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Change for the better

A number of subtle design refinements made a major contribution to the tunnel project's success. So says Symonds project manager Chris Marshall, who points to the critical GINA immersion joint that forms the initial seal between the tunnel elements.

'This is a standard feature on immersed tube tunnels, but here we were working to much tighter tunnel positioning tolerances because of the great length and the inclusion of a main railway line.

'So we added hydraulic jacks behind an oversize, very flexible version of the GINA, which allowed us to align the element positively to achieve millimetric tolerances.'

At the same time the design team eliminated the secondary welded steel plate which is normally cast into the end segment to carry the joint and replaced it with a much simpler mounting frame. Similar attention was directed to the dry match-cast joints between the segments, which have no secondary seals apart from a groutable waterbar.

Marshall explains: 'Originally there were half-joints all around the perimeter. But we knew from previous experience that half-joints are far too prone

to honeycombing, so we switched to a shear key design which worked much better.'

At the other end of the scale, so to speak, 0TC had to consider how the huge elements would behave during the tricky transportation phase. A series of model tank tests were carried out in Holland to confirm both the seaworthiness and controllability of the elements in a wide range of sea conditions.

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