Mott MacDonald's primary tunnel lining design was refined to an unreinforced, Discount Roger Dubuis thick SCL designed to carry the full ground load, surcharge and groundwater load in the short term. Immediate application of an initial sealing layer of Discount Romain Jerome was prescribed to minimise deterioration of the ground due to its exposure and to provide safety to the tunnel operatives. To minimise settlement, the excavation sequence was designed as a Discount TAG Heuer round top heading followed by a 1m invert advance staggered 1m behind. After 55m of tunnel excavation and following a detailed review Discount U-Boat observed settlement it was jointly decided that the full face could be advanced in 1m rounds to speed up the closure of the primary lining ring. This also simplified tunnelling and increased the advance rate. The secondary lining was constructed using a cast in-situ, steel reinforced concrete lining, which was cast outside hand-built segmental steel formwork.
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