BANK STATION UPGRADE | TECHNICAL


Primary SFR sprayed concrete lining domed headwall (min. 350mm)


platform tunnel 3


Northern Line southbound


5/263


a joined-up monitoring and inspection plan was used to address any outstanding risks, learning lessons from previous LU upgrade projects to address serviceability impacts that monitoring alone struggles to capture. From an initial list of 287 civil assets at Bank, only 13


indirectly affected locations required strengthening to withstand the changes in loading and deformation to them, despite the substantial tunnelling planned around the operational railway. With greater understanding of the anticipated movements, it was possible to avoid expensive and intrusive mitigation measures that would impact the travelling public, such as propping, jacking of escalators and speed restrictions.


TUNNEL CONNECTIONS At the concept design stage all proposed tunnel connections to the existing infrastructure were designed using traditional hand mined tunnelling methodologies (also referred to as squareworks). There were four SCL cross passages, connecting to the trackside of the 6.5m-diameter existing Northern Line southbound platform tunnel using squareworks openings. This included two 12-ring (cross passages 1 and 2), one 10-ring (cross passage 3) and one 7-ring wide (cross passage 4) openings. The width of each cast iron ring


is 457mm. A further three adits located between the platforms were designed to be constructed using hand- mining. The squareworks designer at concept was concerned


about failure of the existing cast iron tunnel due to ground movement induced by the new works and propagation of tensile cracks within the tunnel segments in and around the bolt holes and through the depth of the segment pans. The Northern Line connections were identified as requiring extensive strengthening works prior to the tunnelling, noting that internal propping in the platform was not an option as the platform had to be kept operational during construction and was already highly congested. The mitigation proposed a series of steel segment


pans that would sit within the existing cast iron segments just around the openings, to be connected using new bolts through the existing holes. The existing cast iron pans on the Northern Line platforms are infilled with plain concrete, which would have to be removed to install the stiffeners. Clearing the pans in the operational platform tunnel in advance of constructing the cross passages (and adits between the platforms) would have required significant enabling works with notable programme and cost implications.


Above, figure 5: SCL construction sequence at CP1


Below, figure 6: Numerical modelling at connections (Left) Northern Line CP 1 & 2, and (right) DLR CP2


NL NB Platform (5/264, CI)


(SCL, temporary) NL-A1


(hand-mined) NL-CP1 (SCL)


(SCL, temporary) NL-A1C


NL-BC2


Cross passage 5/203 (brick)


NL SB Platform (5/263, CI)


NL-AT2 (SCL, temporary) NL-CP2 (SCL)


DLR-CP2AS (SCL)


DLR-CP2 West (SCL)


(6/270, SGI)


DLR arrival platform


(6/270, PCC)


DLR arrival platform


Escalator 3 upper machine chamber (SCL)


DLR-CC (SCL)


DLR-CP2 East (SCL)


DLR departure platform (6/269, PCC)


DLR cross passage (6/207)


DLR concourse (6/201)


DLR cross passage (6/206)


May 2024 | 27


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