A SWI EMPLOYEE WAS PERFORMING HIS ROUTINE DAILY JOB(FLUSHING AND MAINTENANCE ON MFL PADS/LEACHING). HE HAS PERFORMED THE TASK FOR THE LAST YEAR AND A HALF. THE EMPLOYEE HAD FINISHED OPENING A FLUSH VALVE AND WAS WALKING TO WHERE THE OTHER 2 EMPLOYEES WERE AT THE OTHER VALVE. HE STEPPED ON A SOFT AREA AND HIS LEFT KNEE BENT INWARD AND IT CAUSED HIM TO FALL, LANDING ON HIS SIDE.