EMPLOYEE HAD CLEAN A SPRAY NOZZLE AND WAS TRYINGTO GET THE PLANT OPERATOR'S ATTENTION TO TURN ON THE WATER PUMP. THE EMPLOYEE WAS STANDING ON A 4' HIGH PILE OF SCREENINGS. WHEN HE STEPPED DOWN TO GET OFF THE PILE THE PILE WAS FROZEN AND DIDNOT GIVE AS HE EXPECTED. THIS CAUSED HIM TO STRAIN HIS CALF.