For the casting of the parking area bays, solutions are offered that allow the variation from the typical section to the enlarged section of the tunnel bay by means of telescopic travellers and shells. This solution consists of a shell and a traveller that allow lateral opening by means of a hydraulic system to allow the assembly of supplementary pieces in the vault and thus vary the geometry and width of the shell for casting the parking bays. Afterwards, the formwork can be returned back to the typical section to continue with the tunnel melts. All this inside the tunnel and without the need for additional heavy machinery.