The 675m long tunnel is a Public Private Partnership (PPP) scheme for the country’s National Roads Authority. The twin tunnel is designed to carry four lanes of traffic under the River Shannon and is the centrepiece of a major bypass project for the city.
Five 100m long tunnel units will be built in the casting basin on the north bank of the river. When complete, the basin will be flooded and the tunnel units floated out and sunk into a pre-dredged channel.
The tunnel is completed at each end by cut and cover sections built in situ with approach ramps. Measuring 550m long, 33m wide and 9m deep, the basin has to accommodate all five tunnel elements at once. To complicate matters, the ground conditions on the river banks are poor, with soft alluvial soils to 20m.
The construction sequence begins with rig operators driving piles down to firm strata and installing a row of raking ground anchors at between 2.5m and 3m before excavating the cofferdam to about 6m. This allows a fabricated steel waling beam to be welded inside the pile walls.
The 34m long MP250 super struts are then craned into position and hydraulically pressurised between the waling beams.
Excavation continues to formation level, typically down to 9m, where alluvium gives way to a more competent gravel material. As the excavation reaches full depth, permanent steel and concrete composite U-beams are dropped into the basin to carry the lateral loads. The struts can then be lifted out and moved on to the next section to be excavated.
Due to its exceptional length, the casting basin is being excavated in phases. "The struts are leap-frogged along the basin as the excavation proceeds towards the river," says Groundforce Ireland general manager on site Liam Brew.
The struts cross over a 45m span using 1.2m diameter tubular extensions and special tapered adaptor sections that Groundforce claims are the largest proprietary hydraulic struts available in Europe.
"The extension reduced installation time for 83 precast concrete composite struts from an estimated 20 weeks to just over 12 weeks," says Roadbridge project manager Eamonn Curran. "The hydraulic element also allowed loads to be monitored and checked against the design assumption ensuring safe construction."
Temporary works design was carried out by the Limerick office of White Young Green Ireland.