The main challenge was the length of the tunnel without having an intermediate shaft. This had an impact on the detailed planning and design to ensure working safety and logistics in this relatively small inner tunnel diameter of approximately 3,650 mm. Therefore, a Mixshield TBM with a shield outer diameter of approximately 4,340 mm was used for the project. Logistics were handled by a Multi Service Vehicle (MSV), not by a rail-bound locomotive, which was the first time this had been undertaken in England for this diameter range. A sophisticated safety plan was implemented to ensure additional working safety at all times during the tunnelling progress.
Several solutions were available for inserting a pipeline into a completed casing tunnel. One possibility was pipeline insertion by means of a Pipe Thruster, a thrust unit originally developed for HDD, which is installed in the launch shaft to push the prefabricated pipeline into the casing tunnel.
This procedure was chosen for the River Humber Crossing in England. As described above, for safety reasons welding was not permitted in the tunnel to join the 12-meter-long concrete-coated pipes. After completion of the tunnelling work, all tunnelling equipment was removed from the tunnel and two Herrenknecht Pipe Thrusters with maximum push forces of 500 and 750 tons respectively were erected in the launch shaft. A total of 8 pipe sections, up to 624 m long, were welded together near the launch shaft and pushed into the flooded tunnel with the help of the Pipe Thrusters.
With a length of almost 5 km, this was the world’s longest hydraulically inserted pipeline and this achievement has been recorded in the 2020 Guinness Book of Records.