE CAT D9R DOZER UNIT DZ-03 RIPPING COAL. HE WAS BACKING OVER A 2 FT CHUNK OF COAL. AS THE DOZER BACKED OVER THE COAL CHUNK, THE CHUNK BROKE CAUSING THE DOZER TO BANG DOWN ON THE PIT FLOOR. EMPLOYEE WAS LOOKING OVER RIGHT SHOULDER AS HE TOOK THE JOLT. SEVERE PAIN AND NUMBNESS IN LEFT LEG.