Tunnel struts its stuff

Contractor Roadbridge used six Super MP250 struts from Groundforce Shorco and saved itself eight precious weeks on the project. ‘They reduced the installation time for the 83 precast concrete composite U-beams from an estimated 20 weeks to just over 12 weeks,’ says Roadbridge’s project manager, Eamonn Curran.

Roadbridge is one of four contractors forming the joint venture which is building the ¤570 PPP scheme. The 675m tunnel will be formed from five tunnel units which will be floated out to position and sunk into place, a process known as immersed tube tunnel construction.

The casting basin, 550m long, 33m wide and 9m deep, is constructed by driving in sheet piles, excavating part way down, installing ground anchors, excavating further and installing a horizontal waling beam to the sheet piles.

The 34m long struts are then craned in and hydraulically pressurised between the waling beams on the two sides of sheet piles. Once the excavation gets through the upper layers of alluvium to a gravel material – typically 9m – permanent concrete composite U-beams are dropped in.

The basin is excavated in phases with the struts leap-frogged along the basin as the excavation proceeds.

Groundforce Shorco developed the Super MP250 strut from its standard MP250 strut. Why is it super? Because it can support its maximum 250t load across a clear span of 45m, whereas the standard strut requires vertical propping on spans of more than 25m.