CONTRACTORS EMPLOYEE HAD ARRIVED AT QUARRY. SHORTLY BEFORE ACCIDENT AND WAS TO HELP UN LOAD CONVEYORS AND LEGS FROM FLATBED AND WAS TO HELP SET UP ON FOLLOWING DAY. INJURED WAS WAS GRABBING A BLOCK FROM FALTBED WHILE THE TRUCK DRIVER WAS REMOVING THE HOLD-DOWN STRAP WHEN CONVEYOR LEG FELL FROM TRAILER AND STRUCK HIS ANKLE.