Each ring of the massive 2m wide x 700mm thick segmental lining of the tunnel comprises eight main segments and a key. The lining is reinforced with 100kg/m³ of steel and the eight main segments weigh 20 tonne each. A lining thickness of about 450mm thick would have been suitable for a tunnel this size, said Wittneben, the site manager, "but we needed the extra weight here to counteract buoyancy under such shallow cover."
Each segment has a ‘panelled’ sealing gasket, where gaskets across the 700mm thickness of the segment and between an inner and outer gasket will allow any leaks to be grouted locally. Bolts in the lining are provided to assist ring building only and these are removed after a day or two as the trailing back-up advances and the bolt pockets are filled.
The segments were manufactured by a JV of Dywidag, Hochtief and Strabag in a field casting yard using moulds supplied by CBE of France. Eight grout pumps feed annular grout through a tailskin grouting system to grout up the lining. Each pump operates independently to ensure an even fill of the annulus as the TBM advances.
