TECHNICAL | BANK STATION UPGRADE
During later design stages, DSP undertook detailed Slab NL-RT Piles RT-TS
analyses at each connection to assess the impact on the existing cast iron segmental rings and concluded that the lining stability would be maintained through all stages of formation of the openings in the existing tunnels. LU agreed that a concession against Standards would be acceptable to allow local overstress of the existing cast iron linings, i.e., local cracking close to the openings could be tolerated so long as overall stability of the existing linings was demonstrated. This was because of the operational constraints on installation of in-tunnel propping, the risk of damaging the cast iron by removing the concrete to install the steel pans, and that damage in both those mitigation scenarios might just be delayed or transferred farther down the tunnel. The proposal was that measures to address any
localised serviceability issues would be delayed until after construction of the connections, i.e., if cracking in the cast iron segments occurred it would be repaired. Risks were thoroughly managed through the monitoring, inspection and watchperson measures in place, all managed through the Required Excavation & Support Sheet (RESS) process as normal. In the end, three out of four squarework connections
at the Northern Line level were replaced by an SCL approach at the detailed design stage. Cross passage 4 (CP4), the smallest opening (3.3m wide), was built first. No cracking was observed in the existing cast iron segments from the connection, though one pan was damaged due to concrete removal at a later time. For more details see Nasekhian et al. (2020). Next, construction of the largest SCL connections
(CP1 & 2) took place. Their final opening width was reduced to 10 rings from 12, but were still anticipated to pose a higher risk of localised damage to the cast iron segments due to the relative size of the excavation to the existing asset. An 8.4m-wide SCL passage approached the platform perpendicularly, with a sequence of two top-headings followed by bench and invert (see Figure 5). Cracks occurred in the cast iron flanges at the shoulder and near crown level above the openings during the top-heading/bench excavation, as predicted by the assessment. From their initial appearance they were monitored. Rigidity provided by connection of the SCL lining to the existing tunnel restricted further distortion of the cast iron rings and excavation was concluded without any issues. The final cross passage, also 10 rings wide, was
Top, figure 7: FEA for soil-structure interaction of pile interception Centre, figure 8: Pile interception at Bank
Bottom, figure 9: New triple escalator provided between Central Line platform tunnels 28 | May 2024
constructed using squarework techniques. Similar cracking to that in CP2 was seen in the existing tunnel, illustrating that the methodology was not the deciding factor, but rather the size/width. While there was no risk of collapse (using either methodology), and the repairs were successful, it is recommended that the parent/child relationship in future projects is considered carefully. For upgrade works, this is normally limited by the dimensions of existing tunnels. Given that permanent props are not an option in many cases (and damage is merely delayed to the moment of removal of temporary props in many others) the number of rings in an opening
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