As the shutters each weigh around 240t and are 34m long, they are being split into four sections to facilitate the lowering operation. They will then be reassembled to begin their work supporting the insitu-poured 250mm-thick secondary lining against the tunnel’s concrete segmental rings.
The cast secondary lining will reduce the inside diameter of the tunnel to 7.3m but provide a smooth, low-friction and abrasion-resistant surface to help achieve the 120-year design life of the tunnel. Sewage will not require pumping, but will flow eastwards under gravity along the tunnel.
The shutters will be used for the 5km stretch of tunnel between the work sites at Kirtling Street and Carnwath Road. Excavation on this section was completed in November 2019 by a tunnel boring machine.
Kirtling Street lies at the central point of the 25km-long sewer from where TBMs have tunnelled east and west.
