EMPLOYEE WAS SERVICING EQUIPMENT @ END OF SHIFT. AFTER COMPLETION OF REFUELING, HE REENTERED CAB TO TURN ON BOOM LIGHTS. WHEN EXITING, HIS FOOT SLIPPED & HE FELL TO THE GROUND LANDING ON A ROCK. HE FRACTURED HIS LT UPPER ARM IN 2 PLACES. INVESTIGATION REVEALED NO TREAD LEFT ON WORKBOOTS.