Emp. was standing on the track hoe tracks filling with DEF. When EE finished, EE turned to the left and EE's left foot slipped off the tracks and EE fell landing on the fuel trailer that was parked beside the track hoe. EE suffered 4 fractured ribs and nicked EE's lung on left side. The area received a heavy rain the night before, the area, the tracks, and EE's boots were muddy.