Limerick Tunnel

Specific information

The 5 tunnel elements of the Shannon tunnel in Limerick were constructed at the same time in a casting basin situated in the northern approach, the 1st element was situated nearest to the river and the 5th was furthest away from it. After completion of the tunnel elements the basin was flooded and the elements were held on the bottom of the basin by filling the ballast tanks.

The immersions were done at a rate of one element per week and were strongly dependent on the tidal window. The tidal water level difference could be up to 5,5m at spring tide. Therefore floatation and immersion had to be done in one session. If not, the tunnel elements would run aground in the casting basin at low water. For the last tunnel element special attention was required because of the large transport distance. To gain time this element was temporarily immersed on its secondary supports at a distance of 1,5m from its final position. The two rear support pins were extracted and the element was set down by the lowering water level. At upcoming tide the element floated up again and the final immersion continued. In final position the rear end of this fifth element protruded above water at low water.

Linked to the large tidal water level differences were the high current flows at the immersion site. A transversal winching system was used to keep the tunnel elements in place. The transverse winches were located on the banks at a larger distance of the construction site.

The foundation of the Limerick tunnel was created by sand flowing and a dry closure joint was created so the northern approach could be built directly connected to the last tunnel element.