The worker sustained serious leg injuries when he was struck by an excavator while working on site in April 2007.
An investigation by the Health and Safety Executive revealed that there had been no there had been no segregation between pedestrians and vehicles on site. It also reported a lack of adequate traffic management.
Redworth pleaded guilty a breach of the Health and Safety at Work Act at Bishop Auckland Magistrates' Court.
HM inspector of health and safety Carol Forster said: "Simple precautions could have stopped this incident occurring in the first place and prevented this worker being needlessly injured."
The firm was also ordered to pay £2,321 in costs.