EE ALLEGEDLY SPRAINED LFT KNEE WHILE DESCENDING LADDER OF INGERSOLL-RAND BLASTHOLE DRILL. THE DISTANCE FROM GROUND TO BOTTOM STEP IS 18"". CONDITION OF GROUND ONTO WHICH HE WAS STEPPING WAS A F LAT, GRAVEL PARKING AREA NEAR MAINT SHOP. UPON INVESTIGATION OF THIS ACC, EE VOLUNTARILY RESIGNED. WORKERS COMP IS INVESTIGATING THE ACCIDENT.