The Northern Line extension (NLE) team in London has completed the excavation and installation of the last spheroidal graphite iron (SGI) rings and the casting of the headwalls
The main purpose of the SGI rings is to control tunnel convergence, i.e. to hold the soil around the tunnel at the step plate junction (SPJ). The next stages of construction, waterproofing and secondary lining, are now underway.
Waterproofing consists of applying two layers of membrane all around the tunnel section. Once waterproofing is complete, secondary lining from the SPJs to the two Kennington shafts is able to commence. For this purpose a secondary lining travelling shutter, a bespoke piece of machinery developed to ensure the precision of cast in-situ linings, was procured.
Looking ahead, preparation is ongoing for the 52 hour possessions in September, during which the team will take down, in the SPJs, 120 rings of the existing Northern line tunnel lining at Kennington Green and 106 rings at Kennington Park.
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