Strutting in the collapsed section was different to that being used on neighbouring cut and cover sections where the alignment was straighter.
On the straight sections NLC fixed struts at 4m centres to the 6m wide diaphragm wall panels. Struts were connected symmetrically to prevent uneven forces being applied. Asymmetrical wall panel loading would have introduced rotational moments that could have undermined the structure's integrity.
An alternating connection pattern was applied along each diaphragm wall: two struts were connected to the outer edges of one panel and a single strut to the centre of the next panel.
But the curved tunnel alignment near the TBM launch shaft forced a radial arrangement of struts. This disrupted the symmetry of the strut to panel connection pattern, and would have resulted in uneven loading of diaphragm wall panels.
A more fl exible strutting system was therefore designed, with struts connected to horizontal waling beams which spread loads over a larger area.
Strutting forces were to be further distributed by the installation of I-section spreaders, branching from the strut-ends at 45¦. Struts were supported midspan by king posts connected by longitudinal stringer beams.
Struts, waling beams and king posts were 400mm 2I-sections.