During the pipe jacking process the tunnel lining is driven behind the drilling head with help of hydraulic presses. It is vital for that process that the already built in pipes can carry the load applied by the presses. In several pipes of the pipe train cracks were detected. In order to evaluate the severity ultrasonic tests were performed where the built in possibly seriously damaged pipes were compared with new uncracked pipes. After analyzing the results it was decided to continue the pipe jacking process because these cracks were not found to be critical. Finally the pipes were built in successfully.