Newport Magistrates’ Court heard that a 6.93 tonne block of precast concrete fell on the leg of agency worker Peter Walker, 42 on 15 August 2016.

Walker had started work at the Maerdy Bridge site in South Wales, where Kier was principal contractor on a Network Rail project to replace two bridges, a few days before the accident.

The court 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 concrete block when the block toppled over and his leg was crushed between the fallen unit and the one behind it.

The Office of Rail and Road’s subsequent investigation found that Walker’s injuries could have been prevented if Kier had provided adequate supervision for the job.

Specifically, work on the day of the accident and the previous day involved the excavator moving its boom while Walker was up the ladder, which would have been prevented with proper supervision.

Other examples of poor supervision included a failure to issue a ladder permit, and the excavator moving while the concrete units were suspended, which was expressly forbidden in the lift plan.

The court indicated that sentence will be passed on 17 July.

Ian Prosser, HM Chief Inspector of Railways, said: “Mr Walker suffered devastating injuries in this incident and we are pleased that the guilty plea recognises the seriousness of the offence.

“We are absolutely committed to protecting the health and safety of passengers and railway staff and, as this and previous prosecutions demonstrate, will not hesitate to take enforcement action or prosecute when appropriate.”