Limerick, Ireland

Geocon handled the immersion dimensioning for the five tunnel segments that make up the Limerick Tunnel in Ireland. These units measure approximately 100 by 25 by 8.5 metres (L x W x H). The segments were immersed in September 2008.

Client:
National Road Authorities
Location:
Limerick
Number of elements:
5
Immersion:
2008
Specials:
5 metres tide
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Project information

The five tunnel segments were installed in the River Shannon near Limerick in Southwest Ireland. The tunnel, which has a total length of some 900 metres, links Coonagh and Bunlicky. The immersion was part of the ‘Limerick Southern Ring Road Phase II’ project. This was the final stage in the construction of a ring road around Limerick City, which diverts a considerable share of the traffic in this area away from Limerick city centre.

Before immersing the tunnel segments, the team needed to install so-called tiles in pits dredged especially for this purpose in the riverbed. It was necessary to precisely align these tiles (which can measure up to 6.5 by 6.5 by 1.5 metres) with the tunnel axis under water. Due to strong tidal fluctuations, the team had to account for length variations in the immersed joints. These fluctuations could also be accommodated on the secondary end of the segment through the installation of sliding panels on the tile supports.

Measurement systems

The detailed planning showed that installing the final segment between sheet piling and under the jacks would not be feasible within the space of a single tide. That is why halfway through the immersion operation, the team temporarily allowed the receding water to lower the segment on supporting tiles. The complex positioning operation was subsequently rounded off during the next tide.

Geocon used RTK-GPS and total stations to position the tiles, as well as an underwater gyroscope.

The five tunnel segments were immersed with the aid of self-tracking total stations.


Tunnel being floated out

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