Employee stated that he & 6 co-workers were loading 48"" conveyor belt into a coal hauler on the supply yard. The diesel mini-trac was pulling belt in the coal hauler from back, while 966 loader was holding roll of belt. EE was standing approx 10' in front of loader. As the mini-trac was pulling on the belt, he stepped on the belt, lost his balance, struck his head on the ground.