Sentence was passed by District Judge Brown sitting at Newport Magistrates’ Court on Monday after Kier pleaded guilty to the offence at an earlier hearing.
The court had heard that agency worker Peter Walker, 42, suffered life-changing injuries when a 6.93 tonne block of pre-cast concrete fell on his leg on 15 August 2016.
Walker had started work at the Maerdy Bridge, Cardiff site, where Kier was principal contractor on a Network Rail project to build two bridges, a few days before the accident.
District Judge Brown heard that on the day of the accident the concrete units that would form the bridge were being relocated on the site.
Walker was on a ladder removing lifting chains from a block when it toppled over and his leg was crushed between the fallen unit and the one behind it.
The Office of Rail and Road’s investigation into the incident revealed that Walker’s injuries could have been prevented if Kier had provided adequate supervision for the job.
Ian Prosser, HM Chief Inspector of Railways, said: “Mr Walker suffered devastating injuries in this incident and we are pleased that the severity of the sentence indicates how seriously it is taken by the court.
“We are absolutely committed to protecting the health and safety of workers and, as this prosecution demonstrates, we will not hesitate to take enforcement action when it is necessary.”