Newport Magistrates' Court heard how Peter Walker, 42, was working at the Maerdy Bridge site to replace two bridges on 15 August 2016.
On the day of the accident, pre-cast concrete units that would form the bridge were being relocated on the site. The court heard that Mr Walker was on a ladder removing lifting chains from a concrete block when the 6.93-tonne block toppled over, crushing his leg between the fallen unit and the one behind it.
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An investigation by the Office of Rail and Road (ORR) found that Mr Walker's injuries could have been prevented if Kier had provided adequate supervision for the job. Specifically, work on the day and the previous day involved the excavator moving its boom while Mr 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.
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."
At Newport Magistrates' Court on Tuesday, Kier pleaded guilty to one offence under the Health and Safety at Work etc Act.
The court indicated that the firm will be sentenced on 17 July.
(LM)
