AT APPROX. 7:30 PM, ON SAT ONE CREW WAS WORKING IN THE SLOPE AT 8 FT AND ONE CREW WAS WORKING AT THE PORTAL OF THE SLOPE INSTALLING THE KNUCKLE ROLLER ASSEMBLY WHILE POSITIONING THE ASSEMBLY A T THE PORTAL THE ASSEMBLY, WHICH IS NOT TIED OFF, STARTED SLIDING DOWN THE SLOPE. IN SLOPE SUPERVISOR EE WAS UNABLE TO SEEK SHELTER, FOR HE WAS RIDING THE MUCK CAR TO THE SURFACE. THE ASSEMBL