EMPLOYEE WAS PLACING A ROCK DEFLECTOR ON PHASE 1 SCREEN DECK. HE WAS STANDING ON A PILE OF ROCK FINES WHICH BROKE LOOSE CAUSING HIM TO FALL. HE TWISTED HIS RIGHT KNEE CAUSING A STRAIN. EE WAS INSTRUCTED ON THE IMPORTANCE OF MAKING SURE THAT THE AREA HE IS WORKING IN IS CLEAN AND FREEOF TRIP SLIP HAZARDS.